Dbf4 regulates the Cdc5 Polo-like kinase through a distinct non-canonical binding interaction

An acidic loop in the forkhead-associated domain of the yeast meiosis-specific kinase Mek1 interacts with a specific motif in a subset of Mek1 substrates

The meiosis-specific kinase Mek1 regulates key steps in meiotic recombination in the budding yeast, Saccharomyces cerevisiae. MEK1 limits resection at double-strand break (DSB) ends and is required for preferential strand invasion into homologs, a process known as interhomolog bias. After strand invasion, MEK1 promotes phosphorylation of the synaptonemal complex protein Zip1 that is necessary for DSB repair mediated by a crossover-specific pathway that enables chromosome synapsis. In addition, Mek1 phosphorylation of the meiosis-specific transcription factor, Ndt80, regulates the meiotic recombination checkpoint that prevents exit from pachytene when DSBs are present. Mek1 interacts with Ndt80 through a 5-amino acid sequence, RPSKR, located between the DNA-binding and activation domains of Ndt80. AlphaFold Multimer modeling of a fragment of Ndt80 containing the RPSKR motif and full-length Mek1 indicated that RPSKR binds to an acidic loop located in the Mek1 FHA domain, a noncanonical interaction with this motif. A second protein, the 5'-3' helicase Rrm3, similarly interacts with Mek1 through an RPAKR motif and is an in vitro substrate of Mek1. Genetic analysis using various mutants in the MEK1 acidic loop validated the AlphaFold model, in that they specifically disrupt 2-hybrid interactions with Ndt80 and Rrm3. Phenotypic analyses further showed that the acidic loop mutants are defective in the meiotic recombination checkpoint and, in certain circumstances, exhibit more severe phenotypes compared to the NDT80 mutant with the RPSKR sequence deleted, suggesting that additional, as yet unknown, substrates of Mek1 also bind to Mek1 using an RPXKR motif.

An acidic loop in the FHA domain of the yeast meiosis-specific kinase Mek1 interacts with a specific motif in a subset of Mek1 substrates

The meiosis-specific kinase Mek1 regulates key steps in meiotic recombination in the budding yeast, Saccharomyces cerevisiae. MEK1 limits resection at the double strand break (DSB) ends and is required for preferential strand invasion into homologs, a process known as interhomolog bias. After strand invasion, MEK1 promotes phosphorylation of the synaptonemal complex protein Zip1 that is necessary for DSB repair mediated by a crossover specific pathway that enables chromosome synapsis. In addition, Mek1 phosphorylation of the meiosis-specific transcription factor, Ndt80, regulates the meiotic recombination checkpoint that prevents exit from pachytene when DSBs are present. Mek1 interacts with Ndt80 through a five amino acid sequence, RPSKR, located between the DNA binding and activation domains of Ndt80. AlphaFold Multimer modeling of a fragment of Ndt80 containing the RPSKR motif and full length Mek1 indicated that RPSKR binds to an acidic loop located in the Mek1 FHA domain, a non-canonical interaction with this motif. A second protein, the 5'-3' helicase Rrm3, similarly interacts with Mek1 through an RPAKR motif and is an in vitro substrate of Mek1. Genetic analysis using various mutants in the MEK1 acidic loop validated the AlphaFold model, in that they specifically disrupt two-hybrid interactions with Ndt80 and Rrm3. Phenotypic analyses further showed that the acidic loop mutants are defective in the meiotic recombination checkpoint, and in certain circumstances exhibit more severe phenotypes compared to the NDT80 mutant with the RPSKR sequence deleted, suggesting that additional, as yet unknown, substrates of Mek1 also bind to Mek1 using an RPXKR motif.

The prognostic significance and potential mechanism of DBF4 zinc finger in hepatocellular carcinoma

