Cyclin B

EVOLUTIONARY HOMOLOGS (part 1/3)

Yeast Cyclin B family members

In Saccharomyces cerevisiae, a single cyclin-dependent kinase, Cdc28, regulates both G1/S and G2/M phase transitions by associating with stage-specific cyclins. During progression through S phase and G2/M, Cdc28 is activated by the B-type cyclins Clb1-6. Because of functional redundancy, specific roles for individual Clbs have been difficult to assign. To help genetically define such roles, strains carrying a cdc28(ts) allele, combined with single CLB deletions were studied. It is assumed that by limiting the activity of the kinase, these strains would be rendered more sensitive to loss of individual Clbs. By this approach, a novel phenotype associated with CLB5 mutation was observed. Homozygous cdc28-4(ts) clb5 diploids are not viable at room temperature. Cells are defective in spindle positioning, leading to migration of undivided nuclei into the bud. Occasionally, misplaced spindles are observed in cdc28-4 clb5 haploids; additional deletion of CLB6 causes full penetrance. Thus, CLB5 brings about proper preanaphase spindle positioning, yet the requirement differs in haploids and diploids. The execution point for the defect corresponds to the time of Clb5-dependent kinase activation. Nevertheless, the lethality of cdc28-4 clb5 diploids is not rescued by CLB2 or CLB4 overexpression, indicating a specificity of Clb5 function beyond temporality of expression (Segal, 1998).

The B-type cyclins of S. cerevisiae are diversified with respect to time of expression during the cell cycle as well as biological function. A single Cdk, Cdc28p, is activated by three G1, or CLN, cyclins and six B-type, or CLB, cyclins. The CLN cyclins are required for activation of Clb-Cdc28p kinase in addition to other cell cycle roles. Activated Clb-Cdc28p kinase is then required for initiation of DNA replication, spindle formation, and initiation of mitosis. Cyclins presumably evolved from a single ancestor. This ancestor was proposed to have the ability to carry out multiple roles in regulation of the cell cycle: activation of DNA replication, initiation of mitosis, inhibition of loading of replication origins, and inhibition of mitotic exit. In budding yeast, different CLB genes are primarily responsible for these tasks, with CLB5 and CLB6 largely responsible for activation of replication origins, and CLB1,2 largely responsible for mitotic events. This division of labor correlates with, and could in principle be entirely caused by, differences in the time of expression of CLB5,6 (early) and CLB1,2 (late). CLB3 and CLB4 are expressed at an intermediate time, and consistent with the idea that timing dictates biological function, CLB3 and CLB4 appear to share functions with both CLB5,6 and CLB1,2. According to the idea that timing of expression is the only relevant difference between CLBs, regulated times of expression of different CLBs may simply serve to allow fine tuning of the total Clb 'dose' in the cell. The early-expressed CLB5 coding sequence was replaced with the late-expressed CLB2 coding sequence, at the CLB5 locus. CLB5::CLB2 exhibits almost no rescue of clb5-specific replication defects, although it could rescue clb1 clb2 lethality, and in synchronized cells Clb2p-associated kinase activity from CLB5::CLB2 rises early in the cell cycle, similar to that of Clb5p. Mutagenesis of a potential substrate-targeting domain of CLB5 reduces biological activity without reducing Clb5p-associated kinase activity. Thus, Clb5p may have targeting domains required for CLB5-specific biological activity (Cross, 1999).

Four cyclin-B homologs , CLB1-CLB4, have been cloned from Saccharomyces cerevisiae using the polymerase chain reaction and low stringency hybridization approaches. These genes form two classes based on sequence relatedness: CLB1 and CLB2 show highest homology to the Schizosaccharomyces pombe cyclin-B homolog cdc13 involved in the initiation of mitosis, whereas CLB3 and CLB4 are more highly related to the S. pombe cyclin-B homolog cig1, which appears to play a role in G1 or S phase. CLB1 and CLB2 mRNA levels peak late in the cell cycle, whereas CLB3 and CLB4 are expressed earlier in the cell cycle but peak later than the G1-specific cyclin, CLN1. Analysis of null mutations suggests that the CLB genes exhibit some degree of redundancy, but clb1,2 and clb2,3 cells are inviable. Using clb1,2,3,4 cells rescued by conditional overproduction of CLB1, it has been shown that the CLB genes perform an essential role at the G2/M-phase transition, and also a role in S phase. CLB genes also appear to share a role in the assembly and maintenance of the mitotic spindle. Taken together, these analyses suggest that CLB1 and CLB2 are crucial for mitotic induction, whereas CLB3 and CLB4 might participate additionally in DNA replication and spindle assembly (Richardson, 1992).

Eukaryotic cells ensure the stable propagation of their genome by coupling each round of DNA replication (S phase) to passage through mitosis (M phase). This control is exerted at the initiation of replication, which occurs at multiple origins throughout the genome. Once an origin has initiated, reinitiation is blocked until the completion of mitosis, ensuring that DNA is replicated at most once per cell cycle. Recent studies in several organisms have suggested a model in which S- and M-phase promoting cyclin-dependent kinases prevent reinitiation by blocking the repetition of an early step in the initiation reaction. In budding yeast, this regulation is thought to involve inhibition of prereplicative complex (pre-RC) formation at origins of replication. This inhibition is carried out by S and M phase-promoting Clb kinases. To date, however, there has been no direct demonstration that these kinases can perform such an important function. In this report such a confirmation is provided by showing that ectopic induction in G1 phase of a mitotic Clb (e.g., Clb2) is sufficient to inhibit DNA replication and does so by preventing pre-RC formation. This inhibition requires that Clb2 be induced before Cdc6, an initiation protein required for pre-RC formation; once pre-RCs have formed, Clb2 can no longer inhibit initiation. These results support the notion that during the normal cell cycle reassembly of the pre-RC, and hence reinitiation at an origin, is directly inhibited by S and M phase-promoting cyclin-dependent kinases (Detweiler, 1998).

