(mutant cells were first arrested in the G1 phase by nutrient starvation at 26C and then released to the rich medium at the restrictive temperature (36C)
(mutant cells were first arrested in the G1 phase by nutrient starvation at 26C and then released to the rich medium at the restrictive temperature (36C). containing similar cleavage sites to those of Scc1p/Mcd1p is cleaved specifically in anaphase. This cleavage Combretastatin A4 is essential, although the amount of cleaved product is very small ( 5%). Mis4, another sister chromatid cohesion protein, plays an essential role for loading Rad21 on chromatin. A simple model is presented to explain the specific behavior of fission yeast cohesin and why only a tiny fraction of Rad21 is sufficient to be cleaved for normal anaphase. Rad21, originally identified as a protein involved in DNA repair (Birkenbihl and Subramani 1992, 1995; Tatebayashi et al. 1998). Besides the cohesin complex, several other proteins are known to be required for sister chromatid cohesion. Mis4, homologous to Scc2p in and to Nipped-B in (Rollins et al. 1999), collectively called adherin, is needed for cohesion and becomes essential during the S phase (Furuya et al. 1998). Eco1p/Ctf7p is necessary for the establishment of cohesion only in the S phase (Skibbens et al. 1999; Toth et al. 1999). A fission yeast homolog Eso1 is also needed for sister chromatid cohesion (Tanaka et al. 2000). Other proteins that may interact with cohesin subunits and play a possible role in cohesion are Trf4p in (Castano et al. 1996) and BimD in (Holt and May 1996). These sister chromatid cohesion Cav1.2 proteins Combretastatin A4 are implicated in the normal progression of the S phase. Existence of sister chromatid cohesion proteins was first presumed by the observation that sister chromatid separation may depend on the destruction of proteins other than mitotic cyclins (Holloway et al. 1993; Surana et al. 1993; Irniger et al. 1995). Proteins such as a glue or cohesion protein were thus considered (Miyazaki and Orr-Weaver 1994). Very recent findings in budding yeast suggest that dissociation of the cohesin complex from the chromosomes requires the cleavage of Scc1p by a protease under the control of Esp1p (Ciosk et al. 1998; Uhlmann et al. 1999). Esp1p, which is similar to fission yeast Cut1, is bound to Pds1p, a presumed inhibitor of anaphase similar to fission yeast Cut2. On anaphase proteolysis of Pds1p, which is ubiquitinated by anaphase promoting complex (APC)/cyclosome, Esp1p is liberated to degrade Scc1p, thus resulting in sister chromatid separation (Uhlmann et al. 1999). We are interested in whether the same mechanisms can be applied to fission yeast, as the same set of separin/Cut1 and securin/Cut2 complexes exists (Funabiki et al. 1996; Kumada et al. 1998). On the contrary, most of the cohesin complex was found to be dissociated from chromosomes at prophase, well before anaphase, in (Losada et al. 1998). Also, in human and mouse, most of the cohesin exists outside the chromosomes before metaphase (Schmiesing et al. 1998; Darwiche et al. 1999). In higher eukaryotes, the Combretastatin A4 protein complex other than cohesin may act for sister chromatid cohesion until anaphase. Alternatively, a residual fraction of cohesin remaining on the chromosomes may be sufficient for the association of sister chromatids until anaphase. We therefore addressed the question of whether fission yeast cohesin is dissociated in anaphase, as in budding yeast, or is largely dissociated from chromosomes before mitosis, as in vertebrate cells. To answer this, the cohesin complex had to be identified in fission yeast first. However, only Rad21, a homolog of Scc1p, had been previously investigated (Birkenbihl and Subramani 1995; Tatebayashi et al. 1998) when we began this investigation. By looking for homologs of Smc1p, Smc3p, and Scc3p, all of their counterparts in designated Psm1, Psm3, and Psc3, respectively, were identified, and the presence of the cohesin complex was established by analysis of the immunoprecipitates. Chromatin immunoprecipitation (CHIP) and immunofluorescent and green fluorescent protein (GFP)-tagging microscopy showed that the cohesin complex differed in one important aspect from vertebrates and budding yeast; namely, the majority of cohesin was located in the nucleus and seemed to be bound to chromatin throughout the cell cycle. To our surprise, the level of associated Rad21 detected by CHIP did not change during the highly synchronous progression from metaphase to anaphase. As in budding yeast, however, a small population of Rad21 was cleaved at the onset of anaphase, and this cleavage was essential for proper progression of anaphase. Combretastatin A4 These results are interpreted in the Discussion section, in which we present a simple model to explain the specific behavior of cohesin. Results Identification of fission yeast cohesin?genes To characterize the cohesin complex in (Smc3p. The sequence in the database for cohesin.