The role of the destruction box and its neighbouring lysine residues in cyclin B for anaphase ubiquitin-dependent proteolysis in fission yeast: defining the D-box receptor

Synthesis of ubiquitinated proteins for biochemical and functional analysis

Ubiquitination plays a crucial role in controlling various biological processes such as translation, DNA repair and immune response. Protein degradation for example, is one of the main processes which is controlled by the ubiquitin system and has significant implications on human health. In order to investigate these processes and the roles played by different ubiquitination patterns on biological systems, homogeneously ubiquitinated proteins are needed. Notably, these conjugates that are made enzymatically in cells cannot be easily obtained in large amounts and high homogeneity by employing such strategies. Therefore, chemical and semisynthetic approaches have emerged to prepare different ubiquitinated proteins. In this review, we will present the key synthetic strategies and their applications for the preparation of various ubiquitinated proteins. Furthermore, the use of these precious conjugates in different biochemical and functional studies will be highlighted.

Meiotic Cell Cycle Progression in Mouse Oocytes: Role of Cyclins

All eukaryotic cells, including oocytes, utilize an engine called cyclin-dependent kinase (Cdk) to drive the cell cycle. Cdks are activated by a co-factor called cyclin, which regulates their activity. The key Cdk-cyclin complex that regulates the oocyte cell cycle is known as Cdk1-cyclin B1. Recent studies have elucidated the roles of other cyclins, such as B2, B3, A2, and O, in oocyte cell cycle regulation. This review aims to discuss the recently discovered roles of various cyclins in mouse oocyte cell cycle regulation in accordance with the sequential progression of the cell cycle. In addition, this review addresses the translation and degradation of cyclins to modulate the activity of Cdks. Overall, the literature indicates that each cyclin performs unique and redundant functions at various stages of the cell cycle, while their expression and degradation are tightly regulated. Taken together, this review provides new insights into the regulatory role and function of cyclins in oocyte cell cycle progression.

Overexpression of a Novel Noxo1 Mutant Increases Ros Production and Noxo1 Relocalisation

Noxo1, the organizing element of the Nox1-dependent NADPH oxidase complex responsible for producing reactive oxygen species, has been described to be degraded by the proteasome. We mutated a D-box in Noxo1 to express a protein with limited degradation and capable of maintaining Nox1 activation. Wild-type (wt) and mutated Noxo1 (mut1) proteins were expressed in different cell lines to characterize their phenotype, functionality, and regulation. Mut1 increases ROS production through Nox1 activity affects mitochondrial organization and increases cytotoxicity in colorectal cancer cell lines. Unexpectedly the increased activity of Noxo1 is not related to a blockade of its proteasomal degradation since we were unable in our conditions to see any proteasomal degradation either for wt or mut1 Noxo1. Instead, D-box mutation mut1 leads to an increased translocation from the membrane soluble fraction to a cytoskeletal insoluble fraction compared to wt Noxo1. This mut1 localization is associated in cells with a filamentous phenotype of Noxo1, which is not observed with wt Noxo1. We found that mut1 Noxo1 associates with intermediate filaments such as keratin 18 and vimentin. In addition, Noxo1 D-Box mutation increases Nox1-dependent NADPH oxidase activity. Altogether, Nox1 D-box does not seem to be involved in Noxo1 degradation but rather related to the maintenance of the Noxo1 membrane/cytoskeleton balance.

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.

The 20S as a stand-alone proteasome in cells can degrade the ubiquitin tag

The proteasome, the primary protease for ubiquitin-dependent proteolysis in eukaryotes, is usually found as a mixture of 30S, 26S, and 20S complexes. These complexes have common catalytic sites, which makes it challenging to determine their distinctive roles in intracellular proteolysis. Here, we chemically synthesize a panel of homogenous ubiquitinated proteins, and use them to compare 20S and 26S proteasomes with respect to substrate selection and peptide-product generation. We show that 20S proteasomes can degrade the ubiquitin tag along with the conjugated substrate. Ubiquitin remnants on branched peptide products identified by LC-MS/MS, and flexibility in the 20S gate observed by cryo-EM, reflect the ability of the 20S proteasome to proteolyze an isopeptide-linked ubiquitin-conjugate. Peptidomics identifies proteasome-trapped ubiquitin-derived peptides and peptides of potential 20S substrates in Hi20S cells, hypoxic cells, and human failing-heart. Moreover, elevated levels of 20S proteasomes appear to contribute to cell survival under stress associated with damaged proteins.

A prometaphase mechanism of securin destruction is essential for meiotic progression in mouse oocytes

Successful cell division relies on the timely removal of key cell cycle proteins such as securin. Securin inhibits separase, which cleaves the cohesin rings holding chromosomes together. Securin must be depleted before anaphase to ensure chromosome segregation occurs with anaphase. Here we find that in meiosis I, mouse oocytes contain an excess of securin over separase. We reveal a mechanism that promotes excess securin destruction in prometaphase I. Importantly, this mechanism relies on two phenylalanine residues within the separase-interacting segment (SIS) of securin that are only exposed when securin is not bound to separase. We suggest that these residues facilitate the removal of non-separase-bound securin ahead of metaphase, as inhibiting this period of destruction by mutating both residues causes the majority of oocytes to arrest in meiosis I. We further propose that cellular securin levels exceed the amount an oocyte is capable of removing in metaphase alone, such that the prometaphase destruction mechanism identified here is essential for correct meiotic progression in mouse oocytes.

