A conserved mitotic kinase active at late anaphase-telophase in syncytial Drosophila embryos

The Impact of Centrosome Pathologies on Ovarian Cancer Development and Progression with a Focus on Centrosomes as Therapeutic Target

The impact of centrosome abnormalities on cancer cell proliferation has been recognized as early as 1914 (Boveri, Zur Frage der Entstehung maligner Tumoren. Jena: G. Fisher, 1914), but vigorous research on molecular levels has only recently started when it became fully apparent that centrosomes can be targeted for new cancer therapies. While best known for their microtubule-organizing capabilities as MTOC (microtubule organizing center) in interphase and mitosis, centrosomes are now further well known for a variety of different functions, some of which are related to microtubule organization and consequential activities such as cell division, migration, maintenance of cell shape, and vesicle transport powered by motor proteins, while other functions include essential roles in cell cycle regulation, metabolic activities, signal transduction, proteolytic activity, and several others that are now heavily being investigated for their role in diseases and disorders (reviewed in Schatten and Sun, Histochem Cell Biol 150:303-325, 2018; Schatten, Adv Anat Embryol Cell Biol 235:43-50, 2022a; Schatten, Adv Anat Embryol Cell Biol 235:17-35, 2022b).Cancer cell centrosomes differ from centrosomes in noncancer cells in displaying specific abnormalities that include phosphorylation abnormalities, overexpression of specific centrosomal proteins, abnormalities in centriole and centrosome duplication, formation of multipolar spindles that play a role in aneuploidy and genomic instability, and several others that are highlighted in the present review on ovarian cancer. Ovarian cancer cell centrosomes, like those in other cancers, display complex abnormalities that in part are based on the heterogeneity of cells in the cancer tissues resulting from different etiologies of individual cancer cells that will be discussed in more detail in this chapter.Because of the critical role of centrosomes in cancer cell proliferation, several lines of research are being pursued to target centrosomes for therapeutic intervention to inhibit abnormal cancer cell proliferation and control tumor progression. Specific centrosome abnormalities observed in ovarian cancer will be addressed in this chapter with a focus on targeting such aberrations for ovarian cancer-specific therapies.

Explaining Redundancy in CDK-Mediated Control of the Cell Cycle: Unifying the Continuum and Quantitative Models

In eukaryotes, cyclin-dependent kinases (CDKs) are required for the onset of DNA replication and mitosis, and distinct CDK–cyclin complexes are activated sequentially throughout the cell cycle. It is widely thought that specific complexes are required to traverse a point of commitment to the cell cycle in G1, and to promote S-phase and mitosis, respectively. Thus, according to a popular model that has dominated the field for decades, the inherent specificity of distinct CDK–cyclin complexes for different substrates at each phase of the cell cycle generates the correct order and timing of events. However, the results from the knockouts of genes encoding cyclins and CDKs do not support this model. An alternative “quantitative” model, validated by much recent work, suggests that it is the overall level of CDK activity (with the opposing input of phosphatases) that determines the timing and order of S-phase and mitosis. We take this model further by suggesting that the subdivision of the cell cycle into discrete phases (G0, G1, S, G2, and M) is outdated and problematic. Instead, we revive the “continuum” model of the cell cycle and propose that a combination with the quantitative model better defines a conceptual framework for understanding cell cycle control.

Design and Synthesis of a Novel PLK1 Inhibitor Scaffold Using a Hybridized 3D-QSAR Model

Polo-like kinase 1 (PLK1) plays an important role in cell cycle progression and proliferation in cancer cells. PLK1 also contributes to anticancer drug resistance and is a valuable target in anticancer therapeutics. To identify additional effective PLK1 inhibitors, we performed QSAR studies of two series of known PLK1 inhibitors and proposed a new structure based on a hybridized 3D-QSAR model. Given the hybridized 3D-QSAR models, we designed and synthesized 4-benzyloxy-1-(2-arylaminopyridin-4-yl)-1H-pyrazole-3-carboxamides, and we inspected its inhibitory activities to identify novel PLK1 inhibitors with decent potency and selectivity.

Polo-like Kinase 1 as an emerging drug target: structure, function and therapeutic implications

Polo-like kinase 1 (PLK1) is a conserved mitotic serine-threonine protein kinase, functions as a regulatory protein, and is involved in the progression of the mitotic cycle. It plays important roles in the regulation of cell division, maintenance of genome stability, in spindle assembly, mitosis, and DNA-damage response. PLK1 is consist of a N-terminal serine-threonine kinase domain, and a C-terminal Polo-box domain (regulatory site). The expression of PLK1 is controlled by transcription repressor in the G1 stage and transcription activators in the G2 stage of the cell cycle. Overexpression of PLK1 results in undermining of checkpoints causes excessive cellular division resulting in abnormal cell growth, leading to the development of cancer. Blocking the expression of PLK1 by an antibody, RNA interference, or kinase inhibitors, causes a subsequent reduction in the proliferation of tumour cells and induction of apoptosis in tumour cells without affecting the healthy cells, suggesting an attractive target for drug development. In this review, we discuss detailed information on expression, gene and protein structures, role in different diseases, and progress in the design and development of PLK1 inhibitors. We have performed an in-depth analysis of the PLK1 inhibitors and their therapeutic implications with special focus to the cancer therapeutics.

The dual role of BI 2536, a small-molecule inhibitor that targets PLK1, in induction of apoptosis and attenuation of autophagy in neuroblastoma cells

Neuroblastoma (NB) is the most common extra-cranial solid tumor in childhood with the overall 5 years' survival less than 40%. Polo-like kinase 1 (PLK1) is a serine/threonine-protein kinase expressed during mitosis and over expressed in multiple cancers, including neuroblastoma. We found that higher PLK1 expression related to poor outcome of NB patients. BI2536, a small molecule inhibitor against PLK1, significantly reduced cell viability in a panel of NB cell lines, with IC50 less than 100 nM. PLK1 inhibition by BI 2536 treatment induced cell cycle arrest at G₂/M phase and cell apoptosis in NB cells. Realtime PCR array revealed the PLK1 inhibition related genes, such as BIRC7, TNFSF10, LGALS1 and DAD1 et al. Moreover, autophagy activity was investigated in the NB cells treated with BI 2536. BI 2536 treatment in NB cells increased LC3-II puncta formation and LC3-II expression. Formation of autophagosome induced by BI 2536 was observed by transmission electron microscopy. However, BI2536 abrogated the autophagic flux in NB cells by reducing SQSTM1/p62 expression and AMPKα^T172 phosphorylation. These results provide new clues for the molecular mechanism of cell death induced by BI 2536 and suggest that BI 2536 may act as new candidate drug for neuroblastoma.

