The requirements for protein synthesis and degradation, and the control of destruction of cyclins A and B in the meiotic and mitotic cell cycles of the clam embryo

A conserved role of a DEAD box helicase in mRNA masking

Clam p82 is a member of the cytoplasmic polyadenylation element-binding protein (CPEB) family of RNA-binding proteins and serves dual functions in regulating gene expression in early development. In the oocyte, p82/CPEB is a translational repressor, whereas in the activated egg, it acts as a polyadenylation factor. Coimmunoprecipitations were performed with p82 antibodies in clam oocyte and egg lysates to identify stage-regulated accessory factors. p47 coprecipitates with p82 from oocyte lysates in an RNA-dependent manner and is absent from egg lysate p92-bound material. Clam p47 is a member of the RCK/p54 family of DEAD box RNA helicases. Xp54, the Xenopus homolog, with bona fide helicase activity, is an abundant and integral component of stored mRNP in oocytes (Ladomery et al., 1997). In oocytes, clam p47 and p82/CPEB are found in large cytoplasmic mRNP complexes. Whereas the helicase level is constant during embryogenesis, in contrast to CPEB, clam p47 translocates to nuclei at the two-cell stage. To address the role of this class of helicase in masking, Xp54 was tethered via 3' UTR MS2-binding sites to firefly luciferase, following microinjection of fusion protein and nonadenylated reporter mRNAs into Xenopus oocytes. Tethered helicase repressed luciferase translation three- to fivefold and, strikingly, mutations in two helicase motifs (DEAD--> DQAD and HRIGR-->HRIGQ), activated translation three- to fourfold, relative to MS2. These data suggest that this helicase family represses translation of maternal mRNA in early development, and that its activity may be attenuated during meiotic maturation, prior to cytoplasmic polyadenylation.

Sub-cellular localisation of GFP-tagged tobacco mitotic cyclins during the cell cycle and after spindle checkpoint activation

We have previously shown that the tobacco cyclin B1;1 protein accumulates during the G2 phase of the cell cycle and is subsequently destroyed during mitosis. Here, we investigated the sub-cellular localisation of two different B1-types and one A3-type cyclin during the cell cycle by using confocal imaging and differential interference contrast (DIC) microscopy. The cyclins were visualised as GFP-tagged fusion proteins in living tobacco cells. Both B1-type cyclins were found in the cytoplasm and in the nucleus during G2 but when cells entered into prophase, both cyclins became associated with condensing chromatin and remained on chromosomes until metaphase. As cells exited metaphase, the B1-type cyclins became degraded, as shown by time-lapse images. A stable variant of cyclin B1;1-GFP fusion protein, in which the destruction box had been mutated, maintained its association with the nuclear material at later phases of mitosis such as anaphase and telophase. Furthermore, we demonstrated that cyclin B1;1 protein is stabilised in metaphase-arrested cells after microtubule destabilising drug treatments. In contrast to the B1-type cyclins, the cyclin A3;1 was found exclusively in the nucleus in interphase cells and disappeared earlier than the cyclin B1 proteins during mitosis.

Overexpression of simian virus 40 small-T antigen blocks centrosome function and mitotic progression in human fibroblasts

Recombinant adenoviruses that express high levels of the simian virus 40 (SV40) small-t (ST) antigen have been used to study the requirement for ST to drive cell cycle proliferation of confluent human diploid fibroblasts. This occurs when either large-T (LT) antigen or serum is added to provide a second signal. While cells readily completed S phase in these experiments, they were found to accumulate with 4N DNA content. Cellular and nuclear morphology, as well as the biochemical status of cyclin B complexes, showed that these cells entered mitosis but were blocked prior to mitotic metaphase. The defect appears to reflect an inability of cells overexpressing ST to form organized centrosomes that duplicate and separate normally during the cell cycle and, therefore, the absence of a mitotic spindle. The ability of ST to bind protein phosphatase 2A was required for this pattern, suggesting that altered phosphorylation of key centrosomal components may occur when ST is overexpressed. Although the possible significance of ST effects on the centrosome cycle is not fully understood, these findings suggest that ST could influence chromosomal instability patterns that are a hallmark of SV40-transformed cells and LT expression.

Cell cycle: how, when and why cells get rid of cyclin A

Sequences outside the 'destruction box' direct the degradation of cyclin A to completion before the metaphase-anaphase transition; cyclin A that escapes timely degradation can block the metaphase-anaphase transition, impede anaphase and telophase, and impair a cell's ability to arrest in G1 of the next cell cycle.

Anaphase-promoting complex/cyclosome-dependent proteolysis of human cyclin A starts at the beginning of mitosis and is not subject to the spindle assembly checkpoint

Cyclin A is a stable protein in S and G2 phases, but is destabilized when cells enter mitosis and is almost completely degraded before the metaphase to anaphase transition. Microinjection of antibodies against subunits of the anaphase-promoting complex/cyclosome (APC/C) or against human Cdc20 (fizzy) arrested cells at metaphase and stabilized both cyclins A and B1. Cyclin A was efficiently polyubiquitylated by Cdc20 or Cdh1-activated APC/C in vitro, but in contrast to cyclin B1, the proteolysis of cyclin A was not delayed by the spindle assembly checkpoint. The degradation of cyclin B1 was accelerated by inhibition of the spindle assembly checkpoint. These data suggest that the APC/C is activated as cells enter mitosis and immediately targets cyclin A for degradation, whereas the spindle assembly checkpoint delays the degradation of cyclin B1 until the metaphase to anaphase transition. The "destruction box" (D-box) of cyclin A is 10-20 residues longer than that of cyclin B. Overexpression of wild-type cyclin A delayed the metaphase to anaphase transition, whereas expression of cyclin A mutants lacking a D-box arrested cells in anaphase.

Cyclin A is destroyed in prometaphase and can delay chromosome alignment and anaphase

Mitosis is controlled by the specific and timely degradation of key regulatory proteins, notably the mitotic cyclins that bind and activate the cyclin-dependent kinases (Cdks). In animal cells, cyclin A is always degraded before cyclin B, but the exact timing and the mechanism underlying this are not known. Here we use live cell imaging to show that cyclin A begins to be degraded just after nuclear envelope breakdown. This degradation requires the 26S proteasome, but is not affected by the spindle checkpoint. Neither deletion of its destruction box nor disrupting Cdk binding prevents cyclin A proteolysis, but Cdk binding is necessary for degradation at the correct time. We also show that increasing the levels of cyclin A delays chromosome alignment and sister chromatid segregation. This delay depends on the proteolysis of cyclin A and is not caused by a lag in the bipolar attachment of chromosomes to the mitotic spindle, nor is it mediated via the spindle checkpoint. Thus, proteolysis that is not under the control of the spindle checkpoint is required for chromosome alignment and anaphase.

