A phosphomimetic mutation (S223E) bypasses the requirement for GTPases in PAK1 activation, whereas the constitutive activity of the PAK1 mutant (PAK1 H83,86L), postulated to mimic GTPase-induced structural changes, is abolished by inhibition of S223 phosphorylation. This phosphorylation appears to be essential for autophosphorylation at specific residues and overall activity of PAK1. In this paper, we show that this change is necessary but not sufficient to activate PAK1 and that it is, rather, required for CK2-dependent PAK1 S223 phosphorylation that converts a monomeric PAK1 into a catalytically active form. Activation of the p21-activated kinase 1 (PAK1) is achieved through a conformational change that converts an inactive PAK1 dimer to an active monomer.
