The CRY box: a second APCcdh1-dependent degron in mammalian cdc20

Balance of polo-like kinase Plo1 and monopolar attachment protein 1 (Moa1) regulates fission yeast meiosis

During meiosis, diploid germ cells undergo two successive rounds of chromosome segregation requiring key changes that sister chromatids co-orient in meiosis I and bi-orient in meiosis II. The kinetochore protein MEIKIN/Moa1 is restricted to meiosis I, has the function to properly co-orient sister kinetochores and maintain pericentrometic cohesion. However, the mechanisms governing the Moa1 activity throughout meiosis remain elusive in Schizosaccharomyces pombe. Here, we demonstrate that fission yeast Moa1 is degraded by the APC/C at anaphase I and blocking Moa1 degradation has no effect on cohesin protection and chromosome segregation during meiosis. Blocking Moa1 degradation can be prevented by the elimination of kinetochore Plo1. Conversely, the removal of Plo1 from the kinetochore, which leads to chromosome mis-segregation, can be reversed by maintaining kinetochore Moa1 levels. Therefore, we have observed a feedback relationship between reduced Plo1 enrichment at kinetochores and inhibited Moa1 degradation.

Functional analysis of Cdc20 reveals a critical role of CRY box in mitotic checkpoint signaling

Accurate mitosis is coordinated by the spindle assembly checkpoint (SAC) through the mitotic checkpoint complex (MCC), which inhibits the anaphase-promoting complex or cyclosome (APC/C). As an essential regulator, Cdc20 promotes mitotic exit through activating APC/C and monitors kinetochore-microtubule attachment through activating SAC. Cdc20 requires multiple interactions with APC/C and MCC subunits to elicit these functions. Functionally assessing these interactions within cells requires efficient depletion of endogenous Cdc20, which is highly difficult to achieve by RNA interference (RNAi). Here we generated Cdc20 RNAi-sensitive cell lines which display a penetrant metaphase arrest by a single RNAi treatment. In this null background, we accurately measured the contribution of each known motif of Cdc20 on APC/C and SAC activation. The CRY box, a previously identified degron, was found critical for SAC by promoting MCC formation and its interaction with APC/C. These data reveal additional regulation within the SAC and establish a novel method to interrogate Cdc20.

Biochemical, biophysical, and functional characterisation of the E3 ubiquitin ligase APC/C regulator CDC20 from Arabidopsis thaliana

The Anaphase Promoting Complex (APC/C), a large cullin-RING E3-type ubiquitin ligase, constitutes the ultimate target of the Spindle Assembly Checkpoint (SAC), an intricate regulatory circuit that ensures the high fidelity of chromosome segregation in eukaryotic organisms by delaying the onset of anaphase until each chromosome is properly bi-oriented on the mitotic spindle. Cell-division cycle protein 20 homologue (CDC20) is a key regulator of APC/C function in mitosis. The formation of the APC/C^CDC20 complex is required for the ubiquitination and degradation of select substrates, which is necessary to maintain the mitotic state. In contrast to the roles of CDC20 in animal species, little is known about CDC20 roles in the regulation of chromosome segregation in plants. Here we address this gap in knowledge and report the expression in insect cells; the biochemical and biophysical characterisation of Arabidopsis thaliana (AtCDC20) WD40 domain; and the nuclear and cytoplasmic distribution of full-length AtCDC20 when transiently expressed in tobacco plants. We also show that most AtCDC20 degrons share a high sequence similarity to other eukaryotes, arguing in favour of conserved degron functions in AtCDC20. However, important exceptions were noted such as the lack of a canonical MAD1 binding motif; a fully conserved RRY-box in all six AtCDC20 isoforms instead of a CRY-box motif, and low conservation of key residues known to be phosphorylated by BUB1 and PLK1 in other species to ensure a robust SAC response. Taken together, our studies provide insights into AtCDC20 structure and function and the evolution of SAC signalling in plants.

E3 Ubiquitin Ligase APC/CCdh1 Negatively Regulates FAH Protein Stability by Promoting Its Polyubiquitination

Fumarylacetoacetate hydrolase (FAH) is the last enzyme in the degradation pathway of the amino acids tyrosine and phenylalanine in mammals that catalyzes the hydrolysis of 4-fumarylacetoacetate into acetoacetate and fumarate. Mutations of the FAH gene are associated with hereditary tyrosinemia type I (HT1), resulting in reduced protein stability, misfolding, accelerated degradation and deficiency in functional proteins. Identifying E3 ligases, which are necessary for FAH protein stability and degradation, is essential. In this study, we demonstrated that the FAH protein level is elevated in liver cancer tissues compared to that in normal tissues. Further, we showed that the FAH protein undergoes 26S proteasomal degradation and its protein turnover is regulated by the anaphase-promoting complex/cyclosome-Cdh1 (APC/C)^Cdh1 E3 ubiquitin ligase complex. APC/C^Cdh1 acts as a negative stabilizer of FAH protein by promoting FAH polyubiquitination and decreases the half-life of FAH protein. Thus, we envision that Cdh1 might be a key factor in the maintenance of FAH protein level to regulate FAH-mediated physiological functions.

Phosphorylation of the Anaphase Promoting Complex activator FZR1/CDH1 is required for Meiosis II entry in mouse male germ cell

FZR1/CDH1 is an activator of Anaphase promoting complex/Cyclosome (APC/C), best known for its role as E3 ubiquitin ligase that drives the cell cycle. APC/C activity is regulated by CDK-mediated phosphorylation of FZR1 during mitotic cell cycle. Although the critical role of FZR1 phosphorylation has been shown mainly in yeast and in vitro cell culture studies, its biological significance in mammalian tissues in vivo remained elusive. Here, we examined the in vivo role of FZR1 phosphorylation using a mouse model, in which non-phosphorylatable substitutions were introduced in the putative CDK-phosphorylation sites of FZR1. Although ablation of FZR1 phosphorylation did not show substantial consequences in mouse somatic tissues, it led to severe testicular defects resulting in male infertility. In the absence of FZR1 phosphorylation, male juvenile germ cells entered meiosis normally but failed to enter meiosis II or form differentiated spermatids. In aged testis, male mutant germ cells were overall abolished, showing Sertoli cell-only phenotype. In contrast, female mutants showed apparently normal progression of meiosis. The present study demonstrated that phosphorylation of FZR1 is required for temporal regulation of APC/C activity at meiosis II entry, and for maintenance of spermatogonia, which raised an insight into the sexual dimorphism of FZR1-regulation in germ cells.

Mechanisms for the temporal regulation of substrate ubiquitination by the anaphase-promoting complex/cyclosome

The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit, multifunctional ubiquitin ligase that controls the temporal degradation of numerous cell cycle regulatory proteins to direct the unidirectional cell cycle phases. Several different mechanisms contribute to ensure the correct order of substrate modification by the APC/C complex. Recent advances in biochemical, biophysical and structural studies of APC/C have provided a deep mechanistic insight into the working of this complex ubiquitin ligase. This complex displays remarkable conformational flexibility in response to various binding partners and post-translational modifications, which together regulate substrate selection and catalysis of APC/C. Apart from this, various features and modifications of the substrates also influence their recognition and affinity to APC/C complex. Ultimately, temporal degradation of substrates depends on the kind of ubiquitin modification received, the processivity of APC/C, and other extrinsic mechanisms. This review discusses our current understanding of various intrinsic and extrinsic mechanisms responsible for 'substrate ordering' by the APC/C complex.

