The oxidation product of 2′-deoxyguanosine, 7,8-dihydro-8-oxo- 2′-deoxyguanosine (OG), produces G:C to T:A transversion mutations. The Escherichia coli base excision repair glycosylase MutY plays an important role in preventing OG-associated mutations by removing adenines misincorporated opposite OG lesions during DNA replication. Recently, biallelic mutations in the human MutY homologue (hMYH) have been correlated with the development of colorectal cancer. The two most common mutations correspond to two single amino acid substitutions in the hMYH protein: Y165C and G382D [Al-Tassan, N., et al. (2002) Nat. Genet. 30, 227-232]. Previously, our laboratory analyzed the adenine glycosylase activity of the homologous variant E. coli MutY enzymes, Y82C and G253D [Chmiel, N. H., et al. (2003) J. Mol. Biol. 327, 431-443]. This work demonstrated that both variants have a reduced adenine glycosylase activity and affinity for substrate analogues compared to wild-type MutY. Recent structural work on Bacillus stearothermophilus MutY bound to an OG:A mismatch-containing duplex indicates that both residues aid in recognition of OG [Fromme, J. C., et al. (2004) Nature 427, 652-656]. To determine the extent with which Tyr 82 and Gly 253 contribute to catalysis of adenine removal by E. coli MutY, we made a series of additional modifications in these residues, namely, Y82F, Y82L, and G253A. When the substrate analogue 2′-deoxy-2′-fluoroadenosine (FA) in duplex paired with G or OG is used, both Y82F and G253A showed reduced binding affinity, and G253A was unable to discriminate between OG and G when paired with FA. Additionally, compromised glycosylase activity of Y82F, Y82C, and G253A MutY was observed using the nonoptimal G:A substrate, or at low reaction temperatures. Interestingly, adenine removal from an OG:A-containing DNA substrate by Y82C MutY was also shown to be extremely sensitive to the NaCl concentration. The most surprising result was the remarkably similar activity of Y82L MutY to the WT enzyme under all conditions examined, indicating that a leucine residue may effectively replace tyrosine for intercalation at the OG:A mismatch. The results contained herein provide further insight regarding the intricate roles of Tyr 82 and Gly 253 in the OG recognition and adenine excision functions of MutY.
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