Ste20p from Saccharomyces cerevisiae is a member of the Ste20/p21-activated protein kinase family of protein kinases. The Ste20p kinase is post-translationally modified by phosphorylation in a cell cycle-dependent manner, as judged by the appearance of phosphatase-sensitive species with reduced mobility on SDS-polyacrylamide gel electrophoresis. This modification is maximal during S phase, and correlates with the accumulation of Ste20p fused to green fluorescent protein at the site of bud emergence. Overexpression of Cln2p (a G1 type cyclin), but not Clb2p or Clb5p (G2 B-type cyclins), causes a quantitative shift of Ste20p to the reduced mobility form, and this shift is dependent on Cdc28p activity. The post-translational mobility shift can be generated in vitro by incubation of Ste20p with immunoprecipitated Cln2p kinase complexes, but not by immunoprecipitated Clb2p or Clb5p kinase complexes. Ste20p is therefore a substrate for the Cdc28p kinase, and undergoes a Cln2p-Cdc28p mediated mobility shift as cells initiate budding and DNA replication. In cells that express only the Cln2p G1 cyclin, minor overexpression of Ste20p causes aberrant morphology, suggesting a proper coordination of Ste20p and Cln-Cdc28p activity may be required for the control of cell shape (Wu, 1998).

Chromosome separation during the cell-cycle transition from metaphase to anaphase requires the proteolytic destruction of anaphase inhibitors such as Pds1. pds1 mutants exhibit precocious dissociation of sister chromatids in the presence of these microtubule inhibitors. pds1 mutants are defective in anaphase arrest that normally is imposed by a spindle-damage checkpoint. Proteolysis of Pds1 is mediated by a ubiquitin-protein ligase, the anaphase-promoting complex (APC) or cyclosome. The APC is also necessary for the ubiquitin-dependent degradation of mitotic cyclins in late telophase as cells exit mitosis. Although phosphorylation seems to be involved, it is not clear what activates the APC at the onset of anaphase. In Saccharomyces cerevisiae, chromosome segregation also requires the CDC20 gene, whose product contains WD40 repeats. The functional relationship between the APC and the Cdc20 protein has been investigated. Evidence is presented that strongly suggests that Cdc20 is an essential regulator of APC-dependent proteolysis such that in the absence of Cdc20, cells are unable to degrade either Pds1 at the onset of anaphase or the mitotic cyclin Clb2 during telophase. This notion is consistent with observations that Cdc20 is localized in the nucleus and co-immunoprecipitates with an APC component, Cdc23 (Lim, 1998).

Cln-Cdc28 is the G1 phase cyclin-cyclin dependent kinase dimer in budding yeast Stability of the mitotic B-type cyclin Clb2 is tightly cell cycle-regulated. B-type cyclin proteolysis is initiated during anaphase and persists throughout the G1 phase. Cln-Cdc28 kinase activity at START is required to repress B-type cyclin-specific proteolysis. Clb-dependent kinases, when expressed during G1, are also capable of repressing the B-type cyclin proteolysis machinery. Furthermore, inactivation of Cln- and Clb-Cdc28 kinases is found to be sufficient to trigger Clb2 proteolysis and sister-chromatid separation in G2/M phase-arrested cells, where the B-type cyclin-specific proteolysis machinery is normally inactive. These results suggest that Cln- and Clb-dependent kinases are both capable of repressing B-type cyclin-specific proteolysis and that these kinases are required to maintain the proteolysis machinery in an inactive state in S and G2/M phase-arrested cells. It is proposed that in yeast, as cells pass through START, Cln-Cdc28-dependent kinases inactivate B-type cyclin proteolysis. As Cln-Cdc28-dependent kinases decline during G2, Clb-Cdc28-dependent kinases take over this role, ensuring that B-type cyclin proteolysis is not activated during S phase and early mitosis (Amon, 1997).

Cyclins and cyclin-dependent kinases induce and coordinate the events of the cell cycle, although the mechanisms by which they do so remain largely unknown. In budding yeast, a pathway used by the Clb2 cyclin to control bud growth during mitosis provides a good model system in which to understand how cyclin-dependent kinases control cell-cycle events. In this pathway, Clb2 initiates a series of events that lead to the mitosis-specific activation of the Gin4 protein kinase. A protein called Nap1 is required in vivo for the activation of Gin4, and is able to bind to both Gin4 and Clb2. A simple genetic screen was used to identify additional proteins that function in this pathway. The Cdc42 GTPase and a member of the PAK kinase family called Cla4 both function in the pathway used by Clb2 to control bud growth during mitosis. Cdc42 and Cla4 interact genetically with Gin4 and Nap1, and both are required in vivo for the mitosis-specific activation of the Gin4 kinase. Furthermore, Cla4 undergoes a dramatic hyperphosphorylation in response to the combined activity of Nap1, the Clb2-Cdc28 kinase complex, and the GTP-bound form of Cdc42. Evidence is presented that suggests that the hyperphosphorylated form of Cla4 is responsible for relaying the signal to activate Gin4. Previous studies have suggested that cyclin-dependent kinases control the cell cycle by directly phosphorylating proteins involved in specific events, such as nuclear lamins, microtubule-associated proteins and histones. In contrast, these results demonstrate that the Clb2-Cdc28 cyclin-dependent kinase complex controls specific cell-cycle events through a pathway that involves a GTPase and at least two different kinases. This suggests that cyclin-dependent kinases may control many cell-cycle events through GTPase-linked signaling pathways that resemble the intricate signaling pathway known to control many other cellular events (Tjandra, 1998).

