Reciprocal signaling between heterotrimeric G proteins and the p21-stimulated protein kinase.
p21-activated protein kinase (PAK)-1 phosphorylated Galpha(z), a member of the Galpha(i) family that is found in the brain, platelets, and adrenal medulla. Phosphorylation approached 1 mol of phosphate/mol of Galpha(z) in vitro. In transfected cells, Galpha(z) was phosphorylated both by wild-type PAK1 when stimulated by the GTP-binding protein Rac1 and by constitutively active PAK1 mutants. In vitro, phosphorylation occurred only at Ser(16), one of two Ser residues that are the major substrate sites for protein kinase C (PKC). PAK1 did not phosphorylate other Galpha subunits (i1, i2, i3, o, s, or q). PAK1-phosphorylated Galpha(z) was resistant both to RGSZ1, a G(z)-selective GTPase-activating protein (GAP), and to RGS4, a relatively nonselective GAP for the G(i) and G(q) families of G proteins. Phosphorylation of Ser(27) by PKC did not alter sensitivity to either GAP. The previously described inhibition of G(z) GAPs by PKC is therefore mediated by phosphorylation of Ser(16). Phosphorylation of either Ser(16) by PAK1 or Ser(27) by PKC decreased the affinity of Galpha(z) for Gbetagamma; phosphorylation of both residues by PKC caused no further effect. PAK1 thus regulates Galpha(z) function by attenuating the inhibitory effects of both GAPs and Gbetagamma. In this context, the kinase activity of PAK1 toward several protein substrates was directly inhibited by Gbetagamma, suggesting that PAK1 acts as a Gbetagamma-regulated effector protein. This inhibition of mammalian PAK1 by Gbetagamma contrasts with the stimulation of the PAK homolog Ste20p in Saccharomyces cerevisiae by the Gbetagamma homolog Ste4p/Ste18p.
Wang
J
J
Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9041, USA.
Frost
J A
JA
Cobb
M H
MH
Ross
E M
EM
D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D020930:MAP Kinase Kinase Kinases; D004676:Myelin Basic Protein / Q000378:metabolism; D010766:Phosphorylation; D011485:Protein Binding; D011493:Protein Kinase C / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D020710:RGS Proteins / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D012694:Serine / Q000378:metabolism; D015398:Signal Transduction; D013379:Substrate Specificity; D054462:p21-Activated Kinases; D020830:rac1 GTP-Binding Protein / Q000378:metabolism
D047908:Intracellular Signaling Peptides and Proteins; D004676:Myelin Basic Protein; D020710:RGS Proteins; D029701:Saccharomyces cerevisiae Proteins; D012694:Serine; D017346:Protein Serine-Threonine Kinases; D054462:p21-Activated Kinases; D011493:Protein Kinase C; D020930:MAP Kinase Kinase Kinases; C078754:STE20 protein, S cerevisiae; D020962:Heterotrimeric GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein