Mammalian polo-like kinase 1 (Plk1) has been thought to play a central role in this process. However, how Plk1 contributes to this event remains unknown. Here we demonstrated that Cdc2-dependent phosphorylation on a gamma-TuRC recruitment protein, Nedd1/GCP-WD, at two distinct sites provides both temporal and spatial cues to timely bring about Plk1 functions to the centrosomes and spindles via phosphorylation-dependent interaction with the polo-box domain of Plk1. At centrosomes, Plk1 interacted with a C-terminal phosphorylated motif of Nedd1 to phosphorylate the latter, a step suggested to be important for centrosome-based microtubule nucleation. Immediately following this event, Plk1 also interacted with an N-terminal phosphorylated motif of Nedd1 along the spindles and phosphorylated an Augmin subunit, Hice1, to promote microtubule-based microtubule nucleation. Loss of Nedd1-mediated Hice1 phosphorylation by Plk1 resulted in impaired Augmin interaction with microtubules and diminished gamma-tubulin recruitment to the spindles that ultimately led to defects in proper bipolar spindle formation and chromosome segregation. Taken together, the data provided here demonstrate the underlying mechanisms of how the two distinct Plk1-Nedd1 interactions at the centrosomes and spindles are deciphered into different biochemical and cellular outcomes to achieve normal bipolar spindle formation and mitotic progression. This study may allow us to understand the underlying mechanism of how Plk1 cooperates with other mitotic components to promote timely activation of specific protein complexes at distinct subcellular structures. Aside from the role of Plk1 in the regulation of Nedd1 at centrosomes, we have been focusing on understanding the physiological significance of the Cep192-Plk1 interaction. Our results showed that Plk1 binds to the phosphorylated S995 motif of Cep192. To better understand the significance of this event, we have generated cells expressing a phosphorylation-defective Cep192 S995A mutant. Characterization of this mutant is underway. Preliminary results suggest that the p-S995-dependent interaction between Cep192 and Plk1 is important for proper establishment of spindle bipolarity during M-phase of the cell cycle. In depth analysis of the Cep192 S995A mutant-expressing cells may allow us to comprehend how Aurora A influences the function of Plk1, vice versa. Aside from investigating the role of Plk1 in regulating the function of centrosomes during the late state of the cell cycle, we have begun to investigate how viral accessory proteins can deregulate the function of centrosomes. Notably, some of HIV viral accessory proteins have been shown to bind to mammalian polo-like kinase 4 (Plk4) and alter the capacity of this enzyme in regulating centriole duplication process. However, the molecular mechanisms underlying this event remains unknown. Given that tight control of centriole numbers is essential for the maintenance of genomic stability, this new direction of research may likely be important to understand the mechanism of how virus-induced centrosome aberrations can promote the development of various human disorders, including cancer and microcephaly.
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