Crossover formation and coordinated assembly of synaptonemal complex relies on a direct interaction between Zip1 and Zip3

Several proteins collaborate to promote the crossover recombination events critical for accurate chromosome segregation during meiosis. How these "ZMM" factors (Zip2, Zip3, Zip4, Spo16, Mer3 and MutSγ) collaboratively function remains incompletely understood. We previously reported that Zip3's abundance and activity rely on the synaptonemal complex (SC) component Zip1, and specifically on Zip1's N-terminal residues associated with crossovers and coupling SC assembly to the crossover pathway. Here, we demonstrate that Zip3 co-immunoprecipitates Zip1 from meiotic cells independent of recombination initiation and other ZMMs, and that Zip3's interaction with Zip1 relies on Zip1's N terminal residues. Co-expression and pull-down experiments in bacterial cells demonstrate that Zip1 and Zip3 interact directly. Experiments to identify Zip3 regions required for the Zip1 interaction unexpectedly revealed an incorrectly annotated translational start; we also determined that Zip3's N-terminal structured region is necessary and sufficient for the interaction, and a predicted coil downstream of Zip3's RING domain is essential for specific activities attributed to Zip1's N-terminal tip such as proximity labeling of Zip3 by Zip2 and the coupling of crossover recombination to SC assembly. Finally, we discovered that interaction with Zip1 protects Zip3 not only from proteasome-mediated degradation but also from post-translational modification when another ZMM is absent. We propose that direct interaction with Zip1's N terminus orients Zip3 within a nascent ZMM ensemble in a manner that facilitates crossover formation and the coupling of crossover intermediates to SC assembly and furthermore ensures Zip3 remains both abundant and unmodified until all requisite ZMMs have joined the group.