cdc6: Biological Overview | References
Gene name - Cdc6
Synonyms - CG5971
Cytological map position-66D9-66D9
Function - enzyme - DNA-dependent ATPase
Symbol - Cdc6
FlyBase ID: FBgn0035918
Genetic map position - 3L: 8,624,609..8,627,181 [-]
Classification - AAA-superfamily of ATPases and CDC6 C terminal
Cellular location - nuclear
|Recent literature||Wu, Z., Guo, W., Xie, Y. and Zhou, S. (2016). Juvenile hormone activates the transcription of Cell-division-cycle 6 (Cdc6) for polyploidy-dependent insect vitellogenesis and oogenesis. J Biol Chem 291: 5418-5427. PubMed ID: 26728459
Although juvenile hormone (JH) is known to prevent insect larval metamorphosis and stimulate adult reproduction, the molecular mechanisms of JH action in insect reproduction remain largely unknown. The JH-receptor complex, composed of methoprene-tolerant and steroid receptor co-activator, acts on mini-chromosome maintenance (Mcm) genes Mcm4 and Mcm7 to promote DNA replication and polyploidy for the massive vitellogenin (Vg) synthesis required for egg production in the migratory locust. This study has investigated the involvement of Cell-division-cycle 6 (Cdc6) in JH-dependent vitellogenesis and oogenesis, as Cdc6 is essential for the formation of prereplication complex. Cdc6 was shown to be expressed in response to JH and methoprene-tolerant, and Cdc6 transcription is directly regulated by the JH-receptor complex. Knockdown of Cdc6 inhibits polyploidization of fat body and follicle cells, resulting in the substantial reduction of Vg expression in the fat body as well as severely impaired oocyte maturation and ovarian growth. These data indicate the involvement of Cdc6 in JH pathway and a pivotal role of Cdc6 in JH-mediated polyploidization, vitellogenesis, and oogenesis.
The Cdc6/18 protein has been mainly characterised for its role in the initiation of DNA replication (see Bell, 2002a for a review describing Origin recognition complex and CDC6 function in initiation of DNA replication; see ORC1 information of the role of ORC in DNA replication in Drosophila). Several studies exist, however, which suggest that it may also have a role in controlling the G2/M transition. Studies on the Drosophila Cdc6 (DmCdc6) protein support this dual function for the protein. Its location is consistent with a cellular role post replication initiation since it remains nuclear throughout G1, S and G2 phases. In addition, the level of DmCdc6 protein was reduced to nondetectable levels in S2 cells using RNAi. This causes DNA fragmentation and cell cycle abnormalities which have some similarities with phenotypes previously observed in yeasts and are consistent with the cells entering mitosis with incompletely replicated DNA. Finally, the DmCdc6 protein was stably overexpressed to a high level in S2 cells. Despite a large excess of protein, the effects on the S2 cells were minimal. However, a slight stalling of the cells in the late S phase of the cell cycle was detected, further supporting the proposal that DmCdc6 has a role in controlling the transition from the S to M phases of the cycle (Crevel, 2005).
The Cdc6/cdc18 protein is thought to have two important roles during the progression of the cell cycle. One is in the initiation of DNA replication, and the other is in checkpoint processes controlling the passage of the cell through the later stages of the cell cycle. Of the two, its role in DNA replication has been more extensively studied (reviewed by Bell, 2002b). In this case Cdc6 is one of the earliest proteins required and is involved in the formation of the prereplicative complex (see A model for the state of pre-replication chromatin and cell cycle regulation in human cells from Fujita, 2006). Together with cdt1 (Drosophila homolog; Double parked), Cdc6 binds after the hetero-hexameric origin recognition complex (ORC; see Drosophila Orc2), but its binding precedes and is needed for the loading of the six-membered minichromosome maintenance protein complex (MCM; see Drosophila Disc proliferation abnormal/MCM4). In the absence of Cdc6 no prereplicative complex can be formed. The prereplicative complex then provides the platform from which bulk DNA synthesis is launched. Although this sequence of events has been well studied, the exact biochemical activities catalysed by these proteins remain to be clarified, with the exception of the MCM complex which is thought to be the replicative helicase (Crevel, 2005 and references therein).
