During prophase, the chromatin in the nucleus condenses, and the nucleolus disappears. Centrioles begin moving to the opposite poles or sides of the cell. Some of the fibers that extend from the centromeres cross the cell to form the mitotic spindle.
View original pathway at Reactome.
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Mitotic G2 (gap 2) phase is the second growth phase during eukaryotic mitotic cell cycle. G2 encompasses the interval between the completion of DNA synthesis and the beginning of mitosis. During G2, the cytoplasmic content of the cell increases. At G2/M transition, duplicated centrosomes mature and separate and CDK1:cyclin B complexes become active, setting the stage for spindle assembly and chromosome condensation that occur in the prophase of mitosis (O'Farrell 2001, Bruinsma et al. 2012, Jiang et al. 2014).
The dissolution of the nuclear membrane marks the beginning of the prometaphase. Kinetochores are created when proteins attach to the centromeres. Microtubules then attach at the kinetochores, and the chromosomes begin to move to the metaphase plate.
MASTL (GWL i.e. Greatwall kinase) phosphorylates ARPP19 on serine residue S62 (Gharbi-Ayachi et al. 2010). S62 of human ARPP19 corresponds to serine residue S67 of Xenopus Arpp19, which is phosphorylated by Xenopus Mastl (Mochida et al. 2010).
GORASP1 (GRASP65) and GOLGA2 (GM130) form a complex on cis-Golgi membranes. RAB1A or RAB1B, small RAS GTP-ases, can also associate with this complex through interaction with GOLGA2 (Moyer et al. 2001, Weide et al. 2001). GOLGA2 provides a docking site for the USO1 (p115) homodimer (Nakamura et al. 1995, Seeman et al. 2000). RAB1 also participates in this interaction and facilitates it when in the GTP-bound state (Moyer et al. 2001). Binding of USO1 to GORASP1:GOLGA2:RAB1:GTP complex enables fusion of vesicles originating in the endoplasmic reticulum (ER) with cisternae of cis-Golgi. In mitotic prophase, CDK1 (CDC2) in complex with either CCNB1 (cyclin B1) or CCNB2 (cyclin B2), as both CCNB1 and CCNB2 can localize to Golgi (Jackman et al. 1995, Draviam et al. 2001), phosphorylates GORASP1, GOLGA2 and RAB1 (Bailly et al. 1991, Lowe et al. 1998, Preisinger et al. 2005). Phosphorylation of GOLGA2 and RAB1 impairs their association with USO1, which inhibits thethering and subsequent fusion of ER-originating vesicles with cis-Golgi cisternae, resulting in cessation of ER to Golgi protein trafficking at the start of mitosis and increase in the number of Golgi trafficking vesicles at the expense of Golgi cisternae (Lowe et al. 1998, Seeman et al. 2000, Moyer et al. 2001, Diao et al. 2008).
Phosphorylation of GORASP1 (GRASP65) by cyclin B-associated CDK1 creates a docking site for PLK1. PLK1 is also able to bind to CDK1-phosphorylated RAB1, but not to CDK1-phosphorylated GOLGA2 (Preisinger et al. 2005).
RB1 binds the condensin II complex through interaction with the NCAPD3 subunit of condensin II. This interaction is E2F independent and is important for targeting of the condensin II complex to chromatin (Longworth et al. 2008). RB1 may be particularly important for targeting of the condensin II complex to centromeres (Manning et al. 2010). RB1 deficient cells exhibit chromosome condensation defects and are prone to aneuploidy caused by aberrant chromosomal segregation. Therefore, tumor suppressor role of RB1 is based both on E2F-dependent control of G1/S transition, as well as on E2F-independent maintenance of genomic stability through regulation of mitotic chromosome condensation (Longworth et al. 20008, Coschi et al. 2010, Manning et al. 2010).
The role of RB1 in the maintenance of genomic stability is supported by studies of the childhood eye cancer retinoblastoma and its precursor, retinoma. Retinoma, a quiescent precursor of malignant retinoblastoma with functional loss of both RB1 alleles, is genomically unstable (Dimaras et al. 2008). Also, while the majority of retinoblastoma tumors are caused by the loss-of-function of the tumor suppressor gene RB1, ~2% of retinoblastoma tumors in unilaterally affected patients are initiated by a high level amplification of MYCN gene, in the presence of two functional, unmutated RB1 alleles. These tumors, with normal RB1 and amplified MYCN show a much lower level of genomic instability than retinoblastoma tumors with RB1 loss-of-function (Rushlow et al. 2013).
