Molecular activities of meiosis-specific proteins Hop2, Mnd1, and the Hop2-Mnd1 complex.

The mouse Hop2 and Mnd1 proteins, which can form a stable heterodimeric complex, ensure the proper synapsis of homologous chromosomes in meiosis by acting in concert with Rad51 and Dmc1 to promote the strand invasion (D-loop formation) step of homologous recombination. Hop2 alone promotes D-loop formation, but Mnd1 and the Hop2-Mnd1 complex do not. Here we show that only the heterodimer complex, but not the individual proteins, can stimulate strand invasion by Dmc1. Furthermore, we demonstrate that the interaction with Mnd1 provokes changes in Hop2 that are responsible not only for abrogating the recombinase activity of Hop2 but also for generating a new molecular interface able to physically interact with and stimulate Dmc1. We also show that coiled-coil motifs in Hop2 and Mnd1 are essential for their interaction with each other and that a clearly delineated region near the COOH terminus of both proteins is necessary for both the DNA binding and single-strand annealing by the Hop-Mnd1 heterodimer. Finally, we describe a point mutation in Hop2 that dissociates its strand invasion activity from its ability to bind and anneal DNA.