The checkpoint role of Cdc6/18 (referring to the the cdc18 gene in S. pombe and its homologue, CDC6, in other eukaryotes) is less well understood. In Saccharomyces cerevisiae Cdc6 has been suggested to play a direct role in mitotic exit by inactivating cdc28 kinase complexes (Bueno, 1992). In Schizosaccharomyces pombe it is thought to be needed for the replication arrest checkpoint acting through cds1 (chk2) (Murakami, 2002). A replication checkpoint-related role has also been reported in Xenopus, although in this case it acts through chk1 (Oehlmann, 2004). In Xenopus (Clay-Farrace, 2003) Chk1 also seems to be involved in the observed role of Cdc6 to couple the S and M phases of the cell cycle (Crevel, 2005 and references therein).
Species-specific differences have been reported in the behaviour of the Cdc6/18 during the cell cycle (reviewed by Kearsey, 2003). Cdc6/cdc18 in S. cerevisiae/S. pombe is an unstable protein that is present only in a narrow window of the cell cycle (Baum, 1998; Piatti, 1995), whereas in higher eukaryotes the protein seems to be more stable but has been reported to show cell cycle changes in localisation (Saha, 1998; Petersen, 1999). Several reports have suggested that the protein is nuclear and probably chromatin bound in G1, and then moves to the cytoplasm in S phase. Although it is worth noting that many of the observations providing localisation data were carried out using overexpressed protein, a recent paper (Alexandrow, 2004) has suggested that endogenous Cdc6 in mammalian cells may remain nuclear and chromatin-associated throughout the whole cycle (Crevel, 2005 and references therein).
Differences in Cdc6 behaviour between species might be responsible for the species-specific variations that are observed when Cdc6/18 is overexpressed. Overexpression of cdc18 in S. pombe causes overreplication of the DNA, particularly in combination with cdt1 (Yanow, 2001). In S. cerevisiae a delay in the initiation of M phase is seen, although the overall growth rate is not affected (Elsasser, 1996). In higher eukaryotes the effect of increased expression of Cdc6 is dependent on the phase of the cell cycle in which it is carried out. In nonsynchronised cells retrovirally overexpressed protein shows no measurable effects (Petersen, 2000). However, if high levels of protein are microinjected into cells in the G2 phase of the cell cycle the cells are prevented from entering mitosis (Clay-Farrace, 2003). Finally, adenoviral directed expression in quiescent mammalian cells (from which it is normally absent) causes MCM loading and, if coupled with serum stimulation of the cells, advancement of S phase entry (Cook, 2002; Crevel, 2005 and references therein).
The effect of knocking out or mutating the gene also varies depending on the species under study. In S. pombe, cdc18 null mutants go through mitosis without having first replicated their DNA, producing a cut phenotype that is lethal (Kelly, 1993). Reduction division and lethality is also observed for null mutants in S. cerevisiae (Piatti, 1995). In higher eukaryotes in vivo knockout experiments for Cdc6 have not been reported; however, depletion of the protein in Xenopus extracts causes a block before S phase (Coleman, 1996). Dominant negative mutants for mammalian cells (Walker A and B box mutants) cause a stop during the S phase of the cell cycle (Herbig, 1999), a result that is consistent with what is observed for analogous mutants in S. cerevisiae (Crevel, 2005 and references therein).
Drosophila Cdc6 function was examined in the three areas where behavioural differences have been reported in other species. In terms of cell cycle localisation it was found that both endogenous and overexpressed Cdc6 remain nuclear during S phase. The effect of knocking out Cdc6 in vivo was examined by exploiting the amenability of Drosophila S2 cells to RNAi. Reduction of Cdc6 protein to nondetectable levels causes DNA fragmentation and cell cycle abnormalities that have some similarities with phenotypes previously observed in yeasts. Knockout experiments have not previously been reported for other higher eukaryotic systems. Finally, it has been shown that, consistent with observations in most systems, with the exception of S. pombe, even high-level stable overexpression of Cdc6 in nonsynchronised S2 cells has only minimal effects on cell functioning (Crevel, 2007).
