The active conformation of the PAK1 kinase domain

The p21-activated kinases (PAKs) participate in cytoskeletal control networks, downstream of Rho-family GTPases. A structure of PAKII in an autoregulated, off state showed that a regulatory region, N-terminal to the kinase domain, forces the latter into an inactive conformation, prevents phosphorylation of Thr423 in the activation loop, and promotes dimerization. We have now determined structures at 1.8 angstrom resolution for the free PAK1 kinase domain, with a mutation in the active site that blocks enzymatic activity, and for the same domain with a phosphomimetic mutation in the activation loop. The two very similar structures show that even in the absence of a phosphorylated Thr423, the kinase has an essentially active conformation. When Cdc42 binds the regulatory region and dissociates the dimer, PAK1 will be in an intermediate-active state, with a capacity to phosphorylate itself or other substrates even prior to modification of its activation loop.