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In early mitosis, the duplicated chromosomes are held together by the ring-shaped cohesin complex 1 . Separation of chromosomes during anaphase is triggered by separase—a large cysteine endopeptidase that cleaves the cohesin subunit SCC1 (also known as RAD21 2 – 4 ). Separase is activated by degradation of its inhibitors, securin 5 and cyclin B 6 , but the molecular mechanisms of separase regulation are not clear. Here we used cryogenic electron microscopy to determine the structures of human separase in complex with either securin or CDK1–cyclin B1–CKS1. In both complexes, separase is inhibited by pseudosubstrate motifs that block substrate binding at the catalytic site and at nearby docking sites. As in Caenorhabditis elegans 7 and yeast 8 , human securin contains its own pseudosubstrate motifs. By contrast, CDK1–cyclin B1 inhibits separase by deploying pseudosubstrate motifs from intrinsically disordered loops in separase itself. One autoinhibitory loop is oriented by CDK1–cyclin B1 to block the catalytic sites of both separase and CDK1 9 , 10 . Another autoinhibitory loop blocks substrate docking in a cleft adjacent to the separase catalytic site. A third separase loop contains a phosphoserine 6 that promotes complex assembly by binding to a conserved phosphate-binding pocket in cyclin B1. Our study reveals the diverse array of mechanisms by which securin and CDK1–cyclin B1 bind and inhibit separase, providing the molecular basis for the robust control of chromosome segregation. Structures of separase in complex with either securin or cyclin B–CDK1 shed light on the regulation of chromosome separation during the cell cycle.

Phosphorylation of substrates by cyclin-dependent kinases (CDKs) is the driving force of cell cycle progression. Several CDK-activating cyclins are involved, yet how they contribute to substrate specificity is still poorly understood. Here, we discover that a positively charged pocket in cyclin B1, which is exclusively conserved within B-type cyclins and binds phosphorylated serine- or threonine-residues, is essential for correct execution of mitosis. HeLa cells expressing pocket mutant cyclin B1 are strongly delayed in anaphase onset due to multiple defects in mitotic spindle function and timely activation of the E3 ligase APC/C. Pocket integrity is essential for APC/C phosphorylation particularly at non-consensus CDK1 sites and full in vitro ubiquitylation activity. Our results support a model in which cyclin B1’s pocket facilitates sequential substrate phosphorylations involving initial priming events that assist subsequent pocket-dependent phosphorylations even at non-consensus CDK1 motifs. How cyclins contribute to CDK1 substrate specificity during cell division is poorly understood. Here, the authors show that a phosphate-binding pocket in cyclin B1 is critical for accurate CDK1 substrate phosphorylation ensuring mitotic fidelity.

Cyclin-dependent kinase 1 (CDK1) is the pivotal kinase responsible for initiating cell division. Its activation is dependent on binding to regulatory cyclins, such as CCNB1. Our research demonstrates that copper binding to both CDK1 and CCNB1 is essential for activating CDK1 in cells. Mutations in the copper-binding amino acids of either CDK1 or CCNB1 do not disrupt their interaction but are unable to activate CDK1. We also reveal that CCNB1 facilitates the transfer of copper from ATOX1 to CDK1, consequently activating its kinase function. Disruption of copper transfer through the ATOX1-CCNB1-CDK1 pathway can impede CDK1 activation and halt cell cycle progression. In summary, our findings elucidate a mechanism through which copper promotes CDK1 activation and the G2/M transition in the cell cycle. These results could provide insight into the acquisition of proliferative properties associated with increased copper levels in cancer and offer targets for cancer therapy. Intracellular copper concentration has been linked to cell proliferation. Here, the authors demonstrate that copper binding to both Cyclin-dependent kinase 1 (CDK1) and CCNB1 is essential for activating CDK1 and promoting the G2/M transition in the cell cycle.

Spermatogenesis, which involves mitosis and meiosis of male germ cells, is a highly complicated and coordinately ordered process. Cyclin B1 (CCNB1), an important regulator in cell cycle machinery, is proved essential for mouse embryonic development. However, the role of CCNB1 in mammalian spermatogenesis remains unclear. Here we tested the requirement for CCNB1 using conditional knockout mice lacking CCNB1 in male germ cells. We found that ablation of CCNB1 in gonocytes and spermatogonia led to mouse sterile caused by the male germ cells’ depletion. Gonocyte and spermatogonia without CCNB1 is unable to proliferate normally and apoptosis increased. Moreover, CCNB1 ablation in spermatogonia may promote their differentiation by downregulating Lin28a and upregulating let-7 miRNA. However, ablation of CCNB1 in premeiotic male germ cells did not have an effect on meiosis of spermatocytes and male fertility, suggesting that CCNB1 may be dispensable for meiosis of spermatocytes. Collectively, these results indicate that CCNB1 is critically required for the proliferation of gonocytes and spermatogonia but may be redundant in meiosis of spermatocytes in mouse spermatogenesis.