The Drosophila Cdc6 gene (CG5971) was identified by searching the annotated Flybase gene collection using the sequences of cdc6 genes from several other species. A comparison of the protein sequence with Cdc6/18 homologues from other species shows a high degree of homology; however, the Drosophila protein is 662 amino acids, which is about 100 amino acids longer than the protein from other species. This includes three regions of 20-24 amino acids inserted in the first 124 amino acids of Drosophila Cdc6 and one of 17 amino acids inserted at position 222. By database searching none of these regions seems to contain any particular motif; however, all of these regions are highly charged, either positive or negative depending on the region (Crevel, 2007).
Cdc6/18 knockouts in yeasts cause a block to S phase entry. In higher eukaryotes gene knockouts have not been carried out, but adding dominant negative protein mutants (usually mutated in the walker A and B ATPase motifs that are seen in cdc6) led to slow progression through S phase. To determine the effect of performing a knockout of Cdc6 in Drosophila RNAi in S2 cells was carried out against the DmCdc6 protein. The experiments were carried out with RNAi against two different regions of the protein, and the same results were obtained in each case. Using this technique it was possible to significantly reduce the levels of both Cdc6 protein and RNA in these cells. S2 cells lacking Cdc6 were less healthy than mock treated controls. Analysis of cell counts suggested that the doubling time of the depleted cells was severely compromised. More detailed examination of these cells by FACS analysis revealed that as early as day 3 after RNAi addition there was a significant increase in the numbers of the treated cells that were in the S phase of the cell cycle, suggesting that they were having problems in passing through this stage. In addition, a significant number of cells were seen that had sub-G1 DNA content. The numbers of cells in both of these categories increased at longer time points (Crevel, 2007).
The results suggest that in Drosophila cells DmCdc6 is a nuclear protein in G1, S and G2 phases of the cell cycle. Similar results have been obtained for the endogenous protein in CHO cells (Alexandrow, 2002). These data are somewhat at odds with earlier data suggesting that Cdc6 in higher eukaryotes leaves the nucleus during the S phase, although as pointed out by Alexandrow the earlier data was largely obtained from the study of overexpressed protein, which seems to behave differently to endogenous protein in mammalian systems. Interestingly, in the Drosophila system even heavily overproduced protein is still largely nuclear, although whether this is related to differential control of the protein in different species or is because of the use of different methodology is not clear. In Drosophila a high percentage of the Cdc6 at G1, S and G2 phases of the cycle seems to be tightly associated with chromatin. This prolonged association of Cdc6 with chromatin is consistent with Cdc6 having a role in the cell after the initiation of DNA replication as suggested by others (Oehlmann, 2004; Clay-Farrace, 2003). However, in aphidicolin-blocked cells a reproducible increase is seen in the amount of Drosophila Cdc6 in the nucleoplasm (i.e. detergent soluble). This might reflect a change in the nature of the Cdc6 complex with chromatin related to this second function, although it is not possible to rule out that this is a specific effect caused by the perturbation of the cell cycle due to drug treatment (Crevel, 2007).
Most of the previous studies carried out to look at the effects of overexpression of Cdc6 in higher eukaryotes have involved protein microinjection or transient expression. These studies suggest that the effects of this overexpression in nonsynchronised cells are very limited. Using Drosophila S2 cells it is possible to make stable cell lines that express Cdc6 under an inducible promoter to high levels. The levels and duration of expression in this type of system are more likely to reveal overexpression effects of the protein. Furthermore, the tight chromatin association of the bulk of the Cdc6 overexpressed in this way, and the ability of the overexpressed protein to substitute when the endogenous cellular Cdc6 has been removed by RNAi suggests functionality of the protein. Despite this, in agreement with earlier studies in other systems, even the highest level of overexpression of the protein does not have a drastic effect on the cells -- in particular, no evidence is seen of over replication of chromatin. A small but reproducible lag in the G2/M transition of these cells is seen. In human cells injection of Cdc6 into synchronised G2 cells has been reported to block entry into mitosis, which has been attributed to a role for Cdc6 in linking the S and M phases of the cell cycle. The data with Drosophila are suggestive of a similar mechanism operating in cultured cells (Crevel, 2007).