Nuclear extracts from Saccharomyces cerevisiae cells synchronized in S phase support the semiconservative replication of supercoiled plasmids in vitro. An examination was carried out of the dependence of this reaction on the prereplicative complex that assembles at yeast origins and on S-phase kinases that trigger initiation in vivo. Replication in nuclear extracts is initiated independent of the origin recognition complex (ORC), Cdc6p, and an autonomously replicating sequence (ARS) consensus. Nonetheless, quantitative density gradient analysis has shown that S- and M-phase nuclear extracts consistently promote semiconservative DNA replication more efficiently than G1-phase extracts. The observed semiconservative replication is compromised in S-phase nuclear extracts deficient for the Cdk1 kinase (Cdc28p) but not in extracts deficient for the Cdc7p kinase. In a cdc4-1 G1-phase extract, which accumulates high levels of the specific Clb-Cdk1 inhibitor p40(SIC1), very low levels of semiconservative DNA replication are detected. Recombinant Clb5-Cdc28 restores replication in a cdc28-4 S-phase extract, yet fails to do so in the cdc4-1 G1-phase extract. In contrast, the addition of recombinant Xenopus CycB-Cdc2, which is not sensitive to inhibition by p40(SIC1), restores efficient replication to both extracts. These results suggest that in addition to its well-characterized role in regulating the origin-specific prereplication complex, the Clb-Cdk1 complex modulates the efficiency of the replication machinery itself (Duncker, 1999).

Elaborate regulatory mechanisms might control the precise progression of mitosis. Ubiquitin-dependent proteolysis of Cut2/Pds1, which triggers sister chromatid separation, and Cyclin B are required for sister chromatid separation and exit from mitosis, respectively. Anaphase-promoting complex/cyclosome (APC) specifically ubiquitinates Cut2/Pds1 at metaphase-anaphase transition, and ubiquitinates Cyclin B in late mitosis and G1 phase. Recent genetic and biochemical analyses in yeast, Drosophila, and Xenopus have indicated that APC is activated by the WD-repeat proteins, Cdc20/p55CDC/Fizzy (Cdc20) and Cdh1/Hct1/Fizzy-related (Cdh1: Drosophila homolog Fizzy related) in a substrate-specific manner, whereas APC is inactivated by a spindle assembly checkpoint through Mad2. Whereas Mad1, Mad2, Mad3/Bub1, and Bub3 suppress Cdc20-dependent APC activation, Bub2 localized in the spindle pole body regulates Cdh1-dependent APC activation. Furthermore, Cdc14, whose activity is regulated by Bub2/Byr4 and RENT complex, dephosphorylates Cdh1/Hct1 and inactivates APC, Therefore, APC activity is regulated by at least four distinct mechanisms: activation and inactivation by phosphorylation and dephosphorylation of APC itself; activation by the binding of substrate-specific activators Cdc20 and Cdh1 to APC; suppression of APC activity by the spindle assembly checkpoint through Mad family and Bub family, and regulation of APC activity by Bub2/RENT complex system. However, the precise regulatory mechanism of substrate-specific activation of mammalian APC with the right timing through these complicated mechanisms remains to be elucidated. The binding of the activators Cdc20 and Cdh1 and the inhibitor Mad2 to APC regulate APC activity. Also the phosphorylation of Cdc20 and Cdh1 by Cdc2-Cyclin B (MPF) and phosphorylation of APC by Polo-like kinase and cAMP-dependent protein kinase regulate APC activity. The cooperation of the phosphorylation/dephosphorylation and the regulatory factors in regulation of APC activity may thus control the precise progression of mitosis (Kotani, 1999).

Thus, phosphorylation of Cdc20 is required for Cdc20-dependent APC activation at least in vitro. It has been reported that Cdc20 is expressed during G2 phase and mitosis and binds to APC in early mitotic stages. Cdc20 is clearly phosphorylated during mitosis in HeLa cells, and Cdc20 is phosphorylated by MPF in Xenopus embryos. Actually, the possible Cdc2 phosphorylation site is conserved in budding yeast Cdc20, fission yeast Slp1, and mammalian Cdc20. Therefore, it is most likely that Cdc20 is indeed phosphorylated during mitosis in vivo. Taken together, it is concluded that Cdc20 activates APC when it is phosphorylated by MPF. The activation of APC can be blocked by the binding of Mad2 to pCdc20. Therefore, at least two events may be required to activate APC at metaphase-anaphase transition: release of Mad2 from pCdc20 after spindle assembly checkpoint is released, and phosphorylation of Cdc20 bound to APC by MPF or binding of pCdc20 to APC. Furthermore, at least in vitro APC activation can be suppressed by PKA, which phosphorylates two APC subunits, APC1 and APC3. Thus, dephosphorylation of PKA phosphorylation sites on APC by a specific phosphatase yet unidentified, which may be PP1, might also be required for the onset of anaphase (Kotani, 1999 and references).

Mad2 cannot inhibit Cdh1-induced APC activation, and pAPC (phosphorylated APC) can ubiquitinate Cyclin B even in the presence of Mad2. Thus Mad2 inhibits APC activation only through Cdc20. Further, the binding of Cdc20 or pCdc20 (phosphorylated pCdc20) to APC or pAPC was not affected by Mad2, indicating that Mad2 inhibits the function of pCdc20 but not the binding of pCdc20 to APC. Whereas Mad1, Mad2, Mad3/Bub1, and Bub3 suppress Cdc20-dependent APC activation, Bub2 localized in the spindle pole body regulates Cdh1-dependent APC activation. Further detailed functional analyses of the Mad family and Bub family are required to clarify the molecular mechanisms of spindle assembly checkpoint and of APC regulation (Kotani, 1999 and references).

