How Do so Few Control so Many?

The separation of sister chromatids at the metaphase-to-anaphase transition is triggered by a protease called separase that is activated by the destruction of an inhibitory chaperone (securin). This process is mediated by a ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C), along with a protein called Cdc20. It is vital that separase not be activated before every single chromosome has been aligned on the mitotic spindle. Kinetochores that have not yet attached to microtubules catalyze the sequestration of Cdc20 by an inhibitor called Mad2. Recent experiments shed important insight into how Mad2 molecules bound to centromeres through their association with a protein called Mad1 might be transferred to Cdc20 and thereby inhibit securin’s destruction

Cohesin cleavage is insufficient for centriole disengagement in Drosophila

Medical Research Council; Wellcome Trust; European Research Council

The cohesin ring concatenates sister DNA molecules

Sister chromatid cohesion, which is essential for mitosis, is mediated by a multi-subunit protein complex called cohesin whose Scc1, Smc1, and Smc3 subunits form a tripartite ring structure. It has been proposed that cohesin holds sister DNAs together by trapping them inside its ring. To test this, we used site-specific cross-linking to create chemical connections at the three interfaces between the ring’s three constituent polypeptides, thereby creating covalently closed cohesin rings. As predicted by the ring entrapment model, this procedure produces dimeric DNA/cohesin structures that are resistant to protein denaturation. We conclude that cohesin rings concatenate individual sister minichromosome DNAs

A Central Role for Cohesins in Sister Chromatid Cohesion, Formation of Axial Elements, and Recombination during Yeast Meiosis

AbstractA multisubunit complex, called cohesin, containing Smc1p, Smc3p, Scc1p, and Scc3p, is required for sister chromatid cohesion in mitotic cells. We show here that Smc3p and a meiotic version of Scc1p called Rec8p are required for cohesion between sister chromatids, for formation of axial elements, for reciprocal recombination, and for preventing hyperresection of double-strand breaks during meiosis. Both Rec8p and Smc3p colocalize with chromosome cores independently of synapsis during prophase I and largely disappear from chromosome arms after pachytene but persist in the neighborhood of centromeres until the onset of anaphase II. The eukaryotic cell's cohesion apparatus is required both for the repair of recombinogenic lesions and for chromosome segregation and therefore appears to lie at the heart of the meiotic process

Dependency of the Spindle Assembly Checkpoint on Cdk1 Renders the Anaphase Transition Irreversible

SummaryActivation of anaphase-promoting complex/cyclosome (APC/CCdc20) by Cdc20 is delayed by the spindle assembly checkpoint (SAC). When all kinetochores come under tension, the SAC is turned off and APC/CCdc20 degrades cyclin B and securin, which activates separase [1]. The latter then cleaves cohesin holding sister chromatids together [2]. Because cohesin cleavage also destroys the tension responsible for turning off the SAC, cells must possess a mechanism to prevent SAC reactivation during anaphase, which could be conferred by a dependence of the SAC on Cdk1 [3–5]. To test this, we analyzed mouse oocytes and embryos expressing nondegradable cyclin B together with a Cdk1-resistant form of separase. After biorientation and SAC inactivation, APC/CCdc20 activates separase but the resulting loss of (some) cohesion is accompanied by SAC reactivation and APC/CCdc20 inhibition, which aborts the process of further securin degradation. Cyclin B is therefore the only APC/CCdc20 substrate whose degradation at the onset of anaphase is necessary to prevent SAC reactivation. The mutual activation of tension sensitive SAC and Cdk1 creates a bistable system that ensures complete activation of separase and total downregulation of Cdk1 when all chromosomes have bioriented

Molecular basis for SMC rod formation and its dissolution upon DNA binding.

SMC condensin complexes are central modulators of chromosome superstructure in all branches of life. Their SMC subunits form a long intramolecular coiled coil, which connects a constitutive "hinge" dimerization domain with an ATP-regulated "head" dimerization module. Here, we address the structural arrangement of the long coiled coils in SMC complexes. We unequivocally show that prokaryotic Smc-ScpAB, eukaryotic condensin, and possibly also cohesin form rod-like structures, with their coiled coils being closely juxtaposed and accurately anchored to the hinge. Upon ATP-induced binding of DNA to the hinge, however, Smc switches to a more open configuration. Our data suggest that a long-distance structural transition is transmitted from the Smc head domains to regulate Smc-ScpAB's association with DNA. These findings uncover a conserved architectural theme in SMC complexes, provide a mechanistic basis for Smc's dynamic engagement with chromosomes, and offer a molecular explanation for defects in Cornelia de Lange syndrome

Sister chromatid cohesion establishment during DNA replication termination

Newly copied sister chromatids are tethered together by the cohesin complex, but how sister chromatid cohesion is coordinated with DNA replication is poorly understood. Prevailing models suggest cohesin complexes, bound to DNA before replication, remain behind the advancing replication fork to keep sister chromatids together. By visualizing single replication forks colliding with pre-loaded cohesin complexes, we find that the replisome instead pushes cohesin to where a converging replisome is met. While the converging replisomes are removed during DNA replication termination, cohesin remains on nascent DNA and provides cohesion. Additionally, we show that CMG disassembly during replication termination is vital for proper cohesion in budding yeast. Together, our results support a new model where sister chromatid cohesion is established during DNA replication termination

Evolution of condensin and cohesin complexes driven by replacement of Kite by Hawk proteins

Mitotic chromosome condensation, sister chromatid cohesion, and higher order folding of interphase chromatin are mediated by condensin and cohesin, eukaryotic members of the SMC (structural maintenance of chromosomes)–kleisin protein family. Other members facilitate chromosome segregation in bacteria [1]. A hallmark of these complexes is the binding of the two ends of a kleisin subunit to the apices of V-shaped Smc dimers, creating a tripartite ring capable of entrapping DNA (Figure 1A). In addition to creating rings, kleisins recruit regulatory subunits. One family of regulators, namely Kite dimers (Kleisin interacting winged-helix tandem elements), interact with Smc–kleisin rings from bacteria, archaea and the eukaryotic Smc5-6 complex, but not with either condensin or cohesin [2]. These instead possess proteins containing HEAT (Huntingtin/EF3/PP2A/Tor1) repeat domains whose origin and distribution have not yet been characterized. Using a combination of profile Hidden Markov Model (HMM)-based homology searches, network analysis and structural alignments, we identify a common origin for these regulators, for which we propose the name Hawks, i.e. HEAT proteins associated with kleisins

Scc2 counteracts a Wapl-independent mechanism that releases cohesin from chromosomes during G1

Acknowledgements Maria Demidova conducted initial experiments that this study expanded on. We are grateful to Tomo Tanaka and Seiji Tanaka for supplying reagents. We thank all members of the Nasmyth group for valuable discussions, technical assistance and critical reading of the manuscript. This work was funded by the Wellcome Trust Senior Investigator Award, Grant Ref 107935/Z/15/Z and Cancer Research UK Programme Grant, Grant Ref 26747 to KN. BH is funded by (202062/Z/16/Z).Peer reviewedPublisher PD