The most penetrative event that is observed when the Cdc6 protein is removed from Drosophila cells is the inability of the cells to progress through the S phase of the cell cycle (as observed by FACS analysis). This suggests, unsurprisingly, that as for other organisms the Cdc6 protein plays a vital role in DNA replication. Further analysis of the cells by immunofluorescence does not, however, show cells that are arrested with S phase morphology. A high percentage of the cells, although they do not apparently have fully replicated DNA, have passed into mitosis, as measured by the presence of the phospho-H3 antigen on the chromatin. A few of these cells have a morphology that looks like premature chromatin condensation (PCC). PCC has often been observed during the analysis of larvae carrying mutations in Drosophila replication proteins. In addition, depletion of other replication proteins in S2 cells (cdc45, MCM2, MCM5 and MCM10) has also been reported to show PCC. For many of the Cdc6-depleted Drosophila cells, however, the phenotype looks more severe as the DNA appears to be heavily fragmented, spindles are often present and in some cases subgenome quantities of DNA appear to be surrounded by individual nuclear laminae. This phenotype has some similarities with what has been observed for Drosophila cdt1 RNAi in S2 cells and also with the cut phenotype obtained from knocking out the S. pombe Cdc6 homologue cdc18 (Kelly, 1993). The observed state of these DmCdc6-depleted S2 cells suggests that, in addition to their replication defect, they are also missing a checkpoint to prevent mitotic entry with unreplicated DNA. The observation of spindles and nuclear laminar reformation around subgenomic DNA masses further suggest that several aspects of the cell cycle, not just the chromosome cycle, are also affected. This, therefore, provides further evidence that in Drosophila as well as in other higher eukaryotes the Cdc6 protein may have a second role in the cell cycle concerned with the co-ordination of the S and M phases (Crevel, 2007).
Analysis of the role of Cdc6 in preventing re-replication in C. elegans provides further information of Cdc6 function in metazoans. Genome stability requires that genomic DNA is replicated only once per cell cycle. The replication licensing system ensures that the formation of pre-replicative complexes is temporally separated from the initiation of DNA replication. The replication licensing factors Cdc6 and Cdt1 are required for the assembly of pre-replicative complexes during G1 phase. During S phase, metazoan Cdt1 is targeted for degradation by the CUL4 ubiquitin ligase, and vertebrate Cdc6 is translocated from the nucleus to the cytoplasm (Jiamg, 1999; Petersen, 1999). However, because residual vertebrate Cdc6 remains in the nucleus throughout S phase, it has been unclear whether Cdc6 translocation to the cytoplasm prevents re-replication. The inactivation of C. elegans CUL-4 is associated with dramatic levels of DNA re-replication. C. elegans CDC-6 has been shown to be exported from the nucleus during S phase in response to the phosphorylation of multiple CDK sites. CUL-4 promotes the phosphorylation of CDC-6 and its translocation via negative regulation of the CDK-inhibitor CKI-1. Re-replication can be induced by co-expressing non-exportable CDC-6 with non-degradable CDT-1, indicating that redundant regulation of CDC-6 and CDT-1 prevents re-replication. This demonstrates that Cdc6 translocation is critical for preventing re-replication, and that CUL-4 independently controls both replication licensing factors (Kim, 2007).
Thus C. elegans CDC-6 is exported from the nucleus during S phase, similar to vertebrate Cdc6, suggesting that this is an ancient regulatory mechanism. Further, the strategy to trigger Cdc6 nuclear export is also conserved: the phosphorylation of multiple CDK sites to inactivate N-terminal NLSs. All six N-terminal CDK-sites must be phosphorylated to promote CDC-6 nuclear export. Interestingly, the phosphorylation of T131 is associated with both nuclear export and nucleoli localization. This suggests the possibility that the phosphorylation of a subset of sites, while not sufficient to induce nuclear export, can direct CDC-6 to specific sub-nuclear locations (Kim, 2007).