It was clearly shown that pAPC can ubiquitinate Cyclin B but not Cut2 in vitro, but it remains unclear whether free pAPC actually exists during mitosis in vivo. It is most likely that the majority of pAPC forms a complex with either pCdc20 or Cdh1 that ubiquitinates Cyclin B in vivo. Furthermore, phosphorylation of APC by pPlk may not be essential for APC activation at the metaphase-anaphase transition, but phosphorylation of APC by pPlk is required for complete Cyclin B ubiquitination in later stages of mitosis (Kotani, 1999).

pCdc20 and Cdh1 (but neither Cdc20 nor pCdh1) activates APC in vitro, and the following switching mechanism from pCdc20 to Cdh1 could be speculated. When the cells enter anaphase after Cut2/Pds1 is entirely degraded, the pAPC-pCdc20 complex steadily ubiquitinates Cyclin B and, consequently, MPF activity decreases by the end of mitosis. The pCdc20 can be dephosphorylated by a specific phosphatase and released from pAPC or it may be degraded. When MPF activity is high, Cdh1 is phosphorylated and remains inactive, but when Cdc14 is activated, Cdh1 is dephosphorylated by Cdc14 and binds to and activates APC. Consequently, the pAPC-pCdc20 complex is replaced by the pAPC-Cdh1 (or APCp-Cdh1) complex in late mitosis. The pAPC-Cdh1 or APC-Cdh1 complex further ubiquitinates Cyclin B in late mitosis and G1 phase. Thus, the switch mechanism from pCdc20 to Cdh1 may be dependent upon the MPF activity during mitosis, and the MPF activity itself is controlled by the pCdc20- and Cdh1-dependent APC activity (Kotani, 1999 and references).

Cdc20 has been reported to activate ubiquitination of the factors regulating sister chromatid separation and Cdh1 promotes ubiquitination of mitotic cyclins. However, in Xenopus early embryos, Cdh1 is not expressed, which is consistent with the observation that Cdh1 is not expressed before stage 13 in Drosophila embryos. It was also described that Cdc20 regulates ubiquitination of both Cut2/Pds1 and Cyclin B in the early embryonic cell cycle. Ubiquitin-dependent proteolysis of Cyclin B begins at metaphase-anaphase transition in HeLa cells, which is consistent with the results obtained with clam embryo. These findings are consistent with the observation that pCdc20 can activate ubiquitination not only of Cut2/Pds1, but also of Cyclin B. Thus, pCdc20 alone may be enough for cells to go through mitosis without Cdh1. However, in the somatic cell cycle, Cdh1 in addition to pCdc20, may be required for effective and complete ubiquitination of Cyclin B in later stages of mitosis and G1 phase (Kotani, 1999 and references).

This study has demonstrated that Cdh1 can be phosphorylated by MPF, and Cdh1 but not pCdh1 binds to and activates APC only after dephosphorylation. These in vitro results are consistent with in vivo data in budding yeast that the dephosphorylated form of Cdh1/Hct1 activates APC. Cdh1 is constantly expressed throughout the cell cycle, binds to APC in late mitosis and G1 phase, and is phosphorylated during mitosis in HeLa cells. Cdh1/Hct1 binding to APC is regulated by cyclin-dependent kinases. Cdc14 dephosphorylates Cdh1/Hct1 and inactivates APC. Further, the activation of Cdc14 is regulated by Bub2/Byr4 and RENT complex, although it remains unresolved whether the regulation by RENT complex works in the mammalian system (Kotani, 1999 and references).

It is possible that Cdh1-APC complex activity is maintained until late G1 phase, while it might be diminished by the phosphorylation of Cdh1 with Cdk2, Cdk4, or another unidentified specific kinase that is active at G1/S transition. It has been reported that the low level of Cyclin B is translated even in G1 phase. Therefore, in the G1 phase, APC-Cdh1 complex may effectively ubiquitinate this newly translated Cyclin B to avoid activation of MPF. The possible involvement of G1/S Cdks or other specific kinases in Cdh1-dependent APC inactivation merits further study (Kotani, 1999).

The current findings support the notion that pCdc20 but not Cdc20 activates APC, and that Cdh1 but not pCdh1 binds to and activates APC. Therefore, phosphorylation and dephosphorylation of APC regulatory factors by MPF are critical for their binding to APC and/or APC activation. The MPF activity itself is regulated by the pCdc20- and Cdh1-dependent APC activity, and this feedback control precisely regulates APC activity. Taken together, the APC activity is regulated by the phosphorylation and dephosphorylation of APC and of the regulatory factors, Cdc20 and Cdh1, by MPF, Plk, PKA, and PP1, as well as by the binding of positive and negative regulatory factors, Cdc20, Cdh1, and Mad2, to APC. These elaborate regulatory mechanisms might control the precise progression of mitosis (Kotani, 1999).

The life cycle of most eukaryotic organisms includes a meiotic phase, in which diploid parental cells produce haploid gametes. During meiosis a single round of DNA replication is followed by two rounds of chromosome segregation. In the first, or reductional, division (meiosis I), which is unique to meiotic cells, homologous chromosomes segregate from one another, whereas in the second, or equational, division (Meiosis II) sister centromeres disjoin. Meiotic DNA replication precedes the initiation of recombination by programmed Spo11-dependent DNA double-strand breaks. Recent reports that meiosis-specific cohesion is established during meiotic S phase and that the length of S phase is modified by recombination factors (Spo11 and Rec8) raise the possibility that replication plays a fundamental role in the recombination process. To address how replication influences the initiation of recombination, mutations in the B-type cyclin genes CLB5 and CLB6, which specifically prevent premeiotic replication in the yeast Saccharomyces cerevisiae, were used. clb5 and clb5;clb6 but not clb6 mutants are defective in DSB induction and prior associated changes in chromatin accessibility, heteroallelic recombination, and SC formation. The severity of these phenotypes in each mutant reflects the extent of replication impairment. This assemblage of phenotypes reveals roles for CLB5 and CLB6 not only in DNA replication but also in other key events of meiotic prophase. The elaboration of the proper local substrate for DNA cleavage likely includes many chromosomal morphogenetic events that must occur between replication and DSB formation. Such events probably include the development of sister chromatid cohesion and the establishment of chromosomal pairing. These activities may account for the extended length of meiotic, as opposed to mitotic, S phase. Intriguingly, the Spo11 protein specifically regulates the length of S phase, and this indicates that it may coordinate replication with the initiation of recombination. In conclusion, these results demonstrate that the initiation of recombination and SC formation require B-type cyclin functions, and this requirement may stem from the role of Clb5 and Clb6 in promoting meiotic replication (Smith, 2001).

