Gene name - Cyclin B
Cytological map position - 59A
Function - Regulatory subunit of cyclin dependent kinase - G2-M cyclin
Keywords - cell cycle
Symbol - CycB
Genetic map position - 2-
Classification - Cyclin B
Cellular location - nuclear
Cyclin B, along with its dimerization partner cdc2, plays a significant role in cell division in Drosophila, but this fact is easy to overlook for several reasons. First, mutational studies can be misleading, with respect to the vital role played by Cyclin B during cell division. A biological fail safe mechanism appears to exist between Cyclin B and Cyclin A: a single mutational deficiency in either of these two cyclins involved in the G2-M transition can be compensated for by the unmutated cyclin. One might reasonably suppose that because Cyclin B is thus replaceable, it is less than essential to the process. The compensation is not 100% effective however; mitotic spindles are abnormal and progression through mitosis is delayed in cyclin B deficient embryos (Knoblich, 1993).
Two other factors mitigate against finding a substantial role for Cyclin B in Drosophila development: first, maternal cyclins are involved in development through mitosis 14, at which time they are degraded. Second, and occuring after this time, the dynamics of String protein limit the rate of mitosis. Despite these limiting factors, Cyclins A and B are produced in G2 both before and after cell cycle 14.
The strongest evidence that Cyclin B is crucial to mitosis is provided by the measurement of cyclin levels throughout the process. During cycles 8-13 a progressive increase in the degradation of cyclins at mitosis leads to increasing oscillations of cdc2 kinase activity. Quantitative measurements indicate that less than 10% of Cyclin A and B are degraded at mitosis 5, and that about 30%, 60%, and 80% are degraded at mitoses 8, 11 and 13 respectively. It appears that cyclin synthesis is limiting for mitoses 10-13 (Edgar, 1994).
During interphase 14, programmed degradation of maternal String protein leads to inhibitory phosphorylation of cdc2 and cell cycle arrest. Subsequently, mitoses 14-16 are triggered by pulses of zygotic string transcription. The use of an N-terminally truncated Cyclin B in Drosophila provides evidence for the role of Cyclin B in timing mitosis. Cyclin A is degraded during metaphase and Cyclin B degradation occurs at approximately the metaphase-anaphase transition (Whitfield, 1990). The N-terminally truncated Cyclin B fails to degrade due to absence of a 'destruction box' in the truncated N-terminal region. The truncated Cyclin B results in mitotic delay at late anaphase (Rimmington, 1994). The gene fizzy, homologous to cdc20 in S. cerevisiae, is required for metaphase-anaphase transition in Drosophila, and is required for normal cyclin A and B degradation. It is suggested that fizzy functions to promote the ubiquitin-dependent proteolytic events that occur during mitosis (Dawson, 1995).
Looking to yeast and to vertebrates for clues as to the role of Cyclin B in mitosis, one finds a bewildering array of information. There appears to be a connection between G1-S cyclins and G2-M cyclins. Cdk2 kinase, the partner for G1-S cyclins, is a positive regulator of cdc2-Cyclin B complexes in Xenopus. When cdk2 kinase activity is inhibited by the cdk-specific inhibitor, a block to mitosis occurs, and inactive cdc2-cyclin B accumulates (Guadagno, 1996). Eukaryotic cells have evolved regulatory mechanisms to ensure the strict alternation of DNA replication and mitosis. Cdc2/cyclin B has a role in preventing re-replication of the genome before mitosis. A Xenopus homolog of S. pombe, cdc21, exhibits cell-cycle dependent chromatin binding and phosphorylation in association with S-phase control. Cdc21 remains bound to chromatin during the initiation of DNA replication and is displaced with the progression of the replication fork. Cytoplasmic cdc21 remains underphosphorylated but at the beginning of mitosis the entire pool of cdc21 is hyperphosphorylated, possibly by the cdc2/cyclin B kinase. These properties identify Xenopus cdc21 as a possible component of the DNA licensing factor (Coué, 1996). For more information about licensing factor see the DNA replication site.
