title: Human Ste20 homologue hPAK1 links GTPases to the JNK MAP kinase pathway.
abstract: BACKGROUND
  The Rho-related GTP-binding proteins Cdc42 and Rac1 have been shown to regulate signaling pathways involved in cytoskeletal reorganization and stress-responsive JNK (Jun N-terminal kinase) activation. However, to date, the GTPase targets that mediate these effects have not been identified. PAK defines a growing family of mammalian kinases that are related to yeast Ste20 and are activated in vitro through binding to Cdc42 and Rac1 (PAK: p21 Cdc42-/Rac-activated kinase). Clues to PAK function have come from studies of Ste20, which controls the activity of the yeast mating mitogen-activated protein (MAP) kinase cascade, in response to a heterotrimeric G protein and Cdc42.
  
  
  RESULTS
  To initiate studies of mammalian Ste20-related kinases, we identified a novel human PAK isoform, hPAK1. When expressed in yeast, hPAK1 was able to replace Ste20 in the pheromone response pathway. Chemical mutagenesis of a plasmid encoding hPAK1, followed by transformation into yeast, led to the identification of a potent constitutively active hPAK1 with a substitution of a highly conserved amino-acid residue (L107F) in the Cdc42-binding domain. Expression of the hPAK1(L107F) allele in mammalian 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 demonstrate that hPAK1 is a GTPase effector controlling a downstream MAP kinase pathway in mammalian cells, as Ste20 does in yeast. Thus, PAK and Ste20 kinases play key parts in linking extracellular signals from membrane components, such as receptor-associated G proteins and Rho-related GTPases, to nuclear responses, such as transcriptional activation.
authors.affiliation: Department of Molecular and Cellular Biology, Harvard University, 7 Divinity Avenue, Cambridge, Massachusetts 02138, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Brown
authors.forename: L
authors.initials: L
authors.lastname: Stowers
authors.forename: M
authors.initials: M
authors.lastname: Baer
authors.forename: J
authors.initials: J
authors.lastname: Trejo
authors.forename: S
authors.initials: S
authors.lastname: Coughlin
authors.forename: J
authors.initials: J
authors.lastname: Chant
mesh_terms: D000483:Alleles; D000595:Amino Acid Sequence; D001483:Base Sequence; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000378:metabolism; D003001:Cloning, Molecular; D018076:DNA, Complementary; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D008969:Molecular Sequence Data; D016296:Mutagenesis; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins; D015398:Signal Transduction; D020764:cdc42 GTP-Binding Protein; D054462:p21-Activated Kinases
chemical_list: D018797:Cell Cycle Proteins; D018076:DNA, Complementary; D029701:Saccharomyces cerevisiae Proteins; C094326:PRK1 protein, S cerevisiae; C515903:PAK1 protein, human; D017346:Protein Serine-Threonine Kinases; D054462:p21-Activated Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D020764:cdc42 GTP-Binding Protein