APC/Cdh1 targets PECAM-1 for ubiquitination and degradation in endothelial cells

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a member of the immunoglobulin superfamily and is expressed by hematopoietic and endothelial cells (ECs). Recent studies have shown that PECAM-1 plays a crucial role in promoting the development of the EC inflammatory response in the context of disturbed flow. However, the mechanistic pathways that control PECAM-1 protein stability remain largely unclear. Here, we identified PECAM-1 as a novel substrate of the APC/Cdh1 E3 ubiquitin ligase. Specifically, lentivirus-mediated Cdh1 depletion stabilized PECAM-1 in ECs. Conversely, overexpression of Cdh1 destabilized PECAM-1. The proteasome inhibitor MG132 blocked Cdh1-mediated PECAM-1 degradation. In addition, Cdh1 promoted K48-linked polyubiquitination of PECAM-1 in a destruction box-dependent manner. Furthermore, we demonstrated that compared with pulsatile shear stress (PS), oscillatory shear stress decreased the expression of Cdh1 and the ubiquitination of PECAM-1, therefore stabilizing PECAM-1 to promote inflammation in ECs. Hence, our study revealed a novel mechanism by which fluid flow patterns regulate EC homeostasis via Cdh1-dependent ubiquitination and subsequent degradation of PECAM-1.

Visualizing the complex functions and mechanisms of the anaphase promoting complex/cyclosome (APC/C)

The anaphase promoting complex or cyclosome (APC/C) is a large multi-subunit E3 ubiquitin ligase that orchestrates cell cycle progression by mediating the degradation of important cell cycle regulators. During the two decades since its discovery, much has been learnt concerning its role in recognizing and ubiquitinating specific proteins in a cell-cycle-dependent manner, the mechanisms governing substrate specificity, the catalytic process of assembling polyubiquitin chains on its target proteins, and its regulation by phosphorylation and the spindle assembly checkpoint. The past few years have witnessed significant progress in understanding the quantitative mechanisms underlying these varied APC/C functions. This review integrates the overall functions and properties of the APC/C with mechanistic insights gained from recent cryo-electron microscopy (cryo-EM) studies of reconstituted human APC/C complexes.

Building a Regulatory Network with Short Linear Sequence Motifs: Lessons from the Degrons of the Anaphase-Promoting Complex

The anaphase-promoting complex or cyclosome (APC/C) is a ubiquitin ligase that polyubiquitinates specific substrates at precise times in the cell cycle, thereby triggering the events of late mitosis in a strict order. The robust substrate specificity of the APC/C prevents the potentially deleterious degradation of non-APC/C substrates and also averts the cell-cycle errors and genomic instability that could result from mistimed degradation of APC/C targets. The APC/C recognizes short linear sequence motifs, or degrons, on its substrates. The specific and timely modification and degradation of APC/C substrates is likely to be modulated by variations in degron sequence and context. We discuss the extensive affinity, specificity, and selectivity determinants encoded in APC/C degrons, and we describe some of the extrinsic mechanisms that control APC/C-substrate recognition. As an archetype for protein motif-driven regulation of cell function, the APC/C-substrate interaction provides insights into the general properties of post-translational regulatory systems.

New Functions of APC/C Ubiquitin Ligase in the Nervous System and Its Role in Alzheimer's Disease

The E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) regulates important processes in cells, such as the cell cycle, by targeting a set of substrates for degradation. In the last decade, APC/C has been related to several major functions in the nervous system, including axon guidance, synaptic plasticity, neurogenesis, and neuronal survival. Interestingly, some of the identified APC/C substrates have been related to neurodegenerative diseases. There is an accumulation of some degradation targets of APC/C in Alzheimer’s disease (AD) brains, which suggests a dysregulation of the protein complex in the disorder. Moreover, recently evidence has been provided for an inactivation of APC/C in AD. It has been shown that oligomers of the AD-related peptide, Aβ, induce degradation of the APC/C activator subunit cdh1, in vitro in neurons in culture and in vivo in the mouse hippocampus. Furthermore, in the AD mouse model APP/PS1, lower cdh1 levels were observed in pyramidal neurons in CA1 when compared to age-matched wildtype mice. In this review, we provide a complete list of APC/C substrates that are involved in the nervous system and we discuss their functions. We also summarize recent studies that show neurobiological effects in cdh1 knockout mouse models. Finally, we discuss the role of APC/C in the pathophysiology of AD.

Transcriptional and post-transcriptional regulation of Cdc20 during the spindle assembly checkpoint in S. cerevisiae

The anaphase-promoting complex (APC) is a ubiquitin ligase responsible for promoting the degradation of many cell cycle regulators. One of the activators and substrate-binding proteins for the APC is Cdc20. It has been shown previously that Cdc20 can promote its own degradation by the APC in normal cycling cells mainly through a cis-degradation mode (i.e. via an intramolecular mechanism). However, how Cdc20 is degraded during the spindle assembly checkpoint (SAC) is still not fully clear. In this study, we used a dual-Cdc20 system to investigate this issue and found that the cis-degradation mode is also the major pathway responsible for Cdc20 degradation during the SAC. In addition, we found that there is an inverse relationship between APC^Cdc20 activity and the transcriptional activity of the CDC20 promoter, which likely occurs through feedback regulation by APC^Cdc20 substrates, such as the cyclins Clb2 and Clb5. These findings contribute to our understanding of how the inhibition of APC^Cdc20 activity and enhanced Cdc20 degradation are required for proper spindle checkpoint arrest.

Molecular Mechanisms of Spindle Assembly Checkpoint Activation and Silencing

In eukaryotic cell division, the Spindle Assembly Checkpoint (SAC) plays a key regulatory role by monitoring the status of chromosome-microtubule attachments and allowing chromosome segregation only after all chromosomes are properly attached to spindle microtubules. While the identities of SAC components have been known, in some cases, for over two decades, the molecular mechanisms of the SAC have remained mostly mysterious until very recently. In the past few years, advances in biochemical reconstitution, structural biology, and bioinformatics have fueled an explosion in the molecular understanding of the SAC. This chapter seeks to synthesize these recent advances and place them in a biological context, in order to explain the mechanisms of SAC activation and silencing at a molecular level.