TRAF2 Is a Novel Ubiquitin E3 Ligase for the Na,K-ATPase β-Subunit That Drives Alveolar Epithelial Dysfunction in Hypercapnia

Several acute and chronic lung diseases are associated with alveolar hypoventilation leading to accumulation of CO2 (hypercapnia). The β-subunit of the Na,K-ATPase plays a pivotal role in maintaining epithelial integrity by functioning as a cell adhesion molecule and regulating cell surface stability of the catalytic α-subunit of the transporter, thereby, maintaining optimal alveolar fluid balance. Here, we identified the E3 ubiquitin ligase for the Na,K-ATPase β-subunit, which promoted polyubiquitination, subsequent endocytosis and proteasomal degradation of the protein upon exposure of alveolar epithelial cells to elevated CO2 levels, thus impairing alveolar integrity. Ubiquitination of the Na,K-ATPase β-subunit required lysine 5 and 7 and mutating these residues (but not other lysines) prevented trafficking of Na,K-ATPase from the plasma membrane and stabilized the protein upon hypercapnia. Furthermore, ubiquitination of the Na,K-ATPase β-subunit was dependent on prior phosphorylation at serine 11 by protein kinase C (PKC)-ζ. Using a protein microarray, we identified the tumor necrosis factor receptor-associated factor 2 (TRAF2) as the E3 ligase driving ubiquitination of the Na,K-ATPase β-subunit upon hypercapnia. Of note, prevention of Na,K-ATPase β-subunit ubiquitination was necessary and sufficient to restore the formation of cell-cell junctions under hypercapnic conditions. These results suggest that a hypercapnic environment in the lung may lead to persistent epithelial dysfunction in affected patients. As such, the identification of the E3 ligase for the Na,K-ATPase may provide a novel therapeutic target, to be employed in patients with acute or chronic hypercapnic respiratory failure, aiming to restore alveolar epithelial integrity.

CDK control pathways integrate cell size and ploidy information to control cell division

Maintenance of cell size homeostasis is a property that is conserved throughout eukaryotes. Cell size homeostasis is brought about by the co-ordination of cell division with cell growth and requires restriction of smaller cells from undergoing mitosis and cell division, whilst allowing larger cells to do so. Cyclin-CDK is the fundamental driver of mitosis and therefore ultimately ensures size homeostasis. Here we dissect determinants of CDK activity in vivo to investigate how cell size information is processed by the cell cycle network in fission yeast. We develop a high-throughput single-cell assay system of CDK activity in vivo and show that inhibitory tyrosine phosphorylation of CDK encodes cell size information, with the phosphatase PP2A aiding to set a size threshold for division. CDK inhibitory phosphorylation works synergistically with PP2A to prevent mitosis in smaller cells. Finally, we find that diploid cells of equivalent size to haploid cells exhibit lower CDK activity in response to equal cyclin-CDK enzyme concentrations, suggesting that CDK activity is reduced by increased DNA levels. Therefore, scaling of cyclin-CDK levels with cell size, CDK inhibitory phosphorylation, PP2A, and DNA-dependent inhibition of CDK activity, all inform the cell cycle network of cell size, thus contributing to cell size homeostasis.

Haven't got a glue: Protein surface variation for the design of molecular glue degraders

Molecular glue degraders are small, drug-like compounds that induce interactions between an E3 ubiquitin ligase and a target, which result in ubiquitination and subsequent degradation of the recruited protein. In recent years, serendipitous discoveries revealed that some preclinical and clinical compounds already work as molecular glue degraders, with many more postulated to destabilize their targets through indirect or yet unresolved mechanisms. Here we review strategies by which E3 ubiquitin ligases can be reprogrammed by monovalent degraders, with a focus on molecular glues hijacking cullin-RING ubiquitin ligases. We argue that such drugs exploit the intrinsic property of proteins to form higher-order assemblies, a phenomenon previously seen with disease-causing sequence variations. Modifications of the protein surface by a bound small molecule can change the interactome of the target protein. By inducing interactions between a ligase and a substrate, molecular glue degraders offer an exciting path for the development of novel therapeutics.

Chatterboxes: the structural and functional diversity of cyclins

Proteins of the cyclin family have divergent sequences and execute diverse roles within the cell while sharing a common fold: the cyclin box domain. Structural studies of cyclins have played a key role in our characterization and understanding of cellular processes that they control, though to date only ten of the 29 CDK-activating cyclins have been structurally characterized by X-ray crystallography or cryo-electron microscopy with or without their cognate kinases. In this review, we survey the available structures of human cyclins, highlighting their molecular features in the context of their cellular roles. We pay particular attention to how cyclin activity is regulated through fine control of degradation motif recognition and ubiquitination. Finally, we discuss the emergent roles of cyclins independent of their roles as cyclin-dependent protein kinase activators, demonstrating the cyclin box domain to be a versatile and generalized scaffolding domain for protein-protein interactions across the cellular machinery.

Cyclin A2 degradation during the spindle assembly checkpoint requires multiple binding modes to the APC/C

The anaphase-promoting complex/cyclosome (APC/C) orchestrates cell cycle progression by controlling the temporal degradation of specific cell cycle regulators. Although cyclin A2 and cyclin B1 are both targeted for degradation by the APC/C, during the spindle assembly checkpoint (SAC), the mitotic checkpoint complex (MCC) represses APC/C's activity towards cyclin B1, but not cyclin A2. Through structural, biochemical and in vivo analysis, we identify a non-canonical D box (D2) that is critical for cyclin A2 ubiquitination in vitro and degradation in vivo. During the SAC, cyclin A2 is ubiquitinated by the repressed APC/C-MCC, mediated by the cooperative engagement of its KEN and D2 boxes, ABBA motif, and the cofactor Cks. Once the SAC is satisfied, cyclin A2 binds APC/C-Cdc20 through two mutually exclusive binding modes, resulting in differential ubiquitination efficiency. Our findings reveal that a single substrate can engage an E3 ligase through multiple binding modes, affecting its degradation timing and efficiency.

Aneuploidy in Oocytes Is Prevented by Sustained CDK1 Activity through Degron Masking in Cyclin B1

Successful mitosis requires that cyclin B1:CDK1 kinase activity remains high until chromosomes are correctly aligned on the mitotic spindle. It has therefore been unclear why, in mammalian oocyte meiosis, cyclin B1 destruction begins before chromosome alignment is complete. Here, we resolve this paradox and show that mouse oocytes exploit an imbalance in the ratio of cyclin B1 to CDK1 to control CDK1 activity; early cyclin B1 destruction reflects the loss of an excess of non-CDK1-bound cyclin B1 in late prometaphase, while CDK1-bound cyclin B1 is destroyed only during metaphase. The ordered destruction of the two forms of cyclin B1 is brought about by a previously unidentified motif that is accessible in free cyclin B1 but masked when cyclin B1 is in complex with CDK1. This protects the CDK1-bound fraction from destruction in prometaphase, ensuring a period of prolonged CDK1 activity sufficient to achieve optimal chromosome alignment and prevent aneuploidy.