PHF8, a PHD and Jumonji C domain-containing protein, is recruited to chromatin by binding to dimethylated or trimethylated histone H3 - H3K4me2 and/or H3K4me3. PHF8 demethylates monomethylated histone H4, H4K20me1, a docking site for the condesin II complex (Liu et al. 2010).
CDK1-mediated phosphorylation of GORASP1 (GRASP65) enables GORASP1 to recruit PLK1 (Preisinger et al. 2005). PLK1 phosphorylates GORASP1 on serine residue S189 (Sengupta and Linstedt 2010). This serine residue is near the GORASP1 region involved in GORASP1 dimerization and oligomerization, a process underlying the stacking of cis-Golgi cisternae (Wang et al. 2003). The phosphorylation of S189 by PLK1 impairs Golgi cisternae stacking (tethering), contributing to Golgi unlinking and fragmentation in mitosis, probably by preventing formation of GORASP1 dimers and oligomers (Sutterlin et al. 2001, Sengupta and Linstedt, 2010). Two other potential phosphorylation sites that match PLK1 substrate consensus sequence exist in GORASP1, but their functional significance has not yet been examined (Sengupta and Linstedt, 2010).
Increased activity of CDK1:CCNB1 during the cell cycle promotes PHF8 dissociation from chromatin, while the inhibition of CDK activity promotes binding of PHF8 to chromatin during mitosis. CDK1:CCNB1 complex phosphorylates PHF8 in vitro on serine residues S33 and S84. Mutation of PHF8 phosphorylation sites impairs the dissociation of PHF8 from chromatin and the accumulation of H4K20me1 in prophase (Liu et al. 2010). Positions of CDK1-phosphorylated serine residues in PHF8, S33 and S84, are based on the sequence of PHF8 splicing isoform 2, which was used in the experiments of Liu et al. In PHF8 splicing isoforms 1 and 3, serine residues S69 and S120 are annotated as targets of CDK1-mediated phosphorylation.
Accumulation of monomethylated histone H4 (H4K20me1) is necessary for loading of the condensin II complex on chromatin. Condensin II binds H4K20me1 through HEAT repeats of two condensin II subunits, NCAPD3 and NCAPG2 (Liu et al. 2010). RB1 is required, at least partially, for the successful association of condensin II with chromatin (Longworth et al. 2008). The precise role of RB1 in condensin II loading and the connection, if any, between histone H4 monomethylation and RB1-facilitated loading of the condensin II complex on chromatin has not, however, been elucidated. RB1 family proteins are known to interact with H4K20 trimethylating enzymes Suv4-20h1 and Suv4-20h2 and promote H4K20 trimethylation at pericentric and telomeric heterochromatin (Gonzalo et al. 2005).
Once PLK1 is recruited to the chromatin-bound condensin II complex, it phosphorylates the NCAPD3 subunit of condensin II on serine residue S1419, and possibly other residues. In addition to phosphorylating NCAPD3, PLK1 phosphorylates other condensin II subunits, NCAPG2 and NCAPH2. However, the phosphorylation sites have not yet been determined. PLK1-mediated phosphorylation of the condensin II complex facilitates condensation of prophase chromosomes (Abe et al. 2011).
Phosphorylation of the threonine residue T1415 of condensin II subunit NCAPD3 is required for chromosome condensation in prophase. In vivo, phosphorylation of NCAPD3 threonine residue T1415 is blocked when cells are treated with CDK1 inhibitors. In addition, it was shown that CDK1 in complex with cyclin B1 (CDK1:CCNB1) phosphorylates NCAPD3 at T1415 in vitro (Abe et al. 2011).
SETD8 is a protein-lysine N-methyltransferase that monomethylates H4 histone to produce H4K20me1 (Nishioka et al. 2002, Wu et al. 2010). SETD8 levels peak at G2/M transition, and regulated SETD8 activity is required for normal cell cycle progression (Rice et al. 2002, Wu et al. 2010).
