Cohesin and Monopolin: Two Major Determinants of Chromosome Segregation

In order to reproduce, a cell must segregate a complete set of chromosomes to each of its progeny. Two methods of reproduction evolved in eukaryotes: mitosis and meiosis. Mitosis generates progeny that are quasi-identical to their mother cell, while meiosis generates nonidentical gametes with half the ploidy of the mother cell.

During mitosis, DNA replication is followed by one round of chromosome segregation in which sister chromatids move to opposite poles. Sister chromatid segregation is dependent on sister chromatid cohesion, which is only established during replication and holds sisters together until anaphase. Cohesion allows cells to distinguish sister chromatids from nonsister chromosomes and facilitates sister kinetochore attachment to microtubules emanating from opposite spindle poles. When sister kinetochores attach to opposite poles, cohesion opposes the pulling force of the spindle, creating tension, which stabilizes kinetochore microtubule attachments. At the onset of anaphase, destruction of cohesion allows sister chromatid segregation.

During meiosis, DNA replication is followed by two nuclear divisions. Sister chromatid cohesion plays a pivotal role during both divisions, by ensuring that chromosomes containing homolog centromeres segregate to opposite poles during meiosis I, and chromosomes containing sister centromeres segregate during meiosis II. Three fundamental differences between meiosis I and mitosis are necessary to permit these two rounds of chromosome segregation after one round of DNA replication. First, sister kinetochore attachment to opposite poles is prevented during meiosis I. Second, sister chromatid cohesion distal to cross-overs holds homologs together, promoting attachment of homologs to opposite poles. Finally, during meiosis I, sister chromatid cohesion is destroyed along chromosome arms; however, it persists around centromeres until anaphase II. This prevents sister centromere separation but allows homologs to segregate during meiosis I. The residual centromeric cohesion facilitates sister kinetochore attachment to opposite poles during the mitosis-like meiosis II.

At the start of my Ph.D., the molecular basis of cohesion was unknown. To identify genes involved in cohesion, I screened yeast mutants for premature sister chromatid separation. This screen, together with another two screens in our laboratory, identified seven proteins required for cohesion: Scc1, Scc2, Scc3, Scc4, Smc1, Smc3, and Eco1 (1-3). Further experiments showed that these proteins form three distinct functional groups. Scc1, Scc3, Smc1, and Smc3 form a "cohesin" complex, which binds to chromosomes and presumably holds sister chromatids together (2). This possibility is strongly supported by the observation that proteolytic cleavage of Scc1, one of cohesin's subunits, is required for both dissociation of cohesin from chromosomes and sister chromatid separation at the onset of anaphase (4). A complex distinct from cohesin, formed by Scc2 and Scc4, loads cohesin onto chromosomes (3). Finally, Eco1 is specifically concerned with establishing bonds between sister chromatids during replication (2, 5).

Interestingly, cohesin's cleavable subunit, Scc1, is absent from meiotic cells. Recently it was discovered that Rec8, a meiosis-specific homolog of Scc1, is required to hold both sisters and homologs together during meiosis. Rec8 associates with chromosomes along their entire length during replication. At the onset of anaphase I, Rec8 disappears from chromosome arms, but unlike Scc1 which completely disappears from chromosomes during mitosis, Rec8 persists around centromeres until anaphase II (6, 7). This raises the possibility that differences between Rec8 and Scc1 provide the basis for distinct regulation of cohesion during meiosis and mitosis. By replacing Rec8 with Scc1 during meiosis, I demonstrated that only Rec8-mediated cohesion can survive around centromeres during meiosis I (8).

Two fundamental questions of meiotic chromosome segregation remain unanswered: How is Rec8-mediated centromeric cohesion maintained, and how is attachment of sister kinetochores to a single pole (monopolar attachment) achieved during meiosis I? Rec8 in budding yeast is not required for monopolar attachment (8). Conventional forward genetic and biochemical analyses have been inefficient at discovering genes involved in these processes. Therefore, I initiated a novel functional genomic screen, making use of the mitotic and meiotic expression profiles of all yeast genes (9, 11). Assuming that many of the genes required for meiosis I-type chromosome behavior are expressed exclusively during meiosis I, I selected and with my co-workers deleted 301 genes preferentially expressed during meiosis (8, 12). Our approach identified eight genes required for proper meiotic chromosome segregation.

One of these genes, MAM1, encodes a meiosis I–specific kinetochore protein. Although in mam1-deficient cells spindle pole body duplication, meiotic cell cycle progression, and chromatin association of Rec8 appear normal, spindle elongation and chromosome segregation fail during meiosis I. Chromosomes segregate only during an abnormal meiosis II division, as judged by the presence of four spindle pole bodies. Surprisingly, a large fraction of sister centromeres separate prematurely in mam1-deficient cells at the time of the failed first meiotic division, despite persistence of centromeric Rec8 protein in these cells. These observations can be explained if Mam1 is specifically required to prevent sister kinetochore attachment to opposite spindle poles. According to this hypothesis, in mam1-deficient cells sister kinetochores attach to opposite poles during meiosis I, but the persistence of Rec8 around centromeres prevents efficient chromosome segregation until meiosis II. To test this provocative idea, I prevented persistence of centromeric cohesion during meiosis I in mam1-deficient cells by replacing Rec8 with Scc1. Remarkably, nuclear division during meiosis I was restored; however, sister chromatids, and not homologs, segregated in these cells during meiosis I. Thus, we successfully turned the reductional meiosis I division into a mitosis-like equational division, by replacing meiotic cohesin with mitotic cohesin in mam1-deficient meiotic cells. These results identify Mam1 as the first member of a new class of proteins, the "monopolins," which primarily act in setting up monopolar attachment during meiosis I (8).

Our work was among the first to generate insight into the molecular basis of sister chromatid cohesion and that of meiotic cohesion and kinetochore behaviour. Nevertheless, how cohesin glues chromosomes together remains obscure. Similarly, we must learn more about maintenance of centromeric cohesion and about monopolin's biochemical function if we are to resolve one of biology's fundamental questions: What makes meiosis different from mitosis?

References

1. C. Michaelis et al., Cell 91, 35 (1997).
2. A. Tóth et al., Genes Dev. 13, 320 (1999).
3. R. Ciosk et al., Mol. Cell 5, 243 (2000).
4. F. Uhlmann et al., Nature 400, 37 (1999).
5. R. V. Skibbens et al., Genes Dev. 13, 307 (1999).
6. F. Klein et al., Cell 98, 91 (1999).
7. S. B. Buonomo et al., Cell 103, 387 (2000).
8. A. Tóth et al., Cell 103, 1155 (2000).
9. R. J. Cho et al., Mol. Cell 2, 65 (1998).
10. S. Chu et al., Science 282, 699 (1998).
11. M. Primig et al., Nature Genet. 26, 415 (2000).
12. K. P. Rabitsch et al., Curr. Biol. 11, 1001 (2001).

To Advertise   |   Find Products