Cdk1 Phosphorylates SPAT-1/Bora to Promote Plk1 Activation in C. elegans and Human Cells.

The conserved Bora protein is a Plk1 activator, essential for checkpoint recovery after DNA damage in human cells. Here, we show that Bora interacts with Cyclin B and is phosphorylated by Cyclin B/Cdk1 at several sites. The first 225 amino acids of Bora, which contain two Cyclin binding sites and three conserved phosphorylated residues, are sufficient to promote Plk1 phosphorylation by Aurora A in vitro. Mutating the Cyclin binding sites or the three conserved phosphorylation sites abrogates the ability of the N terminus of Bora to promote Plk1 activation. In human cells, Bora-carrying mutations of the three conserved phosphorylation sites cannot sustain mitotic entry after DNA damage. In C. elegans embryos, mutation of the three conserved phosphorylation sites in SPAT-1, the Bora ortholog, results in a severe mitotic entry delay. Our results reveal a crucial and conserved role of phosphorylation of the N terminus of Bora for Plk1 activation and mitotic entry.

Cdk1 plays matchmaker for the Polo-like kinase and its activator SPAT-1/Bora.

Mitosis is orchestrated by several protein kinases including Cdks, Plks and Aurora kinases. Despite considerable progress toward understanding the individual function of these protein kinases, how their activity is coordinated in space and time during mitosis is less well understood. In a recent article published in the Journal of Cell Biology, we show that CDK-1 regulates PLK-1 activity during mitosis in C. elegans embryos through multisite phosphorylation of the PLK-1 activator SPAT-1 (Aurora Borealis, Bora in human). SPAT-1 variants mutated on CDK-1 phosphorylation sites results in severe delays in mitotic entry, mimicking embryos lacking spat-1 or plk-1 function. We further show that SPAT-1 phosphorylation by CDK-1 promotes its binding to PLK-1 and stimulates PLK-1 phosphorylation on its activator T-loop by Aurora A kinase in vitro. Likewise, we find that phosphorylation of Bora by Cdk1 promotes phosphorylation of human Plk1 by Aurora A suggesting that this mechanism is conserved in humans. These results indicate that Cdk1 regulates Plk1 by boosting its kinase activity. Here we discuss these recent findings and open questions regarding the regulation of Plk1/PLK-1 by Cdk1/CDK-1 and Bora/SPAT-1.

Cdk1 phosphorylates SPAT-1/Bora to trigger PLK-1 activation and drive mitotic entry in C. elegans embryos.

The molecular mechanisms governing mitotic entry during animal development are incompletely understood. Here, we show that the mitotic kinase CDK-1 phosphorylates Suppressor of Par-Two 1 (SPAT-1)/Bora to regulate its interaction with PLK-1 and to trigger mitotic entry in early Caenorhabditis elegans embryos. Embryos expressing a SPAT-1 version that is nonphosphorylatable by CDK-1 and that is defective in PLK-1 binding in vitro present delays in mitotic entry, mimicking embryos lacking SPAT-1 or PLK-1 functions. We further show that phospho-SPAT-1 activates PLK-1 by triggering phosphorylation on its activator T loop in vitro by Aurora A. Likewise, we show that phosphorylation of human Bora by Cdk1 promotes phosphorylation of human Plk1 by Aurora A, suggesting that this mechanism is conserved in humans. Our results suggest that CDK-1 activates PLK-1 via SPAT-1 phosphorylation to promote entry into mitosis. We propose the existence of a positive feedback loop that connects Cdk1 and Plk1 activation to ensure a robust control of mitotic entry and cell division timing.

Coordinating cell polarity with cell division in space and time.

Decisions of when and where to divide are crucial for cell survival and fate, and for tissue organization and homeostasis. The temporal coordination of mitotic events during cell division is essential to ensure that each daughter cell receives one copy of the genome. The spatial coordination of these events is also crucial because the cytokinetic furrow must be aligned with the axis of chromosome segregation and, in asymmetrically dividing cells, the polarity axis. Several recent papers describe how cell shape and polarity are coordinated with cell division in single cells and tissues and begin to unravel the underlying molecular mechanisms, revealing common principles and molecular players. Here, we discuss how cells regulate the spatial and temporal coordination of cell polarity with cell division.

SPAT-1/Bora acts with Polo-like kinase 1 to regulate PAR polarity and cell cycle progression.

During asymmetric cell division, cell polarity and cell cycle progression are tightly coordinated, yet mechanisms controlling both these events are poorly understood. Here we show that the Bora homologue SPAT-1 regulates both PAR polarity and cell cycle progression in C. elegans embryos. We find that, similarly to mammalian cells, SPAT-1 acts with PLK-1 and not with the mitotic kinase Aurora A (AIR-1), as shown in Drosophila. SPAT-1 binds to PLK-1, and depletion of SPAT-1 or PLK-1 leads to similar cell division defects in early embryos, which differ from the defects caused by depletion of AIR-1. Additionally, SPAT-1 and PLK-1 depletion causes impaired polarity with abnormal length of the anterior and posterior PAR domains, and partial plk-1(RNAi) or spat-1(RNAi), but not air-1(RNAi), can rescue the lethality of a par-2 mutant. SPAT-1 is enriched in posterior cells, and this enrichment depends on PAR polarity and PLK-1. Taken together, our data suggest a model in which SPAT-1 promotes the activity of PLK-1 to regulate both cell polarity and cell cycle timing during asymmetric cell division, providing a link between these two processes.

A casein kinase 1 and PAR proteins regulate asymmetry of a PIP(2) synthesis enzyme for asymmetric spindle positioning.

Spindle positioning is an essential feature of asymmetric cell division. The conserved PAR proteins together with heterotrimeric G proteins control spindle positioning in animal cells, but how these are linked is not known. In C. elegans, PAR protein activity leads to asymmetric spindle placement through cortical asymmetry of Galpha regulators GPR-1/2. Here, we establish that the casein kinase 1 gamma CSNK-1 and a PIP(2) synthesis enzyme (PPK-1) transduce PAR polarity to asymmetric Galpha regulation. PPK-1 is posteriorly enriched in the one-celled embryo through PAR and CSNK-1 activities. Loss of CSNK-1 causes uniformly high PPK-1 levels, high symmetric cortical levels of GPR-1/2 and LIN-5, and increased spindle pulling forces. In contrast, knockdown of ppk-1 leads to low GPR-1/2 levels and decreased spindle forces. Furthermore, loss of CSNK-1 leads to increased levels of PIP(2). We propose that asymmetric generation of PIP(2) by PPK-1 directs the posterior enrichment of GPR-1/2 and LIN-5, leading to posterior spindle displacement.