Characterization of the substrate specificity of p21-activated protein kinase (PAK) I

PAK I (p21-activated protein kinase) is a serine/threonine kinase with a molecular mass of 58,000 and has been shown to be a cytostatic protein kinase involved in maintaining cells in a non-dividing state (Rooney,R.D. et al, J. Biol. Chem. 271, 21498-21504, 1996). PAK I can be activated by autophosphorylation upon binding with Cdc42 in the presence of GTPγS or by limited digestion with trypsin to produce an active polypeptide of 37 kDa which consists of the catalytic domain and part of the regulatory domain (Jakobi, R. et al J. Biol. Chem. 271, 6206-6211, 1996). These two activated forms have been characterized and compared. Autophosphorylation of p37 stimulates activity and is on serine and threonine residues. Cdc42 stimulates autophosphorylation of P58 on serine. The K_m values for mixed histone IIAS for the two forms of activated enzyme are similar. Both forms utilize Mg⁺² and Mn⁺² at an optimal concentration of 6 mM and 1 mM, respectively. Both forms phosphorylate the same substrates including histone H4, histone H2B, and myosin light chain.Using synthetic peptides based on the phosphorylation site in the Avian sarcoma virus nuclear capsid protein NC, the recognition determinant of PAK I has been determined to be basic amino acids on the Nterminal side of the phosphorylation site.