Inactivation of M-phase promoting factor at exit from first embryonic mitosis in the rat is independent of cyclin B1 degradation

Exit from M-phase and completion of cell division requires inactivation of M-phase promoting factor (MPF), a heterodimer composed of the regulatory cyclin B1 and the catalytic p34cdc2 kinase. Inactivation of MPF is associated with cyclin B1 degradation that is brought about by the ubiquitin-proteasome pathway. Our study examined the role of the proteasome in the first mitosis of rat embryos and its participation in the regulation of cyclin B1 degradation and MPF inactivation. We show that in the early zygote the proteasome is evenly distributed in the ooplasm and the nucleus, whereas during mitosis it accumulates on the spindle apparatus. We further demonstrate that inhibition of proteasomal catalytic activity prevents 1-cell embryos from undergoing mitosis. This mitotic arrest is associated with the presence of relatively high amounts of cyclin B1, which unexpectedly does not result in elevated MPF activity. Our findings strongly imply that completion of the first embryonic division depends on proteasomal degradation and that cyclin B1 is included among its target proteins. They also provide the first evidence that MPF inactivation at this stage of development is not solely dependent upon cyclin B1 degradation and is insufficient to allow the formation of the 2-cell embryo.

The interplay between cyclin-B-Cdc2 kinase (MPF) and MAP kinase during maturation of oocytes

Throughout oocyte maturation, and subsequently during the first mitotic cell cycle, the MAP kinase cascade and cyclin-B-Cdc2 kinase are associated with the control of cell cycle progression. Many roles have been directly or indirectly attributed to MAP kinase and its influence on cyclin-B-Cdc2 kinase in different model systems; yet a principle theme does not emerge from the published literature, some of which is apparently contradictory. Interplay between these two kinases affects the major events of meiotic maturation throughout the animal kingdom, including the suppression of DNA replication, the segregation of meiotic chromosomes, and the prevention of parthenogenetic activation. Central to many of these events appears to be the control by MAP kinase of cyclin translation and degradation.

DNA damage leads to a Cyclin A-dependent delay in metaphase-anaphase transition in the Drosophila gastrula

BACKGROUND

In response to DNA damage, fission yeast, mammalian cells, and cells of the Drosophila gastrula inhibit Cdk1 to delay the entry into mitosis. In contrast, budding yeast delays metaphase-anaphase transition by stabilization of an anaphase inhibitor, Pds1p. A variation of the second response is seen in Drosophila cleavage embryos; when nuclei enter mitosis with damaged DNA, centrosomes lose gamma-tubulin, spindles lose astral microtubules, chromosomes fail to reach a metaphase configuration, and interphase resumes without an intervening anaphase. The resulting polyploid nuclei are eliminated.

RESULTS

The cells of the Drosophila gastrula can also delay metaphase-anaphase transition in response to DNA damage. This delay accompanies the stabilization of Cyclin A, a known inhibitor of sister chromosome separation in Drosophila. Unlike in cleavage embryos, gamma-tubulin remains at the spindle poles, and anaphase always occurs after the delay. Cyclin A mutants fail to delay metaphase-anaphase transition after irradiation and show an increased frequency of chromosome breakage in the subsequent anaphase.

CONCLUSIONS

DNA damage delays metaphase-anaphase transition in Drosophila by stabilizing Cyclin A. This delay may normally serve to preserve chromosomal integrity during segregation. To our knowledge this is the first report of a metazoan metaphase-anaphase transition being delayed in response to DNA damage. Though mitotic progression is modulated in response to DNA damage in both cleaving and gastruating embryos of Drosophila, different mechanisms operate. These differences are discussed in the context of differential cell cycle regulation in cleavage and gastrula stages.

Re-staging mitosis: a contemporary view of mitotic progression

The process of cell division, or mitosis, has fascinated biologists since its discovery in the late 1870s. Progress through mitosis is traditionally divided into stages that were defined over 100 years ago from analyses of fixed material from higher plants and animals. However, this terminology often leads to ambiguity, especially when comparing different systems. We therefore suggest that mitosis can be re-staged to reflect more accurately the molecular pathways that underlie key transitions.

The spike of S phase cyclin Cig2 expression at the G1-S border in fission yeast requires both APC and SCF ubiquitin ligases

We describe a novel set of oscillation mechanisms for the fission yeast S phase cyclin Cig2, which contains an authentic destruction box and is destroyed at anaphase via the APC/cyclosome (APC/C). Unlike the mitotic cyclin Cdc13, however, Cig2 mRNA and protein peak at the G1/S boundary and decline to low levels in G2 and M phases. We show here that SCF(Pop1, Pop2) plays a role in transcriptional periodicity, as pop mutations result in constitutive cig2(+) transcripts. The instability of Cig2 during G2 and M is independent of either the APC/C or Pop1/Pop2, but requires Skp1, a core component of SCF. These data indicate that the APC/C and SCF control Cig2 levels differentially at different stages of the cell cycle.

Characterization of vertebrate cohesin complexes and their regulation in prophase

In eukaryotes, sister chromatids remain connected from the time of their synthesis until they are separated in anaphase. This cohesion depends on a complex of proteins called cohesins. In budding yeast, the anaphase-promoting complex (APC) pathway initiates anaphase by removing cohesins from chromosomes. In vertebrates, cohesins dissociate from chromosomes already in prophase. To study their mitotic regulation we have purified two 14S cohesin complexes from human cells. Both complexes contain SMC1, SMC3, SCC1, and either one of the yeast Scc3p orthologs SA1 and SA2. SA1 is also a subunit of 14S cohesin in Xenopus. These complexes interact with PDS5, a protein whose fungal orthologs have been implicated in chromosome cohesion, condensation, and recombination. The bulk of SA1- and SA2-containing complexes and PDS5 are chromatin-associated until they become soluble from prophase to telophase. Reconstitution of this process in mitotic Xenopus extracts shows that cohesin dissociation does neither depend on cyclin B proteolysis nor on the presence of the APC. Cohesins can also dissociate from chromatin in the absence of cyclin-dependent kinase 1 activity. These results suggest that vertebrate cohesins are regulated by a novel prophase pathway which is distinct from the APC pathway that controls cohesins in yeast.

A CDE/CHR tandem element regulates cell cycle-dependent repression of cyclin B2 transcription

Cyclin B is an important regulator of progression through the cell division cycle. The oscillating appearance of cyclin B1 and B2 proteins during the cell cycle is in part due to fluctuating mRNA levels. We had identified earlier a tandem promoter element named cell cycle-dependent element (CDE) and cell cycle genes homology region (CHR) which regulates cell cycle-dependent transcription of cdc25C, cyclin A and cdc2. Here we describe that cyclin B2 transcription is repressed through a novel CDE/CHR element in resting and G(1) cells. By relief of this repression in S and G(2) oscillating expression of cyclin B2 mRNA is achieved during the cell cycle.

