A NOVEL REGULATOR OF P21-ACTIVATED KINASES

Proteins of the pal-activated kinase (Pak) family have been implicated in the regulation of gene expression, cytoskeletal architecture, and apoptosis, Although the ability of Cdc42 and Pac GTPases to activate P ak is well established, relatively little else is known about Pak regu lation or the identity of Pak cellular targets. Here we report the ide ntification of two closely related Pak3-binding proteins, possibly ari sing from alternative splicing, designated p50 and p85(Cool-1) (cloned out of library), Both isoforms of Cool contain a Src homology 3 domai n that directly mediates interaction with Pak3 and tandem Dbl homology and pleckstrin homology domains. Despite the presence of the Dbl homo logy-pleckstrin homology motif, a characteristic of Rho family activat ors, activation of Cdc42 or Rac by Cool is not detectable. Instead bin ding of p50(Cool-1), but not p85(Cool-1), to Pak3 represses its activa tion by upstream activators such as the Dbl oncoprotein, indicating a novel mechanism of regulation of Pak signaling.