Mechanism of in vivo activation of the MutLγ-Exo1 complex for meiotic crossover formation

Crossovers generated during the repair of programmed double-strand breaks (DSBs) are essential for fertility to allow accurate homolog segregation during the first meiotic division. Most crossovers arise through the asymmetric cleavage of double-Holliday junction (dHJ) intermediates by the MutLγ endonuclease (Mlh1-Mlh3) and an elusive non-catalytic function of Exo1, and require the Cdc5/PLK1 kinase. Here we show in budding yeast that MutLγ forms a constitutive complex with Exo1, and in meiotic cells transiently contacts the upstream MutSγ (Msh4-Msh5) heterodimer. Once recombination intermediates are committed to the crossover repair pathway, MutLγ-Exo1 associates with sites of DSB hotspots, and Exo1 recruits Cdc5 through a direct interaction that is required for activating MutLγ and crossover formation. Exo1 therefore serves as a non-catalytic matchmaker between Cdc5 and MutLγ. We further show that in vivo, MutLγ associates with the vast majority of DSB hotspots, but at a lower frequency near centromeres, consistent with a strategy to reduce at-risk crossover events in these regions. Our data highlight the tight temporal and spatial control of the activity of this constitutive, potentially harmful, nuclease.