Major histocompatibility complex (MHC) class I molecules play an important role in cell mediated immunity by reporting on intracellular events such as viral infection, the presence of intracellular bacteria or tumor-associated antigens. They bind peptide fragments of these proteins and presenting them to CD8+ T cells at the cell surface. This enables cytotoxic T cells to identify and eliminate cells that are synthesizing abnormal or foreign proteins. MHC class I is a trimeric complex composed of a polymorphic heavy chain (HC or alpha chain) and an invariable light chain, known as beta2-microglobulin (B2M) plus an 8-10 residue peptide ligand. Represented here are the events in the biosynthesis of MHC class I molecules, including generation of antigenic peptides by the ubiquitin/26S-proteasome system, delivery of these peptides to the endoplasmic reticulum (ER), loading of peptides to MHC class I molecules and display of MHC class I complexes on the cell surface.
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Co-immunoprecipitation studies and size exclusion chromatography analysis indicate that the high molecular weight (around 700 to 900 kDa) IKK complex is composed of two kinase subunits (IKK1/CHUK/IKBKA and/or IKK2/IKBKB/IKKB) bound to a regulatory gamma subunit (IKBKG/NEMO) (Rothwarf DMet al. 1998; Krappmann D et al. 2000; Miller BS & Zandi E 2001). Variants of the IKK complex containing IKBKA or IKBKB homodimers associated with NEMO may also exist. Crystallographic and quantitative analyses of the binding interactions between N-terminal NEMO and C-terminal IKBKB fragments showed that IKBKB dimers would interact with NEMO dimers resulting in 2:2 stoichiometry (Rushe M et al. 2008). Chemical cross-linking and equilibrium sedimentation analyses of IKBKG (NEMO) suggest a tetrameric oligomerization (dimers of dimers) (Tegethoff S et al. 2003). The tetrameric NEMO could sequester four kinase molecules, yielding an 2xIKBKA:2xIKBKB:4xNEMO stoichiometry (Tegethoff S et al. 2003). The above data suggest that the core IKK complex consists of an IKBKA:IKBKB heterodimer associated with an IKBKG dimer or higher oligomeric assemblies. However, the exact stoichiometry of the IKK complex remains unclear.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
Proteasomes are usually localized to the cytoplasm but are also present in the nucleus and ER. Houde et al. (2003) investigated the distribution of proteasomes in J774 macrophages and observed them on phagosomes. The proteasomes present on phagosomes may not come directly from ER, but instead assemble on phagosomes to play a function at a precise point during phagolysosomal biogenesis. Houde et al. also observed polyubiquitinated proteins on the cytoplasmic side of phagosomes and highlighted the link between the ubiquitination process and proteasomal degradation; translocated peptides are ubiquitinated and processed by the proteasome complex assembled on the cytoplasmic side of phagosomes, leading to the generation of MHC class I binding peptides.
Soluble antigens are presented to dendritic cells (DCs) in some cases by receptor mediated endocytosis or fluid-phase endocytosis. Burgdorf et al. (2008) suggest that there are two different endocytic compartments for antigen processing: one dedicated to MHC class I (early endosomes) and the other one for MHC class II presentation (lysosomes). Sorting of cargo into these different compartments occurs at the plasma membrane and is likely to depend on the type of endocytic receptor the cargo is interacting with (Burgdorf et al. 2008, Zhuang et al. 2006). The mannose receptor (MR) is the best studied receptor that targets soluble antigens to early endosomes but not to lysosomes (Burgdorf et al. 2006). Antigens taken up by the MR are targeted towards a mildly acidic stable early endosomal compartment for exclusive presentation on MHC I molecules (Burgdorf et al. 2008).
Within the endosome the receptor and cargo separate and the receptor recycles back to the cell surface. Soluble antigens are targeted into the stable early endosome lumen for efficient cross presentation. Early endosomes are mildly acidic and relatively poor in proteases.
Peptides generated by Cathepsin S or IRAP-mediated proteolysis in the endosomes are loaded onto MHC class I molecules, which are internalized and transported to early and late endosomal compartments where antigenic peptides can be loaded. A fraction of the internalized cell surface class I molecules enter MHC class II compartments (MIICs) within endocytic vesicles (Gromme et al. 1999, Kleijmeer et al. 2001). Microscopic analysis has revealed that surface MHC-I molecules are internalized and transported to early and late endosomal compartments (Basha et al. 2008, Lizee et al. 2003). A tyrosine-based endocytic trafficking motif (YXXA) is required for the constitutive internalization of MHC-I molecules from the cell surface into early/late endosomes for peptide loading (Basha et al. 2008, Lizee et al. 2003). Upon entry in to these endosomal compartments the MHC class I complexes exchange their pre-bound peptides with exogenously derived antigenic peptides.
