The crossover function of MutSγ is activated via Cdc7-dependent stabilization of Msh4

Abstract

The MutSγ complex, Msh4-Msh5, binds DNA joint-molecule (JM) intermediates during homologous recombination to promote crossing over and accurate chromosome segregation at the first division of meiosis. MutSγ facilitates the formation and biased resolution of crossoverspecific JM intermediates called double Holliday junctions. Here we show that these activities are governed by regulated proteasomal degradation. MutSγ is initially inactive for crossing over due to an N-terminal degron on Msh4 that renders it unstable. Activation of MutSγ requires the Dbf4-dependent kinase, Cdc7 (DDK), which directly phosphorylates and thereby neutralizes the Msh4 degron. Phosphorylated Msh4 is chromatin bound and requires DNA strand exchange and chromosome synapsis, implying that DDK specifically targets MutSγ that has already bound nascent JMs. Our study establishes regulated protein degradation as a fundamental mechanism underlying meiotic crossover control. the differentiation of crossover sites manifests as the selective retention and accumulation of specific recombination factors. One such factor is MutSγ, a heterodimer of Msh4 and Msh5, two homologs of the DNA mismatch-recognition factor MutS (Manhart and Alani, 2016;Snowden et al., 2004). Msh4 and Msh5 are members the ZMM proteins (Zip1, Zip2, Zip3, Zip4, Msh4, Msh5, Mer3, and Spo16), a diverse set of activities that facilitate crossover-specific events of recombination, and couple these events to chromosome synapsis (Fung et al., 2004;Hunter, 2015;Lynn et al., 2007;Shinohara et al., 2008). As seen in a variety of species, initial numbers of MutSγ immunostaining foci greatly outnumber final crossover numbers (De Muyt et al., 2014; de Vries et al., 1999;Edelmann et al., 1999;Higgins et al., 2008;Kneitz et al., 2000;Yokoo et al., 2012;Zhang et al., 2014). As prophase I progresses, MutSγ is lost from most recombination sites but retained at sites that will go on to mature into crossovers. This patterning process is dependent on the Zip3/RNF212/ZHP-3/HEI10 family of RING E3 ligases (Agarwal and Roeder, Wang et al., 2012;Yokoo et al., 2012;Zhang et al., 2018). Additional evidence implicates the SUMO-modification and ubiquitin-proteasome systems in meiotic crossover control (Ahuja et al., 2017; Rao et al., 2017) and suggests a model in which factors such as MutSγ are selectively stabilized at crossover sites by protecting them from proteolysis. Implicit in this model is the notion that MutSγ may be intrinsically unstable.Here, we show that regulated proteolysis plays a direct and essential role in meiotic crossing over. Msh4 is identified as an intrinsically unstable protein that is targeted for proteasomal degradation by an N-terminal degron thereby rendering MutSγ inactive for crossing over. Activation of MutSγ occurs by neutralizing the Msh4 degron via Cdc7-catalyzed phosphorylation. Thus, a key meiotic pro-crossover factor is activated by attenuating its proteolysis.

Crossing and zipping: molecular duties of the ZMM proteins in meiosis

“…However, the physical interaction observed between Zip4 and Msh5 suggests that these complexes, probably along with Mer3, act synergistically on DNA intermediates to promote stable SEI intermediate formation (Figure 3b-c). During this process, SEI will be progressively stabilized by the MutSγ heterodimer and the phosphorylation of Msh4 protein (He et al 2018). The physical link observed between Zip3 and the MutSγ complex suggests that Zip3 consolidates the mechanism of CO maturation by further stabilizing this complex (Agarwal and Roeder 2000) ( Figure 2b).…”

“…In mice, it has been suggested that the balance of RNF212mediated SUMOylation and HEI10-mediated ubiquitylation determines the stability of MutSγ complex by protecting it from proteolysis at CO-specific DNA intermediates (Qiao et al 2014). Interestingly, a recent study in budding yeast indicates that another type of post-translational modification is important for MutSγ complex stability (He et al 2018).…”