Regulation of the APC and the exit from mitosis

Developmentally regulated expression of calponin isoforms and the effect of h2-calponin on cell proliferation

h2-calponin is found in both smooth muscle and nonmuscle cells, and its function remains to be established. Western blots with specific monoclonal antibodies detected significant expression of h2-calponin in the growing embryonic stomach and urinary bladder and the early pregnant uterus. Although the expression of h1-calponin is upregulated in the stomach and bladder during postnatal development, the expression of h2-calponin is decreased to low levels in quiescent smooth muscle cells. To investigate a hypothesis that h2-calponin regulates the function of the actin cytoskeleton during cytokinesis, a smooth muscle-originated cell line (SM3) lacking calponin was transfected to express either sense or antisense h2-calponin cDNA and the effects on the rates of cell proliferation were examined. Both stable and transient sense cDNA-transfected cells had a significantly decreased proliferation rate compared with the antisense cDNA-transfected or nontransfected cells. Immunofluorescence microscopy showed that the force-expressed h2-calponin was associated with actin-tropomyosin microfilaments. The number of binuclear cells was significantly greater in the sense cDNA-transfected culture, in which h2-calponin was concentrated in a nuclear ring structure formed by actin filaments. The results suggest that h2-calponin may regulate cytokinesis by inhibiting the activity of the actin cytoskeleton.

Spatial regulation of the guanine nucleotide exchange factor Lte1 in Saccharomyces cerevisiae

In budding yeast, activation of the small Ras-like GTPase Tem1 triggers exit from mitosis and cytokinesis. Tem1 is regulated by Bub2/Bfa1, a two-component GTPase-activating protein (GAP), and by Lte1, a putative guanine nucleotide exchange factor. Lte1 is confined to the bud cortex, and its spatial separation from Tem1 at the spindle pole body (SPB) is important to prevent untimely exit from mitosis. The pathways contributing to Lte1 asymmetry have not been elucidated. Here we show that establishment of Lte1 at the cortex occurs by an actin-independent mechanism, which requires activation of Cdc28/Cln kinase at START and Cdc42, a key regulator of cell polarity and cytoskeletal organisation. This defines a novel role for Cdc42 in late mitotic events. In turn, dissociation of Lte1 from the cortex in telophase depends on activation of the Cdc14 phosphatase. Ectopic expression of Cdc14 at metaphase results in premature dephosphorylation of Lte1 coincident with its release from the cortex. In vitro phosphatase assays confirm that Lte1 is a direct substrate for Cdc14. Our results suggest that the asymmetry in Lte1 localisation is imposed by Cdc28-dependent phosphorylation. Finally, we report a mutational analysis undertaken to investigate intrinsic Lte1 determinants for localisation. Our data suggest that an intrameric interaction between the N-and C-terminal regions of Lte1 is important for cortex association.

The destruction box of the cyclin Clb2 binds the anaphase-promoting complex/cyclosome subunit Cdc23

Properly regulated cyclin proteolysis is critical for normal cell cycle progression. A nine-amino acid peptide motif called the destruction box (D box) is present at the N terminus of the yeast mitotic cyclins. This short sequence is required for cyclin ubiquitination and subsequent proteolysis. The anaphase-promoting complex/cyclosome (APC/C) is a multisubunit E3 required for cyclin ubiquitination. We have tested the D box of five mitotic cyclins for interaction with six APC/C subunits. The APC/C subunit Cdc23, but not five other subunits tested, interacted by two-hybrid analysis with the N terminus of wild-type Clb2. None of these subunits interacted with the N termini of the cyclins Clb1, Clb3, or Clb5. Mutations in the D box sequences of Clb2 inhibited interaction with Cdc23 both in vivo and in vitro. Our results provide the first evidence for a direct interaction between an APC/C substrate (Clb2) and an APC/C subunit (Cdc23).

Molecular characterization of plant ubiquitin-conjugating enzymes belonging to the UbcP4/E2-C/UBCx/UbcH10 gene family

The anaphase promoting complex or cyclosome is the ubiquitin-ligase that targets destruction box-containing proteins for proteolysis during the cell cycle. Anaphase promoting complex or cyclosome and its activator (the fizzy and fizzy-related) proteins work together with ubiquitin-conjugating enzymes (UBCs) (E2s). One class of E2s (called E2-C) seems specifically involved in cyclin B1 degradation. Although it has recently been shown that mammalian E2-C is regulated at the protein level during the cell cycle, not much is known concerning the expression of these genes. Arabidopsis encodes two genes belonging to the E2-C gene family (called UBC19 and UBC20). We found that UBC19 is able to complement fission yeast (Schizosaccharomyces pombe) UbcP4-140 mutant, indicating that the plant protein can functionally replace its yeast ortholog for protein degradation during mitosis. In situ hybridization experiments were performed to study the expression of the E2-C genes in various tissues of plants. Their transcripts were always, but not exclusively, found in tissues active for cell division. Thus, the UBC19/20 E2s may have a key function during cell cycle, but may also be involved in ubiquitylation reactions occurring during differentiation and/or in differentiated cells. Finally, we showed that a translational fusion protein between UBC19 and green fluorescent protein localized both in the cytosol and the nucleus in stable transformed tobacco (Nicotiana tabacum cv Bright Yellow 2) cells.

Cell cycle: new functions for Cdc14 family phosphatases

The Cdc14 phosphatase was identified by its requirement for mitotic exit in budding yeast. Cdc14 homologs exist throughout the eukaryotic kingdom, but it was unclear whether their function would also be conserved. Recent analyses in fission yeast, humans and now C. elegans suggest numerous other functions for this family of proteins.

Human p14(Arf): an exquisite sensor of morphological changes and of short-lived perturbations in cell cycle and in nucleolar function

The human Ink4a/Arf tumor suppressor locus encodes two distinct products: p16(Ink4a) which prevents phosphorylation and inactivation of the retinoblastoma protein and, p14(Arf), a nucleolar protein which activates the function of the tumor suppressor p53 protein in the nucleoplasm in response to oncogenic stimulation through an as yet ill-defined mechanism. Here we show that the level of endogenous p14(Arf) and its balance between the nucleolus and the nucleoplasm in HeLa cells are exquisitely sensitive to changes in cell morphology and to short-lived perturbations in cell cycle and in nucleolar function such as those induced by the cyclin-dependent kinase inhibitor, roscovitine, and the casein kinase II and RNA synthesis inhibitor, DRB. Most remarkably, whereas p14(Arf) predominantly concentrates in the nucleolus of interphase cells and transiently disappears between metaphase and early G1 under normal growth conditions, it massively and reversibly accumulates in the nucleoplasm of postmitotic and S-phase cells upon short-term treatment with roscovitine and, at a lesser extent, DRB. In line with the fact that the nuclear level of p53 reaches a peak between mid-G1 and the G1/S border in p53-expressor cells which lack Arf expression, these results provide a clue that, in p53+/Arf+ cells, Arf proteins might serve both to speed and to amplify p53-mediated responses in conditions and cell cycle periods in which the mechanisms involved in p53 stabilization and activation are not fully operational. They further suggest that human endogenous p14(Arf) might activate p53 pathways in physiologic situations by acting inside the nucleoplasm, especially when normal cell cycle progression and nucleolar function are compromised.

