HUMAN STE20 HOMOLOG HPAK1 LINKS GTPASES TO THE JNK MAP KINASE PATHWAY

Background: The Rho-related GTP-binding proteins Cdc42 and Rac1 have b een shown to regulate signaling pathways involved in cytoskeletal reor ganization and stress-responsive JNK (Jun N-terminal kinase) activatio n. However, to date, the GTPase targets that mediate these effects hav e not been identified. PAK defines a growing family of mammalian kinas es that are related to yeast Ste20 and are activated in vitro through binding to Cdc42 and Rac1 (PAK: p21 Cdc42-/Rac-activated kinase). Clue s to PAK function have come from studies of Ste20, which controls the activity of the yeast mating mitogen-activated protein (MAP) kinase ca scade, in response to a heterotrimeric G protein and Cdc42. Results: T o initiate studies of mammalian Ste20-related kinases, we identified a novel human PAK isoform, hPAK1. When expressed in yeast, hPAK1 was ab le to replace Ste20 in the pheromone response pathway. Chemical mutage nesis of a plasmid encoding hPAK1, followed by transformation into yea st, led to the identification of a potent constitutively active hPAK1 with a substitution of a highly conserved amino-acid residue (L107F) i n the Cdc42-binding domain. Expression of the hPAK1(L107F) allele in m ammalian cells led to specific activation of the Jun N-terminal kinase MAP kinase pathway, but not the mechanistically related extracellular signal-regulated MAP kinase pathway. Conclusions: These results demon strate that hPAK1 is a GTPase effector controlling a downstream MAP ki nase pathway in mammalian cells, as Ste20 does in yeast. Thus, PAK and Ste20 kinases play key parts in linking extracellular signals from me mbrane components, such as receptor-associated G proteins and Rho-rela ted GTPases, to nuclear responses, such as transcriptional activation.