PAK promotes morphological changes by acting upstream of Rac

The serine/threonine kinase p21-activated kinase (PAK)has been implicated as a downstream effector of the small GTPases Pac and Cdc42, While these GTPases evidently induce a variety of morphological changes, the role(s) of PAK remains elusive. Here we report that overexpression of beta PAK in PC12 cells induces a Pac phenotype, including cell spreading/ membrane ruffling, and increased lamellipodia formation at growth cones and shafts of nerve growth factor-induced neurites, These effects are still observed in cells expressing kinase-negative or Rac/ Cdc42 binding-deficient PAK mutants, indicating that kinase- and p21-binding domains are not involved. Furthermore, lamellipodia formation in all cell lines, including those expressing Pac binding-deficient PAK, is inhibited significantly by dominant-negative RacN17. Equal inhibition is achieved by blocking PAK interaction with the guanine nucleotide exchange factor PIX using a specific N-terminal PAK fragment, We conclude that PAK, via its N-terminal non-catalytic domain, acts upstream of Pac mediating lamellipodia formation through interaction with PIX.