The ubiquitin-dependent targeting pathway in Saccharomyces cerevisiae plays a critical role in multiple chromatin assembly regulatory steps

In a screen designed to isolate Saccharomyces cerevisiae strains defective for in vitro chromatin assembly, two temperature-sensitive (ts) mutants were obtained: rmc1 and rmc3 (remodeling of chromatin). Cloning of RMC1 and RMC3 revealed a broad role for the ubiquitin-dependent targeting cascade as the ubiquitin-protein ligases (E3s), the anaphase promoting complex (APC; RMC1 encodes APC5) and Rsp5p, respectively, were identified. Genetic studies linked the rmc1/apc5 chromatin assembly defect to APC function: rmc1/apc5 genetically interacted with apc9Delta, apc10Delta, and cdc26Delta mutants. Furthermore, phenotypes associated with the rmc1/apc5 allele were consistent with defects in chromatin metabolism and in APC function: (i) UV sensitivity, (ii) plasmid loss, (iii) accumulation of G2/M cells, and (iv) suppression of the ts defect by growth on glucose-free media and by expression of ubiquitin. On the other hand, the multifunctional E3, Rsp5p, was shown to be required for both in vitro and in vivo chromatin assembly, as well as for the proper transcriptional and translational control of at least histone H3. The finding that the distinctly different E3 enzymes, APC and Rsp5p, both play roles in regulating chromatin assembly highlight the depth of the regulatory networks at play. The significance of these findings will be discussed.

Determinants of Swe1p degradation in Saccharomyces cerevisiae

Swe1p, the sole Wee1-family kinase in Saccharomyces cerevisiae, is synthesized during late G1 and is then degraded as cells proceed through the cell cycle. However, Swe1p degradation is halted by the morphogenesis checkpoint, which responds to insults that perturb bud formation. The Swe1p stabilization promotes cell cycle arrest through Swe1p-mediated inhibitory phosphorylation of Cdc28p until the cells can recover from the perturbation and resume bud formation. Swe1p degradation involves the relocalization of Swe1p from the nucleus to the mother-bud neck, and neck targeting requires the Swe1p-interacting protein Hsl7p. In addition, Swe1p degradation is stimulated by its substrate, cyclin/Cdc28p, and Swe1p is thought to be a target of the ubiquitin ligase SCF(Met30) acting with the ubiquitin-conjugating enzyme Cdc34p. The basis for regulation of Swe1p degradation by the morphogenesis checkpoint remains unclear, and in order to elucidate that regulation we have dissected the Swe1p degradation pathway in more detail, yielding several novel findings. First, we show here that Met30p (and by implication SCF(Met30)) is not, in fact, required for Swe1p degradation. Second, cyclin/Cdc28p does not influence Swe1p neck targeting, but can directly phosphorylate Swe1p, suggesting that it acts downstream of neck targeting in the Swe1p degradation pathway. Third, a screen for functional but nondegradable mutants of SWE1 identified two small regions of Swe1p that are key to its degradation. One of these regions mediates interaction of Swe1p with Hsl7p, showing that the Swe1p-Hsl7p interaction is critical for Swe1p neck targeting and degradation. The other region did not appear to affect interactions with known Swe1p regulators, suggesting that other as-yet-unknown regulators exist.

Aurora-A kinase interacting protein (AIP), a novel negative regulator of human Aurora-A kinase

Aurora kinases have evolved as a new family of mitotic centrosome- and microtubule-associated kinases that regulate the structure and function of centrosomes and spindle. One of its members, Aurora-A, is a potential oncogene. Overexpression of Aurora-A is also implicated in defective centrosome duplication and segregation, leading to aneuploidy and tumorigenesis in various cancer cell types. However, the regulatory pathways for mammalian Aurora-A are not well understood. Exploiting the lethal phenotype associated with the overexpression of Aurora-A in yeast, we performed a dosage suppressor screen in yeast and report here the identification of a novel negative regulator of Aurora-A, named AIP (Aurora-A kinase Interacting Protein). AIP is a ubiquitously expressed nuclear protein that interacts specifically with human Aurora-A in vivo. Ectopic expression of AIP with Aurora-A in NIH 3T3 and COS cells results in the down-regulation of ectopically expressed Aurora-A protein levels, and this down-regulation is demonstrated to be the result of destabilization of Aurora-A through a proteasome-dependent protein degradation pathway. A noninteracting deletion mutant of AIP does not down-regulate Aurora-A protein, suggesting that the interaction is important for the protein degradation. AIP could therefore be a potential useful target gene for anti-tumor drugs.

Cytokinetic actomyosin ring formation and septation in fission yeast are dependent on the full recruitment of the polo-like kinase Plo1 to the spindle pole body and a functional spindle assembly checkpoint

In dividing cells, the assembly and contraction of the cytokinetic actomyosin ring (CAR) is precisely coordinated with spindle formation and chromosome segregation. Despite having a cell wall, the fission yeast Schizosaccharomyces pombe forms a CAR reminiscent of the structure responsible for the cleavage of cells with flexible boundaries. We used the myo2-gc fission yeast strain in which the chromosomal copy of the type II myosin gene, myo2(+), is fused to the gene encoding green fluorescent protein (GFP) to investigate the dynamics of Myo2 recruitment to the cytokinetic actomyosin ring in living cells. Analysis of CAR formation in relation to spindle pole body (SPB) and centromere separation enabled us to pinpoint the timing of Myo2 recruitment into a stable CAR structure to the onset of anaphase A. Depolymerisation of actin with latrunculin B did not affect the timing of Myo2 accumulation at the cell equator (although Myo2 no longer formed a ring), whereas depolymerisation of microtubules with either thiabendazole (TBZ) or methyl 2-benzimidazolecarbamate (MBC) resulted in a delay of up to 90 minutes in CAR formation. Microtubule depolymerisation also delayed the localisation of other CAR components such as actin and Mid1/Dmf1. The delay of cytokinesis in response to loss of microtubule integrity was abolished in cells lacking the spindle assembly checkpoint protein Mad2 or containing non-functional Cdc16, a component of the fission yeast septation initiation network (SIN). The delay was also abolished in cells lacking Zfs1, a component of the previously described S. pombe cytokinesis checkpoint. Recruitment of the polo-related kinase, Plo1, a key regulator of CAR formation, to the SPBs was substantially reduced in TBZ in a Mad2-dependent manner. Loading of Cdc7, a component of the SIN and downstream of Plo1 in the cytokinesis pathway, onto the the SPBs was also delayed in TBZ to the same extent as CAR formation. We conclude that CAR formation is subject to regulation by the spindle assembly checkpoint via the loading of Plo1 onto the SPBs and the consequent activation of the SIN.

