Structural Basis for Avoidance of Promutagenic DNA Repair by MutY Adenine DNA Glycosylase

The highly mutagenic A: oxoG (8-oxoguanine) base pair in DNA most frequently arises by aberrant replication of the primary oxidative lesion C:oxoG. This lesion is particularly insidious because neither of its constituent nucleobases faithfully transmit genetic information from the original C:G base pair. Repair of A: oxoG is initiated by adenine DNA glycosylase, which catalyzes hydrolytic cleavage of the aberrant A nucleobase from the DNA backbone. These enzymes, MutY in bacteria and MUTYH in humans, scrupulously avoid processing of C: oxoG because cleavage of the C residue in C: oxoG would actually promote mutagenic conversion to A: oxoG. Here we analyze the structural basis for rejection of C: oxoG by MutY, using a synthetic crystallography approach to capture the enzyme in the process of inspecting the C: oxoG anti-substrate, with which it ordinarily binds only fleetingly. We find that MutY uses two distinct strategies to avoid presentation of C to the enzyme active site. Firstly, MutY possesses an exo-site that serves as a decoy for C, and secondly, repulsive forces with a key active site residue prevent stable insertion of C into the nucleobase recognition pocket within the enzyme active site.