title: Protein phosphorylation is a regulatory mechanism for O6-alkylguanine-DNA alkyltransferase in human brain tumor cells.
abstract: The biochemical regulation of human O6-alkylguanine-DNA alkyltransferase (AGT), which determines the susceptibility of normal tissues to methylating carcinogens and resistance of tumor cells to many alkylating agents, is poorly understood. We investigated the regulation of AGT by protein phosphorylation in a human medulloblastoma cell line. Incubation of cell extracts with [gamma-32P]ATP resulted in Mg(2+)-dependent phosphorylation of the endogenous AGT. Immunoprecipitation after exposure of the cells to 32P-labeled inorganic phosphate showed that AGT exists as a phosphoprotein under physiological conditions. Western analysis and chemical stability studies showed the AGT protein to be phosphorylated at tyrosine, threonine, and serine residues. Purified protein kinase A (PKA), casein kinase II (CK II), and protein kinase C (PKC) phosphorylated the recombinant AGT protein with a stoichiometry of 0.15, 0.28, and 0.44 (mol phosphate incorporated/mol protein), respectively. Residual phosphorylation of the endogenous AGT by the PKs present in cell homogenates and phosphorylation of the recombinant AGT by purified serine/threonine kinases, PKA, PKC, and CK II reduced AGT activity by 30-65%. Conversely, dephosphorylation of cell extracts by alkaline phosphatases stimulated AGT activity. We also identified consensus phosphorylation motifs for many cellular kinases, including PKA and CK II in the AGT protein. These data provide the first and conclusive evidence of AGT phosphorylation and suggest that reversible phosphorylation may control the activity of this therapeutically important DNA repair protein in human normal and cancer cells.
authors.affiliation: Department of Neurosurgery, The University of Texas M.D. Anderson Cancer Center, Houston 77030, USA. ksrivenu@mdanderson.org
authors.forename: K S
authors.initials: KS
authors.lastname: Srivenugopal
authors.forename: S R
authors.initials: SR
authors.lastname: Mullapudi
authors.forename: J
authors.initials: J
authors.lastname: Shou
authors.forename: T K
authors.initials: TK
authors.lastname: Hazra
authors.forename: F
authors.initials: F
authors.lastname: Ali-Osman
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D001932:Brain Neoplasms / Q000201:enzymology; D047390:Casein Kinase II; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D006706:Homeostasis; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008969:Molecular Sequence Data; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000737:chemistry / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D010710:Phosphates; D011994:Recombinant Proteins; D000255:Adenosine Triphosphate; D019853:O(6)-Methylguanine-DNA Methyltransferase; D047390:Casein Kinase II; D017346:Protein Serine-Threonine Kinases; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D008274:Magnesium