Timing of APC/C substrate degradation is determined by fzy/fzr specificity of destruction boxes

The anaphase promoting complex/cyclosome (APC/C), activated by fzy and fzr, degrades cell cycle proteins that carry RXXL or KEN destruction boxes (d-boxes). APC/C substrates regulate sequential events and must be degraded in the correct order during mitosis and G(1). We studied how d-boxes determine APC/C(fzy)/APC/C(fzr) specificity and degradation timing. Cyclin B1 has an RXXL box and is degraded by both APC/C(fzy) and APC/C(fzr); fzy has a KEN box and is degraded by APC/C(fzr) only. We characterized the degradation of substrates with swapped d-boxes. Cyclin B1 with KEN was degraded by APC/C(fzr) only. Fzy with RXXL could be degraded by APC/C(fzy) and APC/C(fzr). Interestingly, APC/C(fzy)- but not APC/C(fzr)-specific degradation is highly dependent on the location of RXXL. We studied degradation of tagged substrates in real time and observed that APC/C(fzr) is activated in early G(1). These observations demonstrate how d-box specificities of APC/C(fzy) and APC/C(fzr), and the successive activation of APC/C by fzy and fzr, establish the temporal degradation pattern. Our observations can explain further why some endogenous RXXL substrates are degraded by APC/C(fzy), while others are restricted to APC/C(fzr).

Farnesylation of Cenp-F is required for G2/M progression and degradation after mitosis

Farnesyl transferase inhibitors induce G2/M cell cycle delays that cannot be explained by inhibition of the Ras GTPase. Recently, the kinetochore protein Cenp-F has been shown to be farnesylated. Here, we show that ectopic expression of the kinetochore targeting domain of Cenp-F delays progression through G2/M. Significantly, this is dependent on the CAAX farnesylation motif. We also show that localisation of Cenp-F to the nuclear envelope at G2/M and kinetochores in prometaphase is dependent both on its CAAX motif and farnesyl transferase activity. Strikingly, farnesyl transferase activity is also required for Cenp-F degradation after mitosis. Thus, these observations suggest that farnesylation of Cenp-F is required not only for its localisation to the nuclear envelope and kinetochores but also for timely progression through G2/M and its degradation after mitosis. In addition, these observations raise the possibility that the anti-proliferative effects induced by farnesyl transferase inhibitors may be due to inhibition of Cenp-F function and/or turnover.

The oncogenic potential of Kaposi's sarcoma-associated herpesvirus cyclin is exposed by p53 loss in vitro and in vivo

Expression of the Kaposi's sarcoma-associated herpesvirus (KSHV) cyclin D homolog, K cyclin, is thought to contribute to viral oncogenesis. We show that K cyclin expression in primary cells sensitizes to apoptosis and induces growth arrest, both of which are dependent on p53 but independent of E2F1 or p19(ARF). DNA synthesis, but not cytokinesis, continues in K cyclin-expressing cells, leading to multinucleation and polyploidy. Such polyploid cells exhibit pronounced centrosome amplification and consequent aneuploidy. Our data suggest that K cyclin expression leads to cytokinesis defects and polyploidy, which activates p53. However, in the absence of p53, such cells survive and expand as an aneuploid population. Corroborating these findings, in vivo Emu; K cyclin expression cooperates with p53 loss in the induction of lymphomas.

A spindle checkpoint arrest and a cytokinesis failure by the dominant-negative polo-box domain of Plk1 in U-2 OS cells

Polo kinases play critical roles for proper M-phase progression. They are characterized by the presence of two regions of homology in the C-terminal non-catalytic domain, termed polo-box 1 (PB1) and polo-box 2 (PB2). Here we demonstrate that both PB1 and PB2 are required for targeting the catalytic activity of Plk1 to centrosomes, midbody, and kinetochores. Expression of either kinase-inactive PLK1/K82M or the C-terminal plk1 Delta N induced a pre-anaphase arrest with elevated Cdc2 and Plk1 activity. Prophase-arrested cells exhibited randomly oriented spindle structures, whereas metaphase cells exhibited aberrant bipolar spindles with Mad2 localization at kinetochores of misaligned chromosomes. Microtubule nucleation activity of centrosomes was not compromised. In vivo time-lapse studies revealed that expression of plk1 Delta N resulted in repeated cycles of bipolar spindle formation and disruption, suggestive of a defect in spindle stability. A prolonged arrest frequently led to the generation of micronucleated cells in the absence of sisterchromatid separation and centrosome duplication, indicating that micronucleation is not a result of accumulated cytokinesis failures. Interestingly, bypass of the mitotic arrest by dominant-negative spindle checkpoint components led to a failure in completion of cytokinesis. We propose that, in mammalian cells, the polo-box-dependent Plk1 activity is required for proper metaphase/anaphase transition and for cytokinesis.

APC-dependent proteolysis of the mitotic cyclin Clb2 is essential for mitotic exit

Cyclin degradation is central to regulation of the cell cycle. Mitotic exit was proposed to require degradation of the S phase cyclin Clb5 by the anaphase-promoting complex activated by Cdc20 (APC(Cdc20)). Furthermore, Clb5 degradation was thought to be necessary for effective dephosphorylation and activation of the APC regulatory subunit Cdh1 (also known as Hct1) and the cyclin-dependent kinase inhibitor Sic1 by the phosphatase Cdc14, allowing mitotic kinase inactivation and mitotic exit. Here we show, however, that spindle disassembly and cell division occur without significant APC(Cdc20)-mediated Clb5 degradation, as well as in the absence of both Cdh1 and Sic1. We find instead that destruction-box-dependent degradation of the mitotic cyclin Clb2 is essential for mitotic exit. APC(Cdc20) may be required for an essential early phase of Clb2 degradation, and this phase may be sufficient for most aspects of mitotic exit. Cdh1 and Sic1 may be required for further inactivation of Clb2-Cdk1, regulating cell size and the length of G1.

