Mitotic Protein Research Articles

The functional significance of ATM phosphorylation of Bub3 on Serine 135 in sensitivity to DNA damaging agents.

e15048 Background: Identification of biomarkers to assess and modify the sensitivity of cancer cells to radiotherapy and chemotherapy is critical to improve cancer treatment outcome. Budding uninhibited by benzimidazoles 3 (Bub3) is a mitotic checkpoint protein, and it is frequently overexpressed in many cancers and associated with low survival rates. Bub3 is involved in the repair of DNA damage induced by radiotherapy and chemotherapy. We recently identified that the ATM kinase phosphorylated Bub3 on Serine 135 (S135) via the stable isotope labeled amino acid in cell culture -mass spectrometry analysis. This study aims to explore the mechanism of ATM-phosphorylated Bub3 in the DNA damage response (DDR) and its effect on tumor sensitivity to DNA damaging agents. Methods: The radiosensitivity of the cells was detected by clonal formation assay, the proliferation ability of the cells was detected by the MTS assay. The expression of DDR protein γ-H2AX in the nucleus was detected by immunofluorescence assay. Genomic instability was observed by multinuclear formation. Co-immunoprecipitation and Western Blot were used to explore the internal mechanism of ATM phosphorylation of Bub3 in DDR. Results: We showed that ionizing radiation (IR) could induce Bub3 S/TQ (the specific ATM consensus motif) phosphorylation in an ATM dependent manner. Mutation of Bub3 Serine 135 to alanine (S135A) led to a phosphorylation defect. Phenotypic experiments showed hypersensitivity to IR in cells expressing Bub3 S135A. Bub3 S135A prolonged existence of γ-H2AX foci and increased the proportion of cells containing micronuclei. Further, we found that Ku70 and Ku80 showed a significant increase after IR in their interactions with Bub3, while Bub3 S135A mutation significantly reduced the interaction, leading to impaired DNA repair. Conclusions: We demonstrate that Bub3 S135 phosphorylation mediated by ATM is essential for an optimal DDR and disruption of this pathway increases tumor sensitivity to DNA damaging agents.

Trial in progress: A phase 1, multicenter, open-label, dose-exploration and dose-expansion study evaluating the safety, tolerability, pharmacokinetics, and efficacy of AMG650 in subjects with advanced solid tumors.

TPS5600 Background: KIF18A is a mitotic kinesin motor protein that regulates chromosome positioning during cell division and is overexpressed in a subset of human cancers. TP53 mutant unstable aneuploid cancer cells with chromosomal instability (CIN) features are dependent on KIF18A motor activity to prevent lethal multipolar cell division. Preclinical data demonstrate that treatment with AMG 650; an oral, first in class, selective small molecule inhibitor of KIF18A may be safe and tolerable. We are conducting a first-in-human phase 1 study with AMG 650 in adult subjects with locally advanced or metastatic solid tumors with TP53MUT, triple negative breast cancer (TNBC), high grade serous ovarian cancer (HGSOC) or serous like endometrial cancers and other solid tumors. Methods: In this phase 1, multicentric, dose escalation and dose expansion study we evaluate the safety and tolerability of AMG 650 monotherapy in patients with advanced/metastatic solid tumors (NCT04293094). The main objective is to determine the maximum tolerated dose (MTD) and/or recommended phase 2 dose (RP2D) based on emerging safety, efficacy, and pharmacodynamics (PD) data prior to reaching the MTD. Key inclusion criteria include the presence of measurable disease and diagnosis of advanced/metastatic triple negative breast cancer (TNBC), high-grade serous ovarian cancer (HGSOC), serous-like endometrial cancer or other solid tumors with documented TP53 mutations. In the dose expansion phase, participants with locally advanced or metastatic TNBC or HGSOC will be treated with the preliminary RP2D identified from the dose exploration part of the study. Primary endpoints include the incidence of Dose Limiting Toxicities (DLTs),Treatment-Emergent Adverse Events (TEAEs), Serious Adverse Events (SAEs), Treatment-related Adverse Events and the evaluation of the number of participants who experience a clinically significant change from baseline in vital signs, electrocardiogram and laboratory tests parameters. Secondary endpoints include Objective Response Rate, Duration of Response, Progression-free Survival, Clinical Benefit Rate, Time to Response, Time to Progression, Overall Survival (OS), Maximum Plasma Concentration (Cmax) of AMG 650, Time to Maximum Plasma Concentration (Tmax) of AMG 650 as well as Area Under the Plasma Concentration-time Curve (AUC) Over the Dosing Interval for AMG 650. Continuous monitoring of toxicity is conducted. The study began enrolling pts in March 2020 and is ongoing. For more information, please contact Amgen Medical Information: medinfo@amgen.com Clinical trial information: NCT04293094.