Several targets of cdc2-Cyclin B kinase have been identified. The dimer cdc2-Cyclin B targets Wee1 kinase. Wee1 then inhibits cell division by phosphorylating cdc2. In each cell cycle, the mouse Wee1 kinase is phosphorylated at M-phase resulting in inactivation of Wee kinase. The N-terminal domain or entire molecule is extensively phosphorylated by the cdc2-Cyclin B dimer (Honda, 1995). Drosophila Cyclin B has a consensus cAMP-dependent protein kinase site. Evidence from Xenopus suggests that cyclin degradation and exit from mitosis requires Cyclin B/cdc2-dependent activation of the cAMP-PKA pathway. The concentration of cAMP and the activity of PKA decrease at the onset of mitosis and increase at the transition between mitosis and interphase (Grieco, 1996). Proteins of the mitotic apparatus are direct targets of cdc2-Cyclin B dimer.
A human homolog of Xenopus Eg5, (a kinesin-related motor protein) is implicated in the assembly and dynamics of the mitotic spindle. An evolutionarily conserved cdc2 phosphorylation site in HsEg5 is phosphorylated specifically during mitosis in HeLa cells and in vitro by p34cdc2/Cyclin B. Phosphorylation controls the association of this motor protein with the spindle apparatus (Blangy, 1995).
Cdc2/Cyclin B targets the ubiquitin proteins involved in cyclin destruction. Two specific components are required for the ubiquitination of mitotic cyclins: E2-C, a cyclin-selective ubiquitin carrier protein that is constitutively active during the cell cycle, and E3-C, a cyclin-selective ubiquitin ligase (termed the cyclosome) that purifies as part of an approximately 1500-kDa complex and is active only near the end of mitosis. The cyclosome has been separated from its ultimate upstream activator, cdc2, that activates the cyclosome complex by means of phosphorlyation (Lahav-Baratz, 1995).
The cdc2/Cyclin B heterodimer was first characterized as maturation promoting factor, or MPF. Unfertilized frog eggs manifest cytoplasmic activity that can induce immature oocytes to undergo meiotic maturation. MPF activity drives the events of early mitosis such as nuclear breakdown, chromosome condensation and spindle formation by phosphorylating cellular substrates. While cdc2 (the catalytic subunit of the MPF heterodimer) is required to drive the events of early mitosis, it must be inactivated to allow the events of late mitosis to proceed. The metaphase-anaphase transition is a checkpoint during mitosis. At this juncture the cell is able to monitor the integrity of its spindle before proceeding to inactivate MPF and initiate chromosome separation (Murray, 1992 and references). What is the link between phosphorlyation and this checkpoint? A common mitotic error is the attachment of a chromosome to only one spindle pole rather than to both poles. Even a single such chromosomal error delays anaphase in cells. Recently it has been shown that when the tension associated with proper attachment is absent the kinetochore becomes phosphorylated and anaphase is delayed. It has been proposed that the kinetochore protein dephosphorylation caused by tension is the all-clear signal to the checkpoint. The involvement of Cyclin B/cdc2 in the events surrounding this mitotic checkpoint have yet to be documented, but the fact that Cyclin B/cdc2 is degraded at the metaphase-anaphase transition, suggests that MPF is the direct target of this checkpoint (Nicklas, 1995 and Dawson, 1995).
In the Drosophila embryo, Nanos and Pumilio collaborate to repress the translation of hunchback mRNA in the somatic cytoplasm. Both proteins are also required for repression of maternal Cyclin B mRNA in the germline; it has not been clear whether they act directly on Cyclin B mRNA, and if so, whether regulation in the presumptive somatic and germline cytoplasm proceeds by similar or fundamentally different mechanisms. This report shows that Pumilio and Nanos bind to an element in the 3' UTR to repress Cyclin B mRNA. Regulation of Cyclin B and hunchback differ in two significant respects. (1) Pumilio is dispensable for repression of Cyclin B (but not hunchback) if Nanos is tethered via an exogenous RNA-binding domain. Nanos probably acts, at least in part, by recruiting the CCR4-Pop2-NOT deadenylase complex, interacting directly with the NOT4 subunit. (2) Although Nanos is the sole spatially limiting factor for regulation of hunchback, regulation of Cyclin B requires another Oskar-dependent factor in addition to Nanos. Ectopic repression of Cyclin B in the presumptive somatic cytoplasm causes lethal nuclear division defects. It is suggested that a requirement for two spatially restricted factors is a mechanism for ensuring that Cyclin B regulation is strictly limited to the germline (Kadyrova, 2007).