Nuclei and microtubule asters stimulate maturation/M phase promoting factor (MPF) activation in Xenopus eggs and egg cytoplasmic extracts

Although maturation/M phase promoting factor (MPF) can activate autonomously in Xenopus egg cytoplasm, indirect evidence suggests that nuclei and centrosomes may focus activation within the cell. We have dissected the contribution of these structures to MPF activation in fertilized eggs and in egg fragments containing different combinations of nuclei, centrosomes, and microtubules by following the behavior of Cdc2 (the kinase component of MPF), the regulatory subunit cyclin B, and the activating phosphatase Cdc25. The absence of the entire nucleus-centrosome complex resulted in a marked delay in MPF activation, whereas the absence of the centrosome alone caused a lesser delay. Nocodazole treatment to depolymerize microtubules through first interphase had an effect equivalent to removing the centrosome. Furthermore, microinjection of isolated centrosomes into anucleate eggs promoted MPF activation and advanced the onset of surface contraction waves, which are close indicators of MPF activation and could be triggered by ectopic MPF injection. Finally, we were able to demonstrate stimulation of MPF activation by the nucleus-centriole complex in vitro, as low concentrations of isolated sperm nuclei advanced MPF activation in cycling cytoplasmic extracts. Together these results indicate that nuclei and microtubule asters can independently stimulate MPF activation and that they cooperate to enhance activation locally.

Residual Cdc2 activity remaining at meiosis I exit is essential for meiotic M-M transition in Xenopus oocyte extracts

To investigate the regulatory mechanisms of the cell cycle transition from M phase to M phase in meiotic cycles, a Xenopus oocyte extract that performs the M-M transition has been developed. Using the meiotic extract, we found that a low level of Cdc2 activity remained at the exit of meiosis I (MI), due to incomplete degradation of cyclin B. The inactivation of the residual Cdc2 activity induced both entry into S phase and tyrosine phosphorylation on Cdc2 after MI. Quantitative analysis demonstrated that a considerable amount of Wee1 was present at the MI exit and Cdc2 inhibitory phosphorylation during this period was suppressed by the dominance of Cdc2 over Wee1. Consistently, the addition of more than a critical amount of Wee1 to the extract induced Cdc2 inhibitory phosphorylation, changing the M-M transition into an M-S-M transition. Thus, the Cdc2 activity remaining at MI exit is required for suppressing entry into S phase during the meiotic M-M transition period.

Inhibiting proteasome activity causes overreplication of DNA and blocks entry into mitosis in sea urchin embryos

The proteasome has been shown to be involved in exit from mitosis by bringing about destruction of mitotic cyclins. Here, we present evidence that the proteasome is also required for proper completion of S phase and for entry into mitosis in the sea urchin embryonic cleavage cycle. A series of structurally related peptide-aldehydes prevent nuclear envelope breakdown in their order of inhibitory efficacies against the proteasome. Their efficacies in blocking exit from S phase and exit from mitosis correlate well, indicating that the proteasome is involved at both these steps. Mitotic histone HI kinase activation and tyrosine dephosphorylation of p34(cdc2) kinase are blocked by inhibition of the proteasome, indicating that the proteasome plays an important role in the pathway that leads to embryonic p34(cdc2)kinase activation. Arrested embryos continued to incorporate [(3)H]thymidine and characteristically developed large nuclei. Pre-mitotic arrest can be overcome by treatment with caffeine, a manoeuvre that is known to override the DNA replication checkpoint. These data demonstrate that the proteasome is involved in the control of termination of S phase and consequently in the initiation of M phase of the first embryonic cell cycle.

Mitotic regulation of the APC activator proteins CDC20 and CDH1

The ordered activation of the ubiquitin protein ligase anaphase-promoting complex (APC) or cyclosome by CDC20 in metaphase and by CDH1 in telophase is essential for anaphase and for exit from mitosis, respectively. Here, we show that CDC20 can only bind to and activate the mitotically phosphorylated form of the Xenopus and the human APC in vitro. In contrast, the analysis of phosphorylated and nonphosphorylated forms of CDC20 suggests that CDC20 phosphorylation is neither sufficient nor required for APC activation. On the basis of these results and the observation that APC phosphorylation correlates with APC activation in vivo, we propose that mitotic APC phosphorylation is an important mechanism that controls the proper timing of APC(CDC20) activation. We further show that CDH1 is phosphorylated in vivo during S, G2, and M phase and that CDH1 levels fluctuate during the cell cycle. In vitro, phosphorylated CDH1 neither binds to nor activates the APC as efficiently as does nonphosphorylated CDH1. Nonphosphorylatable CDH1 mutants constitutively activate APC in vitro and in vivo, whereas mutants mimicking the phosphorylated form of CDH1 are constitutively inactive. These results suggest that mitotic kinases have antagonistic roles in regulating APC(CDC20) and APC(CDH1); the phosphorylation of APC subunits is required to allow APC activation by CDC20, whereas the phosphorylation of CDH1 prevents activation of the APC by CDH1. These mechanisms can explain the temporal order of APC activation by CDC20 and CDH1 and may help to ensure that exit from mitosis is not initiated before anaphase has occurred.

Timing of calcium and protein synthesis requirements for the first mitotic cell cycle in fertilised Xenopus eggs

Mitosis is governed by the activity of the M-phase promoting factor (MPF). In some systems, particularly early embryos, transient increases in calcium concentration have been shown to be necessary for mitosis and regulate its timing. By microinjection of the calcium buffer, dibromoBAPTA, into fertilised Xenopus eggs, we have assessed whether calcium events are required to initiate MPF activation and inactivation. Since initial experiments showed that this buffer inhibited protein synthesis, we measured when mitosis and cleavage became independent of translation. We found that, after a period of protein synthesis essential for cleavage, there was a phase during which continued translation affected the timing of cleavage, but was not essential for its occurrence. Measurement of MPF activity in single embryos injected with calcium buffer at different times in the first cell cycle, showed that there were two sensitive periods. The first period of sensitivity blocked MPF activation and coincided with the time at which cleavage became completely independent of protein synthesis. The second sensitive period occurred just before histone kinase activity peaked, and was necessary for kinase inactivation. Preventing inactivation in this way arrested egg extracts in mitosis. These results support the view that transient increases in free calcium concentration contribute to mitotic progression by first triggering MPF activation and subsequently, with elevated MPF activity, inducing its inactivation.

Cell cycle-regulated proteolysis of mitotic target proteins

The ubiquitin-dependent proteolysis of mitotic cyclin B, which is catalyzed by the anaphase-promoting complex/cyclosome (APC/C) and ubiquitin-conjugating enzyme H10 (UbcH10), begins around the time of the metaphase-anaphase transition and continues through G1 phase of the next cell cycle. We have used cell-free systems from mammalian somatic cells collected at different cell cycle stages (G0, G1, S, G2, and M) to investigate the regulated degradation of four targets of the mitotic destruction machinery: cyclins A and B, geminin H (an inhibitor of S phase identified in Xenopus), and Cut2p (an inhibitor of anaphase onset identified in fission yeast). All four are degraded by G1 extracts but not by extracts of S phase cells. Maintenance of destruction during G1 requires the activity of a PP2A-like phosphatase. Destruction of each target is dependent on the presence of an N-terminal destruction box motif, is accelerated by additional wild-type UbcH10 and is blocked by dominant negative UbcH10. Destruction of each is terminated by a dominant activity that appears in nuclei near the start of S phase. Previous work indicates that the APC/C-dependent destruction of anaphase inhibitors is activated after chromosome alignment at the metaphase plate. In support of this, we show that addition of dominant negative UbcH10 to G1 extracts blocks destruction of the yeast anaphase inhibitor Cut2p in vitro, and injection of dominant negative UbcH10 blocks anaphase onset in vivo. Finally, we report that injection of dominant negative Ubc3/Cdc34, whose role in G1-S control is well established and has been implicated in kinetochore function during mitosis in yeast, dramatically interferes with congression of chromosomes to the metaphase plate. These results demonstrate that the regulated ubiquitination and destruction of critical mitotic proteins is highly conserved from yeast to humans.

