A novel murine gene encoding a 216-kDa protein is related to a mitotic checkpoint regulator previously identified in Aspergillus nidulans

Thinking within the D box: initial identification of Cdh1-APC substrates in the nervous system

The anaphase-promoting complex (APC) has a well-established role in cell cycle control, but recent exciting evidence has uncovered unexpected neurobiological functions for this complex E3 ubiquitin ligase. With its co-activator Cdh1, APC's effects upon the nervous system range from regulation of axon growth and patterning to development of synapses to neuronal survival. The Cdh1-APC substrates that control these biological processes in neurons are just beginning to be identified. These findings may offer a glimpse of the wide spectrum of neural activities that are orchestrated by Cdh1-APC.

The anaphase-promoting complex: a key factor in the regulation of cell cycle

Events controlling cell division are governed by the degradation of different regulatory proteins by the ubiquitin-dependent pathway. In this pathway, the attachment of a polyubiquitin chain to a substrate by an ubiquitin-ligase targets this substrate for degradation by the 26S proteasome. Two different ubiquitin ligases play an important role in the cell cycle: the SCF (Skp1/Cullin/F-box) and the anaphase-promoting complex (APC). In this review, we describe the present knowledge about the APC. We pay particular attention to the latest results concerning APC structure, APC regulation and substrate recognition, and we discuss the implication of these findings in the understanding the APC function.

Multiple subunits of the Caenorhabditis elegans anaphase-promoting complex are required for chromosome segregation during meiosis I

Two genes, originally identified in genetic screens for Caenorhabditis elegans mutants that arrest in metaphase of meiosis I, prove to encode subunits of the anaphase-promoting complex or cyclosome (APC/C). RNA interference studies reveal that these and other APC/C subunits are essential for the segregation of chromosomal homologs during meiosis I. Further, chromosome segregation during meiosis I requires APC/C functions in addition to the release of sister chromatid cohesion.

The anaphase-promoting complex: new subunits and regulators

Ubiquitin-mediated proteolysis of cell cycle regulators is a crucial process during the cell cycle. The anaphase-promoting complex (APC) is a large, multiprotein complex whose E3-ubiquitin ligase activity is required for the ubiquitination of mitotic cyclins and other regulatory proteins that are targeted for destruction during cell division. The recent identification of new APC subunits and regulatory proteins has begun to reveal some of the intricate mechanisms that govern APC regulation. One mechanism is the use of specificity factors to impose temporal control over substrate degradation. A second mechanism is the APC-mediated proteolysis of specific APC regulators. Finally, components of both the APC and the SCF E3 ubiquitin-ligase complex contain several conserved sequence motifs, including WD-40 repeats and cullin homology domains, which suggest that both complexes may use a similar mechanism for substrate ubiquitination.

Expression of the CDH1-associated form of the anaphase-promoting complex in postmitotic neurons

The anaphase-promoting complex/cyclosome (APC) is a tightly cell cycle-regulated ubiquitin-protein ligase that targets cyclin B and other destruction box-containing proteins for proteolysis at the end of mitosis and in G1. Recent work has shown that activation of the APC in mitosis depends on CDC20, whereas APC is maintained active in G1 via association with the CDC20-related protein CDH1. Here we show that the mitotic activator CDC20 is the only component of the APC ubiquitination pathway whose expression is restricted to proliferating cells, whereas the APC and CDH1 are also expressed in several mammalian tissues that predominantly contain differentiated cells, such as adult brain. Immunocytochemical analyses of cultured rat hippocampal neurons and of mouse and human brain sections indicate that the APC and CDH1 are ubiquitously expressed in the nuclei of postmitotic terminally differentiated neurons. The APC purified from brain contains all core subunits known from proliferating cells and is tightly associated with CDH1. Purified brain APC(CDH1) has a high cyclin B ubiquitination activity that depends less on the destruction box than on the activity of mitotic APC(CDC20). On the basis of these results, we propose that the functions of APC(CDH1) are not restricted to controlling cell-cycle progression but may include the ubiquitination of yet unidentified substrates in differentiated cells.

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.

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.