The dynamic localisation of the Drosophila APC/C: evidence for the existence of multiple complexes that perform distinct functions and are differentially localised

In Drosophila cells, the destruction of cyclin B is spatially regulated. In cellularised embryos, cyclin B is initially degraded on the mitotic spindle and is then degraded in the cytoplasm. In syncytial embryos,only the spindle-associated cyclin B is degraded at the end of mitosis. The anaphase promoting complex/cyclosome (APC/C) targets cyclin B for destruction,but its subcellular localisation remains controversial. We constructed GFP fusions of two core APC/C subunits, Cdc16 and Cdc27. These fusion proteins were incorporated into the endogenous APC/C and were largely localised in the cytoplasm during interphase in living syncytial embryos. Both fusion proteins rapidly accumulated in the nucleus prior to nuclear envelope breakdown but only weakly associated with mitotic spindles throughout mitosis. Thus, the global activation of a spatially restricted APC/C cannot explain the spatially regulated destruction of cyclin B. Instead, different subpopulations of the APC/C must be activated at different times to degrade cyclin B. Surprisingly,we noticed that GFP-Cdc27 associated with mitotic chromosomes, whereas GFP-Cdc16 did not. Moreover, reducing the levels of Cdc16 or Cdc27 by >90%in tissue culture cells led to a transient mitotic arrest that was both biochemically and morphologically distinct. Taken together, our results raise the intriguing possibility that there could be multiple forms of the APC/C that are differentially localised and perform distinct functions.

Defects in G1-S cell cycle control in head and neck cancer: a review

Tumors gradually develop as a result of a multistep acquisition of genetic alterations and ultimately emerge as selfish, intruding and metastatic cells. The genetic defects associated with the process of tumor progression affect control of proliferation, programmed cell death, cell aging, angiogenesis, escape from immune control and metastasis. Fundamental cancer research over the last thirty years has revealed a multitude of genetic alterations which specify more or less separate steps in tumor development and which are collectively responsible for the process of tumor progression. The genes affected play in normal cells a crucial role in control over cell duplication and the interaction between cells, and between cells and their direct surrounding. This is illustrated on control during the G1/S phase of the cell cycle by its ultimate regulators: cyclins and cyclin dependent kinases. These proteins not only control the transition through the G1/S phase of the cell cycle, but also serve as mediators of the interaction between cells, and between cells and their surrounding. Defaults in the regulation of these proteins are associated with tumor progression, and, therefore, serve as targets for therapy. Defaults in those genes are found in various tumor types, although some of those prevail in particular tumor types. In this review emphasis is given to the defaults that occur in head and neck cancer.

MEN, destruction and separation: mechanistic links between mitotic exit and cytokinesis in budding yeast

Cellular events must be executed in a certain sequence during the cell division in order to maintain genome integrity and hence ensure a cell's survival. In M phase, for instance, chromosome segregation always precedes mitotic exit (characterized by mitotic kinase inactivation via cyclin destruction); this is then followed by cytokinesis. How do cells impose this strict order? Recent findings in budding yeast have suggested a mechanism whereby partitioning of chromosomes into the daughter cell is a prerequisite for the activation of mitotic exit network (MEN). So far, however, a regulatory scheme that would temporally link the initiation of cytokinesis to the execution of mitotic exit has not been determined. We propose that the requirement of MEN components for cytokinesis, their translocation to the mother-daughter neck and triggering of this translocation by inactivation of the mitotic kinase may be the three crucial elements that render initiation of cytokinesis dependent on mitotic exit.

Human securin proteolysis is controlled by the spindle checkpoint and reveals when the APC/C switches from activation by Cdc20 to Cdh1

Progress through mitosis is controlled by the sequential destruction of key regulators including the mitotic cyclins and securin, an inhibitor of anaphase whose destruction is required for sister chromatid separation. Here we have used live cell imaging to determine the exact time when human securin is degraded in mitosis. We show that the timing of securin destruction is set by the spindle checkpoint; securin destruction begins at metaphase once the checkpoint is satisfied. Furthermore, reimposing the checkpoint rapidly inactivates securin destruction. Thus, securin and cyclin B1 destruction have very similar properties. Moreover, we find that both cyclin B1 and securin have to be degraded before sister chromatids can separate. A mutant form of securin that lacks its destruction box (D-box) is still degraded in mitosis, but now this is in anaphase. This destruction requires a KEN box in the NH2 terminus of securin and may indicate the time in mitosis when ubiquitination switches from APCCdc20 to APCCdh1. Lastly, a D-box mutant of securin that cannot be degraded in metaphase inhibits sister chromatid separation, generating a cut phenotype where one cell can inherit both copies of the genome. Thus, defects in securin destruction alter chromosome segregation and may be relevant to the development of aneuploidy in cancer.

Inducing precocious anaphase in cultured mammalian cells

The spindle checkpoint, which prevents anaphase onset upon spindle damage or incorrect chromosome alignment, presents a problem for experimental analysis of protein function in anaphase and cytokinesis. This is because the functional disruption of many proteins before anaphase onset can activate this checkpoint, preventing anaphase and subsequent cell cycle events. This paper compares new and old methods of overriding the spindle checkpoint in prometaphase mammalian tissue culture cells.

The APC regulator CDH1 is essential for the progression of embryonic cell cycles in Xenopus

The orderly progression of cell cycle depends on timely destruction of key regulators through ubiquitin-mediated proteolysis. The anaphase-promoting complex (APC) is a major component of this degradation machinery and its activation is regulated by CDC20 and CDH1. We demonstrate here that CDH1 mRNA is ubiquitously expressed in Xenopus embryos of all developmental stages. Loss of CDH1 function during early embryonic cell cycles leads to an immediate and prolonged arrest with low cyclin-dependent kinase activity. In contrast, ectopic overexpression of CDH1 induces cell cycle arrest during the first G(1) phase at the midblastula transition. CDH1-dependent degradation of cyclin A is likely involved in this G(1) arrest. Our findings establish the essential roles of CDH1 in embryonic cell cycles.

Degradation of human Aurora-A protein kinase is mediated by hCdh1

Human Aurora-A is related to a protein kinase originally identified by its close homology to Ipl1p from Saccharomyces cerevisiae and aurora from Drosophila melanogaster, which are key regulators of the structure and function of the mitotic spindle. We previously showed that human Aurora-A is turned over through the anaphase promoting complex/cyclosome (APC/C)-ubiquitin-proteasome pathway. The association of two distinct WD40 repeat proteins known as Cdc20 and Cdh1, respectively, sequentially activates the APC/C. The present study shows that Aurora-A degradation is dependent on hCdh1 in vivo, not on hCdc20, and that Aurora-A is targeted for proteolysis through distinct structural features of the destruction box, the KEN box motifs and its kinase activity.

Transitions regulating the timing of cytokinesis in embryonic cells

Anaphase, mitotic exit, and cytokinesis proceed in rapid succession, and while mitotic exit is a requirement for cytokinesis in yeast, it may not be a direct requirement for furrow initiation in animal cells. In this report, we physically manipulated the proximity of the mitotic apparatus (MA) to the cell cortex in combination with microinjection of effectors of the spindle checkpoint and CDK1 activity to determine how the initiation of cytokinesis is coupled to the onset of anaphase and mitotic exit. Whereas precocious contact between the MA and the cell surface advanced the onset of cytokinesis into early anaphase A, furrowing could not be advanced prior to the metaphase-anaphase transition. Additionally, while cells arrested in anaphase could be induced to initiate cleavage furrows, cells arrested in metaphase could not. Finally, activation of the mitotic checkpoint in one spindle of a binucleate cell failed to arrest cytokinesis induced by the control spindle but did inhibit the formation of furrows between the arrested MA and the control, nonarrested MA. Our experiments suggest that the competence of the mitotic apparatus to initiate cytokinesis is not dependent on cyclin degradation but does require anaphase-promoting complex (APC) activity and, thus, inactivation of the mitotic checkpoint.