Global phosphoproteomics reveals DYRK1A regulates CDK1 activity in glioblastoma cells

Both tumour suppressive and oncogenic functions have been reported for dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A). Herein, we performed a detailed investigation to delineate the role of DYRK1A in glioblastoma. Our phosphoproteomic and mechanistic studies show that DYRK1A induces degradation of cyclin B by phosphorylating CDC23, which is necessary for the function of the anaphase-promoting complex, a ubiquitin ligase that degrades mitotic proteins. DYRK1A inhibition leads to the accumulation of cyclin B and activation of CDK1. Importantly, we established that the phenotypic response of glioblastoma cells to DYRK1A inhibition depends on both retinoblastoma (RB) expression and the degree of residual DYRK1A activity. Moderate DYRK1A inhibition leads to moderate cyclin B accumulation, CDK1 activation and increased proliferation in RB-deficient cells. In RB-proficient cells, cyclin B/CDK1 activation in response to DYRK1A inhibition is neutralized by the RB pathway, resulting in an unchanged proliferation rate. In contrast, complete DYRK1A inhibition with high doses of inhibitors results in massive cyclin B accumulation, saturation of CDK1 activity and cell cycle arrest, regardless of RB status. These findings provide new insights into the complexity of context-dependent DYRK1A signalling in cancer cells.

A single-cell analysis of the Arabidopsis vegetative shoot apex.

The shoot apical meristem allows for reiterative formation of new aerial structures throughout the life cycle of a plant. We use single-cell RNA sequencing to define the cellular taxonomy of the Arabidopsis vegetative shoot apex at the transcriptome level. We find that the shoot apex is composed of highly heterogeneous cells, which can be partitioned into 7 broad populations with 23 transcriptionally distinct cell clusters. We delineate cell-cycle continuums and developmental trajectories of epidermal cells, vascular tissue, and leaf mesophyll cells and infer transcription factors and gene expression signatures associated with cell fate decisions. Integrative analysis of shoot and root apical cell populations further reveals common and distinct features of epidermal and vascular tissues. Our results, thus, offer a valuable resource for investigating the basic principles underlying cell division and differentiation in plants at single-cell resolution.

LY6K-AS lncRNA is a lung adenocarcinoma prognostic biomarker and regulator of mitotic progression.

Recent advances in genomics unraveled several actionable mutational drivers in lung cancer, leading to promising therapies such as tyrosine kinase inhibitors and immune checkpoint inhibitors. However, the tumors' acquired resistance to the newly-developed as well as existing therapies restricts life quality improvements. Therefore, we investigated the noncoding portion of the human transcriptome in search of alternative actionable targets. We identified an antisense transcript, LY6K-AS, with elevated expression in lung adenocarcinoma (LUAD) patients, and its higher expression in LUAD patients predicts poor survival outcomes. LY6K-AS abrogation interfered with the mitotic progression of lung cancer cells resulting in unfaithful chromosomal segregation. LY6K-AS interacts with and stabilizes 14-3-3 proteins to regulate the transcription of kinetochore and mitotic checkpoint proteins. We also show that LY6K-AS regulates the levels of histone H3 lysine 4 trimethylation (H3K4me3) at the promoters of kinetochore members. Cisplatin treatment and LY6K-AS silencing affect many common pathways enriched in cell cycle-related functions. LY6K-AS silencing affects the growth of xenografts derived from wildtype and cisplatin-resistant lung cancer cells. Collectively, these data indicate that LY6K-AS silencing is a promising therapeutic option for LUAD that inhibits oncogenic mitotic progression.

