SUMO Modifies Hop1-HORMAD1 at Distinct Sites to Control Initiation of Meiotic Recombination and Facilitate Inter-homolog Crossing Over

Meiosis is the conduit of heredity, precisely halving chromosome ploidy so that parents can make equal contributions to their offspring while maintaining a stable ploidy through successive generations. Pairing and recombination between homologous chromosomes ensures their accurate segregation at the first meiotic division. Meiotic recombination is initiated by programmed DNA double-stranded breaks (DSBs) that are repaired between homologous chromosomes. A subset of recombination events results in crossovers that connect homologs to enable stable bipolar connection to the spindle apparatus and thereby promote their accurate segregation. Crossovers take place in the context of a zipper-like structure called the synaptonemal complex (SC), which is comprised of the two homologous chromosome axes linked by a central region of densely packed transverse filaments. The conserved meiotic protein Hop1HORMAD is a dynamic component of the chromosome axes that mediates interactions between homologous chromosomes. Hop1HORMAD facilitates DSB formation, promotes preferential DNA strand exchange between homologous chromosomes rather than sister chromatids, and mediates checkpoint signaling by activating the effector kinase, Mek1CHK2. As synapsis ensues, Hop1HORMAD dissociates from homolog axes to attenuate both DSB formation and Mek1CHK2 activation allowing meiotic prophase to progress and crossing over to be completed.Here I show that all known functions of yeast Hop1HORMAD are regulated via post-translational modification by the small ubiquitin-like modifier, SUMO. Mass spectrometry analysis revealed that Hop1 can be modified by SUMO at no less than 17 sites. SUMOylation is dependent on the Hop1 binding partner, Red1, and occurs prior to DSB formation but is sustained throughout meiotic prophase I. Analysis of SUMOylation-defective hop1 alleles reveals that Hop1 SUMOylation is necessary for its localization to chromosomes and its function in DSB formation, and, consequently, for chromosome synapsis and crossing over. I further show that only 2 of the 17 identified SUMOylation sites (K556 and K590) are necessary and sufficient for the DSB function of Hop1. Ongoing analysis aims to address the molecular mechanisms by which Hop1 SUMOylation activates DSB formation. We hypothesize that SUMO modification of Hop1 may facilitate interaction with other DSB factors.Post-DSB roles of Hop1HORMAD are initiated by phosphorylation catalyzed by the PI3K-like kinases Tel1ATM and Mec1ATR, which leads to activation of Mek1. SUMOylation at one or more of the 15 sites that are not required for the DSB function of Hop1 was shown to facilitate the inter-homolog recombination and checkpoint functions of Hop1. Future experiments will investigate how Hop1 SUMOylation promotes its inter-homolog template choice and checkpoint signaling functions