The mitotic checkpoint complex (MCC): looking back and forth after 15 years

The mitotic checkpoint is a specialized signal transduction pathway that contributes to the fidelity of chromosome segregation. The signaling of the checkpoint originates from defective kinetochore-microtubule interactions and leads to formation of the mitotic checkpoint complex (MCC), a highly potent inhibitor of the Anaphase Promoting Complex/Cyclosome (APC/C)—the E3 ubiquitin ligase essential for anaphase onset. Many important questions concerning the MCC and its interaction with APC/C have been intensively investigated and debated in the past 15 years, such as the exact composition of the MCC, how it is assembled during a cell cycle, how it inhibits APC/C, and how the MCC is disassembled to allow APC/C activation. These efforts have culminated in recently reported structure models for human MCC:APC/C supra-complexes at near-atomic resolution that shed light on multiple aspects of the mitotic checkpoint mechanisms. However, confusing statements regarding the MCC are still scattered in the literature, making it difficult for students and scientists alike to obtain a clear picture of MCC composition, structure, function and dynamics. This review will comb through some of the most popular concepts or misconceptions about the MCC, discuss our current understandings, present a synthesized model on regulation of CDC20 ubiquitination, and suggest a few future endeavors and cautions for next phase of MCC research.

APC/C and retinoblastoma interaction: cross-talk of retinoblastoma protein with the ubiquitin proteasome pathway

The ubiquitin (Ub) ligase anaphase promoting complex/cyclosome (APC/C) and the tumour suppressor retinoblastoma protein (pRB) play key roles in cell cycle regulation. APC/C is a critical regulator of mitosis and G₁-phase of the cell cycle whereas pRB keeps a check on proliferation by inhibiting transition to the S-phase. APC/C and pRB interact with each other via the co-activator of APC/C, FZR1, providing an alternative pathway of regulation of G₁ to S transition by pRB using a post-translational mechanism. Both pRB and FZR1 have complex roles and are implicated not only in regulation of cell proliferation but also in differentiation, quiescence, apoptosis, maintenance of chromosomal integrity and metabolism. Both are also targeted by transforming viruses. We discuss recent advances in our understanding of the involvement of APC/C and pRB in cell cycle based decisions and how these insights will be useful for development of anti-cancer and anti-viral drugs.

Molecular basis of APC/C regulation by the spindle assembly checkpoint

In the dividing eukaryotic cell, the spindle assembly checkpoint (SAC) ensures that each daughter cell inherits an identical set of chromosomes. The SAC coordinates the correct attachment of sister chromatid kinetochores to the mitotic spindle with activation of the anaphase-promoting complex (APC/C), the E3 ubiquitin ligase responsible for initiating chromosome separation. In response to unattached kinetochores, the SAC generates the mitotic checkpoint complex (MCC), which inhibits the APC/C and delays chromosome segregation. By cryo-electron microscopy, here we determine the near-atomic resolution structure of a human APC/C–MCC complex (APC/C(MCC)). Degron-like sequences of the MCC subunit BubR1 block degron recognition sites on Cdc20, the APC/C coactivator subunit responsible for substrate interactions. BubR1 also obstructs binding of the initiating E2 enzyme UbcH10 to repress APC/C ubiquitination activity. Conformational variability of the complex enables UbcH10 association, and structural analysis shows how the Cdc20 subunit intrinsic to the MCC (Cdc20(MCC)) is ubiquitinated, a process that results in APC/C reactivation when the SAC is silenced.

Substrate Recognition by the Cdh1 Destruction Box Receptor Is a General Requirement for APC/CCdh1-mediated Proteolysis

The anaphase-promoting complex, or cyclosome (APC/C), is a ubiquitin ligase that selectively targets proteins for degradation in mitosis and the G1 phase and is an important component of the eukaryotic cell cycle control system. How the APC/C specifically recognizes its substrates is not fully understood. Although well characterized degron motifs such as the destruction box (D-box) and KEN-box are commonly found in APC/C substrates, many substrates apparently lack these motifs. A variety of alternative APC/C degrons have been reported, suggesting either that multiple modes of substrate recognition are possible or that our definitions of degron structure are incomplete. We used an in vivo yeast assay to compare the G1 degradation rate of 15 known substrates of the APC/C co-activator Cdh1 under normal conditions and conditions that impair binding of D-box, KEN-box, and the recently identified ABBA motif degrons to Cdh1. The D-box receptor was required for efficient proteolysis of all Cdh1 substrates, despite the absence of canonical D-boxes in many. In contrast, the KEN-box receptor was only required for normal proteolysis of a subset of substrates and the ABBA motif receptor for a single substrate in our system. Our results suggest that binding to the D-box receptor may be a shared requirement for recognition and processing of all Cdh1 substrates.

The Non-Canonical Role of Aurora-A in DNA Replication

Aurora-A is a well-known mitotic kinase that regulates mitotic entry, spindle formation, and chromosome maturation as a canonical role. During mitosis, Aurora-A protein is stabilized by its phosphorylation at Ser51 via blocking anaphase-promoting complex/cyclosome-mediated proteolysis. Importantly, overexpression and/or hyperactivation of Aurora-A is involved in tumorigenesis via aneuploidy and genomic instability. Recently, the novel function of Aurora-A for DNA replication has been revealed. In mammalian cells, DNA replication is strictly regulated for preventing over-replication. Pre-replication complex (pre-RC) formation is required for DNA replication as an initiation step occurring at the origin of replication. The timing of pre-RC formation depends on the protein level of geminin, which is controlled by the ubiquitin–proteasome pathway. Aurora-A phosphorylates geminin to prevent its ubiquitin-mediated proteolysis at the mitotic phase to ensure proper pre-RC formation and ensuing DNA replication. In this review, we introduce the novel non-canonical role of Aurora-A in DNA replication.

Targeting Cdc20 as a novel cancer therapeutic strategy

The Anaphase Promoting Complex (APC, also called APC/C) regulates cell cycle progression by forming two closely related, but functionally distinct E3 ubiquitin ligase sub-complexes, APC(Cdc20) and APC(Cdh1), respectively. Emerging evidence has begun to reveal that Cdc20 and Cdh1 have opposing functions in tumorigenesis. Specifically, Cdh1 functions largely as a tumor suppressor, whereas Cdc20 exhibits an oncogenic function, suggesting that Cdc20 could be a promising therapeutic target for combating human cancer. However, the exact underlying molecular mechanisms accounting for their differences in tumorigenesis remain largely unknown. Therefore, in this review, we summarize the downstream substrates of Cdc20 and the critical functions of Cdc20 in cell cycle progression, apoptosis, ciliary disassembly and brain development. Moreover, we briefly describe the upstream regulators of Cdc20 and the oncogenic role of Cdc20 in a variety of human malignancies. Furthermore, we summarize multiple pharmacological Cdc20 inhibitors including TAME and Apcin, and their potential clinical benefits. Taken together, development of specific Cdc20 inhibitors could be a novel strategy for the treatment of human cancers with elevated Cdc20 expression.

