IDENTIFICATION AND MOLECULAR-CLONING OF A P21(CDC42/RAC1)-ACTIVATED SERINE THREORNINE KINASE THAT IS RAPIDLY ACTIVATED BY THROMBIN IN PLATELETS

The brain-enriched p21(cdc42/rac1)-activated serine/threonine kinase, p65(PAK), was identified and purified on the basis of overlays with [gamma-P-32]GTP-Cdc42 onto SDS-fractionated proteins (Manser, E., Leung, T., Salihuddin, H., Zhao, Z.-S., and Lim, L. (1994) Nature 367, 40-46). In this study, the ubiquitously expressed p21(cdc42/rac1) binding protein with relative molecular weight of 62,000 was purified from rat testes and shown to contain peptides related to PAK. It has thus been designated as the gamma-PAK isoform (alpha- and beta-isoforms being brain enriched). Isolation of gamma-PAK cDNAs show that the kinase is highly conserved with alpha-PAK in both the p21 binding and kinase domains. The purified protein exhibited kinase activity that was activated by GTP-Cdc42 or GTP-Rac1 in vitro. In platelets, a p62 in situ. renaturable kinase was recognized by antibodies raised against gamma-PAK. This thrombin-activated protein kinase appears to coprecipitate with another kinase of M(r) 86,000, suggesting that PAK may be part of a thrombin-responsive signaling complex.