A NOVEL SERINE KINASE ACTIVATED BY RAC1/CDC42HS-DEPENDENT AUTOPHOSPHORYLATION IS RELATED TO PAK65 AND STE20

We identified three proteins in neutrophil cytosol of molecular size 65, 62 and 68 kDa which interact in a GTP-dependent manner with rad and CDC42Hs, but not with rho. Purification of p65 and subsequent peptide sequencing revealed identity to rat brain PAK65 and to yeast STE20 kinase domains, Based on these sequences we screened a human placenta library and cloned the full-length cDNA. The complete amino acid sequence of the human cDNA shares similar to 73 % identity with rat brain PAK65; within the kinase domain the human protein shares >95% and similar to 63% identity with rat PAK65 and yeast STE20 respectively. The new human (h)PAK65 mRNA is ubiquitously expressed and hPAK65 protein is distinct from either human or rat brain PAK65. Recombinant hPAK65 exhibits identical specificity to the endogenous p65; both can bind rad and CDC42Hs in a GTP-dependent manner. The GTP-bound forms of rad and CDC42Hs induce autophosphorylation of hPAK65 on serine residues only, hPAK65 activated by either rad or CDC42Hs is phosphorylated on the same sites. Induction of hPAK65 autophosphorylation by rad or CDC42Hs stimulates hPAK65 kinase activity towards myelin basic protein and once hPAK65 is activated, rad or CDC42Hs are no longer required to keep it active. The affinities of rac/CDC42Hs for the non-phosphorylated and phosphorylated hPAK65 were similar, hPAK65 had only a marginal effect on the intrinsic GTPase activity of CDC42Hs, but significantly affected the binding and GAP activity of p190. These data are consistent with a model in which hPAK65 functions as an effector molecule for rad and CDC42Hs.