Thus Nos and Pum directly regulate maternal CycB mRNA, binding to an NRE in its 3' UTR. Differences in the spacing and arrangement of protein-binding sites within the hb and CycB NREs appear to account for the regulation of hb but not CycB by Brat. For regulation of CycB, the main function of Pum is to recruit Nos, a role that can be bypassed by tethering Nos via an exogenous RNA-binding domain. CycB-bound Nos is then likely to act, at least in part, by recruiting a deadenylase complex, interacting with its NOT4 subunit. Regulation of CycB is limited to the PGCs to avoid the deleterious consequences of repression in the presumptive somatic cytoplasm. The requirement for both Nos plus at least one additional germline-restricted factor may be part of a mechanism to ensure that CycB regulation is strictly limited to the PGCs (Kadyrova, 2007).
The co-crystal structure of human Pum bound to a fragment of the hb NRE shows that a single Pum RBD directly contacts eight nucleotides of the RNA. However, Puf proteins bind with essentially wild-type affinity to many mutant sites, suggesting that all eight nucleotides are not rigidly specified. How, then, do Puf proteins recognize specific mRNA targets in vivo (Kadyrova, 2007)?
Part of the answer appears to be that, within functional NREs, more than eight nucleotides are recognized, at least by Drosophila Pum. Mutations that simultaneously disrupt Pum binding in vitro and regulation in vivo are spread over 20 nts of the hb NRE and 18 nts of the CycB NRE. These extended Pum mutational 'footprints' are too large to be accounted for by binding of a single RBD; it is suggested that two or more Pum RBDs bind each NRE, an idea supported by the detection of two RNA-protein complexes in gel mobility shift experiments using both the CycB and hb NREs. This model disagrees with earlier experiments that suggested only a single Pum RBD binds to the hb NRE. Further biochemical and structural studies will be required to resolve the issue (Kadyrova, 2007).
The distribution of Pum- and Nos-binding sites within the CycB and hb NREs is different. In the former, the Nos binding site lies 5' to the Pum-binding site(s), whereas in the latter, the Nos-binding site is flanked by nucleotides recognized by Pum. It is assumed that the different arrangement of Nos- and Pum-binding sites is responsible for the assembly of Pum-NRE-Nos complexes with different topographies, such that Brat is recruited to hb but not to CycB. Further definition of each RNP structure will ultimately be required to understand the combinatorial assembly of different repressor complexes on each NRE (Kadyrova, 2007).
In addition to the NRE, Pum also binds with high affinity to at least two other sites in the CycB 3' UTR; however, binding to these sites does not mediate translational repression in the PGCs, perhaps because neither supports recruitment of Nos. These sites may simply bind Pum fortuitously, or they may mediate Nos-independent regulation at other stages of development. Pum has been suggested to destabilize bcd mRNA at the anterior of the embryo in a Nos-independent manner. Another Nos-independent function of Pum is the repression of CycB translation throughout the prospective somatic cytoplasm during the early syncitial nuclear cleavages. These processes might be mediated by elements in Fragments A and F of the 3' UTR, that bind Pum but not Nos (Kadyrova, 2007).
A general framework has been provided for understanding how Puf proteins act to control either the translation or stability of target mRNAs (Goldstrohm, 2006). The yeast Puf protein MPT5 interacts directly with Pop2, one of the catalytically active subunits of a large deadenylase complex. Subsequent deadenylation could either silence the mRNA or cause its degradation, depending on other signals in the transcript or the composition of the deadenylase complex (or both). The Puf-Pop2 interaction is conserved across species (including Drosophila), supporting the idea that the mechanism uncovered for MPT5 might generally be applicable to Puf proteins (Kadyrova, 2007).
