The life cycle of HIV-1 is divided into early and late phases, shown schematically in the figure. In the early phase, an HIV-1 virion binds to receptors and co-receptors on the human host cell surface (a), viral and host cell membranes fuse and the viral particle is uncoated (b), the viral genome is reverse transcribed and the viral preintegration complex (PIC) forms (c), the PIC is transported through the nuclear pore into the nucleoplasm (d), and the viral reverse transcript is integrated into a host cell chromosome (e). In the late phase, viral RNAs are transcribed from the integrated viral genome and processed to generate viral mRNAs and full-length viral genomic RNAs (f), the viral RNAs are exported through the nuclear pore into the cytosol (g), viral mRNAs are translated and the resulting viral proteins are post-translationally processed (h), core particles containing viral genomic RNA and proteins assemble at the host cell membrane and immature viral particles are released by budding. The released particles mature to become infectious (j), completing the cycle (Frankel and Young 1998; Miller and Bushman 1997). Most of the crucial concepts used to describe these processes were originally elucidated in studies of retroviruses associated with tumors in chickens, birds, and other animal model systems, and the rapid elucidation of the basic features of the HIV-1 life cycle was critically dependent on the intellectual framework provided by these earlier studies. This earlier work has been very well summarized (e.g., Weiss et al. 1984; Coffin et al. 1997); here for brevity and clarity we focus on experimental studies specific to the HIV-1 life cycle.
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Ott DE, Coren LV, Copeland TD, Kane BP, Johnson DG, Sowder RC, Yoshinaka Y, Oroszlan S, Arthur LO, Henderson LE.; ''Ubiquitin is covalently attached to the p6Gag proteins of human immunodeficiency virus type 1 and simian immunodeficiency virus and to the p12Gag protein of Moloney murine leukemia virus.''; PubMedEurope PMCScholia
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Fujinaga K, Irwin D, Huang Y, Taube R, Kurosu T, Peterlin BM.; ''Dynamics of human immunodeficiency virus transcription: P-TEFb phosphorylates RD and dissociates negative effectors from the transactivation response element.''; PubMedEurope PMCScholia
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Klaver B, Berkhout B.; ''Premature strand transfer by the HIV-1 reverse transcriptase during strong-stop DNA synthesis.''; PubMedEurope PMCScholia
Bushman F, Lewinski M, Ciuffi A, Barr S, Leipzig J, Hannenhalli S, Hoffmann C.; ''Genome-wide analysis of retroviral DNA integration.''; PubMedEurope PMCScholia
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Ferrer M, Kapoor TM, Strassmaier T, Weissenhorn W, Skehel JJ, Oprian D, Schreiber SL, Wiley DC, Harrison SC.; ''Selection of gp41-mediated HIV-1 cell entry inhibitors from biased combinatorial libraries of non-natural binding elements.''; PubMedEurope PMCScholia
Buratowski S.; ''Progression through the RNA polymerase II CTD cycle.''; PubMedEurope PMCScholia
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Huber HE, Richardson CC.; ''Processing of the primer for plus strand DNA synthesis by human immunodeficiency virus 1 reverse transcriptase.''; PubMedEurope PMCScholia
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Lin X, Taube R, Fujinaga K, Peterlin BM.; ''P-TEFb containing cyclin K and Cdk9 can activate transcription via RNA.''; PubMedEurope PMCScholia
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Bukrinsky MI, Sharova N, McDonald TL, Pushkarskaya T, Tarpley WG, Stevenson M.; ''Association of integrase, matrix, and reverse transcriptase antigens of human immunodeficiency virus type 1 with viral nucleic acids following acute infection.''; PubMedEurope PMCScholia
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Morita E, Sandrin V, Alam SL, Eckert DM, Gygi SP, Sundquist WI.; ''Identification of human MVB12 proteins as ESCRT-I subunits that function in HIV budding.''; PubMedEurope PMCScholia
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Choe H, Farzan M, Sun Y, Sullivan N, Rollins B, Ponath PD, Wu L, Mackay CR, LaRosa G, Newman W, Gerard N, Gerard C, Sodroski J.; ''The beta-chemokine receptors CCR3 and CCR5 facilitate infection by primary HIV-1 isolates.''; PubMedEurope PMCScholia
Miller MD, Wang B, Bushman FD.; ''Human immunodeficiency virus type 1 preintegration complexes containing discontinuous plus strands are competent to integrate in vitro.''; PubMedEurope PMCScholia
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Lasky LA, Nakamura G, Smith DH, Fennie C, Shimasaki C, Patzer E, Berman P, Gregory T, Capon DJ.; ''Delineation of a region of the human immunodeficiency virus type 1 gp120 glycoprotein critical for interaction with the CD4 receptor.''; PubMedEurope PMCScholia
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Dvir A, Conaway RC, Conaway JW.; ''A role for TFIIH in controlling the activity of early RNA polymerase II elongation complexes.''; PubMedEurope PMCScholia
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Mousson F, Kolkman A, Pijnappel WW, Timmers HT, Heck AJ.; ''Quantitative proteomics reveals regulation of dynamic components within TATA-binding protein (TBP) transcription complexes.''; PubMedEurope PMCScholia
Zhou M, Halanski MA, Radonovich MF, Kashanchi F, Peng J, Price DH, Brady JN.; ''Tat modifies the activity of CDK9 to phosphorylate serine 5 of the RNA polymerase II carboxyl-terminal domain during human immunodeficiency virus type 1 transcription.''; PubMedEurope PMCScholia
Gallaher WR, Ball JM, Garry RF, Griffin MC, Montelaro RC.; ''A general model for the transmembrane proteins of HIV and other retroviruses.''; PubMedEurope PMCScholia
Jiang M, Mak J, Ladha A, Cohen E, Klein M, Rovinski B, Kleiman L.; ''Identification of tRNAs incorporated into wild-type and mutant human immunodeficiency virus type 1.''; PubMedEurope PMCScholia
Malim MH, Bieniasz PD.; ''HIV Restriction Factors and Mechanisms of Evasion.''; PubMedEurope PMCScholia
Zapp ML, Green MR.; ''Sequence-specific RNA binding by the HIV-1 Rev protein.''; PubMedEurope PMCScholia
Fischer U, Meyer S, Teufel M, Heckel C, Lührmann R, Rautmann G.; ''Evidence that HIV-1 Rev directly promotes the nuclear export of unspliced RNA.''; PubMedEurope PMCScholia
Frontini M, Soutoglou E, Argentini M, Bole-Feysot C, Jost B, Scheer E, Tora L.; ''TAF9b (formerly TAF9L) is a bona fide TAF that has unique and overlapping roles with TAF9.''; PubMedEurope PMCScholia
Fritz CC, Green MR.; ''HIV Rev uses a conserved cellular protein export pathway for the nucleocytoplasmic transport of viral RNAs.''; PubMedEurope PMCScholia
Kao SY, Calman AF, Luciw PA, Peterlin BM.; ''Anti-termination of transcription within the long terminal repeat of HIV-1 by tat gene product.''; PubMedEurope PMCScholia
Parvin JD, Sharp PA.; ''DNA topology and a minimal set of basal factors for transcription by RNA polymerase II.''; PubMedEurope PMCScholia
Bischoff FR, Krebber H, Kempf T, Hermes I, Ponstingl H.; ''Human RanGTPase-activating protein RanGAP1 is a homologue of yeast Rna1p involved in mRNA processing and transport.''; PubMedEurope PMCScholia
Ori A, Banterle N, Iskar M, Iskar M, Andrés-Pons A, Escher C, Khanh Bui H, Sparks L, Solis-Mezarino V, Rinner O, Bork P, Lemke EA, Beck M.; ''Cell type-specific nuclear pores: a case in point for context-dependent stoichiometry of molecular machines.''; PubMedEurope PMCScholia
Dvir A, Tan S, Conaway JW, Conaway RC.; ''Promoter escape by RNA polymerase II. Formation of an escape-competent transcriptional intermediate is a prerequisite for exit of polymerase from the promoter.''; PubMedEurope PMCScholia
Yi R, Bogerd HP, Cullen BR.; ''Recruitment of the Crm1 nuclear export factor is sufficient to induce cytoplasmic expression of incompletely spliced human immunodeficiency virus mRNAs.''; PubMedEurope PMCScholia
Wild C, Oas T, McDanal C, Bolognesi D, Matthews T.; ''A synthetic peptide inhibitor of human immunodeficiency virus replication: correlation between solution structure and viral inhibition.''; PubMedEurope PMCScholia
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Farazi TA, Waksman G, Gordon JI.; ''The biology and enzymology of protein N-myristoylation.''; PubMedEurope PMCScholia
Charneau P, Alizon M, Clavel F.; ''A second origin of DNA plus-strand synthesis is required for optimal human immunodeficiency virus replication.''; PubMedEurope PMCScholia
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Rabut G, Doye V, Ellenberg J.; ''Mapping the dynamic organization of the nuclear pore complex inside single living cells.''; PubMedEurope PMCScholia
Kosinski J, Mosalaganti S, von Appen A, Teimer R, DiGuilio AL, Wan W, Bui KH, Hagen WJ, Briggs JA, Glavy JS, Hurt E, Beck M.; ''Molecular architecture of the inner ring scaffold of the human nuclear pore complex.''; PubMedEurope PMCScholia
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Dubay JW, Roberts SJ, Brody B, Hunter E.; ''Mutations in the leucine zipper of the human immunodeficiency virus type 1 transmembrane glycoprotein affect fusion and infectivity.''; PubMedEurope PMCScholia
McDougal JS, Nicholson JK, Cross GD, Cort SP, Kennedy MS, Mawle AC.; ''Binding of the human retrovirus HTLV-III/LAV/ARV/HIV to the CD4 (T4) molecule: conformation dependence, epitope mapping, antibody inhibition, and potential for idiotypic mimicry.''; PubMedEurope PMCScholia
Kilby JM, Eron JJ.; ''Novel therapies based on mechanisms of HIV-1 cell entry.''; PubMedEurope PMCScholia
Rizzuto CD, Wyatt R, Hernández-Ramos N, Sun Y, Kwong PD, Hendrickson WA, Sodroski J.; ''A conserved HIV gp120 glycoprotein structure involved in chemokine receptor binding.''; PubMedEurope PMCScholia
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Zhang W, Canziani G, Plugariu C, Wyatt R, Sodroski J, Sweet R, Kwong P, Hendrickson W, Chaiken I.; ''Conformational changes of gp120 in epitopes near the CCR5 binding site are induced by CD4 and a CD4 miniprotein mimetic.''; PubMedEurope PMCScholia
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DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding (Wada et al.,1998). Spt5 subunit of DSIF can be phosphorylated by P-TEFb.