DBF4 zinc finger (DBF4) is a critical component involved in DNA replication and cell proliferation. It acts as a positive regulator of the cell division cycle 7 kinase. In this study, our investigation encompassed the impact of DBF4 on hepatocellular carcinoma (HCC) progression and delved into the potential mechanisms. We utilized open-access databases like TCGA and GEO to analyze the association between DBF4 and 33 different tumor types. We also conducted immunohistochemistry experiments to validate the expression of DBF4 in HCC, STAD, COAD, READ, PAAD, and LGG. Furthermore, we employed lentiviral transduction to knockdown DBF4 in HLF and SMMC cells, as well as to overexpress DBF4 in Huh7 cells. Subsequently, we evaluated the impact of DBF4 on proliferation, migration, and invasion of hepatocellular carcinoma cells. RNA sequencing and KEGG pathway enrichment analysis were also conducted to identify potential pathways, which were further validated through WB experiments. Finally, pathway inhibitor was utilized in rescue experiments to confirm whether DBF4 exerts its effects on tumor cells via the implicated pathway. Our findings revealed that DBF4 exhibited significant expression levels in nearly all examined tumors, which were further substantiated by the results of immunohistochemistry analysis. High DBF4 expression was correlated with poor overall survival (OS), disease-specific survival (DSS), progression-free interval (PFI), disease-free interval (DFI), relapse-free interval (RFI) in majority of tumor types, particularly in patients with HCC. In vitro experiments demonstrated that inhibition of DBF4 impaired the proliferative, migratory, and invasive abilities of HCC cells, whereas overexpression of DBF4 promoted these phenotypes. Sequencing results indicated that DBF4 may induce these changes through the ERBB signaling pathway. Further experimental validation revealed that DBF4 activates the ERBB signaling pathway, leading to alterations in the JNK/STAT, MAPK, and PI3K/AKT signaling pathways, thereby impacting the proliferative, migratory, and invasive abilities of tumor cells. Lastly, treatment of Huh7 cells overexpressing DBF4 with the ERBB2 inhibitor dacomitinib demonstrated the ability of ERBB2 inhibition to reverse the promoting effect of DBF4 overexpression on the proliferative, migratory, and invasive abilities of HCC cells. DBF4 plays a pivotal oncogenic role in HCC by promoting the ERBB signaling pathway and activating its downstream PI3K/AKT, JNK/STAT3, and MAPK signaling pathways. DBF4 may serve as a prognostic biomarker for patients with HCC.

Rewiring of the phosphoproteome executes two meiotic divisions in budding yeast

The cell cycle is ordered by a controlled network of kinases and phosphatases. To generate gametes via meiosis, two distinct and sequential chromosome segregation events occur without an intervening S phase. How canonical cell cycle controls are modified for meiosis is not well understood. Here, using highly synchronous budding yeast populations, we reveal how the global proteome and phosphoproteome change during the meiotic divisions. While protein abundance changes are limited to key cell cycle regulators, dynamic phosphorylation changes are pervasive. Our data indicate that two waves of cyclin-dependent kinase (Cdc28Cdk1) and Polo (Cdc5Polo) kinase activity drive successive meiotic divisions. These two distinct phases of phosphorylation are ensured by the meiosis-specific Spo13 protein, which rewires the phosphoproteome. Spo13 binds to Cdc5Polo to promote phosphorylation in meiosis I, particularly of substrates containing a variant of the canonical Cdc5Polo motif. Overall, our findings reveal that a master regulator of meiosis directs the activity of a kinase to change the phosphorylation landscape and elicit a developmental cascade.

Spo13/MEIKIN ensures a Two-Division meiosis by preventing the activation of APC/CAma1 at meiosis I

Genome haploidization at meiosis depends on two consecutive nuclear divisions, which are controlled by an oscillatory system consisting of Cdk1-cyclin B and the APC/C bound to the Cdc20 activator. How the oscillator generates exactly two divisions has been unclear. We have studied this question in yeast where exit from meiosis involves accumulation of the APC/C activator Ama1 at meiosis II. We show that inactivation of the meiosis I-specific protein Spo13/MEIKIN results in a single-division meiosis due to premature activation of APC/CAma1 . In the wild type, Spo13 bound to the polo-like kinase Cdc5 prevents Ama1 synthesis at meiosis I by stabilizing the translational repressor Rim4. In addition, Cdc5-Spo13 inhibits the activity of Ama1 by converting the B-type cyclin Clb1 from a substrate to an inhibitor of Ama1. Cdc20-dependent degradation of Spo13 at anaphase I unleashes a feedback loop that increases Ama1's synthesis and activity, leading to irreversible exit from meiosis at the second division. Thus, by repressing the exit machinery at meiosis I, Cdc5-Spo13 ensures that cells undergo two divisions to produce haploid gametes.

The Polo kinase Cdc5 is regulated at multiple levels in the adaptation response to telomere dysfunction

Telomere dysfunction activates the DNA damage checkpoint to induce a cell cycle arrest. After an extended period of time, however, cells can bypass the arrest and undergo cell division despite the persistence of the initial damage, a process called adaptation to DNA damage. The Polo kinase Cdc5 in Saccharomyces cerevisiae is essential for adaptation and for many other cell cycle processes. How the regulation of Cdc5 in response to telomere dysfunction relates to adaptation is not clear. Here, we report that Cdc5 protein level decreases after telomere dysfunction in a Mec1-, Rad53- and Ndd1-dependent manner. This regulation of Cdc5 is important to maintain long-term cell cycle arrest but not for the initial checkpoint arrest. We find that both Cdc5 and the adaptation-deficient mutant protein Cdc5-ad are heavily phosphorylated and several phosphorylation sites modulate adaptation efficiency. The PP2A phosphatases are involved in Cdc5-ad phosphorylation status and contribute to adaptation mechanisms. We finally propose that Cdc5 orchestrates multiple cell cycle pathways to promote adaptation.