Fusion of the maturing phagosome with the ER mediates the exchange of materials resulting in the formation of a hybrid ER-phagosome compartment (Gagnon et al. 2002, Guermonprez et al. 2003, Houde et al. 2003, Ackerman et al. 2003, Blanchard et al. 2010). This hybrid contains the retrotranslocon factor Sec61 that mediates the access of proteasomes on the cytosolic surface of the phagosome. Using fluorescence imaging, Houde et al. (2003) provided evidence for the role of Sec61 in the retrotranslocation of internalized exogenous proteins from phagosomes to the cytoplasmic face of J774 macrophages. Sec61 factor is a heterotrimeric complex composed of alpha, beta and gamma subunits forming the core of the mammalian ER translocon (Greenfield et al. 1999). Oligomers of the Sec61 complex form a transmembrane channel involved in the retrotranslocation of misfolded proteins from ER to the cytosol for degradation, and thus it has been proposed that Sec61 might be involved in the translocation of proteins in phagosomes to the cytosol (Kasturi et al. 2008).
Endocytic compartments contain many cysteine proteases such as cathepsin S that can generate cross-presented peptides. Cathepsins may participate in the generation of MHC class II-presentated peptides (Villadangos et al. 1999). Shen et al. (2004) demonstrated that cathepsin S contributes to TAP-independent cross-presentation in vivo, showing that ovalbumin was cross-presented by denritic cells (DCs) through both TAP-dependent and TAP-independent pathways. The TAP-independent pathway was sensitive to the cystine protease inhibitor leupeptin, but not to proteasome inhibitors. Further experiments with knockout mice showed that cathepsin S contributed to cross-presentation. DCs lacking cathepsin S lack the TAP-independent pathway (Khor et al. 2008, Shen et al. 2004).
After processing by the proteasome, some of the oligopeptides could be reinternalized to the phagosome lumen of the same phagosome through the TAP complex probably acquired during ER-phagosome fusion. Guermonprez et al. (2003) tested this model and observed abundant TAP complex in early phagosomes and its reduction over time. Electron microscopy analysis of purified phagosomes also showed the insertion of TAP2 into the phagosomal membrane.
While it is established that cathepsin S is involved in antigen processing in endocytotic vesicles, it is less certain whether other proteases present in endocytic vesicles are also involved in generating the peptide fragments. Insulin regulated aminopeptidase (IRAP) is an epitope-trimming zinc-dependent aminopeptidase closely related to ERAP1 and ERAP2. IRAP may be involved in vacuolar processing of the peptide fragments within endosomes (Saveanu et al. 2009, Segura et al. 2009). IRAP is detected predominantly in the early and recycling endosome fractions. Saveanu et al. (2009) observed the physical association of IRAP with internalized class I MHC molecules and suggested that this may favour a direct linkage between peptide trimming and MHC class I loading. They also showed that IRAP-dependent processing of antigens requires active proteasome but not lysosomal proteases, which suggests that this pathway utilizes cytosolic degradation followed by peptide transport into IRAP-containing endosomes.
After internalization, F-actin is depolymerized from the phagosome, and the newly denuded vacuole membrane becomes accessible to early endosomes (Aderem & Underhill. 1999).
Dendritic cells (DCs) and macrophages recognize and phagocytose apoptotic cells/invading microorganisms using a variety of receptors including lectins, Mannose receptor (MR) (Stahl & Ezekowitz. 1998), CD36 (Savill 1997), CD14 (Devitt et al. 1998), scavenger receptor A (SR-A) (Platt et al. 1996) and integrin alphaVbeta5 (Savill et al. 1992, Savilli et al. 1990), then cross-present cell-associated antigens to CD8+ T cells. Ligands on apoptotic cell like sugars, phosphatidylserine and surface-bound thrombospondin (TSP) are recognized by these receptors and induce rearrangements in the actin cytoskeleton that lead to the internalization of the particle.
