Self-regulated Plk1 recruitment to kinetochores by the Plk1-PBIP1 interaction is critical for proper chromosome segregation

The polo-box domain (PBD) of mammalian polo-like kinase 1 (Plk1) is essential in targeting its catalytic activity to specific subcellular structures critical for mitosis. The mechanism underlying Plk1 recruitment to the kinetochores and the role of Plk1 at this site remain elusive. Here, we demonstrate that a PBD-binding protein, PBIP1, is crucial for recruiting Plk1 to the interphase and mitotic kinetochores. Unprecedentedly, Plk1 phosphorylated PBIP1 at T78, creating a self-tethering site that specifically interacted with the PBD of Plk1, but not Plk2 or Plk3. Later in mitosis, Plk1 also induced PBIP1 degradation in a T78-dependent manner, thereby enabling itself to interact with other components critical for proper kinetochore functions. Absence of the p-T78-dependent Plk1 localization induced a chromosome congression defect and compromised the spindle checkpoint, ultimately leading to aneuploidy. Thus, Plk1 self-regulates the Plk1-PBIP1 interaction to timely localize to the kinetochores and promote proper chromosome segregation.     

High Mitotic Activity of Polo-like Kinase 1 Is Required for Chromosome Segregation and Genomic Integrity in Human Epithelial Cells

Protein kinases play key roles in regulating human cell biology, but manifold substrates and functions make it difficult to understand mechanism. We tested whether we could dissect functions of a pleiotropic mitotic kinase, Polo-like kinase 1 (Plk1), via distinct thresholds of kinase activity. We accomplished this by titrating Plk1 activity in RPE1 human epithelial cells using chemical genetics and verifying results in additional lines. We found that distinct activity thresholds are required for known functions of Plk1 including (from low to high activity) bipolar spindle formation, timely mitotic entry, and formation of a cytokinesis cleavage furrow. Subtle losses in Plk1 activity impaired chromosome congression and produced severe anaphase dysfunction characterized by poor separation of chromosome masses. These two phenotypes were separable, suggesting that they stem from distinct phosphorylation events. Impaired chromosome segregation in anaphase was the most sensitive to modest loss in Plk1 activity. Mechanistically, it was associated with unpaired sister chromatids with stretched kinetochores, suggestive of merotelic attachments. The C-terminal Polo box domain of Plk1 was required for its anaphase function, although it was dispensable for forming a bipolar spindle. The ultimate effect of partial inhibition of Plk1 was the formation of micronuclei, an increase in tetraploid progeny, and senescence. These results demonstrate that different thresholds of Plk1 activity can elicit distinct phenotypes, illustrating a general method for separating pleiotropic functions of a protein kinase even when these are executed close in time.     

Selective blockade of cancer cell proliferation and anchorage-independent growth by Plk1 activity–dependent suicidal inhibition of its polo-box domain

Polo-like kinase 1 (Plk1) plays a critical role in proper M-phase progression and cell proliferation. Plk1 is overexpressed in a broad spectrum of human cancers and is considered an attractive anticancer drug target. Although a large number of inhibitors targeting the catalytic domain of Plk1 have been developed, these inhibitors commonly exhibit a substantial level of cross-reactivity with other structurally related kinases, thus narrowing their applicable dose for patient treatment. Plk1 contains a C-terminal polo-box domain (PBD) that is essentially required for interacting with its binding targets. However, largely due to the lack of both specific and membrane-permeable inhibitors, whether PBD serves as an alternative target for the development of anticancer therapeutics has not been rigorously examined. Here, we used an intracellularly expressed 29-mer-long PBIP1-derived peptide (i.e., PBIPtide), which can be converted into a “suicidal” PBD inhibitor via Plk1-dependent self-priming and binding. Using this highly specific and potent system, we showed that Plk1 PBD inhibition alone is sufficient for inducing mitotic arrest and apoptotic cell death in cancer cells but not in normal cells, and that cancer cell–selective killing can occur regardless of the presence or absence of oncogenic RAS mutation. Intriguingly, PBD inhibition also effectively prevented anchorage-independent growth of malignant cancer cells. Thus, targeting PBD represents an appealing strategy for anti-Plk1 inhibitor development. Additionally, PBD inhibition–induced cancer cell–selective killing may not simply stem from activated RAS alone but, rather, from multiple altered biochemical and physiological mechanisms, which may have collectively contributed to Plk1 addiction in cancer cells.     

