MSH-MLH complexes formed at a DNA mismatch are disrupted by the PCNA sliding clamp

In the yeast Saccharomyces cerevisiae, mismatch repair (MMR) is initiated b y the binding of heterodimeric MutS homolog (MSH) complexes to mismatches t hat include single nucleotide and loop insertion/deletion mispairs. In in v itro experiments, the mismatch binding specificity of the MSH2-MSH6 heterod imer is eliminated if ATP is present. However, addition of the MutL homolog complex MLH1-PMS1 to binding reactions containing MSH2-MSH6, ATP, and mism atched substrate results in the formation of a stable ternary complex. The stability of this complex suggests that it represents an intermediate in MM R that is subsequently acted upon by other MMR factors. In support of this idea, we found that the replication processivity factor proliferating cell nuclear antigen (PCNA), which plays a critical role in MMR at step(s) prior to DNA resynthesis, disrupted preformed ternary complexes. These observati ons, in conjunction with experiments performed with streptavidin end-blocke d mismatch substrates, suggested that PCNA interacts with an MSH-MLH comple x formed on DNA mispairs. (C) 2001 Academic Press.