The cyclin-ubiquitin ligase activity of cyclosome/APC is jointly activated by protein kinases Cdk1-cyclin B and Plk

The cyclosome/anaphase-promoting complex is a multisubunit ubiquitin ligase that targets for degradation mitotic cyclins and some other cell cycle regulators in exit from mitosis. It becomes enzymatically active at the end of mitosis. The activation of the cyclosome is initiated by its phosphorylation, a process necessary for its conversion to an active form by the ancillary protein Cdc20/Fizzy. Previous reports have implicated either cyclin-dependent kinase 1-cyclin B or polo-like kinase as the major protein kinase that directly phosphorylates and activates the cyclosome. These conflicting results could be due to the use of partially purified cyclosome preparations or of immunoprecipitated cyclosome, whose interactions with protein kinases or ancillary factors may be hampered by binding to immobilized antibody. To examine this problem, we have purified cyclosome from HeLa cells by a combination of affinity chromatography and ion exchange procedures. With the use of purified preparations, we found that both cyclin-dependent kinase 1-cyclin B and polo-like kinase directly phosphorylated the cyclosome, but the pattern of the phosphorylation of the different cyclosome subunits by the two protein kinases was not similar. Each protein kinase could restore only partially the cyclin-ubiquitin ligase activity of dephosphorylated cyclosome. However, following phosphorylation by both protein kinases, an additive and nearly complete restoration of cyclin-ubiquitin ligase activity was observed. It is suggested that this joint activation may be due to the complementary phosphorylation of different cyclosome subunits by the two protein kinases.

The anaphase-promoting complex: proteolysis in mitosis and beyond

Key events in mitosis such as sister chromatid separation and subsequent inactivation of cyclin-dependent kinase 1 are regulated by ubiquitin-dependent proteolysis. These events are mediated by the anaphase-promoting complex (APC), a cell cycle-regulated ubiquitin ligase that assembles multiubiquitin chains on regulatory proteins such as securin and cyclins and thereby targets them for destruction by the 26S proteasome.

Ca(2+) oscillations promote APC/C-dependent cyclin B1 degradation during metaphase arrest and completion of meiosis in fertilizing mouse eggs

Cyclin B1, the regulatory component of M phase-promoting factor (MPF), is degraded during the metaphase-anaphase transition in an anaphase-promoting complex/cyclosome (APC/C)-dependent process. MPF activity is stable in eggs, and a sperm-triggered Ca(2+) signal is needed to promote cyclin degradation. In frogs, a single Ca(2+) spike promotes cell cycle resumption, but, in mammals, the Ca(2+) signal is more complex, consisting of a series of spikes that stop several hours after sperm fusion. Using dual imaging in mouse eggs, we have examined how the Ca(2+) signal generates cyclin B1 destruction using destructible and nondestructible GFP-tagged constructs. APC/C activity was present in unfertilized eggs, giving cyclin B1 a half-life of 1.15 +/- 0.28 hr. However, APC/C-dependent cyclin degradation was elevated 6-fold when sperm raised cytosolic Ca(2+) levels above 600 nM. This activation was transitory since cyclin B1 levels recovered between Ca(2+) spikes. For continued cyclin degradation at basal Ca(2+) levels, multiple spikes were needed. APC/C-mediated degradation was observed until eggs had completed meiosis with the formation of pronuclei, and, at this time, Ca(2+) spikes stopped. Therefore, the physiological need for a repetitive Ca(2+) signal in mammals is to ensure long-term cyclin destruction during a protracted exit from meiosis.

Cdc5 influences phosphorylation of Net1 and disassembly of the RENT complex

BACKGROUND

In S. cerevisiae, the mitotic exit network (MEN) proteins, including the Polo-like protein kinase Cdc5 and the protein phosphatase Cdc14, are required for exit from mitosis. In pre-anaphase cells, Cdc14 is sequestered to the nucleolus by Net1 as a part of the RENT complex. When cells are primed to exit mitosis, the RENT complex is disassembled and Cdc14 is released from the nucleolus.

RESULTS

Here, we show that Cdc5 is necessary to free nucleolar Cdc14 in late mitosis, that elevated Cdc5 activity provokes ectopic release of Cdc14 in pre-anaphase cells, and that the phosphorylation state of Net1 is regulated by Cdc5 during anaphase. Furthermore, recombinant Cdc5 and Xenopus Polo-like kinase can disassemble the RENT complex in vitro by phosphorylating Net1 and thereby reducing its affinity for Cdc14. Surprisingly, although RENT complexes containing Net1 mutants (Net1(7m) and Net1(19m') lacking sites phosphorylated by Cdc5 in vitro are refractory to disassembly by Polo-like kinases in vitro, net1(7m) and net1(19m') cells grow normally and exhibit only minor defects in releasing Cdc14 during anaphase. However, net1(19m') cells exhibit a synergistic growth defect when combined with mutations in CDC5 or DBF2 (another MEN gene).

CONCLUSIONS

We propose that although Cdc5 potentially disassembles RENT by directly phosphorylating Net1, Cdc5 mediates exit from mitosis primarily by phosphorylating other targets. Our study suggests that Cdc5/Polo is unusually promiscuous and highlights the need to validate Cdc5/Polo in vitro phosphorylation sites by direct in vivo mapping experiments.

Bfa1 can regulate Tem1 function independently of Bub2 in the mitotic exit network of Saccharomyces cerevisiae

In budding yeast, exit from mitosis is achieved by inactivation of Cdc28/Clb2 activity. Although it is not clear at present how mitotic exit is triggered, a growing body of evidence suggests that the Tem1 GTPase plays a critical role in mediating this pathway and that Bfa1 and Bub2 constitute a two-component GTPase-activating protein to negatively regulate Tem1. Here, we have demonstrated that introduction of bfa1 Delta suppresses the growth defects associated with the cdc5-1 mutation significantly better than that of bub2 Delta, suggesting that Bfa1 may have a previously uncharacterized role in this pathway. Overexpression of BFA1 efficiently arrested the cell cycle at postanaphase even in the absence of BUB2, whereas overexpression of BUB2 weakly induced mitotic arrest only in the presence of BFA1. Coimmunoprecipitation and in vitro binding studies indicate that Bfa1 binds strongly to Tem1 independently of Bub2. Provision of GDP+AlF(4)(-), which mimics the GTPase transition state, enhanced the Bub2-Tem1 interaction both in vitro and in vivo. Interestingly, introduction of bfa1 Delta, but not bub2 Delta, greatly increased the interaction between Tem1 and Cdc15, a step that is thought to be critical for activating the mitotic exit network. Our data suggest that, in addition to its role as a putative, two-component GTPase-activating protein with Bub2, Bfa1 also can play a role in the regulation of mitotic exit by directly inhibiting the interaction between Tem1 and Cdc15 even in the absence of Bub2.