Abstract PS5-20: Protein interactome dysregulation analysis reveals putative therapeutic targets for BCI(H/I)-low breast cancers patients

Abstract Background: Hormone receptor-positive (HR+) breast cancer (BC) is a disease of late recurrence. HR+ BC patients who are disease free after five years of adjuvant hormonal therapy, have an ongoing risk of disease recurrence and death for at least 15 years from diagnosis. Multiple trials have demonstrated consistent absolute benefits with extending adjuvant endocrine therapy to 10 years in HR+ patients. In several extended endocrine therapy trials, we have shown that HR+ patients with BCI(HoxB13/IL17BR) [BCI(H/I)]-high biomarker expression benefit from extended endocrine therapy, while those with low BCI(H/I)-low expression do not benefit from such therapy. Currently, there are limited therapeutic options for BCI(H/I)-low patients who are at high risk for late disease recurrence. Previous cell line-based, global scale protein-protein interaction (PPI) dysregulation mapping studies have identified therapeutic pathways within cellular circuits carrying the perturbed protein-protein interaction. To identify exploitable therapeutic vulnerabilities in BCI(H/I)-low BC patients, we performed comparative multiplexed quantitative mass spectrometry-based interactome dysregulation analysis of BCI(H/I)-low with BCI(H/I)-high BC samples and correlated interactome perturbations with drug responses. Methods: 21 BCI(H/I)-high and 23 BCI(H/I)-low frozen tumor samples from ER+ BC patients were identified in the MGH BC tissue repository, and proteomes were mapped using TMT-11 reagents, and the Synchronous Precursor Selection supported MS2/MS3 method on an Orbitrap Fusion and an Orbitrap Lumos. Dysregulations of PPIs were mapped through using bivariate outlier testing on each identified protein pair based on mapping Mahalanobis distances. . Protein identifiers (IDs) were converted to their appropriate gene IDs and gene set enrichment analysis (GSEA) was performed. Using previously established PPI data from 41 human breast cancer cell lines, we performed comparative PPI dysregulation GSEA and leading edge GSEA of the BCI(H/I)-high and -low samples. To identify therapeutic target candidates for BCI(H/I)-low patients, tumor interactome dysregulations were correlated with drug response dysregulations determined in drug screen of 195 cancer therapeutics against 41 breast cancer cell lines. Results: Across 44 samples, we have quantified more than 11,608 proteins with an average of 7,890 proteins per sample. Protein concentration co-regulation analysis resulted in identifying more than 11,908 protein-protein associations (ρ >0.65, FDRq <0.0001). PPI dysregulation analysis revealed an average of more than 1596 dysregulated PPIs per tumor sample. GSEA of the 21 BCI(H/I)-high proteome samples revealed a positive association with the Bowie tamoxifen response, and the α- and γ-interferon response C2 gene sets, while the 23 BCI-H/I-Low samples revealed a positive association (p=0.03, FDRq=0.052) with the hedgehog signaling and the early estrogen Hallmark gene sets. Protein-protein interaction GSEA identified 85 significant gene sets (p <0.016, FDRq <0.25) that were more frequently dysregulated in the BCI-H/I-low samples as compared with the BCI(H/I)-high samples. Dysregulated pathways in BCI-H/I-low samples included cell cycle regulation, anaphase promoting complex-mediated degradation of mitotic proteins, and ERK and RAS signaling. Correlation of the BCI(H/I)-low interactome perturbations with cell line model drug responses to a library of 195 therapeutics revealed a significant correlation with PI3K inhibitor drug response. Conclusion: Quantitative mass spectrometry-based protein-protein interaction dysregulation identified the PI3K- pathway as a putative exploitable therapeutic vulnerability in BCI(H/I)-low HR+ BC patients at risk for late disease recurrence. Citation Format: Dennis Sgroi, Johannes Kreuzer, Robert Morris, Xcanda Herrera Lopez, Wayland Chiu, Wilhelm Haas. Protein interactome dysregulation analysis reveals putative therapeutic targets for BCI(H/I)-low breast cancers patients [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-20.

The Mitotic Crosslinking Protein PRC1 Acts Like a Mechanical Dashpot to Resist Microtubule Sliding

Cell division in eukaryotes requires the regulated assembly of the spindle apparatus. The proper organization of microtubules within the spindle is driven by motor proteins that exert forces to slide filaments, while non-motor proteins crosslink filaments into higher order motifs such as overlapping bundles. It is not clear how active and passive forces are integrated to produce regulated mechanical outputs within spindles. Here we employ simultaneous optical trapping and TIRF microscopy to directly measure the frictional forces produced by the mitotic crosslinking protein PRC1 that resist microtubule sliding.