Microtubules are necessary for proper Reticulon localization during mitosis

During mitosis, the structure of the Endoplasmic Reticulum (ER) displays a dramatic reorganization and remodeling, however, the mechanism driving these changes is poorly understood. Hairpin-containing ER transmembrane proteins that stabilize ER tubules have been identified as possible factors to promote these drastic changes in ER morphology. Recently, the Reticulon and REEP family of ER shaping proteins have been shown to heavily influence ER morphology by driving the formation of ER tubules, which are known for their close proximity with microtubules. Here, we examine the role of microtubules and other cytoskeletal factors in the dynamics of a Drosophila Reticulon, Reticulon-like 1 (Rtnl1), localization to spindle poles during mitosis in the early embryo. At prometaphase, Rtnl1 is enriched to spindle poles just prior to the ER retention motif KDEL, suggesting a possible recruitment role for Rtnl1 in the bulk localization of ER to spindle poles. Using image analysis-based methods and precise temporal injections of cytoskeletal inhibitors in the early syncytial Drosophila embryo, we show that microtubules are necessary for proper Rtnl1 localization to spindles during mitosis. Lastly, we show that astral microtubules, not microfilaments, are necessary for proper Rtnl1 localization to spindle poles, and is largely independent of the minus-end directed motor protein dynein. This work highlights the role of the microtubule cytoskeleton in Rtnl1 localization to spindles during mitosis and sheds light on a pathway towards inheritance of this major organelle.

Regulating a key mitotic regulator, polo-like kinase 1 (PLK1)

During cell division, duplicated genetic material is separated into two distinct daughter cells. This process is essential for initial tissue formation during development and to maintain tissue integrity throughout an organism's lifetime. To ensure the efficacy and efficiency of this process, the cell employs a variety of regulatory and signaling proteins that function as mitotic regulators and checkpoint proteins. One vital mitotic regulator is polo-like kinase 1 (PLK1), a highly conserved member of the polo-like kinase family. Unique from its paralogues, it functions specifically during mitosis as a regulator of cell division. PLK1 is spatially and temporally enriched at three distinct subcellular locales; the mitotic centrosomes, kinetochores, and the cytokinetic midbody. These localization patterns allow PLK1 to phosphorylate specific downstream targets to regulate mitosis. In this review, we will explore how polo-like kinases were originally discovered and diverged into the five paralogues (PLK1-5) in mammals. We will then focus specifically on the most conserved, PLK1, where we will discuss what is known about how its activity is modulated, its role during the cell cycle, and new, innovative tools that have been developed to examine its function and interactions in cells.

The Centrioles, Centrosomes, Basal Bodies, and Cilia of Drosophila melanogaster

Centrioles play a key role in the development of the fly. They are needed for the correct formation of centrosomes, the organelles at the poles of the spindle that can persist as microtubule organizing centers (MTOCs) into interphase. The ability to nucleate cytoplasmic microtubules (MTs) is a property of the surrounding pericentriolar material (PCM). The centriole has a dual life, existing not only as the core of the centrosome but also as the basal body, the structure that templates the formation of cilia and flagellae. Thus the structure and functions of the centriole, the centrosome, and the basal body have an impact upon many aspects of development and physiology that can readily be modeled in Drosophila Centrosomes are essential to give organization to the rapidly increasing numbers of nuclei in the syncytial embryo and for the spatially precise execution of cell division in numerous tissues, particularly during male meiosis. Although mitotic cell cycles can take place in the absence of centrosomes, this is an error-prone process that opens up the fly to developmental defects and the potential of tumor formation. Here, we review the structure and functions of the centriole, the centrosome, and the basal body in different tissues and cultured cells of Drosophila melanogaster, highlighting their contributions to different aspects of development and cell division.

Plk1 inhibition leads to a failure of mitotic division during the first mitotic division in pig embryos

PURPOSE

This study was conducted to examine the dynamic distribution of polo-like 1 kinase (Plk1) and the possible role it plays in first mitotic division during early porcine embryo development.

METHODS

Indirect immunofluorescence and confocal microscopy imaging techniques combined with western blot analyses were used to study the dynamic expression and subcellular localization of Plk1 protein in pig parthenogenetic embryos. Finally, a selective Plk1 inhibitor, GSK461364, was used to evaluate the potential role of Plk1 during this special stage.

RESULTS

The results showed that Plk1 upon expression exhibited specific dynamic intracellular localization, which closely correlated with the α-tubulin distribution during the first mitotic division. GSK461364 treatment resulted in cleavage failure, with majority of the GSK461364-treated embryos being arrested in prometaphase. Further results of the subcellular structure examination showed that GSK461364 treatment led to a significantly higher proportion of the treated embryos having abnormal spindles and misarranged chromosomes at the prometaphase stage.

CONCLUSIONS

Thus, these results indicated that Plk1 is essential for porcine embryos to complete the first mitotic division. Furthermore, Plk1 regulation was associated with effects on spindle assembly and chromosome arrangement.

Efficacy and mechanism of action of volasertib, a potent and selective inhibitor of Polo-like kinases, in preclinical models of acute myeloid leukemia

Polo-like kinase 1 (Plk1), a member of the Polo-like kinase family of serine/threonine kinases, is a key regulator of multiple steps in mitosis. Here we report on the pharmacological profile of volasertib, a potent and selective Plk inhibitor, in multiple preclinical models of acute myeloid leukemia (AML) including established cell lines, bone marrow samples from AML patients in short-term culture, and subcutaneous as well as disseminated in vivo models in immune-deficient mice. Our results indicate that volasertib is highly efficacious as a single agent and in combination with established and emerging AML drugs, including the antimetabolite cytarabine, hypomethylating agents (decitabine, azacitidine), and quizartinib, a signal transduction inhibitor targeting FLT3. Collectively, these preclinical data support the use of volasertib as a new therapeutic approach for the treatment of AML patients, and provide a foundation for combination approaches that may further improve and prolong clinical responses.

Interdomain allosteric regulation of Polo kinase by Aurora B and Map205 is required for cytokinesis

Aurora B phosphorylation of the Polo kinase activation loop disrupts its binding to Map205 and central spindle microtubules, allowing it to be recruited to the site of cytokinesis.

Drosophila melanogaster Polo and its human orthologue Polo-like kinase 1 fulfill essential roles during cell division. Members of the Polo-like kinase (Plk) family contain an N-terminal kinase domain (KD) and a C-terminal Polo-Box domain (PBD), which mediates protein interactions. How Plks are regulated in cytokinesis is poorly understood. Here we show that phosphorylation of Polo by Aurora B is required for cytokinesis. This phosphorylation in the activation loop of the KD promotes the dissociation of Polo from the PBD-bound microtubule-associated protein Map205, which acts as an allosteric inhibitor of Polo kinase activity. This mechanism allows the release of active Polo from microtubules of the central spindle and its recruitment to the site of cytokinesis. Failure in Polo phosphorylation results in both early and late cytokinesis defects. Importantly, the antagonistic regulation of Polo by Aurora B and Map205 in cytokinesis reveals that interdomain allosteric mechanisms can play important roles in controlling the cellular functions of Plks.

