Plexin-bound Rac1 binds to and stimulates the kinase activity of PAK. PAK dimers are arranged in head-to-tail fashion, in which the catalytic domain binds the kinase inhibitory (KI) domain and is supported by associated PAK-interacting exchange factor (PIX) dimers. Upon Rac1 binding the kinase undergoes conformational change that allows autophosphorylation. Phosphorylation of serine residues disables the KI-domain-kinase interaction and thereby reduces the affinity of PIX.