Biochemical and structural studies reveal the substrate specificity and catalytic mechanism of MYG1 as a two-metal ion-dependent 3′→5′ exonuclease

Nucleases are a class of enzymes that specifically cleave nucleic acids in all living organisms. They play crucial roles in essential biological processes, including the regulation of gene expression, DNA damage repair, and RNA processing and degradation. MYG1 (melanocyte proliferating gene 1) is a highly conserved eukaryotic protein that exhibits 3′→5′ exonuclease activity. This study systematically characterizes the enzymatic properties of MYG1 and determines its structures in complexes with metal ions and various mono- and poly-(deoxy)nucleotides. The functional roles of key residues involved in metal ion binding and substrate binding in the catalytic reaction are examined through site-directed mutagenesis, enzymatic activity assay, and structure determination. Our biochemical and structural data together demonstrate that MYG1 is a Mn²+- or Mg²+-dependent 3′→5′ exonuclease capable of cleaving a variety of nucleic acids with different structures. It exhibits the highest activity for single-stranded RNA and a nucleotide preference for U in single-stranded RNA and dT in single-stranded DNA. Mechanistically, MYG1 functions as a dimer, with the active site formed by the catalytic domain of monomer 1 and the substrate-binding domain of monomer 2, and cleaves nucleic acids through a two-metal ion-mediated catalytic mechanism. These findings establish a molecular basis for further investigations into the biological functions and molecular mechanisms of MYG1 within cells and its potential roles in human diseases.