SYCP3 regulates strand invasion activities of RAD51 and DMC1.

The synaptonemal complex is a higher-ordered proteinaceous architecture formed between homologous chromosomes. SYCP3 is a major component of the lateral/axial elements in the synaptonemal complex and is essential for meiotic recombination. Previous genetic studies showed that SYCP3 functions in meiotic homologous recombination biased to interhomologous chromosomes, by regulating the strand invasion activities of the RAD51 and DMC1 recombinases. However, the mechanism by which SYCP3 regulates RAD51- and DMC1-mediated strand invasion remains elusive. In this study, we found that SYCP3 significantly suppresses the RAD51-mediated, but not the DMC1-mediated, strand invasion reaction by competing with HOP2-MND1, which is an activator for both RAD51 and DMC1. A SYCP3 mutant with defective RAD51 binding does not inhibit the RAD51-mediated homologous recombination in human cells. Therefore, SYCP3 may promote the DMC1-driven homologous recombination by attenuating the RAD51 activity during meiosis.