Cohesion between sister chromatids opposes the splitting force exerted by microtubules, and loss of this cohesion is responsible for the subsequent separation of sister chromatids during anaphase. We describe three chromosmal proteins that prevent premature separation of sister chromatids in yeast. Two, Smc1p and Smc3p, are members of the SMC family, which are putative ATPases with coiled-coil domains. A third protein, which we call Scc1p, binds to chromosomes during S phase, dissociates from them at the metaphase-to-anaphase transition, and is degraded by the anaphase promoting complex. Association of Scc1p with chromatin depends on Smc1p. Proteins homologous to Scc1p exist in a variety of eukaryotic organisms including humans. A common cohesion apparatus might be used by all eukaryotic cells during both mitosis and meiosis.

Cohesion between sister chromatids depends on a multisubunit cohesin complex that binds to chromosomes around DNA replication and dissociates from them at the onset of anaphase. Scc2p, though not a cohesin subunit, is also required for sister chromatid cohesion. We show here that Scc2p forms a complex with a novel protein, Scc4p, which is also necessary for sister cohesion. In scc2 or scc4 mutants, cohesin complexes form normally but fail to bind both to centromeres and to chromosome arms. Our data suggest that a major role for the Scc2p/Scc4p complex is to facilitate the loading of cohesin complexes onto chromosomes.

Sister chromatid cohesion is crucial for chromosome segregation during mitosis. Loss of cohesion very possibly triggers sister separation at the metaphase → anaphase transition. This process depends on the destruction of anaphase inhibitory proteins like Pds1p (Cut2p), which is thought to liberate a sister-separating protein Esp1p (Cut1p). By looking for mutants that separate sister centromeres in the presence of Pds1p, this and a previous study have identified six proteins essential for establishing or maintaining sister chromatid cohesion. Four of these proteins, Scc1p, Scc3p, Smc1p, and Smc3p, are subunits of a 'Cohesin' complex that binds chromosomes from late G 1 until the onset of anaphase. The fifth protein, Scc2p, is not a stoichiometric Cohesin subunit but it is required for Cohesin's association with chromosomes. The sixth protein, Eco1p(Ctf7p), is not a Cohesin subunit. It is necessary for the establishment of cohesion during DNA replication but not for its maintenance during G 2 and M phases.

Cohesion between sister chromatids during G2 and M phases depends on the "cohesin" protein Scc1p (Mcd1p). Loss of cohesion at the metaphase to anaphase transition is accompanied by Scc1p's dissociation from chromatids, which depends on proteolysis of Pds1p mediated by a ubiquitin protein ligase called the anaphase promoting complex (APC). We show that destruction of Pds1p is the APC's sole role in triggering Scc1p's dissociation from chromatids and that Pds1p forms a stable complex with a 180 kDa protein called Esp1p, which is essential for the dissociation of Scc1p from sister chromatids and for their separation. We propose that the APC promotes sister separation not by destroying cohesins but instead by liberating the "sister-separating" Esp1 protein from its inhibitor Pds1p.

Entry into anaphase and exit from mitosis depend on a ubiquitin-protein ligase complex called the anaphase-promoting complex (APC) or cyclosome. At least 12 different subunits were detected in the purified particle from budding yeast, including the previously identified proteins Apc1p, Cdc16p, Cdc23p, Cdc26p, and Cdc27p. Five additional subunits purified in low nanogram amounts were identified by tandem mass spectrometric sequencing. Apc2p, Apc5p, and the RING-finger protein Apc11p are conserved from yeast to humans. Apc2p is similar to the cullin Cdc53p, which is a subunit of the ubiquitin-protein ligase complex SCFCdc4 required for the initiation of DNA replication.