A Small Ras-like protein Ray/Rab1c modulates the p53-regulating activity of PRPK

Novel TP53RK variants cause varied clinical features of Galloway-Mowat syndrome without nephrotic syndrome in three unrelated Chinese patients

Objectives

Galloway-Mowat syndrome-4 (GAMOS4) is a very rare renal-neurological disease caused by TP53RK gene mutations. GAMOS4 is characterized by early-onset nephrotic syndrome, microcephaly, and brain anomalies. To date, only nine GAMOS4 cases with detailed clinical data (caused by eight deleterious variants in TP53RK) have been reported. This study aimed to examine the clinical and genetic characteristics of three unrelated GAMOS4 patients with TP53RK gene compound heterozygous mutations.

Methods

Whole-exome sequencing (WES) was used to identify four novel TP53RK variants in three unrelated Chinese children. Clinical characteristics such as biochemical parameters and image findings of patients were also evaluated. Furthermore, four studies of GAMOS4 patients with TP53RK variants were reviewed. In addition, clinical and genetic features were described after a retrospective analysis of clinical symptoms, laboratory data, and genetic test results.

Results

The three patients showed facial abnormalities, developmental delays, microcephaly, and aberrant cerebral imaging. Furthermore, patient 1 had slight proteinuria, while patient 2 had epilepsy. However, none of the individuals had nephrotic syndrome, and all were alive for more than 3 years of age. This is the first study to assess four variants in the TP53RK gene (NM_033550.4: c.15_16dup/p.A6Efs*29, c.745A > G/p.R249G, c.185G > A/p.R62H, and c.335A > G/p.Y112C).

Conclusion

The clinical characteristics of the three children with TP53RK mutations are significantly different from the known GAMOS4 traits, including early nephrotic syndrome and mortality mainly occurring in the first year of life. This study provides insights into the pathogenic TP53RK gene mutation spectrum and clinical phenotypes of GAMOS4.

p53 Related Protein Kinase is Required for Arp2/3-Dependent Actin Dynamics of Hemocytes in Drosophila melanogaster

Cells extend membrane protrusions like lamellipodia and filopodia from the leading edge to sense, to move and to form new contacts. The Arp2/3 complex sustains lamellipodia formation, and in conjunction with the actomyosin contractile system, provides mechanical strength to the cell. Drosophila p53-related protein kinase (Prpk), a Tsc5p ortholog, has been described as essential for cell growth and proliferation. In addition, Prpk interacts with proteins associated to actin filament dynamics such as α-spectrin and the Arp2/3 complex subunit Arpc4. Here, we investigated the role of Prpk in cell shape changes, specifically regarding actin filament dynamics and membrane protrusion formation. We found that reductions in Prpk alter cell shape and the structure of lamellipodia, mimicking the phenotypes evoked by Arp2/3 complex deficiencies. Prpk co-localize and co-immunoprecipitates with the Arp2/3 complex subunit Arpc1 and with the small GTPase Rab35. Importantly, expression of Rab35, known by its ability to recruit upstream regulators of the Arp2/3 complex, could rescue the Prpk knockdown phenotypes. Finally, we evaluated the requirement of Prpk in different developmental contexts, where it was shown to be essential for correct Arp2/3 complex distribution and actin dynamics required for hemocytes migration, recruitment, and phagocytosis during immune response.

Proteomic analysis reveals brain Rab35 as a potential biomarker of mitragynine withdrawal in rats

Mitragyna speciosa, also known as kratom, has been used for mitigating the severity of opioid withdrawal in humans. Its main indole alkaloid, mitragynine, has been considered as a pharmacotherapy for pain conditions and opioid replacement therapy. However, at high doses, chronic mitragynine may also have an addiction potential. The effects of chronic action of mitragynine in the brain are still unknown. The present study developed a mitragynine withdrawal model in rats and used it for a proteomic analysis of mitragynine withdrawal effects. Mitragynine (30 mg/kg, i.p.) was administered daily over a period of 14 days and then withdrawn. A proteomic analysis revealed that from a total of 1524 proteins identified, 31 proteins were upregulated, and 3 proteins were downregulated in the mitragynine withdrawal model. The Rab35 protein expression increased most profoundly in the mitragynine withdrawal group as compared to vehicle group. Therefore, it is proposed that Rab35 in the brain might be considered as a potential biomarker during mitragynine withdrawal and might be valuable target protein in developing new pharmacotherapies in the future.