In humans and Xenopus, ectopically-expressed Cdc6 is completely exported to the cytoplasm during S phase; in contrast, a substantial fraction of endogenous Cdc6 remains nuclear localized during S phase (Alexandrow, 2004). Strikingly, a similar result is observed in C. elegans with a substantial fraction of endogenous CDC-6 remaining in the nucleus during S phase, while ectopically-expressed CDC-6 appears exclusively cytoplasmic. The reason(s) for these differential localizations are not understood (Kim, 2007).
The presence of nuclear-localized Cdc6 during S phase in mammalian cells has led to the proposal that Cdc6 translocation is not important for restraining DNA replication licensing. Further, there is currently no evidence for a functional role of Cdc6 translocation in preventing re-replication. In contrast, this study observed that non-exportable CDC-6 can synergize with deregulated CDT-1 to induce re-replication. This implies that CDC-6 translocation is a redundant safeguard to prevent the re-initiation of DNA replication, and provides the first evidence in any organism of a functional role for phosphorylation-dependent CDC-6 nuclear export (Kim, 2007).
In S. pombe, the overexpression of the Cdc6 ortholog (Cdc18) is sufficient to induce significant re-replication (Nishitani, 1995). In contrast, overexpression of Cdc6 does not induce re-replication in S. cerevisiae, Drosophila, and humans. In humans, co-overexpression of wild-type Cdt1 and Cdc6 in cells that lack a cell cycle checkpoint produces only modest re-replication in a subset of cells (Kim, 2007 and references therein).
This study observed that co-expression of non-degradable CDT-1 and non-exportable CDC-6 produced significant re-replication in a subset of early-stage C. elegans embryos. In contrast, overexpression of combinations of deregulated and wild-type CDT-1 or CDC-6 did not induce re-replication. This indicates that redundant regulation of CDT-1 and CDC-6 prevents re-replication. The failure to observe re-replication in every embryonic cell expressing deregulated CDT-1 and CDC-6 suggests the presence of additional safeguards in the early embryo to prevent DNA re-replication. Expression of combinations of wild-type and deregulated CDT-1 and CDC-6 produced embryonic lethality that was not associated with increased DNA levels. The cause of this lethality is unclear, but may arise from changes in the timing of DNA replication, which is known to produce embryonic arrest (Kim, 2007).
Inactivation of CUL-4 produces dramatic levels of re-replication that is associated with a failure to degrade CDT-1. However, overexpressing Cdt1 in fission yeast does not induce re-replication, and overexpressing human Cdt1 several log-fold higher than the endogenous protein produces only modest re-replication in a subset of cells. Given the negligible or limited effects of greatly overexpressing Cdt1 in other organisms, it was hard to reconcile the substantial re-replication associated with merely failing to degrade CDT-1 during S phase in cul-4(RNAi) animals (Kim, 2007).
This work reveals that the CDC-6 replication licensing factor is also deregulated in cul-4(RNAi) animals. CDC-6 remains nuclear throughout S phase in cul-4(RNAi) animals, and this is correlated with a failure to phosphorylate CDC-6 on CDK sites. CUL-4 negatively regulates the levels of the CDK-inhibitor CKI-1. The negative regulation of CKIs of the CIP/KIP family by CUL4 is conserved in Drosophila and humans (Higa, 2006). cki-1 RNAi suppresses re-replication in cul-4 mutants without affecting CDT-1 accumulation, indicating that CKI-1 is independently required for the induction of re-replication. Significantly, the presence of CKI-1 is required for the block on CDC-6 phosphorylation and nuclear export in cul-4(gk434) cells. These results suggest that CUL-4 promotes CDC-6 nuclear export by negatively regulating CKI-1 levels, thereby allowing CDK(s) to phosphorylate CDC-6 and induce its nuclear export. The evidence that CDK(s) are the relevant kinases is that CDC-6 is phosphorylated on CDK-consensus sites and the phosphorylation is blocked by a CDK inhibitor. In yeast and mammals, CDK activity prevents re-replication, and siRNA co-depletion of CDK1 and CDK2 in human cells induces limited re-replication. These results suggest that in metazoa, Cdc6 is one of the critical targets of CDKs for preventing re-replication. This work further indicates that CUL-4 is a master regulator that restrains DNA replication through two independent pathways: mediating CDT-1 degradation and promoting CDC-6 nuclear export via the negative regulation of CKI-1 (Kim, 2007).