Targeting cell cycle kinases and kinesins in anticancer drug development

The cell cycle is regulated by kinases such as the cyclin-dependent kinases (CDKs) and non-CDKs, which include Aurora and polo-like kinases, as well as checkpoint proteins. Mitotic kinesins are involved in the establishment of the mitotic spindle formation and function, and also play a role in cell cycle control. The disruption of the cell cycle is a hallmark of malignancy. Genetic or epigenetic events result in the upregulation of these kinases and mitotic kinesins in a myriad of tumour types, suggesting that their inhibition could result in preferential targeting of malignant cells. Such findings make the development of these inhibitors a rational and attractive new area for cancer therapeutics. Although challenges of potency and non-specificity have hampered their progress through the clinic, several novel compounds are presently in various phases of clinical trial evaluation.

Proliferation state and polo-like kinase1 dependence of tumorigenic colon cancer cells

Tumor-initiating cells are responsible for tumor maintenance and relapse in solid and hematologic cancers. Although tumor-initiating cells were initially believed to be mainly quiescent, rapidly proliferating tumorigenic cells were found in breast cancer. In colon cancer, the proliferative activity of the tumorigenic population has not been defined, although it represents an essential parameter for the development of more effective therapeutic strategies. Here, we show that tumorigenic colon cancer cells can be found in a rapidly proliferating state in vitro and in vivo, both in human tumors and mouse xenografts. Inhibitors of polo-like kinase1 (Plk1), a mitotic kinase essential for cell proliferation, demonstrated maximal efficiency over other targeted compounds and chemotherapeutic agents in inducing death of colon cancer-initiating cells in vitro. In vivo, Plk1 inhibitors killed CD133(+) colon cancer cells leading to complete growth arrest of colon cancer stem cell-derived xenografts, whereas chemotherapeutic agents only slowed tumor progression. While chemotherapy treatment increased CD133(+) cell proliferation, treatment with Plk1 inhibitors eliminated all proliferating tumor-initiating cells. Quiescent CD133(+) cells that survived the treatment with Plk1 inhibitors could be killed by subsequent Plk1 inhibition when they exited from quiescence. Altogether, these results provide a new insight into the proliferative status of colon tumor-initiating cells both in basal conditions and in response to therapy and indicate Plk1 inhibitors as potentially useful in the treatment of colorectal cancer.

Roles of Polo-like kinase 3 in suppressing tumor angiogenesis

Angiogenesis is essential for promoting growth and metastasis of solid tumors by ensuring blood supply to the tumor mass. Targeting angiogenesis is therefore an attractive approach to therapeutic intervention of cancer. Tumor angiogenesis is a process that is controlled by a complex network of molecular components including sensors, signaling transducers, and effectors, leading to cellular responses under hypoxic conditions. Positioned at the center of this network are the hypoxia-inducible factors (HIFs). HIF-1 is a major transcription factor that consists of two subunits, HIF-1α and HIF-1β. It mediates transcription of a spectrum of gene targets whose products are essential for mounting hypoxic responses. HIF-1α protein level is very low in the normoxic condition but is rapidly elevated under hypoxia. This dramatic change in the cellular HIF-1α level is primarily regulated through the proteosome-mediated degradation process. In the past few years, scientific progress has clearly demonstrated that HIF-1α phosphorylation is mediated by several families of protein kinases including GSK3β and ERKs both of which play crucial roles in the regulation of HIF-1α stability. Recent research progress has identified that Polo-like kinase 3 (Plk3) phosphorylates HIF-1α at two previously unidentified serine residues and that the Plk3-mediated phosphorylation of these residues results in destabilization of HIF-1α. Plk3 has also recently been found to phosphorylate and stabilize PTEN phosphatase, a known regulator of HIF-1α and tumor angiogenesis. Given the success of targeting protein kinases and tumor angiogenesis in anti-cancer therapies, Plk3 could be a potential molecular target for the development of novel and effective therapeutic agents for cancer treatment.

Expression, adverse prognostic significance and therapeutic small molecule inhibition of Polo-like kinase 1 in multiple myeloma

The amplified myeloma centrosome has been identified as a therapeutic target. The present study explored the expression and prognostic significance of the centrosome-associated protein PLK1 in myeloma and the effect of BI 2536, a potent and selective inhibitor of PLK1, on myeloma cells. High plasma cell expression of PLK1 protein in myeloma patient bone marrow biopsies is an independent adverse prognostic factor (HR=2.3, p=0.003 unadjusted; HR=1.9, p=0.03 in multivariable model). BI 2536 inhibits myeloma cell lines at nanomolar concentrations, and is therapeutic for xenografts in NOD/SCID mice. PLK1 inhibition is a potential new strategy for the treatment of multiple myeloma.

Efficient inhibition of human colorectal carcinoma growth by RNA interference targeting polo-like kinase 1 in vitro and in vivo

Polo-like kinase 1 (PLK1) showing a high expression in various kinds of tumors is considered a candidate target for cancer therapy. The aim of our study was to explore the effects of silencing PLK1 gene on human colorectal carcinoma cell line HCT-116 in vitro and in vivo. In vitro, the plasmids generating short hairpin RNA (shRNA)-targeting PLK1 were transfected into HCT-116 by using FugeneHD reagent, and the silencing potency was measured by RT-PCR, western blot, flow cytometry, and Caspase-Glo 3/7 assay, respectively. In vivo, the growth inhibition capacity of PLK1-shRNA on HCT-116 xenograft was measured in nude mice. Then, the silencing effect of PLK1 was analyzed by RT-PCR, western blot, and immunohistochemistry, respectively. Apoptosis, angiogenesis, and proliferation in tumor tissues were measured by TUNEL, CD31, and PCNA stain, respectively. The RNA interference targeting PLK1 significantly decreased the expression of PLK1 in vitro. More importantly, anti-PLK1 treatment in HCT-116 xenograft decreased tumor weight by 81.58% compared with the control group (p<0.001), accompanied with decreased PLK1 mRNA and protein expression, increased cell apoptosis, and reduced angiogenesis and proliferation (p<0.001). Our study showed that knockdown of PLK1 by shRNA might be the potential therapeutic approach against human colorectal carcinoma.

p53-dependent repression of polo-like kinase-1 (PLK1)

PLK1 is a critical mediator of G₂/M cell cycle transition that is inactivated and depleted as part of the DNA damage-induced G₂/M checkpoint. Here we show that downregulation of PLK1 expression occurs through a transcriptional repression mechanism and that p53 is both necessary and sufficient to mediate this effect. Repression of PLK1 by p53 occurs independently of p21 and of arrest at G₁/S where PLK1 levels are normally repressed in a cell cycle-dependent manner through a CDE/CHR element. Chromatin immunoprecipitation analysis indicates that p53 is present on the PLK1 promoter at two distinct sites termed p53RE1 and p53RE2. Recruitment of p53 to p53RE2, but not to p53RE1, is stimulated in response to DNA damage and/or p53 activation and is coincident with repression-associated changes in the chromatin. Downregulation of PLK1 expression by p53 is relieved by the histone deacetylase inhibitor, trichostatin A, and involves recruitment of histone deacetylase to the vicinity of p53RE2, further supporting a transcriptional repression mechanism. Additionally, wild type, but not mutant, p53 represses expression of the PLK1 promoter when fused upstream of a reporter gene. Silencing of PLK1 expression by RNAi interferes with cell cycle progression consistent with a role in the p53-mediated checkpoint. These data establish PLK1 as a direct transcriptional target of p53, independently of p21, that is required for efficient G₂/M arrest.

