During the first meiotic division, homologous chromosomes (homologs) have to separate to opposite poles of the cell to ensure the right complement in the progeny. Homologous recombination provides a mechanism for a genome-wide homology search and physical linkage among the homologs before their orderly segregation. Rad51 and Dmc1 recombinases are the major players in these processes. Disruption of meiosis-specific HOP2 or MND1 genes leads to severe defects in homologous synapsis and an early-stage recombination failure resulting in sterility. Here we show that mouse Hop2 can efficiently form D-loops, the first recombination intermediates, but this activity is abrogated upon association with Mnd1. Furthermore, the Hop2-Mnd1 heterodimer physically interacts with Rad51 and Dmc1 recombinases and stimulates their activity up to 35-fold. Our data reveal an interplay among Hop2, Mnd1 and Rad51 and Dmc1 in the formation of the first recombination intermediates during meiosis.

Meiosis produces haploid gametes through a succession of chromosomal events, including pairing, synapsis, and recombination. Mechanisms that orchestrate these events remain poorly understood. We found that the SUMO (small ubiquitin-like modifier)–modification and ubiquitin-proteasome systems regulate the major events of meiotic prophase in mouse. Interdependent localization of SUMO, ubiquitin, and proteasomes along chromosome axes was mediated largely by RNF212 and HEI10, two E3 ligases that are also essential for crossover recombination. RNF212-dependent SUMO conjugation effected a checkpointlike process that stalls recombination by rendering the turnover of a subset of recombination factors dependent on HEI10-mediated ubiquitylation. We propose that SUMO conjugation establishes a precondition for designating crossover sites via selective protein stabilization. Thus, meiotic chromosome axes are hubs for regulated proteolysis via SUMO-dependent control of the ubiquitin-proteasome system.

Meiotic recombination between homologous chromosomes ensures their proper segregation at the first division of meiosis and is the main force shaping genetic variation of genomes. The HOP2 and MND1 genes are essential for this recombination: Their disruption results in severe defects in homologous chromosome synapsis and an early-stage failure in meiotic recombination. The mouse Hop2 and Mnd1 proteins form a stable heterodimer (Hop2/Mnd1) that greatly enhances Dmc1-mediated strand invasion. In order to elucidate the mechanism by which Hop2/Mnd1 stimulates Dmc1, we identify several intermediate steps in the homologous pairing reaction promoted by Dmc1. We show that Hop2/Mnd1 greatly stimulates Dmc1 to promote synaptic complex formation on long duplex DNAs, a step previously revealed only for bacterial homologous recombinases. This synaptic alignment is a consequence of the ability of Hop2/Mnd1 to (1) stabilize Dmc1-single-stranded DNA (ssDNA) nucleoprotein complexes, and (2) facilitate the conjoining of DNA molecules through the capture of double-stranded DNA by the Dmc1-ssDNA nucleoprotein filament. To our knowledge, Hop2/Mnd1 is the first homologous recombinase accessory protein that acts on these two separate and critical steps in mammalian meiotic recombination.[Keywords: DNA repair; Dmc1 recombinase; homologous recombination; strand invasion; synaptic complex] Supplemental material is available at http://www.genesdev.org. Homologous recombination serves a critical function in the repair of DNA double-strand breaks (DSBs) and in the proper segregation of homologous chromosomes in meiosis (Kleckner 1996;Roeder 1997). Failure to establish a physical connection through chiasmata causes missegregation of chromosomes at prophase I and results in meiotic cell apoptosis or aneuploid gametes. The Dmc1 recombinase, a eukaryotic homolog of the bacterial RecA protein, is expressed exclusively in meiotic cells and is a major player in meiotic homologous recombination. It promotes the search for homology and catalyzes the invasion of a single-stranded end generated by the 5Ј resection of DSBs introduced by Spo11 into a homologous unbroken double-stranded DNA (dsDNA) to form joint molecules through strand invasion (D-loop formation) (Li et al. 1997;Masson et al. 1999;Hong et al. 2001;Masson and West 2001;Neale and Keeney 2006). The interaction between Hop2, Mnd1, and Dmc1 and/or Rad51, the ubiquitously expressed eukaryotic homolog of RecA, is crucial for the progression of meiotic homologous recombination. Biochemical studies have shown that the Hop2/Mnd1 complex physically interacts with and stimulates Dmc1 and Rad51 strand invasion activity (Chen et al. 2004;Petukhova et al. 2005;Pezza et al. 2006). The cooperation between Dmc1/Rad51 and Hop2/ Mnd1 is likely to be crucial in vivo, since without Hop2 and/or Mnd1, in yeast (Leu et al. 1998;Gerton and DeRisi 2002; Roeder 2002, 2003;Zierhut et al. 2004;Henry et al. 2006), Arabidopsis thaliana (Domenichini et al. 2006;Kerzendorfer et al. 2006;Panoli et al. 2006), and mouse (Pe...

Recent studies in simple model organisms have shown that centromere pairing is important for ensuring high-fidelity meiotic chromosome segregation. However, this process and the mechanisms regulating it in higher eukaryotes are unknown. Here we present the first detailed study of meiotic centromere pairing in mouse spermatogenesis and link it with key events of the G2/metaphase I transition. In mouse we observed no evidence of the persistent coupling of centromeres that has been observed in several model organisms. We do however find that telomeres associate in non-homologous pairs or small groups in B type spermatogonia and pre-leptotene spermatocytes, and this association is disrupted by deletion of the synaptonemal complex component SYCP3. Intriguingly, we found that, in mid prophase, chromosome synapsis is not initiated at centromeres, and centromeric regions are the last to pair in the zygotene-pachytene transition. In late prophase, we first identified the proteins that reside at paired centromeres. We found that components of the central and lateral element and transverse filaments of the synaptonemal complex are retained at paired centromeres after disassembly of the synaptonemal complex along diplotene chromosome arms. The absence of SYCP1 prevents centromere pairing in knockout mouse spermatocytes. The localization dynamics of SYCP1 and SYCP3 suggest that they play different roles in promoting homologous centromere pairing. SYCP1 remains only at paired centromeres coincident with the time at which some kinetochore proteins begin loading at centromeres, consistent with a role in assembly of meiosis-specific kinetochores. After removal of SYCP1 from centromeres, SYCP3 then accumulates at paired centromeres where it may promote bi-orientation of homologous centromeres. We propose that, in addition to their roles as synaptonemal complex components, SYCP1 and SYCP3 act at the centromeres to promote the establishment and/or maintenance of centromere pairing and, by doing so, improve the segregation fidelity of mammalian meiotic chromosomes.