Affinity purification and characterization of humanO6-alkylguanine-DNA aikyltransferase complexed with BCNU-treated, synthetic oligonudeotide

Abstract
Tumor cells resistant to chloroethylnitrosourea (CENU) therapy contain high levels of O6-alkylguanine DNA-alkytransferase (GATase), a DNA repair enzyme that aborts DNA interstrand cross-linking by removing CENU-induced O6-alkylguanine adducts. Because the transferase binds covalently to CENU-treated oligonucleotides, we reacted partially purified GATase from cultured human lymphoblasts with a BCNU-trated, 35S-5''-end-labeled, synthetic oligonucleotide designed to have a polyadenylated 3'' terminus. Immunoprobing Western blots of this reaction mixture with GATase-specific monoclonal antibody indicated that 25-30% of the transferase became complexed. We purified this complex by affinity chromatography with oligo(dT) cellulose, recovering homogenous material that appeared as a discrete 35-kDa Coomassie blue or sliver-stained band after SDS-polyacrylamide gel electrophoresis. Autoradiography and Western immunoblotting confirmed that this band contained both the radiolabeled oligonucleotide and the GATase protein.

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