UBIQUITINATION AND DEGRADATION OF POLO-LIKE KINASE

Abstract

The actions of many proteins are involved in the initiation and regulation of the cell cycle. Understanding the function of these proteins is essential to events such as tumorigenesis, cancer progression and the regulation of cellular growth. Specific proteins involved in cell cycle regulation have been extensively studied by many laboratories in recent years. One class of critical cell cycle regulators includes cyclins and cyclin dependent kinases. Another family of cell cycle regulated kinases, the Polo-Like Kinases (Plk's), have recently been implicated in the regulation of mitosis. Polo and its homologs have been shown to be involved in multiple aspects of the cell cycle. Polo-Like Kinase is localized at the spindle poles, centromeres and midbody during mitosis and can activate cdc25, the regulator of the maturation promoting factor (MPF), in Xenopus laevis. Mammalian Plk protein levels are low in G1, accumulate during S phase and peak at the G2M transition, coincident with the appearance of a phosphorylated form of Plk. Following mitosis, there is an abrupt dephosphorylation and disappearance of Plk 1 protein with kinetics similar to the loss of cyclin B. This loss of Plk protein cannot be totally accounted for by variations in the synthesis rate of Plk protein, but could possibly be explained by either expulsion from the cell in the midbody and/or degradation by the ubiquitin proteasome pathway. Both in vitro and in vivo experiments indicate that the loss of Plk protein is mostly a result of ubiquitin proteasome-mediated degradation. In vitro synthesized Plk protein can be degraded by partially purified proteasome preparations. This in vitro degradation can be blocked by specific proteasome inhibitors. The loss of Plk protein detected in vivo after release from a mitotic block was also prevented by specific proteasome inhibitors. Furthermore, poly-ubiquitinated Plk was detected in cell lysates. It is concluded that Plk protein levels are apparently regulated by a combination of changes in synthesis rates, expulsion in the midbody and specific post-mitotic degradation through the ubiquitination-proteasome pathway.