CDK1 has been known to be the sole cyclin-dependent kinase (CDK) partner of cyclin B1 to drive mitotic progression 1 . Here we demonstrate that CDK5 is active during mitosis and is necessary for maintaining mitotic fidelity. CDK5 is an atypical CDK owing to its high expression in post-mitotic neurons and activation by non-cyclin proteins p35 and p39 2 . Here, using independent chemical genetic approaches, we specifically abrogated CDK5 activity during mitosis, and observed mitotic defects, nuclear atypia and substantial alterations in the mitotic phosphoproteome. Notably, cyclin B1 is a mitotic co-factor of CDK5. Computational modelling, comparison with experimentally derived structures of CDK–cyclin complexes and validation with mutational analysis indicate that CDK5–cyclin B1 can form a functional complex. Disruption of the CDK5–cyclin B1 complex phenocopies CDK5 abrogation in mitosis. Together, our results demonstrate that cyclin B1 partners with both CDK5 and CDK1, and CDK5–cyclin B1 functions as a canonical CDK–cyclin complex to ensure mitotic fidelity. Cyclin B1 is a mitotic co-factor of CDK5.

Aberrant levels of the G2/M cyclin cyclin B1 (gene CCNB1) have been associated with multiple cancers; however, the literature lacks a focused and comprehensive analysis of the regulation of this important regulator of cell proliferation in cancer. Through this work, we performed a pancancer analysis of the levels of CCNB1 and dissected aspects of regulation and how this correlates with cancer prognosis. We comprehensively evaluated the expression and promoter methylation of CCNB1 across 38 cancers based on RNA sequencing data obtained from the Cancer Genome Atlas (TCGA). The correlation of CCNB1 with prognosis and the tumor microenvironment was explored. Using lung adenocarcinoma data, we studied the potential upstream noncoding RNAs involved in the regulation of CCNB1 and validated the protein levels and prognostic value of CCNB1 for this disease site. CCNB1 was highly expressed, and promoter methylation was reduced in most cancers. Gene expression of CCNB1 correlated positively with poor prognosis of tumor patients, and these results were confirmed at the protein level using lung adenocarcinoma. CCNB1 expression was associated with the infiltration of T helper cells, and this further correlated with poor prognosis for certain cancers, including renal clear cell carcinoma and lung adenocarcinoma. Subsequently, we identified a specific upstream noncoding RNA contributing to CCNB1 overexpression in lung adenocarcinoma through correlation analysis, expression analysis and survival analysis. This study provides a comprehensive analysis of the expression and methylation status of CCNB1 across several forms of cancer and provides further insight into the mechanistic pathways regulating Cyclin B1 in the tumorigenesis process.

B cyclins regulate G2-M transition. Because human somatic cells continue to cycle after reduction of cyclin B1 (cycB1) or cyclin B2 (cycB2) by RNA interference (RNAi), and because cycB2 knockout mice are viable, the existence of two genes should be an optimization. To explore this idea, we generated HeLa BD™ Tet-Off cell lines with inducible cyclin B1- or B2-EGFP that were RNAi resistant. Cultures were treated with RNAi and/or doxycycline (Dox) and bromodeoxyuridine. We measured G2 and M transit times and 4C cell accumulation. In the absence of ectopic B cyclin expression, knockdown (kd) of either cyclin increased G2 transit. M transit was increased by cycB1 kd but decreased by cycB2 depletion. This novel difference was further supported by time-lapse microscopy. This suggests that cycB2 tunes mitotic timing, and we speculate that this is through regulation of a Golgi checkpoint. In the presence of endogenous cyclins, expression of active B cyclin-EGFPs did not affect G2 or M phase times. As previously shown, B cyclin co-depletion induced G2 arrest. Expression of either B cyclin-EGFP completely rescued knockdown of the respective endogenous cyclin in single kd experiments, and either cyclin-EGFP completely rescued endogenous cyclin co-depletion. Most of the rescue occurred at relatively low levels of exogenous cyclin expression. Therefore, cycB1 and cycB2 are interchangeable for ability to promote G2 and M transition in this experimental setting. Cyclin B1 is thought to be required for the mammalian somatic cell cycle, while cyclin B2 is thought to be dispensable. However, residual levels of cyclin B1 or cyclin B2 in double knockdown experiments are not sufficient to promote successful mitosis, yet residual levels are sufficient to promote mitosis in the presence of the dispensible cyclin B2. We discuss a simple model that would explain most data if cyclin B1 is necessary.