Inhibition of APC-mediated proteolysis by the meiosis-specific protein kinase Ime2

Proteolysis triggered by the anaphase-promoting complex (APC) is needed for sister chromatid separation and the exit from mitosis. APC is a ubiquitin ligase whose activity is tightly controlled during the cell cycle. To identify factors involved in the regulation of APC-mediated proteolysis, a Saccharomyces cerevisiae GAL-cDNA library was screened for genes whose overexpression prevented degradation of an APC target protein, the mitotic cyclin Clb2. Genes encoding G1, S, and mitotic cyclins were identified, consistent with previous data showing that the cyclin-dependent kinase Cdk1 associated with different cyclins is a key factor for inhibiting APC(Cdh1) activity from late-G1 phase until mitosis. In addition, the meiosis-specific protein kinase Ime2 was identified as a negative regulator of APC-mediated proteolysis. Ectopic expression of IME2 in G1 arrested cells inhibited the degradation of mitotic cyclins and of other APC substrates. IME2 expression resulted in the phosphorylation of Cdh1 in G1 cells, indicating that Ime2 and Cdk1 regulate APC(Cdh1) in a similar manner. The expression of IME2 in cycling cells inhibited bud formation and caused cells to arrest in mitosis. We show further that Ime2 itself is an unstable protein whose proteolysis occurs independently of the APC and SCF (Skp1/Cdc53/F-box) ubiquitin ligases. Our findings suggest that Ime2 represents an unstable, meiosis-specific regulator of APC(Cdh1).

Human Mps1 kinase is required for the spindle assembly checkpoint but not for centrosome duplication

Budding yeast Mps1p kinase has been implicated in both the duplication of microtubule-organizing centers and the spindle assembly checkpoint. Here we show that hMps1, the human homolog of yeast Mps1p, is a cell cycle-regulated kinase with maximal activity during M phase. hMps1 localizes to kinetochores and its activity and phosphorylation state increase upon activation of the mitotic checkpoint. By antibody microinjection and siRNA, we demonstrate that hMps1 is required for human cells to undergo checkpoint arrest in response to microtubule depolymerization. In contrast, centrosome (re-)duplication as well as cell division occur in the absence of hMps1. We conclude that hMps1 is required for the spindle assembly checkpoint but not for centrosome duplication.

Post-transcriptional suppression of gene expression in Xenopus embryos by small interfering RNA

Double-stranded RNA (dsRNA) induces gene-specific silencing in organisms from fungi to animals, a phenomenon known as RNA interference (RNAi). RNAi represents an evolutionarily conserved system to protect against aberrant expression of genes and a powerful tool for gene manipulation. Despite reports that RNAi can be induced in vertebrates, severe sequence-non-specific effects of long dsRNA have been documented in various systems. It has recently been shown in cultured mammalian cells that small interfering RNAs (siRNAs) of 21-23 nt can mediate RNAi but bypass the non-specific response induced by longer dsRNAs. However, the effectiveness of siRNAs has not been demonstrated in living vertebrates. In addition, the mechanism of siRNA suppression of gene expression in vertebrate cells remains to be elucidated. Here we show that synthetic 21 nt siRNAs can specifically inhibit the expression of exogenously introduced as well as endogenous genes in the embryos of Xenopus laevis. siRNAs significantly reduced the steady-state amount of both the mRNA and protein of the cognate gene target. Moreover, co-injection of siRNA with the target RNA transcript specifically suppressed the activity of the latter. Taken together, our findings establish siRNA-mediated post-transcriptional suppression of gene expression in Xenopus embryos.

The gamma-tubulin complex protein Alp4 provides a link between the metaphase checkpoint and cytokinesis in fission yeast

BACKGROUND

The progression of cytokinesis requires cyclin B destruction by the anaphase promoting complex (APC/C) and, in fission yeast, activation of the septation initiation network (SIN) is also essential. The gamma-tubulin complex (gamma-TuC) localizes to the centrosome throughout the cell cycle and is directly involved in the organization of the mitotic spindle.

RESULTS

We have previously shown that the mutant defective in alp4+ (Spc97/GCP2) displays bipolar spindle defects due to a failure in the recruitment of the gamma-TuC on to the spindle pole body (SPB, the centrosome equivalent). Here we show that in these mutants the Mad2 checkpoint is activated, yet septation proceeds due to the untimely activation of the SIN. The Sid1 kinase, the downstream effector of the SIN, is recruited prematurely to both, instead of only one, of the SPBs, which triggers septation despite the presence of monopolar spindles. Remarkably, cyclin B levels, which would normally have declined, remain high at the SPB in septated mutant cells.

CONCLUSIONS

We propose a novel role of the gamma-TuC in inhibiting activation of the SIN until cyclin B is destroyed. Given the ubiquitous existence of the gamma-TuC, this mechanism may be conserved throughout evolution and function to couple cytokinesis to mitotic exit.

Multiple telophase arrest bypassed (tab) mutants alleviate the essential requirement for Cdc15 in exit from mitosis in S. cerevisiae

BACKGROUND

The Mitotic Exit Network (MEN) proteins - including the protein kinase Cdc15 and the protein phosphatase Cdc14 - are essential for exit from mitosis in Saccharomyces cerevisiae. To identify downstream targets of the MEN, we sought telophase arrest bypassed (tab) mutations that bypassed the essential requirement for CDC15. Previous studies identified net1(tab2-1) and CDC14(TAB6-1) as mutations in the RENT complex subunits Net1 and Cdc14, respectively, and revealed that the MEN acts by promoting release of Cdc14 from its nucleolar Net1 anchor during anaphase. However, the remaining tab mutants were not characterized.

RESULTS

Fourteen out of fifteen tab mutants were mapped to three recessive (tab1-tab3) and three dominant (TAB5-TAB7) linkage groups. We show that net1(tab2-1) enables growth of tem1Delta, cdc15Delta, dbf2Delta dbf20Delta, and mob1Delta, but not cdc5Delta or cdc14Delta, arguing that whereas the essential task of the first four genes is to promote exit from mitosis, CDC5 possesses additional essential function(s). net1(tab2-1) but not CDC14(TAB6-1) resulted in a high rate of chromosome loss, indicating that Net1 promotes accurate chromosome segregation in addition to its multiple known roles. Finally, TAB1 was shown to be MTR10, a gene encoding nuclear transport receptor/adaptor. In some of the tab mutants including mtr10(tab1-1), defective nuclear export of the ribosomal protein Rpl11b was observed. Furthermore, the transport-defective -31 allele of the karyopherin SRP1, but not the transport competent -49 allele, exhibited a tab phenotype.