The C-terminal domain (CTD) of POLR2A contains about 52 repeats of the consensus heptad YSPTSPS. Serines-2 and 5 of the heptads are phosphorylated in RNA polymerase II initiating transcription of protein coding genes. The exact repeats that are phosphorylated are not known.
HIV can infect non-dividing cells, implying that the PIC must be able to traverse the nuclear membrane. In contrast, simple retroviruses such as MLV can only infect cells once they have passed through mitosis, potentially because they require breakdown of the nucleus to access chromosomal integration sites. The mechanism of nuclear localization is controversial. A variety of proposals have been made for nuclear localization sequences (NLS) in the PIC, but most of those have now been shown to be dispensible for HIV integration. According to a new idea from Yamashita and Emerman, it may be that the PIC is imported into the nucleus by a default pathway, while MLV PICs are retained in the cytoplasm because capsid protein is stably associated with PICs.
Nef amino terminal myristoylation has been shown to be critical for many of Nef's functions. As expected myristoylated Nef can be identified as co-fractionating with cell membranes and cytoskeletal components.
The HIV protein known as gp41 is a transmembrane protein which is considered the major mediator of fusion of extracellular virions to the target cells in the host. HIV gp120 and gp41 proteins form non-covalently linked oligomers on the surface of virions. The gp41 subunit of the oligomer is anchored in the viral membrane and contains a non-polar fusion peptide at its N-terminus. Upon CD4 and receptor binding, gp120 undergoes a second conformation change. The conformation change exposes gp41 which continues to mediate fusion of the viral envelope with the host plasma membrane. Electron microscopy and circular dichroism measurements of the gp41 protein suggest a rod-like conformation with a high alpha-helical content. Although some studies suggest that gp41must dissociate from gp120 in order to cause fusion between HIV envelope and the target cell plasma membrane, evidence on this point is not conclusive.
The minus strand strong stop DNA (-sssDNA) is transferred to the 3' end of the HIV-1 genomic RNA, where the 3' end of the -sssDNA anneals to the viral genomic R sequence motif (Ghosh et al. 1995; Klaver and Berkhout 1994; Ohi and Clever 2000; Telesnitsky and Goff 1997). Viral NC (nucleocapsid) protein may play a role in this transfer (Driscoll and Hughes 2000).
To catalyze DNA synthesis, retroviral reverse transcriptase requires a primer strand to extend and a template strand to copy. For HIV-1, the primer is the 3'-end of a partially unwound lysine(3) tRNA annealed to the PBS (primer binding site) 179 bases from the 5' end of the retroviral genomic RNA (Isel et al. 1995). Reverse transcription of the viral genomic RNA proceeds from the bound tRNA primer to the 5' end of the viral RNA, yielding a minus-strand strong-stop DNA (-sssDNA) complementary to the R and U5 elements of the HIV-1 viral genome, as shown in the figure below (Telesnitsky and Goff 1997; Jonckheere et al. 2000). The reaction takes place in the host cell cytosol, and is catalyzed by the reverse transcriptase activity of the HIV-1 RT heterodimer.
NucleoCapsid (NC) protein prevents self-priming by generating or stabilizing a thermodynamically favored RNA-DNA heteroduplex instead of the kinetically favored TAR hairpin seen in reverse transcription experiments in vitro (Driscoll and Hughes 2000).
After the second jump, elongation of the plus and minus strands continues. The elongation process requires strand displacement, which RT can mediate, at least in vitro (Huber et al. 1989; Hottiger et al. 1994; Rausch and Le Grice 2004). The final product is a blunt-ended linear duplex DNA with a discontinuity in its "plus" strand at the site of the cPPT sequence motif.
The mechanism by which the integration reaction is completed has not been fully clarified. Unfolding of the integration intermediate resulting from the IN-catalyzed transesterification produces a branched DNA molecule. Denaturation of the host DNA between the points of joining produces DNA gaps at each host-virus DNA junction. How these gaps are repaired is unclear. Well studied host cell gap repair enzymes can carry out this repair step on model virus-host DNA junctions in vitro, providing candidate enzymes. However, efforts to show importance in vivo are complicated by the fact that the functions are either redundant or lethal when mutated.
Because the strand transfer complex formed at the completion of integration is quite stable, there may be a requirement for a disassembly step to remove integrase and potentially other proteins to allow access of the gap repair machinery. In order to complete the last stages of integration, the viral proteins must be removed, and the gaps at the host virus DNA junctions repaired. The sequence in which the dissembly of PIC occus is not yet understood.
Once the viral gp120 protein has bound to cellular CD4, its bridging sheet region becomes exposed/formed as a result of conformation changes in the V1 and V2 loops as well as a conformational change in the gp120 core domain. Once this region is exposed, it is free to bind the HIV co-receptors CCR5 or CXCR4 (also known as chemokine receptors). Different viruses use different co-receptors (CCR5 or CXCR4) for entry, and many studies investigated the structural determinants of interaction between gp120 and the co-receptor. Studies of CCR5 binding by gp120 revealed that active regions in the second extracellular loop (ECL2), the N-terminal extracellular domain (specifically the NYYTSE motif) and at the junction between the fifth transmembrane domain and third cytoplasmic loop of the receptor are important for viral attachment and subsequent fusion. The N-terminal region likely interacts with the core of gp120 (bridging sheet and adjacent regions) and the base of V3, while ECL2 may be important for interacting with the tip of V3. The transmembrane 5 / cytoplasmic loop 3 junction of CCR5 has been shown to influence the conformation of the receptor which allows for subsequent binding of gp120 (Wang et al.,1999). Deletion of the V3 loop in gp120 abolished Env interaction with co-receptor without affecting the binding of soluble gp120 to CD4, underscoring the importance of this loop in chemokine receptor, but not CD4, binding. Furthermore, the V3 loop is a major determinant of coreceptor specificity, with amino acid at positions 11 and 25 being partly predictive of CCR5 or CXCR4 use. Single amino acid changes in V3 can alter coreceptor use, however sequences outside of V3 can also contribute to coreceptor specificity.
