Homologous pairing during meiotic prophase is essential for homologous recombination and for chromosome segregation

Homologous pairing during meiotic prophase is essential for homologous recombination and for chromosome segregation during meiosis I; yet, it is largely unknown how homologues approach each other. A chromosomal rearrangement called the telomere bouquet in early meiotic prophase is thought to play a key role in homologous pairing (Scherthan, 2001; Harper et al., 2004). It forms as telomeres cluster to a small region on the nuclear envelope (NE). Though the telomere bouquet is conserved in most organisms, the mechanism of its formation is not well understood. Fission yeast, Schizosaccharomyces pombe, serves as a good model organism to study telomere bouquet formation because it has a conspicuous method for clustering telomeres. Telomeres are in subclusters scattered on the inner NE in mitotic cells and reorganized upon pheromone sensing (Chikashige et al., 1997), as telomeres start to cluster on the NE adjacent to the SPB. Such reorganization persists during premeiotic S phase and meiotic prophase, as the horsetail-shaped nucleus is driven back and forth by the microtubule arrays attached to the SPB. Therefore, the telomere bouquet could help the homologues approach each other, and the horsetail movement stretches them to facilitate their alignment. Although many proteins have been identifi ed in the telomere complex and the SPB, the linking components between them for bouquet formation are not well understood, and the forces that cluster telomeres have not been described. Rap1p is a telomere binding protein that is essential for telomere clustering (Chikashige and Hiraoka, 2001; Kanoh and Ishikawa, 2001). Sad1p, a spindle pole body (SPB) protein (Hagan and Yanagida, 1995) could function as a transmembrane linker for the bouquet formation. It has a transmembrane domain and a Sad1/UNClike domain, which is an essential part of the UNC-84 protein for nuclear migration in Caenorhabditis elegans (Lee et al., 2002). These two domains are also in the SPB half-bridge protein Mps3p/Nep98p (Nishikawa et al., 2003), the reciprocal best hit with Sad1p in budding yeast. Furthermore, Sad1p interacts with Kms1p (Miki et al., 2004), a protein that is required for telomere clustering (Shimanuki et al., 1997), and with dynein light chain Dlc1p, which is required for the horsetail movement (Miki et al., 2002). Cytoplasmic microtubules are thought to be involved in clustering telomeres in fi ssion yeast. However, this may not be the case in other organisms. To identify more genes for bouquet formation, we developed a visual screen by monitoring heterochromatin reorganization upon meiosis. One mutant we found, im295, was defective in bouquet formation and was an allele to bqt2 that was identifi ed in genome-wide screens by systematically deleting upregulated genes during meiotic prophase independently in two other groups (Martin-Castellanos et al., 2005; Chikashige et al., 2006). Using multiple GFPand mCherry-tagged markers for telomeres, the SPB, and the NE, we cytologically proved that Bqt2p specifi cally functions as a linkage component between telomeres and the SPB. It plays a key role in initiating telomere clustering by generating Sad1p foci proximate to the telomere foci upon pheromone sensing. Bqt2p is essential for initiating telomere clustering upon pheromone sensing in fi ssion yeast