EGF Stimulates Rab35 Activation and Gastric Cancer Cell Migration by Regulating DENND1A-Grb2 Complex Formation

Aims: The aim of this study was to reveal the specific molecular mechanisms by which DENND1A accepts EGF signaling and activates Rab35 in gastric cancer.

Methods: The expression of proteins related to DENND1A was examined by western blot analysis. Activation of Rab35 was assessed by GST-pulldown. The interaction of DENND1A and Grb2 was assessed by GST-pulldown and co-immunoprecipitation assays. The relationship between DENND1A and cell migration and invasion was detected using wound healing and transwell by gene overexpression and RNA interference.

Results: EGF stimulation significantly promoted cell migration, whereas transfection with siRab35 partially inhibited EGF-promoted cell migration. DENND1A is also involved in these processes and active Rab35. Moreover, DENND1A binds to the N-terminal and C-terminal SH3 domains of Grb2 through PRD. Of special interest is the observation that EGFR can recruit Grb2-DENND1A complex under EGF stimulation. Further results reveal that the higher the expression of DENND1A, the shorter progression-free survival of gastric cancer patients.

Conclusion: In summary, we confirmed that EGF-Grb2-DENND1A-Rab35 signaling pathway with the interaction of DENND1A and Grb2 as a regulatory center could regulate gastric cancer cell migration and invasion. Ultimately, the expression level of DENND1A predicts the survival status of gastric cancer patients and may become one of the important targets for the treatment of gastric cancer.

Targeting PRPK and TOPK for skin cancer prevention and therapy

Solar ultraviolet (sUV) irradiation is a major environmental carcinogen that can cause inflammation and skin cancer. The costs and morbidity associated with skin cancer are increasing, and therefore identifying molecules that can help prevent skin carcinogenesis is important. In this study, we identified the p53-related protein kinase (PRPK) as a novel oncogenic protein that is phosphorylated by the T-LAK cell-originated protein kinase (TOPK). Knockdown of TOPK inhibited PRPK phosphorylation and conferred resistance to solar-simulated light (SSL)-induced skin carcinogenesis in mouse models. In the clinic, acute SSL irradiation significantly increased epidermal thickness as well as total protein and phosphorylation levels of TOPK and PRPK in human skin tissues. We identified two PRPK inhibitors, FDA-approved rocuronium bromide (Zemuron^®) or betamethasone 17-valerate (Betaderm^®) that could attenuate TOPK-dependent PRPK signaling. Importantly, topical application of either rocuronium bromide or betamethasone decreased SSL-induced epidermal hyperplasia, neovascularization, and cutaneous squamous cell carcinoma (cSCC) development in SKH1 (Crl: SKH1-Hr^hr) hairless mice by inhibiting PRPK activation, and also reduced expression of the proliferation and oncogenesis markers, COX-2, cyclin D1, and MMP-9. This study is the first to demonstrate that targeting PRPK could be useful against sUV-induced cSCC development.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

Targeting PRPK Function Blocks Colon Cancer Metastasis

The biological functions of the p53-related protein kinase (PRPK) remain unclear. We have previously demonstrated that PRPK is phosphorylated by the T-LAK cell-originated protein kinase (TOPK) and that phosphorylated PRPK (p-PRPK) promotes colon cancer metastasis. Here, we analyzed colon adenocarcinomas from 87 patients and found that higher expression levels of p-PRPK were associated with later stages of metastatic dissemination (stage III and IV) as compared with earlier stages (stages I and II). Indeed, levels of p-PRPK were higher in metastatic versus malignant human colon adenocarcinomas. Knocking down PRPK expression attenuated colorectal liver and lung metastasis of colon cancer cells in vivo An in vitro kinase assay indicated that active PRPK does not phosphorylate p53 directly. We found that PRPK phosphorylates survivin, a regulator of colon cancer metastasis. PRPK phosphorylates survivin at Thr34, which is important for survivin stability. Taken together, our data strongly suggest that the PRPK signaling pathway promotes colon cancer metastasis by modulating survivin stability, and that PRPK could be a new prognostic marker for the survival of colon cancer patients. In addition, we identified an FDA-approved bacteriostatic antibiotic, fusidic acid sodium salt (fusidic acid or FA) as an inhibitor of PRPK, and show that FA combined with 5-fluorouracil (5-FU) inhibited PRPK activity and colon cancer metastasis to the lung in mice. We contend that the combination of FA with 5-FU could be an alternative therapeutic strategy to traditional chemotherapy for colon cancer patients with poor prognosis. Mol Cancer Ther; 17(5); 1101-13. ©2018 AACR.