The gp41 glycoprotein contains N- and C-terminal heptad repeats, which form a stable six-helical bundle. This six-helix bundle represents a fusion-active gp41 core, and its conformation is critical for membrane fusion. Among the interactions necessary for the six helix bundle conformation is the formation of a salt bridge between the Asp632 residue in the C-terminal heptad repeat and the Lys574 terminal in the N-terminal coiled-coil. Disruption of this interaction has been found to lead to destabilization of the six helix bundle formation, with a subsequent severe reduction in viral fusion activity. Also, the N-terminal heptad repeat alone was found to be important in viral fusion, as removal or truncation of this repeat reduced the fusion activity of the peptide even when the adjacent, full length N-terminal fusion peptide was in place. The bundle itself is formed during the fusion process, prior to pore formation but after insertion of the gp41 fusion peptide into the target cell membrane. Upon insertion of the fusion peptide, the three N-terminal helices of gp41 adjacent to the target cell membrane and three C-terminal helices adjacent to the viral membrane undergo a conformational change which brings them into close proximity with one another, creating a six-helix bundle and leading to eventual fusion.
CD4, located on the host cell membrane, is the main cellular receptor for the HIV protein gp120, which aids in mediating viral entry into target cells. The initial step in this cascade of events is the binding of viral gp120 protein to its host receptor, CD4. The key binding sites in CD4 for interaction with gp120 are located in the amino-terminal part of the CD4 molecule, distal to the transmembrane domain. The gp120 protein forms an oligomer (trimer) on the viral membrane with each gp120 protein containing variable domains (known as loops) and conservative domains. The V3 loop is also often obscured by gp120 glycosylation. Crystallization studies of CD4 suggest that the molecule has two immunoglobulin like domains important for the CD4/gp120 interaction, with one of the domains (D1) playing a more prominent role. Further studies suggest the Phe 43 and Arg 59 residues of CD4 play a major role in complex formation. Crystallization of gp120 shows that the polypeptide chain is folded into two major domains (an "inner" and "outer" domain with respect to the N and C termini), with the distal end of the “outer� domain containing the V3 loop. Studies of CD4 complexed with gp120 show that CD4 is bound to gp120 in a depression which is formed at the interface between the inner and outer domains. The complex itself is held together through van der Waals forces and hydrogen bonding.
HIV-1 infection of target cells depends on the sequential interaction of the gp120 glycoprotein with the cellular CD4 receptor as well as members of the chemokine receptor family, such as CCR5. Upon interaction with the cellular CD4 receptor, gp120 undergoes a conformation change which allows interaction with these chemokine receptors to occur. Studies indicate that upon binding to CD4, this conformational change results in a repositioning of V1 and V2 loops of gp120, and exposes or forms the "bridging sheet domain" epitopes, which are then available for co-receptor (chemokine receptor) binding along with other domains of gp120. These epitopes are recognized by 17b, a member of a class of antibodies that recognize CD4-induced (CD4i) epitopes (Kwong et al., 1998, Rizzuto et al., 1998, Zhang et al., 1999).
With the removal of all viral genomic RNA and tRNA, the PBS sequence at the 3' end of the plus-strand strong-stop DNA (+sssDNA) is free to pair with the complementary PBS sequence at the 3' end of the minus-strand DNA, to generate a circular structure (Telesnitsky and Goff 1997).
HIV-1 genomic RNA contains a centrally located PPT (cPPT) within the pol gene that, like 3'PPT, is spared by RNase H during minus-strand DNA synthesis and persists to prime plus-strand DNA synthesis. This ribonucleotide primes the synthesis of a plus-strand DNA extending through the U3 and R regions of the HIV sequence and terminating in the PBS region (the tRNA primer-binding site). This DNA segment is known as plus-strand strong-stop DNA (+sssDNA) (Telesnitsky and Goff 1997; Pullen et al. 1993; Huber and Richardson 1990). cPPT priming is important for efficient viral replication (Alizon et al. 1992; Rausch and Le Grice 2004). Several features of cPPT priming in vivo remain to be clarified.
Upon completion of reverse transcription, the viral integrase protein (IN) becomes bound to the ends of the viral DNA. This is inferred by the fact that this is the site of integrase action, and several biochemical studies have documented integrase interactions with the terminal DNA.
Fusion of HIV with target cell plasma membranes is mediated largely by the gp41 glycoprotein. This glycoprotein contains a stretch of strongly hydrophobic amino acids flanked by a series of polar amino acids at its N terminus. Subsequent to the second conformation change in gp120, the N-terminal fusion peptide of gp41 adopts a position which brings it into close proximity with the target cell plasma membrane. As gp41 is found in trimers within the viral membrane, the resulting structure of this conformational change is often referred to as a “prong�, in which three N-terminal peptides extend towards the target cell plasma membrane. The process of fusion begins at this time, with the N-terminus of gp41 inserting itself into the membrane of the target cell.
As the reverse transcriptase activity of the HIV-1 RT heterodimer catalyzes the synthesis of minus-strand strong stop DNA (-sssDNA), the RNaseH activity of the same RT heterodimer catalyzes the degradation of the complementary viral genomic RNA sequences. Degradation of this RNA is required for the efficient transfer of the -sssDNA to the 5' end of the viral genomic RNA. The RNase H active site is positioned within the HIV-1 RT heterodimer so as to attack the RNA strand of the RNA:DNA duplex at a point 18 bases behind the site of reverse transcription (Furfine and Reardon 1991; Ghosh et al. 1995; Gopalakrishnan et al. 1992; Wohrl and Moelling 1990). The rate of RNase H cleavage is substantially lower than the rate of DNA synthesis, however (Kati et al. 1992), and may further depend on RT stalling and structural features of the viral genomic RNA template. The product of these combined DNA synthesis and RNA degradation events is a DNA strand still duplexed with extended viral genomic RNA fragments.
Insertion of the N-terminal fusion peptide of the HIV gp41 protein is the first step in the fusion of viral and target cell membranes. Substitutions of polar amino acids at residues 2, 9, 15 and 26 of the N terminus of this peptide completely eliminated its ability to cause fusion, implicating these residues in gp41’s role in insertion and fusion. Studies have also shown that mutations in a stretch of residues from 36-64(568 to 596 of ENV protein) caused gp41 to become partially or completely defective in mediating membrane fusion, suggesting that conformation of the peptide is important for proper insertion and fusion to occur.
Prior to integration, two nucleotides are removed from each 3' end of the linear viral DNA, thereby exposing recessed 3' hydroxyls. This reaction may serve to remove heterogenous extra bases from the viral DNA end, and to stabilize the IN-DNA complex. The chemistry of cleavage is a simple hydrolysis by single-step transesterification.
The first chemical step of integration involves a single step transesterification, in which the recessed 3' hydroxyl of the viral DNA becomes covalently joined to a protruding 5' end in the target DNA. This step at the same time cleaves the target DNA.
With the transition of gp41 into the six-helix bundle, fusion of the viral and target cell membranes begins to take place. The specifics of fusion are not completely clear, but it is understood that fusion proceeds after insertion of the gp41 fusion peptide, which results in curvature of viral and target cell membranes. This results in a state of hemi-fusion, where only the outer lipid bilayers of each membrane are fused, whereas membrane leaflets that are distal with respect to the intermembrane gap remain separate at this stage. Hemi-fusion allows the exchange of lipids between the contacting leaflets, whereas the exchange of aqueous content between the virus and the cell remains blocked. The next step in fusion is the merger of the distal leaflets, leading to the formation of a nascent fusion pore, which leads to mixing of viral and cellular contents. Studies of fusion of Influenza virus suggested that multiple hairpin structures may form a narrow fusion pore which subsequently expands to a larger opening. In the case of HIV, this larger opening allows for passage of the Matrix-surrounded viral core out of the virus and into the host cell cytoplasm.
Retroviruses use cellular tRNAs as primers for reverse transcription of the viral genomic RNA (Mak and Kleiman 1997). The primer tRNA is selectively packaged during assembly of retrovirus particles. In the case of HIV-1, lysine tRNAs are preferentially incorporated during retroviral packaging, and lysine tRNA 3, the specific isoacceptor form that serves as a primer for reverse transcription, anneals to the PBS (primer binding site) within the U5 region of the viral genomic RNA. This association appears to be mediated by the viral reverse transcriptase (RT) protein, possibly its "thumb" and "connection" domains (Jiang et al. 1993; Mak et al. 1994; Mishima and Steitz 1995).
