Mitosis specific serine phosphorylation and downregulation of one of the focal adhesion protein, paxillin.

Mitotic cells typically lack well-formed focal adhesions. As an approach to explore the dynamic process regulating the focal adhesion assembly, we examined states of focal adhesion proteins during mitosis of the cell cycle. We found that the amount of paxillin was significantly reduced during mitosis of the cell cycle, whereas other focal adhesion proteins including talin, vinculin and Focal Adhesion Kinase did not. Proteolytic degradation appeared to be involved in the mitotic reduction, but transcriptional and/or translational controls of the mRNA were not essential for this downregulation. Moreover, concurrent with the decreased protein level, phosphorylation status of paxillin altered during mitosis; mitotic paxillin was phosphorylated primarily on serine and dephosphorylated on tyrosine while interphase one was phosphorylated both on serine and tyrosine. We found that mitotic phosphorylation created an electrophoretically slow-migrating population of paxillin which was barely detected in interphase cells. This mitotic specific modification occurred with both alpha and beta isoforms of paxillin. We also examined the fate of paxillin protein by changing its protein amount. We found that majority of paxillin overexpressed was subjected to the specific modification but not to the downregulation in the mitotic arrested cells. On the other hand, paxillin exogenously expressed at a moderate level was subjected to both the mitotic modification and downregulation. Collectively, we concluded that paxillin's specific serine phosphorylation together with the proteolytic downregulation of a limited fraction of paxillin is taken place during the mitosis of the cell cycle.