The mismatch repair endonuclease MutL alpha tethers duplex regions of DNA together and relieves DNA torsional tension

摘要

In eukaryotic mismatch repair, MutS homologs recognize mismatches and recruit the MutL alpha endonuclease which introduces a nick in the newly replicated, error-containing DNA strand. The nick occurs in response to the mismatch, but at a site up to several hundred base pairs away. The MutL alpha nick promotes mismatch excision by an exonuclease (Exo1) or removal by the strand displacement activity of a DNA polymerase which may work in conjunction with a flap endonuclease. Models have suggested that MutL homolog endonucleases form oligomeric complexes which facilitate and are activated by strand capture mechanisms, although such models have never been explicitly tested. We present evidence that the mismatch repair MutL alpha endonuclease is activated by DNA-DNA associations and that it can use this property to overcome DNA torsional barriers. Using DNA ligation and pull-down experiments, we determined that the MutL alpha endonuclease associates two DNA duplexes. Using nuclease assays, we determined that this activity stimulates MutL alpha's endonuclease function. We also observe that MutL alpha enhances a topoisomerase without nicking the DNA itself. Our data provide a mechanistic explanation for how MutL proteins interact with DNA during mismatch repair, and how MutL homologs participate in other processes, such as recombination and trinucleotide repeat expansions.