Adjacent cisternae of the Golgi apparatus are stacked and linked by tubules to from a Golgi ribbon (Nakamura et al. 2012). GORASP1 (GRASP65), a protein localizing to membranes of cis-Golgi cisternae, enables stacking by in trans dimerization/oligomerization through its PDZ domains (Tang et al. 2010). In mitosis, GORASP1 is phosphorylated by CDK1 and PLK1 (Preisinger et al. 2005). PLK1-mediated phosphorylation of GORASP1 prevents stacking of Golgi cisternae and contributes to unlinking and fragmentation of the Golgi apparatus, probably by interfering with GORASP1 oligomerization (Wang et al. 2003, Sengupta and Linstedt 2010). Similarly, GORASP2 (GRASP55), localized to median Golgi cisternae, promotes stacking by trans-oligomerization. Trans-oligomerization of GORASP2 is prevented by mitotic phosphorylation of GORASP2 downstream of MEK/ERK cascade, and contributes to the Golgi fragmentation in prophase (Xiang and Wang 2010).
USO1 (p115) protein, localizing to membranes of ER to Golgi transport vesicles, binds GOLGA2 (GM130), localizing to membranes of cis-Golgi cisternae. Binding of USO1 to GOLGA2 enables tethering of ER to Golgi transport vesicles to cis-Golgi cisternae, and is facilitated by a Ras-related GTPase RAB1. Fusion of ER to Golgi transport vesicles with cis-Golgi succeeds tethering and depends on STX5 (syntaxin-5). In mitosis, phosphorylation of GOLGA2 by cyclin B-activated CDK1 prevents USO1 docking. This results in cessation of ER to Golgi transport. Halting ER to Golgi transport increases the number of transport vesicles at the expense of Golgi cisternae, since transport vesicles keep budding from the ER but are unable to fuse with Golgi cisternae and deliver their content (Lowe et al. 1998, Seeman et al. 2000, Diao et al. 2008).
GORASP2 (GRASP55) localizes to the median region of Golgi, where it forms a complex with BLZF1 (Golgin 45) and RAB2A GTPase (Short et al. 2001). Similar to GORASP1, GORASP2 is involved in the maintenance of Golgi structure and positively regulates stacking of Golgi cisternae (Xiang and Wang 2010). In addition, GORASP2, probably through its association with RAB2A GTPase, regulates trafficking through the Golgi (Short et al. 2001). In G2 and mitotic prophase, GORASP2 is phosphorylated by MEK1/2 activated MAP kinases. Monophosphorylated MAPK3 (ERK1) isoform, MAPK3 3 i.e. ERK1b (known as ERK1c in rat), likely activated by a MEK1 isoform MEK1b (Shaul et al. 2009), as well as MAPK1 (ERK2) are implicated in GORASP2 phosphorylation during mitosis (Jesch et al. 2001, Colanzi et al. 2003, Shaul and Seger 2006, Feinstein and Linstedt 2007, Duran et al. 2008, Feinstein and Linstedt 2008). Threonine residues T222 and T225 were implicated as targets of MAPK mediated GORASP2 phosphorylation in studies that used directional mutagenesis (Jesch et al. 2001, Feinstein and Linstedt 2008). However both T222 and T225 were simultaneously mutated in these studies and their roles have not been individually investigated. Using mass spectroscopy, T225 but not T222 was identified as a GORASP2 residue phosphorylated by mitotic cytosol (Duran et al. 2008). T249 residue of GORASP2 was also phosphorylated by mitotic cytosol, but the involvement of ERKs in T249 phosphorylation has not been examined (Duran et al. 2008).
MCPH1 (microcephalin) binds condensin II complex through direct interaction with NCAPG2 and possibly NCAPD3 condensin II subunits (Wood et al. 2008, Yamashita et al. 2011). MCPH1 binding sequesters condensin II by preventing loading of condensin II on chromatin. Simultaneous binding of MCPH1 to the SET oncogene may contribute to condensin II sequestering (Leung et al. 2011). Mutations in MCPH1 are a cause of microchephaly inhereted in an autosomally recessive manner. MCPH1 deficient cells show premature chromosome condensation (PCC) phenotype, with metaphase-like chromosomes apparent in prophase, before nuclear envelope breakdown (Wood et al. 2008).
