Gene name - Cyclin A
Cytological map position - 68E1--68E2
Function - Activates cdc2 kinase
Keywords - cell cycle
Symbol - CycA
Genetic map position - 3-
Classification - G2-M cyclin
Cellular location - nuclear
In Drosophila, Cyclin A and Cyclin B appear to be functionally indistinguishable. However, differences between these cyclins exist, with respect to distribution and degradation patterns. The functional overlap between Cyclin A and B, both of which regulate the cyclin dependent kinase cdc2, is discussed in the Cyclin B site.
Following cellularization, Cyclin B, like Cyclin A accumulates during G2. Cyclin A distribution peaks in prophase, and then declines. In contrast, Cyclin B is maintained at its maximum from prophase until metaphase. Prophase cells often show a more uniform distribution of Cyclin B throughout the cell, and degradation of the protein does not occur until the metaphase-anaphase transition, well after the disappearance of Cyclin A. The accumulation and degradation of Cyclin A is similar in both colchicine treated cells and untreated control cells. However, Cyclin B accumulation seems to be independent of spindle function, and its degradation is inhibited by colchicine treatment (Whitfield, 1990). Expression of a modified form of Cyclin A, which cannot be degraded during mitosis, reveals that persistence of Cyclin A delays metaphase, suggesting a requirement for the degregadation of Cyclin A before entry into mitosis. Persistence of Cyclin B, whose degradation is required for completion of mitosis, does not effect entry into mitosis. Persistence of Cyclin A also results in abnormal spindle structures (Sigrist, 1995).
As with Cyclin B, a review of vertebrate studies highlights the functional differences between Cyclin A and B. From such perspectives, the two cyclins appear to be functionally worlds apart. Cyclin A plays a dual role in vertebrates, involved in both S phase and in the G2-M transition. In association with E2F it helps to terminate activation of the transcription of genes involved in DNA replication. Later in the cycle, depletion of Cyclin A arrests cells in G2. Thus Cyclin A is required for progression from G2 to M (Pagano, 1992). In Drosophila, no S phase role has been detected for Cyclin A. The requirement for Cyclin A in the G2-M phase transition is not well characterized: the targets are not yet known. Cyclin A peaks in prophase and then declines. These kinetics suggests little if any involvement of Cyclin A in the metaphase/anaphase checkpoint (see Cyclin B), but is consistent with an earlier requirement for the onset of mitosis.
In fly embryos, Cyclin E is the normal inducer of S phase in G1 cells. Stable G1 quiescence requires the downregulation both of cyclin E and of other factors that can bypass the normal regulation of cell cycle progression. High-level expression of cyclin A triggers the G1/S transition in wild-type embryos and in mutant embryos lacking cyclin E. Three types of control downregulates this activity of cyclin A: (1) cyclin destruction limits the accumulation of cyclin A protein in G1; (2) inhibitory phosphorylation of cdc2, the kinase partner of cyclin A, reduces the S-phase promoting activity of cyclin A in G1 and (3) Roughex (Rux), a protein with unknown biochemical function, limits cyclin A function in G1. Overexpression of rux blocks S phase induction by coexpressed cyclin A and promotes the degradation of cyclin A. Rux also prevents a stable cyclin A mutant from inducing S phase, indicating that inhibition does not require cyclin destruction. This inhibition also drives the nuclear localization of stable cyclin A. It is therefore considered likely that Roughex effects on the nuclear translocation of cyclin A would target cyclin A for degradation. It is concluded that cyclin A can drive the G1/S transition, but this function is suppressed by three types of control: cyclin A destruction, inhibitory phosphorylation of cdc2, and inhibition by Rux. The partly redundant contributions of these three inhibitory mechanisms safeguard the stability of G1 quiescence until the induction of cyclin E. The action of Rux during G1 resembles the action of inhibitors of mitotic kinases present during G1 in yeast, although no obvious sequence similarity exists (Sprenger, 1997).
Studies of the grapes (grp) gene, which encodes a homolog of the Schizosaccharomyces pombe Chk1 kinase, provide clues about the role of Cyclin A in coordinating mitotic events early in Drosophila development. Grapes is known to regulate a cell-cycle checkpoint that delays mitosis in response to inhibition of DNA replication. Grp is also required in the undisturbed early embryonic cycles: in its absence, mitotic abnormalities appear in cycle 12 and chromosomes fail to fully separate in subsequent cycles. In other systems, Chk1 kinase phosphorylates and suppresses the activity of the Cdc25 phosphatase String: the resulting failure to remove inhibitory phosphate from cyclin-dependent kinase 1 (Cdk1) prevents entry into mitosis. Because in Drosophila embryos Cdk1 lacks inhibitory phosphate during cycles 11- 13, it is not clear that known actions of Grp/Chk1 suffice in these cycles. In early cell cycles, the loss of grp compromises Cyclin A proteolysis and delays mitotic disjunction of sister chromosomes. These defects occur prior to previously reported grp phenotypes. It is concluded that Grp activates Cyclin A degradation, and functions to time the disjunction of chromosomes in the early embryo. As Cyclin A destruction is required for sister chromosome separation: a failure in Grp-promoted cyclin destruction can also explain the mitotic phenotype. The mitotic failure described previously for cycle 12 grp embryos might be a more severe form of the phenotypes that are describe here in earlier embryos and it is suggested that the underlying defect is reduced degradation of Cyclin A (Su, 1999).