Genetic approach to evaluate specificity of small molecule drug candidates inhibiting PLK1 using zebrafish

During the preclinical drug discovery process it remains a challenge to enable early elimination of candidate molecules that may have non-specific, off-target activities. Here, we use whole zebrafish embryo assays coupled with genetic analysis to address this issue. PLK1 (Polo-like kinase 1) is one of the key regulators that control mitotic entry, spindle assembly, chromosome segregation, and cytokinesis in the cell cycle. Since plk1 expression is abnormally up-regulated in several tumors, it is regarded as a good target for cancer therapy. A number of small-molecule inhibitors targeting PLK1 have been developed as reagents and anticancer drug candidates. It will be interesting to determine if these inhibitors indeed specifically target PLK1 in vivo. Bioinformatics analysis revealed that the zebrafish and human genomes share high homology across all PLK family members. In particular, PLK1 has a nearly identical 3-D structure between zebrafish and human. We selected three published PLK1 inhibitors, LFM-A13, ON01910, and thiazole-carboxamide 10A in our assay. When added at 2-cell stage, all of these inhibitors prevented embryos from dividing and caused cells to fuse into one large cell. When added at the later stage during zygotic mRNA transcription program initiation, embryos survived for 3 days but showed different phenotypes for each compound. Embryos treated with LFM-A13 appeared relatively normal. Embryos treated with ON01910 failed to properly develop trunk and tail regions while the head structure was unaffected. Embryos treated with thiazole-carboxamide 10A had a shorter body axis and deformed head structure. To determine which inhibitor is more selectively targeting PLK1, we inhibited PLK1 activity using anti-sense morpholino. Comparative analysis indicated that thiazole-carboxamide 10A could faithfully phenocopy zebrafish embryos genetically deficient of plk1. These findings demonstrate that these three PLK1 inhibitors, although well established by in vitro studies, have different off-target activities in vivo, and that thiazole-carboxamide 10A appears most specific to PLK1. Our studies suggest that zebrafish should be generally useful as an efficient in vivo model to evaluate specificity of small molecules designed to regulate any conserved target proteins through comparative analysis of genetic phenotypes.

Polo kinase interacts with RacGAP50C and is required to localize the cytokinesis initiation complex

The assembly and constriction of an actomyosin contractile ring in cytokinesis is dependent on the activation of Rho at the equatorial cortex by a complex, here termed the cytokinesis initiation complex, between a microtubule-associated kinesin-like protein (KLP), a member of the RacGAP family, and the RhoGEF Pebble. Recently, the activity of the mammalian Polo kinase ortholog Plk1 has been implicated in the formation of this complex. We show here that Polo kinase interacts directly with the cytokinesis initiation complex by binding RacGAP50C. We find that a new domain of Polo kinase, termed the intermediate domain, interacts directly with RacGAP50C and that Polo kinase is essential for localization of the KLP-RacGAP centralspindlin complex to the cell equator and spindle midzone. In the absence of Polo kinase, RacGAP50C and Pav-KLP fail to localize normally, instead decorating microtubules along their length. Our results indicate that Polo kinase directly binds the conserved cytokinesis initiation complex and is required to trigger centralspindlin localization as a first step in cytokinesis.

The balance of Polo-like kinase 1 in tumorigenesis

Polo-like kinase 1 (Plk1) belongs to a family of conserved serine/threonine kinases with a polo-box domain, which have similar but non-overlapping functions in the cell cycle progression. Plk1 plays a key role to ensure the normal mitosis. Interestingly, overexpression of Plk1 is associated with tumor development and could serve as a prognostic marker for many cancers. Due to Plk1 overexpression, several Plk1 inhibitors have been developed and tested for the cancer treatment. However, in a recent study, it has been suggested that down-regulation of Plk1 could also induce aneuploidy and tumor formation in vivo. Therefore, a normal level of Plk1 is important for mitosis. And caution should be taken when Plk1 inhibitors are used in the clinical trial and their side effects including tumorigenesis should be carefully evaluated.

Sequestration of Polo kinase to microtubules by phosphopriming-independent binding to Map205 is relieved by phosphorylation at a CDK site in mitosis

The conserved Polo kinase controls multiple events in mitosis and cytokinesis. Although Polo-like kinases are regulated by phosphorylation and proteolysis, control of subcellular localization plays a major role in coordinating their mitotic functions. This is achieved largely by the Polo-Box Domain, which binds prephosphorylated targets. However, it remains unclear whether and how Polo might interact with partner proteins when priming mitotic kinases are inactive. Here we show that Polo associates with microtubules in interphase and cytokinesis, through a strong interaction with the microtubule-associated protein Map205. Surprisingly, this interaction does not require priming phosphorylation of Map205, and the Polo-Box Domain of Polo is required but not sufficient for this interaction. Moreover, phosphorylation of Map205 at a CDK site relieves this interaction. Map205 can stabilize Polo and inhibit its cellular activity in vivo. In syncytial embryos, the centrosome defects observed in polo hypomorphs are enhanced by overexpression of Map205 and suppressed by its deletion. We propose that Map205-dependent targeting of Polo to microtubules provides a stable reservoir of Polo that can be rapidly mobilized by the activity of Cdk1 at mitotic entry.

Polo-like kinase 1 is essential for early embryonic development and tumor suppression

Polo-like kinases (Plks) are serine/threonine kinases that are highly conserved in organisms from yeasts to humans. Previous reports have shown that Plk1 is critical for all stages of mitosis and may play a role in DNA replication during S phase. While much work has focused on Plk1, little is known about the physiological function of Plk1 in vivo. To address this question, we generated Plk1 knockout mice. Plk1 homozygous null mice were embryonic lethal, and early Plk1(-/-) embryos failed to survive after the eight-cell stage. Immunocytochemistry studies revealed that Plk1-null embryos were arrested outside the mitotic phase, suggesting that Plk1 is important for proper cell cycle progression. It has been postulated that Plk1 is a potential oncogene, due to its overexpression in a variety of tumors and tumor cell lines. While the Plk1 heterozygotes were healthy at birth, the incidence of tumors in these animals was threefold greater than that in their wild-type counterparts, demonstrating that the loss of one Plk1 allele accelerates tumor formation. Collectively, our data support that Plk1 is important for early embryonic development and may function as a haploinsufficient tumor suppressor.