Rab35 GTPase and cancer: Linking membrane trafficking to tumorigenesis

Rab35 is a small GTPase that is involved in many cellular processes, including membrane trafficking, cell polarity, lipid homeostasis, immunity, phagocytosis and cytokinesis. Recent studies showed that activating mutations confer Rab35 with oncogenic properties. Conversely, downregulation of Rab35 inverts apico-basal cell polarity and promotes cell migration. Here we review Rab35's known functions in membrane trafficking and signaling, cell division and cell migration in cancer cells and discuss the importance of Rab35-dependent membrane trafficking in cancer progression.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

Rab35 Functions in Axon Elongation Are Regulated by P53-Related Protein Kinase in a Mechanism That Involves Rab35 Protein Degradation and the Microtubule-Associated Protein 1B

Rab35 is a key protein for cargo loading in the recycling endosome. In neuronal immortalized cells, Rab35 promotes neurite differentiation. Here we describe that Rab35 favors axon elongation in rat primary neurons in an activity-dependent manner. In addition, we show that the p53-related protein kinase (PRPK) negatively regulates axonal elongation by reducing Rab35 protein levels through the ubiquitin-proteasome degradation pathway. PRPK-induced Rab35 degradation is regulated by its interaction with microtubule-associated protein 1B (MAP1B), a microtubule stabilizing binding protein essential for axon elongation. Consistently, axon defects found in MAP1B knock-out neurons were reversed by Rab35 overexpression or PRPK inactivation suggesting an epistatic relationship among these proteins. These results define a novel mechanism to support axonal elongation, by which MAP1B prevents PRPK-induced Rab35 degradation. Such a mechanism allows Rab35-mediated axonal elongation and connects the regulation of actin dynamics with membrane trafficking. In addition, our study reveals for the first time that the ubiquitin-proteasome degradation pathway regulates a Rab GTPase.

SIGNIFICANCE STATEMENT

Rab35 is required for axonal outgrowth. We define that its protein levels are negatively regulated by p53-related protein kinase (PRPK). We show that microtubule-associated protein 1B (MAP1B) interacts with PRPK, preventing PRPK-dependent Rab35 proteasome degradation. We demonstrate that Rab35 regulates Cdc42 activity in neurons. This is the first evidence showing that a Rab protein is regulated by degradation dependent on the ubiquitin-proteasome system.

The T-LAK Cell-originated Protein Kinase Signal Pathway Promotes Colorectal Cancer Metastasis

Approximately 90% of all cancer deaths arise from the metastatic dissemination of primary tumors. Metastasis is the most lethal attribute of colorectal cancer. New data regarding the molecules contributing to the metastatic phenotype, the pathways they control and the genes they regulate are very important for understanding the processes of metastasis prognosis and prevention in the clinic. The purpose of this study was to investigate the role of T-LAK cell-originated protein kinase (TOPK) in the promotion of colorectal cancer metastasis. TOPK is highly expressed in human metastatic colorectal cancer tissue compared with malignant adenocarcinoma. We identified p53-related protein kinase (PRPK) as a new substrate of TOPK. TOPK binds with and phosphorylates PRPK at Ser250 in vitro and ex vivo. This site plays a critical role in the function of PRPK. Cell lines stably expressing mutant PRPK (S250A), knockdown TOPK, knockdown PRPK or knockdown of both TOPK and PRPK significantly inhibited liver metastasis of human HCT116 colon cancer cells in a xenograft mouse model. Therefore, we conclude that TOPK directly promotes metastasis of colorectal cancer by modulating PRPK. Thus, these findings may assist in the prediction of prognosis or development of new therapeutic strategies against colon cancer.

