Msh2-Msh3 DNA-binding is not sufficient to promote trinucleotide repeat expansions in Saccharomyces cerevisiae.

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 deletion loops (in/dels) up to ∼ 17 nucleotides (nt.) and base-base mispairs, respectively; the two complexes have overlapping specificity for small (1-2 nt.) in/dels. The DNA-binding specificity for the two complexes resides in their respective mispair binding domains (MBDs) and have 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 tracts 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.