Red1p, Hop1p, and Mek1p are components of the axial element protein cores in synaptonemal complexes (5). Synaptonemal complexes are found at synapses between homologous chromosomes during meiosis, and form when sister chromatids condense upon axial elements (5). Maintenance of appropriate stoichiometry between Red1p, Hop1p, and Mek1p is important for effective chromosome segregation and for the meiotic recombination checkpoint (3). Red1p, Hop1p and Mek1p are also necessary for normal levels of double-strand break (DSB) formation (6), and are required to ensure that crossovers occur between homologous chromosomes and not between sister chromatids (3). Red1p is a multifunctional protein required for maximum activity of Mek1p, interaction between Mek1p and Hop1p, sister chromatid cohesion, establishment of interhomolog bias at double Holliday junctions, and for proper timing of the first meiotic division (3, 7, 8, 9).

Red1p is also required for the localization of Ddc1p to chromosomes, and Dmc1p localization to DSBs. Further, Red1p may be required to limit Spo11p to the core area of meiotic recombination hotspots, possibly by preventing additional Spo11p from binding to the flanking regions (10, 6).

RED1 is induced early in meiosis, and Red1p is present on chromosomes during zygotene prior to Hop1p, remains on chromosomes during pachytene after Hop1p has left, and begins to dissociate from chromosomes in late pachytene or early diplotene (8, 11). Condensin, a multisubunit protein complex that plays a central role in the condensation of chromosomes, is required for the proper chromosome localization of Red1p (12). Although it has been suggested that Red1p is phosphorylated by Mek1p, it now appears that phosphorylation of Red1p depends on meiotic recombination, but not Mek1p (11, 3). Red1p is dephosphorylated by Glc7p (11).

red1 null mutants display reduced numbers of initiating chromosomal breaks and interhomolog double Holliday junctions, but display approximately normal levels of intersister double Holliday junctions (9). This leads to an overall reduction in levels of meiotic recombination, and the crossovers that do occur do not promote proper meiotic chromosome segregation (7, 5). red1 mutants also display reduced spore viability due to massive nondisjunction, reduced levels of Dmc1p loading onto DSBs during meiosis, and have a partial defect in sister chromatid cohesion (3, 5). Deletion of RED1 suppresses the prophase arrest observed in several mutants such as mnd1 null, rad50S, dmc1 null, and sae2 null which are defective for meiotic DSB repair, suggesting that lack of Red1p allows DSB repair to proceed, probably via a mitosis-like intersister-exchange pathway independent of the meiosis-specific repair machinery (9). Red1p overproduction alters meiotic chromosome morphology, decreases the number of DSBs, decreases the level of meiotic recombination, and suppresses the sporulation defects in mutants such as zip1, zip2, dmc1, and hop2 that undergo pachytene checkpoint-mediated arrest (13).

Red1p is capable of homo-oligomerization through its carboxy terminus (8), and exhibits similarity to Kluyveromyces lactis RED1 (13).

Last updated: 2005-10-14