Rab35 GTPase: A Central Regulator of Phosphoinositides and F-actin in Endocytic Recycling and Beyond

Rab35 is one of the first discovered members of the large Rab GTPase family, yet it received little attention for 10 years being considered merely as a Rab1-like GTPase. In 2006, Rab35 was recognized as a unique Rab GTPase localized both at the plasma membrane and on endosomes, playing essential roles in endocytic recycling and cytokinesis. Since then, Rab35 has become one of the most studied Rabs involved in a growing number of cellular functions, including endosomal trafficking, exosome release, phagocytosis, cell migration, immunological synapse formation and neurite outgrowth. Recently, Rab35 has been acknowledged as an oncogenic GTPase with activating mutations being found in cancer patients. In this review, we provide a comprehensive summary of known Rab35-dependent cellular functions and detail the few Rab35 effectors characterized so far. We also review how the Rab35 GTP/GDP cycle is regulated, and emphasize a newly discovered mechanism that controls its tight activation on newborn endosomes. We propose that the involvement of Rab35 in such diverse and apparently unrelated cellular functions can be explained by the central role of this GTPase in regulating phosphoinositides and F-actin, both on endosomes and at the plasma membrane.

Kinome Screen Identifies PFKFB3 and Glucose Metabolism as Important Regulators of the Insulin/Insulin-like Growth Factor (IGF)-1 Signaling Pathway

The insulin/insulin-like growth factor (IGF)-1 signaling pathway (ISP) plays a fundamental role in long term health in a range of organisms. Protein kinases including Akt and ERK are intimately involved in the ISP. To identify other kinases that may participate in this pathway or intersect with it in a regulatory manner, we performed a whole kinome (779 kinases) siRNA screen for positive or negative regulators of the ISP, using GLUT4 translocation to the cell surface as an output for pathway activity. We identified PFKFB3, a positive regulator of glycolysis that is highly expressed in cancer cells and adipocytes, as a positive ISP regulator. Pharmacological inhibition of PFKFB3 suppressed insulin-stimulated glucose uptake, GLUT4 translocation, and Akt signaling in 3T3-L1 adipocytes. In contrast, overexpression of PFKFB3 in HEK293 cells potentiated insulin-dependent phosphorylation of Akt and Akt substrates. Furthermore, pharmacological modulation of glycolysis in 3T3-L1 adipocytes affected Akt phosphorylation. These data add to an emerging body of evidence that metabolism plays a central role in regulating numerous biological processes including the ISP. Our findings have important implications for diseases such as type 2 diabetes and cancer that are characterized by marked disruption of both metabolism and growth factor signaling.

Rab35 is required for Wnt5a/Dvl2-induced Rac1 activation and cell migration in MCF-7 breast cancer cells

The small GTPases regulate many major biological processes in both tumorigenesis and tumor progression such as cell survival, actin cytoskeleton organization, cell polarity and movement. Wnt5a, a non-canonical Wnt family member, is implicated in the activation of small GTPases in breast cancer. We previously demonstrated that Wnt5a signaling stimulates the migration of breast cancer cells MDA-MB-231 via activating RhoA. However, we found here that RhoA activation was not enhanced by Wnt5a in breast cancer cells MCF-7. The conflicting results prompted us to further probe novel small GTPases in response to Wnt5a and investigate the mechanisms whereby cell migration is regulated. We showed here that Wnt5a dose dependently activated Dvl2, Rab35 and Rac1 and subsequently promoted the migration of MCF-7 cells, which was, however, abolished by knocking down Wnt5a expression via small interfering RNA (siRNA) transfection. Dvl2 siRNA significantly decreased background and Wnt5a-induced Rab35/Rac1 activation and, consequently, cell migration. Rab35 short hairpin RNA (shRNA) remarkably inhibited background and Wnt5a-induced Rac1 activation and cell migration. Additionally, blockade of Rac1 activation with Rac1 siRNA suppressed background and Wnt5a-induced cell migration. Co-immunoprecipitation and immunofluorescence assays showed that Dvl2 bound to Rab35 in mammalian cells. Taken together, we demonstrated that Wnt5a promotes breast cancer cell migration via the Dvl2/Rab35/Rac1 signaling pathway. These findings implicate Wnt5a signaling in regulating small GTPases, which could be targeted for manipulating breast cancer cell migration.