Xe-p9, a Xenopus Suc1/Cks protein, is essential for the Cdc2-dependent phosphorylation of the anaphase- promoting complex at mitosis

Degradation of mitotic cyclins on exit from M phase occurs by ubiquitin-mediated proteolysis. The ubiquitination of mitotic cyclins is regulated by the anaphase-promoting complex (APC) or cyclosome. Xe-p9, the Xenopus homolog of the Suc1/Cks protein, is required for some step in mitotic cyclin destruction in Xenopus egg extracts. Specifically, if p9 is removed from interphase egg extracts, these p9-depleted extracts are unable to carry out the proteolysis of cyclin B after entry into mitosis and thus remain arrested in M phase. To explore the molecular basis of this defect, we depleted p9 from extracts that had already entered M phase and thus contained an active APC. We found that ubiquitin-mediated proteolysis of cyclin B was not compromised under these circumstances, suggesting that p9 is not directly required for ubiquitination or proteolysis. Further analysis of extracts from which p9 had been removed during interphase showed that, at the beginning of mitosis, these extracts are unable to carry out the hyperphosphorylation of the Cdc27 component of the APC, which coincides with the initial activation of the APC. p9 can be found in a complex with a small fraction of the Cdc27 protein during M phase but not interphase. The phosphorylation of the Cdc27 protein (either associated with the APC or in an isolated, bacterially expressed form) by recombinant Cdc2/cyclin B is strongly enhanced by p9. Our results indicate that p9 directly regulates the phosphorylation of the APC by Cdc2/cyclin B. These studies indicate that the Suc1/Cks protein modulates substrate recognition by a cyclin-dependent kinase.

BIMAAPC3, a component of the Aspergillus anaphase promoting complex/cyclosome, is required for a G2 checkpoint blocking entry into mitosis in the absence of NIMA function

Temperature sensitive (ts) nimA mutants of Aspergillus nidulans arrest at a unique point in G2 which is post activation of CDC2. Here we show that this G2 arrest is due to loss of nimA function and that it is dependent on BIMAAPC3, a component of the anaphase promoting complex/cyclosome (APC/C). Whereas nimA single mutants arrested in G2 with decondensed chromatin and interphase microtubule arrays, nimA, bimAAPC3 double mutants arrested growth with condensed chromatin and aster-like microtubule arrays. nimA, bimAAPC3 double mutants entered mitosis with kinetics similar to bimAAPC3 single mutants and wild-type cells, indicating a checkpoint-like role for BIMAAPC3 in G2. Even cells which had been depleted for NIMA protein and which contained insignificant levels of NIMA kinase activity entered mitosis on inactivation of bimAAPC3. BIMAAPC3 was present in a >25S complex containing BIMEAPC1, and bimAAPC3 mutants were sensitive to elevated CYCLIN B expression, consistent with BIMAAPC3 being a component of the APC/C. Inactivation of bimAAPC3 had little affect on the steady state levels of the B-type cyclin, NIMECyclin B. Our results indicate that BIMAAPC3, and most likely the APC/C itself, is activated in G2 in nimA mutants. We propose that APC/C activation is part of a novel, late G2 checkpoint, which responds to a defective process or structure in nimA mutants, and which prevents inappropriate entry into mitosis.

PKA and MPF-activated polo-like kinase regulate anaphase-promoting complex activity and mitosis progression

Ubiquitin-mediated proteolysis is the key to cell cycle control. Anaphase-promoting complex/cyclosome (APC) is a ubiquitin ligase that targets cyclin B and factors regulating sister chromatid separation for proteolysis by the proteasome and, consequently, regulates metaphase-anaphase transition and exit from mitosis. Here we report that Cdc2-cyclin B-activated Polo-like kinase (Plk) specifically phosphorylates at least three components of APC and activates APC to ubiquitinate cyclin B in the in vitro-reconstituted system. Conversely, protein kinase A (PKA) phosphorylates two subunits of APC but suppresses APC activity. PKA is superior to Plk in its regulation of APC, and Plk activity peaks whereas PKA activity is falling at metaphase. These results indicate that Plk and PKA regulate mitosis progression by controlling APC activity.