Mechanisms Underlying Plk1 Polo-Box Domain-Mediated Biological Processes and Their Physiological Significance

Mammalian polo-like kinase 1 (Plk1) has been studied intensively as a key regulator of various cell cycle events that are critical for proper M-phase progression. The polo-box domain (PBD) present in Plk1’s C-terminal non-catalytic region has been shown to play a central role in targeting the N-terminal kinase domain of Plk1 to specific subcellular locations. Subsequent studies reveal that PBD binds to a phosphorylated motif generated by one of the two mechanisms - self-priming by Plk1 itself or non-self-priming by a Pro-directed kinase, such as Cdc2. Here, we comparatively review the differences in the biochemical steps of these mechanisms and discuss their physiological significance. Considering the diverse functions of Plk1 during the cell cycle, a better understanding of how the catalytic activity of Plk1 functions in concert with its cis-acting PBD and how this coordinated process is intricately regulated to promote Plk1 functions will be important for providing new insights into different mechanisms underlying various Plk1-mediated biological events that occur at the multiple stages of the cell cycle.     

NudC is required for Plk1 targeting to the kinetochore and chromosome congression

The equal distribution of chromosomes during mitosis is critical for maintaining the integrity of the genome. Essential to this process are the capture of spindle microtubules by kinetochores and the congression of chromosomes to the metaphase plate . Polo-like kinase 1 (Plk1) is a mitotic kinase that has been implicated in microtubule-kinetochore attachment, tension generation at kinetochores, tension-responsive signal transduction, and chromosome congression . The tension-sensitive substrates of Plk1 at the kinetochore are unknown. Here, we demonstrate that human Nuclear distribution protein C (NudC), a 42 kDa protein initially identified in Aspergillus nidulans and shown to be phosphorylated by Plk1 , plays a significant role in regulating kinetochore function. Plk1-phosphorylated NudC colocalizes with Plk1 at the outer plate of the kinetochore. Depletion of NudC reduced end-on microtubule attachments at kinetochores and resulted in defects in chromosome congression at the metaphase plate. Importantly, NudC-deficient cells exhibited mislocalization of Plk1 and the Kinesin-7 motor CENP-E from prometaphase kinetochores. Ectopic expression of wild-type NudC, but not NudC containing mutations in the Plk1 phosphorylation sites, recovered Plk1 localization at the kinetochore and rescued chromosome congression. Thus, NudC functions as both a substrate and a spatial regulator of Plk1 at the kinetochore to promote chromosome congression.     

The Functional Significance of Posttranslational Modifications on Polo-Like Kinase 1 Revealed by Chemical Genetic Complementation

Mitosis is coordinated by carefully controlled phosphorylation and ubiquitin-mediated proteolysis. Polo-like kinase 1 (Plk1) plays a central role in regulating mitosis and cytokinesis by phosphorylating target proteins. Yet, Plk1 is itself a target for posttranslational modification by phosphorylation and ubiquitination. We developed a chemical-genetic complementation assay to evaluate the functional significance of 34 posttranslational modifications (PTMs) on human Plk1. To do this, we used human cells that solely express a modified analog-sensitive Plk1 (Plk1^AS) and complemented with wildtype Plk1. The wildtype Plk1 provides cells with a functional Plk1 allele in the presence of 3-MB-PP1, a bulky ATP-analog inhibitor that specifically inhibits Plk1^AS. Using this approach, we evaluated the ability of 34 singly non-modifiable Plk1 mutants to complement Plk1^AS in the presence of 3-MB-PP1. Mutation of the T-loop activating residue T210 and adjacent T214 are lethal, but surprisingly individual mutation of the remaining 32 posttranslational modification sites did not disrupt the essential functions of Plk1. To evaluate redundancy, we simultaneously mutated all phosphorylation sites in the kinase domain except for T210 and T214 or all sites in the C-terminal polo-box domain (PBD). We discovered that redundant phosphorylation events within the kinase domain are required for accurate chromosome segregation in anaphase but those in the PBD are dispensable. We conclude that PTMs within the T-loop of Plk1 are essential and nonredundant, additional modifications in the kinase domain provide redundant control of Plk1 function, and those in the PBD are dispensable for essential mitotic functions of Plk1. This comprehensive evaluation of Plk1 modifications demonstrates that although phosphorylation and ubiquitination are important for mitotic progression, many individual PTMs detected in human tissue may have redundant, subtle, or dispensable roles in gene function.     