Regulation of APC activity by phosphorylation and regulatory factors

Ubiquitin-dependent proteolysis of Cut2/Pds1 and Cyclin B is 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. However, the exact regulatory mechanism of substrate-specific activation of mammalian APC with the right timing remains to be elucidated. We found that not only the binding of the activators Cdc20 and Cdh1 and the inhibitor Mad2 to APC, but also the phosphorylation of Cdc20 and Cdh1 by Cdc2-Cyclin B and that 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.

Cdc20 associates with the kinase aurora2/Aik

Cdc20/fizzy family proteins are involved in activation of the anaphase-promoting complex/cyclosome, which catalyzes the ubiquitin-dependent proteolysis of cell cycle regulatory proteins such as anaphase inhibitors and mitotic cyclins, leading to chromosome segregation and exit from mitosis. Previous work has shown that human Cdc20 (hCdc20/p55CDC) associates with one or more kinases. We report here that Cdc20-associated myelin basic protein kinase activity peaks sharply in early M phase (embryonic cells) or in G2 phase (somatic cells). In HeLa cells, Cdc20 is associated with the kinase aurora2/Aik. Aurora2/Aik is a member of the aurora/Ipl1 family of kinases that, like Cdc20, previously has been shown to be localized at mitotic spindle poles and is involved in regulating chromosome segregation and maintaining genomic stability. The demonstration that Cdc20 is associated with aurora2/Aik suggests that some function of Cdc20 is carried out or regulated through its association with aurora2/Aik.

Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae

The spindle checkpoint arrests cells in mitosis in response to defects in the assembly of the mitotic spindle or errors in chromosome alignment. We determined which spindle defects the checkpoint can detect by examining the interaction of mutations that compromise the checkpoint (mad1, mad2, and mad3) with those that damage various structural components of the spindle. Defects in microtubule polymerization, spindle pole body duplication, microtubule motors, and kinetochore components all activate the MAD-dependent checkpoint. In contrast, the cell cycle arrest caused by mutations that induce DNA damage (cdc13), inactivate the cyclin proteolysis machinery (cdc16 and cdc23), or arrest cells in anaphase (cdc15) is independent of the spindle checkpoint.

Temporal and spatial control of cyclin B1 destruction in metaphase

The proteolysis of key regulatory proteins is thought to control progress through mitosis. Here we analyse cyclin B1 degradation in real time and find that it begins as soon as the last chromosome aligns on the metaphase plate, just after the spindle-assembly checkpoint is inactivated. At this point, cyclin B1 staining disappears from the spindle poles and from the chromosomes. Cyclin B1 destruction can subsequently be inactivated throughout metaphase if the spindle checkpoint is reimposed, and this correlates with the reappearance of cyclin B1 on the spindle poles and the chromosomes. These results provide a temporal and spatial link between the spindle-assembly checkpoint and ubiquitin-mediated proteolysis.

Ca2+ is required for phosphorylation of clam p82/CPEB in vitro: implications for dual and independent roles of MAP and Cdc2 kinases

During early development gene expression is controlled principally at the translational level. Oocytes of the surf clam Spisula solidissima contain large stockpiles of maternal mRNAs which are translationally dormant or masked until meiotic maturation. Fertilisation of the oocyte leads to rapid polysomal recruitment of the abundant cyclin and ribonucleotide reductase mRNAs at about the time they undergo cytoplasmic polyadenylation. Clam p82, a 3' UTR RNA-binding protein, and a member of the CPEB (cytoplasmic polyadenylation element binding protein) family, functions as a translational masking factor in oocytes and as a polyadenylation factor in fertilised eggs. In meiotically maturing clam oocytes, p82/CPEB is rapidly phosphorylated on multiple residues to a 92-kDa apparent size, prior to its degradation during the first cell cleavage. Here we examine the protein kinase(s) that phosphorylates clam p82/CPEB using a clam oocyte activation cell-free system that responds to elevated pH, mirroring the pH rise that accompanies fertilisation. We show that p82/CPEB phosphorylation requires Ca2+ (<100 microM) in addition to raised pH. Examination of the calcium dependency combined with the use of specific inhibitors implicates the combined and independent actions of cdc2 and MAP kinases in p82/CPEB phosphorylation. Calcium is necessary for both the activation and the maintenance of MAP kinase, whose activity is transient in vitro, as in vivo. While cdc2 kinase plays a role in the maintenance of MAP kinase activity, it is not required for the activation of MAP kinase. We propose a model of clam p82/CPEB phosphorylation in which MAP kinase initially phosphorylates clam p82/CPEB, at a minor subset of sites that does not alter its migration, and cdc2 kinase is necessary for the second wave of phosphorylation that results in the large mobility size shift of clam p82/CPEB. The possible roles of phosphorylation for the function and regulation of p82/CPEB are discussed.

Subunits and substrates of the anaphase-promoting complex

The initiation of anaphase and exit from mitosis depend on a ubiquitination complex called the anaphase-promoting complex (APC) or cyclosome. The APC is composed of more than 10 constitutive subunits and associates with additional regulatory factors in mitosis and during the G1 phase of the cell cycle. At the metaphase-anaphase transition the APC ubiquitinates proteins such as Pds1 in budding yeast and Cut2 in fission yeast whose subsequent degradation by the 26S proteasome is essential for the initiation of sister chromatid separation. Later in anaphase and telophase the APC promotes the inactivation of the mitotic cyclin-dependent protein kinase 1 by ubiquitinating its activating subunit cyclin B. The APC also mediates the ubiquitin-dependent proteolysis of several other mitotic regulators, including other protein kinases, APC activators, spindle-associated proteins, and inhibitors of DNA replication.