At the beginning of mitosis, MASTL (GWL, Greatwall kinase) is activated by phosphorylation at several key sites. Many of these sites, including functionally important threonine residues T194, T207 and T741 (corresponding to Xenopus residues T193, T206 and T748), are proline directed, matching CDK1 consensus sequence, and thus probably phosphorylated by CDK1, as shown by in vitro studies (Yu et al. 2006. Blake-Hodek et al. 2012). Phosphorylation of the serine residue S875 (S883 in Xenopus) is implicated as critical for the mitotic function of MASTL (Vigneron et al. 2011) and likely occurs through autophosphorylation (Blake-Hodek et al. 2012). Other kinases, such as PLK1 (Vigneron et al. 2011) and other MASTL phosphorylation sites may also be involved in mitotic activation of MASTL (Yu et al. 2006, Vigneron et al. 2011, Blake-Hodek et al. 2012). Phosphorylation of the serine residue S102 (S101 in Xenopus) is functionally important but the responsible kinase has not been identified (Blake-Hodek et al. 2012).
ARPP19 and ENSA, activated by MASTL (GWL) mediated phosphorylation, bind and inhibit PP2A complexed with the regulatory subunit PPP2R2D (B55-delta). Inhibition of PP2A-PPP2R2D phosphatase activity allows mitotis entry and mainetance by preventing dephosphorylation of CDK1 mitotic targets (Mochida et al. 2010, Gharbi-Ayachi et al. 2010).
NEK9 functions as a homodimer and becomes catalytically active in mitosis through phosphorylation (Roig et al. 2002). While threonine T333 of NEK9 is phosphorylated in both interphase and mitotic cells (Roig et al. 2005, Bertran et al. 2011), serine residues S29, S750 and S869 of NEK9 are phosphorylated only in mitotic cells. S29, S750 and S869 sites are proline directed and match the CDK1 consensus sequence (Bertran et al. 2011). CDK1:CCNB complex was shown to phosphorylate NEK9 in vitro (Roig et al. 2002).
NEK9 serine residues S29, S750 and S869, which are likely targets of CDK1:CCNB-mediated phosphorylation in mitosis, can be recognized by the polo-box domain (PBD) of PLK1 when phosphorylated. Phosphorylation of S869 appears to be crucial for the interaction of NEK9 and PLK1 (Bertran et al. 2011). PLK1 phosphorylates threonine T210 of NEK9 in vitro. T210 is located in the kinase activation loop of NEK9 and T210 phosphorylation is necessary for NEK9 kinase activity. While T210 can be autophosphorylated in vitro, when NEK9 is incubated in the presence of excess ATP and Mg2+ (Roig et al. 2005), mitotic phosphorylation of T210 requires both CDK1 and PLK1 activity (Bertran et al. 2011).
NEK9, activated by CDK1- and PLK1-mediated phosphorylation, phosphorylates NEK6 on serine residue S206, and NEK7 on serine residue S195. S206 and S195 are located in the activation loop of NEK6 and NEK7, respectively. NEK6 activation is dependent on S206 phosphorylation, although phosphorylation at threonine T202 may augment NEK6 kinase activity. NEK7 activity also depends on phosphorylation of S195. NEK9 remains tightly associated with NEK6 (as well as NEK7) after phosphorylation, and may direct NEK6/NEK7 to specific target (Belham et al. 2003). In addition, irrespective of phosphorylation, binding of the non-catalytic C-terminus of NEK9 to NEK7 (as well as NEK6), relieves autoinhibitory conformation of NEK7/NEK6. The autoinhibitory conformation of NEK7 depends on the formation of a hydrogen bond between tyrosine Y97 (tyrosine Y108 in NEK6) and leucine L180. This Y97-involving hydrogen bond prevents the formation of a salt bridge between lysine K63 and glutamate E82 of NEK7, which is essential for catalysis. Binding of NEK9 is thought to disrupt the hydrogen bond between Y97 and L180 of NEK7 (Y108 and L191 of NEK6) and allow NEK7/NEK6 to achieve active conformation (Richards et al. 2009).
Phosphorylation of NUP98 by NEK6 (and/or NEK7) promotes nuclear envelope permeabilization by initiating nuclear pore complex (NPC) disassembly. Two NUP98 serine residues, S591 and S822 (referring to NUP98 splice variant NUP98-4; these residues correspond to S608 and S839 of NUP98 splice variant NUP98-3), are phosphorylated on NUP98 isolated from mitotic HeLa cells (human cervical cancer cell line). These serine residues match the NEK6 target site consensus and are phosphorylated by NEK6 in vitro. Both sites can also be phosphorylated in vitro by NEK7 and weakly by NEK2. As NEK7 but not NEK2 was shown to be involved, with NEK6, in nuclear envelope permeabilization, NEK2 is not shown as the NUP98 kinase. Phosphorylated NUP98 dissociates from the NPC (Laurell et al. 2011). As NUP98 localizes to both sides of the NPC, cytosolic and nucleoplasmic (Griffis et al. 2003), the reaction shows a portion of NUP98 being released to the cytosol, and a portion of NUP98 dissociating into the nucleus, similar to what is observed by immunocytochemistry (Laurell et al. 2011).