Following fission of the phagosome vesicle from the plasma membrane, the phagosome undergoes maturation by a series of fusion and fission events, fusing with late endosomes and ultimately lysosomes, to form a phagolysosome.
fuses sequentially with sorting/early endosomes, late endosomes and finally lysosomes to acquire degradative capabilities. This process is referred to as phagosome maturation.
After the fusion of phagosome and lysosome lumen acidity rises and the activity of lysosomal proteases increases, conferring proteolytic ability. However antigens must be spared from complete proteolytic destruction. Dendritic cells (DCs) achieve this by regulating the level of proteolysis and phagosomal acidification. DCs recruit the NADPH oxidase NOX2 to the phagosome and mediate sustained production of low levels of reactive oxygen species (ROS), causing active alkalization of the phagosomal lumen (Savina et al. 2006, Ramachandra et al. 2009). NOX2 consumes oxygen and protons (pumped by V-ATPase or other H+ voltage-gated channels) to produce ROS and this lowers phagosome acidity. This apparently reduces antigen proteolysis to a level that allows processing but does not fully destroy the antigenic peptides, favouring increased MHC-I cross presentation (Ramachandra et al. 2009). Rab27a controls the recruitment of NOX2 to DC phagosomes (Savina et al. 2007).
Endocytosed antigens must leave the endocytic structure to enter into the MHC I pathway before exhaustive degradation within lysosomes. The canonical pathway is the transporter associated with antigen processing (TAP)-dependent cytosolic pathway, which involves the translocation of endocytosed antigens into the cytosol where they are degraded into antigenic peptides by the proteasome and transported to ER through TAP. This hypothesis comes from indirect evidences showing that proteasome inhibitors block cross presentation of certain antigens (Amigorena et al. 2010, Burgdorf et al. 2008) . According to this model antigens are translocated into the cytosol by an undefined mechanism. There are less well-characterized mechanisms for the delivery of exogenous antigens into the cytosol. Certain peptides with highly positively charged sequences derived from HIV tat protein or the Antennapedia homeodomain (AntHD) protein seem to penetrate into the cytosol directly across the plasma membrane (Monu et al. 2007, Vendeville et al. 2004). It is also proposed that some exogenous antigens can be exchanged between neighboring cells through gap junctions, leading to cross presentation by the recipient cell (Monu et al. 2007, Neijssen et al. 2005). Once internalized antigens are routed into the cytosol, they follow the conventional pathway of proteasome digestion and TAP mediated transport of peptides into the ER lumen.
Exogenous antigens are thought to be processed for cross-presentation in much the same manner as endogenous proteins once they enter the cytosolic pathway (Rock et al. 2010). Immunoproteasome components are the major proteases involved in generating the antigenic fragments. The precurssor peptides are further trimmed by cytosolic aminopeptidases and shuttled to ER through TAP for MHC class I loading.
Peptides translocated back into the phagosomal lumen are loaded onto MHC class I molecules. Guermonprez et al. (2003) detected the components of the peptide loading complex (TAP, tapasin, calreticulin and ERp57) together with MHC class I in purified early phagosomes. They observed peptide loading in the presence of ATP in purified phagosomes expressing HLA-A2, incubated with iodinated S-9-L peptide. From these experiments they concluded that TAP-imported peptides can be loaded on MHC class I molecules in the lumen of phagosomes. Houde et al. (2003) also observed OVA peptide SIINFEKL:MHC-I complex in the phagosome lumen.
Upon phagolysosomal proteolysis the phagocytosed proteins still need to gain access to the cytosol for proteasomal access, transport into the ER and loading onto MHC class I. The mechanism for this escape of phagocytosed antigens remain unknown.
Sar1p-GTP recruits the cytoplasmic Sec23p-Sec24p complex. (Though not represented in the subsequent steps, Sec23p-Sec24p would bind to members of the p24 protein family of possible cargo receptors, and together with Sar1p bind the appropiate v-SNARE, and Rab-GTP.)
The interaction between the two compartments could involve either direct fusion of ER stacks to phagosomes (Phgs) or vesicular intermediates. In both cases, a fusion event between the ER or ER-derived membrane vesicles and Phgs must occur. The SNARE SEC22B localizes to the ER-Golgi intermediate compartment (ERGIC) and interacts with SNARE syntaxin 4 (STX4) on phagosomes (Phgs), mediating the recruitment of subset of ER components including transporter associated with antigen processing (TAP), to phagosomes (Cebrian et al. 2011).