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In mouse oocytes, an excess of cyclin B1 preserves CDK1 activity

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A motif in non-CDK1-bound cyclin B1 confers preferential APC/C targeting

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Non-CDK1-bound cyclin B1 is gradually destroyed before CDK1-bound cyclin B1

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Prolonged CDK1 activity assists the spindle checkpoint and prevents aneuploidy

APC/C: current understanding and future perspectives

The separation of sister chromatids at anaphase, which is regulated by an E3 ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC/C), is arguably the most important irrevocable event during the cell cycle. The APC/C and cyclin-dependent kinase 1 (Cdk1) are just two of the many significant cell cycle regulators and exert control through ubiquitylation and phosphorylation, respectively. The temporal and spatial regulation of the APC/C is achieved by multiple mechanisms, including phosphorylation, interaction with the structurally related co-activators Cdc20 and Cdh1, loading of distinct E2 ubiquitin-conjugating enzymes, binding with inhibitors and differential affinities for various substrates. Since the discovery of APC/C 25 years ago, intensive studies have uncovered many aspects of APC/C regulation, but we are still far from a full understanding of this important cellular machinery. Recent high-resolution cryogenic electron microscopy analysis and reconstitution of the APC/C have greatly advanced our understanding of molecular mechanisms underpinning the enzymatic properties of APC/C. In this review, we will examine the historical background and current understanding of APC/C regulation.

Hidden targets of ubiquitin proteasome system: To prevent diabetic nephropathy

Diabetic nephropathy (DN) is the major cause of end stage renal failure. Although, several therapeutic targets have emerged to prevent the progression of DN, the number of people with DN still continues to rise worldwide, suggesting an urgent need of novel targets to prevent DN completely. Currently, the role of ubiquitin proteasome system (UPS) has been highlighted in the pathogenesis and progression of various diseases like obesity, insulin resistance, atherosclerosis, cancers, neurodegerative disorders and including secondary complications of diabetes. UPS mainly involves in protein homeostatis through ubiquitination (post translational modification) and proteasomal degradation of various proteins. Ubiquitination, not only involves in proteasomal degradation, but also directs the substrate proteins to participate in multitude of cell signalling pathways. However, very little is known about ubiquitination and UPS in the progression of DN. This review mainly focuses on UPS and its components including E2 conjugating enzymes, E3 ligases and deubiquitinases (DUBs) in the development of DN and thus may help us to find novel therapeutic targets with in UPS to prevent DN completely in future.

The destruction box is involved in the degradation of the NTE family proteins by the proteasome

Neuropathy target esterase (NTE) and NTE-related esterase (NRE) are endoplasmic reticulum (ER) membrane-anchored proteins belonging to the NTE protein family. NTE and NRE are degraded by macroautophagy and by the ubiquitin-proteasome pathway. However, the regulation of NTE and NRE by proteasome has not been well understood. Western blotting showed that the deletion of the regulatory region of NTE and NRE led to protein accumulation compared with that of the corresponding wild-type proteins. Further, deletion and site-directed mutagenesis experiments demonstrated that the destruction (D) box was required for the proteasomal degradation of NTE and NRE. However, unlike the deletion of the regulatory region, the deletion of the D box did not affect the subcellular localisation of NTE or NRE or disrupt the ER. Moreover, the deletion of the D box or the regulatory region of NTE has similar inhibitory effects on cell growth, which are greater than those produced by the full-length NTE. Here, for the first time, we show that the D box is involved in the regulation of NTE family proteins by the proteasome but not in their subcellular localisation. In addition, these results suggest that the NTE overexpression-mediated inhibition of cell growth is related to active protein levels but not to its ER disruption effect.

Mps1 kinase regulates tumor cell viability via its novel role in mitochondria

Targeting mitotic kinase monopolar spindle 1 (Mps1) for tumor therapy has been investigated for many years. Although it was suggested that Mps1 regulates cell viability through its role in spindle assembly checkpoint (SAC), the underlying mechanism remains less defined. In an endeavor to reveal the role of high levels of mitotic kinase Mps1 in the development of colon cancer, we unexpectedly found the amount of Mps1 required for cell survival far exceeds that of maintaining SAC in aneuploid cell lines. This suggests that other functions of Mps1 besides SAC are also employed to maintain cell viability. Mps1 regulates cell viability independent of its role in cytokinesis as the genetic depletion of Mps1 spanning from metaphase to cytokinesis affects neither cytokinesis nor cell viability. Furthermore, we developed a single-cycle inhibition strategy that allows disruption of Mps1 function only in mitosis. Using this strategy, we found the functions of Mps1 in mitosis are vital for cell viability as short-term treatment of mitotic colon cancer cell lines with Mps1 inhibitors is sufficient to cause cell death. Interestingly, Mps1 inhibitors synergize with microtubule depolymerizing drug in promoting polyploidization but not in tumor cell growth inhibition. Finally, we found that Mps1 can be recruited to mitochondria by binding to voltage-dependent anion channel 1 (VDAC1) via its C-terminal fragment. This interaction is essential for cell viability as Mps1 mutant defective for interaction fails to main cell viability, causing the release of cytochrome c. Meanwhile, deprivation of VDAC1 can make tumor cells refractory to loss of Mps1-induced cell death. Collectively, we conclude that inhibition of the novel mitochondrial function Mps1 is sufficient to kill tumor cells.

Neuropathy Target Esterase Is Degraded by the Ubiquitin-Proteasome Pathway with ARA54 as the Ubiquitin Ligase

Neuropathy target esterase (NTE) is an endoplasmic reticulum membrane-associated phospholipase B, which is essential for embryonic and nervous system development. However, the regulation of NTE at the protein level had not been thoroughly investigated. Our previous study showed that NTE was degraded not only by the macroautophagy-lysosome pathway but also by the ubiquitin-proteasome pathway. Here we further reveal that androgen receptor-associated protein 54 (ARA54) regulated the ubiquitin-proteasome degradation of NTE. We find that deletion of the regulatory domain of NTE, which possesses a putative destruction box and thus is essential for its degradation by the proteasome, prevented its degradation by the proteasome. In addition, we demonstrate that ARA54, which has a RING finger domain and E3 ligase activity, interacts directly with NTE. Overexpression of ARA54 downregulates the protein level of NTE, and knockdown of ARA54 inhibits the degradation of NTE. The mutation in the RING domain of ARA54 blocks the degradation of NTE by ARA54, which indicates that the RING domain is essential for ARA54's E3 activity. These findings suggest that ARA54 acts as the ubiquitin ligase to regulate the ubiquitin-proteasome degradation of NTE.