The Spo13/Meikin pathway confines the onset of gamete differentiation to meiosis II in yeast

Sexual reproduction requires genome haploidization by the two divisions of meiosis and a differentiation program to generate gametes. Here, we have investigated how sporulation, the yeast equivalent of gamete differentiation, is coordinated with progression through meiosis. Spore differentiation is initiated at metaphase II when a membrane-nucleating structure, called the meiotic plaque, is assembled at the centrosome. While all components of this structure accumulate already at entry into meiosis I, they cannot assemble because centrosomes are occupied by Spc72, the receptor of the γ-tubulin complex. Spc72 is removed from centrosomes by a pathway that depends on the polo-like kinase Cdc5 and the meiosis-specific kinase Ime2, which is unleashed by the degradation of Spo13/Meikin upon activation of the anaphase-promoting complex at anaphase I. Meiotic plaques are finally assembled upon reactivation of Cdk1 at entry into metaphase II. This unblocking-activation mechanism ensures that only single-copy genomes are packaged into spores and might serve as a paradigm for the regulation of other meiosis II-specific processes.

Cell-cycle phospho-regulation of the kinetochore

The kinetochore is a mega-dalton protein assembly that forms within centromeric regions of chromosomes and directs their segregation during cell division. Here we review cell cycle-mediated phosphorylation events at the kinetochore, with a focus on the budding yeast Saccharomyces cerevisiae and the insight gained from forced associations of kinases and phosphatases. The point centromeres found in the budding yeast S. cerevisiae are one of the simplest such structures found in eukaryotes. The S. cerevisiae kinetochore comprises a single nucleosome, containing a centromere-specific H3 variant Cse4^CENP-A, bound to a set of kinetochore proteins that connect to a single microtubule. Despite the simplicity of the budding yeast kinetochore, the proteins are mostly homologous with their mammalian counterparts. In some cases, human proteins can complement their yeast orthologs. Like its mammalian equivalent, the regulation of the budding yeast kinetochore is complex: integrating signals from the cell cycle, checkpoints, error correction, and stress pathways. The regulatory signals from these diverse pathways are integrated at the kinetochore by post-translational modifications, notably phosphorylation and dephosphorylation, to control chromosome segregation. Here we highlight the complex interplay between the activity of the different cell-cycle kinases and phosphatases at the kinetochore, emphasizing how much more we have to understand this essential structure.

Exo1 recruits Cdc5 polo kinase to MutLγ to ensure efficient meiotic crossover formation

Crossovers generated during the repair of programmed meiotic double-strand breaks must be tightly regulated to promote accurate homolog segregation without deleterious outcomes, such as aneuploidy. The Mlh1-Mlh3 (MutLγ) endonuclease complex is critical for crossover resolution, which involves mechanistically unclear interplay between MutLγ and Exo1 and polo kinase Cdc5. Using budding yeast to gain temporal and genetic traction on crossover regulation, we find that MutLγ constitutively interacts with Exo1. Upon commitment to crossover repair, MutLγ-Exo1 associate with recombination intermediates, followed by direct Cdc5 recruitment that triggers MutLγ crossover activity. We propose that Exo1 serves as a central coordinator in this molecular interplay, providing a defined order of interaction that prevents deleterious, premature activation of crossovers. MutLγ associates at a lower frequency near centromeres, indicating that spatial regulation across chromosomal regions reduces risky crossover events. Our data elucidate the temporal and spatial control surrounding a constitutive, potentially harmful, nuclease. We also reveal a critical, noncatalytic role for Exo1, through noncanonical interaction with polo kinase. These mechanisms regulating meiotic crossovers may be conserved across species.

Meiosis I Kinase Regulators: Conserved Orchestrators of Reductional Chromosome Segregation

Research over the last two decades has identified a group of meiosis-specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring that sister kinetochores face towards the same pole (mono-orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk.

Polo kinase recruitment via the constitutive centromere-associated network at the kinetochore elevates centromeric RNA

The kinetochore, a multi-protein complex assembled on centromeres, is essential to segregate chromosomes during cell division. Deficiencies in kinetochore function can lead to chromosomal instability and aneuploidy-a hallmark of cancer cells. Kinetochore function is controlled by recruitment of regulatory proteins, many of which have been documented, however their function often remains uncharacterized and many are yet to be identified. To identify candidates of kinetochore regulation we used a proteome-wide protein association strategy in budding yeast and detected many proteins that are involved in post-translational modifications such as kinases, phosphatases and histone modifiers. We focused on the Polo-like kinase, Cdc5, and interrogated which cellular components were sensitive to constitutive Cdc5 localization. The kinetochore is particularly sensitive to constitutive Cdc5 kinase activity. Targeting Cdc5 to different kinetochore subcomplexes produced diverse phenotypes, consistent with multiple distinct functions at the kinetochore. We show that targeting Cdc5 to the inner kinetochore, the constitutive centromere-associated network (CCAN), increases the levels of centromeric RNA via an SPT4 dependent mechanism.