As the reverse transcriptase activity of the HIV-1 RT heterodimer catalyzes the extension of the minus-strand DNA, the RNaseH activity catalyzes the degradation of the complementary viral genomic RNA sequences. Telesnitsky and Goff (1993) observed that two defective forms of reverse transcriptase can complement to restore retroviral infectivity. The RNase H active site is positioned within the HIV-1 RT heterodimer so as to attack the RNA strand of the RNA:DNA duplex at a point 18 bases behind the site of reverse transcription (Furfine and Reardon 1991; Ghosh et al. 1995; Gopalakrishnan et al. 1992; Wohrl and Moelling 1990). The rate of RNase H cleavage is substantially lower than the rate of DNA synthesis and the level of its activity in vivo is unclear, however (Kati et al. 1992). The product of these combined DNA synthesis and RNA degradation events is a DNA strand still duplexed with extended viral genomic RNA fragments.
Following the integrase-mediated strand transfer reaction of autointegration, the integration complex must be disassembled and the gapped intermediate repaired, just as in normal integration.
Nuclear export of the unspliced and partially spliced HIV-1 transcripts requires the association of the HIV-1 Rev protein with a cis-acting RNA sequence known as the Rev Response Element (RRE) located within the env gene. The RRE forms a stem loop structure that associates with an arginine-rich RNA binding motif (ARM) within Rev.
Crm1 is a nucleocytoplasmic transport factor that is believed to interact with nucleoporins facilitating docking of the RRE-Rev-CRM1-RanGTP complex to the nuclear pore and the translocation of the complex across the nuclear pore complex (see Cullen 1998) Crm1 has been found in complex with two such nucleoporins, CAN/Nup214 and Nup88 which have been shown to be components of the human nuclear pore complex (Fornerod et al., 1997).
This HIV-1 event was inferred from the corresponding human RNA Pol II transcription event.
FCP1 dephosphorylates RNAP II in ternary elongation complexes as well as in solution and, therefore, is thought to function in the recycling of RNAP II during the transcription cycle. Biochemical experiments suggest that human FCP1 targets CTDs that are phosphorylated at serine 2 (CTD-serine 2) and/or CTD-serine 5. It is also observed to stimulate elongation independent of its catalytic activity. Dephosphorylation of Ser2 - phosphorylated Pol II results in hypophosphorylated form that disengages capping enzymes (CE).
Pol II pausing is believed to result from reversible backtracking of the Pol II enzyme complex by ~2 to 4 nucleotides. This leads to misaligned 3'-OH terminus that is unable to be an acceptor for the incoming NTPs in synthesis of next phosphodiester bond (reviewed by Shilatifard et al., 2003).
At the beginning of this reaction, 1 molecule of 'FACT complex', 1 molecule of 'HIV-1 early elongation complex with hyperphosphorylated Pol II CTD', 1 molecule of 'Elongin Complex', 1 molecule of 'TFIIH', 1 molecule of 'RNA polymerase II elongation factor ELL', and 1 molecule of 'TFIIS protein' are present. At the end of this reaction, 1 molecule of 'HIV-1 elongation complex' is present.
This reaction takes place in the nucleus (Hill and Sundquist 2013).
This HIV-1 event was inferred from the corresponding human RNA Pol II transcription event. DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160) (Wada et al. 1998). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding. Spt5 subunit of DSIF can be phosphorylated by P-TEFb (Ivanov et al. 2000).
The association between Tat, TAR and P-TEFb is believed to bring the catalytic subunit of P-TEFb(Cyclin T1:Cdk9) in close proximity to Pol II where it hyperphosphorylates the CTD of Pol II (Herrmann et al., 1995; Zhou et al. 2000). In the presence of Tat, P-TEFb(Cyclin T1:CDK9) has been shown to phosphorylate serine 5 in addition to serine 2 suggesting that modification of the substrate specificity of CDK9 may play a role in the ability of Tat to promote transcriptional elongation (Zhou et al. 2000).
This HIV-1 event was inferred from the corresponding human RNA Pol II transcription event.
NELF complex is a ~ 300 kDa multiprotein complex composed of 5 peptides (A - E): ~66,61,59,58 and 46 kDa (Yamaguchi et al 1999). All these peptides are required for NELF-mediated inhibition of Pol II elongation. NELF complex has been reported to bind to the pre-formed DSIF:RNA Pol II complex that may act as a scaffold for its binding. NELF-A is suspected to be involved in Wolf-Hirschhorn syndrome. Binding of DSIF:NELF to RNA Pol II CTD results in abortive termination of early elongation steps by the growing transcripts.
RNA Pol II arrest is believed to be a result of irreversible backsliding of the enzyme by ~7-14 nucleotides. It is suggested that, arrest leads to extrusion of displaced transcripts 3'-end through the small pore near the Mg2+ ion. Pol II arrest may lead to abortive termination of elongation due to irreversible trapping of the 3'-end of the displaced transcript in the pore (reviewed by Shilatifard et al., 2003).
After assembly of the complete RNA polymerase II-preinitiation complex, the next step is separation of the two DNA strands. This isomerization step is known as the closed-to-open complex transition and occurs prior to the initiation of mRNA synthesis. In the RNA polymerase II system this step requires the hydrolysis of ATP or dATP into Pi and ADP or dADP (in contrast to the other RNA polymerase systems) and is catalyzed by the XPB subunit of TFIIH. The region of the promoter, which becomes single-stranded , spans from –10 to +2 relative to the transcription start site.
Negative supercoiling in the promoter region probably induces transient opening events and can alleviate requirement of TFIIE, TFIIH and ATP-hydrolysis for open complex formation. ATP is also used in this step by the cdk7-subunit of TFIIH to phosphorylate the heptad repeats of the C-terminal domain of the largest subunit of RNA polymerase II (RPB1) on serine-2
Phosphorylation of serine 5 residue at the CTD of pol II largest subunit is an important step signaling the end of initiation and escape into processive elongation processes. Cdk7 protein subunit of TFIIH phosphorylates RNA Pol II CTD serine 5 residues on its heptad repeats (Buratowski 2009).
At the beginning of this reaction, 1 molecule of 'HIV-1 transcription complex containing transcript to +30' is present. At the end of this reaction, 1 molecule of 'HIV-1 transcription complex containing extruded transcript to +30' is present.
This reaction takes place in the 'nucleus' (Buratowski 2009).
Formation of the third phosphodiester bond creates a 4-nt product. This commits the initiation complex to promoter escape. The short 4-nt transcript is still loosely associated with the RNA polymerase II initiation complex and can dissociate to yield abortive products, which are not further extended. Inhibition of ATP-hydrolysis by TFIIH does not lead to collapse of the open region any longer. The transcription complex has lost the sensitivity to single-stranded oligo-nucleotide inhibition. However, ATP-hydrolysis and TFIIH are required for efficient promoter escape. The open region ("transcription bubble") expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +4.
RNA polymerase II transcription complexes are susceptible to transcriptional stalling and arrest, when extending nascent transcripts to 30-nt. This susceptibility depends on presence on down-stream DNA, the particular DNA-sequence of the template and presence of transcription factors. Transcription factor TFIIH remains associated to the RNA pol II elongation complex until position +30. At this stage transcription elongation factor TFIIS can rescue stalled transcription elongation complexes. The transcription bubble varies between 13- and 22-nt in size.
Formation of phosphodiester bonds nine and ten creates RNA products, which do not dissociate from the RNA pol II initiation complex. The transcription complex has enter the productive elongation phase. TFIIH and ATP-hydrolysis are required for efficient promoter escape. The open region (“transcription bubble�) expands concomitant with the site of RNA-extension. The region upstream from the transcription start site (-9 to -3) collapses to the double-stranded state. TFIIH remains associated to the RNA pol II initiation complex.
At the beginning of this reaction, 1 molecule of 'HIV-1 open pre-initiation complex', and 2 molecules of 'NTP' are present. At the end of this reaction, 1 molecule of 'HIV-1 initiation complex' is present.
Formation of the second phosphodiester bond creates a 3-nt product. This short transcript is still loosely associated with the RNA polymerase II initiation complex and can dissociate to yield abortive products, which are not further extended. The transcription complex still requires continued ATP-hydrolysis by TFIIH and remains sensitive to single-stranded oligo-nucleotide inhibition.
The open region (“transcription bubble�) expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +3.
At the beginning of this reaction, 1 molecule of 'mRNA capping enzyme', and 1 molecule of 'HIV-1 transcription complex with (ser5) phosphorylated CTD containing extruded transcript to +30' are present. At the end of this reaction, 1 molecule of 'RNA Pol II with phosphorylated CTD: CE complex' is present.
At the beginning of this reaction, 1 molecule of 'HIV-1 initiation complex' is present. At the end of this reaction, 1 molecule of 'HIV-1 initiation complex with phosphodiester-PPi intermediate' is present.
At the beginning of this reaction, 1 molecule of 'RNA Pol II with phosphorylated CTD: CE complex' is present. At the end of this reaction, 1 molecule of 'RNA Pol II with phosphorylated CTD: CE complex with activated GT' is present.
At the beginning of this reaction, 1 molecule of 'HIV-1 initiation complex with phosphodiester-PPi intermediate' is present. At the end of this reaction, 1 molecule of 'HIV-1 transcription complex', and 1 molecule of 'pyrophosphate' are present.
Formation of the second phosphodiester bond creates a 3-nt product. This transcript is still loosely associated with the RNA polymerase II initiation complex and can dissociate to yield abortive products, which are not further extended. At this stage pausing by RNA polymerase II may result in repeated slippage and reextension of the nascent RNA. The transcription complex still requires continued ATP-hydrolysis by TFIIH for efficient promoter escape. Basal transcription factor TFIIE dissociates from the initiation complex before position +10.
Basal transcription factor TFIIF may reassociate and can stimulate transcription elongation at multiple stages. The open region (“transcription bubble�) expands concomitant with the site of RNA-extension, eventually reaching an open region from -9 to +9.
At the beginning of this reaction, 1 molecule of 'DSIF:NELF:early elongation complex after limited nucleotide addition' is present. At the end of this reaction, 1 molecule of 'Early elongation complex with separated aborted transcript' is present.
TFIIS reactivates arrested RNA Pol II directly interacting with the enzyme resulting in endonucleolytic excision of nascent transcript ~7-14 nucleotides upstream of the 3' end. This reaction is catalyzed by the catalytic site and results in the generation of a new 3'-OH terminus that could be used for re-extension from the correctly base paired site (reviewed by Shilatifard et al., 2003).
Recovery from pausing occurs spontaneously after a variable length of time as the enzyme spontaneously slides forward again. This renders the transcript's 3'-OH terminus realigned with the catalytic Mg2+ site of the enzyme. TFIIS is capable of excising the nascent transcript at 2 or 3 nucleotides upstream of the transcript's 3'-end to reinitiate processive elongation (reviewed by Shilatifard et al., 2003).
The capping enzyme interacts with the Spt5 subunit of transcription elongation factor DSIF. This interaction may couple the capping reaction with promoter escape or elongation, thereby acting as a "checkpoint" to assure that capping has occurred before the polymerase proceeds to make the rest of the transcript.
This HIV-1 event was inferred from the corresponding human RNA Pol II transcription event. High-resolution structures of free, catalytically active yeast Pol II and of an elongating form reveal that Pol II elongation complex includes features like: - RNA-DNA hybrid, an unwound template ahead of 3'-OH terminus of growing transcript and an exit groove at the base of the CTD, possibly for dynamic interaction of processing and transcriptional factors. - a cleft or channel created by Rpb1 and Rpb2 subunits to accommodate DNA template, extending to Mg2+ ion located deep in the enzyme core -a 50 kDa "clamp" with open confirmation in free polymerase, allowing entry of DNA strands but closed in the processive elongation phase. The clamp is composed of portions of Rpb1,Rpb2 and Rpb3 , five loops or "switches" that change from unfolded to well-folded structures stabilizing the elongation complex, and a long "bridging helix" that emanates from Rpb1 subunit, crossing near the Mg2+ ion. The bridging helix is thought to "bend" to push on the base pair at the 3'-end of RNA-DNA hybrid like a ratchet, translocating Pol II along the DNA (Cramer et al.,2001; Gnatt et al.,2001).In addition to its dynamic biochemical potential, Pol II possess a repertoire of functions to serve as a critical platform of recruiting and coordinating the actions of a host of additional enzyme and proteins involved in various pathways.
The association between Tat, TAR and P-TEFb is believed to bring the catalytic subunit of P-TEFb(Cyclin T1:Cdk9) in close proximity to Pol II where it hyperphosphorylates the CTD of Pol II (Herrmann et al., 1995; Zhou et al. 2000). In the presence of Tat, P-TEFb(Cyclin T1:CDK9) has been shown to phosphorylate serine 5 in addition to serine 2 suggesting that modification of the substrate specificity of CDK9 may play a role in the ability of Tat to promote transcriptional elongation (Zhou et al. 2000).
At the beginning of this reaction, 1 molecule of 'FACT complex', 1 molecule of 'Elongin Complex', 1 molecule of 'TFIIH', 1 molecule of 'RNA polymerase II elongation factor ELL', 1 molecule of 'Tat-containing early elongation complex with hyperphosphorylated Pol II CTD ( phospho-NELF phospho DSIF)', and 1 molecule of 'TFIIS protein' are present. At the end of this reaction, 1 molecule of 'HIV-1 elongation complex containing Tat' is present.
Tat associates with the Cyclin T1 subunit of P-TEFb (Cyclin T1:Cdk9) through a region of cysteine-rich and core sequences referred to as the ARM domain within Tat (Wei et al., 1998; see also Herrmann 1995). This interaction is believed to involve metal ions stabilized by cysteine residues in both proteins (Bieniasz et al., 1998; Garber et al., 1998).
Pol II pausing is believed to result from reversible backtracking of the Pol II enzyme complex by ~2 to 4 nucleotides. This leads to misaligned 3'-OH terminus that is unable to be an acceptor for the incoming NTPs in synthesis of next phosphodiester bond (reviewed by Shilatifard et al., 2003).
RNA Pol II arrest is believed to be a result of irreversible backsliding of the enzyme by ~7-14 nucleotides. It is suggested that, arrest leads to extrusion of displaced transcripts 3'-end through the small pore near the Mg2+ ion. Pol II arrest may lead to abortive termination of elongation due to irreversible trapping of the 3'-end of the displaced transcript in the pore (reviewed by Shilatifard et al., 2003).
TFIIS reactivates arrested RNA Pol II directly interacting with the enzyme resulting in endonucleolytic excision of nascent transcript ~7-14 nucleotides upstream of the 3' end. This reaction is catalyzed by the catalytic site and results in the generation of a new 3'-OH terminus that could be used for re-extension from the correctly base paired site (reviewed by Shilatifard et al., 2003).
Recovery from pausing occurs spontaneously after a variable length of time as the enzyme spontaneously slides forward again. This renders the transcript's 3'-OH terminus realigned with the catalytic Mg2+ site of the enzyme. TFIIS is capable of excising the nascent transcript at 2 or 3 nucleotides upstream of the transcript's 3'-end to reinitiate processive elongation (reviewed by Shilatifard et al., 2003).
At the beginning of this reaction, 1 molecule of 'HIV-1 Tat-containing arrested processive elongation complex' is present. At the end of this reaction, 1 molecule of 'HIV-1 Tat-containing aborted elongation complex after arrest' is present. This reaction takes place in the 'nucleus'.
At the beginning of this reaction, 1 molecule of 'HIV-1 transcription complex containing 4-9 nucleotide long transcript' is present. At the end of this reaction, 1 molecule of 'TFIIH', 1 molecule of 'TFIIE', 1 molecule of 'HIV-1 template DNA:4-9 nucleotide transcript hybrid', and 1 molecule of 'RNA Polymerase II (unphosphorylated):TFIIF complex' are present.
At the beginning of this reaction, 1 molecule of 'HIV-1 Promoter Escape Complex' is present. At the end of this reaction, 1 molecule of 'TFIIA', 1 molecule of 'TFIIH', 1 molecule of 'HIV-1 template DNA containing promoter with transcript of 2 or 3 nucleotides', 1 molecule of 'TFIIE', 1 molecule of 'TFIID', 1 molecule of 'TFIIB', and 1 molecule of 'RNA Polymerase II (unphosphorylated):TFIIF complex' are present.
At the beginning of this reaction, 1 molecule of 'HIV-1 transcription complex' is present. At the end of this reaction, 1 molecule of 'TFIIA', 1 molecule of 'TFIIH', 1 molecule of 'TFIIE', 1 molecule of 'TFIID', 1 molecule of 'TFIIB', 1 molecule of 'RNA Polymerase II (unphosphorylated):TFIIF complex', and 1 molecule of 'HIV-1 template DNA with first transcript dinucleotide, opened to +8 position' are present.