Ubiquitin-specific protease 22 is a deubiquitinase of CCNB1

The elevated level of CCNB1 indicates more aggressive cancer and poor prognosis. However, the factors that cause CCNB1 upregulation remain enigmatic. Herein, we identify USP22 as a CCNB1 interactor and discover that both USP22 and CCNB1 are dramatically elevated with a strong positive correlation in colon cancer tissues. USP22 stabilizes CCNB1 by antagonizing proteasome-mediated degradation in a cell cycle-specific manner. Phosphorylation of USP22 by CDK1 enhances its activity in deubiquitinating CCNB1. The ubiquitin ligase anaphase-promoting complex (APC/C) targets USP22 for degradation by using the substrate adapter CDC20 during cell exit from M phase, presumably allowing CCNB1 degradation. Finally, we discover that USP22 knockdown leads to slower cell growth and reduced tumor size. Our study demonstrates that USP22 is a CCNB1 deubiquitinase, suggesting that targeting USP22 might be an effective approach to treat cancers with elevated CCNB1 expression.

Specificity of the anaphase-promoting complex: a single-molecule study

Biological processes require specific enzymatic reactions, paradoxically involving short recognition sequences. As an example, cell-cycle timing depends on a sequence of ubiquitylation events mediated by the anaphase-promoting complex (APC) based on short redundant motifs. To understand the origin of specificity, we designed single-molecule fluorescence assays that capture transient ubiquitylation reactions. We find that the APC-mediated ubiquitylation involves a highly processive initial reaction on the substrate, followed by multiple encounters and reactions at a slower rate. The initial ubiquitylation greatly enhances the substrate's binding affinity in subsequent reactions, by both increasing the on-rate and decreasing the off-rate. We postulate that these cycles of positive feedback enable high specificity for substrates with short recognition motifs in a complex cellular environment.

Spatiotemporal regulation of the anaphase-promoting complex in mitosis

The appropriate timing of events that lead to chromosome segregation during mitosis and cytokinesis is essential to prevent aneuploidy, and defects in these processes can contribute to tumorigenesis. Key mitotic regulators are controlled through ubiquitylation and proteasome-mediated degradation. The APC/C (anaphase-promoting complex; also known as the cyclosome) is an E3 ubiquitin ligase that has a crucial function in the regulation of the mitotic cell cycle, particularly at the onset of anaphase and during mitotic exit. Co-activator proteins, inhibitor proteins, protein kinases and phosphatases interact with the APC/C to temporally and spatially control its activity and thus ensure accurate timing of mitotic events.

Co-activator independent differences in how the metaphase and anaphase APC/C recognise the same substrate

The Anaphase Promoting Complex or Cyclosome (APC/C) is critical to the control of mitosis. The APC/C is an ubiquitin ligase that targets specific mitotic regulators for proteolysis at distinct times in mitosis, but how this is achieved is not well understood. We have addressed this question by determining whether the same substrate, cyclin B1, is recognised in the same way by the APC/C at different times in mitosis. Unexpectedly, we find that distinct but overlapping motifs in cyclin B1 are recognised by the APC/C in metaphase compared with anaphase, and this does not depend on the exchange of Cdc20 for Cdh1. Thus, changes in APC/C substrate specificity in mitosis can potentially be conferred by altering interaction sites in addition to exchanging Cdc20 for Cdh1.

Selective protein degradation: a rheostat to modulate cell-cycle phase transitions

Plant growth control has become a major focus due to economic reasons and results from a balance of cell proliferation in meristems and cell elongation that occurs during differentiation. Research on plant cell proliferation over the last two decades has revealed that the basic cell-cycle machinery is conserved between human and plants, although specificities exist. While many regulatory circuits control each step of the cell cycle, the ubiquitin proteasome system (UPS) appears in fungi and metazoans as a major player. In particular, the UPS promotes irreversible proteolysis of a set of regulatory proteins absolutely required for cell-cycle phase transitions. Not unexpectedly, work over the last decade has brought the UPS to the forefront of plant cell-cycle research. In this review, we will summarize our knowledge of the function of the UPS in the mitotic cycle and in endoreduplication, and also in meiosis in higher plants.

Visualization of mitotic arrest of cell cycle with bioluminescence imaging in living animals

PURPOSE

Visualization of the cell cycle in living subjects has long been a big challenge. The present study aimed to noninvasively visualize mitotic arrest of the cell cycle with an optical reporter in living subjects.

PROCEDURES

An N-terminal cyclin B1-luciferase fusion construct (cyclin B-Luc) controlled by the cyclin B promoter, as a mitosis reporter, was generated. HeLa or HCT116 cells stably expressing cyclin B-Luc reporter were used to evaluate its cell cycle-dependent regulation and ubiquitination-mediated degradation. We also evaluated its feasibility to monitor the mitotic arrest caused by Taxotere both in vitro and in vivo.

RESULTS

We showed that the cyclin B-Luc fusion protein was regulated in a cell cycle-dependent manner and accumulated in the mitotic phase (M phase) in cellular assays. The regulation of cyclin B-Luc reporter was mediated by proteasome ubiquitination. In the present study, in vitro imaging showed that antimitotic reagents like Taxotere upregulated the reporter through cell cycle arrest in the M phase. Noninvasive longitudinal bioluminescence imaging further demonstrated an upregulation of the reporter consistent with mitotic arrest induced in tumor xenograft models. Induction of this reporter was also observed with a kinesin spindle protein inhibitor, which causes cell cycle blockage in the M phase.

