Crystal structure of the human O-6-alkylguanine-DNA alkyltransferase

The mutagenic and carcinogenic effects of simple alkylating agents are main ly due to O-6-alkylation of guanine in DNA, This lesion results in transiti on mutations, In both prokaryotic and eukaryotic cells, repair is effected by direct reversal of the damage by a suicide protein, O-6-alkylguanine-DNA alkyltransferase. The alkyltransferase removes the alkyl group to one of i ts own cysteine residues. However, this mechanism for preserving genomic in tegrity limits the effectiveness of certain alkylating anticancer agents, A high level of the alkyltransferase in many tumour cells renders them resis tant to such drugs, Here we report the X-ray structure of the human alkyltr ansferase solved using the technique of multiple wavelength anomalous dispe rsion, This structure explains the markedly different specificities towards various O-6-alkyl lesions and inhibitors when compared with the Escherichi a coli protein (for which the structure has already been determined), It is also used to interpret the behaviour of certain mutant alkyltransferases t o enhance biochemical understanding of the protein. Further examination of the various models proposed for DNA binding is also permitted, This structu re may be useful for the design and refinement of drugs as chemoenhancers o f alkylating agent chemotherapy.