In the early elongation phase, shorter transcripts typically of ~30 nt in length are generated due to random termination of elongating nascent transcripts. This abortive cessation of elongation has been observed mainly in the presence of DSIF-NELF bound to Pol II complex. (Reviewed in Conaway et al.,2000; Shilatifard et al., 2003 ).
At the beginning of this reaction, 1 molecule of 'HIV-1 arrested processive elongation complex' is present. At the end of this reaction, 1 molecule of 'HIV-1 aborted elongation complex after arrest' is present.
At the beginning of this reaction, 1 molecule of 'HIV-1 open pre-initiation complex' is present. At the end of this reaction, 1 molecule of 'HIV-1 closed pre-initiation complex' is present.
Phosphorylation of the RD subunit of NEFL by P-TEFb(Cyclin T1:Cdk9) results in the dissociation of NEFL from TAR as well as the conversion of NEFL to an elongation factor (Fujinaga et al., 2004)
The HIV capsid protein (p24) surrounds the viral genome and associated proteins to make up the viral core. Dissolution of the viral capsid allows for release of the viral RNA and other proteins such as Vpr into the cytoplasm, which will subsequently form the Reverse Transcription Complex. Dissolution of capsid proteins may be caused by interaction with cellular proteins, e.g. TRIM5, or may occur in a similar fashion to that of matrix dissolution; as a reaction to a change in pH. Indeed, studies observing capsid assembly and conformation show that this protein-protein interaction is heavily influenced by even small changes in pH (pH7.0 to 6.8).
Concomitant with the completion of reverse transcription, the pre-integration complex is formed by shedding of some viral proteins from the viral core, and binding of cellular proteins, thereby yielding complexes capable of integration. The terminal cleavage reaction takes place in the cytoplasm, where two nucleotides are removed from each viral DNA 3' end. This serves to remove heterogeneous extra bases from the viral DNA ends occasionally added by reverse transcription, thereby yielding a homogeneous substrate for downstream steps, and also serves to stablilize the PIC. The DNA in PICs is considerably compacted relative to its length when fully extended, probably due to binding of proteins in addition to the viral integrase. These proteins are not fully clarified, due to the difficulty of biochemical analysis of small amounts of material, but candidates include the viral NC and MA proteins, and the cellular HMGA, BAF, and PSIP1/LEDGF/p75 proteins. Purified integrase is capable of carrying out the terminal cleavage and initial strand transfer reactions.
After fusion of the viral membrane with the target cell membrane, the viral core, which is surrounded by a layer of Matrix (p17) proteins, is exposed to the cytoplasm. Disintegration of the Matrix layer allows for the conical-shaped viral core to be fully released, and allow for viral capsid dissociation and eventually reverse transcription. Dissociation of the Matrix layer is not well characterized, but is believed to occur due to disruption of protein-protein interactions as a result of the conditions of the cytoplasm (including pH), which differ from that of the internal viral structure.
The cleaved and assembled gp41:gp121 complexes are transport to teh plasma membrane. This complex ultimately arrives via the cellular secretion pathway. Env is an integral membrane protein shuttled through the ER and Golgi where it was glycosylated and cleaved into the gp41 and gp120 subunits. The trimeric complex is brought to the plasma membrane by the host vesicular transport system. Only 7-14 trimers are present per virion.
RNase H catalyzes the precise cleavage of the bonds linking the primer tRNA attached to the minus-strand DNA, the 3' PPT RNA primer to the plus-strand strong-stop DNA, and the cPPT primer to the stretch of plus-strand DNA whose synthesis it primed. In each case, precise cleavage near the RNA-DNA junction occurs (Pullen et al. 1992). HIV-1 RT is the only reverse transcriptase that cleaves the tRNA:DNA junction so as to leave a ribo A residue from the tRNA at the 5' end of the minus strand.
While a single RT heterodimer could in principle catalyze DNA synthesis and primer RNA:DNA bond cleavage, evidence from several in vitro systems suggests that separate RT heterodimers are likely to catalyze these two reactions (Rausch and Le Grice 2004).
Reverse transcription complex is a transitory structure where reverse transcription takes place. Initially, it is likely identical to the RNA-protein complex found inside the virion core. Upon maturation, it may shed some HIV proteins (such as MA or Vpr) and incorporate cellular proteins (such as INI1 or PML).
There are numerous N-linked glycosylation sites that are important for infectivity of human immunodeficiency virus type 1. With more than 20 consensus N-linked glycosylation sites in gp120 it is expected that a number are important for virion function.
How the PIC finds favored sites on target DNA has not been fully clarified. Active genes are favored for integration, and favored sequences at the site of integration also influence the reaction. Studies of cells depeleted in PSIP1/LEDGF/p75 suggest that this protein acts as a tethering factor binding HIV PICs near integration target DNA. Access of PICs to sites on chromosomes may be significant, since centromeric alphoid repeats are disfavored for integration, perhaps due to wrapping in compact centromeric heterochromatin. Nucleosomes bound to the integration template also affect target site selection and integration complex binding.
The 1-LTR circle can be formed by either of two pathways. The first involves a failure to complete reverse transcription; the second, annotated here, follows the completion of reverse transcription and is mediated by cellular enzymes. In this pathway, the action of host cell homologous recombination enzymes on the long terminal repeat (LTR) termini of the viral DNA results in formation of a single LTR. This reaction probably takes place after partial or complete disassembly of the PIC to expose the viral DNA. Repair of this intermediate as in the late stages of homologous recombination pathways results in formation of the 1-LTR circle. Mutations in the Mre11/Rad50/NBS pathway influence the formation of 1-LTR circles.
The Ku protein can be found bound to active PICs in the cytoplasm. However, ligation of the viral DNA ends to form 2-LTR circles takes place in the nucleus.
Following the integrase-mediated strand transfer reaction of autointegration, the integration complex must be disassembled and the gapped intermediate repaired, just as in normal integration.
Viral DNA that does not become integrated can undergo another fate, which is to have the two viral DNA ends joined together to form a 2-LTR circle. This reaction requires Ku, XRCC4 and ligase 4.
CRM1 associates directly with Rev through the Rev nuclear export signal (NES) domain and acts as the nuclear export receptor for the Rev-RRE ribonucleoprotein complex.
The rate of RNase H cleavage is substantially lower than the rate of DNA synthesis (Kati et al. 1992), so the product of the combined DNA synthesis and RNA degradation events catalyzed by the RT heterodimer mediating minus-strand DNA synthesis is a DNA segment still duplexed with extended viral genomic RNA fragments. Other RT heterodimers bind the remaining RNA:DNA heteroduplexes and their RNase H domains further degrade the viral genomic RNA (Wisniewski et al. 2000a, b). Two PPT (polypurine tract) sequence motifs in the template, one immediately 5' to the U3 sequence and one located within the pol gene in the center of the viral genome, are spared from degradation (Charneau et al. 1992; Julias et al. 2004; Pullen et al. 1993).
The rate of RNase H cleavage is substantially lower than the rate of DNA synthesis (Kati et al. 1992), so the product of the combined DNA synthesis and RNA degradation events catalyzed by the RT heterodimer mediating minus-strand strong stop DNA (-sssDNA) synthesis is a DNA segment still duplexed with extended viral genomic RNA fragments. In vitro, other RT heterodimers bind the remaining RNA:DNA heteroduplexes and their RNase H domains further degrade the viral genomic RNA (Wisniewski et al. 2000a, b).
The fate of the discontinuous viral DNA duplex synthesized in the cytosol of an infected cell by HIV-1 reverse transcriptase is not entirely clear. Studies of some viral systems suggest that this discontinuous structure is required for passage of the viral duplex DNA into the nucleus while there are evidence contrary to this observation. Studies in vitro indicate that human nuclear flap endonuclease and DNA ligase can remove the flap and seal the plus-strand discontinuity in HIV-1 DNA (Miller et al. 1995; Rausch and Le Grice 2004; Rumbaugh et al. 1998), although role of flap is not yet clear.
Monoubiquitinated N-myristoyl Gag polyprotein associates with the ESCRT-1 complex at an endosomal membrane (Eastman et al. 2005; Martin-Serrano et al. 2003; Stuchell et al. 2004).
