Catalytic domain of the Protein Serine/Threonine Kinase, p21-activated kinase 3Serine/threonine kinases (STKs), p21-activated kinase (PAK) 3, catalytic (c) domain. STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. The PAK subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. PAKs are Rho family GTPase-regulated kinases that serve as important mediators in the function of Cdc42 (cell division cycle 42) and Rac. PAKs from higher eukaryotes are classified into two groups (I and II), according to their biochemical and structural features. PAK3 belongs to group I. Group I PAKs contain a PBD (p21-binding domain) overlapping with an AID (autoinhibitory domain), a C-terminal catalytic domain, SH3 binding sites and a non-classical SH3 binding site for PIX (PAK-interacting exchange factor). PAK3 is highly expressed in the brain. It is implicated in neuronal plasticity, synapse formation, dendritic spine morphogenesis, cell cycle progression, neuronal migration, and apoptosis. Inactivating mutations in the PAK3 gene cause X-linked non-syndromic mental retardation, the severity of which depends on the site of the mutation.