Evidence is presented that the major Schizosaccharomyces pombe CDK, Cdc2-cyclin B, influences recombinational repair of radiation-induced DNA double-strand breaks at two distinct stages during the G2 phase. At an early stage in homologous recombination, a defect in Cdc2 kinase activity caused by a single amino acid change in cyclin B affects the formation of Rhp51 (Rad51sp) foci in response to ionizing radiation in a process that is redundant with the function of Rad50. At a late stage in homologous recombination, low Cdc2-cyclin B activity prevents the proper regulation of topoisomerase III (Top3) function, disrupting a recombination step that occurs after the assembly of Rhp51 foci. This effect of Cdc2-cyclin B kinase on Top3 function is mediated by the BRCT-domain-containing checkpoint protein Crb2, thus linking checkpoint proteins directly with recombinational repair in G2. These data suggest a model in which CDK activity links processing of recombination intermediates to cell cycle progression via checkpoint proteins (Caspari, 2002).

In fission yeast the mitotic B type cyclin Cdc13/Cdc2 kinase associates with replication origins in vivo. This association is dependent on the origin recognition complex (ORC) established as chromosomes are replicated, and is maintained during G2 and early mitosis. Cells expressing an orp2 (ORC2) allele that reduces binding of Cdc13 to replication origins are acutely prone to chromosomal reduplication. In synchronized endoreduplicating cells, following Cdc13 ablation, replication origins are coordinately licensed prior to each successive round of S phase with the same periodicity as in a normal cell cycle. Thus, ORC bound mitotic Cyclin B/Cdc2 kinase imposes the dependency of S phase on an intervening mitosis but not the temporal licensing of replication origins between each S phase (Wuarin, 2002).

These results suggest that the Cdc13/Cdc2 complex acts locally at replication origins to inhibit the formation of pre-RCs by phosphorylation of pre-RC components. These data support a simple model to explain why only G1 but not G2 nuclei have the potential to replicate. Namely, activation of the anaphase-promoting complex (APC), a specialized E3 ubiquitin ligase, during mitosis leads both to the dissolution of cohesion between sister chromatids and the removal of the mitotic Cyclin B/Cdc2 kinase from the origins of replicated nuclei. In this manner, licensing of replication origins and sister chromatid separation may be temporally coordinated. In addition, in endoreduplicating cells lacking Cdc13, replication origins are periodically and coordinately licensed prior to each successive S phase with the same timing as in a normal cell cycle. Thus, the mechanisms that determine the temporal licensing of replication origins can be experimentally distinguished from the controls that maintain the dependency of S phase on an intervening mitosis. Furthermore, since cells lacking Cdc13 undergo complete genome duplications, it was reasoned that the controls that maintain the correct order of origin firing within S phase and those that ensure each origin fires only once, each rereplicative S phase must remain intact in the absence of Cdc13. In this respect, the experimentally induced rereplication cycle in fission yeast described here closely resembles the naturally occurring endoreduplication cycles observed in plants, flies, and certain human tissues (Wuarin, 2002).

In eukaryotes, entry into mitosis is induced by cyclin B-bound Cdk1, which is held in check by the protein kinase, Wee1. In budding yeast, Swe1 (Wee1 ortholog) is targeted to the bud neck through Hsl1 (Nim1-related kinase) and its adaptor Hsl7, and is hyperphosphorylated prior to ubiquitin-mediated degradation. Hsl1 and Hsl7 are required for proper localization of Cdc5 (Polo-like kinase homolog) to the bud neck and Cdc5-dependent Swe1 phosphorylation. Mitotic cyclin (Clb2)-bound Cdc28 (Cdk1 homolog) directly phosphorylates Swe1 and this modification serves as a priming step to promote subsequent Cdc5-dependent Swe1 hyperphosphorylation and degradation. Clb2-Cdc28 also facilitated Cdc5 localization to the bud neck through the enhanced interaction between the Clb2-Cdc28-phosphorylated Swe1 and the polo-box domain of Cdc5. It is proposed that the concerted action of Cdc28/Cdk1 and Cdc5/Polo on their common substrates is an evolutionarily conserved mechanism that is crucial for effectively triggering mitotic entry and other critical mitotic events (Asano, 2005).

This study delineates how Swe1 regulation is orchestrated by multiple components as cells progress through the cell cycle. Cla4-dependent septin filament formation early in the cell cycle permits assembly of a platform consisting of Hsl1 (Nim1-related kinase) and its adaptor Hsl7, a critical step that is required for the recruitment of Clb2-Cdc28-phosphorylated Swe1 and Cdc5 later in the cell cycle. Phosphorylated Swe1 further promotes Cdc5 localization to the platform by providing a docking site for the polo-box domain of Cdc5. The data show that both the Hsl1-Hsl7 platform and the primed Swe1 are two crucial elements for Cdc5-dependent Swe1 hyperphosphorylation and subsequent degradation at the bud neck. This coordinated, multistep, Swe1 regulation clearly provides a means to monitor the completion of earlier cell cycle events and to effectively bring about Swe1 destruction at the time of mitotic entry. Once unleashed from the Swe1-imposed G2 delay, Clb-Cdc28 can induce mitotic entry unimpeded (Asano, 2005).