Whole-genome doubling confers unique genetic vulnerabilities on tumour cells.

SummaryWhole genome doubling (WGD) occurs early in tumorigenesis and generates genetically unstable tetraploid cells that fuel tumor development1,2. Cells that undergo WGD (WGD+) must adapt to accommodate their abnormal tetraploid state; however, the nature of these adaptations, and whether they confer vulnerabilities that can subsequently be exploited therapeutically, is unclear. Using sequencing data from ~10,000 primary human cancer samples and essentiality data from ~600 cancer cell lines, we show that WGD gives rise to common genetic traits that are accompanied by unique vulnerabilities. We reveal that WGD+ cells are more dependent on spindle assembly checkpoint signaling, DNA replication factors, and proteasome function than WGD– cells. We also identify KIF18A, which encodes for a mitotic kinesin, as being specifically required for the viability of WGD+ cells. While loss of KIF18A is largely dispensable for accurate chromosome segregation during mitosis in WGD– cells, its loss induces dramatic mitotic errors in WGD+ cells, ultimately impairing cell viability. Collectively, our results reveal new strategies to specifically target WGD+ cancer cells while sparing the normal, non-transformed WGD– cells that comprise human tissue.

Identification of dysregulated pathways underlying HTLV-1-associated myelopathy/tropical spastic paraparesis through co-expression network analysis.

Human T cell lymphotropic virus-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a pathogen-caused disease which is associated with the progressive neurological disorder. HAM/TSP affects the expression level of several proteins and dysregulates some biological pathways. To identify the interaction patterns among expressed genes in HAM/TSP patients, weighted gene co-expression network analysis (WGCNA) was applied. Three microarray datasets regarding HAM/TSP were merged, and the co-expression network was constructed among genes. A total of 38 modules were identified. Three preserved modules in HAM/TSP in comparison to the healthy subjects which also had the most connected proteins and enriched in the biological pathways were selected. These modules were enriched in pathways related to immune systems, cell cycle, viral infection, and neuronal systems. Moreover, the involvement of novel immunological-related proteins including C1QB, GBP5, PSME1, SERPING1, and UBE2C; neurological-related proteins including TUBA4A, TUBB8, and TP63; and also proteins including TRPC6, PRKG2, OPRD1, PRKACA, and TUBB4A involved in the cGMP-PKG signaling pathway, thyroid hormone synthesis, and recruitment of mitotic centrosome proteins and complexes were found. Therefore, tracing these proteins and the identified modules can shed light on the pathogenesis mechanism of HAM/TSP and help to findpotential therapeutic targets. However, further experimental validation should be performed to confirm the proposed functional players.

Targeted exome sequencing for the identification of common mutational signatures and potential driver mutations for brain metastases and prognosis.

Brain metastases (BMs) are malignancies in the central nervous system with poor prognosis. Genetic landscapes of the primary tumor sites have been extensively profiled; however, mutations associated with BMs are poorly understood. In the present study, target exome sequencing of 560 cancer-associated genes in samples from 52 patients with brain metastasis from various primary sites was performed. Recurrent mutations for BMs from distinct origins were identified. There were both genetic homogeneity and heterogeneity between BMs and primary lung tumor tissues. The mutation rate of the major cancer driver gene, TP53, was consistently high in both the primary lung cancer sites and BMs, while some genetic alterations, associated with DNA damage response deficiency, were specifically enriched in BMs. The mutational signatures enriched in BMs could serve as actionable targets for treatment. The mutation in the primary site of the potential brain metastasis driver gene, nuclear mitotic apparatus protein 1 (NUMA1), affected the progression-free survival time of patients with lung cancer, and patients with the NUMA1 mutation in BMs had a good prognosis. This suggested that the occurrence and clinical outcome of brain metastases could be independent of each other.

MAD2B-mediated cell cycle reentry of podocytes is involved in the pathogenesis of FSGS.

