A novel zinc snap motif conveys structural stability to 3-methyladenine DNA glycosylase I.

The Escherichia coli 3-methyladenine DNA glycosylase I (TAG) is a DNA repair enzyme that excises 3-methyladenine in DNA and is the smallest member of the helix-hairpin-helix (HhH) superfamily of DNA glycosylases. Despite many studies over the last 25 years, there has been no suggestion that TAG was a metalloprotein. However, here we establish by heteronuclear NMR and other spectroscopic methods that TAG binds 1 eq of Zn2+ extremely tightly. A family of refined NMR structures shows that 4 conserved residues contributed from the amino- and carboxyl-terminal regions of TAG (Cys4, His17, His175, and Cys179) form a Zn2+ binding site. The Zn2+ ion serves to tether the otherwise unstructured amino- and carboxyl-terminal regions of TAG. We propose that this unexpected "zinc snap" motif in the TAG family (CX(12-17)HX(approximately 150)HX(3)C) serves to stabilize the HhH domain thereby mimicking the functional role of protein-protein interactions in larger HhH superfamily members.