Frank Uhlmann

Sister-chromatid separation at anaphase onset is promoted by cleavage of the cohesin subunit Scc1

Cohesion between sister chromatids is established during DNA replication and depends on a multiprotein complex called cohesin. Attachment of sister kinetochores to the mitotic spindle during mitosis generates forces that would immediately split sister chromatids were it not opposed by cohesion. Cohesion is essential for the alignment of chromosomes in metaphase but must be abolished for sister separation to start during anaphase. In the budding yeast Saccharomyces cerevisiae, loss of sister-chromatid cohesion depends on a separating protein (separin) called Esp1 and is accompanied by dissociation from the chromosomes of the cohesion subunit Scc1. Here we show that Esp1 causes the dissociation of Scc1 from chromosomes by stimulating its cleavage by proteolysis. A mutant Scc1 is described that is resistant to Esp1-dependent cleavage and which blocks both sister-chromatid separation and the dissociation of Scc1 from chromosomes. The evolutionary conservation of separins indicates that the proteolytic cleavage of cohesion proteins might be a general mechanism for triggering anaphase.

Cohesin relocation from sites of chromosomal loading to places of convergent transcription.

Sister chromatids, the products of eukaryotic DNA replication, are held together by the chromosomal cohesin complex after their synthesis. This allows the spindle in mitosis to recognize pairs of replication products for segregation into opposite directions. Cohesin forms large protein rings that may bind DNA strands by encircling them, but the characterization of cohesin binding to chromosomes in vivo has remained vague. We have performed high resolution analysis of cohesin association along budding yeast chromosomes III–VI. Cohesin localizes almost exclusively between genes that are transcribed in converging directions. We find that active transcription positions cohesin at these sites, not the underlying DNA sequence. Cohesin is initially loaded onto chromosomes at separate places, marked by the Scc2/Scc4 cohesin loading complex, 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 the sites of cohesin binding and therefore probably sister chromatid cohesion, a key architectural feature of mitotic chromosomes, display surprising flexibility. Cohesin localization to places of convergent transcription is conserved in fission yeast, suggesting that it is a common feature of eukaryotic chromosomes.

Cohesion between sister chromatids must be established during DNA replication

BACKGROUND Cohesion between sister chromatids, which opposes the splitting force exerted by the mitotic spindle during metaphase, is essential for their segregation to opposite poles of the cell during anaphase. In Saccharomyces cerevisiae, cohesion depends on a set of chromosomal proteins called cohesins, which include structural maintenance of chromosomes 1p (Smc1p), Smc3p and sister chromatid cohesion 1p (Scc1p). Strains with mutations in the genes encoding these proteins separate sister chromatids prematurely and fail to align them in metaphase. This leads to missegregation of chromosomes in the following anaphase. RESULTS In a normal cell cycle, Scc1p was synthesized and recruited to chromosomes at the onset of S phase. Using cells that expressed Scc1p exclusively from a galactose-inducible promoter, we showed that if Scc1p was synthesised only after completion of S phase, it still bound to chromosomes but failed to promote sister chromatid cohesion. CONCLUSIONS Cohesion between sister chromatids must be established during DNA replication, possibly following the passage of a replication fork. Furthermore, Scc1p (and other cohesins) are needed both for maintaining cohesion during mitosis and for establishing it during S phase. Establishment of sister chromatid cohesion is therefore an essential but hitherto neglected aspect of S phase.

Disjunction of Homologous Chromosomes in Meiosis I Depends on Proteolytic Cleavage of the Meiotic Cohesin Rec8 by Separin

It has been proposed but never proven that cohesion between sister chromatids distal to chiasmata is responsible for holding homologous chromosomes together while spindles attempt to pull them toward opposite poles during metaphase of meiosis I. Meanwhile, the mechanism by which disjunction of homologs is triggered at the onset of anaphase I has remained a complete mystery. In yeast, cohesion between sister chromatid arms during meiosis depends on a meiosis-specific cohesin subunit called Rec8, whose mitotic equivalent, Sccl, is cleaved at the metaphase to anaphase transition by an endopeptidase called separin. We show here that cleavage of Rec8 by separin at one of two different sites is necessary for the resolution of chiasmata and the disjunction of homologous chromosomes during meiosis.

Eco1-Dependent Cohesin Acetylation During Establishment of Sister Chromatid Cohesion

Replicated chromosomes are held together by the chromosomal cohesin complex from the time of their synthesis in S phase onward. This requires the replication fork–associated acetyl transferase Eco1, but Eco1s mechanism of action is not known. We identified spontaneous suppressors of the thermosensitive eco1-1 allele in budding yeast. An acetylation-mimicking mutation of a conserved lysine in cohesins Smc3 subunit makes Eco1 dispensable for cell growth, and we show that Smc3 is acetylated in an Eco1-dependent manner during DNA replication to promote sister chromatid cohesion. A second set of eco1-1 suppressors inactivate the budding yeast ortholog of the cohesin destabilizer Wapl. Our results indicate that Eco1 modifies cohesin to stabilize sister chromatid cohesion in parallel with a cohesion establishment reaction that is in principle Eco1-independent.