Polo-like kinase 3 functions as a tumor suppressor and is a negative regulator of hypoxia-inducible factor-1 alpha under hypoxic conditions

Polo-like kinase 3 (Plk3) is an important mediator of the cellular responses to genotoxic stresses. In this study, we examined the physiologic function of Plk3 by generating Plk3-deficient mice. Plk3(-/-) mice displayed an increase in weight and developed tumors in various organs at advanced age. Many tumors in Plk3(-/-) mice were large in size, exhibiting enhanced angiogenesis. Plk3(-/-) mouse embryonic fibroblasts were hypersensitive to the induction of hypoxia-inducible factor-1 alpha (HIF-1 alpha) under hypoxic conditions or by nickel and cobalt ion treatments. Ectopic expression of the Plk3-kinase domain (Plk3-KD), but not its Polo-box domain or a Plk3-KD mutant, suppressed the nuclear accumulation of HIF-1 alpha induced by nickel or cobalt ions. Moreover, hypoxia-induced HIF-1 alpha expression was tightly associated with a significant down-regulation of Plk3 expression in HeLa cells. Given the importance of HIF-1 alpha in mediating the activation of the "survival machinery" in cancer cells, these studies strongly suggest that enhanced tumorigenesis in Plk3-null mice is at least partially mediated by a deregulated HIF-1 pathway.

The mammalian centrosome and its functional significance

Primarily known for its role as major microtubule organizing center, the centrosome is increasingly being recognized for its functional significance in key cell cycle regulating events. We are now at the beginning of understanding the centrosome’s functional complexities and its major impact on directing complex interactions and signal transduction cascades important for cell cycle regulation. The centrosome orchestrates entry into mitosis, anaphase onset, cytokinesis, G1/S transition, and monitors DNA damage. Recently, the centrosome has also been recognized as major docking station where regulatory complexes accumulate including kinases and phosphatases as well as numerous other cell cycle regulators that utilize the centrosome as platform to coordinate multiple cell cycle-specific functions. Vesicles that are translocated along microtubules to and away from centrosomes may also carry enzymes or substrates that use centrosomes as main docking station. The centrosome’s role in various diseases has been recognized and a wealth of data has been accumulated linking dysfunctional centrosomes to cancer, Alstrom syndrome, various neurological disorders, and others. Centrosome abnormalities and dysfunctions have been associated with several types of infertility. The present review highlights the centrosome’s significant roles in cell cycle events in somatic and reproductive cells and discusses centrosome abnormalities and implications in disease.

Molecular and structural basis of polo-like kinase 1 substrate recognition: Implications in centrosomal localization

Polo-like kinase (Plk1) is crucial for cell cycle progression through mitosis. Here we present the molecular and structural mechanisms that regulate the substrate recognition of Plk1 and influence its centrosomal localization and activity. Our work shows that Plk1 localization is controlled not only by the polo box domain (PBD); remarkably, the kinase domain is also involved in Plk1 targeting mechanism to the centrosome. The crystal structures of the PBD in complex with Cdc25C and Cdc25C-P target peptides reveal that Trp-414 is fundamental in their recognition regardless of its phosphorylation status. Binding measurements demonstrate that W414F mutation abolishes molecular recognition and diminishes centrosomal localization. Therefore, Plk1 centrosomal localization is not controlled by His-538 and Lys-540, the residues involved in phosphorylated target binding. The different conformations of the loop, which connects the polo boxes in the apo and the PBD-Cdc25C and PBD-Cdc25C-P complex structures, together with changes in the proline adjacent to the phosphothreonine in the target peptide, suggest a regulatory mechanism to detect binding of unphosphorylated or phosphorylated target substrates. Altogether, these data propose a model for the interaction between Plk1 and Cdc25C.

The genomic repertoire for cell cycle control and DNA metabolism in S. purpuratus

A search of the Strongylocentrotus purpuratus genome for genes associated with cell cycle control and DNA metabolism shows that the known repertoire of these genes is conserved in the sea urchin, although with fewer family members represented than in vertebrates, and with some cases of echinoderm-specific gene diversifications. For example, while homologues of the known cyclins are mostly encoded by single genes in S. purpuratus (unlike vertebrates, which have multiple isoforms), there are additional genes encoding novel cyclins of the B and K/L types. Almost all known cyclin-dependent kinases (CDKs) or CDK-like proteins have an orthologue in S. purpuratus; CDK3 is one exception, whereas CDK4 and 6 are represented by a single homologue, referred to as CDK4. While the complexity of the two families of mitotic kinases, Polo and Aurora, is close to that found in the nematode, the diversity of the NIMA-related kinases (NEK proteins) approaches that of vertebrates. Among the nine NEK proteins found in S. purpuratus, eight could be assigned orthologues in vertebrates, whereas the ninth is unique to sea urchins. Most known DNA replication, DNA repair and mitotic checkpoint genes are also present, as are homologues of the pRB (two) and p53 (one) tumor suppressors. Interestingly, the p21/p27 family of CDK inhibitors is represented by one homologue, whereas the INK4 and ARF families of tumor suppressors appear to be absent, suggesting that these evolved only in vertebrates. Our results suggest that, while the cell cycle control mechanisms known from other animals are generally conserved in sea urchin, parts of the machinery have diversified within the echinoderm lineage. The set of genes uncovered in this analysis of the S. purpuratus genome should enhance future research on cell cycle control and developmental regulation in this model.

Targeting polo-like kinase 1 for cancer therapy

Human polo-like kinase 1 (PLK1) is essential during mitosis and in the maintenance of genomic stability. PLK1 is overexpressed in human tumours and has prognostic potential in cancer, indicating its involvement in carcinogenesis and its potential as a therapeutic target. The use of different PLK1 inhibitors has increased our knowledge of mitotic regulation and allowed us to assess their ability to suppress tumour growth in vivo. We address the structural features of the kinase domain and the unique polo-box domain of PLK1 that are most suited for drug development and discuss our current understanding of the therapeutic potential of PLK1.

Polo-like kinase (Plk) 1 as a target for prostate cancer management

Prostate cancer (PCa) is the most commonly occurring cancer in American men, next to skin cancer. Existing treatment options and surgical intervention are unable to effectively manage this cancer. Therefore, continuing efforts are ongoing to establish novel mechanism-based targets and strategies for its management. The serine/threonine kinases Polo-like kinase (Plk) 1 plays a key role in mitotic entry of proliferating cells and regulates many aspects of mitosis which are necessary for successful cytokinesis. Plk1 is over-expressed in many tumor types with aberrant elevation frequently constituting a prognostic indicator of poor disease outcome. This review discusses the studies which indicate that Plk1 could be an excellent target for the treatment as well as chemoprevention of prostate cancer.