CDK1 activity promotes the nuclear pore complex (NPC) disassembly in mitosis (Muhlhausser and Kutay 2007). While NUP98 is probably not the only nucleoporin phosphorylated by CDK1 at mitotic entry, NUP98 is the best characterized CDK1 target among nuclear pore complex components. NUP98 threonine residues T529, T536, and T653, as well as serine residues S595 and S606 were found to be phosphorylated when NUP98 was isolated from mitotic HeLa cells (human cervical carcinoma cell line); these five sites match the CDK1 target site consensus and are phosphorylated by CDK1:CCNB in vitro (Laurell et al. 2011). The NUP98 splicing isoform NUP98-4 was used in the study by Laurell et al. 2011 and the indicated positions of phosphorylated amino acid residues refer to this isoform. An additional splicing isoform NUP98-3, the product of an alternative splicing site in exon10 of the NUP98 gene, which is 17 amino acids longer than NUP98-4, could also be a part of the NPC. CDK1-phosphorylated residues in NUP98-3 would be threonines T546, T553 and T670, and serines S612 and S623.
In mitotic prophase, chromatin detaches from the nuclear envelope, and this contributes to the nuclear envelope breakdown. VRK1 (and possibly VRK2) mediated phosphorylation of BANF1 (BAF), a protein that simultaneously interacts with DNA, LEM-domain inner nuclear membrane proteins, and lamins (Zheng et al. 2000, Shumaker et al. 2001, Haraguchi et al. 2001, Mansharamani and Wilson 2005, Brachner et al. 2005) is considered to be one of the key steps in the detachment of the nuclear envelope from chromatin (Bengtsson and Wilson 2006, Nichols et al. 2006, Gorjanacz et al. 2007).
BANF1 (BAF i.e. barrier-to-autointegration factor) is a DNA-binding protein that was initially discovered as a regulator of retroviral integration (Lee and Craigie 1994, Lee and Craigie 1998). BANF1 (BAF) binds DNA non-specifically as a homodimer (Zheng et al. 2000). Proteins of the inner nuclear membrane that possess a LEM domain, TMPO (LAP2beta), EMD (emerin), LEMD3 (MAN1) and LEMD2 (LEM2), form three-way complexes with BANF1 and lamins - intermediary filaments of the nucleoplasm (Shumaker et al. 2001, Holaska et al. 2003, Mansharamani and Wilson 2005, Brachner et al. 2005). These complexes are thought to be important for the structure of the nuclear lamina and also enable attachment of chromatin to the nuclear envelope (Haraguchi et al. 2001, Dechat et al. 2004).
In mitosis, VRK1 (and to a lesser extent VRK2) serine/threonine kinase phosphorylates BANF1 (BAF) on serine residue S4 and threonine residues T2 and T3 (Nichols et al. 2006, Gorjanacz et al. 2007, Asencio et al. 2012). Only VRK2 isoform VRK2-2 which can localize to the nucleus (Blanco et al. 2006) is annotated as BANF1 kinase. Phosphorylated BANF1 (BAF) dissociates from chromatin and the inner nuclear membrane proteins (Bengtsson and Wilson 2006), allowing chromatin to detach from the nuclear envelope.
VRK1 and VRK2 are autophosphorylated but not all autophosphorylation sites have been mapped and the impact of autophosphorylation on catalytic activity has not been determined.
After the initiation of DNA condensation in prophase of mitosis, NuMA (NUMA1) is phosphorylated on threonine residue 2055 by the complex of Cdc2 (CDK1) kinase and Cyclin B1 (CCNB1). After the nuclear envelope breakdown, phosphorylated NuMA rapidly moves to the centrosomal region (Compton and Luo 1995, Hsu and Yeh 1996, Kotak et al. 2013). Another phosphorylation event occurs when NuMA associates with the mitotic spindle (Gaglio et al. 1995; Hsu and Yeh 1996). While CCNB1:p-T160-CDK1-dependent phosphorylation appears to plays an essential role in the targeting of NuMA to the spindle apparatus (Compton and Luo 1995, Hsu and Yeh 1996, Kotak et al. 2013), there may be additional protein kinases that promote the release of NuMA from the nuclear compartment at nuclear envelope breakdown (Saredi et al. 1997).