The molecular basis for phosphodependent substrate targeting and regulation of Plks by the Polo-box domain

Polo-like kinases (Plks) perform crucial functions in cell-cycle progression and multiple stages of mitosis. Plks are characterized by a C-terminal noncatalytic region containing two tandem Polo boxes, termed the Polo-box domain (PBD), which has recently been implicated in phosphodependent substrate targeting. We show that the PBDs of human, Xenopus, and yeast Plks all recognize similar phosphoserine/threonine-containing motifs. The 1.9 A X-ray structure of a human Plk1 PBD-phosphopeptide complex shows that the Polo boxes each comprise beta6alpha structures that associate to form a 12-stranded beta sandwich domain. The phosphopeptide binds along a conserved, positively charged cleft located at the edge of the Polo-box interface. Mutations that specifically disrupt phosphodependent interactions abolish cell-cycle-dependent localization and provide compelling phenotypic evidence that PBD-phospholigand binding is necessary for proper mitotic progression. In addition, phosphopeptide binding to the PBD stimulates kinase activity in full-length Plk1, suggesting a conformational switching mechanism for Plk regulation and a dual functionality for the PBD.     

Plk1-targeted small molecule inhibitors: molecular basis for their potency and specificity

Members of polo-like kinases (collectively, Plks) have been identified in various eukaryotic organisms and play pivotal roles in cell proliferation. They are characterized by the presence of a distinct region of homology in the C-terminal noncatalytic domain, called polo-box domain (PBD). Among them, Plk1 and its functional homologs in other organisms have been best characterized because of its strong association with tumorigenesis. Plk1 is overexpressed in a wide spectrum of cancers in humans, and is thought to be an attractive anti-cancer drug target. Plk1 offers, within one molecule, two functionally different drug targets with distinct properties-the N-terminal catalytic domain and the C-terminal PBD essential for targeting the catalytic activity of Plk1 to specific subcellular locations. In this review, we focused on discussing the recent development of small-molecule and phosphopeptide inhibitors for their potency and specificity against Plk1. Our effort in understanding the binding mode of various inhibitors to Plk1 PBD are also presented.     

Interaction of chromatin-associated Plk1 and Mcm7

Plk1 is a multifunctional protein kinase involved in regulation of mitotic entry, chromosome segregation, centrosome maturation, and mitotic exit. Plk1 is a target of DNA damage checkpoints and aids resumption of the cell cycle during recovery from G2 arrest. The polo-box domain (PBD) of Plk1 interacts with phosphoproteins and localizes Plk1 to some mitotic structures. In a search for proteins that interact with the PBD of Plk1, we identified two of the minichromosome maintenance (MCM) proteins, Mcm2 and Mcm7. Co-immunoprecipitation and immunoblot analysis showed an interaction between full-length Plk1 and all other members of the MCM2-7 protein complex. Endogenous Plk1 co-immunoprecipitates with basal forms of Mcm7 as well as with slower migrating forms of Mcm7, induced in response to DNA damage. The strongest interaction between endogenous Plk1 and Mcm7 was detected in a soluble chromatin fraction. These findings suggest a new function for Plk1 in coordination of DNA replication and mitotic events.     

Plk1 activation by Ste20-like kinase (Slk) phosphorylation and polo-box phosphopeptide binding assayed with the substrate translationally controlled tumor protein (TCTP)