In this context, it is surprising that Pum is dispensable if Nanos is tethered to CycB via MS2 CP. It is suggested that yeast Puf proteins both recognize target mRNAs and recruit the deadenylase, but that in the Drosophila germline these functions are partitioned, with Pum primarily responsible for target mRNA recognition and Nos primarily responsible for effector recruitment. This model has the attraction of attributing an important role to Nos, which is essential for Puf-mediated regulation in Drosophila, and probably other metazoans as well. What, then, might be the role of the conserved interaction between Pum and Pop2? One possibility is that it acts cooperatively with Nos to recruit the deadenylase; unlike CycB, other mRNA targets (e.g. hb) might require recruitment by both Nos and Pum to ensure efficient deadenylation. Another possibility is that it plays an essential role for mRNAs regulated by Pum but not Nos (Kadyrova, 2007).
Oscillations in CycB activity underlie normal cell cycle progression. During the early embryonic syncitial nuclear cleavages, degradation in the vicinity of the nuclei is thought to deplete CycB locally. Recent work has shown that Pum can inappropriately repress de novo translation of CycB mRNA during the initial nuclear cleavages if not antagonized by the PNG kinase, resulting in mitotic failure. This early Pum-dependent repression is thought to be Nos-independent, as it occurs efficiently in the anterior, where Nos activity is undetectable (Kadyrova, 2007).
The results show that if CycB is inappropriately subjected to Pum+Nos-dependent repression via the hb NRE, CycB is locally depleted, resulting in mitotic failure during nuclear division cycles 10-13. Since it is thought to be the case during the early cycles (1-7), de novo synthesis of CycB apparently is required to counteract the local degradation that probably occurs during M phase of each cycle. The CycB NRE must therefore be precisely tuned to repress translation only in the PGCs and not in the presumptive somatic cytoplasm (Kadyrova, 2007).
Osk is the limiting factor for assembly of pole plasm in the embryo; the results suggest that it stimulates the accumulation or activity of at least one factor in addition to Nos that is required for repression of CycB in the PGCs. The existence of a co-factor is inferred from the finding that ectopic Nos can repress CycB in the somatic cytoplasm only in the presence of ectopic Osk. Regulation of CycB may depend on more than one germline-restricted factor to ensure that potentially deleterious repression does not occur in the somatic cytoplasm (Kadyrova, 2007).
A germline Nos co-factor might act in a variety of ways. It could bind to the CycB NRE adjacent to Pum and Nos, substituting functionally for Brat, which is recruited to the Pum-hb NRE-Nos complex. The 50 nt CycB NRE is inactivated by a truncation at both ends that leaves the Pum- and Nos-binding sites intact, consistent with the idea that another factor binds to the element. Another possibility is that the co-factor is a germline-specific component of the adenylation/deadenylation machinery, as is the case for the GLD-2 cytoplasmic poly(A)-polymerase in C. elegans. Distinguishing among these ideas awaits identification of the cofactor (Kadyrova, 2007).
The Drosophila embryo is a promising model for isolating gene products that coordinate S phase and mitosis. Increasing maternal Cyclin B dosage to up to six copies (six cycB) increases Cdk1-Cyclin B (CycB) levels and activity in the embryo, delays nuclear migration at cycle 10, and produces abnormal nuclei at cycle 14. The level of CycB in the embryo inversely correlates with the ability to lengthen interphase as the embryo transits from preblastoderm to blastoderm stages and defines the onset of a checkpoint that regulates mitosis when DNA replication is blocked with aphidicolin. A screen for modifiers of the six cycB phenotypes identified 10 new suppressor deficiencies. In addition, heterozygote dRPA2 (a DNA replication gene) mutants suppressed only the abnormal nuclear phenotype at cycle 14. Reduction of dRPA2 also restored interphase duration and checkpoint efficacy to control levels. It is proposed that lowered dRPA2 levels activate Grp/Chk1 to counteract excess Cdk1-CycB activity and restore interphase duration and the ability to block mitosis in response to aphidicolin. These results suggest an antagonistic interaction between DNA replication checkpoint activation and Cdk1-CycB activity during the transition from preblastoderm to blastoderm cycles (Crest, 2007).
It has been proposed (model 1) that the ability of the DNA replication checkpoint to regulate the entry into M-phase is not active before cycle 11 in wild-type (two cycB) embryos. This model is based on observations that the earliest defects in interphase duration can be detected only after cycle 10 in grp (dChk1) or mei-41 (dATR) mutant embryos. In this view, before cycle 10 in two cycB embryos, nuclei have normal morphology because the DNA replication machinery is abundant. The exponentially increasing numbers of nuclei, however, titrate components of this machinery to a critical level after cycle 10, thereby inducing the checkpoint activation, interphase extension, and delay of M-phase in a Grp- and Mei-41-dependent manner (Crest, 2007).