CTDNEP1:CNEP1R1 serine/threonine protein phosphatase complex consists of the catalytic subunit CTDNEP1 (Dullard) and the regulatory subunit CNEP1R1 (TMEM188) and is evolutionarily conserved from yeast to mammals (Kim et al. 2007, Han et al. 2012). CTDNEP1:CNEP1R1 and its yeast counterpart NEM1:SPO7 localize to the nuclear envelope and the endoplasmic reticulum membrane. CTDNEP1:CNEP1R1 dephosphorylates lipins (LPIN1, LPIN2 and LPIN3), which act as phosphatidate phosphatases, dephosphorylating phosphatidate (PA) and converting it to diacylglycerol (DAG). The yeast NEM1:SPO7 complex dephosphorylates yeast lipin orthologue PAH1 (SMP2, PAP1). CTDNEP1:CNEP1R1 shows a preference for the phosphorylated serine S106 of lipins. S106 phosphorylation is insulin-induced, and could be mediated by CDK1, as it is proline-directed (Wu et al. 2011). CDC28, a yeast homolog of CDK1, was shown to phosphorylate PAH1, while NEM1:SPO7 removes CDC28-introduced phosphate groups. Lipin phosphorylation regulates lipin localization, with phosphorylated lipins being soluble and dephosphorylated lipins being membrane-bound (Grimsey et al. 2008, Choi et al. 2011). The association of lipins with the nuclear envelope brings lipins in proximity to its substrate, PA, thereby enabling lipin catalytic activity (Karanasios et al. 2010). Catalytic activity of PAH1 regulates the morphology and dynamics of endoplasmic reticulum and nuclear membranes in yeast. In C. elegans and in human cell lines, lipin catalytic activity is needed for mitotic progression as it facilitates depolymerization of the nuclear lamina and nuclear envelope breakdown (Santos-Rosa et al. 2005, Kim et al. 2007, Gorjanacz et al. 2009, Golden et al. 2009, Choi et al. 2011, Mall et al. 2012).
Lipins (LPIN1, LPIN2, LPIN3) possess several proline-directed phosphorylation sites that can be phosphorylated by CDK1 (Grimsey et al. 2008), including S106. Serine S106 in lipins is a preferred target for dephosphorylation by the evolutionarilly conserved CTDNEP1:CNEP1R1 complex (ortholog of yeast NEM1:SPO7 complex). Lipin phosphorylation regulates lipin localization, with phosphorylated lipins being soluble and dephosphorylated lipins being membrane-bound. The yeast ortholog of CDK1, CDC28, as well as human CDK1 can phosphorylate yeast lipin PAH1, inducing its dissociation from the nuclear envelope and endoplasmic reticulum membrane (Choi et al. 2011).
Lipin proteins LPIN1, 2, and 3, associated with the nuclear envelope, can each catalyze the hydrolysis of phosphatidate to yield 1,2-diacyl-glycerol and orthophosphate. The activities of LPIN1 and LPIN2 have been established experimentally (Grimsey et al. 2008); that of LPIN3 is inferred from its structural similarities both to its human paralogues and to its mouse ortholog (Donkor et al. 2007).
PKC contains an N-terminal C2 like domain, a pseudosubstrate (PS), DAG binding (C1) domain and a C-terminal kinase domain. The PS sequence resembles an ideal substrate with the exception that it contains an alanine residue instead of a substrate serine residue, is bound to the kinase domain in the resting state. As a result, PKC is maintained in a closed inactive state, which is inaccessible to cellular substrates (Colon-Gonzalez & Kazanietz 2006). Diacylglycerol (DAG) produced by activated lipins (LPIN1, LPIN2, LPIN3) leads to the activation of PKC (PRKCA and PRKCB) and their translocation from the nucleoplasm to the nuclear envelope where they can phosphorylate lamins (Mall et al. 2012). PKCs are tethered to the membrane through DAG binding to the C1 domain and this confers a high-affinity interaction between PKC and the membrane. This leads to a massive conformational change that releases the PS domain from the catalytic site and the system becomes both competent and accessible (Colon-Gonzalez & Kazanietz 2006).