A subunit of the anaphase-promoting complex is a centromere-associated protein in mammalian cells

Sister chromatids in early mitotic cells are held together mainly by interactions between centromeres. The separation of sister chromatids at the transition between the metaphase and the anaphase stages of mitosis depends on the anaphase-promoting complex (APC), a 20S ubiquitin-ligase complex that targets proteins for destruction. A subunit of the APC, called APC-alpha in Xenopus (and whose homologs are APC-1, Cut4, BIME, and Tsg24), has recently been identified and shown to be required for entry into anaphase. We now show that the mammalian APC-alpha homolog, Tsg24, is a centromere-associated protein. While this protein is detected only during the prophase to the anaphase stages of mitosis in Chinese hamster cells, it is constitutively associated with the centromeres in murine cells. We show that there are two forms of this protein in mammalian cells, a soluble form associated with other components of the APC and a centromere-bound form. We also show that both the Tsg24 protein and the Cdc27 protein, another APC component, are bound to isolated mitotic chromosomes. These results therefore support a model in which the APC by ubiquitination of a centromere protein regulates the sister chromatid separation process.

How proteolysis drives the cell cycle

Oscillations in the activity of cyclin-dependent kinases (CDKs) promote progression through the eukaryotic cell cycle. This review examines how proteolysis regulates CDK activity-by degrading CDK activators or inhibitors-and also how proteolysis may directly trigger the transition from metaphase to anaphase. Proteolysis during the cell cycle is mediated by two distinct ubiquitin-conjugation pathways. One pathway, requiring CDC34, initiates DNA replication by degrading a CDK inhibitor. The second pathway, involving a large protein complex called the anaphase-promoting complex or cyclosome, initiates chromosome segregation and exit from mitosis by degrading anaphase inhibitors and mitotic cyclins. Proteolysis therefore drives cell cycle progression not only by regulating CDK activity, but by directly influencing chromosome and spindle dynamics.

20S cyclosome complex formation and proteolytic activity inhibited by the cAMP/PKA pathway

The 20S cyclosome complex (also known as the anaphase-promoting complex) has ubiquitin ligase activity and is required for mitotic cyclin destruction and sister chromatid separation. The formation and activation of the 20S cyclosome complex is regulated by an unknown mechanism. Here we show that Cut4 (ref. 6) is an essential component of the cyclosome in fission yeast. Cut4 shares sequence similarity with BimE, a protein that regulates mitosis in Aspergillus nidulans. Mutations in cut4 result in hypersensitivity to cyclic AMP and to stress-inducing heavy metals, inhibition of the onset of anaphase, disruption of the 20S complex, and inhibition of mitotic cyclin ubiquitination. These phenotypes are fully suppressed by cAMP phosphodiesterase and the protein kinase A (PKA) regulatory subunit and weakly suppressed by Sti1 (an activator of the Hsp70 and Hsp90 chaperones). Suppression correlates with the amount of 20S complex, indicating that cyclosome formation and activation is inhibited by the cAMP/PKA pathway.

The Aspergillus nidulans bimE (blocked-in-mitosis) gene encodes multiple cell cycle functions involved in mitotic checkpoint control and mitosis

The bimE (blocked-in-mitosis) gene appears to function as a negative mitotic regulator because the recessive bimE7 mutation can override certain interphase-arresting treatments and mutations, causing abnormal induction of mitosis. We have further investigated the role of bimE in cell cycle checkpoint control by: (1) coordinately measuring mitotic induction and DNA content of bimE7 mutant cells; and (2) analyzing epistasis relationships between bimE7 and 16 different nim mutations. A combination of cytological and flow cytometric techniques was used to show that bimE7 cells at restrictive temperature (44 degrees C) undergo a normal, although somewhat slower cell cycle prior to mitotic arrest. Most bimE7 cells were fully reversible from restrictive temperature arrest, indicating that they are able to enter mitosis normally, and therefore require bimE function in order to finish mitosis. Furthermore, epistasis studies between bimE7 and mutations in cdc2 pathway components revealed that the induction of mitosis caused by inactivation of bimE requires functional p34cdc2 kinase, and that mitotic induction by bimE7 depends upon several other nim genes whose functions are not yet known. The involvement of bimE in S phase function and mitotic checkpoint control was suggested by three lines of evidence. First, at restrictive temperature the bimE7 mutation slowed the cell cycle by delaying the onset or execution of S phase. Second, at permissive temperature (30 degrees C) the bimE7 mutation conferred enhanced sensitivity to the DNA synthesis inhibitor hydroxyurea. Finally, the checkpoint linking M phase to the completion of S phase was abolished when bimE7 was combined with two nim mutations that cause arrest in G1 or S phase. A model for bimE function based on these findings is presented.