Msh2-Msh3 interferes with DNA metabolismin vivo

Abstract

Mismatch repair (MMR) is a highly conserved repair pathway essential to safeguard the genome from errors that occur during DNA replication. In Saccharomyces cerevisiae, two Msh complexes initiate MMR, either Msh2-Msh3 or Msh2-Msh6. These heterodimeric complexes recognize a number of DNA structures with varying affinities and play critical roles in DNA metabolism outside of post-replicative MMR; they contribute to genome stability through homologous recombination, double strand break repair (DSBR), and the DNA damage response. In contrast to its role in promoting genome stability, Msh2-Msh3 is associated with genome instability through trinucleotide repeat (TNR) expansions. Msh2-Msh3's non-canonical activity in TNR expansions appears to be an unfortunate consequence of its intrinsic ability to bind to distinct DNA structures. Msh2-Msh3 binds to a wide range of DNA structures, including branched intermediates with 3' or 5' ss/ds DNA. Whereas the binding to 3' ssDNA tails leads to repair in a specialized form of DSBR called 3' non-homologous tail removal (3' NHTR), in vitro evidence suggests that Msh2-Msh3 binding to 5' flap structures interferes with the cleavage activity of the structure-specific endonuclease Rad27 (Fen1 in humans). The endonucleolytic function of Rad27 promotes 5' ssDNA flap processing during Okazaki fragment maturation and long-patch base excision repair (LP-BER). Here we show that elevated Msh2-Msh3 levels interfere with both DNA replication and BER in vivo in an expression and ATP-dependent manner, consistent with the hypothesis that protein abundance and ATPase activities in Msh3 are key drivers of Msh2-Msh3-mediated genomic instability.