A major reserve of MHC-I in dendritic cells reside within the endocytic recycling compartments (ERC). MHC-I trafficking to the ERC is regulated by the activity of Rab11a and subsequent trafficking from ERC to phagosomes is controlled by TLR-MyD88-IKK2-dependent phosphorylation of phagosomal SNAP23. Toll-like receptor (TLR) signalling regulate cross-presentation as they regulate phagocytosis and phagolysosomal fusion (Nair et al. 2011). MHC-I bearing ERC are enriched with R-SNAREs like RAB11a, VAMP3, and VAMP8. These SNARE molecules can interact with Q-SNARE SNAP23 present on phagosomes and this mediates membrane fusion. This interaction of SNAP23 with R-SNAREs require phosphorylation of SNAP23 (on Ser-95) by IKK2, and IKK2 is activated by TLR signalling. SNAP23 phosphorylation may increase SNAP23 binding to SNAREs. It may also regulate platelet and mast cell secretion (Karim et al. 2013, Suziki & Verma 2008).
In DCs subset of ER proteins including MHC-I peptide loading complex (PLC) and transporter associated with antigen processing (TAP) transit to phagosomes via the intermediate compartment ER-Golgi intermediate compartment (ERGIC) (Cebrian et al. 2011). TAP exits the ER in COPII vesicles in association with MHC class I, and that peptide translocation by TAP and binding to class I can occur in post-ER compartments (Ghanem et al. 2010). SEC22B, an ER-resident SNARE is required for the transport of PLC from ERGIC (Cebrian et al. 2011), but this step does not deliver MHC-I (Nair-Gupta et al. 2014). Instead, MHC-I are recruited from an endosomal recycling compartment (ERC), which is marked by Rab11a, VAMP3/cellubrevin, and VAMP8/endobrevin that holds large reserves of MHC-I. This step is dependent on TLR signalling (Nair-Gupta et al. 2014).
Recycling endosome-localized R-SNARE protein like RAB11a, VAMP3, and VAMP8 dock with target phagosome membrane Q-SNARE protein SNAP23. MHC-I present on the recycling endosomes would be delivered to phagosomes during this process.
Thimet oligopeptidase (THOP1, EP24.15) is a zinc-dependent peptidase of the metallopeptidase M3 family (Pierotti et al. 1990). It was first described as a neuropeptide-degrading enzyme present in the soluble fraction of brain homogenates (Orlowski et al. 1983). However, its predominant location in the cytosol and nucleus suggests that extracellular degradation of neuropeptides and hormones is not its primary function (Fontenele-Neto et al. 2001). THOP1 can metabolize peptides within cells, thereby affecting antigen presentation and G protein-coupled receptor signal transduction. It was shown in vivo to participate in antigen presentation through MHC-I (Silva et al. 1999, Kim et al. 2003, Yorl et al. 2003) and in vitro to bind (Portaro et al. 1999) or degrade (Saric et al. 2001) some MHC-I associated peptides. THOP1 can degrade a broad range of intracellular peptides containing 5–17 amino acids (Oliveira et al. 2001, Berti et al. 2009). Substrate size is restricted because its catalytic center is located in a deep channel (Ray et al. 2004). THOP1 can both degrade existing peptides and generate new peptides, making it a versatile enzyme for regulating intracellular peptide function including antigen presentation (Berti et al. 2009, Russo et al. 2012).
Cytotoxic T lymphocytes (CTLs) recognize peptides presented by HLA class I molecules on the cell surface. The C terminus of these CTL epitopes is considered to be produced by the proteasome, but is complemented by THOP1 and other cytosolic endopeptidases such as Nardilysin (Kessler et al. 2011, Oliveira & van Hall 2015).
MHC class I heterodimers are only stable in peptide bound form and only as a trimer (with bound peptide) present on the cell surface. Class I MHC molecules prefer nonapeptides, and less frequently use octa- or deca-peptides. The peptide binding groove in MHC class I molecules is formed by the intimate association of the alpha1 and alpha2 domains of the heavy chain. Structural studies have revealed that the alpha1:alpha2 domains form a single peptide binding groove consisting of 2 parallel helices on a floor of 8 beta-strands. Hydrogen bonding networks are established in the binding groove with the antigenic peptide main chain and terminal atoms that enable largely sequence independent ligation. Upon peptide binding the class I MHC molecule releases from the peptide loading complex (PLC) and clusters at ER exit sites and is finally exported to the cell surface. MHC I molecules bound to low-affinity peptides are not transferred to the cell surface and are instead cycled back to ER. They can proceed to the cell surface only when they become bound to high-affinity peptide (Howe et al, 2009; Garstka et al, 2007). Calreticulin binds to these low-affinity peptide bound class I molecules and mediate the retrieval from golgi apparatus to ER and for effcient presentation of a model antigen (Howe et al, 2009).
