MutL homolog 3 (Mlh3) is a member of a family of proteins conserved during evolution and having dual roles in DNA mismatch repair and meiosis. The pathway in eukaryotes consists of the DNA-binding components, which are the homologs of the bacterial MutS protein (MSH 2 6), and the MutL homologs, which bind to the MutS homologs and are essential for the repair process. Three of the six homologs of MutS that function in these processes, Msh2, Msh3 and Msh6, are involved in the mismatch repair of mutations, frameshifts and replication errors, and two others, Msh4 and Msh5, have specific roles in meiosis. Of the four MutL homologs, Mlh1, Mlh3, Pms1 and Pms2, three are involved in mismatch repair and at least two, Pms2 and Mlh1, are essential for meiotic progression in both yeast and mice. To assess the role of Mlh3 in mammalian meiosis, we have generated and characterized Mlh3(-/-) mice. Here we show that Mlh3(-/-) mice are viable but sterile. Mlh3 is required for Mlh1 binding to meiotic chromosomes and localizes to meiotic chromosomes from the mid pachynema stage of prophase I. Mlh3(-/-) spermatocytes reach metaphase before succumbing to apoptosis, but oocytes fail to complete meiosis I after fertilization. Our results show that Mlh3 has an essential and distinct role in mammalian meiosis.

The intranuclear spindle of yeast has an electron-opaque body at each pole . These spindle plaques lie on the nuclear envelope . During mitosis the spindle elongates while the nuclear membranes remain intact. After equatorial constriction there are two daughted nuclei, each with one spindle plaque . The spindle plaque then duplicates so that two side-by-side plaques are produced . These move rapidly apart and rotate so that they bracket a stable 0 .8 µm spindle. Later, during mitosis, this spindle elongates, etc . Yeast cells placed on sporulation medium soon enter meiosis . After 4 hr the spindle plaques of the more mature cells duplicate, producing a stable side-by-side arrangement . Subsequently the plaques move apart to bracket a 0.8 µm spindle which immediately starts to elongate . When this meiosis I spindle reaches its maximum length of 3-5 µm, each of the plaques at the poles of the spindle duplicates and the resulting side-by-side plaques increase in size . The nucleus does not divide . The large side-by-side plaques separate and bracket a short spindle of about 1 µm which elongates gradually to 2 or 3 µm . Thus there are two spindles within one nucleus at meiosis II . To the side of each of the four plaques a bulge forms on the nucleus . The four bulges enlarge while the original nucleus shrinks . These four developing ascospore nuclei are partially surrounded by cytoplasm and by a prospore wall which originates from the cytoplasmic side of the spindle plaque . Eventually the spore nuclei pinch off and the spore wall closes . In some of the larger yeast cells this development is completed after 8 hr on sporulation medium.

The behavior of meiotic chromosomes differs in several respects from that of their mitotic counterparts, resulting in the generation of genetically distinct haploid cells. This has been attributed in part to a meiosisspecific chromatin-associated protein structure, the synaptonemal complex. This complex consist of two parallel axial elements, each one associated with a pair of sister chromatids, and a transverse filament located between the synapsed homologous chromosomes. Recently, a different protein structure, the cohesin complex, was shown to be associated with meiotic chromosomes and to be required for chromosome segregation. To explore the functions of the two different protein structures, the synaptonemal complex and the cohesin complex, in mammalian male meiotic cells, we have analyzed how absence of the axial element affects early meiotic chromosome behavior. We find that the synaptonemal complex protein 3 (SCP3) is a main determinant of axial-element assembly and is required for attachment of this structure to meiotic chromosomes, whereas SCP2 helps shape the in vivo structure of the axial element. We also show that formation of a cohesincontaining chromosomal core in meiotic nuclei does not require SCP3 or SCP2. Our results also suggest that the cohesin core recruits recombination proteins and promotes synapsis between homologous chromosomes in the absence of an axial element. A model for early meiotic chromosome pairing and synapsis is proposed.The eukaryotic cell cycle ensures that chromosomes are correctly replicated and symmetrically divided between daughter cells. Errors in the chromosomal segregation process can generate aneuploid cells, which are either not viable or contribute to cancer development, infertility, or other aspects of human disease. Two different strategies for cell division are active in eukaryotic organisms, mitosis and meiosis. Meiosis differs in several respects from mitosis; for example, meiotic cells undergo two cell divisions (M1 and M2) without an intervening DNA replication step, resulting in the generation of haploid cells. Furthermore, homologous chromosomes (each consisting of two sister chromatids) recombine and synapse in prophase I. The homologs are then separated at anaphase I, while the sister chromatids remain associated until the second meiotic division (33, 54).How can the differences between mitotic and meiotic chromosomal behavior be explained? Our understanding of the mechanisms that regulate chromosome synapsis has increased tremendously over the past few years, and two different protein complexes have been shown to take part in these processes, the cohesin complex and the synaptonemal complex (SC) (25,45). We now know that sister chromatids in mitotic cells remain associated by protein complexes called cohesins (14, 26), which consist of at least four different subunits (SMC1, SMC3, SCC1, and SCC3). SMC1 and SMC3 have been shown to bind DNA in vitro (2, 3). Cohesin complexes become attached to chromosomes in somatic cells in the G 1 phase and are deposite...

The eukaryotic RecA homologues RAD51 and DMC1 function in homology recognition and formation of joint-molecule recombination intermediates during yeast meiosis. The precise immunolocalization of these two proteins on the meiotic chromosomes of plants and animals has been complicated by their high degree of identity at the amino acid level. With antibodies that have been immunodepleted of cross-reactive epitopes, we demonstrate that RAD51 and DMC1 have identical distribution patterns in extracts of mouse spermatocytes in successive prophase I stages, suggesting coordinate functionality. Immunofluorescence and immunoelectron microscopy with these antibodies demonstrate colocalization of the two proteins on the meiotic chromosome cores at early prophase I. We also show that mouse RAD51 and DMC1 establish protein–protein interactions with each other and with the chromosome core component COR1(SCP3) in a two-hybrid system and in vitro binding analyses. These results suggest that the formation of a multiprotein recombination complex associated with the meiotic chromosome cores is essential for the development and fulfillment of the meiotic recombination process.

Meiosis is a critical stage of gametogenesis in which alignment and synapsis of chromosomal pairs occur, allowing for the recombination of maternal and paternal genomes. Here we show that FK506 binding protein (Fkbp6) localizes to meiotic chromosome cores and regions of homologous chromosome synapsis. Targeted inactivation of Fkbp6 in mice results in aspermic males and the absence of normal pachytene spermatocytes. Moreover, we identified the deletion of Fkbp6 exon 8 as the causative mutation in spontaneously male sterile as/as mutant rats. Loss of Fkbp6 results in abnormal pairing and misalignments between homologous chromosomes, nonhomologous partner switches, and autosynapsis of X chromosome cores in meiotic spermatocytes. Fertility and meiosis are normal in Fkbp6 mutant females. Thus, Fkbp6 is a component of the synaptonemal complex essential for sex-specific fertility and for the fidelity of homologous chromosome pairing in meiosis.