MOLECULAR-CLONING OF A NEW MEMBER OF THE P21-CDC42 RAC-ACTIVATED KINASE (PAK) FAMILY/

A number of ''target'' proteins for the Rho family of small GTP-bindin g proteins have now been identified, including the protein kinases ACK and p65(PAK) (Manser, E., Leung, T., Salihuddin, H., Zhao, Z.-S., and Lim, L. (1994) Nature 367, 40-46). The purified serine/threonine kina se p65(PAK) has been shown to be directly activated by GTP-Rac1 or GTP -Cdc42. Here we report the cDNA sequence encoding a new brain-enriched PAK isoform beta-PAK, which shares 79% amino acid identity with the p reviously described alpha-isoform. Their mRNAs are differentially expr essed in the brain, with alpha-PAK mRNA being particularly abundant in motor-associated regions. In vitro translation products of the alpha- and beta-PAK cDNAs exhibited relative molecular masses of 68,000 and 65,000, respectively, by SDS-polyacrylamide analysis. A specific beta- PAK peptide sequence was obtained from rat brain-purified p65(PAK). Re combinant alpha- and beta-PAKs exhibited an increase in kinase activit y mediated by GTP-p21 induced autophosphorylation. Cdc42 was a more po tent activator in vitro of alpha-PAK kinase, and the fully activated e nzyme is 300 times more active than the unphosphorylated form. Interes tingly the down-regulation in the binding of p21s to recombinant beta- PAK and brain p65(PAK), which is observed upon kinase activation does not occur with recombinant alpha-PAK.