When syncytial embryos are exposed to cycloheximide, a protein synthesis inhibitor, nuclear cycles arrest in interphase and cyclin A levels decline whereas Cyclin B is stable under these conditions. This suggests that a steady state of synthesis and degradation maintains the interphase levels of Cyclin A during the syncytial divisions. grp mutants were found to be deficient in Cyclin A degradation in the presence of cycloheximide. This defect is seen in embryos in cycles 4- 8, earlier than other reported grp phenotypes and is therefore unlikely to be secondary to these phenotypes. It is inferred that Grp normally destabilizes Cyclin A (Su, 1999).
In wild-type embryos, Cyclin A is unstable not only in interphase but also during mitotic arrest caused by microtubule destabilization. Thus, in embryos treated with colchicine, blocking protein synthesis with cycloheximide leads to a decline in Cyclin A levels, whereas cyclin B is stable under these conditions. grp embryos are compromised for Cyclin A proteolysis at such a colchicine-induced mitotic arrest. As for interphase destruction of Cyclin A, this defect is seen during early syncytial cycles, before the onset of other reported grp phenotypes. It is concluded that Grp promotes Cyclin A degradation in colchicine-arrested early embryos (Su, 1999).
If normal levels of Cyclin A are maintained by a steady state of synthesis and destruction, it would be expected that the levels of Cyclin A would be high in grp embryos as a result of increased stability. Western blotting shows that grp embryos have slightly higher levels of Cyclin A than wild-type embryos (1.5-3-fold). Although this relatively small increase suggests that significant degradation of Cyclin A still occurs in grp embryos, this degradation is not apparent at the arrests induced by cycloheximide or by colchicine (Su, 1999).
Since Grp is required for destruction of Cyclin A in an arrested mitosis, it was determined whether or not mitosis is disrupted in grp embryos. The mitosis-specific phosphorylation of histone H3 (PH3) apparently acts as an in vivo reporter for cyclin/Cdk activity and its disappearance at the end of mitosis requires cyclin destruction. During syncytial cycles of wild-type embryos, PH3 staining is continuous along the length of the chromosome arms from metaphase until late anaphase, when loss of the epitope near kinetochores leads to graded staining. In embryos from grp1 homozygous females or from grp1/Df females, the gradient of PH3 is seen on chromosomes in early anaphase. Thus, PH3 loss is advanced with respect to chromosome segregation in grp embryos. This defect is seen at the earliest cycles scored (cycle 4 in grp1/grp1 and cycle 3 in grp1/Df), well before the onset of previously reported defects in grp embryos (cycle 11) (Su, 1999).
The loss of PH3 during early anaphase in grp embryos could be due to the premature loss of PH3, a scenario opposite of that expected for a mutation that stabilizes Cyclin A during mitosis. Alternatively, timely loss of PH3 staining but delayed chromosome separation would produce the same mis-coordination. Analysis of grp embryos supports the latter hypothesis: an increase in the ratio of embryos with unsegregated chromosomes (prophase/metaphase) to those with segregated chromosomes (anaphase/telophase) has been found. It ia concluded that Grp is required for timely chromosome segregation in syncytial mitoses (Su, 1999).
The mitotic phenotype in grp embryos can be understood as follows: ordinarily, the mitotic cyclins are degraded in a sequence during exit from mitosis; Cyclin A is degraded before the metaphase-anaphase transition; cyclin B is degraded at the beginning of anaphase and cyclin B3 towards the end of anaphase. The disappearance of PH3 can be prevented by stabilization of any of these mitotic cyclins; thus, it appears that loss of PH3 marks the completion of this sequence. It is suggested that grp embryos are specifically defective in the early initiation of Cyclin A degradation but they do degrade Cyclin A, perhaps in conjunction with the B cyclins. Eventual destruction of Cyclin A would explain the ability of grp-deficient nuclei to exit mitosis and lose PH3 staining, events that can be inhibited by stable Cyclin A. Destruction of Cyclin A in conjunction with the B cyclins would explain why the length of mitosis is not increased in grp embryos. Because the expression of a stable form of Cyclin A prevents chromosome disjunction, it has been suggested that the failure to degrade Cyclin A early during mitosis in grp embryos delays chromosome disjunction. The delay in chromosome separation abbreviates anaphase and, when the abbreviation is severe, decondensation of chromosomes and entry into the next interphase occurs before the separating chromosomes reach the spindle poles (Su, 1999).