The interaction between the two compartments involves either direct fusion of ER stacks to phagosomes (Phgs) or vesicular intermediates. In both cases, a fusion event between the ER or ER-derived membrane vesicles and Phgs must occur. The SNARE SEC22B localizes to the ER-Golgi intermediate compartment (ERGIC) and interacts with SNARE syntaxin 4 (STX4) on phagosomes (Phgs), mediating the recruitment of subset of ER components including transporter associated with antigen processing (TAP), to phagosomes (Cebrian et al. 2011).
When the loaded peptide is of a sufficiently high-affinity the trimeric complex is transported to a special ER exit site by a putative cargo receptor, B cell receptor-associated protein (BAP31), where it translocates to the cell surface, possibly by transport in COPII-coated vesicles.
Interaction of E3 with both substrate and E2-Ub, brings them into proximity so that ubiquitin is transferred from E2 to the substrate. In most cases the transfer of ubiquitin is direct from E2 to substrate, but in a small subset of E3s, it occurs via a covalent E3-Ub thioester intermediate (Raymond et al. 2009).
Upon interaction of Beta-2-microglobin (B2M) with MHC class I Heavy Chain (HC), calnexin is fully replaced by its soluble ortholog calreticulin (CRT) and this complex is incorporated into the peptide loading complex (PLC). PLC is a multiprotein complex that includes CRT, ERp57 and the additional components tapasin, transporter associated with antigen processing (TAP) and Bap31. The stoichometry of components in PLC remains unclear. The PLC loads antigenic peptides onto MHC class I molecules; components of the PLC cooperate to stabilize the MHC class I complex and optimally load peptides. Tapasin is a type I transmembrane protein that interacts directly with TAP and tethers the MHC complex to it. TAP facilitates the transport of peptides from the cytosol to the ER lumen. B cell receptor–associated protein (Bap31), a putative cargo receptor, associates with HC and acts as a retrograde transporter, carrying peptide-loaded class I MHC molecules.
Ttransporter associated with antigen processing (TAP) is a heterodimeric complex, composed of TAP1 and TAP2 proteins, members of the ATP-binding cassette (ABC) superfamily. TAP consists of two transmembrane domains (TMDs) and two cytosolic nucleotide-binding domains. Peptide binding to the cytosolic-facing cavity formed by the TMDs causes it to undergo a conformational change that induces ATP hydrolysis, forcing the opening of a pore and translocation of the peptide into the ER lumen. TAP transports peptides in the range of 8-16 amino acids into the ER, which is the peptide length typically generated by the immunoproteasome.
The newly synthesized MHC class I heavy chain (HC) translocates into the endoplasmic reticulum (ER) and binds rapidly to calnexin (CNX), a transmembrane calcium-dependent lectin with chaperone activity. CNX recognizes and binds to an N-linked monoglycosylated Glc1Man9GlcNAc2 carbohydrate group found attached to the conserved Asn-86 in the HC. Interaction of HC with CNX promotes the formation of intrachain disulfide bonds. Another candidate ER chaperon protein is immunoglobulin binding protein (BiP), found to associate with HC in the absence of CNX. These chaperones can cooperate in protein folding and prevention of aggregation.
Interaction of calnexin (CNX) with MHC class I HC stabilizes it and facilitates the association of the beta2 microglobulin component (B2M). The two chains are linked noncovalently via interaction of B2M and the alpha3 domain of MHC HC. After formation of the HC:B2M heterodimer, the MHC complex dissociates from CNX.
Endoplasmic reticulum resident protein 57 (ERp57), is a member of the protein disulphide isomerase (PDI) family of thiol oxidoreducatases. It associates with Calnexin (CNX), and its soluble homolog calreticulin (CRT) and is recruited to MHC Class I Heavy Chain (HC). ERp57 is involved in the formation of HC disulphide bonds.