Phosphorylation of the Cohesin Subunit Scc1 by Polo/Cdc5 Kinase Regulates Sister Chromatid Separation in Yeast

At the onset of anaphase, a caspase-related protease (separase) destroys the link between sister chromatids by cleaving the cohesin subunit Scc1. During most of the cell cycle, separase is kept inactive by binding to an inhibitory protein called securin. Separase activation requires proteolysis of securin, which is mediated by an ubiquitin protein ligase called the anaphase-promoting complex. Cells regulate anaphase entry by delaying securin ubiquitination until all chromosomes have attached to the mitotic spindle. Though no longer regulated by this mitotic surveillance mechanism, sister separation remains tightly cell cycle regulated in yeast mutants lacking securin. We show here that the Polo/Cdc5 kinase phosphorylates serine residues adjacent to Scc1 cleavage sites and strongly enhances their cleavage. Phosphorylation of separase recognition sites may be highly conserved and regulates sister chromatid separation independently of securin.

Identification of cis-acting sites for condensin loading onto budding yeast chromosomes

Eukaryotic chromosomes reach their stable rod-shaped appearance in mitosis in a reaction dependent on the evolutionarily conserved condensin complex. Little is known about how and where condensin associates with chromosomes. Here, we analyze condensin binding to budding yeast chromosomes using high-resolution oligonucleotide tiling arrays. Condensin-binding sites coincide with those of the loading factor Scc2/4 of the related cohesin complex. The sites map to tRNA and other genes bound by the RNA polymerase III transcription factor TFIIIC, and ribosomal protein and SNR genes. An ectopic B-box element, recognized by TFIIIC, constitutes a minimal condensin-binding site, and TFIIIC and the Scc2/4 complex promote functional condensin association with chromosomes. A similar pattern of condensin binding is conserved along fission yeast chromosomes. This reveals that TFIIIC-binding sites, including tRNA genes, constitute a hitherto unknown chromosomal feature with important implications for chromosome architecture during both interphase and mitosis.

Degradation of a cohesin subunit by the N-end rule pathway is essential for chromosome stability.

Cohesion between sister chromatids is established during DNA replication and depends on a protein complex called cohesin. At the metaphase–anaphase transition in the yeast Saccharomyces cerevisiae, the ESP1-encoded protease separin cleaves SCC1, a subunit of cohesin with a relative molecular mass of 63,000 (Mr 63K). The resulting 33K carboxy-terminal fragment of SCC1 bears an amino-terminal arginine—a destabilizing residue in the N-end rule. Here we show that the SCC1 fragment is short-lived (t1/2 ≈ 2 min), being degraded by the ubiquitin/proteasome-dependent N-end rule pathway. Overexpression of a long-lived derivative of the SCC1 fragment is lethal. In ubr1Δ cells, which lack the N-end rule pathway, we found a highly increased frequency of chromosome loss. The bulk of increased chromosome loss in ubr1Δ cells is caused by metabolic stabilization of the ESP1-produced SCC1 fragment. This fragment is the first physiological substrate of the N-end rule pathway that is targeted through its N-terminal residue. A number of yeast proteins bear putative cleavage sites for the ESP1 separin, suggesting other physiological substrates and functions of the N-end rule pathway.

Cdc14 phosphatase induces rDNA condensation and resolves cohesin-independent cohesion during budding yeast anaphase.

At anaphase onset, the protease separase triggers chromosome segregation by cleaving the chromosomal cohesin complex. Here, we show that cohesin destruction in metaphase is sufficient for segregation of much of the budding yeast genome, but not of the long arm of chromosome XII that contains the rDNA repeats. rDNA in metaphase, unlike most other sequences, remains in an undercondensed and topologically entangled state. Separase, concomitantly with cleaving cohesin, activates the phosphatase Cdc14. We find that Cdc14 exerts two effects on rDNA, both mediated by the condensin complex. Lengthwise condensation of rDNA shortens the chromosome XII arm sufficiently for segregation. This condensation depends on the aurora B kinase complex. Independently of condensation, Cdc14 induces condensin-dependent resolution of cohesin-independent rDNA linkage. Cdc14-dependent sister chromatid resolution at the rDNA could introduce a temporal order to chromosome segregation.

A model for ATP hydrolysis-dependent binding of cohesin to DNA.

BACKGROUND Cohesion between sister chromatids is promoted by the chromosomal cohesin complex that forms a proteinaceous ring, large enough in principle to embrace two sister strands. The mechanism by which cohesin binds to DNA, and how sister chromatid cohesion is established, is unknown. RESULTS Biochemical studies of cohesin have largely been limited to protein isolated from soluble cellular fractions. Here, we characterize cohesin purified from budding yeast chromatin, suggesting that chromosomal cohesin is sufficiently described by its known distinctive ring structure. We present evidence that the two Smc subunits of cohesin by themselves form a ring, closed at interacting ATPase head domains. A motif in the Smc1 subunit implicated in ATP hydrolysis is essential for loading cohesin onto DNA. In addition to functional ATPase heads, an intact cohesin ring structure is indispensable for DNA binding, suggesting that ATP hydrolysis may be coupled to DNA transport into the cohesin ring. DNA is released in anaphase when separase cleaves cohesins Scc1 subunit. We show that a cleavage fragment of Scc1 disrupts the interaction between the two Smc heads, thereby opening the ring. CONCLUSIONS We present a model for cohesin binding to chromatin by ATP hydrolysis-dependent transport of DNA into the cohesin ring. After DNA replication, two DNA strands may be trapped to promote sister chromatid cohesion. In anaphase, Scc1 cleavage opens the ring to release sister chromatids.