The mitotic protein kinase Plk1 catalyzes events associated with centrosome maturation, kinetocore function, spindle formation, and cytokinesis and is a target for anticancer drug design. It is composed of a N-terminal kinase domain and a C-terminal polo-box domain (PBD). The PBD domain serves to localize the kinase on cognate phosphorylated substrates, and this binding relieves the inhibition of the kinase by the PBD. Similar to many protein kinases, Plk1 is activated by phosphorylation on a threonine residue, Thr210, in the activation segment. In this work, we describe expression in Escherichia coli cells and purification of full-length Plk1 in quantities suitable for structural studies and use this material for quantitative characterization of the activation events with the substrate translationally controlled tumour protein (TCTP). The presence of the PBD-binding phosphopeptide enhances phosphorylation by the activating Ste20-like kinase (Slk). Native Plk1 exhibits a basal catalytic efficiency k cat/ K(M) of 9.9 x 10 (-5) s (-1) microM (-1). Association with a polo-box-binding phosphopeptide increased the catalytic efficiency by 11x largely through an increase in k(cat) with no change in K(M). Phosphorylation by Slk increases catalytic efficiency by 202x with a 2.3-fold reduction in K(M) and 88-fold increase in k(cat). Phosphorylation and the presence of the PBD-binding phosphopeptide result in an increase in catalytic efficiency of 1515x with a 2.3-fold decrease in K(M) and a 705-fold increase in k(cat) over the unmodified Plk1. Knowledge of kinase regulatory mechanisms and the structures of the Plk1 individual domains has allowed for a model to be proposed for these activatory events.     

Mammalian polo-like kinase 1-dependent regulation of the PBIP1-CENP-Q complex at kinetochores

Mammalian polo-like kinase 1 (Plk1) plays a pivotal role during M-phase progression. Plk1 localizes to specific subcellular structures through the targeting activity of the C-terminal polo-box domain (PBD). Disruption of the PBD function results in improper bipolar spindle formation, chromosome missegregation, and cytokinesis defect that ultimately lead to the generation of aneuploidy. It has been shown that Plk1 recruits itself to centromeres by phosphorylating and binding to a centromere scaffold, PBIP1 (also called MLF1IP and CENP-U[50]) through its PBD. However, how PBIP1 itself is targeted to centromeres and what roles it plays in the regulation of Plk1-dependent mitotic events remain unknown. Here, we demonstrated that PBIP1 directly interacts with CENP-Q, and this interaction was mutually required not only for their stability but also for their centromere localization. Plk1 did not appear to interact with CENP-Q directly. However, Plk1 formed a ternary complex with PBIP1 and CENP-Q through a self-generated p-T78 motif on PBIP1. This complex formation was central for Plk1-dependent phosphorylation of PBIP1-bound CENP-Q and delocalization of the PBIP1-CENP-Q complex from mitotic centromeres. This study reveals a unique mechanism of how PBIP1 mediates Plk1-dependent phosphorylation event onto a third protein, and provides new insights into the mechanism of how Plk1 and its recruitment scaffold, PBIP1-CENP-Q complex, are localized to and delocalized from centromeres.     

Mechanisms of mammalian polo-like kinase 1 (Plk1) localization: Self-versus non-self-priming

Mammalian polo-like kinase 1 (Plk1) has been studied intensively as a key element in regulating diverse mitotic events during M-phase progression. Plk1 is spatially regulated through the targeting activity of the conserved polo-box domain (PBD) present in the C-terminal non-catalytic region. Over the years, studies have demonstrated that the PBD forms a phospho-epitope binding module and the PBD-dependent interaction is critical for proper subcellular localization of Plk1. The current prevailing model is that the PBD binds to a phospho-epitope generated by Cdc2 or other Pro-directed kinases. Here we discuss a recent finding that Plk1 also self-promotes its localization by generating its own PBD-docking site.     

p53 is not directly relevant to the response of Polo-like kinase 1 inhibitors

Polo-like kinase 1 (Plk1) is elementary for cell proliferation and its deregulation is involved in tumorigenesis. Plk1 has been established as one of the most attractive targets for molecular cancer therapy. In fact, multiple small molecule inhibitors targeting either the kinase domain or the Polo-box binding domain (PBD) of Plk1 have been identified and intensively investigated. Intriguingly, Plk1 depletion affects more cancer cells than normal cells. It is also reported that the cytotoxicity induced by Plk1 inhibition is elevated in cancer cells with defective p53. The data lead to the hypothesis that p53 might be a predictive marker for the response of Plk1 inhibition. In this study, we demonstrate that there is no obvious different cytotoxic response between cancer cells with and without functional p53, including the isogenic colon cancer cell lines HCT116p53(+/+) and HCT116p53(-/-), breast cancer cell line MCF7, lung cancer cell line A549 and cervical carcinoma cell line HeLa, after treatment with either siRNA against Plk1, the kinase domain inhibitors BI 2536 and BI 6727 or the PBD inhibitor Poloxin. We suggest that the p53 status is not a predictor for the response of Plk1 inhibition, at least not directly. Yet, the long-term outcomes of losing p53, such as genome instability, could be associated with the cytotoxicity of Plk1 inhibition. Further studies are required to investigate whether other circumstances of cancer cells, such as DNA replication/damage stress, mitotic stress, and metabolic stress, which make possibly the survival of cancer cells more dependent on Plk1 function, are responsible for the sensitivity of Plk1 inhibition.     

