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.

Ana1/Cep295 helps recruit Polo/PLK1 to centrioles to promote mitotic PCM assembly and centriole elongation

Polo kinase (PLK1) is a master cell cycle regulator that is recruited to various subcellular structures, often by its Polo-Box domain (PBD), which binds to phosphorylated S-pS/pT motifs. Polo/PLK1 has multiple functions at centrioles and centrosomes, and we previously showed that in Drosophila phosphorylated Sas-4 initiates Polo/PLK1 recruitment to newly formed centrioles, while phosphorylated Spd-2 recruits Polo/PLK1 to the Pericentriolar Material (PCM) that assembles around mother centrioles in mitosis. Here, we show that Ana1 (Cep295 in humans) also helps to recruit Polo to mother centrioles in Drosophila. If Ana1-dependent Polo/PLK1 recruitment is impaired, mother centrioles can still duplicate, disengage from their daughters and form functional cilia, but they can no longer efficiently assemble mitotic PCM or elongate during G2. We conclude that Ana1 helps recruit Polo/PLK1 to mother centrioles to specifically promote mitotic centrosome assembly and centriole elongation in G2, but not centriole duplication, centriole disengagement or cilia assembly.

Redistribution of centrosomal proteins by centromeres and Polo kinase controls partial nuclear envelope breakdown in fission yeast

Proper mitotic progression in Schizosaccharomyces pombe requires partial nuclear envelope breakdown (NEBD) and insertion of the spindle pole body (SPB – yeast centrosome) to build the mitotic spindle. Linkage of the centromere to the SPB is vital to this process, but why that linkage is important is not well understood. Utilizing high-resolution structured illumination microscopy (SIM), we show that the conserved SUN-domain protein Sad1 and other SPB proteins redistribute during mitosis to form a ring complex around SPBs, which is a precursor for localized NEBD and spindle formation. Although the Polo kinase Plo1 is not necessary for Sad1 redistribution, it localizes to the SPB region connected to the centromere, and its activity is vital for redistribution of other SPB ring proteins and for complete NEBD at the SPB to allow for SPB insertion. Our results lead to a model in which centromere linkage to the SPB drives redistribution of Sad1 and Plo1 activation that in turn facilitate partial NEBD and spindle formation through building of a SPB ring structure.

Aurora kinases promote mitotic progression and asymmetric cell division through activation of Polo kinase

Organ development and integrity requires the maintenance of a defined and restricted pool of dividing stem cells. This process requires the coupling of mitosis progression to the establishment of stem cells polarization and spindle orientation to ensure that they retain the ability to proliferate while their descendant cells do not proliferate excessively. How this coupling occurs is unknown. Here we show that downstream of Aurora kinases, Polo activity levels are crucial for timely mitotic progression independently from the Spindle Assembly Checkpoint (SAC). In addition, Polo functions downstream of Aurora A to ensure cortical polarity and spindle orientation in neural stem cells, thereby preventing excessive cellular proliferation in developing larval brains, and suppressing tumor development. In addition, induction of aneuploidy in polo mutant brain by inactivation of the SAC leads to an increase in tumor development Altogether, our results reveal that the Aurora-Polo kinase axis is an essential module coupling mitotic progression to asymmetric division in NSCs.

Redistribution of centrosomal proteins by centromeres and Polo kinase controls nuclear envelope breakdown

Proper mitotic progression in Schizosaccharomyces pombe requires partial nuclear envelope breakdown (NEBD) and insertion of the spindle pole body (SPB – yeast centrosome) to build the mitotic spindle. Linkage of the centromere to the SPB is vital to this process, but why that linkage is important is not well understood. Utilizing high-resolution structured illumination microscopy (SIM), we show that the conserved SUNprotein Sad1 and other SPB proteins redistribute during mitosis to form a ring complex around SPBs, which is a precursor for NEBD and spindle formation. Although the Polo kinase Plo1 is not necessary for Sad1 redistribution, it localizes to the SPB region connected to the centromere, and its activity is vital for SPB ring protein redistribution and for complete NEBD to allow for SPB insertion. Our results lead to a model in which centromere linkage to the SPB drives redistribution of Sad1 and Plo1 activation that in turn facilitate NEBD and spindle formation through building of an SPB ring structure.SummaryNuclear envelope breakdown is necessary for fission yeast cells to go through mitosis. Bestul et al. show that the SUN protein, Sad1, is vital in carrying out this breakdown and is regulated by the centromere and Polo kinase.

Silencing the spindle assembly checkpoint: Let’s play Polo!

Silencing of the spindle assembly checkpoint involves two protein phosphatases, PP1 and PP2A-B56, that are thought to extinguish checkpoint signaling through dephosphorylation of a checkpoint scaffold at kinetochores. In this issue, Cordeiro et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.202002020) now show that a critical function of these phosphatases in checkpoint silencing is removal of Polo kinase at kinetochores, which would otherwise autonomously sustain the checkpoint.

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.

Ana1 recruits PLK1 to mother centrioles to promote mitotic PCM assembly and centriole elongation

Polo kinase (PLK1) is a master cell cycle regulator that is recruited to various subcellular structures by its Polo-Box domain (PBD), which binds to phosphorylated S-pS/pT motifs. Polo has multiple functions at centrioles and centrosomes, and we previously showed that phosphorylated Sas-4 initiates Polo recruitment to newly formed centrioles, while phosphorylated Spd-2 recruits Polo to the mitotic Pericentriolar Material (PCM) that assembles around mother centrioles. Here, we investigate whether additional proteins recruit Polo to centrioles and/or centrosomes, and find that Ana1 (Cep295 in mammals) helps recruit Polo to mother centrioles. If this function is impaired, mother centrioles can still duplicate and disengage from their daughters, but they can no longer efficiently assemble a mitotic PCM or elongate their centrioles in G2. Thus, Ana1 is part of a sequential phosphorylation cascade that recruits Polo to centrioles to drive mitotic centrosome assembly and centriole elongation in G2, but not centriole duplication or disengagement.