Control of Cyclin B translation

In maturing mouse oocytes, protein synthesis is required for meiotic maturation subsequent to germinal vesicle breakdown (GVBD). While the number of different proteins that must be synthesized for this progression to occur is unknown, at least one of them appears to be cyclin B1, the regulatory subunit of M-phase-promoting factor. The mechanism of cyclin B1 mRNA translational control during mouse oocyte maturation has been investigated. TU-rich cytoplasmic polyadenylation element (CPE), a cis element in the 3' UTR of cyclin B1 mRNA, mediates translational repression in GV-stage oocytes. The CPE is also necessary for cytoplasmic polyadenylation, which stimulates translation during oocyte maturation. The injection of oocytes with a cyclin B1 antisense RNA, which probably precludes the binding of a factor to the CPE, delays cytoplasmic polyadenylation as well as the transition from GVBD to metaphase II. CPEB (related to Drosophila Orb) is an RNA recognition motif and zinc finger-containing protein that has a strong specificity for the CPE. CPEB, which interacts with the cyclin B1 CPE and is present throughout meiotic maturation, becomes phosphorylated at metaphase I. These data indicate that CPEB is involved in both the repression and the stimulation of cyclin B1 mRNA and suggest that the phosphorylation of this protein could be involved in regulating its activity (Tay, 2000).

In Xenopus development, the expression of several maternal mRNAs is regulated by cytoplasmic polyadenylation. CPEB (Drosophila homolog: Orb) and CPEB associated factor maskin, two factors that control polyadenylation-induced translation are present on the mitotic apparatus of animal pole blastomeres in embryos. Cyclin B1 protein and mRNA, whose translation is regulated by polyadenylation, are colocalized with CPEB and maskin. CPEB interacts with microtubules and is involved in the localization of cyclin B1 mRNA to the mitotic apparatus. Agents that disrupt polyadenylation-induced translation inhibit cell division and promote spindle and centrosome defects in injected embryos. Two of these agents inhibit the synthesis of cyclin B1 protein, and one, which has little effect on this process, disrupts the localization of cyclin B1 mRNA and protein. These data suggest that CPEB-regulated mRNA translation is important for the integrity of the mitotic apparatus and for cell division (Groisman, 2000).

The observation that CPEB and maskin are localized on animal pole spindles and centrosomes was completely unexpected. Because the spindles of metaphase II-arrested Xenopus eggs are associated with the animal pole cortex, it seems probable that CPEB and maskin either move along a microtubule array to the mitotic apparatus formed in this region, or became passively incorporated with microtubules as the spindles are formed. In either case, these proteins form a gradient along the spindles, with the greatest concentration nearest the centrosomes. Because CPEB (especially) and maskin bind tubulin in vitro, they probably interact directly with the mitotic apparatus. This is almost certainly the case with CPEB because deletion of a region that includes a PEST domain abrogates both tubulin binding in vitro and centrosome localization in vivo. While PEST domains have been implicated in rapid protein degradation, they are also sites of protein-protein interaction. In contrast to CPEB and maskin, the other factors involved in polyadenylation-induced translation [eIF4E, poly(A) polymerase, and CPSF] are coincident with, but not particularly concentrated on, the mitotic apparatus. The transient nature of protein kinase Eg2 colocalization with CPEB on centrosomes could have a crucial impact on polyadenylation-induced translation. For example, poly(A) tail length is probably controlled by a dynamic equilibrium between elongation and contraction. One mechanism by which this equilibrium could be shifted in one direction or the other is to modulate CPEB activity; such activity could be regulated by the cell cycle-controlled localization of Eg2 on centrosomes. Irrespective of this possibility, the observation that general translation factors are associated with spindles and centrosomes indicates that regulated protein synthesis, in close proximity to the mitotic apparatus, could occur (Groisman, 2000).

The finding that CPE-containing mRNAs are concentrated on the mitotic apparatus in the animal pole brings up two obvious questions: how do they get there, and what purpose do they serve? While this paper does not address the mechanism of RNA localization per se, the sequence-specificity would almost certainly be dependent upon CPEB. Indeed, this is strongly suggested by the observation that CPEBdelta4 acts as a dominant negative mutation for cyclin B1 mRNA localization. However, maskin might also play an important role in this process. The carboxy half of maskin is composed of a coiled-coil domain that is 70% identical with a similar motif in Drosophila dTACC (Gergely, 2000), a member of the TACC (transforming acidic coiled-coil) family of proteins. dTACC, like maskin, is localized in the oocyte, in this case to the anterior pole; it is centrosomal in embryos, and it is essential for Bicoid mRNA localization (Gergely, 2000). Neither maskin nor dTACC contain obvious RNA binding motifs, suggesting that their involvement in RNA localization is mediated by other factors. Orb, the Drosophila homolog of CPEB, is also important for mRNA localization, but whether it interacts with dTACC is not known (Groisman, 2000).

The treatment of embryos with cycloheximide, blocks cell division in S-phase, which results in centrosome replication. This result has been confirmed by injecting a maskin-derived eIF4E blocking peptide that, like cycloheximide, inhibits protein synthesis in general. However, the results demonstrating that CPEB antibody, CPEB dominant negative mutant deltaN, and cordycepin each inhibit cell division, induce the formation of multiple centrosomes, and destroy the integrity of the mitotic apparatus, suggest that the synthesis of key factors is mediated by cytoplasmic polyadenylation. It is important to note that these agents, all of which prevent polyadenylation-induced translation in injected oocytes, specifically inhibit the expression of the relatively few (perhaps a dozen or so are known) mRNAs that undergo poly(A) elongation, one of which is cyclin B1. Because CPEB antibody and cordycepin also inhibit cyclin B1 synthesis in embryos, it follows that the translation of this message is regulated by polyadenylation at this stage of development as well. Therefore, while cell cycle progression is clearly governed by cyclin synthesis and destruction, these data demonstrate that regulated cyclin B1 mRNA translation is also important for cell division in early embryos. In the vegetal region, however, this may not be the case because the injection of CPEB antibody into a vegetal pole blastomere of a 16 cell embryo has no obvious deleterious effect on cell division (Groisman, 2000).