The 26S proteasome complex consists of the 20S catalytic core particle which harbours the proteolytically active sites and the regulatory 19S particle which is responsible for substrate interaction. This process generates a vast number (perhaps hundreds) of different peptides, depending on the length and sequence of the substrate protein. Only a small fraction of these peptides (nearly 10%) form the exact length to be presented by class I MHC; most (approximately 70%) are too short to bind. The remaining proteasome products (10-20%) are N-terminally extended precursors that require additional cleavage by cytosolic aminopeptidases for presentation by MHC class I molecules.
Activated ubiquitin is transferred from E1 to the active site cystine of ubiquitin conjugating enzymes (E2s) via a trans-esterification reaction. E2s catalyze covalent attachment of ubiquitin to target proteins. They all share an active-site ubiquitin-binding cysteine residue and are distinguished by the presence of a ubiquitin-conjugating catalytic (UBC) fold required for binding of distinct ubiquitin ligases or E3s. Once conjugated to ubiquitin, the E2 molecule binds one of several E3s (Glickman et al. 2002).
Ubiquitin is activated in an ATP-dependent manner, catalyzed by E1 ubiquitin-activating enzymes. In the first step of ubiquitin activation, the E1 enzyme binds ATP, Mg+2 and ubiquitin, and catalyses ubiquitin C-terminal acyl-adenylation (Ubiquitin-AMP). In the second step, the catalytic cystine in the E1 attacks the ubiquitin-adenylate (Ub-AMP) to form the activated ubiquitin-E1 thioester-bonded complex and an AMP leaving group. The intermediate reaction involving the formation of the ubiquitin-AMP complex is not represented here.
Monoubiquitinated substrate acquires additional ubiquitin modifications in the form of multiple single attachments or a ubiquitin chain. Polyubiquitin chains added through K48 residue of ubiquitin typically targets the substrate for degradation (Raymond et al. 2009).
Ubiquitin E3 ligases confer specificity to ubiquitination by recognizing target substrates and mediating transfer of ubiquitin from an E2 ubiquitin-conjugating enzyme to substrate (Raymond et al. 2009). E3 ligases includes a large, diverse set of proteins characterized by several defining motifs which include a HECT (homologous to E6-associated C-terminus), RING (Really Interesting New Gene) and U-box domains. The E3 ligases can be multisubunit complexes rather than a single polypeptide. Presently three different kinds of E3 complexes have been described called SCF, APC, and VHL. E3 ligases binds to both substrate and an E2 thioesterified with ubiquitin (E2-Ub).
Transporter associated with antigen processing (TAP) prefers peptides 8-16 residues long, slightly longer than the canonical 8-10 residue peptides that fit into class I MHC binding sites. These longer peptides are further trimmed at their N-termini by the ER-associated aminopeptidase (ERAP). ERAP trims peptides to a length of 8-10 residues, suitable for loading into the MHC class I binding groove.
Upon peptide binding the class I MHC molecule releases from the peptide loading complex (PLC) and clusters at ER exit sites and is finally exported to the cell surface. MHC I molecules bound to low-affinity peptides are not transferred to the cell surface and are instead cycled back to ER. They can proceed to the cell surface only when they become bound to high-affinity peptide (Howe et al. 2009, Garstka et al. 2007). Calreticulin binds to these low-affinity peptide bound class I molecules and mediate the retrieval from golgi apparatus to ER and for effcient presentation of a model antigen (Howe et al. 2009).
Some peptides generated by the 26S proteasome are too long to bind to MHC class I molecules. These N-terminal extended precursor peptides may be trimmed by cytosolic aminopeptidases, such as Tripeptidyl peptidase II (TPP2), puromycin-sensitive aminopeptidase (PSA), bleomycin hydrolase (BH), and leucine aminopeptidase (LAP).
Cargo must pass through the various compartments of the Golgi complex to reach the plasma membrane. This may occur through either of the two well known models, cisternal maturation or vesicular transport (Refer to review articles Glick et al. 2009, Jackson 2009, Nakano et al. 2010).
Before the cargo vesicle can fuse with target membrane, the COPII protein coat must be disassembled and its components released into cytosol. This uncoating is triggered by hydrolysis of the bound GTP to produce Sar1p-GDP, which has decreased affinity for the vesicle membrane. Disassociation of Sar1p-GDP from the membrane is followed by the release of the other COPII subunits.
Sec13-Sec31 binds the prebudding complex and this Sec13-Sec31 heterotetramer forms the outer structural scaffold of the coat. Sec13-Sec31 is likely to crosslink the prebudding complexes generating COPII-coated vesicles.