In mammals, oocytes are arrested in prophase of meiosis I for long periods of time. Prophase arrest is critical for reproduction because it allows oocytes to grow to their full size to support meiotic maturation and embryonic development. Prophase arrest requires the inhibitory phosphorylation of the mitotic kinase CDK1. Whether prophase arrest is also regulated at the translational level is unknown. Here, we show that prophase arrest is regulated by translational control of dormant cyclin B1 mRNAs. Using Trim-Away, we identify two mechanisms that maintain cyclin B1 dormancy and thus prophase arrest. First, a complex of the RNA-binding proteins DDX6, LSM14B and CPEB1 directly represses cyclin B1 translation through interacting with its 3’UTR. Second, cytoplasmic poly(A)-binding proteins (PABPCs) indirectly repress the translation of cyclin B1 and other poly(A)-tail-less or short-tailed mRNAs by sequestering the translation machinery on long-tailed mRNAs. Together, we demonstrate how RNA-binding proteins coordinately regulate prophase arrest, and reveal an unexpected role for PABPCs in controlling mRNA dormancy. Mammalian oocytes must be maintained in prophase long enough to grow to full size. Here, the authors identify two mechanisms that repress cyclin B1 translation to maintain prophase arrest, and reveal a new aspect of cytoplasmic poly(A)-binding proteins in controlling translation.

Suppression of human megakaryocytes by dengue virus (DENV) infection significantly reduces the platelet count that eventually leads to thrombocytopenia, severe dengue and death. To understand DENV interactions with megakaryocytes, we investigated the cell cycle in leukemic human megakaryocytic in vitro cell line (MEG-01 cells). Megakaryocytes are known for complex endomitotic cell cycle leading to their polyploidy state. Our study shows that DENV uses these polyploid cells for its replication. Understanding the modulation of DENV-mediated cell cycle regulation in megakaryocytes is therefore highly important. We show that DENV2 (serotype 2) infection significantly modulates cell cycle signaling. Our protein profile microarray data showed significant upregulation of several cell cycle regulatory proteins including CDK4, CDK1, Cyclin B1 and others or downregulation of Chk1, GSK3-beta, CUL-3, and E2F-3. Quantitative real-time PCR and immunoblotting analyses further confirmed the upregulation of CDK4, CDK1, and Cyclin B1 upon DENV2 infection. Gene silencing of CDK4, CDK1 and Cyclin B1 showed significant reduction in DENV2 loads. Immunoprecipitation analysis further revealed an enhanced interaction between Cyclin B1 and CDK1 upon DENV2 infection that perhaps suggest the substantial changes noted in cell cycle regulation. Overall, our study suggests that DENV2 modulates cell cycle signaling in megakaryocytes and interferes with the critical regulatory proteins that may eventually lead to changes in endomitosis process. In conclusion, we report an important molecular insight regarding DENV2-mediated cell cycle modulation in human megakaryocytes.

ABSTRACT Nuclear factor of activated T‐cells, cytoplasmic 4 (NFATc4), a transcription factor of the NFAT family, has been reported to participate in the tumorigenesis and progression of several cancers. However, the function and regulation of NFATc4 in lung adenocarcinoma (LUAD) remain poorly understood. Here, we report for the first time that NFATc4 is significantly overexpressed in LUAD tissues, and high NFATc4 expression correlates with lymphatic metastasis, advanced tumor stage, and poor prognosis in patients. Subsequent functional studies revealed that NFATc4 depletion inhibits LUAD cell viability, proliferation, and tumor growth by inducing cell cycle arrest in the G2/M phase and apoptosis. A mechanistic study shows that NFATc4 knockdown leads to significant enrichment of cellular process‐related pathways and differentially expressed genes, especially downregulated genes Cyclin B1 (CCNB1) and cyclin‐dependent kinase 1 (CDK1). NFATc4 directly binds to the CCNB1 promoter to regulate the CCNB1/CDK1 pathway, resulting in cell cycle arrest and inhibition of cell proliferation. This study identifies NFATc4/CCNB1/CDK1 as a novel regulatory pathway involved in LUAD development and provides a potential prognostic biomarker and molecular therapeutic target for LUAD. NFATc4 is upregulated in LUAD, and its high expression correlates with the malignant progression of patients. NFATc4 enhances LUAD cell viability and proliferation by regulating the G2/M phase transition and apoptosis. Functionally, NFATc4 directly binds to the CCNB1 promoter, modulating the CCNB1/CDK1 pathway, which results in cell cycle arrest and inhibition of cell proliferation.