The regulated loading of the replicative helicase minichromosome maintenance proteins 2-7 (MCM2-7) onto replication origins is a prerequisite for replication fork establishment and genomic stability. Origin recognition complex (ORC), Cdc6, and Cdt1 assemble two MCM2-7 hexamers into one double hexamer around dsDNA. Although the MCM2-7 hexamer can adopt a ring shape with a gap between Mcm2 and Mcm5, it is unknown which Mcm interface functions as the DNA entry gate during regulated helicase loading. This study established that the Saccharomyces cerevisiae MCM2-7 hexamer assumes a closed ring structure, suggesting that helicase loading requires active ring opening. Using a chemical biology approach, it was shown that ORC-Cdc6-Cdt1-dependent helicase loading occurs through a unique DNA entry gate comprised of the Mcm2 and Mcm5 subunits. Controlled inhibition of DNA insertion triggers ATPase-driven complex disassembly in vitro, while in vivo analysis establishes that Mcm2/Mcm5 gate opening is essential for both helicase loading onto chromatin and cell cycle progression. Importantly, it was demonstrated that the MCM2-7 helicase becomes loaded onto DNA as a single hexamer during ORC/Cdc6/Cdt1/MCM2-7 complex formation prior to MCM2-7 double hexamer formation. This study establishes the existence of a unique DNA entry gate for regulated helicase loading, revealing key mechanisms in helicase loading, which has important implications for helicase activation (Samel, 2014).
Search PubMed for articles about Drosophila Cdc6
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Alexandrow, M. G. and Hamlin, J. L. (2004). Cdc6 chromatin affinity is unaffected by serine-54 phosphorylation, S-phase progression, and overexpression of cyclin A. Mol. Cell. Biol. 24: 1614-1627. PubMed ID: 14749377
Baum, B., Nishitani, H., Yanow, S. and Nurse, P. (1998). Cdc18 transcription and proteolysis couple S phase to passage through mitosis. EMBO J. 17: 5689-5698. PubMed ID: 9755169
Bell, S. P. (2002a). The origin recognition complex: from simple origins to complex functions. Genes Dev. 16(6): 659-72. PubMed ID: 11914271
Bell, S. P. and Dutta, A. (2002b). DNA replication in eukaryotic cells. Annu. Rev. Biochem. 71: 333-374. PubMed ID: 12045100
Bueno, A. and Russell, P. (1992). Dual functions of CDC6: a yeast protein required for DNA replication also inhibits nuclear division. EMBO J. 11: 2167-2176. PubMed ID: 1600944
Clay-Farrace, L., Pelizon, C., Santamaria, D. M., Pines, J. and Laskey, R. A. (2003). Human replication protein Cdc6 prevents mitosis through a checkpoint mechanism that implicates Chk1. EMBO J. 22: 704-712. PubMed ID: 12554670
Coleman, T. R., Carpenter, P. B. and Dunphy, W. G. (1996). The Xenopus Cdc6 protein is essential for the initiation of a single round of DNA replication in cell-free extracts. Cell 87: 53-63. PubMed ID: 8858148
Cook, J. G., Park, C. H., Burke, T. W., Leone, G., DeGregori, J., Engel, A. and Nevins, J. R. (2002). Analysis of Cdc6 function in the assembly of mammalian prereplication complexes. Proc. Natl. Acad. Sci. 99: 1347-1352. PubMed ID: 11805305
Crevel, G,, Mathe, E. and Cotterill, S. (2005). The Drosophila Cdc6/18 protein has functions in both early and late S phase in S2 cells. J. Cell. Sci. 118: 2451-9. PubMed ID: 15923658
Elsasser, S., Lou, F., Wang, B., Campbell, J. L. and Jong, A. (1996). Interaction between yeast Cdc6 protein and B-type cyclin/Cdc28 kinases. Mol. Biol. Cell 7: 1723-1735. PubMed ID: 8930895
