Meiosis comprises a pair of specialized nuclear divisions that produce haploid germ cells. To accomplish this, sister chromatids must segregate together during the first meiotic division (meiosis I), which requires that sister chromatid cohesion persists at centromeres. The factors that protect centromeric cohesion during meiosis I have remained elusive. Here we identify Sgo1 (shugoshin), a protector of the centromeric cohesin Rec8 in fission yeast. We also identify a homologue of Sgo1 in budding yeast. We provide evidence that shugoshin is widely conserved among eukaryotes. Moreover, we identify Sgo2, a paralogue of shugoshin in fission yeast, which is required for faithful mitotic chromosome segregation. Localization of Sgo1 and Sgo2 at centromeres requires the kinase Bub1, identifying shugoshin as a crucial target for the kinetochore function of Bub1. These findings provide insights into the evolution of meiosis and kinetochore regulation during mitosis and meiosis.

Sister chromatids, the products of eukaryotic DNA replication, are held together after their synthesis by the chromosomal cohesin complex. This allows the spindle in mitosis to recognise pairs of replication products for segregation into opposite direction1-6. Cohesin forms large protein rings that may bind DNA strands by encircling7, but the characterisation of cohesin binding to chromosomes in vivo has remained vague. Here, we present high resolution analysis of cohesin association along budding yeast chromosomes III -VI. Cohesin localises almost exclusively between genes transcribed in converging direction. We find that not the underlying sequence, but active transcription positions cohesin at these sites. Cohesin is initially loaded onto chromosomes at separate places, marked by the Scc2/Scc4 cohesin loading complex8, from where it appears to slide to its more permanent locations. But even after sister chromatid cohesion is established changes in transcription lead to repositioning of cohesin. Thus a key architectural feature of mitotic chromosomes, the sites of cohesin binding and therefore most likely sister chromatid cohesion, display surprising flexibility. Cohesin localisation to places of convergent transcription is conserved in fission yeast, suggesting that it is a common feature of eukaryotic chromosomes.Correspondence and requests for materials should be addressed to F.U. (e-mail: frank.uhlmann@cancer.org.uk).. * these authors contributed equally Supplementary Information accompanies the paper on Nature's website (http://www.nature.com). Cohesin association with yeast and human chromosomes has been studied4,9-15, but the defining characteristics of association sites, and how cohesin gets to these sites, remained unclear. We analysed cohesin binding to chromosome VI of the budding yeast Saccharomyces cerevisiae by chromatin immunoprecipitation (ChIP) followed by hybridisation to a high-density oligonucleotide array16. The pattern of association in metaphase was similar for all cohesin subunits analysed, Scc1, Scc3, Smc3, and Pds5 ( Fig. 1, and Supplementary Figure S1). It was also similar before the establishment of sister chromatid cohesion, in cells arrested with the replication inhibitor hydroxyurea (Ref. 9, and Supplementary Figure S1). Cohesin bound 28 distinct sites, each spanning 1-4 kilobases (kb) in width. The intensity of association varied, with the strongest peaks found around the centromere, consistent with previous analyses9-11. The distance between neighbouring cohesin association sites ranged from 2 to 35 kb. Almost all cohesin association sites were centred in intergenic regions where genes from opposite strands converged (Fig. 1a), as previously suggested17. Using an additional high-density array, we also mapped the association of Scc1 with chromosomes III, IV, and V (Supplementary Table 1 and Figure S2). 91% (276 of 304) cohesin association sites identified lie at intergenic regions between converging genes, and of 328 convergent intergene regions 84% were bound by cohes...

Sister chromatid cohesion, mediated by a complex called cohesin, is crucial--particularly at centromeres--for proper chromosome segregation in mitosis and meiosis. In animal mitotic cells, phosphorylation of cohesin promotes its dissociation from chromosomes, but centromeric cohesin is protected by shugoshin until kinetochores are properly captured by the spindle microtubules. However, the mechanism of shugoshin-dependent protection of cohesin is unknown. Here we find a specific subtype of serine/threonine protein phosphatase 2A (PP2A) associating with human shugoshin. PP2A colocalizes with shugoshin at centromeres and is required for centromeric protection. Purified shugoshin complex has an ability to reverse the phosphorylation of cohesin in vitro, suggesting that dephosphorylation of cohesin is the mechanism of protection at centromeres. Meiotic shugoshin of fission yeast also associates with PP2A, with both proteins collaboratively protecting Rec8-containing cohesin at centromeres. Thus, we have revealed a conserved mechanism of centromeric protection of eukaryotic chromosomes in mitosis and meiosis.