PLK1/NF-κB feedforward circuit antagonizes the mono-ADP-ribosyltransferase activity of PARP10 and facilitates HCC progression

Dysregulation of PARP10 has been implicated in various tumor types and plays a vital role in delaying hepatocellular carcinoma (HCC) progression. However, the mechanisms controlling the expression and activity of PARP10 in HCC remain mostly unknown. The crosstalk between PLK1, PARP10, and NF-κB pathway in HCC was determined by performing different in vitro and in vivo assays, including mass spectrometry, kinase, MARylation, chromatin immunoprecipitation, and luciferase reporter measurements. Functional examination was performed by using small chemical drug, cell culture, and mice HCC models. Correlation between PLK1, NF-κB, and PARP10 expression was determined by analyzing clinical samples of HCC patients with using immunohistochemistry. PLK1, an important regulator for cell mitosis, directly interacts with and phosphorylates PARP10 at T601. PARP10 phosphorylation at T601 significantly decreases its binding to NEMO and disrupts its inhibition to NEMO ubiquitination, thereby enhancing the transcription activity of NF-κB toward multiple target genes and promoting HCC development. In turn, NF-κB transcriptionally inhibits the PARP10 promoter activity and leads to its downregulation in HCC. Interestingly, PLK1 is mono-ADP-ribosylated by PARP10 and the MARylation of PLK1 significantly inhibits its kinase activity and oncogenic function in HCC. Clinically, the expression levels of PLK1 and phosphor-p65 show an inverse correlation with PARP10 expression in human HCC tissues. These findings are the first to uncover a PLK1/PARP10/NF-κB signaling circuit that underlies tumorigenesis and validate PLK1 inhibitors, alone or with NF-κB antagonists, as potential effective therapeutics for PARP10-expressing HCC.

Distinct surfaces on Cdc5/PLK Polo-box domain orchestrate combinatorial substrate recognition during cell division

Polo-like kinases (Plks) are key cell cycle regulators. They contain a kinase domain followed by a polo-box domain that recognizes phosphorylated substrates and enhances their phosphorylation. The regulatory subunit of the Dbf4-dependent kinase complex interacts with the polo-box domain of Cdc5 (the sole Plk in Saccharomyces cerevisiae) in a phosphorylation-independent manner. We have solved the crystal structures of the polo-box domain of Cdc5 on its own and in the presence of peptides derived from Dbf4 and a canonical phosphorylated substrate. The structure bound to the Dbf4-peptide reveals an additional density on the surface opposite to the phospho-peptide binding site that allowed us to propose a model for the interaction. We found that the two peptides can bind simultaneously and non-competitively to the polo-box domain in solution. Furthermore, point mutations on the surface opposite to the phosphopeptide binding site of the polo-box domain disrupt the interaction with the Dbf4 peptide in solution and cause an early anaphase arrest phenotype distinct from the mitotic exit defect typically observed in cdc5 mutants. Collectively, our data illustrates the importance of non-canonical interactions mediated by the polo-box domain and provide key mechanistic insights into the combinatorial recognition of substrates by Polo-like kinases.

Functions and regulation of the Polo-like kinase Cdc5 in the absence and presence of DNA damage

Polo-like kinases are essential cell cycle regulators that are conserved from yeast to humans. Unlike higher eukaryotes, who express multiple Polo-like kinase family members that perform many important functions, budding yeast express only a single Polo-like kinase, Cdc5, which is the homolog of mammalian cell cycle master regulator Polo-like kinase 1. Cdc5 is a fascinating multifaceted protein that is programmed to target its many substrates in a timely, sequential manner to ensure proper cell cycle progression. Over the years, many lessons about Polo-like kinase 1 have been learned by studying Cdc5 in budding yeast. Cdc5 has been well documented in regulating mitotic entry, chromosome segregation, mitotic exit, and cytokinesis. Cdc5 also plays important roles during cell division after DNA damage. Here, we briefly review the many functions of Cdc5 and its regulation in the absence and presence of DNA damage.

Randomized CRISPR-Cas Transcriptional Perturbation Screening Reveals Protective Genes against Alpha-Synuclein Toxicity

The genome-wide perturbation of transcriptional networks with CRISPR-Cas technology has primarily involved systematic and targeted gene modulation. Here, we developed PRISM (Perturbing Regulatory Interactions by Synthetic Modulators), a screening platform that uses randomized CRISPR-Cas transcription factors (crisprTFs) to globally perturb transcriptional networks. By applying PRISM to a yeast model of Parkinson's disease (PD), we identified guide RNAs (gRNAs) that modulate transcriptional networks and protect cells from alpha-synuclein (αSyn) toxicity. One gRNA identified in this screen outperformed the most protective suppressors of αSyn toxicity reported previously, highlighting PRISM's ability to identify modulators of important phenotypes. Gene expression profiling revealed genes differentially modulated by this strong protective gRNA that rescued yeast from αSyn toxicity when overexpressed. Human homologs of top-ranked hits protected against αSyn-induced cell death in a human neuronal PD model. Thus, high-throughput and unbiased perturbation of transcriptional networks via randomized crisprTFs can reveal complex biological phenotypes and effective disease modulators.

