p21-activated kinase PAK phosphorylates desmin at sites different from those for Rho-assooiated kinase

p21-activated kinase (PAK) and Rho-associated kinase (Rho-kinase) have been shown to induce Ca2+-independent contraction of smooth muscle. PAK-induced contraction of Triton-skinned smooth muscle correlates with increased phos phorylation of caldesmon and desmin, although the role of desmin phosphoryl ation has remained obscure. Here we report that desmin serves as an excelle nt substrate for PAK in vitro. PAK phosphorylated desmin in a GTP Cdc42/Rac -dependent manner. Phosphorylation of desmin by PAK dramatically inhibited its filament-forming ability. PAK phosphorylated mainly serine residues of the head domain of desmin, and the major phosphorylation sites differed fro m those for Rho-kinase. These results suggest that different site-specific phosphorylation of desmin via two divergent protein kinases downstream of R ho family GTPases would seem to increase the regulatory potential for organ ization of desmin filaments. (C) 2000 Academic Press.