CONCLUSIONS

Our results demonstrate that the cyclin B-Luc reporter can be used to image whether compounds are capable, in vivo, of causing an M phase arrest and/or altering cyclin B turnover. This reporter can also be potentially used in high-throughput screening efforts aimed at discovering novel molecules that will cause cell cycle arrest at the M phase in cultivated cell lines and animal models.

The Giardia cell cycle progresses independently of the anaphase-promoting complex

Most cell cycle regulation research has been conducted in model organisms representing a very small part of the eukaryotic domain. The highly divergent human pathogen Giardia intestinalis is ideal for studying the conservation of eukaryotic pathways. Although Giardia has many cell cycle regulatory components, its genome lacks all anaphase-promoting complex (APC) components. In the present study, we show that a single mitotic cyclin in Giardia is essential for progression into mitosis. Strikingly, Giardia cyclin B lacks the conserved N-terminal motif required for timely degradation mediated by the APC and ubiquitin conjugation. Expression of Giardia cyclin B in fission yeast is toxic, leading to a prophase arrest, and this toxicity is suppressed by the addition of a fission yeast degradation motif. Cyclin B is degraded during mitosis in Giardia cells, but this degradation appears to be independent of the ubiquitination pathway. Other putative APC substrates, aurora and polo-like kinases, also show no evidence of ubiquitination. This is the first example of mitosis not regulated by the APC and might reflect an evolutionary ancient form of cell cycle regulation.

The anaphase promoting complex contributes to the degradation of the S. cerevisiae telomerase recruitment subunit Est1p

Telomerase is a multi-subunit enzyme that reverse transcribes telomere repeats onto the ends of linear eukaryotic chromosomes and is therefore critical for genome stability. S. cerevisiae telomerase activity is cell-cycle regulated; telomeres are not elongated during G1 phase. Previous work has shown that Est1 protein levels are low during G1 phase, preventing telomerase complex assembly. However, the pathway targeting Est1p for degradation remained uncharacterized. Here, we show that Est1p stability through the cell cycle mirrors that of Clb2p, a known target of the Anaphase Promoting Complex (APC). Indeed, Est1p is stabilized by mutations in both essential and non-essential components of the APC. Mutations of putative Destruction boxes (D-boxes), regions shown to be important for recognition of known APC substrates, stabilize Est1p, suggesting that Est1p is likely to be targeted for degradation directly by the APC. However, we do not detect degradation or ubiquitination of recombinant Est1p by the APC in vitro, suggesting either that the recombinant protein lacks necessary post-translational modification and/or conformation, or that the APC affects Est1p degradation by an indirect mechanism. Together, these studies shed light on the regulation of yeast telomerase assembly and demonstrate a new connection between telomere maintenance and cell cycle regulation pathways.

Anaphase-promoting complex/cyclosome-mediated proteolysis of Ams2 in the G1 phase ensures the coupling of histone gene expression to DNA replication in fission yeast

Histone transcription and deposition are tightly regulated with the DNA replication cycle to maintain genetic integrity. Ams2 is a GATA-containing transcription factor responsible for core histone gene expression and for CENP-A loading at centromeres in fission yeast. Ams2 levels are cell cycle-regulated, and after the S phase Ams2 is degraded by the SCF(pof3) ubiquitin ligase; however, the regulation of Ams2 in G(1) or meiosis is poorly understood. Here we show that another ubiquitin ligase, the anaphase-promoting complex/cyclosome (APC/C) targets Ams2 for destruction in G(1). Ubiquitylation and destruction of Ams2 is dependent upon a coactivator Cdh1/Ste9 and the KEN box in the C terminus of Ams2. We also find that stabilization of Ams2 sensitizes cells to the anti-microtubule drug thiabendazole and the histone deacetylase inhibitor tricostatin A when a histone deacetylase gene hst4 is deleted, suggesting that histone acetylation together with Ams2 stability ensures the coupling of mitosis to DNA replication. Furthermore, in meiosis, the failure of the APC/C-mediated destruction of Ams2 is deleterious, and pre-meiotic DNA replication is barely completed. These data suggest that Ams2 destruction via both the APC/C and the SCF ubiquitin ligases underlies the coordination of histone expression and DNA replication.

Role of cyclin B1 levels in DNA damage and DNA damage-induced senescence

The cyclin B1-Cdk1 complex is a key regulator of mitotic entry. A large number of proteins are phosphorylated by the cyclin B1-Cdk1 complex prior to mitotic entry. Regulation of the mitotic events is linked to the control of the activity of the cyclin B1-Cdk1 complex to make cells enter mitosis, arrest at G2-phase, or skip mitosis. The roles of cyclin B1 levels in DNA damage are described. The ATM/ATR pathway acts as a molecular switch for regulating cell fates, flipping between cell death via progress into mitosis and polyploidization via sustained G2 arrest upon DNA damage, where cyclin B1 degradation is important for inducing polyploidization. The decrease in cyclin B1 levels that is induced by DNA damage leads to polyploidization in DNA damage-induced senescence. A useful method for monitoring the expression level of cyclin B1 throughout cell cycle progression in living cells is also presented.

Fulfilling the metabolic requirements for cell proliferation

The activity of key metabolic enzymes is regulated by the ubiquitin ligases that control the function of the cyclins; therefore the activity of these ubiquitin ligases explains the coordination of cell-cycle progression with the supply of substrates necessary for cell duplication. APC/C (anaphase-promoting complex/cyclosome)-Cdh1, the ubiquitin ligase that controls G(1)- to S-phase transition by targeting specific degradation motifs in cell-cycle proteins, also regulates the glycolysis-promoting enzyme PFKFB3 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3) and GLS1 (glutaminase 1), a critical enzyme in glutaminolysis. A decrease in the activity of APC/C-Cdh1 in mid-to-late G(1) releases both proteins, thus explaining the simultaneous increase in the utilization of glucose and glutamine during cell proliferation. This occurs at a time consistent with the point in G(1) that has been described as the nutrient-sensitive restriction point and is responsible for the transition from G(1) to S. PFKFB3 is also a substrate at the onset of S-phase for the ubiquitin ligase SCF (Skp1/cullin/F-box)-β-TrCP (β-transducin repeat-containing protein), so that the activity of PFKFB3 is short-lasting, coinciding with a peak in glycolysis in mid-to-late G(1), whereas the activity of GLS1 remains high throughout S-phase. The differential regulation of the activity of these proteins indicates that a finely-tuned set of mechanisms is activated to fulfil specific metabolic demands at different stages of the cell cycle. These findings have implications for the understanding of cell proliferation in general and, in particular, of cancer, its prevention and treatment.

