The anaphase-promoting complex: it's not just for mitosis any more

TPR subunits of the anaphase-promoting complex mediate binding to the activator protein CDH1

BACKGROUND

Chromosome segregation and mitotic exit depend on activation of the anaphase-promoting complex (APC) by the substrate adaptor proteins CDC20 and CDH1. The APC is a ubiquitin ligase composed of at least 11 subunits. The interaction of APC2 and APC11 with E2 enzymes is sufficient for ubiquitination reactions, but the functions of most other subunits are unknown.

RESULTS

We have biochemically characterized subcomplexes of the human APC. One subcomplex, containing APC2/11, APC1, APC4, and APC5, can assemble multiubiquitin chains but is unable to bind CDH1 and to ubiquitinate substrates. The other subcomplex contains all known APC subunits except APC2/11. This subcomplex can recruit CDH1 but fails to support any ubiquitination reaction. In vitro, the C termini of CDC20 and CDH1 bind to the closely related TPR subunits APC3 and APC7. Homology modeling predicts that these proteins are similar in structure to the peroxisomal import receptor PEX5, which binds cargo proteins via their C termini. APC activation by CDH1 depends on a conserved C-terminal motif that is also found in CDC20 and APC10.

CONCLUSIONS

APC1, APC4, and APC5 may connect APC2/11 with TPR subunits. TPR domains in APC3 and APC7 recruit CDH1 to the APC and may thereby bring substrates into close proximity of APC2/11 and E2 enzymes. In analogy to PEX5, the different TPR subunits of the APC might function as receptors that interact with the C termini of regulatory proteins such as CDH1, CDC20, and APC10.

Differential expression, localization and activity of two alternatively spliced isoforms of human APC regulator CDH1

The timely destruction of key regulators through ubiquitin-mediated proteolysis ensures the orderly progression of the cell cycle. The APC (anaphase-promoting complex) is a major component of this degradation machinery and its activation is required for the execution of critical events. Recent studies have just begun to reveal the complex control of the APC through a regulatory network involving WD40 repeat proteins CDC20 and CDH1. In the present paper, we report on the identification and characterization of human CDH1beta, a novel alternatively spliced isoform of CDH1. Both CDH1alpha and CDH1beta can bind to the APC and stimulate the degradation of cyclin B1, but they are differentially expressed in human tissues and cells. CDH1alpha contains a nuclear localization signal which is absent in CDH1beta. Intracellularly, CDH1alpha appears in the nucleus whereas CDH1beta is a predominantly cytoplasmic protein. The forced overexpression of CDH1alpha in cultured cells correlates with the reduction of nuclear cyclin A, but the steady-state amount of cyclin A does not change noticeably in CDH1beta-overexpressed cells. In Xenopus embryos, ectopic overexpression of human CDH1alpha, but not of CDH1beta, induces cell-cycle arrest during the first G(1) phase at the mid-blastula transition. Taken together, our findings document the differential expression, subcellular localization and cell-cycle-regulatory activity of human CDH1 isoforms.

Controlling transcription by destruction: the regulation of yeast Gcn4p stability

The Gcn4 protein, a member of the AP-1 family of transcription factors, is involved in the expression of more than 500 genes in the budding yeast Saccharomyces cerevisiae. A key role of Gcn4p is the increased expression of many amino acid biosynthesis genes in response to amino acid starvation. The accumulation of this transcription activator is mainly induced by efficient translation of the GCN4 ORF and by stabilisation of the Gcn4 protein. Under normal growth conditions, Gcn4p is a highly unstable protein, thereby resembling many eukaryotic transcription factors, including mammalian Jun and Myc proteins. Gcn4p is degraded by ubiquitin-dependent proteolysis mediated by the Skp1/cullin/F-box (SCF) ubiquitin ligase, which recognises specifically phosphorylated substrates. Two cyclin-dependent protein kinases, Pho85p and Srb10p, have crucial functions in regulating Gcn4p phosphorylation and degradation. The past few years have revealed many novel insights into these regulatory processes. Here, we summarise current knowledge about the factors and mechanisms regulating Gcn4p stability.

Methods designed for the identification and characterization of in vitro and in vivo chromatin assembly mutants in Saccharomyces cerevisiae

Assembly of DNA into chromatin allows for the formation of a barrier that protects naked DNA from protein and chemical agents geared to degrade or metabolize DNA. Chromatin assembly occurs whenever a length of DNA becomes exposed to the cellular elements, whether during DNA synthesis or repair. This report describes tools to study chromatin assembly in the model system Saccharomyces cerevisiae. Modifications to an in vitro chromatin assembly assay are described that allowed a brute force screen of temperature sensitive (ts) yeast strains in order to identify chromatin assembly defective extracts. This screen yielded mutations in genes encoding two ubiquitin protein ligases (E3s): RSP5, and a subunit of the Anaphase Promoting Complex (APC), APC5. Additional modifications are described that allow for a rapid analysis and an in vivo characterization of yeast chromatin assembly mutants, as well as any other mutant of interest. Our analysis suggests that the in vitro and invivo chromatin assembly assays are responsive to different cellular signals, including cell cycle cues that involve different molecular networks.