ON01910, a non-ATP-competitive small molecule inhibitor of Plk1, is a potent anticancer agent

Elevated expression of polo-like kinase1 (Plk1) has been reported in many human tumors, and inhibition of Plk1 activity results in their mitotic arrest and apoptosis. Here we describe the profile of ON01910, a small molecule inhibitor of Plk1 activity, which induces mitotic arrest of tumor cells characterized by spindle abnormalities leading to their apoptosis. This compound was not ATP-competitive, but competed for the substrate binding site of the enzyme. In vivo, this compound did not exhibit hematotoxicity, liver damage, or neurotoxicity, and was a potent inhibitor of tumor growth in a variety of xenograft nude mouse models. ON01910 showed strong synergy with several chemotherapeutic agents, often inducing complete regression of tumors.

Polo kinase and progression through M phase in Drosophila: a perspective from the spindle poles

Genes for the mitotic kinases Polo and Aurora A were first identified in Drosophila through screens of maternal effect lethal mutations for defects in spindle pole behaviour. These enzymes have been shown to be highly conserved and required for multiple functions in mitosis. Polo is stabilized at the centrosome by association with Hsp90. It is required for centrosome maturation on M-phase entry in order to recruit the gamma-tubulin ring complex and activate the abnormal spindle protein, Asp. These events facilitate the nucleation of minus ends of microtubules at the centrosome. The localization of Polo at the kinetochore and the mid-zone of the central spindle together with the phenotypes of polo mutants point to functions at the metaphase to anaphase transition and in cytokinesis. The latter are mediated, at least in part, through the Pavarotti kinesin-like motor protein and its conserved counterparts in other metazoans.

Polo-like kinases (Plks) and cancer

Deregulated centrosome duplication or maturation often results in increased centrosome size and/or centrosome number, both of which show a positive and significant correlation with aneuploidy and chromosomal instability, thus contributing to cancer formation. Given the role of Polo-like kinases (Plks) in the centrosome cycle, it is not unexpected that deregulated expression of Plks is detected in many types of cancer and is associated with oncogenesis. Extensive studies have shown that Plk1 expression is elevated in non-small-cell lung cancer, head and neck cancer, esophageal cancer, gastric cancer, melanomas, breast cancer, ovarian cancer, endometrial cancer, colorectal cancer, gliomas, and thyroid cancer. Plk1 gene and protein expression has been proposed as a new prognostic marker for many types of malignancies, and Plk1 is a potential target for cancer therapy. In contrast to Plk1, several studies have observed that Plk3 expression is negatively correlated with the development of certain cancers.

The maternal-effect gene futile cycle is essential for pronuclear congression and mitotic spindle assembly in the zebrafish zygote

Embryos have been successfully used for the general study of the cell cycle. Although there are significant differences between the early embryonic and the somatic cell cycle in vertebrates, the existence of specialised factors that play a role during the early cell cycles has remained elusive. We analysed a lethal recessive maternal-effect mutant, futile cycle (fue), isolated in a maternal-effect screen for nuclear division defects in the zebrafish (Danio rerio). The pronuclei fail to congress in zygotes derived from homozygous fue mothers. In addition, a defect in the formation of chromosomal microtubules prevents mitotic spindle assembly and thus chromosome segregation in fue zygotes. However, centrosomal functions do not appear to be affected in fue embryos, suggesting this mutant blocks a subset of microtubule functions. Cleavage occurs normally for several divisions resulting in many anucleate cells, thus showing that nuclear- and cell division can be uncoupled genetically. Therefore, we propose that in mitotic spindle assembly chromosome-dependent microtubule nucleation is essential for the coupling of nuclear and cell division.

Cell cycle arrest and apoptosis induced by human Polo-like kinase 3 is mediated through perturbation of microtubule integrity

Human Polo-like kinase 3 (Plk3, previously termed Prk or Fnk) is involved in regulation of cell cycle progression through the M phase (B. Ouyang, H. Pan, L. Lu, J. Li, P. Stambrook, B. Li, and W. Dai, J. Biol. Chem. 272:28646-28651, 1997). Here we report that in most interphase cells endogenous Plk3 was predominantly localized around the nuclear membrane. Double labeling with Plk3 and gamma-tubulin, the latter a major component of pericentriole materials, revealed that Plk3 was closely associated with centrosomes and that its localization to centrosomes was dependent on the integrity of microtubules. Throughout mitosis, Plk3 appeared to be localized to mitotic apparatus such as spindle poles and mitotic spindles. During telophase, a significant amount of Plk3 was also detected in the midbody. Ectopic expression of Plk3 mutants dramatically changed cell morphology primarily due to their effects on microtubule dynamics. Expression of a constitutively active Plk3 (Plk3-A) resulted in rapid cell shrinkage, which led to formation of cells with an elongated, unsevered, and taxol-sensitive midbody. In contrast, cells expressing a kinase-defective Plk3 (Plk3(K52R)) mutant exhibited extended, deformed cytoplasmic structures, the phenotype of which was somewhat refractory to taxol treatment. Expression of both Plk3-A and Plk3(K52R) induced apparent G(2)/M arrest followed by apoptosis, although the kinase-defective mutant was less effective. Taken together, our studies strongly suggest that Plk3 plays an important role in the regulation of microtubule dynamics and centrosomal function in the cell and that deregulated expression of Plk3 results in cell cycle arrest and apoptosis.

Identification of the human homologue of the early-growth response gene Snk, encoding a serum-inducible kinase

Murine serum inducible kinase (mSnk) was recently cloned and characterized as an early-growth response gene involved in cell proliferation. Here we report the isolation and characterization of its human homologue, named hSnk. Sequence comparison shows that hSnk is highly conserved and its deduced protein sequence shares a significant amino acid identity with mSnk and rSnk proteins, as well as with other polo family kinase gene products. A survey of hSnk expression reveals that while a wide variety of human tissues express a low to moderate level of hSnk transcripts, fetal tissues, testis, and spleen express the most abundant hSnk transcripts. In addition, serum stimulation rapidly induces hSnk expression in fibroblast cells, reaching the peak level of induction within one hour post treatment. Considering that Plk and Prk, two other known human polo-family kinases, control cell cycle checkpoint and cell cycle progression, our current observations suggest that hSnk may also play an important role in cells undergoing rapid cell division or having a high mitotic index.