Fundamental cell cycle kinases collaborate to ensure timely destruction of the synaptonemal complex during meiosis

The synaptonemal complex (SC) is a proteinaceous macromolecular assembly that forms during meiotic prophase I and mediates adhesion of paired homologous chromosomes along their entire lengths. Although prompt disassembly of the SC during exit from prophase I is a landmark event of meiosis, the underlying mechanism regulating SC destruction has remained elusive. Here, we show that DDK (Dbf4-dependent Cdc7 kinase) is central to SC destruction. Upon exit from prophase I, Dbf4, the regulatory subunit of DDK, directly associates with and is phosphorylated by the Polo-like kinase Cdc5. In parallel, upregulated CDK1 activity also targets Dbf4. An enhanced Dbf4-Cdc5 interaction pronounced phosphorylation of Dbf4 and accelerated SC destruction, while reduced/abolished Dbf4 phosphorylation hampered destruction of SC proteins. SC destruction relieved meiotic inhibition of the ubiquitous recombinase Rad51, suggesting that the mitotic recombination machinery is reactivated following prophase I exit to repair any persisting meiotic DNA double-strand breaks. Taken together, we propose that the concerted action of DDK, Polo-like kinase, and CDK1 promotes efficient SC destruction at the end of prophase I to ensure faithful inheritance of the genome.

Control of Mus81 nuclease during the cell cycle

DNA replication and homologous recombination involve the formation of branched DNA structures that physically link chromosomes. Such DNA-based connections, which arise during S-phase, are typically disengaged prior to entry into mitosis, in order to ensure proper chromosome segregation. Exceptions can, however, occur: replication stress, or elevated levels of DNA damage, may cause cells to enter mitosis with unfinished replication as well as carrying recombination intermediates, such as Holliday junctions. Hence, cells are equipped with pathways that recognize and process branched DNA structures, and evolved mechanisms to enhance their function when on the verge of undergoing cell division. One of these pathways utilizes the structure-selective endonuclease Mus81, which is thought to facilitate the resolution of replication and recombination intermediates. Mus81 function is known to be enhanced upon entry into M phase in budding yeast and human cells. Based on recent findings, we discuss here an updated model of Mus81 control during the cell cycle.

DDK Promotes Tumor Chemoresistance and Survival via Multiple Pathways

DBF4-dependent kinase (DDK) is a two-subunit kinase required for initiating DNA replication at individual origins and is composed of CDC7 kinase and its regulatory subunit DBF4. Both subunits are highly expressed in many diverse tumor cell lines and primary tumors, and this is correlated with poor prognosis. Inhibiting DDK causes apoptosis of tumor cells, but not normal cells, through a largely unknown mechanism. Firstly, to understand why DDK is often overexpressed in tumors, we identified gene expression signatures that correlate with DDK high- and DDK low-expressing lung adenocarcinomas. We found that increased DDK expression is highly correlated with inactivation of RB1-E2F and p53 tumor suppressor pathways. Both CDC7 and DBF4 promoters bind E2F, suggesting that increased E2F activity in RB1 mutant cancers promotes increased DDK expression. Surprisingly, increased DDK expression levels are also correlated with both increased chemoresistance and genome-wide mutation frequencies. Our data further suggest that high DDK levels directly promote elevated mutation frequencies. Secondly, we performed an RNAi screen to investigate how DDK inhibition causes apoptosis of tumor cells. We identified 23 kinases and phosphatases required for apoptosis when DDK is inhibited. These hits include checkpoint genes, G2/M cell cycle regulators, and known tumor suppressors leading to the hypothesis that inhibiting mitotic progression can protect against DDKi-induced apoptosis. Characterization of one novel hit, the LATS2 tumor suppressor, suggests that it promotes apoptosis independently of the upstream MST1/2 kinases in the Hippo signaling pathway.

Dbf4-dependent kinase and the Rtt107 scaffold promote Mus81-Mms4 resolvase activation during mitosis

DNA repair by homologous recombination is under stringent cell cycle control. This includes the last step of the reaction, disentanglement of DNA joint molecules (JMs). Previous work has established that JM resolving nucleases are activated specifically at the onset of mitosis. In case of budding yeast Mus81‐Mms4, this cell cycle stage‐specific activation is known to depend on phosphorylation by CDK and Cdc5 kinases. Here, we show that a third cell cycle kinase, Cdc7‐Dbf4 (DDK), targets Mus81‐Mms4 in conjunction with Cdc5—both kinases bind to as well as phosphorylate Mus81‐Mms4 in an interdependent manner. Moreover, DDK‐mediated phosphorylation of Mms4 is strictly required for Mus81 activation in mitosis, establishing DDK as a novel regulator of homologous recombination. The scaffold protein Rtt107, which binds the Mus81‐Mms4 complex, interacts with Cdc7 and thereby targets DDK and Cdc5 to the complex enabling full Mus81 activation. Therefore, Mus81 activation in mitosis involves at least three cell cycle kinases, CDK, Cdc5 and DDK. Furthermore, tethering of the kinases in a stable complex with Mus81 is critical for efficient JM resolution.

