title: Activation of an S6/H4 kinase (PAK 65) from human placenta by intramolecular and intermolecular autophosphorylation.
abstract: The S6/H4 kinase purified from human placenta catalyzes phosphorylation of the S6 ribosomal protein, histone H4, and myelin basic protein. In vitro activation of the p60 S6/H4 kinase requires removal of an autoinhibitory domain by mild trypsin digestion and autophosphorylation of the catalytic domain (p40 S6/H4 kinase). The two autophosphorylation/autoactivation sites contain the sequences SSMVGTPY (site 1) and SVIDPVPAPVGDSHVDGAAK (site 2). These sequences identify S6H4 kinase as the rac-activated PAK65 (Martin, G. A., Bollag, G., McCormick, F. and Abo, A. (1995) EMBO J. 14, 1971-1978). Site 1 phosphorylation is most rapid, but activation does not occur until site 2 is autophosphorylated. The site 1 phosphorylation occurs by an intramolecular mechanism whereas site 2 autophosphorylation occurs by an intermolecular mechanism. A model is proposed in which phosphorylation of sites 1 and 2 occurs sequentially. The model proposes that trypsin treatment of the inactive holoenzyme removes an inhibitory rac-binding domain which blocks MgATP access to the catalytic site. The pseudosubstrate domain at site 1 is autophosphorylated and subsequent bimolecular autophosphorylation at site 2 fully opens the catalytic site. Phosphorylation by a regulatory protein kinase may occur at site 2 in vivo.
authors.affiliation: Department of Biological Sciences, University of North Texas, Denton 76203-5018, USA.
authors.forename: G E
authors.initials: GE
authors.lastname: Benner
authors.forename: P B
authors.initials: PB
authors.lastname: Dennis
authors.forename: R A
authors.initials: RA
authors.lastname: Masaracchia
mesh_terms: D000595:Amino Acid Sequence; D004789:Enzyme Activation; D005260:Female; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D010449:Peptide Mapping; D010766:Phosphorylation; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D019893:Ribosomal Protein S6 Kinases; D013379:Substrate Specificity
chemical_list: D017346:Protein Serine-Threonine Kinases; D019893:Ribosomal Protein S6 Kinases