In addition to agents that affect CPEB activity, maskin antibody injection also results in spindle/centrosome defects in embryos. These defects are similar to those observed in Drosophila embryos that have mutations in dTACC. Because dTACC has no obvious eIF4E binding domain, it is difficult to determine whether this protein and maskin are functionally homologous. However, these results suggest that they have at least partially overlapping activities (Groisman, 2000 and references therein).

The observation that cyclin B1 mRNA and protein, together with CPEB and maskin, is concentrated on spindles and centrosomes might suggest that these structures are the sites of translational control. While consistent with this possibility, the relative amount of this message, or the amounts of these proteins for that matter, that actually resides on the mitotic apparatus is difficult to estimate, and therefore the functional importance of this concentration is not necessarily clear-cut. However, CPEBdelta4, which is fully capable of binding CPE-containing mRNA but is defective for microtubule binding, mostly destroys the concentration of cyclin B1 mRNA and protein on the mitotic apparatus while having little effect on overall cyclin levels. This result suggests that the inhibition of cell division, which results from CPEBdelta4 injection, is due to the loss of cyclin B1 synthesis on the mitotic apparatus. Therefore, it is proposed that in animal pole blastomeres of Xenopus embryos, the translation of cyclin B1 mRNA is regulated locally, on or near spindles and centrosomes (Groisman, 2000).

Local cyclin mRNA translation might be necessary to ensure that the protein product is delivered to the sight at which it is needed. This would appear to be the case in Drosophila embryos, where both cyclin mRNA and protein are detected on the mitotic apparatus. Given that Xbub3 mRNA is also associated with spindles, it is suspected that local synthesis of this checkpoint control protein is also an important regulatory event. Additional CPE-containing mRNAs may also be found on spindles, and experiments to address this possibility are presently underway (Groisman, 2000).

The synthesis and destruction of cyclin B drives mitosis in eukaryotic cells. Cell cycle progression is also regulated at the level of cyclin B translation. In cycling extracts from Xenopus embryos, progression into M phase requires the polyadenylation-induced translation of cyclin B1 mRNA. Polyadenylation is mediated by the phosphorylation of CPEB by Aurora, a kinase whose activity oscillates with the cell cycle. Exit from M phase seems to require deadenylation and subsequent translational silencing of cyclin B1 mRNA by Maskin, a CPEB and eIF4E binding factor, whose expression is cell cycle regulated. These observations suggest that regulated cyclin B1 mRNA translation is essential for the embryonic cell cycle. Mammalian cells also display a cell cycle-dependent cytoplasmic polyadenylation, suggesting that translational control by polyadenylation might be a general feature of mitosis in animal cells (Groisman, 2002).

At the midblastula transition, the Xenopus laevis embryonic cell cycle is remodeled from rapid alternations between S and M phases to become the complex adult cell cycle. Cell cycle remodeling occurs after zygotic transcription initiates and is accompanied by terminal downregulation of maternal cyclins A1 and B2. The disappearance of both cyclin A1 and B2 proteins is preceded by the rapid deadenylation of their mRNAs. A specific mechanism triggers this deadenylation. This mechanism depends upon discrete regions of the 3' untranslated regions and requires zygotic transcription. Together, these results strongly suggest that zygote-dependent deadenylation of cyclin A1 and cyclin B2 mRNAs is responsible for the downregulation of these proteins. These studies also raise the possibility that zygotic control of maternal cyclins plays a role in establishing the adult cell cycle (Audic, 2001).

Translational activation of dormant cyclin B1 mRNA stored in oocytes is a prerequisite for the initiation or promotion of oocyte maturation in many vertebrates. Using a monoclonal antibody against the domain highly homologous to that of Drosophila Pumilio, it has been shown for the first time in any vertebrate that a homolog of Pumilio is expressed in Xenopus oocytes. This 137-kDa protein binds to the region including the sequence UGUA at nucleotides 1335-1338 in the 3'-untranslated region of cyclin B1 mRNA, which is close to but does not overlap the cytoplasmic polyadenylation elements (CPEs). Physical in vitro association of Xenopus Pumilio with a Xenopus homolog of Nanos (Xcat-2) was demonstrated by a protein pull-down assay. The results of immunoprecipitation experiments have shown in vivo interaction between Xenopus Pumilio and CPE-binding protein (CPEB: Drosophila homolog Orb), a key regulator of translational repression and activation of mRNAs stored in oocytes. This evidence provides a new insight into the mechanism of translational regulation through the 3'-end of mRNA during oocyte maturation. These results also suggest the generality of the function of Pumilio as a translational regulator of dormant mRNAs in both invertebrates and vertebrates (Nakahata, 2001).

