Abstract
Mismatch repair (MMR) is a highly conserved DNA repair pathway that recognizes mispairs that occur spontaneously during DNA replication and coordinates their repair. In Saccharomyces cerevisiae, Msh2-Msh3 and Msh2-Msh6 initiate MMR by recognizing and binding insertion or deletion (in/del) loops up to ∼17 nucleotides (nt.) and base-base mispairs, respectively; the 2 complexes have overlapping specificity for small (1-2 nt.) in/dels. The DNA-binding specificity for the 2 complexes resides in their respective mispair binding domains (MBDs) and has distinct DNA-binding modes. Msh2-Msh3 also plays a role in promoting CAG/CTG trinucleotide repeat (TNR) expansions, which underlie many neurodegenerative diseases such as Huntington's disease and myotonic dystrophy type 1. Models for Msh2-Msh3's role in promoting TNR tract expansion have invoked its specific DNA-binding activity and predict that the TNR structure alters its DNA binding and downstream activities to block repair. Using a chimeric Msh complex that replaces the MBD of Msh6 with the Msh3 MBD, we demonstrate that Msh2-Msh3 DNA-binding activity is not sufficient to promote TNR expansions. We propose a model for Msh2-Msh3-mediated TNR expansions that requires a fully functional Msh2-Msh3 including DNA binding, coordinated ATP binding, and hydrolysis activities and interactions with Mlh complexes that are analogous to those required for MMR.