Cytosolic N-myristoyl Gag polyprotein is conjugated with a single molecule of ubiquitin. Conjugation is typically to one of two lysine residues in the p6 domain of Gag but can be to lysine residues in the MA, CA, NC, and SP2 domains of the protein. The specific host cell E2 and E3 proteins that mediate Gag ubiquitination have not been identified. The same studies that first identified the p6 ubiquitination sites in Gag also called the biological significance of Gag ubiquitination into question by demonstrating that Gag proteins in which the p6 ubiquitination sites had been removed by mutagenesis could still assemble efficiently into infectious viral particles (Ott et al. 1998, 2000). More recent work, however, has identified additional ubiquitination sites throughout the carboxyterminal region of the Gag polyprotein, and when all of these sites are removed by mutagenesis, both viral assembly involving the mutant Gag polyprotein and infectivity of the resulting viral particles are sharply reduced (Gottwein et al. 2006).
The amino terminal glycine residue of HIV-1 Gag polyprotein is myristoylated (Henderson et al. 1992). Myristoylation of newly synthesized Gag occurs in the cytosol of the infected host cell, with myristoyl-CoA as the myristate donor and the host cell NMT2 enzyme as the catalyst. Human cells express two isoforms of N-myristoyl transferase (NMT) (Giang and Cravatt 1998). The argumant that the second isoform catalyzes this reaction is indirect, based on the the observations that a stable enzyme:substrate complex forms transiently during the reaction (Farazi et al. 2001), and that Gag polyprotein can be found complexed with NMT2 (but not NMT1) in HIV-1-infected human cells (Hill and Skowronski 2005).
Gag is translated from the unspliced viral RNA on free ribosomes in the cytoplasm. The products of the pro and pol genes are also synthesized from the unspliced viral RNA, but never as parts of an independent polyprotein. They are initially contained within the Gag-Pro or Gag-Pro-Pol fusion protein, the product of translational readthrough
The proteolytic events that cleave Gag and Gag-Pro-Pol are well characterized, but the event that triggers the protease is not well characterized. The PRGag, that is assembled in the immature virion weakly dimerizes, once PR is cleaved from the proprotein PR dimerizes and becomes an efficient protease. This assembly step may be part of the switch. Once the protease becomes active in the immature virion MA, CA, SP1, NC, SP2, P6, PR, RT, and IN are produced. This event, the production of these fragments would be the switch from immature to mature.
Once transported to the plasma membrane the VPU protein will be incorporated into the assembling virus. The Vpu accessory protein is found to be required for efficient virion release from some cell lines but completely dispensible in others.
Assembling Gag molecules are largely derived from the rapidly diffusing cytoplasmic pool. Gag membrane targeting requires myristoylation and a subset of GAG molecules are shuttled to the plasma membrane in this way.
Gag assembly leads to formation of the immature lattice. The Gag molecules in the immature virion are extended and oriented radially, with their amino-terminal MA domains bound to the inner membrane leaflet and their carboxy- terminal p6 domains facing the interior of the particle. The GAGPol Pro molecules have arrived at the site of viral assembly in fewer numbers than the Gag protein (20:1). The trimeric gp41:gp120 complex is brought to the plasma membrane by the host vesicular transport system. Only 7-14 trimers per virion. VPU has followed the same ER:Golgi path. Vif, Nef, and Vpr are packaged along with the the HIV genome.
The events that lead to the viral component assembly and the recruitment of the ESCRT host machinery are well-characterized. The exact steps that release the immature viral particle are not. Membrane fission is an energy intensive process and an active area of study.
The human ESCRT pathway comprises more than 30 different proteins, and this complexity is expanded further by associated regulatory and ubiquitylation machinery. Functional studies have identified a minimal core set of human ESCRT proteins, machinery that is essential for HIV-1 budding. ESCRT-1 recruitment follows an unusal path. The PTAP motif in p6 mimics the ESCRT-1 recruitment motif, bypassing the need for ESCRT-0. The TSG101/ ESCRT-I and ALIX both function by recruiting downstream ESCRT-III and VPS4 complexes, which in turn mediate membrane fission and ESCRT factor recycling.
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DataNodes
DNA as smaller
circlesDNA as an inverted
circlehairpin
complex:CCR5/CXCR4elongation complex after limited
nucleotide additioncomplex with separated aborted
transcriptPolymerase II
(phosphorylated):TFIIF:capped pre-mRNAaborted elongation complex after
arrestarrested processive
elongation complexpaused processive
elongation complexprocessive
elongation complexelongation complex
after arrestprocessive
elongation complexpre-mRNA:CBC:RNA Pol II (phosphorylated)
complexpre-initiation
complexelongation complex with hyperphosphorylated
Pol II CTDcomplex containing
Tatcomplex with phosphodiester-PPi
intermediatepre-initiation
complexprocessive
elongation complexDNA:4-9 nucleotide
transcript hybridcontaining promoter with transcript of
2 or 3 nucleotideswith first transcript dinucleotide, opened to +8
positioncomplex containing 11 nucleotide long
transcriptcomplex containing 3 nucleotide long
transcriptcomplex containing 4 nucleotide long
transcriptcomplex containing 4-9 nucleotide long
transcriptcomplex containing 9 nucleotide long
transcriptcomplex containing extruded transcript
to +30complex containing
transcript to +30complex with (ser5) phosphorylated CTD containing extruded
transcript to +303' ends of viral
DNA in PIC3' ends of viral
DNA in PICto host genomic DNA
with staggered endsT1:Cdk9)-containing elongation complex with separated and uncleaved
transcriptPol II
(hypophosphorylated) complex bound to DSIF proteinPol II
(hypophosphorylated):capped pre-mRNA complexPolymerase II
(unphosphorylated):TFIIF complexphosphorylated CTD: CE complex with
activated GTphosphorylated CTD:
CE complexTranscription Complex) with RNA
templatecomplementary PBS seqments in +sssDNA
and -strand DNARNA template and
minus sssDNAcontaining discontinuous plus
strand flapsssDNA transferred to 3'-end of viral
RNA templatesssDNA:tRNA
primer:RNA templatestrand DNA synthesis initiated
from 3'-endminus sssDNA:tRNA
primer:RNA templatemultimer-bound HIV-1
mRNA:Crm1:Ran:GTP:NPCHIV-1
mRNA:Crm1:Ran:GTPHIV-1
mRNA:Crm1:Ran:GTPHIV-1 mRNA:CRM1
complexelongation complex
prior to separationelongation complex with hyperphosphorylated Pol II CTD ( phospho-NELF
phospho DSIF)elongation complex with hyperphosphorylated Pol II CTD and
phospho-NELFelongation complex with hyperphosphorylated
Pol II CTDsurrounded by
Matrix layerCD4:gp120 bound to
CCR5/CXCR4fusogenically
activated gp41coreceptor binding
sitesforming hairpin
structurefusion peptide in
insertion complexN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinProtein
(UniProt:P04601)proteins:XRCC4:DNA
ligase IV complexAnnotated Interactions
DNA as smaller
circlesDNA as an inverted
circlehairpin
complex:CCR5/CXCR4elongation complex after limited
nucleotide additionelongation complex after limited
nucleotide additioncomplex with separated aborted
transcriptPolymerase II
(phosphorylated):TFIIF:capped pre-mRNAaborted elongation complex after
arrestarrested processive
elongation complexarrested processive
elongation complexarrested processive
elongation complexpaused processive
elongation complexpaused processive
elongation complexprocessive
elongation complexprocessive
elongation complexprocessive
elongation complexprocessive
elongation complexprocessive
elongation complexprocessive
elongation complexelongation complex
after arrestprocessive
elongation complexprocessive
elongation complexprocessive
elongation complexpre-mRNA:CBC:RNA Pol II (phosphorylated)
complexpre-mRNA:CBC:RNA Pol II (phosphorylated)
complexpre-initiation
complexpre-initiation
complexelongation complex with hyperphosphorylated
Pol II CTDelongation complex with hyperphosphorylated
Pol II CTDcomplex containing
Tatcomplex containing
Tatcomplex with phosphodiester-PPi
intermediatecomplex with phosphodiester-PPi
intermediatepre-initiation
complexpre-initiation
complexpre-initiation
complexprocessive
elongation complexprocessive
elongation complexDNA:4-9 nucleotide
transcript hybridcontaining promoter with transcript of
2 or 3 nucleotideswith first transcript dinucleotide, opened to +8
positioncomplex containing 11 nucleotide long
transcriptcomplex containing 11 nucleotide long
transcriptcomplex containing 3 nucleotide long
transcriptcomplex containing 3 nucleotide long
transcriptcomplex containing 4 nucleotide long
transcriptcomplex containing 4 nucleotide long
transcriptcomplex containing 4-9 nucleotide long
transcriptcomplex containing 9 nucleotide long
transcriptcomplex containing 9 nucleotide long
transcriptcomplex containing extruded transcript
to +30complex containing extruded transcript
to +30complex containing
transcript to +30complex containing
transcript to +30complex with (ser5) phosphorylated CTD containing extruded
transcript to +30complex with (ser5) phosphorylated CTD containing extruded
transcript to +303' ends of viral
DNA in PIC3' ends of viral
DNA in PIC3' ends of viral
DNA in PIC3' ends of viral
DNA in PIC3' ends of viral
DNA in PIC3' ends of viral
DNA in PIC3' ends of viral
DNA in PIC3' ends of viral
DNA in PICto host genomic DNA
with staggered endsto host genomic DNA
with staggered endsto host genomic DNA
with staggered endsT1:Cdk9)-containing elongation complex with separated and uncleaved
transcriptNucleoCapsid (NC) protein prevents self-priming by generating or stabilizing a thermodynamically favored RNA-DNA heteroduplex instead of the kinetically favored TAR hairpin seen in reverse transcription experiments in vitro (Driscoll and Hughes 2000).
