A structural and functional analysis of mismatch repair proteins in meiosis

A number of the mismatch repair proteins facilitate recombination during meiosis. Among these are the MutS homologues, Msh4p and Msh5p, the MutL homologues M1h1p and M1h3p, and the exonuclease Exo1p. Msh4p and Msh5p are meiosis specific proteins, which function to promote crossing over, although the mechanism is unknown. M1h1p and M1h3p are involved in the same pathway, but are thought to act later during meiosis as the meiotic phenotypes of m1h1Delta and m1h3Delta are less severe than those of msh4Delta or msh5Delta. Structure and function studies of M1h1p have begun to elucidate its roles in mitosis and meiosis. In particular it has been shown that ATP binding by M1h1p is important for both its mitotic and meiotic roles, whereas ATP hydrolysis is only partially required for mitotic mismatch repair (MMR) and not at all for meiotic recombination.;In this study it has been demonstrated that ATP binding by M1h3p is essential for its meiotic crossover function, suggesting that the ATP-mediated conformational change is essential for function and/or interactions. In contrast, ATP hydrolysis is not required for crossing over. However, when both M1h1p and M1h3p are unable to hydrolyse ATP, a defect was observed in crossing over suggesting that some recycling of the M1h1p-M1h3p complex is necessary for functionality.;Exo1p reduces processing of the HIS4 DSB and also crossing over to approximately 50 % of the levels observed in a wild-type. Here we show that both the 5'-' exonuclease and the 5' flap endonuclease activities of Exolp are required to maintain crossing over. We propose that reduced resection of DSBs produces less stable strand invasions, which consequently reduces crossing over. However, not all DSBs are dependent on Exol fro processing. Additionally, Exo1p is required for the removal of DNA flaps produced from over-replication of DNA during meiotic recombination.