Degradation of mouse NTE-related esterase by macroautophagy and the proteasome

NTE-related esterase (NRE) is a novel endoplasmic reticulum-anchored lysophospholipase with high homology to neuropathy target esterase (NTE). However, little is known about the regulation of NRE protein. In the current study, we investigated the degradation pathways of mouse NRE (mNRE) in mammalian cells. Based on experiments with inhibitors and inducer of protein degradation pathways, we provide here the first evidence that mNRE is degraded by macroautophagy as well as by the proteasome. Moreover, the contribution of protein domains to the degradation of mNRE was investigated, which showed that the transmembrane and regulatory domain played a role in the degradation of mNRE by macroautophagy and the proteasome respectively. In contrast the C-terminal catalytic domain was not involved in both degradation pathways of mNRE. These findings showed for the first time that the degradation pathways in controlling mNRE quantity and may provide further insight into structure and regulation of mNRE.

Molecular basis for the differential use of glucose and glutamine in cell proliferation as revealed by synchronized HeLa cells

During cell division, the activation of glycolysis is tightly regulated by the action of two ubiquitin ligases, anaphase-promoting complex/cyclosome-Cdh1 (APC/C-Cdh1) and SKP1/CUL-1/F-box protein-β-transducin repeat-containing protein (SCF-β-TrCP), which control the transient appearance and metabolic activity of the glycolysis-promoting enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, isoform 3 (PFKFB3). We now demonstrate that the breakdown of PFKFB3 during S phase occurs specifically via a distinct residue (S(273)) within the conserved recognition site for SCF-β-TrCP. Glutaminase 1 (GLS1), the first enzyme in glutaminolysis, is also targeted for destruction by APC/C-Cdh1 and, like PFKFB3, accumulates after the activity of this ubiquitin ligase decreases in mid-to-late G1. However, our results show that GLS1 differs from PFKFB3 in that its recognition by APC/C-Cdh1 requires the presence of both a Lys-Glu-Asn box (KEN box) and a destruction box (D box) rather than a KEN box alone. Furthermore, GLS1 is not a substrate for SCF-β-TrCP and is not degraded until cells progress from S to G2/M. The presence of PFKFB3 and GLS1 coincides with increases in generation of lactate and in utilization of glutamine, respectively. The contrasting posttranslational regulation of PFKFB3 and GLS1, which we have verified by studies of ubiquitination and protein stability, suggests the different roles of glucose and glutamine at distinct stages in the cell cycle. Indeed, experiments in which synchronized cells were deprived of either of these substrates show that both glucose and glutamine are required for progression through the restriction point in mid-to-late G1, whereas glutamine is the only substrate essential for the progression through S phase into cell division.

The SCF-FBXW5 E3-ubiquitin ligase is regulated by PLK4 and targets HsSAS-6 to control centrosome duplication

Deregulated centrosome duplication can result in genetic instability and contribute to tumorigenesis. Here, we show that centrosome duplication is regulated by the activity of an E3-ubiquitin ligase that employs the F-box protein FBXW5 (ref. 3) as its targeting subunit. Depletion of endogenous FBXW5 or overexpression of an F-box-deleted mutant version results in centrosome overduplication and formation of multipolar spindles. We identify the centriolar protein HsSAS-6 (refs 4,5) as a critical substrate of the SCF-FBXW5 complex. FBXW5 binds HsSAS-6 and promotes its ubiquitylation in vivo. The activity of SCF-FBXW5 is in turn negatively regulated by Polo-like kinase 4 (PLK4), which phosphorylates FBXW5 at Ser 151 to suppress its ability to ubiquitylate HsSAS-6. FBXW5 is a cell-cycle-regulated protein with expression levels peaking at the G1/S transition. We show that FBXW5 levels are controlled by the anaphase-promoting (APC/C) complex, which targets FBXW5 for degradation during mitosis and G1, thereby helping to reset the centrosome duplication machinery. In summary, we show that a cell-cycle-regulated SCF complex is regulated by the kinase PLK4, and that this in turn restricts centrosome re-duplication through degradation of the centriolar protein HsSAS-6.

Processive ubiquitin chain formation by the anaphase-promoting complex

Progression through mitosis requires the sequential ubiquitination of cell cycle regulators by the anaphase-promoting complex, resulting in their proteasomal degradation. Although several mechanisms contribute to APC/C regulation during mitosis, the APC/C is able to discriminate between its many substrates by exploiting differences in the processivity of ubiquitin chain assembly. Here, we discuss how the APC/C achieves processive ubiquitin chain formation to trigger the sequential degradation of cell cycle regulators during mitosis.

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.

Structures of APC/C(Cdh1) with substrates identify Cdh1 and Apc10 as the D-box co-receptor

The ubiquitylation of cell-cycle regulatory proteins by the large multimeric anaphase-promoting complex (APC/C) controls sister chromatid segregation and the exit from mitosis. Selection of APC/C targets is achieved through recognition of destruction motifs, predominantly the destruction (D)-box and KEN (Lys-Glu-Asn)-box. Although this process is known to involve a co-activator protein (either Cdc20 or Cdh1) together with core APC/C subunits, the structural basis for substrate recognition and ubiquitylation is not understood. Here we investigate budding yeast APC/C using single-particle electron microscopy and determine a cryo-electron microscopy map of APC/C in complex with the Cdh1 co-activator protein (APC/C(Cdh1)) bound to a D-box peptide at ∼10 Å resolution. We find that a combined catalytic and substrate-recognition module is located within the central cavity of the APC/C assembled from Cdh1, Apc10--a core APC/C subunit previously implicated in substrate recognition--and the cullin domain of Apc2. Cdh1 and Apc10, identified from difference maps, create a co-receptor for the D-box following repositioning of Cdh1 towards Apc10. Using NMR spectroscopy we demonstrate specific D-box-Apc10 interactions, consistent with a role for Apc10 in directly contributing towards D-box recognition by the APC/C(Cdh1) complex. Our results rationalize the contribution of both co-activator and core APC/C subunits to D-box recognition and provide a structural framework for understanding mechanisms of substrate recognition and catalysis by the APC/C.