The dynamics of signal amplification by macromolecular assemblies for the control of chromosome segregation

The control of chromosome segregation relies on the spindle assembly checkpoint (SAC), a complex regulatory system that ensures the high fidelity of chromosome segregation in higher organisms by delaying the onset of anaphase until each chromosome is properly bi-oriented on the mitotic spindle. Central to this process is the establishment of multiple yet specific protein-protein interactions in a narrow time-space window. Here we discuss the highly dynamic nature of multi-protein complexes that control chromosome segregation in which an intricate network of weak but cooperative interactions modulate signal amplification to ensure a proper SAC response. We also discuss the current structural understanding of the communication between the SAC and the kinetochore; how transient interactions can regulate the assembly and disassembly of the SAC as well as the challenges and opportunities for the definition and the manipulation of the flow of information in SAC signaling.

Functional characterization of Anaphase Promoting Complex/Cyclosome (APC/C) E3 ubiquitin ligases in tumorigenesis

The Anaphase Promoting Complex/Cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase that primarily governs cell cycle progression. APC/C is composed of at least 14 core subunits and recruits its substrates for ubiquitination via one of the two adaptor proteins, Cdc20 or Cdh1, in M or M/early G1 phase, respectively. Furthermore, recent studies have shed light on crucial functions for APC/C in maintaining genomic integrity, neuronal differentiation, cellular metabolism and tumorigenesis. To gain better insight into the in vivo physiological functions of APC/C in regulating various cellular processes, particularly development and tumorigenesis, a number of mouse models of APC/C core subunits, coactivators or inhibitors have been established and characterized. However, due to their essential role in cell cycle regulation, most of the germline knockout mice targeting the APC/C pathway are embryonic lethal, indicating the need for generating conditional knockout mouse models to assess the role in tumorigenesis for each APC/C signaling component in specific tissues. In this review, we will first provide a brief introduction of the ubiquitin-proteasome system (UPS) and the biochemical activities and cellular functions of the APC/C E3 ligase. We will then focus primarily on characterizing genetic mouse models used to understand the physiological roles of each APC/C signaling component in embryogenesis, cell proliferation, development and carcinogenesis. Finally, we discuss future research directions to further elucidate the physiological contributions of APC/C components during tumorigenesis and validate their potentials as a novel class of anti-cancer targets.

The APC/C activator FZR1 is essential for meiotic prophase I in mice

Fizzy-related 1 (FZR1) is an activator of the Anaphase promoting complex/cyclosome (APC/C) and an important regulator of the mitotic cell division cycle. Using a germ-cell-specific conditional knockout model we examined its role in entry into meiosis and early meiotic events in both sexes. Loss of APC/C(FZR1) activity in the male germline led to both a mitotic and a meiotic testicular defect resulting in infertility due to the absence of mature spermatozoa. Spermatogonia in the prepubertal testes of such mice had abnormal proliferation and delayed entry into meiosis. Although early recombination events were initiated, male germ cells failed to progress beyond zygotene and underwent apoptosis. Loss of APC/C(FZR1) activity was associated with raised cyclin B1 levels, suggesting that CDK1 may trigger apoptosis. By contrast, female FZR1Δ mice were subfertile, with premature onset of ovarian failure by 5 months of age. Germ cell loss occurred embryonically in the ovary, around the time of the zygotene-pachytene transition, similar to that observed in males. In addition, the transition of primordial follicles into the growing follicle pool in the neonatal ovary was abnormal, such that the primordial follicles were prematurely depleted. We conclude that APC/C(FZR1) is an essential regulator of spermatogonial proliferation and early meiotic prophase I in both male and female germ cells and is therefore important in establishing the reproductive health of adult male and female mammals.

Formation of multiprotein assemblies in the nucleus: the spindle assembly checkpoint

Specific interactions within the cell must occur in a crowded environment and often in a narrow time-space framework to ensure cell survival. In the light that up to 10% of individual protein molecules present at one time in mammalian cells mediate signal transduction, the establishment of productive, specific interactions is a remarkable achievement. The spindle assembly checkpoint (SAC) is an evolutionarily conserved and essential self-monitoring system of the eukaryotic cell cycle that ensures the high fidelity of chromosome segregation by delaying the onset of anaphase until all chromosomes are properly bi-oriented on the mitotic spindle. The function of the SAC involves communication with the kinetochore, an essential multiprotein complex crucial for chromosome segregation that assembles on mitotic or meiotic centromeres to link centromeric DNA with microtubules. Interactions in the SAC and kinetochore-microtubule network often involve the reversible assembly of large multiprotein complexes in which regions of the polypeptide chain that exhibit low structure complexity undergo a disorder-to-order transition. The confinement and high density of protein molecules in the cell has a profound effect on the stability, folding rate, and biological functions of individual proteins and protein assemblies. Here, I discuss the role of large and highly flexible surfaces that mediate productive intermolecular interactions in SAC signaling and postulate that macromolecular crowding contributes to the exquisite regulation that is required for the timely and accurate segregation of chromosomes in higher organisms.

CDC20 downregulation impairs spindle morphology and causes reduced first polar body emission during bovine oocyte maturation

The cell division cycle protein 20 (CDC20) is an essential regulator of cell division, encoded by the CDC20 gene. However, the role of CDC20 in bovine oocyte maturation is unknown. In this study, CDC20 morpholino antisense oligonucleotides (MOs) were microinjected into the cytoplasm of bovine oocytes to block the translation of CDC20 mRNA. CDC20 downregulation significantly reduced the rate of first polar body emission (PB1). Further analysis indicated that oocytes treated with CDC20 MO arrested before or at meiotic stage I with abnormal spindles. To further confirm the functions of CDC20 during oocyte meiotic division, CDC20 MOs were microinjected into oocytes together with a supplementary PB1. The results showed that newly synthesized CDC20 was not necessary at the meiosis II-to-anaphase II transition. Our data suggest that CDC20 is required for spindle assembly, chromosomal segregation, and PB1 extrusion during bovine oocyte maturation.

The HECT type ubiquitin ligase NEDL2 is degraded by anaphase-promoting complex/cyclosome (APC/C)-Cdh1, and its tight regulation maintains the metaphase to anaphase transition

NEDD4-like ubiquitin ligase 2 (NEDL2) is a HECT type ubiquitin ligase. NEDL2 enhances p73 transcriptional activity and degrades ATR kinase in lamin misexpressed cells. Compared with the important functions of other HECT type ubiquitin ligase, there is less study concerning the function and regulation of NEDL2. Using primary antibody immunoprecipitation and mass spectrometry, we identify a list of potential proteins that are putative NEDL2-interacting proteins. The candidate list contains many of mitotic proteins, especially including several subunits of anaphase-promoting complex/cyclosome (APC/C) and Cdh1, an activator of APC/C. Cdh1 can interact with NEDL2 in vivo and in vitro. Cdh1 recognizes one of the NEDL2 destruction boxes (R(740)GSL(743)) and targets it for degradation in an APC/C-dependent manner during mitotic exit. Overexpression of Cdh1 reduces the protein level of NEDL2, whereas knockdown of Cdh1 increases the protein level of NEDL2 but has no effect on the NEDL2 mRNA level. NEDL2 associates with mitotic spindles, and its protein level reaches a maximum in mitosis. The function of NEDL2 during mitosis is essential because NEDL2 depletion prolongs metaphase, and overexpression of NEDL2 induces chromosomal lagging. Elevated expression of NEDL2 protein and mRNA are both found in colon cancer and cervix cancer. We conclude that NEDL2 is a novel substrate of APC/C-Cdh1 as cells exit mitosis and functions as a regulator of the metaphase to anaphase transition. Its overexpression may contribute to tumorigenesis.

