THE ROLE OF 2 CONSERVED AMINO-ACIDS, GLUTAMINE-90 AND ASPARAGINE-137, IN O-6-METHYLGUANINE-DNA METHYLTRANSFERASE STABILITY, ACTIVITY AND SUBSTRATE-SPECIFICITY

To assess the possibility that two conserved amino acids (glutamine 90 and asparagine 137) in O-6-methylguanine-DNA methyltransferase (MGMT) are involved in protein-substrate contact and/or discrimination between favored and non-favored substrates, families of proteins mutant at these two sites were expressed in alkyltransferase-deficient bacteria and analyzed for stability, ability to repair O-6-methylguanine (MG)-containing DNA, and ability to differentially repair a preferred (MG-containing DNA) versus a non-preferred (free base MG) substrate. All seven proteins mutant at glutamine 90 (except a proline mutant) were stable in bacteria and repaired MG-containing DNA (> 50% of wild-type levels). A representative glutamine 90 mutant protein was not, however, significantly different from the wild-type protein in the preferential repair of MG-containing DNA versus MG free base. Of eight proteins mutant at asparagine 137, only glutamine and serine mutants repaired MG-containing DNA to any degree (8.5% and 0.8% of wild-type respectively) and only the glutamine mutant protein was detectable in bacterial sonicates by Western blot analysis. Alanine and leucine mutant alkyl-transferases, inactive and unstable as non-fusion proteins, could, however, be stably expressed in bacteria as glutathione S-transferase fusion proteins, although the proteins were still inactive in repair. These results suggest that while glutamine 90 has no direct role in MG-DNA methyltransferase-mediated repair or free base/lesioned DNA substrate specificity, asparagine 137 is important in both the stability and activity of the protein and may contribute to the formation or function of the active site of the protein.