TI - Cytoskeletal changes regulated by the PAK4 serine/threonine kinase are mediated by LIM kinase 1 and cofilin . AB - PAK4 is the recently identified member of the PAK family of serine/threonine kinases . PAK4 differs from other members of the PAK family in sequence and in many of its functions . Previously , we have shown that an important function of this kinase is to mediate the induction of filopodia in response to the Rho GTPase Cdc42 . Here we show that PAK4 also regulates the activity of the protein kinase LIM kinase 1 ( LIMK1 ) . PAK4 was shown to interact specifically with LIMK1 in binding assays . Immune complex kinase assays revealed that both wild-type and constitutively active PAK4 phosphorylated LIMK1 even more strongly than PAK1 , and activated PAK4 stimulated LIMK1's ability to phosphorylate cofilin . Immunofluorescence experiments revealed that PAK4 and LIMK1 cooperate to induce cytoskeletal changes in C2C12 cells . Furthermore , dominant negative LIMK1 and a mutant cofilin inhibited the specific cytoskeletal and cell shape changes that were induced in response to a recently characterized constitutively activated PAK4 mutant .