APC/C(Cdh1)-dependent degradation of Cdc20 requires a phosphorylation on CRY-box by Polo-like kinase-1 during somatic cell cycle

Cdc20 is an activator of the anaphase-promoting complex (APC/C), and APC/C(Cdc20) is essential for metaphase-anaphase transition. To allow progression beyond mitosis, Cdc20 is degraded through KEN-box-dependent APC/C(Cdh1) activity. Mammalian Cdc20 contains the CRY box, a second APC/C(Cdh1)-dependent degron, but the molecular mechanism in degradation process remains undefined. Polo-like kinase-1 (Plk1) is an essential mitotic kinase regulating various targets in kinetochore, centrosome, and midbody for proper mitotic progression. Plk1 directly bound to Cdc20 and phosphorylates it on serine-170 located in CRY-box. Whereas wild-type Cdc20 was degraded according to progress cell cycle beyond mitosis, the phosphorylation-defective mutant, which serine-170 was changed into alanine, was not destroyed in early G1 phase. The phosphorylation on serine-170 by Plk1 was important for ubiquitination and Cdh1-dependent proteolysis. However, this modification by Plk1 on CRY box had no effect on the subcellular localization of Cdc20 and the formation of APC/C-inhibitory checkpoint complexes under spindle assembly checkpoint. This mechanism will be the first finding of inhibitory phosphorylation related to Cdc20 instability.

Myosin 16 levels fluctuate during the cell cycle and are downregulated in response to DNA replication stress

Myosins comprise a highly conserved superfamily of eukaryotic actin-dependent motor proteins implicated in a large repertoire of functions in both the cytoplasm and the nucleus. Class XVI myosin, MYO16, reveals expression in most somatic as well as meiotic cells with prominent localization in the nucleus, excepting the nucleolus; however, the role(s) of Myo16 in the nucleus remain unknown. In this report, we investigated Myo16 abundance during transit through the cell cycle. Immunolocalization, immunoblot, flow cytometric and quantitative RT-PCR studies performed in Rat2 cells indicate that Myo16 mRNA and protein abundance are cell cycle regulated: in the unperturbed cell cycle, each rises to peak levels in late G1 and thereon through S-phase and each decays as cells enter M-phase. Notably, RNA interference-induced Myo16 depletion results in altered cell cycle distribution as well as in large-scale cell death. In response to DNA replication stress (impaired replication fork progression as a consequence of DNA damage, lack of sufficient deoxynucleotides, or inhibition of DNA polymerases), Myo16 protein shows substantial loss. Attenuation of replication stress (aphidicolin or hydroxyurea) is followed by a recovery of Myo16 expression and resumption of S-phase progression. Collectively, these observations suggest that Myo16 may play a regulatory role in cell cycle progression.

The ubiquitin proteasome system - implications for cell cycle control and the targeted treatment of cancer

Two families of E3 ubiquitin ligases are prominent in cell cycle regulation and mediate the timely and precise ubiquitin-proteasome-dependent degradation of key cell cycle proteins: the SCF (Skp1/Cul1/F-box protein) complex and the APC/C (anaphase promoting complex or cyclosome). While certain SCF ligases drive cell cycle progression throughout the cell cycle, APC/C (in complex with either of two substrate recruiting proteins: Cdc20 and Cdh1) orchestrates exit from mitosis (APC/C(Cdc20)) and establishes a stable G1 phase (APC/C(Cdh1)). Upon DNA damage or perturbation of the normal cell cycle, both ligases are involved in checkpoint activation. Mechanistic insight into these processes has significantly improved over the last ten years, largely due to a better understanding of APC/C and the functional characterization of multiple F-box proteins, the variable substrate recruiting components of SCF ligases. Here, we review the role of SCF- and APC/C-mediated ubiquitylation in the normal and perturbed cell cycle and discuss potential clinical implications of SCF and APC/C functions. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

APC(FZR1) prevents nondisjunction in mouse oocytes by controlling meiotic spindle assembly timing

The APC activator FZR1 has a role in controlling the timing of meiosis I spindle assembly. Oocytes lacking FZR1 undergo accelerated meiosis I, associated with earlier spindle assembly checkpoint satisfaction and APC^CDC20 activity, resulting in high rates of aneuploidy.

FZR1 is an anaphase-promoting complex (APC) activator best known for its role in the mitotic cell cycle at M-phase exit, in G1, and in maintaining genome integrity. Previous studies also established that it prevents meiotic resumption, equivalent to the G2/M transition. Here we report that mouse oocytes lacking FZR1 undergo passage through meiosis I that is accelerated by ∼1 h, and this is due to an earlier onset of spindle assembly checkpoint (SAC) satisfaction and APC^CDC20 activity. However, loss of FZR1 did not compromise SAC functionality; instead, earlier SAC satisfaction was achieved because the bipolar meiotic spindle was assembled more quickly in the absence of FZR1. This novel regulation of spindle assembly by FZR1 led to premature bivalent attachment to microtubules and loss of kinetochore-bound MAD2. Bivalents, however, were observed to congress poorly, leading to nondisjunction rates of 25%. We conclude that in mouse oocytes FZR1 controls the timing of assembly of the bipolar spindle and in so doing the timing of SAC satisfaction and APC^CDC20 activity. This study implicates FZR1 as a major regulator of prometaphase whose activity helps to prevent chromosome nondisjunction.

Regulation of APC/C-Cdh1 and its function in neuronal survival

Neurons are post-mitotic cells that undergo an active downregulation of cell cycle-related proteins to survive. The activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that regulates cell cycle progression in proliferating cells, plays a relevant role in post-mitotic neurons. Recent advances in the study of the regulation of APC/C have documented that the APC/C-activating cofactor, Cdh1, is essential for the function(s) of APC/C in neuronal survival. Here, we review the normal regulation of APC/C activity in proliferating cells and neurons. We conclude that in neurons the APC/C-Cdh1 complex actively downregulates the stability of the cell cycle protein cyclin B1 and the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3. Keeping these proteins destabilized is critical both for preventing the aberrant reentry of post-mitotic neurons into the cell cycle and for maintaining their reduced antioxidant status. Further understanding of the pathophysiological regulation of these proteins by APC/C-Cdh1 in neurons will be important for the search for novel therapeutic targets against neurodegeneration.