Since Grp promotes Cyclin A degradation, genetic interactions might be detected between grp and Cyclin A. A test was performed to see whether grp mutants are sensitive to levels of Cyclin A. Homozygous grp flies are viable but female sterile. When a single copy of a heat-inducible Cyclin A transgene is introduced, however, homozygous grp1 progeny are not recovered. Thus, even without induction, the presence of a heat-inducible Cyclin A transgene (which by itself is viable as a heterozygote or homozygote) causes grp1 to behave as a recessive lethal. The observed synthetic lethality suggests that the Grp deficiency sensitizes the fly to low levels of Cyclin A expression from the transgene. In contrast to this strong interaction with increased Cyclin A levels, a reduction in the dose of Cyclin A fails to suppress the grp1 allele. It is perhaps not surprising that reduction of Cyclin A by half does not suppress a null allele of grp. Reduction in the dose of Cyclin A does suppress the lethality of mei-41 mutations. The mei-41 gene is a homolog of the gene ATM, which is mutated in the genetic disorder ataxia-telangiectasia; mei-41 is thought to act upstream of grp in the checkpoint pathway, and mutations in mei-41 result in a phenotype like grp, but less severe. Importantly, the suppression of the mei-41 embryonic lethality by cyclin reduction occurs without restoring interphase length. This result shows that the mitotic defect is not an inevitable consequence of premature entry into mitosis as previously thought. It is suggested that the mitotic defect is an anaphase failure as a result of defective metaphase destruction of Cyclin A (Su, 1999).
If Grp promotes the metaphase-anaphase transition, why is it dispensable at most stages of development? Before cell cycle 12, grp embryos exhibit a defective mitosis with delayed sister chromosome separation; despite this, mitosis is successful. From this observation, two inferences can be made: (1) mitosis can tolerate a limited disruption in the timing of events and, (2) as anaphase occurs in the absence of Grp, there must be a backup Grp-independent mechanism that promotes sister separation slightly later. The mitotic mis-coordination in grp embryos gets progressively more severe, and Cyclin A levels increase progressively during the syncytial cycles. This correlation, together with the ability of reduced Cyclin A dose to suppress mei-41 lethality, leads the authors to suggest that the consequence of a defect in the grp/mei-41 pathway increases in severity as Cyclin A increases, until anaphase fails at mitosis 12 and 13 (Su, 1999).
The current model for Chk1 function involves the phosphorylation and inhibition of Cdc25, in part by the binding of 14-3-3 protein to the phosphorylated Cdc25 and sequestration in the cytoplasm where it is ineffective in counteracting the nuclear kinases Wee1 and Mik1. Thus, inhibitory phosphorylation of Cdk1 prevents its activation and the cell arrests in G2. Although this action of Chk1 appears general, it is possible that Chk1 activity has other consequences. Indeed, there is no substantial accumulation of inhibitory phosphate on Cdk1, and the Cdc25Stg protein is constitutively present and nuclear during interphase of syncytial cycles 11- 13 when a grp-dependent mechanism regulates the entry into mitosis. The results presented here suggest that Grp may function to destabilize Cyclin A. When a Grp-dependent cell-cycle checkpoint is induced by blocking S phase with aphidicolin in cleavage-stage Drosophila embryos, Cdc25Stg is destabilized. Thus, whether it is direct or indirect, Grp promotes the destruction of two cell-cycle proteins, Cdc25Stg and Cyclin A. It is suggested that promotion of the metaphase- anaphase transition represents a second function of the grp/mei-41 pathway, distinct from the checkpoint arrest of entry into mitosis. Nevertheless, a common mechanism might be involved because blocking entry into mitosis and promoting exit from mitosis both involve inhibition of cyclin/Cdk1 activity (Su, 1999).
In summary, Grp is required for normal Cyclin A turnover in the early Drosophila embryo. It was also found that grp mutant embryos show a delay in the timing of the metaphase- anaphase transition. Stable versions of Cyclin A block chromosome separation at the metaphase plate, at least in cellularized Drosophila embryos, suggesting that proteolysis of Cyclin A is required for this process. Thus, the proposal that Grp activates Cyclin A proteolysis can explain the mitotic phenotype as a consequence of at least temporary persistence of Cyclin A (Su, 1999).
Bases 5' UTR - 298+
Bases 3' UTR - 670
In aligned fly and clam Cyclin A sequences within a 60 amino acid region, residues at 82% of the positions are similar and 72% are identical. Within this same region, 47% of the positions are identical among all the cyclins, whether A or B type, or whether of yeast, mollusc, echinoderm or insect origin. Conservation among this group of proteins extends throughout the carboxy-terminal two-thirds of these proteins (Lehner, 1989).
date revised: 15 Nov 97
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