Intron/exon organization and polymorphisms of the PLK3/PRK gene in human lung carcinoma cell lines

PLK3/PRK, a conserved polo family protein serine/threonine kinase, plays a significant role at the onset of mitosis and mitotic progression. Recently, PLK3/PRK has been shown to induce apoptosis when overexpressed in cell lines and is also implicated in cell proliferation and tumor development. Forty lung tumor cell lines were used for single-strand confirmation polymorphism (SSCP) analysis and DNA sequencing to examine the mutational status of PLK3/PRK. No missense or nonsense mutations were revealed in the lung carcinoma cell lines examined. However, three polymorphisms were identified as: a G to A at position 720, an A to G at 1053, and a G to C at 1275. Intron/exon boundaries were determined by amplification of genomic DNA with PLK3/PRK exon-specific primers. The amplification products with increased size relative to the cDNA were sequenced. Fifteen exons throughout the open reading frame were characterized. None of the introns were exceptionally large, typically ranging from 100-300 basepairs in length. These results suggest that although PLK3/PRK expression is downregulated in a majority of lung carcinoma samples, mutational inactivation of the coding sequence of the PLK3/PRK gene appears to be a rare event in lung cancer.

Sak serine-threonine kinase acts as an effector of Tec tyrosine kinase

The murine sak gene encodes a putative serine-threonine kinase which is homologous to the members of the Plk/Polo family. Although Sak protein is presumed to be involved in cell growth mechanism, efforts have failed to demonstrate its kinase activity. Little has been, therefore, elucidated how Sak is regulated and how Sak contributes to cell proliferation. Tec is a cytoplasmic protein-tyrosine kinase (PTK) which becomes activated by the stimulation of cytokine receptors, lymphocyte surface antigens, heterotrimeric G protein-linked receptors, and integrins. To clarify the in vivo function of Tec, we have tried to isolate the second messengers of Tec by using the yeast two-hybrid screening. One of such Tec-binding proteins turned out to be Sak. In human kidney 293 cells, Sak became tyrosine-phosphorylated by Tec, and the serine-threonine kinase activity of Sak was detected only under the presence of Tec, suggesting Sak to be an effector molecule of Tec. In addition, Tec activity efficiently protects Sak from the "PEST" sequence-dependent proteolysis. Internal deletion of the PEST sequences led to the stabilization of Sak proteins, and expression of these mutants acted suppressive to cell growth. Our data collectively supports a novel role of Sak acting in the PTK-mediated signaling pathway.

Reactive oxygen species-induced phosphorylation of p53 on serine 20 is mediated in part by polo-like kinase-3

Upon exposure of cells to hydrogen peroxide (H(2)O(2)) phosphorylation of p53 was rapidly induced in human fibroblast GM00637, and this phosphorylation occurred on serine 9, serine 15, serine 20, but not on serine 392. In addition, H(2)O(2)-induced phosphorylation of p53 was followed by induction of p21, suggesting functional activation of p53. Induction of phosphorylation of p53 on multiple serine residues by H(2)O(2) was caffeine-sensitive and blocked in ATM(-/-) cells. Polo-like kinase-3 (Plk3) activity was also activated upon H(2)O(2) treatment, and this activation was ATM-dependent. Recombinant His(6)-Plk3 phosphorylated glutathione S-transferase (GST)-p53 fusion protein but not GST alone. When phoshorylated in vitro by His(6)-Plk3, but not by the kinase-defective mutant His6-Plk3(K52R), GST-p53 was recognized by an antibody specifically to serine 20-phosphorylated p53, indicating that serine 20 is an in vitro target of Plk3. Also serine 20-phosphorylated p53 was coimmunoprecipitated with Plk3 in cells treated with H(2)O(2). Furthermore, although H(2)O(2) strongly induced serine 15 phosphorylation of p53, it failed to induce serine 20 phosphorylation in Plk3-dificient Daudi cells. Ectopic expression of a Plk3 dominant negative mutant, Plk3(K52R), in GM00637 cells suppressed H(2)O(2)-induced serine 20 phosphorylation. Taken together, our studies strongly suggest that the oxidative stress-induced activation of p53 is at least in part mediated by Plk3.

Adhesion induced expression of the serine/threonine kinase Fnk in human macrophages

Members of the polo subfamily of protein kinases play crucial roles in cell proliferation. To study the function of this family in more detail, we isolated the cDNA of human Fnk (FGF-inducible kinase) which codes for a serine/threonine kinase of 646 aa. Despite the homology to the proliferation-associated polo-like kinase (Plk), tissue distribution of Fnk transcripts and expression kinetics differed clearly. In contrast to Plk no correlation between cell proliferation and Fnk gene expression was found. Instead high levels of Fnk mRNA were detectable in blood cells undergoing adhesion. The transition of monocytes from peripheral blood to matrix bound macrophages was accompanied by increasing levels of Fnk with time in culture. Neither treatment of monocytes with inducers of differentiation nor withdrawal of serum did influence Fnk mRNA levels significantly, suggesting that cell attachment triggers the onset of Fnk gene transcription. The idea that Fnk is part of the signalling network controlling cellular adhesion was supported by the analysis of the cytoplasmic distribution of the Fnk protein and the influence of its overexpression on the cellular architecture. Fnk as fusion protein with GFP localized at the cellular membrane in COS cells. Dysregulated Fnk gene expression disrupted the cellular f-actin network and induced a spherical morphology. Furthermore, Fnk binds to the Ca2+/integrin-binding protein Cib in two-hybrid-analyses and co-immunoprecipitation in assays. Moreover, both proteins were shown to co-localize in mammalian cells. The homology of Cib with calmodulin and with calcineurin B suggests that Cib might be a regulatory subunit of polo-like kinases.

A new genetic method for isolating functionally interacting genes: high plo1(+)-dependent mutants and their suppressors define genes in mitotic and septation pathways in fission yeast

We describe a general genetic method to identify genes encoding proteins that functionally interact with and/or are good candidates for downstream targets of a particular gene product. The screen identifies mutants whose growth depends on high levels of expression of that gene. We apply this to the plo1(+) gene that encodes a fission yeast homologue of the polo-like kinases. plo1(+) regulates both spindle formation and septation. We have isolated 17 high plo1(+)-dependent (pld) mutants that show defects in mitosis or septation. Three mutants show a mitotic arrest phenotype. Among the 14 pld mutants with septation defects, 12 mapped to known loci: cdc7, cdc15, cdc11 spg1, and sid2. One of the pld mutants, cdc7-PD1, was selected for suppressor analysis. As multicopy suppressors, we isolated four known genes involved in septation in fission yeast: spg1(+), sce3(+), cdc8(+), and rho1(+), and two previously uncharacterized genes, mpd1(+) and mpd2(+). mpd1(+) exhibits high homology to phosphatidylinositol 4-phosphate 5-kinase, while mpd2(+) resembles Saccharomyces cerevisiae SMY2; both proteins are involved in the regulation of actin-mediated processes. As chromosomal suppressors of cdc7-PD1, we isolated mutations of cdc16 that resulted in multiseptation without nuclear division. cdc16(+), dma1(+), byr3(+), byr4(+) and a truncated form of the cdc7 gene were isolated by complementation of one of these cdc16 mutations. These results demonstrate that screening for high dose-dependent mutants and their suppressors is an effective approach to identify functionally interacting genes.

