Structural and immunological comparison of indigenous human O super(6)-methylguanine-DNA methyltransferase with that encoded by a cloned cDNA

O super(6)-Methylguanine-DNA methyltransferase, a ubiquitous and unusual DNA repair protein, eliminates mutagenic and cytotoxic O super(6)-alkylguanine from DNA by transferring the alkyl group to one of its cysteine residues in a second-order suicide reaction. This 22-kDa protein was immunoaffinity-purified to homogeneity from cultured human lymphoblasts (CEM-CCRF line) and compared with the O super(6)-methylguanine-DNA methyltransferase purified to homogeneity from Escherichia coli) expressing a cloned human cDNA. A unique cysteine residue at position 145 was identified as the methyl acceptor site.