GPS-ARM: computational analysis of the APC/C recognition motif by predicting D-boxes and KEN-boxes

Anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase incorporated with Cdh1 and/or Cdc20 recognizes and interacts with specific substrates, and faithfully orchestrates the proper cell cycle events by targeting proteins for proteasomal degradation. Experimental identification of APC/C substrates is largely dependent on the discovery of APC/C recognition motifs, e.g., the D-box and KEN-box. Although a number of either stringent or loosely defined motifs proposed, these motif patterns are only of limited use due to their insufficient powers of prediction. We report the development of a novel GPS-ARM software package which is useful for the prediction of D-boxes and KEN-boxes in proteins. Using experimentally identified D-boxes and KEN-boxes as the training data sets, a previously developed GPS (Group-based Prediction System) algorithm was adopted. By extensive evaluation and comparison, the GPS-ARM performance was found to be much better than the one using simple motifs. With this powerful tool, we predicted 4,841 potential D-boxes in 3,832 proteins and 1,632 potential KEN-boxes in 1,403 proteins from H. sapiens, while further statistical analysis suggested that both the D-box and KEN-box proteins are involved in a broad spectrum of biological processes beyond the cell cycle. In addition, with the co-localization information, we predicted hundreds of mitosis-specific APC/C substrates with high confidence. As the first computational tool for the prediction of APC/C-mediated degradation, GPS-ARM is a useful tool for information to be used in further experimental investigations. The GPS-ARM is freely accessible for academic researchers at: http://arm.biocuckoo.org.

Structural insights into anaphase-promoting complex function and mechanism

The anaphase-promoting complex or cyclosome (APC/C) controls sister chromatid segregation and the exit from mitosis by catalysing the ubiquitylation of cyclins and other cell cycle regulatory proteins. This unusually large E3 RING-cullin ubiquitin ligase is assembled from 13 different proteins. Selection of APC/C targets is controlled through recognition of short destruction motifs, predominantly the D box and KEN box. APC/C-mediated coordination of cell cycle progression is achieved through the temporal regulation of APC/C activity and substrate specificity, exerted through a combination of co-activator subunits, reversible phosphorylation and inhibitory proteins and complexes. Recent structural and biochemical studies of the APC/C are beginning to reveal an understanding of the roles of individual APC/C subunits and co-activators and how they mutually interact to mediate APC/C functions. This review focuses on the findings showing how information on the structural organization of the APC/C provides insights into the role of co-activators and core APC/C subunits in mediating substrate recognition. Mechanisms of regulating and modulating substrate recognition are discussed in the context of controlling the binding of the co-activator to the APC/C, and the accessibility and conformation of the co-activator when bound to the APC/C.

REV7 is required for anaphase-promoting complex-dependent ubiquitination and degradation of translesion DNA polymerase REV1

REV1 is a Y-family polymerase specialized for replicating across DNA lesions at the stalled replication folk. Due to the high error rate of REV1-dependent translesion DNA synthesis (TLS), tight regulation of REV1 activity is essential. Here, we show that human REV1 undergoes proteosomal degradation mediated by the E3 ubiquitin ligase known as anaphase-promoting complex (APC). REV1 associates with APC. Overexpression of APC coactivator CDH1 or CDC20 promotes polyubiquitination and proteosomal degradation of REV1. Surprisingly, polyubiquitination of REV1 also requires REV7, a TLS accessory protein that interacts with REV1 and other TLS polymerases. The N-terminal region of REV1 contains both the APC degron and an additional REV7-binding domain. Depletion of REV7 by RNA interference stabilizes REV1 by preventing polyubiquitination, whereas overexpression of REV7 augments REV1 degradation. Taken together, our findings suggest a role of REV7 in governing REV1 stability and interplay between TLS and APC-dependent proteolysis.

N-2-Propenyl-(5-dimethylamino)-1-naphthalene Sulfonamide, a Novel Fluorescent Monomer for the Molecularly Imprinted Polymer-Based Detection of 2,4-Dinitrotoluene in the Gas Phase

Fluorescent molecularly imprinted polymers (MIP) specific for 2,4-dinitrotoluene (DNT) have been synthesised using a novel monomer N-2-propenyl-(5-dimethylamino)-1-naphthalene sulfonamide. Three formats of the polymer were produced: a traditional bulk monolith ground into particles, a flexible, but highly cross-linked plasticiser-modified free standing membrane, and a hybrid material consisting of particles embedded in a poly(acrylonitrile) phase inversed film. Within all materials, a clearly defined imprinting effect was observed upon exposure to DNT vapour at room temperature. In all cases, preferential rebinding of DNT to the molecularly imprinted materials (3–5 times) over their non-imprinted (NIP) equivalents was evident within <10 min of contact with the DNT vapour stream. Fluorographic images of the fluorescent polymers showed the DNT binding-induced quenching to be significantly higher in the MIP material than in the non-imprinted control polymer.

APC/C(Cdh1)-mediated degradation of the F-box protein NIPA is regulated by its association with Skp1

NIPA (Nuclear Interaction Partner of Alk kinase) is an F-box like protein that targets nuclear Cyclin B1 for degradation. Integrity and therefore activity of the SCF^NIPA E3 ligase is regulated by cell-cycle-dependent phosphorylation of NIPA, restricting substrate ubiquitination to interphase. Here we show that phosphorylated NIPA is degraded in late mitosis in an APC/C^Cdh1-dependent manner. Binding of the unphosphorylated form of NIPA to Skp1 interferes with binding to the APC/C-adaptor protein Cdh1 and therefore protects unphosphorylated NIPA from degradation in interphase. Our data thus define a novel mode of regulating APC/C-mediated ubiquitination.

Novel functions for the anaphase-promoting complex in neurobiology

In recent years, diverse and unexpected neurobiological functions have been uncovered for the major cell cycle-regulated ubiquitin ligase, the anaphase-promoting complex (APC). Functions of the APC in the nervous system range from orchestrating neuronal morphogenesis and synapse development to the regulation of neuronal differentiation, survival, and metabolism. The APC acts together with the coactivating proteins Cdh1 and Cdc20 in neural cells to target specific substrates for ubiquitination and consequent degradation by the proteasome. As we continue to unravel APC functions and mechanisms in neurobiology, these studies should advance our understanding of the molecular mechanisms of neuronal connectivity, with important implications for the study of brain development and disease.

APC/CCdh1-dependent proteolysis of USP1 regulates the response to UV-mediated DNA damage

APC/C^Cdh1-dependent degradation of USP1 allows for PCNA monoubiquitination and subsequent recruitment of trans-lesion synthesis polymerase to UV repair sites.

Targeted protein destruction of critical cellular regulators during the G1 phase of the cell cycle is achieved by anaphase-promoting complex/cyclosome^Cdh1 (APC/C^Cdh1), a multisubunit E3 ubiquitin ligase. Cells lacking Cdh1 have been shown to accumulate deoxyribonucleic acid (DNA) damage, suggesting that it may play a previously unrecognized role in maintaining genomic stability. The ubiquitin-specific protease 1 (USP1) is a known critical regulator of DNA repair and genomic stability. In this paper, we report that USP1 was degraded in G1 via APC/C^Cdh1. USP1 levels were kept low in G1 to provide a permissive condition for inducing proliferating cell nuclear antigen (PCNA) monoubiquitination in response to ultraviolet (UV) damage before DNA replication. Importantly, expression of a USP1 mutant that cannot be degraded via APC/C^Cdh1 inhibited PCNA monoubiquitination during G1, likely compromising the recruitment of trans-lesion synthesis polymerase to UV repair sites. Thus, we propose a role for APC/C^Cdh1 in modulating the status of PCNA monoubiquitination and UV DNA repair before S phase entry.