TIF1gamma, a novel member of the transcriptional intermediary factor 1 family

The Anaphase-Promoting Complex/Cyclosome (APC/C) is an ubiquitin ligase that functions during mitosis. Here we identify the transcriptional regulator, Transcriptional Intermediary Factor 1γ, TIF1γ as an APC/C-interacting protein that regulates APC/C function. TIF1γ is not a substrate for APC/C-dependent ubiquitylation but instead, associates specifically with the APC/C holoenzyme and Cdc20 to affect APC/C activity and progression through mitosis. RNA interference studies indicate that TIF1γ knockdown results in a specific reduction in APC/C ubiquitin ligase activity, the stabilization of APC/C substrates, and an increase in the time taken for cells to progress through mitosis from nuclear envelope breakdown (NEBD) to anaphase. TIF1γ knockdown cells are also characterized by the inappropriate presence of cyclin A at metaphase, and an increase in the number of cells that fail to undergo metaphase-to-anaphase transition. Expression of a siRNA-resistant TIF1γ species relieves the mitotic phenotype imposed by TIF1γ knockdown and allows for mitotic progression. Binding studies indicate that TIF1γ is also a component of the APC/C-Mitotic Checkpoint Complex (MCC), but is not required for MCC dissociation from the APC/C once the Spindle Assembly Checkpoint (SAC) is satisfied. TIF1γ inactivation also results in chromosome misalignment at metaphase, and SAC activation; inactivation of the SAC relieves the mitotic block imposed by TIF1γ knockdown. Together these data define novel functions for TIF1γ during mitosis and suggest that a reduction in APC/C ubiquitin ligase activity promotes SAC activation.

Dual roles of p82, the clam CPEB homolog, in cytoplasmic polyadenylation and translational masking

In the transcriptionally inert maturing oocyte and early embryo, control of gene expression is largely mediated by regulated changes in translational activity of maternal mRNAs. Some mRNAs are activated in response to poly(A) tail lengthening; in other cases activation results from de-repression of the inactive or masked mRNA. The 3' UTR cis-acting elements that direct these changes are defined, principally in Xenopus and mouse, and the study of their trans-acting binding factors is just beginning to shed light on the mechanism and regulation of cytoplasmic polyadenylation and translational masking. In the marine invertebrate, Spisula solidissima, the timing of activation of three abundant mRNAs (encoding cyclin A and B and the small subunit of ribonucleotide reductase, RR) in fertilized oocytes correlates with their cytoplasmic polyadenylation. However, in vitro, mRNA-specific unmasking occurs in the absence of polyadenylation. In Walker et al. (in this issue) we showed that p82, a protein defined as selectively binding the 3' UTR masking elements, is a homolog of Xenopus CPEB (cytoplasmic polyadenylation element binding protein). In functional studies reported here, the elements that support polyadenylation in clam egg lysates include multiple U-rich CPE-like motifs as well as the nuclear polyadenylation signal AAUAAA. This represents the first detailed analysis of invertebrate cis-acting cytoplasmic polyadenylation signals. Polyadenylation activity correlates with p82 binding in wild-type and CPE-mutant RR 3' UTR RNAs. Moreover, since anti-p82 antibodies specifically neutralize polyadenylation in egg lysates, we conclude that clam p82 is a functional homolog of Xenopus CPEB, and plays a positive role in polyadenylation. Anti-p82 antibodies also result in specific translational activation of masked mRNAs in oocyte lysates, lending support to our original model of clam p82 as a translational repressor. We propose therefore that clam p82/CPEB has dual functions in masking and cytoplasmic polyadenylation.

The clam 3' UTR masking element-binding protein p82 is a member of the CPEB family

During early development gene expression is controlled principally at the translational level. Oocytes of the surf clam Spisula solidissima contain large stockpiles of maternal mRNAs that are translationally dormant or masked until meiotic maturation. Activation of the oocyte by fertilization leads to translational activation of the abundant cyclin and ribonucleotide reductase mRNAs at a time when they undergo cytoplasmic polyadenylation. In vitro unmasking assays have defined U-rich regions located approximately centrally in the 3' UTRs of these mRNAs as translational masking elements. A clam oocyte protein of 82 kDa, p82, which selectively binds the masking elements, has been proposed to act as a translational repressor. Importantly, mRNA-specific unmasking in vitro occurs in the absence of poly(A) extension. Here we show that clam p82 is related to Xenopus CPEB, an RNA-binding protein that interacts with the U-rich cytoplasmic polyadenylation elements (CPEs) of maternal mRNAs and promotes their polyadenylation. Cloned clam p82/CPEB shows extensive homology to Xenopus CPEB and related polypeptides from mouse, goldfish, Drosophila and Caenorhabditis elegans, particularly in their RNA-binding C-terminal halves. Two short N-terminal islands of sequence, of unknown function, are common to vertebrate CPEBs and clam p82. p82 undergoes rapid phosphorylation either directly or indirectly by cdc2 kinase after fertilization in meiotically maturing clam oocytes, prior to its degradation during the first cell cleavage. Phosphorylation precedes and, according to inhibitor studies, may be required for translational activation of maternal mRNA. These data suggest that clam p82 may be a functional homolog of Xenopus CPEB.

p34cdc2 in invasive breast cancer: relationship to DNA content, Ki67 index and c-erbB-2 expression

AIMS

One-hundred and eighty-eight cases of human mammary carcinoma were examined immunohistochemically for their expression of Ki67, p34cdc2 and c-erbB-2. DNA image cytometry was performed to evaluate DNA ploidy, Auer type, S-phase fraction (SPF), 5c exceeding rate (5cER) and 2c deviation index (2cDI).

METHODS AND RESULTS

One-hundred and sixty-eight cases were invasive ductal carcinomas, 20 were of invasive lobular type. Routinely assessed oestrogen and progesterone receptor scores were available. The results were analysed statistically in comparison to tumour type, histopathological grade, lymph node status, menopausal status, patient age and overall survival. Ki67 (P < 0.002) and c-erbB-2 (P < 0.0001) correlated well with overall survival (P < 0.0008) and grade (P < 0.038) but not with lymph node status and tumour type. p34cdc2 showed a trend towards a positive correlation with Ki67 (P < 0.058) and a significant negative correlation with receptor status (P < 0.008) but with none of the other parameters examined.

CONCLUSIONS

No association between the DNA measured parameters (Auer type, SPF, 5cER and 2cDI) and survival was found. Our results suggest that c-erbB-2 and Ki67 are parameters which might, in combination with receptor status, help to define subgroups with different outcomes.

Cell cycle -dependent proteolysis in plants. Identification Of the destruction box pathway and metaphase arrest produced by the proteasome inhibitor mg132

It is widely assumed that mitotic cyclins are rapidly degraded during anaphase, leading to the inactivation of the cell cycle-dependent protein kinase Cdc2 and allowing exit from mitosis. The proteolysis of mitotic cyclins is ubiquitin/26S proteasome mediated and requires the presence of the destruction box motif at the N terminus of the proteins. As a first attempt to study cyclin proteolysis during the plant cell cycle, we investigated the stability of fusion proteins in which the N-terminal domains of an A-type and a B-type tobacco mitotic cyclin were fused in frame with the chloramphenicol acetyltransferase (CAT ) reporter gene and constitutively expressed in transformed tobacco BY2 cells. For both cyclin types, the N-terminal domains led the chimeric cyclin-CAT fusion proteins to oscillate in a cell cycle-specific manner. Mutations within the destruction box abolished cell cycle-specific proteolysis. Although both fusion proteins were degraded after metaphase, cyclin A-CAT proteolysis was turned off during S phase, whereas that of cyclin B-CAT was turned off only during the late G2 phase. Thus, we demonstrated that mitotic cyclins in plants are subjected to post-translational control (e.g., proteolysis). Moreover, we showed that the proteasome inhibitor MG132 blocks BY2 cells during metaphase in a reversible way. During this mitotic arrest, both cyclin-CAT fusion proteins remained stable.