An alternative (model 2) is proposed: Checkpoint function is active all the time, but before cycle 10 checkpoint activity is too low and is overridden by the high level of maternal Cdk1-CycB. The difference between the two models is that in model 1, checkpoint is activated by a critical amount of the replication machinery. In model 2, at a critical concentration of Cdk1-CycB, this kinase (Cdk1) can no longer override checkpoint activity. Several observations are not compatible with model 1, but are with model 2. First, here this study shows that checkpoint activity does not depend on a specific number of nuclei, number of rounds of divisions, or time after fertilization, but on the amount of CycB-Cdk1. It occurs earlier in one cycB embryos with fewer nuclei and later in six cycB embryos with more nuclei (Crest, 2007).
Second, model 1 does not explain interphase extension before cycle 10 and the slight increase of interphase even in grp mutant embryos after cycle 10 (Crest, 2007).
Third, Grp protein is required for the degradation of cyclin A in the presence of cycloheximide as early as cycle 4. Although the effect of Grp on CycA degradation may be different from its effect on replication checkpoint activation, this observation suggests that Grp is present and functional before cycle 10. How then do nuclei enter M-phase at the normal time when aph. is applied before cycle 10, i.e., show no DNA replication checkpoint? It is reasoned that successful execution of the checkpoint requires the inhibitory effect of Grp to overcome the M-phase-promoting effect of Cdk1-CycB. Early embryos might have too few nuclei, thus limited numbers of replication forks, to trigger sufficient Grp/Chk1 activity necessary to overcome the relatively high levels of Cdk1-CycB. Despite this situation, nuclear morphology is normal because replication machinery is not limited before cycle 10, S-phase is rapidly completed, and nuclei can successfully undergo a normal mitosis (Crest, 2007).
Fourth, during normal S-phase of either yeast or mammalian cells, a low level of replication checkpoint activity is observed. This low level of checkpoint activity can be detected as phosphorylation on Chk1 in S-phase cells without any replication stress or DNA damage. Physiological regulation of Chk1 is under the control of similar factors of the DNA replication checkpoint machinery, and thus it was proposed that DNA replication per se generates lesions that activated the checkpoint pathway. Alternatively, but not mutually exclusively, this constitutively low level of replication checkpoint activation may be due to transient signaling from the replication forks, which does not lead to cell cycle arrest, but serves as a mechanism to coordinate the firing of replication origins, thereby moderating the rate of S-phase (Crest, 2007).
Model 2 not only accounts for observations with the aph. experiments, but also accounts for observation of the CycB effects on the replication checkpoint effect. In one cycB embryos the Grp/Chk1 effect is observed earlier, presumably because levels of Cdk1-CycB become limiting earlier, whereas in six cycB embryos, these occur later. Using PH3 staining on anaphase chromosomes as a measurement for Chk1-dependent Cyclin A degradation indicates that Chk1 is not functioning in six cycB embryos before cycle 11, possibly because it is overridden by the abundance of Cdk1-CycB in these embryos. It is proposed that in six cycB embryos at cycle 11 or later, Cdk1-CycB activity is still too high and forces the nuclei into mitosis at a time when the DNA replication machinery is limited, resulting in precocious M-phase and abnormal nuclei (Crest, 2007).
This study addresses how dRPA2 might suppress the six cycB phenotype at cycle 14. dRPA2 is a subunit of a highly conserved heterotrimeric complex of proteins that make up the RPA complex. All three subunits contain DNA-binding domains, which stabilize ssDNA as it is unwound at the replication fork. This stabilized DNA allows for Cdc45 and DNA polymerase-α to initiate DNA replication. Additional roles for RPA have been implicated in DNA repair and recombination (Crest, 2007).