Protein kinase C (PRKCA and PRKCB), activated by lipin-generated diacylglycerol (DAG), phosphorylates C-terminal tails of lamins (serine S395 in lamins A, B and C, and also serine S405 of lamin B), leading to depolymerization of lamin filaments (Hocevar et al. 1993, Goss et al. 1994, Mall et al. 2012).
Phosphorylation of the N-termini of lamins by CDK1 (serine S23 of lamin B, serin S22 of lamin A and C) probably happens consequentially with phosphorylation of C-termini of lamins by PKC, and contributes to the depolymerization of lamin filaments and solubilization of the nuclear lamina (Ward and Kirschner 1990, Peter et al. 1990, Heald and McKeon 1990, Mall et al. 2012).
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II:Nucleosome with
H4K20me1transport vesicle fused with
cis-GolgiCondensin
II:NucleosomeCondensin
II:Nucleosomep-S206-NEK6/
p-S195-NEK7Annotated Interactions
II:Nucleosome with
H4K20me1II:Nucleosome with
H4K20me1transport vesicle fused with
cis-GolgiCondensin
II:NucleosomeCondensin
II:NucleosomeCondensin
II:NucleosomeCondensin
II:NucleosomeIn mitotic prophase, CDK1 (CDC2) in complex with either CCNB1 (cyclin B1) or CCNB2 (cyclin B2), as both CCNB1 and CCNB2 can localize to Golgi (Jackman et al. 1995, Draviam et al. 2001), phosphorylates GORASP1, GOLGA2 and RAB1 (Bailly et al. 1991, Lowe et al. 1998, Preisinger et al. 2005). Phosphorylation of GOLGA2 and RAB1 impairs their association with USO1, which inhibits thethering and subsequent fusion of ER-originating vesicles with cis-Golgi cisternae, resulting in cessation of ER to Golgi protein trafficking at the start of mitosis and increase in the number of Golgi trafficking vesicles at the expense of Golgi cisternae (Lowe et al. 1998, Seeman et al. 2000, Moyer et al. 2001, Diao et al. 2008).
The role of RB1 in the maintenance of genomic stability is supported by studies of the childhood eye cancer retinoblastoma and its precursor, retinoma. Retinoma, a quiescent precursor of malignant retinoblastoma with functional loss of both RB1 alleles, is genomically unstable (Dimaras et al. 2008). Also, while the majority of retinoblastoma tumors are caused by the loss-of-function of the tumor suppressor gene RB1, ~2% of retinoblastoma tumors in unilaterally affected patients are initiated by a high level amplification of MYCN gene, in the presence of two functional, unmutated RB1 alleles. These tumors, with normal RB1 and amplified MYCN show a much lower level of genomic instability than retinoblastoma tumors with RB1 loss-of-function (Rushlow et al. 2013).
BANF1 (BAF i.e. barrier-to-autointegration factor) is a DNA-binding protein that was initially discovered as a regulator of retroviral integration (Lee and Craigie 1994, Lee and Craigie 1998). BANF1 (BAF) binds DNA non-specifically as a homodimer (Zheng et al. 2000). Proteins of the inner nuclear membrane that possess a LEM domain, TMPO (LAP2beta), EMD (emerin), LEMD3 (MAN1) and LEMD2 (LEM2), form three-way complexes with BANF1 and lamins - intermediary filaments of the nucleoplasm (Shumaker et al. 2001, Holaska et al. 2003, Mansharamani and Wilson 2005, Brachner et al. 2005). These complexes are thought to be important for the structure of the nuclear lamina and also enable attachment of chromatin to the nuclear envelope (Haraguchi et al. 2001, Dechat et al. 2004).
In mitosis, VRK1 (and to a lesser extent VRK2) serine/threonine kinase phosphorylates BANF1 (BAF) on serine residue S4 and threonine residues T2 and T3 (Nichols et al. 2006, Gorjanacz et al. 2007, Asencio et al. 2012). Only VRK2 isoform VRK2-2 which can localize to the nucleus (Blanco et al. 2006) is annotated as BANF1 kinase. Phosphorylated BANF1 (BAF) dissociates from chromatin and the inner nuclear membrane proteins (Bengtsson and Wilson 2006), allowing chromatin to detach from the nuclear envelope.
VRK1 and VRK2 are autophosphorylated but not all autophosphorylation sites have been mapped and the impact of autophosphorylation on catalytic activity has not been determined.
p-S206-NEK6/
p-S195-NEK7p-S206-NEK6/
p-S195-NEK7