Regulation of the anaphase-promoting complex/cyclosome by bimAAPC3 and proteolysis of NIMA

Surprisingly, although highly temperature-sensitive, the bimA1(APC3) anaphase-promoting complex/cyclosome (APC/C) mutation does not cause arrest of mitotic exit. Instead, rapid inactivation of bimA1(APC3) is shown to promote repeating oscillations of chromosome condensation and decondensation, activation and inactivation of NIMA and p34(cdc2) kinases, and accumulation and degradation of NIMA, which all coordinately cycle multiple times without causing nuclear division. These bimA1(APC3)-induced cell cycle oscillations require active NIMA, because a nimA5 + bimA1(APC3) double mutant arrests in a mitotic state with very high p34(cdc2) H1 kinase activity. NIMA protein instability during S phase and G2 was also found to be controlled by the APC/C. The bimA1(APC3) mutation therefore first inactivates the APC/C but then allows its activation in a cyclic manner; these cycles depend on NIMA. We hypothesize that bimAAPC3 could be part of a cell cycle clock mechanism that is reset after inactivation of bimA1(APC3). The bimA1(APC3) mutation may also make the APC/C resistant to activation by mitotic substrates of the APC/C, such as cyclin B, Polo, and NIMA, causing mitotic delay. Once these regulators accumulate, they activate the APC/C, and cells exit from mitosis, which then allows this cycle to repeat. The data indicate that bimAAPC3 regulates the APC/C in a NIMA-dependent manner.

The ubiquitin system

The selective degradation of many short-lived proteins in eukaryotic cells is carried out by the ubiquitin system. In this pathway, proteins are targeted for degradation by covalent ligation to ubiquitin, a highly conserved small protein. Ubiquitin-mediated degradation of regulatory proteins plays important roles in the control of numerous processes, including cell-cycle progression, signal transduction, transcriptional regulation, receptor down-regulation, and endocytosis. The ubiquitin system has been implicated in the immune response, development, and programmed cell death. Abnormalities in ubiquitin-mediated processes have been shown to cause pathological conditions, including malignant transformation. In this review we discuss recent information on functions and mechanisms of the ubiquitin system. Since the selectivity of protein degradation is determined mainly at the stage of ligation to ubiquitin, special attention is focused on what we know, and would like to know, about the mode of action of ubiquitin-protein ligation systems and about signals in proteins recognized by these systems.

The plant cell cycle in context

Biological scientists are eagerly confronting the challenge of understanding the regulatory mechanisms that control the cell division cycle in eukaryotes. New information will have major implications for the treatment of growth-related diseases and cancer in animals. In plants, cell division has a key role in root and shoot growth as well as in the development of vegetative storage organs and reproductive tissues such as flowers and seeds. Many of the strategies for crop improvement, especially those aimed at increasing yield, involve the manipulation of cell division. This review describes, in some detail, the current status of our understanding of the regulation of cell division in eukaryotes and especially in plants. It also features an outline of some preliminary attempts to exploit transgenesis for manipulation of plant cell division.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Regulation of the cyclin B degradation system by an inhibitor of mitotic proteolysis

The initiation of anaphase and exit from mitosis depend on the anaphase-promoting complex (APC), which mediates the ubiquitin-dependent proteolysis of anaphase-inhibiting proteins and mitotic cyclins. We have analyzed whether protein phosphatases are required for mitotic APC activation. In Xenopus egg extracts APC activation occurs normally in the presence of protein phosphatase 1 inhibitors, suggesting that the anaphase defects caused by protein phosphatase 1 mutation in several organisms are not due to a failure to activate the APC. Contrary to this, the initiation of mitotic cyclin B proteolysis is prevented by inhibitors of protein phosphatase 2A such as okadaic acid. Okadaic acid induces an activity that inhibits cyclin B ubiquitination. We refer to this activity as inhibitor of mitotic proteolysis because it also prevents the degradation of other APC substrates. A similar activity exists in extracts of Xenopus eggs that are arrested at the second meiotic metaphase by the cytostatic factor activity of the protein kinase mos. In Xenopus eggs, the initiation of anaphase II may therefore be prevented by an inhibitor of APC-dependent ubiquitination.

Differential subcellular localization of protein phosphatase-1 alpha, gamma1, and delta isoforms during both interphase and mitosis in mammalian cells

Protein phosphatase-1 (PP-1) is involved in the regulation of numerous metabolic processes in mammalian cells. The major isoforms of PP-1, alpha, gamma1, and delta, have nearly identical catalytic domains, but they vary in sequence at their extreme NH2 and COOH termini. With specific antibodies raised against the unique COOH-terminal sequence of each isoform, we find that the three PP-1 isoforms are each expressed in all mammalian cells tested, but that they localize within these cells in a strikingly distinct and characteristic manner. Each isoform is present both within the cytoplasm and in the nucleus during interphase. Within the nucleus, PP-1 alpha associates with the nuclear matrix, PP-1 gamma1 concentrates in nucleoli in association with RNA, and PP-1 delta localizes to nonnucleolar whole chromatin. During mitosis, PP-1 alpha is localized to the centrosome, PP-1 gamma1 is associated with microtubules of the mitotic spindle, and PP-1 delta strongly associates with chromosomes. We conclude that PP-1 isoforms are targeted to strikingly distinct and independent sites in the cell, permitting unique and independent roles for each of the isoforms in regulating discrete cellular processes.

In vivo regulation of cyclin A/Cdc2 and cyclin B/Cdc2 through meiotic and early cleavage cycles in starfish

In starfish, fertilization occurs naturally at late meiosis I. In the absence of fertilization, however, oocytes complete meiosis I and II, resulting in mature eggs arrested at the pronucleus stage, which are still fertilizable. In this study, we isolated cDNAs of starfish cyclin A and Cdc2, and monitored extensively the cell cycle dynamics of cyclin A and cyclin B levels and their associated Cdc2 kinase activity, Tyr phosphorylation of Cdc2, and Cdc25 phosphorylation states throughout meiotic and early embryonic cleavage cycles in vivo. In meiosis I, cyclin A was undetectable and cyclin B/Cdc2 alone exhibited histone H1 kinase activity, while thereafter both cyclin A/Cdc2 and cyclin B/Cdc2 kinase activity oscillated along with the cell cycle. Cyclin B-, but not cyclin A-, associated Cdc2 was subjected to regulation via Tyr phosphorylation, and phosphorylation states of Cdc25 correlated with cyclin B/Cdc2 kinase activity with some exceptions. Between meiosis I and II and at the pronucleus stage, cyclin A and B levels remained low, Cdc2 Tyr phosphorylation was undetectable, and Cdc25 remained phosphorylated depending on MAP kinase activity, showing a good correlation between these two stages. Upon fertilization of mature eggs, Cdc2 Tyr phosphorylation reappeared and Cdc25 was dephosphorylated. In the first cleavage cycle, under conditions which prevented Cdc25 activity, cyclin A/Cdc2 was activated with a normal time course and then cyclin B/Cdc2 was activated with a significant delay, resulting in the delayed completion of M-phase. Thus, in contrast to meiosis I, both cyclin A and cyclin B appear to be involved in the embryonic cleavage cycles. We propose that regulation of cyclin A/Cdc2 and cyclin B/Cdc2 is characteristic of meiotic and early cleavage cycles.