Conserved CDC20 cell cycle functions are carried out by two of the five isoforms in Arabidopsis thaliana

Background

The CDC20 and Cdh1/CCS52 proteins are substrate determinants and activators of the Anaphase Promoting Complex/Cyclosome (APC/C) E3 ubiquitin ligase and as such they control the mitotic cell cycle by targeting the degradation of various cell cycle regulators. In yeasts and animals the main CDC20 function is the destruction of securin and mitotic cyclins. Plants have multiple CDC20 gene copies whose functions have not been explored yet. In Arabidopsis thaliana there are five CDC20 isoforms and here we aimed at defining their contribution to cell cycle regulation, substrate selectivity and plant development.

Methodology/Principal Findings

Studying the gene structure and phylogeny of plant CDC20s, the expression of the five AtCDC20 gene copies and their interactions with the APC/C subunit APC10, the CCS52 proteins, components of the mitotic checkpoint complex (MCC) and mitotic cyclin substrates, conserved CDC20 functions could be assigned for AtCDC20.1 and AtCDC20.2. The other three intron-less genes were silent and specific for Arabidopsis. We show that AtCDC20.1 and AtCDC20.2 are components of the MCC and interact with mitotic cyclins with unexpected specificity. AtCDC20.1 and AtCDC20.2 are expressed in meristems, organ primordia and AtCDC20.1 also in pollen grains and developing seeds. Knocking down both genes simultaneously by RNAi resulted in severe delay in plant development and male sterility. In these lines, the meristem size was reduced while the cell size and ploidy levels were unaffected indicating that the lower cell number and likely slowdown of the cell cycle are the cause of reduced plant growth.

Conclusions/Significance

The intron-containing CDC20 gene copies provide conserved and redundant functions for cell cycle progression in plants and are required for meristem maintenance, plant growth and male gametophyte formation. The Arabidopsis-specific intron-less genes are possibly “retrogenes” and have hitherto undefined functions or are pseudogenes.

Essential role of protein phosphatase 2A in metaphase II arrest and activation of mouse eggs shown by okadaic acid, dominant negative protein phosphatase 2A, and FTY720

Vertebrate eggs arrest at second meiotic metaphase. The fertilizing sperm causes meiotic exit through Ca(2+)-mediated activation of the anaphase-promoting complex/cyclosome (APC/C). Although the loss in activity of the M-phase kinase CDK1 is known to be an essential downstream event of this process, the contribution of phosphatases to arrest and meiotic resumption is less apparent, especially in mammals. Therefore, we explored the role of protein phosphatase 2A (PP2A) in mouse eggs using pharmacological inhibition and activation as well as a functionally dominant-negative catalytic PP2A subunit (dn-PP2Ac-L199P) coupled with live cell imaging. We observed that PP2A inhibition using okadaic acid induced events normally observed at fertilization: degradation of the APC/C substrates cyclin B1 and securin resulting from loss of the APC/C inhibitor Emi2. Although sister chromatids separated, chromatin remained condensed, and polar body extrusion was blocked as a result of a rapid spindle disruption, which could be ameliorated by non-degradable cyclin B1, suggesting that spindle integrity was affected by CDK1 loss. Similar cell cycle effects to okadaic acid were also observed using dominant-negative PP2Ac. Preincubation of eggs with the PP2A activator FTY720 could block many of the actions of okadaic acid, including Emi2, cyclin B1, and securin degradation and sister chromatid separation. Therefore, in conclusion, we used okadaic acid, dn-PP2Ac-L199P, and FTY720 on mouse eggs to demonstrate that PP2A is needed to for both continued metaphase arrest and successful exit from meiosis.

The APC/C activator FZR1 coordinates the timing of meiotic resumption during prophase I arrest in mammalian oocytes

FZR1, an activator of the anaphase-promoting complex/cyclosome (APC/C), is recognized for its roles in the mitotic cell cycle. To examine its meiotic function in females we generated an oocyte-specific knockout of the Fzr1 gene (Fzr1Δ/Δ). The total number of fully grown oocytes enclosed in cumulus complexes was 35-40% lower in oocytes from Fzr1Δ/Δ mice and there was a commensurate rise in denuded, meiotically advanced and/or fragmented oocytes. The ability of Fzr1Δ/Δ oocytes to remain prophase I/germinal vesicle (GV) arrested in vitro was also compromised, despite the addition of the phosphodiesterase milrinone. Meiotic competency of smaller diameter oocytes was also accelerated by Fzr1 loss. Cyclin B1 levels were elevated ~5-fold in Fzr1Δ/Δ oocytes, whereas securin and CDC25B, two other APC/CFZR1 substrates, were unchanged. Cyclin B1 overexpression can mimic the effects of Fzr1 loss on GV arrest and here we show that cyclin B1 knockdown in Fzr1Δ/Δ oocytes affects the timing of meiotic resumption. Therefore, the effects of Fzr1 loss are mediated, at least in part, by raised cyclin B1. Thus, APC/CFZR1 activity is required to repress cyclin B1 levels in oocytes during prophase I arrest in the ovary, thereby maintaining meiotic quiescence until hormonal cues trigger resumption.

Orderly inactivation of the key checkpoint protein mitotic arrest deficient 2 (MAD2) during mitotic progression

Anaphase is promoted by the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) only when all the chromosomes have achieved bipolar attachment to the mitotic spindles. Unattached kinetochores or the absence of tension between the paired kinetochores activates a surveillance mechanism termed the spindle-assembly checkpoint. A fundamental principle of the checkpoint is the activation of mitotic arrest deficient 2 (MAD2). MAD2 then forms a diffusible complex called mitotic checkpoint complex (designated as MAD2(MCC)) before it is recruited to APC/C (designated as MAD2(APC/C)). Large gaps in our knowledge remain on how MAD2 is inactivated after the checkpoint is satisfied. In this study, we have investigated the regulation of MAD2-containing complexes during mitotic progression. Using selective immunoprecipitation of checkpoint components and gel filtration chromatography, we found that MAD2(MCC) and MAD2(APC/C) were regulated very differently during mitotic exit. Temporally, MAD2(MCC) was broken down ahead of MAD2(APC/C). The inactivation of the two complexes also displayed different requirements of proteolysis; although APC/C and proteasome activities were dispensable for MAD2(MCC) inactivation, they are required for MAD2(APC/C) inactivation. In fact, the degradation of CDC20 is inextricably linked to the breakdown of MAD2(APC/C). These data extended our understanding of the checkpoint complexes during checkpoint silencing.