Self-regulated mechanism of Plk1 localization to kinetochores: lessons from the Plk1-PBIP1 interaction

Mammalian polo-like kinase 1 (Plk1) has been studied extensively as a critical element in regulating various mitotic events during M-phase progression. Plk1 function is spatially regulated through the targeting activity of the conserved polo-box domain (PBD) present in the C-terminal non-catalytic region. Recent progress in our understanding of Plk1 localization to the centromeres shows that Plk1 self-regulates its initial recruitment by phosphorylating a centromeric component PBIP1 and generating its own PBD-binding site. Paradoxically, Plk1 also induces PBIP1 delocalization and degradation from the mitotic kinetochores late in the cell cycle, consequently permitting itself to bind to other kinetochore components. Thus, PBIP1-dependent self-recruitment of Plk1 to the interphase centromeres serves as a prelude to the efficient delivery of Plk1 itself to other kinetochore components whose interactions with Plk1 are vital for proper mitotic progression.     

Polo-box domains confer target specificity to the Polo-like kinase family

Polo-like kinases (Plks) contain a conserved Polo-box domain, shown to bind to phosphorylated Ser-pSer/pThr-Pro motifs. The Polo-box domain of Plk-1 mediates substrate interaction and plays an important role in subcellular localization. Intriguingly, the major interactions between the PBD and the optimal recognition peptide are mediated by highly conserved residues in the PBD, suggesting there is little target specificity conveyed by the various PBDs. However, here we show that the affinity of the purified Plk1-3 PBDs to both a physiological Cdc25C derived phospho-peptide and an optimal recognition phospho-peptide differs significantly among family members. To decipher the role of the PBDs and kinase domains in inferring Plk specificity, we exchanged the PBD of Plk1 (PBD1) with the PBD of Plk2, 3, or 4 (PBD2-4). The resulting hybrid proteins can restore bipolar spindle formation and centrosome maturation in Plk1-depleted U2OS cells to various degrees. In these experiments PBD2 was most efficient in complementing PBD-function. Using the MPM2 antibody that recognizes a large set of mitotic phospho-proteins, we could show that PBD1 and PBD2 display some limited overlap in target recognition. Thus, PBDs convey a significant deal of target specificity, indicating that there is only a limited amount of functional redundancy possible within the Plk family.     

Functional independency between catalytic activity and subcellular targeting of Polo-like kinase-1: Phenotypes of ectopic overexpression of various mutants

The mammalian polo-like kinase-1 (Plk1) plays key role in the progression of the M-phase. As cells enter the M-phase, Plk1 is phosphorylated by an upstream kinase on Thr-210 in N-terminal region, which is essential for Plk1 activation. In addition to catalytic activation, the subcellular localization is also an important task for Plk1, and the C-terminal polo-boxes are responsible for Plk1 targeting. Although both catalytic activation and targeting might be essential and work concurrently, it is still unclear which process precedes the other in mitotic progression, and which is more essential for the function of Plk1 through the M-phase. In order to investigate this mechanism, this study introduced several point mutations in the catalytic and polo-box domains, and overexpressed these constructs in HeLa cells. Although the cellular effects by ectopic expression were different in the cells containing various Plk1 constructs, phosphorylation itself, not the catalytic activity, was found to be important for the progression of the M-phase. Plk1 phosphorylation was also responsible for targeting by polo-box, and compensated for the polo-box mutation. These results suggested that Plk1 is phosphorylated in the starting M-phase, and this phosphorylation induces polo-box targeting through a possible conformational change. In this situation, catalytic activation does not appear to be a critical factor for the function of Plk1.     

