Protein phosphorylation is a regulatory mechanism for O6-alkylguanine-DNA alkyltransferase in human brain tumor cells.

Published

Journal Article

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.

Full Text

Duke Authors

Cited Authors

  • Srivenugopal, KS; Mullapudi, SR; Shou, J; Hazra, TK; Ali-Osman, F

Published Date

  • January 2000

Published In

Volume / Issue

  • 60 / 2

Start / End Page

  • 282 - 287

PubMed ID

  • 10667577

Pubmed Central ID

  • 10667577

Electronic International Standard Serial Number (EISSN)

  • 1538-7445

International Standard Serial Number (ISSN)

  • 0008-5472

Language

  • eng