Ligand stimulation of CD95 induces activation of Plk3 followed by phosphorylation of caspase-8

Upon interaction of the CD95 receptor with its ligand, sequential association of the adaptor molecule FADD (MORT1), pro-forms of caspases-8/10, and the caspase-8/10 regulator c-FLIP leads to the formation of a death-inducing signaling complex. Here, we identify polo-like kinase (Plk) 3 as a new interaction partner of the death receptor CD95. The enzymatic activity of Plk3 increases following interaction of the CD95 receptor with its ligand. Knockout (KO) or knockdown of caspase-8, CD95 or FADD prevents activation of Plk3 upon CD95 stimulation, suggesting a requirement of a functional DISC for Plk3 activation. Furthermore, we identify caspase-8 as a new substrate for Plk3. Phosphorylation occurs on T273 and results in stimulation of caspase-8 proapoptotic function. Stimulation of CD95 in cells expressing a non-phosphorylatable caspase-8-T273A mutant in a rescue experiment or in Plk3-KO cells generated by CRISPR/Cas9 reduces the processing of caspase-8 prominently. Low T273 phosphorylation correlates significantly with low Plk3 expression in a cohort of 95 anal tumor patients. Our data suggest a novel mechanism of kinase activation within the Plk family and propose a new model for the stimulation of the extrinsic death pathway in tumors with high Plk3 expression.

Centrosome-Dependent Bypass of the DNA Damage Checkpoint by the Polo Kinase Cdc5

Cell-cycle checkpoints are essential feedback mechanisms that promote genome integrity. However, in the face of unrepairable DNA lesions, bypass mechanisms can suppress checkpoint activity and allow cells to resume proliferation. The molecular mechanisms underlying this biological response are currently not understood. Taking advantage of unique separation-of-function mutants, we show that the Polo-like kinase (PLK) Cdc5 uses a phosphopriming-based interaction mechanism to suppress G2/M checkpoint arrest by targeting Polo kinase activity to centrosomes. We also show that key subunits of the evolutionarily conserved RSC complex are critical downstream effectors of Cdc5 activity in checkpoint suppression. Importantly, the lethality and checkpoint defects associated with loss of Cdc5 Polo box activity can be fully rescued by artificially anchoring Cdc5 kinase domain to yeast centrosomes. Collectively, our results highlight a previously unappreciated role for centrosomes as key signaling centers for the suppression of cell-cycle arrest induced by persistent or unrepairable DNA damage.

Understanding the Polo Kinase machine

The Polo Kinase is a central regulator of cell division required for several events of mitosis and cytokinesis. In addition to a kinase domain (KD), Polo-like kinases (Plks) comprise a Polo-Box domain (PBD), which mediates protein interactions with targets and regulators of Plks. In all organisms that contain Plks, one Plk family member fulfills several essential functions in the regulation of cell division, and here we refer to this conserved protein as Polo Kinase (Plk1 in humans). The PBD and the KD are capable of both cooperation and mutual inhibition in their functions. Crystal structures of the PBD, the KD and, recently, a PBD-KD complex have helped understanding the inner workings of the Polo Kinase. In parallel, an impressive array of molecular mechanisms has been found to mediate the regulation of the protein. Moreover, the targeting of Polo Kinase in the development of anti-cancer drugs has yielded several molecules with which to chemically modulate Polo Kinase to study its biological functions. Here we review our current understanding of the protein function and regulation of Polo Kinase as a fascinating molecular device in control of cell division.

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

DNA replication checkpoint signaling depends on a Rad53-Dbf4 N-terminal interaction in Saccharomyces cerevisiae

Dbf4-dependent kinase (DDK) and cyclin-dependent kinase (CDK) are essential to initiate DNA replication at individual origins. During replication stress, the S-phase checkpoint inhibits the DDK- and CDK-dependent activation of late replication origins. Rad53 kinase is a central effector of the replication checkpoint and both binds to and phosphorylates Dbf4 to prevent late-origin firing. The molecular basis for the Rad53-Dbf4 physical interaction is not clear but occurs through the Dbf4 N terminus. Here we found that both Rad53 FHA1 and FHA2 domains, which specifically recognize phospho-threonine (pT), interacted with Dbf4 through an N-terminal sequence and an adjacent BRCT domain. Purified Rad53 FHA1 domain (but not FHA2) bound to a pT Dbf4 peptide in vitro, suggesting a possible phospho-threonine-dependent interaction between FHA1 and Dbf4. The Dbf4-Rad53 interaction is governed by multiple contacts that are separable from the Cdc5- and Msa1-binding sites in the Dbf4 N terminus. Importantly, abrogation of the Rad53-Dbf4 physical interaction blocked Dbf4 phosphorylation and allowed late-origin firing during replication checkpoint activation. This indicated that Rad53 must stably bind to Dbf4 to regulate its activity.