A functional genomic screen identifies a role for TAO1 kinase in spindle-checkpoint signalling

Defects in chromosome-microtubule attachment trigger spindle-checkpoint activation and delay mitotic progression. How microtubule attachment is sensed and integrated into the steps of checkpoint-signal amplification is poorly understood. In a functional genomic screen targeting human kinases and phosphatases, we identified a microtubule affinity-regulating kinase kinase, TAO1 (also known as MARKK) as an important regulator of mitotic progression, required for both chromosome congression and checkpoint-induced anaphase delay. TAO1 interacts with the checkpoint kinase BubR1 and promotes enrichment of the checkpoint protein Mad2 at sites of defective attachment, providing evidence for a regulatory step that precedes the proposed Mad2-Mad1 dependent checkpoint-signal amplification step. We propose that the dual functions of TAO1 in regulating microtubule dynamics and checkpoint signalling may help to coordinate the establishment and monitoring of correct congression of chromosomes, thereby protecting genomic stability in human cells.     

Phosphorylation- and polo-box-dependent binding of Plk1 to Bub1 is required for the kinetochore localization of Plk1

Polo-like kinase 1 (Plk1) is required for the generation of the tension-sensing 3F3/2 kinetochore epitope and facilitates kinetochore localization of Mad2 and other spindle checkpoint proteins. Here, we investigate the mechanism by which Plk1 itself is recruited to kinetochores. We show that Plk1 binds to budding uninhibited by benzimidazole 1 (Bub1) in mitotic human cells. The Plk1-Bub1 interaction requires the polo-box domain (PBD) of Plk1 and is enhanced by cyclin-dependent kinase 1 (Cdk1)-mediated phosphorylation of Bub1 at T609. The PBD-dependent binding of Plk1 to Bub1 facilitates phosphorylation of Bub1 by Plk1 in vitro. Depletion of Bub1 in HeLa cells by RNA interference (RNAi) diminishes the kinetochore localization of Plk1. Ectopic expression of the wild-type Bub1, but not the Bub1-T609A mutant, in Bub1-RNAi cells restores the kinetochore localization of Plk1. Our results suggest that phosphorylation of Bub1 at T609 by Cdk1 creates a docking site for the PBD of Plk1 and facilitates the kinetochore recruitment of Plk1.     

Identification of novel and selective non-peptide inhibitors targeting the polo-box domain of polo-like kinase 1

A series of non-peptide inhibitors targeting the polo-box domain (PBD) of polo-like kinase 1 (Plk1) was designed based on the potent and selective minimal tripeptide Plk1 PBD inhibitor. Seven compounds were designed, synthesized and evaluated for fluorescence polarization (FP) assay. The most promising compound 10 bound to Plk1 PBD with IC₅₀ of 3.37 μM and had no binding to Plk2 PBD or Plk3 PBD at 100 μM. Molecular docking study was performed and possible binding mode was proposed. MM/GBSA binding free energy calculation were in agreement with the observed experimental results. These novel non-peptide selective Plk1 PBD inhibitors provided new lead compounds for further optimization.     

Proteomic screen defines the Polo-box domain interactome and identifies Rock2 as a Plk1 substrate

Polo-like kinase-1 (Plk1) phosphorylates a number of mitotic substrates, but the diversity of Plk1-dependent processes suggests the existence of additional targets. Plk1 contains a specialized phosphoserine-threonine binding domain, the Polo-box domain (PBD), postulated to target the kinase to its substrates. Using the specialized PBD of Plk1 as an affinity capture agent, we performed a screen to define the mitotic Plk1-PBD interactome by mass spectrometry. We identified 622 proteins that showed phosphorylation-dependent mitosis-specific interactions, including proteins involved in well-established Plk1-regulated processes, and in processes not previously linked to Plk1 such as translational control, RNA processing, and vesicle transport. Many proteins identified in our screen play important roles in cytokinesis, where, in mammalian cells, the detailed mechanistic role of Plk1 remains poorly defined. We go on to characterize the mitosis-specific interaction of the Plk1-PBD with the cytokinesis effector kinase Rho-associated coiled-coil domain-containing protein kinase 2 (Rock2), demonstrate that Rock2 is a Plk1 substrate, and show that Rock2 colocalizes with Plk1 during cytokinesis. Finally, we show that Plk1 and RhoA function together to maximally enhance Rock2 kinase activity in vitro and within cells, and implicate Plk1 as a central regulator of multiple pathways that synergistically converge to regulate actomyosin ring contraction during cleavage furrow ingression.