Polo-like kinase1, a new target for antisense tumor therapy

The Polo-like kinase 1 (Plk1) is a highly conserved mitotic serine/threonine kinase which is commonly overexpressed in cancer cell lines. Plk1 positively regulates mitotic progression by activating the CDC25C-CDK1 amplification loop and by regulating late mitotic events, primarily the ubiquitin-dependent proteolysis. In the present study, an antisense strategy against Plk1 mRNA was developed to specifically inhibit cell proliferation of cancer cells in cell culture and in the nude-mouse tumor model. Among 41 phosphorothioate antisense oligodeoxynucleotides tested, the 20-mer JWG2000 strongly inhibited expression of Plk1 in cultured A549 cells, leading to loss of cell viability, and exhibited anti-tumor activity in nude mice A549 xenograft. JWG2000 did not inhibit growth and viability of primary human mesangial cells and human amnion fibroblasts.

PRK, a cell cycle gene localized to 8p21, is downregulated in head and neck cancer

The human PRK gene encodes a protein serine/threonine kinase of the polo family and plays an essential role in regulating meiosis and mitosis. We have previously shown that PRK expression is downregulated in a significant fraction of lung carcinomas. Our current studies reveal that PRK mRNA expression is downregulated in a majority (26 out of 35 patients) of primary head and neck squamous-cell carcinomas (HNSCC) compared with adjacent uninvolved tissues from the same patients, regardless of stage. In addition, PRK transcripts were undetectable in one of the two HNSCC cell lines analyzed. Ectopic expression of PRK, but not a PRK deletion construct, in transformed A549 fibroblast cells suppresses their proliferation. Furthermore, fluorescence in situ hybridization analyses show that the PRK gene localizes to chromosome band 8p21, a region that exhibits a high frequency of loss of heterozygosity in a variety of human cancers, including head and neck cancers, and that is proposed to contain two putative tumor suppressor genes. Considering that PRK plays an important role in the regulation of the G2/M transition and cell cycle progression, our current studies suggest that deregulated expression of PRK may contribute to tumor development. Genes Chromosomes Cancer 27:332-336, 2000.

The polo-like kinase PLK-1 is required for nuclear envelope breakdown and the completion of meiosis in Caenorhabditis elegans

The Polo-like kinases are key regulatory molecules required during the cell cycle for the successful completion of mitosis. We have cloned a C. elegans homolog of the Drosophila melanogaster polo gene (designated plk-1 for C. elegans polo-like kinase-1) and present the subcellular localization of the PLK-1 protein during the meiotic and mitotic cell cycles in C. elegans oocytes and embryos, respectively. Disruption of PLK-1 expression by RNA-mediated interference (RNAi) disrupts normal oocyte and embryonic development. Inspection of oocytes revealed a defect in nuclear envelope breakdown (NEBD) before ovulation. This defect in NEBD was also observed in oocytes that were depleted of the cyclin-dependent kinase NCC-1 (C. elegans homolog of Cdc2). The plk-1 RNAi oocytes were fertilized; however the resulting embryos were unable to separate their meiotic chromosomes or form and extrude polar bodies. These defects led to embryonic arrest as single cells. genesis 26:26-41, 2000. Published 2000 Wiley-Liss, Inc.

The physical association and phosphorylation of Cdc25C protein phosphatase by Prk

prk encodes a protein serine/threonine kinase involved in regulating M phase functions during the cell cycle. We have expressed His6-Prk and His6-Cdc25C proteins using the baculoviral vector expression system. Purified recombinant His6-Prk, but not a kinase-defective mutant His6-PrkK52R, is capable of strongly phosphorylating His6-Cdc25C in vitro. Co-immunoprecipitation and affinity column chromatography experiments demonstrate that GST-Prk and native Cdc25C interact. When co-infected with His6-Prk and His6-Cdc25C recombinant baculoviruses, sf-9 cells produce His6-Cdc25C antigen with an additional slower mobility band on denaturing polyacrylamide gels compared with cells infected with His6-Cdc25C baculovirus alone. In addition, His6-Cdc25C immunoprecipitated from sf-9 cells co-infected with His6-Prk and His6-Cdc25C baculoviruses, but not with His6-PrkK52R and His6-Cdc25C baculoviruses, contains a greatly enhanced kinase activity that phosphorylates His6-Cdc25C in vitro. Moreover, phosphopeptide mapping shows that His6-Prk phosphorylates His6-Cdc25C at two sites in vitro and that the major phosphorylation site co-migrates with the one that is phosphorylated in vivo in asynchonized cells. Further studies reveal that His6-Prk phosphorylates Cdc25C on serine216, a residue also phosphorylated by Chk1 and Chk2. Together, these observations strongly suggest that Prk's role in mitosis is at least partly mediated through direct regulation of Cdc25C.

Caenorhabditis elegans contains structural homologs of human prk and plk

We and others have recently reported cloning and characterization of human prk and plk, members of the polo family of protein serine/threonine kinases that includes the budding yeast cdc5 and Drosophila melanoganster polo. The cdc5 gene is essential for cell cycle progression through mitosis and controls adaptation to the yeast DNA damage checkpoint. Here we report the identification of two new cdc5 homologs from Ceanorhabditis elegans, named plc1 and plc2. The deduced amino acid sequences of Plc1 and Plc2 share strong homology with both human Prk and Plk. plc1 and plc2 genes are closely linked on chromosome III and share 40% residue identity, suggesting that gene duplication followed by independent evolution gives rise to multiple polo homologous genes within a species. Similar to polo family members in other species, two distinct domains are present in Plc1 and Plc2 with the N-terminal half being the putative kinase domain. Interestingly, Plc2, unlike Plc1, contains a less conserved polo box within the C-terminal half of the protein, suggesting a functional division between these two kinases.

Polo-like kinases: positive regulators of cell division from start to finish

Present in organisms ranging from yeast to man, homologues of the Drosophila Polo kinase control multiple stages of cell division. At the onset of mitosis, Polo-like kinases (Plks) function in centrosome maturation and bipolar spindle formation, and they contribute to the activation of cyclin-dependent kinase (Cdk)1-cyclin B. Subsequently, they are required for the inactivation of Cdk1 and exit from mitosis. In the absence of Plk function, mitotic cyclins fail to be destroyed, indicating that Plks are important regulators of the anaphase-promoting complex/cyclosome (APC/C), a key component of the ubiquitin-dependent proteolytic degradation pathway. Finally, recent evidence implicates Plks in the temporal and spatial coordination of cytokinesis.