Reprogramming the cell cycle for endoreduplication in rodent trophoblast cells

Differentiation of trophoblast giant cells in the rodent placenta is accompanied by exit from the mitotic cell cycle and onset of endoreduplication. Commitment to giant cell differentiation is under developmental control, involving down-regulation of Id1 and Id2, concomitant with up-regulation of the basic helix-loop-helix factor Hxt and acquisition of increased adhesiveness. Endoreduplication disrupts the alternation of DNA synthesis and mitosis that maintains euploid DNA content during proliferation. To determine how the mammalian endocycle is regulated, we examined the expression of the cyclins and cyclin-dependent kinases during the transition from replication to endoreduplication in the Rcho-1 rat choriocarcinoma cell line. We cultured these cells under conditions that gave relatively synchronous endoreduplication. This allowed us to study the events that occur during the transition from the mitotic cycle to the first endocycle. With giant cell differentiation, the cells switched cyclin D isoform expression from D3 to D1 and altered several checkpoint functions, acquiring a relative insensitivity to DNA-damaging agents and a coincident serum independence. The initiation of S phase during endocycles appeared to involve cycles of synthesis of cyclins E and A, and termination of S was associated with abrupt loss of cyclin A and E. Both cyclins were absent from gap phase cells, suggesting that their degradation may be necessary to allow reinitiation of the endocycle. The arrest of the mitotic cycle at the onset of endoreduplication was associated with a failure to assemble cyclin B/p34(cdk1) complexes during the first endocycle. In subsequent endocycles, cyclin B expression was suppressed. Together these data suggest several points at which cell cycle regulation could be targeted to shift cells from a mitotic to an endoreduplicative cycle.

Cell cycle arrest in cdc20 mutants of Saccharomyces cerevisiae is independent of Ndc10p and kinetochore function but requires a subset of spindle checkpoint genes

The spindle checkpoint ensures accurate chromosome segregation by inhibiting anaphase onset in response to altered microtubule function and impaired kinetochore function. In this study, we report that the ability of the anti-microtubule drug nocodazole to inhibit cell cycle progression in Saccharomyces cerevisiae depends on the function of the kinetochore protein encoded by NDC10. We examined the role of the spindle checkpoint in the arrest in cdc20 mutants that arrest prior to anaphase with an aberrant spindle. The arrest in cdc20 defective cells is dependent on the BUB2 checkpoint and independent of the BUB1, BUB3, and MAD spindle checkpoint genes. We show that the lesion recognized by Bub2p is not excess microtubules, and the cdc20 arrest is independent of kinetochore function. We show that Cdc20p is not required for cyclin proteolysis at two points in the cell cycle, suggesting that CDC20 is distinct from genes encoding integral proteins of the anaphase promoting complex.

Accumulation of cyclin B1, activation of cyclin B1-dependent kinase and induction of programmed cell death in human epidermoid carcinoma KB cells treated with taxol

Cyclin B1 plays a critical role in regulating cell-cycle progression from G2 through M phase (including exit from M phase). In this study, we investigated the relationship between taxol-induced M-phase arrest, disruption of the cyclin B1-regulation pathway and apoptosis in KB cells. Continuous exposure of KB cells to 0.5 microg/ml taxol caused mitotic arrest and >90% cell death at 48 hr. Mitotic blockade peaked at 24 hr, with 68% of cells in mitosis at that time compared with 3% at baseline, and decreased thereafter. Apoptosis assessed by morphological changes and DNA ladder fragmentation was a later event, peaking at 48 hr (later time points were not studied). Taxol also caused an increase in cyclin B1 accumulation, as assessed by Western blot analysis, and stimulated cyclin B1-dependent kinase. Cyclin B1 accumulation and kinase stimulation peaked at 12 and 24 hr, respectively, at which times they were 5-fold and 90-fold higher than in control untreated cells. These effects decreased thereafter. All taxol-induced cellular effects were abrogated by the protein and RNA synthesis inhibitors cycloheximide and actinomycin D. In contrast, the endonuclease inhibitors aurintricarboxilic acid and zinc markedly inhibited taxol-induced DNA ladder fragmentation without altering taxol-induced cell-cycle arrest, cyclin B1 accumulation, activation of cyclin B1 kinase activity and cytotoxicity. We conclude that taxol-induced stimulation of cyclin B1-dependent kinase activity parallels mitotic arrest, is more pronounced than mitotic arrest and precedes the induction of programmed cell death.

The polo-like kinase Plx1 is required for M phase exit and destruction of mitotic regulators in Xenopus egg extracts

Polo-like kinases (Plks), named after the Drosophila gene product polo, have been implicated in the regulation of multiple aspects of mitotic progression, including the activation of the Cdc25 phosphatase, bipolar spindle formation and cytokinesis. Genetic analyses performed in yeast and Drosophila suggest a function for Plks at late stages of mitosis, but biochemical data to support such a function in vertebrate organisms are lacking. Here we have taken advantage of Xenopus egg extracts for exploring the function of Plx1, a Xenopus Plk, during the cell cycle transition from M phase to interphase (I phase). We found that the addition of a catalytically inactive Plx1 mutant to M phase-arrested egg extracts blocked their Ca2+-induced release into interphase. Concomitantly, the proteolytic destruction of several targets of the anaphase-promoting complex and the inactivation of the Cdc2 protein kinase (Cdk1) were prevented. Moreover, the M to I phase transition could be abolished by immunodepletion of Plx1, but was restored upon the addition of recombinant Plx1. These results demonstrate that the exit of egg extracts from M phase arrest requires active Plx1, and they strongly suggest an important role for Plx1 in the activation of the proteolytic machinery that controls the exit from mitosis.