Rationale: Focal segmental glomerulosclerosis (FSGS) is characterized by the dysfunction of “post-mitotic” podocytes. The reentry of podocytes in the cell cycle will ultimately result in cell death. Mitotic arrest deficient 2-like protein 2 (MAD2B), an inhibitor of anaphase-promoting complex (APC)/cyclosome, precisely controls the metaphase to anaphase transition and ordered cell cycle progression. However, the role of MAD2B in FSGS podocyte injury remains unknown.Methods: To explore MAD2B function in podocyte cell cycle reentry, we used conditional mutant mice lacking MAD2B selectively in podocytes in ADR-induced FSGS murine model. Additionally, KU-55933, a specific inhibitor of ataxia-telangiectasia mutated (ATM) was utilized in vivo and in vitro to explore the role of ATM in regulating MAD2B.Results: The expression of MAD2B in podocytes was dramatically increased in patients with FSGS and ADR-treated mice along with podocyte cell cycle reentry. Podocyte-specific knockout of MAD2B effectively attenuated proteinuria, podocyte injury, and prevented the aberrant cell cycle reentry. By bioinformatics analysis we revealed that ATM kinase is a key upstream regulator of MAD2B. Furthermore, inhibition of ATM kinase abolished MAD2B-driven cell cycle reentry and alleviated podocyte impairment in FSGS murine model. In vitro studies by site-directed mutagenesis and immunoprecipitation we revealed ATM phosphorylated MAD2B and consequently hampered the ubiquitination of MAD2B in a phosphorylation-dependent manner.Conclusions: ATM kinase-MAD2B axis importantly contributes to the cell cycle reentry of podocytes, which is a novel pathogenic mechanism of FSGS, and may shed light on the development of its therapeutic approaches.

Use of Mitotic Protein Kinase Inhibitors and Phospho-Specific Antibodies to Monitor Protein Phosphorylation During the Cell Cycle.

Reversible protein phosphorylation regulates the transitions between different phases of the cell cycle ensuring proper segregation of the duplicated genome into two daughter cells. Protein kinases and protein phosphatases establish the appropriate phosphorylation stoichiometries in diverse substrates maintaining genomic stability as a cell undergoes this complex process. Along with regulating common substrates, these opposing enzymes regulate one another by fine-tuning each other's activity both spatially and temporally throughout mitosis. Protein phosphatase catalytic subunits work together with regulatory proteins, which control their localization, activity, and specificity. Protein phosphatase 1 (PP1) recognizes its regulatory proteins via a short linear interaction motif (SLIM) called the "RVxF" motif. A subset of proteins with these "RVxF" motifs contain a phosphorylatable amino acid (S/T) at the 'x' position.Here, we describe methods to generate, affinity purify and utilize phospho-specific antibodies to monitor phosphorylation sites during the cell cycle and the appropriate use of mitotic kinase inhibitors. More specifically, we employ phospho-specific antibodies, which recognize phosphorylated RVp[S/T]F motif-containing proteins, to monitor the phosphorylation status of these motifs throughout the cell cycle. Furthermore, we use mitotic kinase inhibitors to examine the effect of kinase inhibition on the phosphorylation status of multiple RV[S/T]F motifs using these phospho-specific antibodies.

Bistable, Biphasic Regulation of PP2A-B55 Accounts for the Dynamics of Mitotic Substrate Phosphorylation

SUMMARYThe phosphorylation of mitotic proteins is bistable, which contributes to the decisiveness of the transitions into and out of M phase. The bistability in substrate phosphorylation has been attributed to bistability in the activation of the cyclin-dependent kinase Cdk1. However, more recently it has been suggested that bistability also arises from positive feedback in the regulation of the Cdk1-counteracting phosphatase PP2A-B55. Here, we demonstrate biochemically using Xenopus laevis egg extracts that the Cdk1-counter-acting phosphatase PP2A-B55 functions as a bistable switch, even when the bistability of Cdk1 activation is suppressed. In addition, Cdk1 regulates PP2A-B55 in a biphasic manner; low concentrations of Cdk1 activate PP2A-B55 and high concentrations inactivate it. As a consequence of this incoherent feedforward regulation, PP2A-B55 activity rises concurrently with Cdk1 activity during interphase and suppresses substrate phosphorylation. PP2A-B55 activity is then sharply downregulated at the onset of mitosis. During mitotic exit, Cdk1 activity initially falls with no obvious change in substrate phosphorylation; dephosphorylation then commences once PP2A-B55 spikes in activity. These findings suggest that changes in Cdk1 activity are permissive for mitotic entry and exit but that the changes in PP2A-B55 activity are the ultimate trigger.