Prophase destruction of Emi1 by the SCF(betaTrCP/Slimb) ubiquitin ligase activates the anaphase promoting complex to allow progression beyond prometaphase

Progression through mitosis occurs because cyclin B/Cdc2 activation induces the anaphase promoting complex (APC) to cause cyclin B destruction and mitotic exit. To ensure that cyclin B/Cdc2 does not prematurely activate the APC in early mitosis, there must be a mechanism delaying APC activation. Emi1 is a protein capable of inhibiting the APC in S and G2. We show here that Emi1 is phosphorylated by Cdc2, and on a DSGxxS consensus site, is subsequently recognized by the SCF(betaTrCP/Slimb) ubiquitin ligase and destroyed, thus providing a delay for APC activation. Failure of betaTrCP-dependent Emi1 destruction stabilizes APC substrates and results in mitotic catastrophe including centrosome overduplication, potentially explaining mitotic deficiencies in Drosophila Slimb/betaTrCP mutants. We hypothesize that Emi1 destruction relieves a late prophase checkpoint for APC activation.

Mitotic entry: tipping the balance

In the eukaryotic cell cycle, the G2-M transition is regulated by a counteracting kinase/phosphatase mechanism. A novel player, Tome-1, has now been found to trigger entry into mitosis by interfering with this kinase/phosphatase 'switch'.

Mnd2 and Swm1 are core subunits of the Saccharomyces cerevisiae anaphase-promoting complex

The anaphase-promoting complex (APC) is a multisubunit E3 ubiquitin ligase that regulates the metaphase-anaphase transition and exit from mitosis in eukaryotic cells. Eleven subunits have been previously identified in APC from budding yeast. We have identified two additional subunits, Mnd2 and Swm1, by mass spectrometry. Both Mnd2 and Swm1 were found specifically associated with a highly purified preparation of APC from haploid yeast whole cell extract. Moreover, the APC co-purified with epitope-tagged Mnd2 and Swm1. Both proteins were present in APC preparations from haploid cells arrested in G(1), S, and M phases and from meiotic diploid cells, indicating that they are constitutive components of the complex throughout the yeast cell cycle. Mnd2 interacted strongly with Cdc23, Apc5, and Apc1 when coexpressed in an in vitro transcription/translation reaction. Swm1 also interacted with Cdc23 and Apc5 in this system. Previous studies described meiotic defects for mutations in MND2 and SWM1. Here, we show that mnd2delta and swm1delta haploid strains exhibit slow growth and accumulation of G(2)/M cells comparable with that seen in apc9delta or apc10Delta strains and consistent with an APC defect. Taken together, these results demonstrate that Swm1 and Mnd2 are functional components of the yeast APC.

Synergistic inhibition of APC/C by glucose and activated Ras proteins can be mediated by each of the Tpk1-3 proteins in Saccharomyces cerevisiae

Proteolysis triggered by the anaphase-promoting complex/cyclosome (APC/C) is essential for the progression through mitosis. APC/C is a highly conserved ubiquitin ligase whose activity is regulated during the cell cycle by various factors, including spindle checkpoint components and protein kinases. The cAMP-dependent protein kinase (PKA) was identified as negative regulator of APC/C in yeast and mammalian cells. In the yeast Saccharomyces cerevisiae, PKA activity is induced upon glucose addition or by activated Ras proteins. This study shows that glucose and the activated Ras2(Val19) protein synergistically inhibit APC/C function via the cAMP/PKA pathway in yeast. Remarkably, Ras2 proteins defective in the interaction with adenylate cyclase fail to influence APC/C, implying that its function is regulated exclusively by PKA, but not by alternative Ras pathways. Furthermore, it is shown that the three PKAs in yeast, Tpk1, Tpk2 and Tpk3, have redundant functions in regulating APC/C in response to glucose medium. Single or double deletions of TPK genes did not prevent inhibition of APC/C, suggesting that each of the Tpk proteins can take over this function. However, Tpk2 seems to inhibit APC/C function more efficiently than Tpk1 and Tpk3. Finally, evidence is provided that Cdc20 is involved in APC/C regulation by the cAMP/PKA pathway.