State of the APC/C: organization, function, and structure

The ubiquitin-proteasome protein degradation system is involved in many essential cellular processes including cell cycle regulation, cell differentiation, and the unfolded protein response. The anaphase-promoting complex/cyclosome (APC/C), an evolutionarily conserved E3 ubiquitin ligase, was discovered 15 years ago because of its pivotal role in cyclin degradation and mitotic progression. Since then, we have learned that the APC/C is a very large, complex E3 ligase composed of 13 subunits, yielding a molecular machine of approximately 1 MDa. The intricate regulation of the APC/C is mediated by the Cdc20 family of activators, pseudosubstrate inhibitors, protein kinases and phosphatases and the spindle assembly checkpoint. The large size, complexity, and dynamic nature of the APC/C represent significant obstacles toward high-resolution structural techniques; however, over the last decade, there have been a number of lower resolution APC/C structures determined using single particle electron microscopy. These structures, when combined with data generated from numerous genetic and biochemical studies, have begun to shed light on how APC/C activity is regulated. Here, we discuss the most recent developments in the APC/C field concerning structure, substrate recognition, and catalysis.

Structure, function and mechanism of the anaphase promoting complex (APC/C)

The complex molecular events responsible for coordinating chromosome replication and segregation with cell division and growth are collectively known as the cell cycle. Progression through the cell cycle is orchestrated by the interplay between controlled protein synthesis and degradation and protein phosphorylation. Protein degradation is primarily regulated through the ubiquitin proteasome system, mediated by two related E3 protein ubiquitin ligases, the Skp1 cullin F-box (SCF) and the anaphase promoting complex (also known as the cyclosome) (APC/C). The APC/C is a multi-subunit cullin-RING E3 ubiquitin ligase that regulates progression through the mitotic phase of the cell cycle and controls entry into S phase by catalysing the ubiquitylation of cyclins and other cell cycle regulatory proteins. Selection of APC/C targets is controlled through recognition of short destruction motifs, predominantly the D-box and KEN-box. APC/C-mediated coordination of cell cycle progression is achieved through the temporal regulation of APC/C activity and substrate specificity, exerted through a combination of co-activator subunits, reversible phosphorylation and inhibitory proteins and complexes. The aim of this article is to discuss the APC/C from a structural and mechanistic perspective. Although an atomic structure of the APC/C is still lacking, a combination of genetic, biochemical, electron microscopy studies of intact APC/C and crystallographic analysis of individual subunits, together with analogies to evolutionarily related E3 ligases of the RING family, has provided deep insights into the molecular mechanisms of catalysis and substrate recognition, and structural organisation of the APC/C.

Regulated degradation of the APC coactivator Cdc20

Background

Cdc20 is a highly conserved activator of the anaphase-promoting complex (APC), promoting cell-cycle-regulated ubiquitination and proteolysis of a number of critical cell-cycle-regulatory targets including securin and mitotic cyclins. APC-Cdc20 activity is tightly regulated, and this regulation is likely important for accurate cell cycle control. One significant component of Cdc20 regulation is thought to be Cdc20 proteolysis. However, published literature suggests different mechanisms and requirements for Cdc20 proteolysis. The degree to which Cdc20 proteolysis is cell-cycle regulated, the dependence of Cdc20 proteolysis on Cdc20 destruction boxes (recognition sequences for APC-mediated ubiqutination, either by Cdc20 or by the related Cdh1 APC activator), and the need for APC itself for Cdc20 proteolysis all have been disputed to varying extents. In animals, Cdc20 proteolysis is thought to be mediated by Cdh1, contributing an intrinsic order of APC activation by Cdc20 and then by Cdh1. One report suggests a Cdh1 requirement for Cdc20 proteolysis in budding yeast; this idea has not been tested further.

Results

We characterized Cdc20 proteolysis using Cdc20 expressed from its endogenous locus; previous studies generally employed strongly overexpressed Cdc20, which can cause significant artifacts. We analyzed Cdc20 proteolysis with or without mutations in previously identified destruction box sequences, using varying methods of cell cycle synchronization, and in the presence or absence of Cdh1. Cdc20 instability is only partially dependent on destruction boxes. A much stronger dependence on Cdh1 for Cdc20 proteolysis was observed, but Cdh1-independent proteolysis was also clearly observed. Cdc20 proteolysis independent of both destruction boxes and Cdh1 was especially detectable around the G1/S transition; Cdh1-dependent proteolysis was most notable in late mitosis and G1.

Conclusions

Cdc20 proteolysis is under complex control, with different systems operating at different points in the cell cycle. This complexity is likely to explain apparent conflicts in previously published literature on this subject. A major mode of control of Cdc20 proteolysis occurs in late mitosis/early G1 and is Cdh1-dependent, as in animal cells; this mode may contribute to the known sequential activation of the APC by Cdc20 followed by Cdh1. An independent mode of Cdc20 proteolysis, independent of destruction boxes and Cdh1, occurs at G1/S; we do not know the mechanism or function of this mode of proteolysis, but speculate that it may contribute to sharpening and restricting activation of APC-Cdc20 to early mitosis.

Interplay between Cdh1 and JNK activity during the cell cycle

The ubiquitin ligase APC/C(Cdh1) coordinates degradation of key cell cycle regulators. We report here that a nuclear-localized portion of the stress-activated kinase JNK is degraded by the APC/C(Cdh1) during exit from mitosis and the G1 phase of the cell cycle. Expression of a non-degradable JNK induces prometaphase-like arrest and aberrant mitotic spindle dynamics. Moreover, JNK phosphorylates Cdh1 directly, during G2 and early mitosis, changing its subcellular localization and attenuating its ability to activate the APC/C during G2/M. This regulatory mechanism between JNK and Cdh1 reveals an important function for JNK during the cell cycle.

The dynamic ubiquitin ligase duo: Cdh1-APC and Cdc20-APC regulate neuronal morphogenesis and connectivity

The proper development and patterning of axons, dendrites, and synapses is essential for the establishment of accurate neuronal circuits in the brain. A major goal in neurobiology is to identify the mechanisms and principles that govern these fundamental developmental events of neuronal circuit formation. In recent years, exciting new studies have suggested that ubiquitin signaling pathways may play crucial roles in the control of neuronal connectivity. Among E3 ubiquitin ligases, Cdh1-anaphase promoting complex (Cdh1-APC) and Cdc20-APC have emerged as key regulators of diverse aspects of neuronal connectivity, from axon and dendrite morphogenesis to synapse differentiation and remodeling.

The emerging role of APC/CCdh1 in controlling differentiation, genomic stability and tumor suppression

Deregulation of the G1/G0 phase of the cell cycle can lead to cancer. During G1, most cells commit alternatively to DNA replication and division, or to cell-cycle exit and differentiation. The anaphase-promoting complex or cyclosome (APC/C) activated by Cdh1 coordinately eliminates positive cell-cycle regulators as well as inhibitors of differentiation, thereby coupling cell-cycle exit and differentiation. Misregulation of Cdh1 thus has the potential to promote both cell-cycle re-entry and either perturbed differentiation or dedifferentiation. In addition, APC/C(Cdh1) is required to maintain genomic stability. As a result, loss of Cdh1 can contribute to tumorigenesis in the form of proliferation of poorly differentiated and genetically unstable cells.