Because the strand transfer complex formed at the completion of integration is quite stable, there may be a requirement for a disassembly step to remove integrase and potentially other proteins to allow access of the gap repair machinery.
In order to complete the last stages of integration, the viral proteins must be removed, and the gaps at the host virus DNA junctions repaired. The sequence in which the dissembly of PIC occus is not yet understood.
Studies of CCR5 binding by gp120 revealed that active regions in the second extracellular loop (ECL2), the N-terminal extracellular domain (specifically the NYYTSE motif) and at the junction between the fifth transmembrane domain and third cytoplasmic loop of the receptor are important for viral attachment and subsequent fusion. The N-terminal region likely interacts with the core of gp120 (bridging sheet and adjacent regions) and the base of V3, while ECL2 may be important for interacting with the tip of V3. The transmembrane 5 / cytoplasmic loop 3 junction of CCR5 has been shown to influence the conformation of the receptor which allows for subsequent binding of gp120 (Wang et al.,1999). Deletion of the V3 loop in gp120 abolished Env interaction with co-receptor without affecting the binding of soluble gp120 to CD4, underscoring the importance of this loop in chemokine receptor, but not CD4, binding. Furthermore, the V3 loop is a major determinant of coreceptor specificity, with amino acid at positions 11 and 25 being partly predictive of CCR5 or CXCR4 use. Single amino acid changes in V3 can alter coreceptor use, however sequences outside of V3 can also contribute to coreceptor specificity.
This reaction takes place in the nucleus (Hill and Sundquist 2013).
Negative supercoiling in the promoter region probably induces transient opening events and can alleviate requirement of TFIIE, TFIIH and ATP-hydrolysis for open complex formation. ATP is also used in this step by the cdk7-subunit of TFIIH to phosphorylate the heptad repeats of the C-terminal domain of the largest subunit of RNA polymerase II (RPB1) on serine-2
This reaction takes place in the 'nucleus' (Buratowski 2009).
This reaction takes place in the 'nucleus'.
The open region (“transcription bubble�) expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +3.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
Basal transcription factor TFIIF may reassociate and can stimulate transcription elongation at multiple stages. The open region (“transcription bubble�) expands concomitant with the site of RNA-extension, eventually reaching an open region from -9 to +9.
This reaction takes place in the 'nucleus'.
- RNA-DNA hybrid, an unwound template ahead of 3'-OH terminus of growing transcript and an exit groove at the base of the CTD, possibly for dynamic interaction of processing and transcriptional factors.
- a cleft or channel created by Rpb1 and Rpb2 subunits to accommodate DNA template, extending to Mg2+ ion located deep in the enzyme core
-a 50 kDa "clamp" with open confirmation in free polymerase, allowing entry of DNA strands but closed in the processive elongation phase.
The clamp is composed of portions of Rpb1,Rpb2 and Rpb3 , five loops or "switches" that change from unfolded to well-folded structures stabilizing the elongation complex, and a long "bridging helix" that emanates from Rpb1 subunit, crossing near the Mg2+ ion. The bridging helix is thought to "bend" to push on the base pair at the 3'-end of RNA-DNA hybrid like a ratchet, translocating Pol II along the DNA (Cramer et al.,2001; Gnatt et al.,2001).In addition to its dynamic biochemical potential, Pol II possess a repertoire of functions to serve as a critical platform of recruiting and coordinating the actions of a host of additional enzyme and proteins involved in various pathways.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
While a single RT heterodimer could in principle catalyze DNA synthesis and primer RNA:DNA bond cleavage, evidence from several in vitro systems suggests that separate RT heterodimers are likely to catalyze these two reactions (Rausch and Le Grice 2004).
Pol II
(hypophosphorylated) complex bound to DSIF proteinPol II
(hypophosphorylated) complex bound to DSIF proteinPol II
(hypophosphorylated):capped pre-mRNA complexPol II
(hypophosphorylated):capped pre-mRNA complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexphosphorylated CTD: CE complex with
activated GTphosphorylated CTD: CE complex with
activated GTphosphorylated CTD:
CE complexphosphorylated CTD:
CE complexTranscription Complex) with RNA
templateTranscription Complex) with RNA
templatecomplementary PBS seqments in +sssDNA
and -strand DNAcomplementary PBS seqments in +sssDNA
and -strand DNAcomplementary PBS seqments in +sssDNA
and -strand DNARNA template and
minus sssDNARNA template and
minus sssDNAcontaining discontinuous plus
strand flapcontaining discontinuous plus
strand flapsssDNA transferred to 3'-end of viral
RNA templatesssDNA transferred to 3'-end of viral
RNA templatesssDNA transferred to 3'-end of viral
RNA templatesssDNA:tRNA
primer:RNA templatesssDNA:tRNA
primer:RNA templatesssDNA:tRNA
primer:RNA templatestrand DNA synthesis initiated
from 3'-endstrand DNA synthesis initiated
from 3'-endstrand DNA synthesis initiated
from 3'-endminus sssDNA:tRNA
primer:RNA templateminus sssDNA:tRNA
primer:RNA templatemultimer-bound HIV-1
mRNA:Crm1:Ran:GTP:NPCmultimer-bound HIV-1
mRNA:Crm1:Ran:GTP:NPCHIV-1
mRNA:Crm1:Ran:GTPHIV-1
mRNA:Crm1:Ran:GTPHIV-1
mRNA:Crm1:Ran:GTPHIV-1
mRNA:Crm1:Ran:GTPHIV-1
mRNA:Crm1:Ran:GTPHIV-1 mRNA:CRM1
complexHIV-1 mRNA:CRM1
complexelongation complex
prior to separationelongation complex
prior to separationelongation complex with hyperphosphorylated Pol II CTD ( phospho-NELF
phospho DSIF)elongation complex with hyperphosphorylated Pol II CTD ( phospho-NELF
phospho DSIF)elongation complex with hyperphosphorylated Pol II CTD and
phospho-NELFelongation complex with hyperphosphorylated Pol II CTD and
phospho-NELFelongation complex with hyperphosphorylated Pol II CTD and
phospho-NELFelongation complex with hyperphosphorylated
Pol II CTDelongation complex with hyperphosphorylated
Pol II CTDelongation complex with hyperphosphorylated
Pol II CTDsurrounded by
Matrix layersurrounded by
Matrix layerCD4:gp120 bound to
CCR5/CXCR4CD4:gp120 bound to
CCR5/CXCR4fusogenically
activated gp41fusogenically
activated gp41coreceptor binding
sitescoreceptor binding
sitesforming hairpin
structureforming hairpin
structurefusion peptide in
insertion complexfusion peptide in
insertion complexN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinN-myristoyl GAG
(P04591) proteinProtein
(UniProt:P04601)Protein
(UniProt:P04601)Protein
(UniProt:P04601)proteins:XRCC4:DNA
ligase IV complexproteins:XRCC4:DNA
ligase IV complexproteins:XRCC4:DNA
ligase IV complex