Cytoskeletal changes regulated by the PAK4 serine/threonine kinase are mediated by LIM kinase 1 and cofilin

PAK4 is the most recently identified member of the PAR family of serine/thr eonine kinases. PAK4 differs from other members of the PAK family in sequen ce and in many of its functions. Previously, we have shown that an importan t function of this kinase is to mediate the induction of filopodia in respo nse to the Rho GTPase Cdc42. Here we show that PAR4 also regulates the acti vity of the protein kinase LIM kinase 1 (LIMK1). PAK4 was shown to interact specifically with LIMK1 in binding assays. Immune complex kinase assays re vealed that both wild-type and constitutively active PAK4 phosphorylated LI MK1 even more strongly than PAK1, and activated PAK4 stimulated LIMK1's abi lity to phosphorylate cofilin. Immunofluorescence experiments revealed that PAK4 and LIMK1 cooperate to induce cytoskeletal changes in C2C12 cells. Fu rthermore, dominant negative LIMK1 and a mutant cofilin inhibited the speci fic cytoskeletal and cell shape changes that were induced in response to a recently characterized constitutively activated PAK4 mutant.