A HOMOLOGY MODEL OF THE 3-DIMENSIONAL STRUCTURE OF HUMAN O-6-ALKYLGUANINE-DNA ALKYLTRANSFERASE BASED ON THE CRYSTAL-STRUCTURE OF THE C-TERMINAL DOMAIN OF THE ADA PROTEIN FROM ESCHERICHIA-COLI

O-6-Alkylguanine-DNA alkyltransferase (EC 2.1.1.63) repairs O-6-alkylg uanine lesions in DNA. A homology model of the human protein (hAT) was built, based on the crystal structure of the C-terminal domain of the Ada protein, which carries out a similar repair in Escherichia coli. Sequence alignments of known O-6-alkylguanine-DNA alkyltransferases we re used to aid the model building using QUANTA and CHARMm software. De spite low homology in the N-terminal half (hAT residues 1-85), a well- defined topology over this region in Ada permitted successful modellin g. The C-terminal half of hAT (residues 92-207) was modelled almost en tirely by residue-for-residue superposition onto the Ada structure up to residue hAT(175). The model was solvated to a residue radius of 0.8 Angstrom and then minimized using CHARMm. This structural model was u sed to rationalize findings from site-directed mutagenesis experiments on hAT, to make further predictions on the relationship between struc ture and function for the alkyltransferase family of proteins, and to explain the specificity towards known small-molecule inhibitors of the protein.