CONCLUSIONS

Transport-defective mutations in two karyopherins can bypass cdc15Delta, suggesting that the function of the MEN is to promote mitotic exit by regulating nuclear transport.

The role of cell cycle-regulated expression in the localization of spatial landmark proteins in yeast

In Saccharomyces cerevisiae, Bud8p and Bud9p are homologous plasma membrane glycoproteins that appear to mark the distal and proximal cell poles, respectively, as potential sites for budding in the bipolar pattern. Here we provide evidence that Bud8p is delivered to the presumptive bud site (and thence to the distal pole of the bud) just before bud emergence, and that Bud9p is delivered to the bud side of the mother-bud neck (and thence to the proximal pole of the daughter cell) after activation of the mitotic exit network, just before cytokinesis. Like the delivery of Bud8p, that of Bud9p is actin dependent; unlike the delivery of Bud8p, that of Bud9p is also septin dependent. Interestingly, although the transcription of BUD8 and BUD9 appears to be cell cycle regulated, the abundance of BUD8 mRNA peaks in G2/M and that of BUD9 mRNA peaks in late G1, suggesting that the translation and/or delivery to the cell surface of each protein is delayed and presumably also cell cycle regulated. The importance of time of transcription in localization is supported by promoter-swap experiments: expression of Bud8p from the BUD9 promoter leads to its localization predominantly to the sites typical for Bud9p, and vice versa. Moreover, expression of Bud8p from the BUD9 promoter fails to rescue the budding-pattern defect of a bud8 mutant but fully rescues that of a bud9 mutant. However, although expression of Bud9p from the BUD8 promoter fails to rescue a bud9 mutant, it also rescues only partially the budding-pattern defect of a bud8 mutant, suggesting that some feature(s) of the Bud8p protein is also important for Bud8p function. Experiments with chimeric proteins suggest that the critical element(s) is somewhere in the extracytoplasmic domain of Bud8p.

Overexpression of the secreted factor Mig30 expressed in the Spemann organizer impairs morphogenetic movements during Xenopus gastrulation

The Spemann organizer secretes several antagonists of growth factors during gastrulation. We describe a novel secreted protein, Mig30, which is expressed in the anterior endomesoderm of the Spemann organizer. Mixer-inducible gene 30 (Mig30) was isolated as a target of Mixer, a homeobox gene required for endoderm development. The Mig30 gene encodes a secreted protein containing a cysteine-rich domain and an immunoglobulin-like domain that belongs to the insulin-like growth factor-binding protein family. Overexpression of Mig30 in the dorsal region results in the retardation of morphogenetic movements during gastrulation and leads to microcephalic embryos. Overexpression of Mig30 also inhibits activin-induced elongation of ectodermal explants without affecting gene expression patterns in mesoderm and endoderm. These results suggest that Mig30 is involved in the regulation of morphogenetic movements during gastrulation in the extracellular space of the Spemann organizer.

fzr-1 and lin-35/Rb function redundantly to control cell proliferation in C. elegans as revealed by a nonbiased synthetic screen

We report here a synthetic-lethal screen in Caenorhabditis elegans that overcomes a number of obstacles associated with the analysis of functionally redundant genes. Using this approach, we have identified mutations that synthetically interact with lin-35/Rb, a SynMuv gene and the sole member of the Rb/pocket protein family in C. elegans. Unlike the original SynMuv screens, our approach is completely nonbiased and can theoretically be applied to any situation in which a mutation fails to produce a detectable phenotype. From this screen we have identified fzr-1, a gene that synthetically interacts with lin-35 to produce global defects in cell proliferation control. fzr-1 encodes the C. elegans homolog of Cdh1/Hct1/FZR, a gene product shown in other systems to regulate the APC cyclosome. We have also uncovered genetic interactions between fzr-1 and a subset of class B SynMuv genes, and between lin-35 and the putative SCF regulator lin-23. We propose that lin-35, fzr-1, and lin-23 function redundantly to control cell cycle progression through the regulation of cyclin levels.

Complementary whole-genome technologies reveal the cellular response to proteasome inhibition by PS-341

Although the biochemical targets of most drugs are known, the biological consequences of their actions are typically less well understood. In this study, we have used two whole-genome technologies in Saccharomyces cerevisiae to determine the cellular impact of the proteasome inhibitor PS-341. By combining population genomics, the screening of a comprehensive panel of bar-coded mutant strains, and transcript profiling, we have identified the genes and pathways most affected by proteasome inhibition. Many of these function in regulated protein degradation or a subset of mitotic activities. In addition, we identified Rpn4p as the transcription factor most responsible for the cell's ability to compensate for proteasome inhibition. Used together, these complementary technologies provide a general and powerful means to elucidate the cellular ramifications of drug treatment.

Separase, polo kinase, the kinetochore protein Slk19, and Spo12 function in a network that controls Cdc14 localization during early anaphase

In budding yeast, the phosphatase Cdc14, a key regulator of exit from mitosis, is released from its inhibitor Cfi1/Net1 in the nucleolus during anaphase. A signaling cascade, known as the mitotic exit network (MEN), controls this release. We have identified a regulatory network, the FEAR (Cdc fourteen early anaphase release) network that promotes Cdc14 release from the nucleolus during early anaphase. The FEAR network is comprised of the polo kinase Cdc5, the separase Esp1, the kinetochore-associated protein Slk19, and Spo12. We also show that the FEAR network initiates Cdc14 release from Cfi1/Net1 during early anaphase, and MEN maintains Cdc14 in the released state during late anaphase. We propose that one function of Cdc14 released by the FEAR network is to stimulate MEN activity.

SRC1: an intron-containing yeast gene involved in sister chromatid segregation

Analysis of a three-member gene family in the yeast Saccharomyces cerevisiae has allowed the discovery of a new gene that comprises two contiguous open reading frames previously annotated as YML034w and YML033w. The gene contains a small intron with two alternative 5' splicing sites. It is specifically transcribed during G(2)/M in the cell cycle and after several hours of meiosis induction. Splicing of the mRNA is partially dependent on NAM8 but does not vary during meiosis or the cell cycle. Deletion of the gene induces a shortening of the anaphase and aggravates the phenotype of scc1 and esp1 conditional mutants, which suggests a direct role of the protein in sister chromatid separation.

Polo boxes and Cut23 (Apc8) mediate an interaction between polo kinase and the anaphase-promoting complex for fission yeast mitosis

The fission yeast plo1(+) gene encodes a polo-like kinase, a member of a conserved family of kinases which play multiple roles during the cell cycle. We show that Plo1 kinase physically interacts with the anaphase-promoting complex (APC)/cyclosome through the noncatalytic domain of Plo1 and the tetratricopeptide repeat domain of the subunit, Cut23. A new cut23 mutation, which specifically disrupts the interaction with Plo1, results in a metaphase arrest. This arrest can be rescued by high expression of Plo1 kinase. We suggest that this physical interaction is crucial for mitotic progression by targeting polo kinase activity toward the APC.