Dbf4: the whole is greater than the sum of its parts

Together with cyclin-dependent kinases, the Dbf4-dependent kinase (DDK) is essential to activate the Mcm2-7 helicase and, hence, initiate DNA replication in eukaryotes. Beyond its role as the regulatory subunit of the DDK complex, the Dbf4 protein also regulates the activity of other cell cycle kinases to mediate the checkpoint response and prevent premature mitotic exit under stress. Two features that are unusual in DNA replication proteins characterize Dbf4. The first is its evolutionary divergence; the second is how its conserved motifs are combined to form distinct functional units. This structural plasticity appears to be at odds with the conserved functions of Dbf4. In this review, we summarize recent genetic, biochemical and structural work delineating the multiple interactions mediated by Dbf4 and its various functions during the cell cycle. We also discuss how the limited sequence conservation of Dbf4 may be an advantage to regulate the activities of multiple cell cycle kinases.

Binding of Drosophila Polo kinase to its regulator Matrimony is noncanonical and involves two separate functional domains

Drosophila melanogaster Polo kinase physically interacts with, and is repressed by, the Matrimony (Mtrm) protein during oogenesis. Females heterozygous for a deletion of the mtrm gene display defects in chromosome segregation at meiosis I. However, a complete absence of Mtrm results in both meiotic catastrophe and female sterility. We show that three phosphorylated residues in an N-terminal region in Mtrm are required for Mtrm::Polo binding. However, this binding is noncanonical; it does not require either a complete S-pS/pT-P motif in Mtrm or key residues in the Polo-box domain of Polo that allow Polo to bind phosphorylated substrates. By using fluorescence cross-correlation spectroscopy to characterize the Mtrm::Polo interaction in vivo, we show that a sterile α-motif (SAM) domain located at the C terminus of Mtrm increases the stability of Mtrm::Polo binding. Although Mtrm's C-terminal SAM domain is not required to rescue the chromosome segregation defects observed in mtrm/+ females, it is essential to prevent both meiotic catastrophe and the female sterility observed in mtrm/mtrm females. We propose that Polo's interaction with the cluster of phosphorylated residues alone is sufficient to rescue the meiosis I defect. However, the strengthening of Mtrm::Polo binding mediated by the SAM domain is necessary to prevent meiotic catastrophe and ensure female fertility. Characterization of the Mtrm::Polo interaction, as well as that of other Polo regulators, may assist in the design of a new class of Polo inhibitors to be used as targeted anticancer therapeutic agents.

Structure-function relationship of the Polo-like kinase in Trypanosoma brucei

Polo-like kinases (Plks) play multiple roles in mitosis and cytokinesis in eukaryotes and are characterized by the C-terminal Polo-box domain (PBD), which is implicated in binding to Plk substrates, targeting Plk and regulating Plk activity. The Plk homolog in Trypanosoma brucei (TbPLK) possesses a similar architecture, but it lacks the crucial residues involved in substrate binding and regulates cytokinesis but not mitosis. Little is known about the regulation of TbPLK and the role of the PBD in TbPLK localization and function. Here, we addressed the requirement of the kinase activity and the PBD for TbPLK localization and function through coupling RNAi of endogenous TbPLK with ectopic expression of TbPLK mutants. We demonstrate that the kinase activity and phosphorylation of two threonine residues, Thr198 and Thr202, in the activation loop (T-loop) of the kinase domain are essential for TbPLK function but not for TbPLK localization. Deletion of the PBD abolishes TbPLK localization, but the PBD itself is not correctly targeted, indicating that TbPLK localization requires both the PBD and the kinase domain. Surprisingly, the kinase domain of TbPLK, but not the PBD, binds to its substrates TbCentrin2 and p110, suggesting that TbPLK might interact with its substrate through different mechanisms. Finally, the PBD interacts with the kinase domain of TbPLK and inhibits its activity, and this inhibition is relieved when Thr198 is phosphorylated. Together, these results suggest an essential role of T-loop phosphorylation in TbPLK activation and crucial roles of the PBD in regulating TbPLK activity and localization.

Saccharomyces cerevisiae Dbf4 has unique fold necessary for interaction with Rad53 kinase

Dbf4 is a conserved eukaryotic protein that functions as the regulatory subunit of the Dbf4-dependent kinase (DDK) complex. DDK plays essential roles in DNA replication initiation and checkpoint activation. During the replication checkpoint, Saccharomyces cerevisiae Dbf4 is phosphorylated in a Rad53-dependent manner, and this, in turn, inhibits initiation of replication at late origins. We have determined the minimal region of Dbf4 required for the interaction with the checkpoint kinase Rad53 and solved its crystal structure. The core of this fragment of Dbf4 folds as a BRCT domain, but it includes an additional N-terminal helix unique to Dbf4. Mutation of the residues that anchor this helix to the domain core abolish the interaction between Dbf4 and Rad53, indicating that this helix is an integral element of the domain. The structure also reveals that previously characterized Dbf4 mutants with checkpoint phenotypes destabilize the domain, indicating that its structural integrity is essential for the interaction with Rad53. Collectively, these results allow us to propose a model for the association between Dbf4 and Rad53.