The Cdc14 phosphatase and the FEAR network control meiotic spindle disassembly and chromosome segregation

During meiosis, DNA replication is followed by two consecutive rounds of chromosome segregation. Cells lacking the protein phosphatase CDC14 or its regulators, SPO12 and SLK19, undergo only a single meiotic division, with some chromosomes segregating reductionally and others equationally. We find that this abnormal chromosome behavior is due to an uncoupling of meiotic events. Anaphase I spindle disassembly is delayed in cdc14-1, slk19Delta, or spo12Delta mutants, but the chromosome segregation cycle continues, so that both meiotic chromosome segregation phases take place on the persisting meiosis I spindle. Our results show that Cdc14, Slk19, and Spo12 are not only required for meiosis I spindle disassembly but also play a pivotal role in establishing two consecutive chromosome segregation phases, a key feature of the meiotic cell cycle.

Nuclear localization of the cell cycle regulator CDH1 and its regulation by phosphorylation

The anaphase-promoting complex activated by CDC20 and CDH1 is a major ubiquitination system that controls the destruction of cell cycle regulators. Exactly how ubiquitination is regulated in time and space is incompletely understood. Here we report on the cell cycle-dependent localization of CDH1 and its regulation by phosphorylation. CDH1 localizes dynamically to the nucleus during interphase and to the centrosome during metaphase and anaphase. The nuclear accumulation of CDH1 correlates with a reduction in the steady-state amount of cyclin A, but not of cyclin E. A nuclear localization signal conserved in various species was identified in CDH1, and it sufficiently targets green fluorescent protein to the nucleus. Interestingly, a CDH1-4D mutant mimicking the hyperphosphorylated form was constitutively found in the cytoplasm. In further support of the notion that phosphorylation inhibits nuclear import, the nuclear localization signal of CDH1 with two phospho-accepting serine/threonine residues changed into aspartates was unable to drive heterologous protein into the nucleus. On the other hand, abolition of the cyclin-binding ability of CDH1 has no influence on its nuclear localization. Taken together, our findings document the phosphorylation-dependent localization of CDH1 in vertebrate cells.

Orchestrating nuclear functions: ubiquitin sets the rhythm

The nucleus of the eukaryotic cell must carry out many functions simultaneously. These tasks include ensuring that the cell is continuously supplied with an appropriate, changing set of proteins on its way through cell divisions and differentiation. During these processes, the integrity of the genetic material must be maintained against a constant onslaught of damaging physiological and environmental factors. Fulfilling these complex tasks requires the dynamic integration and synchronization of different nuclear functions. Protein modification by ubiquitin is proving to be a crucial tool for nuclear functioning, and is emerging as a decisive mechanism that enables the concerted regulation of nuclear pathways.

Developmental defects observed in hypomorphic anaphase-promoting complex mutants are linked to cell cycle abnormalities

In C. elegans, mutants in the anaphase-promoting complex or cyclosome (APC/C) exhibit defects in germline proliferation, the formation of the vulva and male tail, and the metaphase to anaphase transition of meiosis I. Oocytes lacking APC/C activity can be fertilized but arrest in metaphase of meiosis I and are blocked from further development. To examine the cell cycle and developmental consequences of reducing but not fully depleting APC/C activity, we analyzed defects in embryos and larvae of mat-1/cdc-27 mutants grown at semi-permissive temperatures. Hypomorphic embryos developed to the multicellular stage but were slow to complete meiosis I and displayed aberrant meiotic chromosome separation. More severely affected embryos skipped meiosis II altogether and exhibited striking defects in meiotic exit. These latter embryos failed to produce normal eggshells or establish normal asymmetries prior to the first mitotic division. In developing larvae, extended M-phase delays in late-dividing cell lineages were associated with defects in the morphogenesis of the male tail. This study reveals the importance of dosage-specific mutants in analyzing molecular functions of a ubiquitously functioning protein within different cell types and tissues, and striking correlations between specific abnormalities in cell cycle progression and particular developmental defects.

Doc1 mediates the activity of the anaphase-promoting complex by contributing to substrate recognition

The anaphase-promoting complex (APC) is a multisubunit E3 ubiquitin ligase that targets specific cell cycle-related proteins for degradation, regulating progression from metaphase to anaphase and exit from mitosis. The APC is regulated by binding of the coactivator proteins Cdc20p and Cdh1p, and by phosphorylation. We have developed a purification strategy that allowed us to purify the budding yeast APC to near homogeneity and identify two novel APC-associated proteins, Swm1p and Mnd2p. Using an in vitro ubiquitylation system and a native gel binding assay, we have characterized the properties of wild-type and mutant APC. We show that both the D and KEN boxes contribute to substrate recognition and that coactivator is required for substrate binding. APC lacking Apc9p or Doc1p/Apc10 have impaired E3 ligase activities. However, whereas Apc9p is required for structural stability and the incorporation of Cdc27p into the APC complex, Doc1p/Apc10 plays a specific role in substrate recognition by APC-coactivator complexes. These results imply that Doc1p/Apc10 may play a role to regulate the binding of specific substrates, similar to that of the coactivators.