In six cycB embryos, during the blastoderm cycles, elevated levels of Cdk1-CycB override Chk1 and the nuclei divide before DNA replication is completed, leading to abnormal nuclei. It is speculated that reducing dRPA2 is likely to slow DNA replication because RPA can facilitate DNA replication by unwinding dsDNA and by modulating the activities of several enzymes, such as DNA helicases, DNA polymerases, and primases. This would result in less RPA coated, primed DNA and ssDNA. Such a DNA structure may potentiate the TopBP1-mediated ATR-ATRIP kinase activation, leading to stronger Chk1 activation. Thus in dRPA2/six cycB embryos, a stronger Chk1 activation would have a stronger inhibitory effect on Cdk1-CycB activity that cancels out the effect caused by extra Cdk1-CycB. This interpretation for the suppressive effect of RPA2 on the six cycB phenotype suggests an antagonistic relationship between the DNA replication checkpoint activation and Cdk1-CycB activity in regulating the transition from the preblastoderm cycles to the blastoderm cycles (Crest, 2007).
The earliest divisions in the embryo (with the exception of mammals) are maternally regulated until the zygotic genome takes over. On the basis of many observations with different animals such as Xenopus and Drosophila, a simple concept developed: Early cell cycles are invariant, synchronous, and lack both gap phases until the transition to zygotic control occurs. The time point at which this change happens has been called the midblastula transition (MBT). It is assigned to the specific cycle when zygotic transcription is activated. Furthermore, G2 is induced in Drosophila and G1 and G2 in Xenopus into the abbreviated cell cycles and cell division is patterned (Crest, 2007).
This concept is attractive; however, as with many simple concepts, the more that is learned the harder it is to accept the concept at face value. For example, synchronous divisions have never been observed in Caenorhabditis elegans. Even in Drosophila, the simplification that early cell cycles are synchronous and equal in length is incorrect since interphase durations steadily increase after cycle 7 and metasynchronous mitoses are observed as early as cycle 4. These changes occur long before cycle 14, the time that has been designated by many as the MBT. The data presented in this study clearly demonstrate a change in the maternal program as the embryo develops: A DNA replication checkpoint is first detectable after cycle 10, but becomes increasingly robust in the subsequent cycles, indicating that the ability to regulate M-phase by checkpoints is not completely'off' or 'on'. In addition, it was found that the DNA replication checkpoint is detectable earlier in one cycB and later in six cycB embryos, clearly indicating that changes do not have to occur at a specific stage. A gradual attainment of full checkpoint function is also supported by the fact that aph. injections before cycle 11 can stall/delay the nuclear cycle, but not the centrosomal cycle. These results are not compatible with the idea of an invariant maternal program for pre-MBT cycles (Crest, 2007).
Another simplification is that changes that define the MBT are events that envelope the entire embryo. In sea urchins the deceleration of the cell cycle in the micromeres occurs long before these changes have been observed in the macromeres. Thus an MBT has not been proposed for this animal. But differences are also observed among the blastoderm cycles in Drosophila, where divisions in the middle of the embryo are slower than at the poles, correlating with their nuclear densities (Crest, 2007).
The initiation of the zygotic program also does not occur suddenly from off to on. fushi tarazu (ftz) transcripts are first observed at cycle 9 in one or the other embryo and in one or the other nucleus. Transcription gradually increases over the next three cycles (cycles 9-12). This gradual increase is a consequence of the dose-dependent repressor Tramtrack (TTK), where with one gene dose of maternal ttk, initial transcription of ftz occurs one cycle earlier, and conversely extra copies of ttk result in initial transcription of ftz one cycle later. These data are interpreted as a decline of TTK during cycles 8-10 to a threshold level where TTK repression is insufficient, enabling low-level transcription of ftz (Crest, 2007).
Despite the many observations that do not fit the MBT concept, textbooks, reviews, research articles, and grant proposals still hold on to it, hindering progress in the understanding of how maternally controlled development declines and terminates and zygotic programming gradually takes over (Crest, 2007).
Bases in 5' UTR -123 or more
Bases in 3' UTR - 776
Within a central region spanning 206 amino acid residues, the Drosophila Cyclins A and B share 35% identity, whereas clam Cyclin A and Drosophila Cyclin A share 53% identity as do clam Cyclin B and Drosophila Cyclin B. In particular, the Drosophila Cyclin B sequence contains a consensus cAMP-dependent protein kinase site, a feature common to all other Cyclin B sequences (Whitfield, 1990).
date revised: 21 APR 97
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