Polo kinases establish links between meiotic chromosomes and cytoskeletal forces essential for homolog pairing

During meiosis, chromosomes must find and align with their homologous partners. SUN and KASH-domain protein pairs play a conserved role by establishing transient linkages between chromosome ends and cytoskeletal forces across the intact nuclear envelope (NE). In C. elegans, a pairing center (PC) on each chromosome mediates homolog pairing and linkage to the microtubule network. We report that the polo kinases PLK-1 and PLK-2 are targeted to the PC by ZIM/HIM-8-pairing proteins. Loss of plk-2 inhibits chromosome pairing and licenses synapsis between nonhomologous chromosomes, indicating that PLK-2 is required for PC-mediated interhomolog interactions. plk-2 is also required for meiosis-specific phosphorylation of SUN-1 and establishment of dynamic SUN/KASH (SUN-1/ZYG-12) modules that promote homolog pairing. Our results provide key insights into the regulation of homolog pairing and reveal that targeting of polo-like kinases to the NE by meiotic chromosomes establishes the conserved linkages to cytoskeletal forces needed for homology assessment.

Independent modulation of the kinase and polo-box activities of Cdc5 protein unravels unique roles in the maintenance of genome stability

Polo-like kinases (PLKs) are evolutionarily conserved kinases essential for cell cycle regulation. These kinases are characterized by the presence of a C-terminal phosphopeptide-interaction domain, the polo-box domain (PBD). How the functional domains of PLKs work together to promote cell division is not understood. To address this, we performed a genetic screen to identify mutations that independently modulate the kinase and PBD activities of yeast PLK/Cdc5. This screen identified a mutagenic hotspot in the F-helix region of Cdc5 kinase domain that allows one to control kinase activity in vivo. These mutations can be systematically engineered into other major eukaryotic cell cycle kinases to similarly regulate their activity in live cells. Here, using this approach, we show that the kinase activity of Cdc5 can promote the execution of several stages of mitosis independently of PBD activity. In particular, we observe that the activation of Cdc14 and execution of mitotic exit are uniquely sensitive to the modulation of Cdc5 kinase activity. In contrast, PBD-defective mutants are capable of completing mitosis but are unable to maintain spindle pole body integrity. Consistent with this defect, PBD-deficient cells progressively double the size of their genome and ultimately lose genome integrity. Collectively, these results highlight the specific contributions of Cdc5 functional domains to cell division and reveal unexpected mechanisms controlling spindle pole body behavior and genome stability.

A synthetic human kinase can control cell cycle progression in budding yeast

The DDK kinase complex, composed of Cdc7 and Dbf4, is required for S-phase progression. The two component proteins show different degrees of sequence conservation between human and yeast. Here, we determine that Saccharomyces cerevisiae bearing human CDC7 and DBF4 grows comparably to cells with yeast DDK under standard growth conditions. HsDrf1 (a second human Dbf4-like protein) does not support growth, suggesting that HsDbf4 is the true ortholog of ScDbf4. Both human subunits are required to complement yeast cdc7Δ or dbf4Δ due to the inability of human Cdc7 or Dbf4 to interact with the corresponding yeast protein. Flow cytometry indicates normal cell cycle progression for yeast containing human DDK. However, yeast containing human DDK is sensitive to long-term exposure to hydroxyurea and fails to sporulate, suggesting that human DDK substitutes for some, but not all, of yeast DDK's functions. We mapped the region of Cdc7 required for species-specific function of DDK to the C-terminus of Cdc7 by substituting the yeast C-terminal 55 amino acid residues in place of the equivalent human residues. The resulting hybrid protein supported growth of a cdc7Δ strain only in the presence of ScDBF4. The strain supported by the hybrid CDC7 was not sensitive to HU and formed tetrads. Together, our data indicate that DDK's targeting of its essential substrate is conserved between species, whereas the interactions within DDK are species specific.

Regulation of the final stage of mitosis by components of the pre-replicative complex and a polo kinase

The accurate division of duplicated DNA is essential for maintenance of genomic stability in proliferating eukaryotic cells. Errors in DNA replication and chromosomal segregation may lead to cell death or genomic mutations that lead to oncogenic properties. Thus, tight regulation of DNA replication and mitosis is essential for maintaining genomic integrity. Cell division cycle 6 (Cdc6) is an essential factor for initiating DNA replication. Recent work shows that phosphorylation of Cdc6 by polo-like kinase 1 (Plk1), one of the essential mitotic kinases, regulates mitotic exit mediated by Cdk1 and separase. Here we discuss how pre-replicative complex factors are connected with Plk1 and affect mitotic exit.