A GTPase-independent Mechanism of p21-activated Kinase Activation

p21-activated kinases (PAKs) are serine/threonine kinases that have been identified as targets for the small GTPases Rac and Cdc42. PAKs have been implicated in cytoskeletal regulation, stimulation of mitogen-activated protein kinase cascades, and in control of the phagocyte NADPH oxidase. Membrane targeting of PAK1 induced increased kinase activity in a GTPase-independent manner, suggesting that other mechanisms for PAK regulation exist. We observed concentration- and time-dependent activation of PAK1 by sphingosine and several related long chain sphingoid bases but not by ceramides or a variety of other lipids. Although phospholipids were generally ineffective, phosphatidic acid and phosphatidylinositol also had stimulatory effects on PAK1. Lipid stimulation induced a similar level of PAK1 activity as did stimulation by GTPases, and the patterns of PAK1 autophosphorylation determined after partial tryptic digestion and two-dimensional peptide analysis were similar with each class of activator. Lipid stimulation of PAK1 activity was dependent upon intact PAK kinase activity, as indicated by studies with a kinase-dead PAK1 mutant. Treatment of COS-7 cells expressing wild type PAK1 with sphingosine, fumonisin B, or sphingomyelinase, all of which are able to elevate the levels of free sphingosine, induced increased activity of PAK1 as determined using a p47 phox peptide substrate. Studies using PAK1 mutants suggest that lipids act at a site overlapping or identical to the GTPase-binding domain on PAK. The inactive sphingosine derivative N,N -dimethylsphingosine was an effective inhibitor of PAK1 activation in response to either sphingosine or Cdc42. Our results demonstrate a novel GTPase-independent mechanism of PAK activation and, additionally, suggest that PAK(s) may be important mediators of the biological effects of sphingolipids.