Moonlighting at the Centrosome: RXRα Turns to Plk1

One of the hardest working mitotic proteins, Polo-like kinase 1 (PLK1), functions at mitotic entry, cytokinesis, and many steps in between. In this issue, Xie etal. (2020) describe a centrosome-specific interaction between PLK1 and Retinoid X Receptor-α and they test selective inhibition of this interaction as an anti-mitotic cancer therapy.

CBIO-06. MKlp2 INHIBITION AS A NOVEL ANTIMITOTIC FOR GLIOBLASTOMA

Abstract Despite innovations in therapy and surgery, glioblastoma remains one of the deadliest tumor types with a 5-year survival rate of approximately 5%. There is a desperate need for new therapeutic strategies to block the growth and survival of glioblastoma. To circumvent toxicities associated with traditional antimitotics, there has been a growing interest in developing therapies that impede mitosis-specific microtubule functions, such as the inhibition of kinesins, mitotic motor proteins that travel along microtubules. Our work with motor kinesin-like protein 2 (MKlp2), in combination with reports from published literature, suggest a previously undescribed role for MKlp2 in early mitosis that would make its inhibition effective at impeding glioblastoma cell growth without neurologic side effects. The objective of this study was to further define this novel function of MKlp2 and to determine the efficacy of MKlp2 inhibition in glioblastoma. Our data indicate that MKlp2 inhibition induced a spindle assembly checkpoint-mediated arrest, suggesting that function of MKlp2 is important for recovery progression to anaphase in addition to its well-known function in late mitosis. In addition, MKlp2 inhibition with a small molecule MKlp2 inhibitor significantly decreased short-term and long-term survival of human glioblastoma patient-derived lines. Furthermore, our data indicate that it is loss of this early mitotic function that is responsible for the anti-proliferative effects caused by MKlp2 inhibition. Ongoing work will continue to delineate the mechanism of MKlp2 in early mitosis and determine the effect of small molecule MKlp2 inhibition in vivo, all the while exploring a new therapeutic strategy for glioblastoma

Black phosphorus quantum dots reverse the malignant potential and enhance chemosensitivity of human renal cell carcinoma cells by targeting histone deacetylase 1 signal pathway

AbstractTargeted anti‐cancer therapy selectively inhibits the growth of malignant cells by interfering with cancer‐specific signaling pathways, and inflicts minimal toxicity to normal tissues. Here, the current study demonstrates black phosphorus quantum dots (BPQDs) can target histone deacetylase 1 (HDAC1), which is overexpressed in multiple tumors and involved in cancer progression. The BPQDs inhibit HDAC1 activity and impair HDAC1‐mediated deacetylation of the mitotic spindle protein Eg5 in human renal cell carcinoma (RCC) cells, thereby disrupting the mitotic spindle structure and stalling cell cycle progression. Since HDAC1 regulates multiple aspects of cancer progression, BPQDs can neutralize the malignant potential of cancer cells by suppressing their proliferation and migration, and inducing apoptosis. Further in vitro assays demonstrate BPQDs significantly increase the sensitivity of RCC cells to the clinical chemotherapeutic agent, paclitaxel (PTX). Moreover, animal experiments also indicate the combined approach with both BPQDs and PTX displays better efficacy than PTX alone. These findings demonstrate that BPQDs have potential applications in targeted anti‐cancer therapy and can be more effectively treated with chemotherapy.

Revealing PAK2's Function in the Cell Division through MKLP1's Interactome.

Cell division-related proteins are essential for the normal development and differentiation of cells and may be related to the occurrence of cancer and the drug resistance mechanism of cancer cells. The mitotic kinesin-like protein 1 (MKLP1) is a kinesin protein that has been involved in the assembly of the midzone/midbody during mitosis and cytokinesis. In this study, we found that the tail domain of MKLP1 exhibited an autoinhibitory effect on its motor activity. Overexpression of the tail domain in HEK293 cells blocked cytokinesis and caused bi-/multinucleation. It is possible that protein binding to the MKLP1 tail relieves this autoinhibition and induces the motility of MKLP1. We used the GST pull-down assay followed by the LC-MS/MS analysis and identified 54 MKLP1 tail domain-specific binding proteins. Further, we confirmed the MS result by coimmunoprecipitation and FRET that a serine/threonine kinase, p21-activated kinase 2 (PAK2), binding to MKLP1. Endogenous PAK2 expression was found to be identical to that of MKLP1 in HEK293 cells during cytokinesis. Finally, functional studies indicated that when PAK2 expression was downregulated by siRNA, MKLP1 underwent a change in its localization away from the midbody, and cell cytokinesis was subsequently impeded. This study presents a novel regulatory mechanism that PAK2 promotes the activation of MKLP1 and contributes to complete cell cytokinesis.

Mitotic checkpoint protein Mad1 is required for early Nup153 recruitment to chromatin and nuclear envelope integrity.

Nucleoporin Nup153 is a multifunctional protein and a known binding partner of mitotic checkpoint protein Mad1 (also known as MAD1L1). The functional relevance of their interaction has remained elusive. Here, we have further dissected the interface and functional interplay of Nup153 and Mad1. Using in situ proximity ligation assays, we found that the presence of a nuclear envelope (NE) is a prerequisite for the Nup153-Mad1 association. Time-lapse microscopy revealed that depletion of Mad1 delayed recruitment of Nup153 to anaphase chromatin, which was often accompanied by a prolongation of anaphase. Furthermore, as seen by electron microscopic and three-dimensional structured illumination investigations, Nup153 and Mad1 depletion led to alterations in NE architecture, characterised by a change of membrane curvature at nuclear pore complexes (NPCs) and an expansion of the spacing between inner and outer nuclear membranes. Nup153 depletion, but not Mad1 depletion, caused defects in interphase NPC assembly, with partial displacement of cytoplasmic nucleoporins and a reduction in NPC density. Taken together, our results suggest that Nup153 has separable roles in NE and NPC formation: in post-mitotic NE re-formation in concert with Mad1 and in interphase NPC assembly, independent of Mad1.

Mitochondrial survivin reduces oxidative phosphorylation in cancer cells by inhibiting mitophagy.

Survivin (also known as BIRC5) is a cancer-associated protein that is pivotal for cellular life and death - it is an essential mitotic protein and an inhibitor of apoptosis. In cancer cells, a small pool of survivin localises to the mitochondria, the function of which remains to be elucidated. Here, we report that mitochondrial survivin inhibits the selective form of autophagy called 'mitophagy', causing an accumulation of respiratory-defective mitochondria. Mechanistically, the data reveal that survivin prevents recruitment of the E3-ubiquitin ligase Parkin to mitochondria and their subsequent recognition by the autophagosome. The data also demonstrate that cells in which mitophagy has been blocked by survivin expression have an increased dependency on glycolysis. As these effects were found exclusively in cancer cells, they suggest that the primary act of mitochondrial survivin is to steer cells towards the implementation of the Warburg transition by inhibiting mitochondrial turnover, which enables them to adapt and survive.This article has an associated First Person interview with the first author of the paper.

The mitotic protein NuMA plays a spindle-independent role in nuclear formation and mechanics.

Eukaryotic cells typically form a single, round nucleus after mitosis, and failures to do so can compromise genomic integrity. How mammalian cells form such a nucleus remains incompletely understood. NuMA is a spindle protein whose disruption results in nuclear fragmentation. What role NuMA plays in nuclear integrity, and whether its perceived role stems from its spindle function, are unclear. Here, we use live imaging to demonstrate that NuMA plays a spindle-independent role in forming a single, round nucleus. NuMA keeps the decondensing chromosome mass compact at mitotic exit and promotes a mechanically robust nucleus. NuMA's C terminus binds DNA in vitro and chromosomes in interphase, while its coiled-coil acts as a central regulatory and structural element: it prevents NuMA from binding chromosomes at mitosis, regulates its nuclear mobility, and is essential for nuclear formation. Thus, NuMA plays a structural role over the cell cycle, building and maintaining the spindle and nucleus, two of the cell's largest structures.