Proximity labeling reveals new insights into the relationships between meiotic recombination proteins in S. cerevisiae

Several protein ensembles facilitate MutS{gamma} crossover recombination and the associated process of synaptonemal complex (SC) assembly during meiosis, but the physical and functional relationships between the components involved remain obscure. We have employed proximity labeling as a phenotypic tool to discern functional relationships between meiotic recombination and SC proteins in S. cerevisiae, and to gain deeper insight into molecular deficits of crossover-defective mutants. We find that recombination initiation (Spo11) and the Mer3 helicase are dispensable for proximity labeling of the Zip3 E3 ligase by components of the ZZS ensemble (Zip2, Zip4 and Spo16) but are required for proximity labeling of Zip3 by Msh4, consistent with the possibility that MutS{gamma} joins Zip3 only after a specific recombination intermediate has been generated. Proximity labeling analysis of crossover-defective zip1 mutants suggests a key shared defect is a failure to assemble an early recombination ensemble where ZZS can properly engage Zip3. We furthermore discovered that Zip3s abundance within the meiotic cell is uniquely dependent on the presence of Zip1, and that the post-translational modification of Zip3 is promoted by most MutS{gamma} pathway proteins but countered by Zip1. Based on this and additional data, we propose a model whereby Zip1 stabilizes a functional, unmodified form of Zip3 until intermediate steps in recombination are complete. We also find that SC structural protein Ecm11 is proximity labeled by ZZS complex proteins in a Zip4-dependent manner, but by Zip3 and Msh4, at least in part, via a distinct pathway. Finally, streptavidin pulldowns followed by mass spectrometry on eleven different proximity labeling strains uncovers shared proximity targets of MutS{gamma}-associated proteins, some with known meiotic functions and others not yet implicated in a meiotic activity, highlighting the potential power of proximity labeling as a discovery tool.