Novel DNA damage checkpoints mediating cell death induced by the NEDD8-activating enzyme inhibitor MLN4924

MLN4924 is an investigational small-molecule inhibitor of the NEDD8-activating enzyme (NAE) in phase I clinical trials. NAE inhibition prevents the ubiquitination and proteasomal degradation of substrates for cullin-RING ubiquitin E3 ligases that support cancer pathophysiology, but the genetic determinants conferring sensitivity to NAE inhibition are unknown. To address this gap in knowledge, we conducted a genome-wide siRNA screen to identify genes and pathways that affect the lethality of MLN4924 in melanoma cells. Of the 154 genes identified, approximately one-half interfered with components of the cell cycle, apoptotic machinery, ubiquitin system, and DNA damage response pathways. In particular, genes involved in DNA replication, p53, BRCA1/BRCA2, transcription-coupled repair, and base excision repair seemed to be important for MLN4924 lethality. In contrast, genes within the G(2)-M checkpoint affected sensitivity to MLN4924 in colon cancer cells. Cell-cycle analysis in melanoma cells by flow cytometry following RNAi-mediated silencing showed that MLN4924 prevented the transition of cells from S-G(2) phase after induction of rereplication stress. Our analysis suggested an important role for the p21-dependent intra-S-phase checkpoint and extensive rereplication, whereas the ATR-dependent intra-S-phase checkpoint seemed to play a less dominant role. Unexpectedly, induction of the p21-dependent intra-S-phase checkpoint seemed to be independent of both Cdt1 stabilization and ATR signaling. Collectively, these data enhance our understanding of the mechanisms by which inhibition of NEDD8-dependent ubiquitination causes cell death, informing clinical development of MLN4924.

The activity of an ancient atypical protein kinase is stimulated by ADP-ribose in vitro

The piD261/Bud32 protein kinases are universal amongst the members of the Eucarya and Archaea. Despite the fact that phylogenetic analyses indicate that the piD261/Bud32 protein kinases descend directly from the primordial ancestor of the "eukaryotic" protein kinase superfamily, our knowledge of their physiological role is relatively fragmentary and largely limited to two eucaryal representatives: piD261/Bud32 from yeast and the p53-related protein kinase from humans. A deduced archaeal homolog, SsoPK5, is encoded by open reading frame sso0433 from the acidothermophile Sulfolobus solfataricus. Recombinantly-expressed SsoPK5 exhibited protein kinase activity, with a noticeable preference for phosphorylating proteins of acidic character and for Mn(2+) as cofactor. The protein kinase also can phosphorylate itself on serine and threonine residues. The activity of rSsoPK5 was increased several-fold upon preincubation with either millimolar concentrations of 5'-AMP or submicromolar concentrations of ADP-ribose. Other mono- and di-nucleotides were ineffective. While activation was enhanced by the presence of ATP, no autophosphorylation of the protein kinase could be detected prior to addition of exogenous substrate proteins. We therefore suggest that ADP-ribose acts by evoking a conformational transition in the enzyme. Activation by ADP-ribose represents a potential regulatory link between chromatin remodeling and the activity of SsoPK5.

Regulation of intestinal epithelial cells transcriptome by enteric glial cells: impact on intestinal epithelial barrier functions

BACKGROUND

Emerging evidences suggest that enteric glial cells (EGC), a major constituent of the enteric nervous system (ENS), are key regulators of intestinal epithelial barrier (IEB) functions. Indeed EGC inhibit intestinal epithelial cells (IEC) proliferation and increase IEB paracellular permeability. However, the role of EGC on other important barrier functions and the signalling pathways involved in their effects are currently unknown. To achieve this goal, we aimed at identifying the impact of EGC upon IEC transcriptome by performing microarray studies.