The Polo-like kinase Cdc5p and the WD-repeat protein Cdc20p/fizzy are regulators and substrates of the anaphase promoting complex in Saccharomyces cerevisiae

Proteolysis mediated by the anaphase promoting complex (APC) has a crucial role in regulating the passage of cells through anaphase. Destruction of the anaphase inhibitor Pds1p is necessary for separation of sister chromatids, whereas destruction of the mitotic cyclin Clb2p is important for disassembly of the mitotic spindle, cytokinesis and re-replication of the genome. Pds1p proteolysis precedes that of Clb2p by at least 15 min, which helps to ensure that cells never re-replicate their genome before they have separated sister chromatids at the previous mitosis. What triggers Pds1p proteolysis and why does it not also trigger that of Clb2p? Apart from sharing a dependence on the APC, these two proteolytic events differ in their dependence on other cofactors. Pds1p proteolysis depends on a WD-repeat protein called Cdc20p, whereas Clb2p proteolysis depends on another, related WD protein called Hct1/Cdh1p. On the other hand, destruction of Clb2p, but not that of Pds1p, depends on the Polo-like kinase, Cdc5p. Cdc20p is essential for separation of sister chromatids, whereas Cdc5p is not. We show that both Cdc5p and Cdc20p are unstable proteins whose proteolysis is regulated by the APC. Both proteins accumulate during late G2/M phase and disappear at a late stage of anaphase. Accumulation of Cdc20p contributes to activation of Pds1p proteolysis in metaphase, whereas accumulation of Cdc5p facilitates the activation of Clb2p proteolysis.

A molecular rhythm mediating circadian clock output in Drosophila

Analysis of the Drosophila lark gene indicates that it encodes an RNA-binding protein that functions as a regulatory element of the circadian clock output pathway controlling adult eclosion. We now demonstrate that the lark RNA-binding protein oscillates in abundance during the circadian cycle; importantly, the phasing of the lark rhythm is consistent with gene-dosage studies, which indicate that the protein behaves as a repressor molecule. The lark protein rhythm persists in constant conditions (continuous darkness and constant temperature) and is eliminated by period gene null mutations, confirming that it is under clock control and suggesting that it acts as an output mechanism that mediates the temporal regulation of adult eclosion. We also show that lark protein oscillates in abundance within a defined group of neuropeptide (CCAP) -containing neurons of the ventral nervous system (VNS), which in other insects are thought to comprise cellular elements of the clock output pathway regulating eclosion.

Roles of ubiquitin-mediated proteolysis in cell cycle control

Selective degradation of cyclins, inhibitors of cyclin-dependent kinases and anaphase inhibitors is responsible for several major cell cycle transitions. The degradation of these cell cycle regulators is controlled by the action of ubiquitin-protein-ligase complexes, which target the regulators for degradation by the 26S proteasome. Recent results indicate that two types of multisubunit ubiquitin ligase complexes, which are connected to the protein kinase regulatory network of the cell cycle in different ways, are responsible for the specific and programmed degradation of many cell cycle regulators.

Thrombopoietin-induced polyploidization of bone marrow megakaryocytes is due to a unique regulatory mechanism in late mitosis

Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. However, the mechanism underlying this polyploidization remains totally unknown. It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication. We carried out immunohistochemical studies on mouse bone marrow megakaryocytes during thrombopoietin- induced polyploidization and found that during this process megakaryocytes indeed enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis. It was clearly observed that multiple spindle poles were formed as the polyploid megakaryocytes entered mitosis; the nuclear membrane broke down during prophase; the sister chromatids were aligned on a multifaced plate, and the centrosomes were symmetrically located on either side of each face of the plate at metaphase; and a set of sister chromatids moved into the multiple centrosomes during anaphase A. We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B. Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to a skipping of mitosis, and that the megakaryocytes must have a unique regulatory mechanism in anaphase, e.g., factors regulating anaphase such as microtubule motor proteins might be involved in this polyploidization process.

CDC20 and CDH1: a family of substrate-specific activators of APC-dependent proteolysis

Proteolysis mediated by the anaphase-promoting complex (APC) triggers chromosome segregation and exit from mitosis, yet its regulation is poorly understood. The conserved Cdc20 and Cdh1 proteins were identified as limiting, substrate-specific activators of APC-dependent proteolysis. CDC20 was required for the degradation of the APC substrate Pds1 but not for that of other APC substrates, such as Clb2 and Ase1. Conversely, cdh1Delta mutants were impaired in the degradation of Ase1 and Clb2 but not in that of Pds1. Overexpression of either CDC20 or CDH1 was sufficient to induce APC-dependent proteolysis of the appropriate target in stages of the cell cycle in which substrates are normally stable.

Intracellular pH increase driven by an Na+/H+ exchanger upon activation of surf clam oocytes

Intracellular pH (pHi) measurements were performed in surf clam (Spisula solidissima) oocytes before and after artificial activation or fertilization [evidenced by germinal vesicle breakdown (GVBD)] by the dimethyloxazolidinedione (DMO) and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) methods. Results using both methods showed increases of pHi of 0.3 pH unit after activation by excess K+. Using BCECF, we found an increase of similar magnitude after fertilization or after the addition of serotonin. By contrast, GVBD did not occur when the pHi was increased to similar or even higher levels by exposing the oocytes to ammonia. In sodium-free seawater, excess K+ induced GVBD but the pHi of K+-activated oocytes decreased significantly below the resting level of unactivated oocytes. The pHi increases in K+-activated oocytes were otherwise proportional to the external Na+ concentration. The amiloride derivatives dimethylamiloride and hexamethylene amiloride (at 10-50 microM) efficiently inhibited the K+-induced increase of pHi but did not block GVBD. These two derivatives were able, however, to retard K+-induced GVBD, hexamethylene amiloride being the more efficient. This retardation of K+-induced GVBD could be abolished by the simultaneous addition of ammonia. Taken altogether, these results show that a pHi increase, driven by a typical Na+/H+ exchanger, follows activation of surf clam oocytes but that this pHi increase is neither sufficient nor required for GVBD, though it does allow its progression at an optimal rate.

In vivo regulation of cytostatic activity in Xenopus metaphase II-arrested oocytes

Metaphase II arrest of Xenopus oocyte is characterized by the presence of M-phase-promoting factor (MPF) and of a microtubular spindle, both of which are stable in the presence of protein synthesis inhibitors. We studied in vivo this equilibrium state that is settled during meiotic maturation. At time of germinal vesicle breakdown (GVBD), cdc2 kinase and MAP kinase activities are stimulated. A component of the cyclin ubiquitin ligase, CDC27, is phosphorylated at the same time and remains phosphorylated until fertilization, indicating that an important component of the ligase complex is modified as early as GVBD. During a first period extending from GVBD until the cortical anchorage of the metaphase II spindle, homogeneous pools of cdc2 kinase and mitogen-activated protein (MAP) kinase activities are present in oocyte and are strictly dependent on protein turnover, since protein synthesis inhibition induces their total inactivation and drives oocytes into interphase. The metaphase II spindle, once anchored into the cortex, is no more sensitive to protein synthesis inhibition, likewise MAP kinase activity. During this cellular arrest, cdc2 kinase is divided into two distinctly regulated pools. The first one contains cyclin B that actively turns over and is subjected to a microtubular checkpoint. The second one is stable. Alteration of intracellular compartmentation of metaphase II oocytes either by gentle centrifugation or by cold shock inactivates MAP kinase and targets all cyclin B molecules for full destruction. We therefore suggest that MAP kinase participates to the cytostatic activity by preventing part of cyclin B molecules from entering the ubiquitination/degradation machinery which is still turned on in metaphase II oocytes.