The cytoplasmic tails of MHC class I molecules do not possess any of the known ER export sequences. BAP31 acts as a cargo receptor by exporting peptide-loaded class I MHC molecules to the ER exit sites or ER/Golgi intermediate compartment. Sec24 of the Sec23-Sec24/Sar1-GTP complex catches cargo by direct contact, forming the prebudding complex. Sar1/Sec23-Sec24 can bring about curvature of the membrane in the formation of vesicle. The cargo (antigen-bound MHC) accumulates within the COPII vesicle by binding to the Sec24 polypeptide of the COPII coat.
After passing through the Golgi complex, secretory cargo is packaged into post-Golgi transport intermediates (post-Golgi), which translocate plus-end directed along microtubules to the plasma membrane.
There at least two classes of clathrin coated vesicles in cells, one predominantly Golgi-associated, involved in budding from the trans-Golgi network and the other at the plasma membrane. Here the clathrin-coated vesicles emerging from the Golgi apparatus are triggered by the heterotetrameric adaptor protein complex, AP-1 at the trans-Golgi network membrane. The cargo can be transmembrane, membrane associated or golgi luminal proteins. Each step in the vesicle sculpting pathway, gathers cargo and clathrin triskeletons, until a complete vesicular sphere is formed. With the scission of the membrane the vesicle is released and eventually losses its clathrin coat.
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DataNodes
bound class I
MHC:BAP31 oligomerpeptide:MHC:B2M peptide loading
complexrecognizing
apoptotic cellsrecognizing
apoptotic cellsproteasomal
degradationParticulate antigen
(Ag)Particulate antigen
(Ag)Particulate antigen
(Ag)class I
HC:Calnexin/BiP:Erp57class I
HC:Calnexin/BiPAnnotated Interactions
bound class I
MHC:BAP31 oligomerbound class I
MHC:BAP31 oligomerpeptide:MHC:B2M peptide loading
complexpeptide:MHC:B2M peptide loading
complexrecognizing
apoptotic cellsrecognizing
apoptotic cellsproteasomal
degradationproteasomal
degradationproteasomal
degradationParticulate antigen
(Ag)Particulate antigen
(Ag)Particulate antigen
(Ag)Particulate antigen
(Ag)Particulate antigen
(Ag)class I
HC:Calnexin/BiP:Erp57class I
HC:Calnexin/BiP:Erp57fuses sequentially with sorting/early endosomes, late endosomes and finally lysosomes to acquire degradative capabilities. This process is referred to as phagosome maturation.
There are less well-characterized mechanisms for the delivery of exogenous antigens into the cytosol. Certain peptides with highly positively charged sequences derived from HIV tat protein or the Antennapedia homeodomain (AntHD) protein seem to penetrate into the cytosol directly across the plasma membrane (Monu et al. 2007, Vendeville et al. 2004). It is also proposed that some exogenous antigens can be exchanged between neighboring cells through gap junctions, leading to cross presentation by the recipient cell (Monu et al. 2007, Neijssen et al. 2005).
Once internalized antigens are routed into the cytosol, they follow the conventional pathway of proteasome digestion and TAP mediated transport of peptides into the ER lumen.
Cytotoxic T lymphocytes (CTLs) recognize peptides presented by HLA class I molecules on the cell surface. The C terminus of these CTL epitopes is considered to be produced by the proteasome, but is complemented by THOP1 and other cytosolic endopeptidases such as Nardilysin (Kessler et al. 2011, Oliveira & van Hall 2015).
MHC I molecules bound to low-affinity peptides are not transferred to the cell surface and are instead cycled back to ER. They can proceed to the cell surface only when they become bound to high-affinity peptide (Howe et al, 2009; Garstka et al, 2007). Calreticulin binds to these low-affinity peptide bound class I molecules and mediate the retrieval from golgi apparatus to ER and for effcient presentation of a model antigen (Howe et al, 2009).
MHC I molecules bound to low-affinity peptides are not transferred to the cell surface and are instead cycled back to ER. They can proceed to the cell surface only when they become bound to high-affinity peptide (Howe et al. 2009, Garstka et al. 2007). Calreticulin binds to these low-affinity peptide bound class I molecules and mediate the retrieval from golgi apparatus to ER and for effcient presentation of a model antigen (Howe et al. 2009).
class I
HC:Calnexin/BiPclass I
HC:Calnexin/BiP