Role of the Mammalian Polo-Like Kinase 3(Plk3) in Cell Cycle Regulation and DNA Damage Checkpoints
In order for cell division to be completed with a high degree of fidelity, the cell division cycle is controlled by cell cycle promoting proteins, that promote a highly ordered step-wise progression through the cell cycle, and checkpoint proteins, that halt cell cycle progression in response to genotoxic stress. Deregulation of these two protein groups leads to malignant transformation. Recent evidence suggests that one family of proteins, the polo-like kinases (Plks), is comprised of members having both cell cycle promoting and checkpoint functions. These serine/threonine kinases are components of many signaling networks that regulate mitotic entry, centrosome maturation and separation, chromosome segregation, mitotic exit, and cytokinesis. Of the four mammalian Plk homologues, Plk3 is the only known Plk that is expressed throughout the entire cell cycle and after DNA damage. Plk3 appears to keep a constant vigil as a checkpoint protein that responds to a variety of genotoxic stress. Since Plk3 phosphorylates Chk2, Cdc25A, and Cdc25C, the goal of this study was to determine how Plk3 phosphorylation of a checkpoint protein, Chk2, and the checkpoint targets, Cdc25C and Cdc25A, affects their function and in turn affects cell cycle regulation. We demonstrated that Plk3 phosphorylates Chk2 on serine 62 and serine 73 to prime Chk2 for phosphorylation and activation by the ataxia-telangiectasia mutated (ATM) protein. We determined that Plk3 phosphorylates Cdc25C to promote its nuclear accumulation, and possible nuclear sequestration. Lastly, the results of these studies revealed that in vitro, Plk3 phosphorylates two residues (S513 and S519) on the Cdc25A proximal C-terminal domain. Interestingly, these two phosphorylation sites are adjacent to K514 and R520, two residues shown to be important in cyclin B binding to Cdc25A. In vivo, phospho-mimetics of S513 and S519 promote ionizing radiation (IR) mediated Cdc25A instability in asynchronous cells and constitutive binding of cyclin B1 in response to a spindle checkpoint activator, nocodazole. In support of these results, we have also shown that IR- treated Plk3 deficient mouse embryonic fibroblasts (MEFs) contain more Cdc25A than Plk3 proficient cells. Thus we have described pathways and demonstrated molecular mechanisms by which Plk3 serves as a checkpoint protein.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:cell cycle polo like kinase cdc25a plk3
Date of Publication:01/01/2006