RESULTS

EGC induced significant changes in gene expression profiling of proliferating IEC after 24 hours of co-culture. 116 genes were identified as differentially expressed (70 up-regulated and 46 down-regulated) in IEC cultured with EGC compared to IEC cultured alone. By performing functional analysis of the 116 identified genes using Ingenuity Pathway Analysis, we showed that EGC induced a significant regulation of genes favoring both cell-to-cell and cell-to-matrix adhesion as well as cell differentiation. Consistently, functional studies showed that EGC induced a significant increase in cell adhesion. EGC also regulated genes involved in cell motility towards an enhancement of cell motility. In addition, EGC profoundly modulated expression of genes involved in cell proliferation and cell survival, although no clear functional trend could be identified. Finally, important genes involved in lipid and protein metabolism of epithelial cells were shown to be differentially regulated by EGC.

CONCLUSION

This study reinforces the emerging concept that EGC have major protective effects upon the IEB. EGC have a profound impact upon IEC transcriptome and induce a shift in IEC phenotype towards increased cell adhesion and cell differentiation. This concept needs to be further validated under both physiological and pathophysiological conditions.

Androgen-related expression of G-proteins in ovarian cancer

BACKGROUND

Epidemiological and in vitro data implicate androgens in the aetiology of ovarian cancer, but the mechanisms by which this is mediated are unclear. In this study, we wished to examine the effects of androgens on gene expression in ovarian cancer.

METHODS

The expression of androgen receptor (AR) in OVCAR3 and OSEC2 cells was confirmed using immunoblotting and response to androgens was measured using flow cytometric assessment of S-phase fraction. The differential gene expression between androgen stimulated and unstimulated OVCAR3 ovarian cancer cells was examined with a cDNA microarray. The upregulation of a subset of these genes was then confirmed with reverse transcriptase PCR in both OVCAR3 and OSEC2, an ovarian epithelial cell line. Finally, the clinical significance of this upregulation was investigated by examining the expression of Rab25 and Rab35, two G-protein-related molecules in an ovarian cancer tissue microarray (TMA).

RESULTS

OVCAR3 and OSEC2 cells were shown to express the AR and showed an increase in S-phase fraction in response to androgen treatment. Treatment of OVCAR3 cells with androgen resulted in a significant upregulation of 121 genes. These findings were confirmed for a subset of seven monomeric G-protein-related genes in both OVCAR3 and OSEC2 cells. After staining for Rab25 and Rab35, the majority of TMA sections examined showed expression for Rab25 (92%) and Rab35 (95%). The expression of Rab25 correlated with histological grade, and expression was higher in endometrioid (median histoscore 10.5) than serous (7.5) or mucinous (5.3) tumours. The expression of Rab25 correlated positively with AR expression supporting its role as an androgen responsive gene in ovarian cancer.

CONCLUSIONS

These results suggest that androgens can effect expression of the oncogenic GTPases in ovarian cancer. We propose that the androgen responsive Rab35 may have clinical importance as a biomarker of AR function.

A mitotic kinase TOPK enhances Cdk1/cyclin B1-dependent phosphorylation of PRC1 and promotes cytokinesis

A MAPKK-like mitotic kinase, TOPK, implies the formation of mitotic spindles and spindle midzone and accomplishing cytokinesis, however, its underlying mechanism remains unclear. A microtubule bundling protein, PRC1, plays a pivotal role in the formation of mitotic spindles and spindle midzone. Because of their functional resemblance, we attempted to clarify the links between these two molecules. TOPK supported mitotic advance via the cdk1/cyclin B1-dependent phosphorylation of PRC1. TOPK induced the phosphorylation of PRC1 at T481 in vivo, however, TOPK did not phosphorylate PRC1 in vitro. TOPK induced the phosphorylation of PRC1 at T481 only when the cdk1/cyclin B1 existed simultaneously in vitro. Both the enzymatic activity of TOPK and association competence of TOPK with PRC1 were mandatory for this phosphorylation. TOPK binds to cdk1/cyclin B1, microtubules and PRC1 via its unique region near the C terminus. TOPK co-localized closely with cdk1 throughout the cell cycle in vivo. Collectively, these data indicate that TOPK, which makes a kinase-substrate complex with cdk1/cyclin B1 and PRC1 on microtubules during mitosis, enhances the cdk1/cyclin B1-dependent phosphorylation of PRC1 and thereby strongly promotes cytokinesis.