Structural basis for phosphodependent substrate selection and orientation by the SCFCdc4 ubiquitin ligase

Cell cycle progression depends on precise elimination of cyclins and cyclin-dependent kinase (CDK) inhibitors by the ubiquitin system. Elimination of the CDK inhibitor Sic1 by the SCFCdc4 ubiquitin ligase at the onset of S phase requires phosphorylation of Sic1 on at least six of its nine Cdc4-phosphodegron (CPD) sites. A 2.7 A X-ray crystal structure of a Skp1-Cdc4 complex bound to a high-affinity CPD phosphopeptide from human cyclin E reveals a core CPD motif, Leu-Leu-pThr-Pro, bound to an eight-bladed WD40 propeller domain in Cdc4. The low affinity of each CPD motif in Sic1 reflects structural discordance with one or more elements of the Cdc4 binding site. Reengineering of Cdc4 to reduce selection against Sic1 sequences allows ubiquitination of lower phosphorylated forms of Sic1. These features account for the observed phosphorylation threshold in Sic1 recognition and suggest an equilibrium binding mode between a single receptor site in Cdc4 and multiple low-affinity CPD sites in Sic1.

Cyclin destruction in mitosis: a crucial task of Cdc20

Proteolytic destruction of cyclins is a fundamental process for cell division. At the end of mitosis, degradation of mitotic cyclins results in the inactivation of cyclin-dependent kinases. Cyclin proteolysis is triggered by the anaphase-promoting complex/cyclosome (APC/C), a multi-subunit complex which contains ubiquitin ligase activity. Recent data in yeast demonstrated that a partial degradation of the mitotic cyclin Clb2, mediated by APC/C and its activator protein Cdc20, is essential and sufficient for the mitotic exit. Remarkably, a complete inactivation of cyclin-dependent kinases seems to be not essential. This review discusses recent novel insights into cyclin destruction and its implications for the mitotic exit.

Cell cycle regulatory E3 ubiquitin ligases as anticancer targets

Disregulation of the cell cycle and proliferation play key roles in cellular transformation and tumorigenesis. Such processes are intimately tied to the concentration, localization and activity of enzymes, adapters, receptors, and structural proteins in cells. Ubiquitination of these cellular regulatory proteins, governed by specific enzymes in the ubiquitin (Ub) conjugation cascade, has profound effects on their various functions, most commonly through proteasome targeting and degradation. This review will focus on a variety of E3 Ub ligases as potential oncology drug targets, with particular emphasis on the role of these molecules in the regulation of stability, localization, and activity of key proteins such as tumor suppressors and oncoproteins. E3 ubiquitin ligases that have established roles in cell cycle and apoptosis, such as the anaphase-promoting complex (APC), the Skp-1-Cul1-F-box class, and the murine double minute 2 (MDM2) protein, in addition to more recently discovered E3 ubiquitin ligases which may be similarly important in tumorigenesis, (e.g. Smurf family, CHFR, and Efp), will be discussed. We will present evidence to support E3 ligases as good biological targets in the development of anticancer therapeutics and address challenges in drug discovery for these targets.

The Doc1 subunit is a processivity factor for the anaphase-promoting complex

Ubiquitin-mediated proteolysis of securin and mitotic cyclins is essential for exit from mitosis. The final step in ubiquitination of these and other proteins is catalysed by the anaphase-promoting complex (APC), a multi-subunit ubiquitin-protein ligase (E3). Little is known about the molecular reaction resulting in APC-dependent substrate ubiquitination or the role of individual APC subunits in the reaction. Using a well-defined in vitro system, we show that highly purified APC from Saccharomyces cerevisiae ubiquitinates a model cyclin substrate in a processive manner. Analysis of mutant APC lacking the Doc1/Apc10 subunit (APC(doc1 Delta)) indicates that Doc1 is required for processivity. The specific molecular defect in APC(doc1 Delta) is identified by a large increase in apparent K(M) for the cyclin substrate relative to the wild-type enzyme. This suggests that Doc1 stimulates processivity by limiting substrate dissociation. Addition of recombinant Doc1 to APC(doc1 Delta) fully restores enzyme function. Doc1-related domains are found in mechanistically distinct ubiquitin-ligase enzymes and may generally stimulate ubiquitination by contributing to substrate-enzyme affinity.