UNPHOSPHORYLATED AND TYROSINE-PHOSPHORYLATED FORMS OF A FOCAL ADHESION PROTEIN, PAXILLIN, ARE SUBSTRATES FOR CALPAIN-II IN-VITRO - IMPLICATIONS FOR THE POSSIBLE INVOLVEMENT OF CALPAIN-II IN MITOSIS-SPECIFIC DEGRADATION OF PAXILLIN

Cell-to-substratum adhesion becomes weakened during mitosis of the cell cycle in fibroblasts. The level of one focal adhesion protein, paxillin, is greatly reduced in mitotic-arrested cells. We show here the possible involvement of calpain II, known to be localized in focal adhesion plaques, in the degradation of paxillin. Paxillin is tyrosine-phosphorylated during interphase of the cell cycle by protein tyrosine kinases (PTK) such as c-Src and Csk, and becomes dephosphorylated during mitosis. Our data, however, indicate that tyrosine phosphorylation of paxillin does not affect the rate of paxillin degradation by calpain in vitro.