The actual biological roles of XPum are completely unknown at present, but it can be speculated that XPum plays an important role in translational control of cyclin B1 mRNA, as in Drosophila. CPEB directly binds to maskin, a protein that can also bind directly to the cap-binding translation initiation factor elF-4E, which leads to translational repression. The dissociation of maskin from elF-4E allows elF-4G to bind to elF-4E, which brings elF-3 and the 40 S ribosomal subunit to the mRNA to initiate translation via cap-ribose methylation. Recent studies have also shown that a progesterone-induced early phosphorylation of CPEB at serine 174 is catalyzed by Eg2 and that this phosphorylation recruits cleavage and polyadenylation specificity factor into an active cytoplasmic polyadenylation complex. Thus, CPEB plays a key role in both translational repression and activation of mRNAs stored in oocytes. XPum is physically associated with CPEB in oocytes. In cooperation with CPEB, XPum may control the CPEB/maskin-mediated translational masking and unmasking to assure the highly coordinated successive translational activation of masked mRNAs during oocyte maturation. Further studies are required to understand the biological significance of the interactions among XPum, CPEB, and cyclin B1 mRNA, as well as to elucidate the functions of XPum in oocytes (Nakahata, 2001).

Protein synthesis of cyclin B by translational activation of the dormant mRNA stored in oocytes is required for normal progression of maturation. In Xenopus it has been shown that the cytoplasmic polyadenylation element (CPE) in the 3'-untranslated region (UTR) of cyclin B1 mRNA is responsible for both translational repression (masking) and activation (unmasking) of the mRNA (Mendez and Richter, 2001; Richter, 2000). The CPE is bound by a CPE-binding protein. In this study, the involvement of Xenopus Pumilio (XPum), a cyclin B1 mRNA-binding protein, was investigated in mRNA-specific translational activation. XPum exhibits high homology to mammalian counterparts, with amino acid identity close to 90%, even if the conserved RNA-binding domain is excluded. XPum is bound, in mature oocytes, to the unphosphorylated form of cytoplasmic polyadenylation element (CPE)-binding protein (CPEB) through the RNA-binding domain. In addition to the CPE, the XPum-binding sequence of cyclin B1 mRNA acts as a cis-element for translational repression. Injection of anti-XPum antibody accelerated oocyte maturation and synthesis of cyclin B1, and, conversely, over-expression of XPum retarded oocyte maturation and translation of cyclin B1 mRNA, which was accompanied by inhibition of poly(A) tail elongation. The injection of antibody and the over-expression of XPum, however, had no effect on translation of Mos mRNA, which also contains the CPE. These findings provide the first evidence that XPum is a translational repressor specific to cyclin B1 in vertebrates. It is proposed that in cooperation with the CPEB-maskin complex, the master regulator common to the CPE-containing mRNAs, XPum acts as a specific regulator that determines the timing of translational activation of cyclin B1 mRNA by its release from phosphorylated CPEB during oocyte maturation (Nakahata, 2003).

One possible mechanism of translational activation of cyclin B1 mRNA is that a dissociation of XPum from phosphorylated CPEB during oocyte maturation induces destabilization of the CPEB-maskin-eIF4E complex and provides a cue that leads to unmasking of cyclin B1 mRNA by the mechanism common to CPE-containing mRNAs. In this respect, it is noteworthy that phosphorylation of CPEB on Ser210, which occurs about the time of cyclin B1 translation, is sufficient for selective translational activation of cyclin B1. While this phenomenon has been explained in relation to degradation of CPEB, it is also conceivable that the later phosphorylation of CPEB induces release of XPum from the CPEB-maskin-eIF4E complex and that this event triggers translational activation of cyclin B1. Consistent with this possibility, it has been demonstrated that phosphorylation of CPEB is required for its dissociation from a large ribonucleoprotein complex upon oocyte maturation, prior to degradation (Nakahata, 2003).

Previous work has provided evidence for E2F-dependent transcription control of both G1/S- and G2/M-regulated genes. Analysis of the G2-regulated cdc2 and cyclin B1 genes reveals the presence of both positive- and negative-acting E2F promoter elements. Additional elements provide both positive (CCAAT and Myb) and negative (CHR) control. Chromatin immunoprecipitation assays identify multiple interactions of E2F proteins that include those previously shown to activate and repress transcription. E2F1, E2F2, and E2F3 were found to bind to the positive-acting E2F site in the cdc2 promoter, whereas E2F4 binds to the negative-acting site. Binding of an activator E2F is dependent on an adjacent CCAAT site that is bound by the NF-Y transcription factor and binding of a repressor E2F is dependent on an adjacent CHR element, suggesting a role for cooperative interactions in determining both activation and repression. Finally, the kinetics of B-Myb interaction with the G2-regulated promoters coincides with the activation of the genes, and RNAi-mediated reduction of B-Myb inhibits expression of cyclin B1 and cdc2. The ability of B-Myb to interact with the cdc2 promoter is dependent on an intact E2F binding site. These results thus point to a role for E2Fs, together with B-Myb, which is an E2F-regulated gene expressed at G1/S, in linking the regulation of genes at G1/S and G2/M (Zhu, 2004).

Mutation of Cyclin B

Two B-type cyclins (B1 and B2), have been identified in mammals. Proliferating cells express both cyclins, which bind to and activate p34cdc2. To test whether the two B-type cyclins have distinct roles, lines of transgenic mice were generated, one lacking cyclin B1 and the other lacking cyclin B2. Cyclin B1 proves to be an essential gene; no homozygous B1-null pups are born. In contrast, nullizygous B2 mice develop normally and do not display any obvious abnormalities. Both male and female cyclin B2-null mice are fertile, which is unexpected in view of the high levels and distinct patterns of expression of cyclin B2 during spermatogenesis. The expression of cyclin B1 overlaps the expression of cyclin B2 in the mature testis, but not vice versa. Cyclin B1 can be found both on intracellular membranes and free in the cytoplasm, in contrast to cyclin B2, which is membrane-associated. These observations suggest that cyclin B1 may compensate for the loss of cyclin B2 in the mutant mice, and implies that cyclin B1 is capable of targeting the p34cdc2 kinase to the essential substrates of cyclin B2 (Brandeis, 1998).

Activation of cyclin-dependent kinases

Continued: CyclinB Evolutionary homologs  part 2/3 |  part 3/3

Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.