Rca1 inhibits APC-Cdh1(Fzr) and is required to prevent cyclin degradation in G2

We demonstrate that Rca1 is an essential inhibitor of the anaphase-promoting complex/cyclosome (APC) in Drosophila. APC activity is restricted to mitotic stages and G1 by its activators Cdc20-Fizzy (Cdc20(Fzy)) and Cdh1-Fizzy-related (Cdh1(Fzr)), respectively. In rca1 mutants, cyclins are degraded prematurely in G2 by APC-Cdh1(Fzr)-dependent proteolysis, and cells fail to execute mitosis. Overexpression of Cdh1(Fzr) mimics the rca1 phenotype, and coexpression of Rca1 blocks this Cdh1(Fzr) function. We show that Rca1 and Cdh1(Fzr) are in a complex that also includes the APC component Cdc27. Previous studies have shown that phosphorylation of Cdh1 prevents its interaction with the APC. Our data reveal a different mode of APC regulation by Rca1 at the G2 stage, when low Cdk activity is unable to inhibit Cdh1(Fzr) interaction.

Regulation of the anaphase-promoting complex by the dual specificity phosphatase human Cdc14a

Two forms of the anaphase-promoting complex (APC) mediate the degradation of critical cell cycle regulators. APC(Cdc20) promotes sister-chromatid separation by ubiquitinating securin, whereas APC(Cdh1) ubiquitinates mitotic cyclins, allowing the exit from mitosis. Here we show that phosphorylation of human Cdh1 (hCdh1) by cyclin B-Cdc2 alters the conformation of hCdh1 and prevents it from activating APC. A human homologue of yeast Cdc14, human Cdc14a (hCdc14a), dephosphorylates hCdh1 and activates APC(Cdh1). In contrast, hCdc14a does not affect the activity of APC(Cdc20). hCdc14a is a major phosphatase for hCdh1 and localizes to centrosomes in HeLa cells. Therefore, hCdc14a may promote the activation of APC(Cdh1) and exit from mitosis in mammalian cells.

Prospore membrane formation linked to the leading edge protein (LEP) coat assembly

In yeast, the differentiation process at the end of meiosis generates four daughter cells inside the boundaries of the mother cell. A meiosis-specific plaque (MP) at the spindle pole bodies (SPBs) serves as the starting site for the formation of the prospore membranes (PSMs) that are destined to encapsulate the post-meiotic nuclei. Here we report the identification of Ady3p and Ssp1p, which are functional components of the leading edge protein (LEP) coat, that covers the ring-shaped opening of the PSMs. Ssp1p is required for the assembly of the LEP coat, which consists of at least three proteins (Ssp1p, Ady3p and Don1p). The assembly of the LEP coat starts with the formation of cytosolic precursors, which then bind in an Ady3p-dependent manner to the SPBs. Subsequent processes at the SPBs leading to functional LEP coats require Ssp1p and the MP components. During growth of the PSMs, the LEP coat functions in formation of the cup-shaped membrane structure that is indispensable for the regulated cellularization of the cytoplasm around the post-meiotic nuclei.

APC/C-mediated destruction of the centrosomal kinase Nek2A occurs in early mitosis and depends upon a cyclin A-type D-box

Nek2 is a NIMA-related kinase implicated in regulating centrosome structure at the G(2)/M transition. Two splice variants have been identified that exhibit distinct patterns of expression during cell cycle progression and development. Here we show that Nek2A, but not Nek2B, is destroyed upon entry into mitosis coincident with cyclin A destruction and in the presence of an active spindle assembly checkpoint. Destruction of Nek2A is mediated by the proteasome and is dependent upon the APC/C-Cdc20 ubiquitin ligase. Nek2 activity is not required for APC/C activation. Nek2A destruction in early mitosis is regulated by a motif in its extreme C-terminus which bears a striking resemblance to the extended destruction box (D-box) of cyclin A. Complete stabilization of Nek2A requires deletion of this motif and mutation of a KEN-box. Destruction of Nek2A is not inhibited by the cyclin B-type D-box, but the C-terminal domain of Nek2A inhibits destruction of both cyclins A and B. We propose that recognition of substrates by the APC/C-Cdc20 in early mitosis depends upon possession of an extended D-box motif.

APC2 Cullin protein and APC11 RING protein comprise the minimal ubiquitin ligase module of the anaphase-promoting complex

In mitosis, the anaphase-promoting complex (APC) regulates the onset of sister-chromatid separation and exit from mitosis by mediating the ubiquitination and degradation of the securin protein and mitotic cyclins. With the use of a baculoviral expression system, we have reconstituted the ubiquitin ligase activity of human APC. In combination with Ubc4 or UbcH10, a heterodimeric complex of APC2 and APC11 is sufficient to catalyze the ubiquitination of human securin and cyclin B1. However, the minimal APC2/11 ubiquitin ligase module does not possess substrate specificity, because it also ubiquitinates the destruction box deletion mutants of securin and cyclin B1. Both APC11 and UbcH10 bind to the C-terminal cullin homology domain of APC2, whereas Ubc4 interacts with APC11 directly. Zn(2+)-binding and mutagenesis experiments indicate that APC11 binds Zn(2+) at a 1:3 M ratio. Unlike the two Zn(2+) ions of the canonical RING-finger motif, the third Zn(2+) ion of APC11 is not essential for its ligase activity. Surprisingly, with Ubc4 as the E2 enzyme, Zn(2+) ions alone are sufficient to catalyze the ubiquitination of cyclin B1. Therefore, the Zn(2+) ions of the RING finger family of ubiquitin ligases may be directly involved in catalysis.

Regulation of the Bub2/Bfa1 GAP complex by Cdc5 and cell cycle checkpoints

During mitosis, a ras-related GTPase (Tem1) binds GTP and activates a signal transduction pathway to allow mitotic exit. During most of the cell cycle, Tem1 function is antagonized by a GTPase-activating protein complex, Bfa1/Bub2. How the Bfa1/Bub2 complex is regulated is not well understood. We find that Polo/Cdc5 kinase acts upstream of Bfa1/Bub2 in the mitotic exit network. Cdc5 phosphorylates Bfa1 and acts to antagonize Bfa1 function to promote mitotic exit. Bfa1 is regulated by multiple cell cycle checkpoints. The spindle assembly and spindle orientation checkpoints inhibit Bfa1 phosphorylation. DNA damage does not inhibit Bfa1 phosphorylation and instead causes a Rad53- and Dun1-dependent modification of Bfa1. Regulation of Bfa1 may therefore be a key step controlled by multiple checkpoint pathways to ensure a mitotic arrest.