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.

Erythrocyte tropomodulin isoforms with and without the N-terminal actin-binding domain

Erythrocyte tropomodulin (E-Tmod or Tmod1) of 41 kDa is a tropomyosin (TM)-binding protein that caps the slow-growing end of the actin filaments. Its N-terminal half is flexible, whereas the C-terminal half has a single domain structure. E-Tmod/TM5 complex may function as a "molecular ruler" generating actin protofilaments of ∼37 nm. Here we report the discovery of a short isoform of 29 kDa that lacks the N-terminal actin-binding domain (N-ABD) but retains the C-terminal actin-binding domain (C-ABD). E-Tmod29 can be generated by alternative splicing from an upstream promoter or by multiple transcriptional start sites from a downstream promoter. Promoter switching leads to a surge of E-Tmod41 in reticulocytes, which degrades quickly in the cytosol. We expressed recombinant isoforms in Escherichia coli and tested their binding toward TM5, G-actin, and F-actin. Solid-phase binding assays show that, without the N-terminal 102 residues, E-Tmod29 binds to TM5 or G-actin more strongly than E-Tmod41 does, but barely binds to F-actin after TM5 binding. Differential bindings explain the distinct localizations of E-Tmod29 in the cytosol and E-Tmod41 on the membrane. Sequential bindings and immunofluorescent staining further suggest that 1) TM5 binding to E-Tmod41 may open up the flexible N-terminal half, exposing N-ABD and unblocking C-ABD; 2) N-ABD binds to F-actin and C-ABD binds to G-actin; and 3) F-actin binding to N-ABD may prevent G-actin from binding to C-ABD. E-Tmod29 may thus modulate the availability of TM5 and G-actin for E-Tmod41 to construct the protofilament-based membrane skeletal network for circulating erythrocytes.

E3 ubiquitin ligase APC/C-Cdh1 accounts for the Warburg effect by linking glycolysis to cell proliferation

Cell proliferation is known to be accompanied by activation of glycolysis. We have recently discovered that the glycolysis-promoting enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, isoform 3 (PFKFB3), is degraded by the E3 ubiquitin ligase APC/C-Cdh1, which also degrades cell-cycle proteins. We now show in two different cell types (neoplastic and nonneoplastic) that both proliferation and aerobic glycolysis are prevented by overexpression of Cdh1 and enhanced by its silencing. Furthermore, we have coexpressed Cdh1 with PFKFB3--either wild-type or a mutant form resistant to ubiquitylation by APC/C-Cdh1--or with the glycolytic enzyme 6-phosphofructo-1-kinase and demonstrated that whereas glycolysis is essential for cell proliferation, its initiation in the presence of active Cdh1 does not result in proliferation. Our experiments indicate that the proliferative response, regardless of whether it occurs in normal or neoplastic cells, is dependent on a decrease in the activity of APC/C-Cdh1, which activates both proliferation and glycolysis. These observations have implications for cell proliferation, neoplastic transformation, and the prevention and treatment of cancer.

Role of the IRF-1 enhancer domain in signalling polyubiquitination and degradation

The interferon regulated transcription factor IRF-1 is a tumour suppressor protein that is activated in response to viral infection and cell signalling activated by double stranded DNA lesions. IRF-1 has a short half-life (t(0.5) 20-40 min) allowing rapid changes in steady state levels by modulating its rate of degradation and/or synthesis. However, little is known about the pathway(s) leading to IRF-1 protein degradation or what determines the rate of degradation in cells. Here we establish a role for discrete motifs in the enhancer domain of IRF-1 in directing polyubiquitination and degradation. By studying the structure of the enhancer domain as related to its role in the turnover of IRF-1 we have demonstrated that this region is not subject to modification by ubiquitin but rather that it contains both an ubiquitination signal and a distinct degradation signal. Removal of the C-terminal 70 amino acids from IRF-1 inhibits both its degradation and polyubiquitination, whereas removal of the C-terminal 25 amino acids inhibits degradation of the protein but does not prevent its ubiquitination. Furthermore, consistent with the C-terminus being involved in targeting or recognition by an E3-ligase or associated protein(s) the enhancer domain can act in trans to inhibit IRF-1 ubiquitination by endogenous E3-ligase activity. The identification of structural determinants that signals IRF-1 polyubiquitination and which can be uncoupled from IRF-1 degradation lends support to the idea that the degradation of selective substrates can be regulated at multiple steps in the ubiquitin-proteasome system.

The APC/C E3 ligase remains active in most post-mitotic Arabidopsis cells and is required for proper vasculature development and organization

Selective protein degradation via the ubiquitin-26S proteasome is a major mechanism underlying DNA replication and cell division in all eukaryotes. In particular, the APC/C (anaphase promoting complex or cyclosome) is a master ubiquitin protein ligase (E3) that targets PDS1/SECURIN and cyclin B for degradation allowing sister chromatid separation and exit from mitosis, respectively. Interestingly, it has been found that the APC/C remains active in differentiated neurons in which the E3 ligase regulates axon growth, neuronal survival and synaptic functions. However, despite these recent findings, the role of APC/C in differentiated cells and the regulation of its activity beyond cell division is still poorly understood. Here, we investigate the activity and function of APC/C in the model plant Arabidopsis thaliana. We used cyclin reporter constructs to follow APC/C activity during plant development and found that this E3 ligase remains active in most post-mitotic plant cells. Strikingly, hypomorphic mutant lines, in which the APC/C activity is reduced, exhibited several developmental abnormalities, including defects in cotyledon vein patterning and internode elongation leading to a characteristic broomhead-like phenotype. Histological analyses revealed an increased amount of vascular tissue, most notably xylem and lignified sclerenchyma, indicating a role for APC/C in plant vasculature development and organization.