Fujita, M. (2006). Cdt1 revisited: complex and tight regulation during the cell cycle and consequences of deregulation in mammalian cells. Cell Div. 1: 22. PubMed ID: 17042960
Herbig, U., Marlar, C. A. and Fanning, E. (1999). The Cdc6 nucleotide-binding site regulates its activity in DNA replication in human cells. Mol. Biol. Cell 10: 2631-2645. PubMed ID: 10436018
Higa, L. A., et al. (2006). Involvement of CUL4 ubiquitin E3 ligases in regulating CDK inhibitors Dacapo/p27Kip1 and cyclin E degradation. Cell Cycle. 5: 71-77. PubMed ID: 16322693
Jiang, W., Wells, N. J. and Hunter, T. (1999). Multistep regulation of DNA replication by Cdk phosphorylation of HsCdc6. Proc. Natl. Acad. Sci. 96: 6193-6198. PubMed ID: 10339564
Kearsey, S. E. and Cotterill, S. (2003). Enigmatic variations: divergent modes of regulating eukaryotic DNA replication. Mol. Cell. 12: 1067-1075. PubMed ID: 14636567
Kelly, T. J., Martin, G. S., Forsburg, S. L., Stephen, R. J., Russo, A. and Nurse, P. (1993). The fission yeast cdc18+ gene product couples S phase to START and mitosis. Cell 74: 371-382. PubMed ID: 7916658
Kim, J., Feng, H. and Kipreos, E. T. (2007). C. elegans CUL-4 prevents rereplication by promoting the nuclear export of CDC-6 via a CKI-1-dependent pathway. Curr. Biol. 17(11): 966-72. PubMed ID: 17509881
Murakami, H., Yanow, S. K., Griffiths, D., Nakanishi, M. and Nurse, P. (2002). Maintenance of replication forks and the S-phase checkpoint by Cdc18p and Orp1p. Nat. Cell Biol. 4: 384-388. PubMed ID: 11988741
Nishitani, H. and Nurse, P. (1995). p65cdc18 plays a major role controlling the initiation of DNA replication in fission yeast. Cell 83: 397-405. PubMed ID: 8521469
Oehlmann, M., Score, A. J. and Blow, J. J. (2004). The role of Cdc6 in ensuring complete genome licensing and S phase checkpoint activation. J. Cell Biol. 165: 181-190. PubMed ID: 15096526
Petersen, B. O., et al. (1999). Phosphorylation of mammalian CDC6 by cyclin A/CDK2 regulates its subcellular localization. EMBO J. 18: 396-410. PubMed ID: 9889196
Petersen, B. O., Wagener, C., Marinoni, F., Kramer, E. R., Melixetian, M., Denchi, E. L., Gieffers, C., Matteucci, C., Peters, J. M. and Helin, K. (2000). Cell cycle- and cell growth-regulated proteolysis of mammalian CDC6 is dependent on APC-CDH1. Genes Dev. 14: 2330-2343. PubMed ID: 10995389
Piatti, S., Lengauer, C. and Nasmyth, K. (1995). Cdc6 is an unstable protein whose de novo synthesis in G1 is important for the onset of S phase and for preventing a 'reductional' anaphase in the budding yeast Saccharomyces cerevisiae. EMBO J. 14: 3788-3799. PubMed ID: 7641697
Saha, P., Chen, J., Thome, K. C., Lawlis, S. J., Hou, Z. H., Hendricks, M., Parvin, J. D. and Dutta, A. (1998). Human CDC6/Cdc18 associates with ORC1 and cyclin-cdk and is selectively eliminated from the nucleus at the onset of S phase. Mol. Cell. Biol. 18: 2758-2767. PubMed ID: 9566895
Samel, S. A., Fernandez-Cid, A., Sun, J., Riera, A., Tognetti, S., Herrera, M. C., Li, H. and Speck, C. (2014). A unique DNA entry gate serves for regulated loading of the eukaryotic replicative helicase MCM2-7 onto DNA. Genes Dev 28: 1653-1666. PubMed ID: 25085418
Yanow, S. K., Lygerou, Z. and Nurse, P. (2001). Expression of Cdc18/Cdc6 and Cdt1 during G2 phase induces initiation of DNA replication. EMBO J. 20: 4648-4656. PubMed ID: 11532929
date revised: 18 October 2014
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