TI - Identification of a mouse p21Cdc42/Rac activated kinase . AB - We have isolated a novel member of the mammalian PAK ( p21 activated kinase ) and yeast Ste20 serine/threonine kinase family from a mouse fibroblast cDNA library , designated mPAK-3 . Expression of mPAK-3 in Saccharomyces cerevisiae partially restores mating function in ste20 null cells . Like other PAKs , mPAK-3 contains a putative Cdc42Hs/Rac binding sequence and when transiently expressed in COS cells , full-length mPAK-3 binds activated ( GTP gamma S ( guanosine 5'-3-O- ( thio-triphosphate ) -bound ) glutathione S-transferase ( GST ) -Cdc42Hs and GST-Rac1 . As expected for a putative target molecule , mPAK-3 does not bind to an effector domain mutant of Cdc42Hs . Furthermore , activated His-tagged Cdc42Hs and His-tagged Rac stimulate mPAK-3 autophosphorylation and phosphorylation of myelin basic protein by mPAK-3 in vitro . Interestingly , the amino terminal region of mPAK-3 contains potential SH3-binding sites and we find that mPAK-3 , expressed in vitro and in vivo , shows highly specific binding to the SH3 domain of phospholipase C-gamma and atleast one SH3 domain in the adapter protein Nck . These results raise the possibility of an additional level of regulation of the PAK family in vivo .