Identification and characterization of chicken neuropathy target esterase

Neuropathy target esterase (NTE) was proposed as the initial target during the process of organophosphate-induced delayed neuropathy (OPIDN) and adult hens are the animal model of OPIDN. However, little has been known about the sequence and characteristics of chicken NTE. Here, we firstly identified the full length cDNA of chicken NTE (cNTE), which contained an open reading frame of 3966 nucleotides encoding 1321 amino acids. Chicken NTE had two distinct regions, one was the regulatory domain (cNTER) and the other was the catalytic domain (cNEST). Over-expression of cNTER in mammalian cells did not show any NTE activity, whereas cNEST had NTE activity. Cells expressing cNTER tagged with green fluorescent protein (GFP) showed accumulation of cNTER-GFP in an endoplasmic reticulum-like localization pattern. In addition, macroautophagy and the proteasome pathways were found to be involved in the degradation of cNTER, but not cNEST. These results first showed that cNTE was an ER-anchored protein and degraded by macroautophagy as well as the proteasome.

In vitro assays for the anaphase-promoting complex/cyclosome (APC/C) in Xenopus egg extracts

The anaphase-promoting complex/cyclosome (APC/C), a large (20S) multisubunit E3 ligase, has an essential role to ubiquitylate numerous substrates at specific times during mitosis and G1 phase as well as in meiosis. The deregulation of the APC/C causes cell death or genomic instability, which is a hallmark of cancers. Although 13 years have passed since its discovery, the molecular mechanisms of the APC/C-dependent ubiquitylation and proteolysis are still poorly understood. The development of in vitro systems enables the identification of new substrates and investigation of the molecular mechanisms by which the APC/C recognizes its substrates. This chapter describes in vitro assays reconstituted in Xenopus egg extracts.

The hunt for cyclin

It is 25 years since Tim Hunt discovered cyclin, the oscillating protein that drives activation of cyclin-dependent kinases and entry into mitosis (Evans et al., 1983).

Anaphase-promoting complex/cyclosome-CDH1-mediated proteolysis of the forkhead box M1 transcription factor is critical for regulated entry into S phase

The forkhead box M1 (FoxM1) transcription factor is overexpressed in many cancers, and in mouse models it is required for tumor progression. FoxM1 activates expression of the cell cycle genes required for both S and M phase progression. Here we demonstrate that FoxM1 is degraded in late mitosis and early G(1) phase by the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. FoxM1 interacts with the APC/C complex and its adaptor, Cdh1. Expression of Cdh1 stimulated degradation of the FoxM1 protein, and depletion of Cdh1 resulted in stabilization of the FoxM1 protein in late mitosis and in early G(1) phase of the cell cycle. Cdh1 has been implicated in regulating S phase entry. We show that codepletion of FoxM1 inhibits early S phase entry observed in Cdh1-depleted cells. The N-terminal region of FoxM1 contains both destruction box (D box) and KEN box sequences that are required for targeting by Cdh1. Mutation of either the D box sequence or the KEN box sequence stabilized FoxM1 and blocked Cdh1-induced proteolysis. Cells expressing a nondegradable form of FoxM1 entered S phase rapidly following release from M phase arrest. Together, our observations show that FoxM1 is one of the targets of Cdh1 in late M or early G(1) phase and that its proteolysis is important for regulated entry into S phase.

The spindle checkpoint functions of Mad3 and Mad2 depend on a Mad3 KEN box-mediated interaction with Cdc20-anaphase-promoting complex (APC/C)

Mitotic progression is driven by proteolytic destruction of securin and cyclins. These proteins are labeled for destruction by an ubiquitin-protein isopeptide ligase (E3) known as the anaphase-promoting complex or cyclosome (APC/C). The APC/C requires activators (Cdc20 or Cdh1) to efficiently recognize its substrates, which are specified by destruction (D box) and/or KEN box signals. The spindle assembly checkpoint responds to unattached kinetochores and to kinetochores lacking tension, both of which reflect incomplete biorientation of chromosomes, by delaying the onset of anaphase. It does this by inhibiting Cdc20-APC/C. Certain checkpoint proteins interact directly with Cdc20, but it remains unclear how the checkpoint acts to efficiently inhibit Cdc20-APC/C activity. In the fission yeast, Schizosaccharomyces pombe, we find that the Mad3 and Mad2 spindle checkpoint proteins interact stably with the APC/C in mitosis. Mad3 contains two KEN boxes, conserved from yeast Mad3 to human BubR1, and mutation of either of these abrogates the spindle checkpoint. Strikingly, mutation of the N-terminal KEN box abolishes incorporation of Mad3 into the mitotic checkpoint complex (Mad3-Mad2-Slp1 in S. pombe, where Slp1 is the Cdc20 homolog that we will refer to as Cdc20 hereafter) and stable association of both Mad3 and Mad2 with the APC/C. Our findings demonstrate that this Mad3 KEN box is a critical mediator of Cdc20-APC/C inhibition, without which neither Mad3 nor Mad2 can associate with the APC/C or inhibit anaphase onset.

The anaphase-promoting complex/cyclosome controls repair and recombination by ubiquitylating Rhp54 in fission yeast

Homologous recombination (HR) is important for maintaining genome integrity and for the process of meiotic chromosome segregation and the generation of variation. HR is regulated throughout the cell cycle, being prevalent in the S and G2 phases and suppressed in the G1 phase. Here we show that the anaphase-promoting complex/cyclosome (APC/C) regulates homologous recombination in the fission yeast Schizosaccharomyces pombe by ubiquitylating Rhp54 (an ortholog of Rad54). We show that Rhp54 is a novel APC/C substrate that is destroyed in G1 phase in a KEN-box- and Ste9/Fizzy-related manner. The biological consequences of failing to temporally regulate HR via Rhp54 degradation are seen in haploid cells only in the absence of antirecombinase Srs2 function and are more extensive in diploid cells, which become sensitive to a range of DNA-damaging agents, including hydroxyurea, methyl methanesulfonate, bleomycin, and UV. During meiosis, expression of nondegradable Rhp54 inhibits interhomolog recombination and stimulates sister chromatid recombination. We thus propose that it is critical to control levels of Rhp54 in G1 to suppress HR repair of double-strand breaks and during meiosis to coordinate interhomolog recombination.