Mammalian genomes are spatially organized by CCCTC-binding factor (CTCF) and cohesin into chromatin loops and topologically associated domains, which have important roles in gene regulation and recombination. By binding to specific sequences, CTCF defines contact points for cohesin-mediated long-range chromosomal cis-interactions. Cohesin is also present at these sites, but has been proposed to be loaded onto DNA elsewhere and to extrude chromatin loops until it encounters CTCF bound to DNA. How cohesin is recruited to CTCF sites, according to this or other models, is unknown. Here we show that the distribution of cohesin in the mouse genome depends on transcription, CTCF and the cohesin release factor Wings apart-like (Wapl). In CTCF-depleted fibroblasts, cohesin cannot be properly recruited to CTCF sites but instead accumulates at transcription start sites of active genes, where the cohesin-loading complex is located. In the absence of both CTCF and Wapl, cohesin accumulates in up to 70 kilobase-long regions at 3'-ends of active genes, in particular if these converge on each other. Changing gene expression modulates the position of these 'cohesin islands'. These findings indicate that transcription can relocate mammalian cohesin over long distances on DNA, as previously reported for yeast cohesin, that this translocation contributes to positioning cohesin at CTCF sites, and that active genes can be freed from cohesin either by transcription-mediated translocation or by Wapl-mediated release.

Cohesin is a chromosome-associated multisubunit protein complex that is highly conserved in eukaryotes and has close homologs in bacteria. Cohesin mediates cohesion between replicated sister chromatids and is therefore essential for chromosome segregation in dividing cells. Cohesin is also required for efficient repair of damaged DNA and has important functions in regulating gene expression in both proliferating and post-mitotic cells. Here we discuss how cohesin associates with DNA, how these interactions are controlled during the cell cycle; how binding of cohesin to DNA may mediate sister chromatid cohesion, DNA repair, and gene regulation; and how defects in these processes can lead to human disease.Eukaryotic cells pass their genomes from one cell generation to the next by first replicating their entire nuclear DNA during S phase and then segregating the resulting sister chromatids from each other later during mitosis or meiosis. The segregation step is carried out with the help of the spindle apparatus, which captures replicated chromosomes in a bipolar fashion during prometaphase. In anaphase, the spindle then moves the sister chromatids into opposite directions so that two genetically identical daughter cells can be formed during cytokinesis. This mechanism of chromosome segregation critically depends on the fact that sister chromatids remain physically connected with each other from the time of their synthesis in S phase until they are separated much later in anaphase. Without this cohesion, sister chromatids could be separated from each other before chromosomes were attached to both poles of the spindle, and an equal distribution of sister chromatids into forming daughter cells would not be possible. Identifying the molecular mechanisms of sister chromatid cohesion is therefore of central importance for understanding the cell division cycle of eukaryotic cells.During DNA replication, sister chromatids become automatically intertwined with each other at sites where replication forks collide (Sundin and Varshavsky 1980), and these catenations need to be resolved by topoisomerases before sister chromatids can separate in anaphase (DiNardo et al. 1984). It was therefore initially plausible to assume that cohesion may be mediated by DNA catenation (Murray and Szostak 1985). Catenations can indeed mediate some cohesion between sister chromatids if the activity of topoisomerases is experimentally inhibited (Vagnarelli et al. 2004;Toyoda and Yanagida 2006). However, it is unclear whether catenations are also required for sister chromatid cohesion under physiological conditions; i.e., when topoisomerases are active. For entire chromosomes, it has so far been impossible to address this question, because it is presently not possible to create replicated chromosomes in which sister chromatids are not catenated with each other. However, it is now well established that catenation cannot be sufficient for cohesion, because several proteins have been identified that are essential for cohesion that do not appear to reg...

Mammalian genomes contain several billion base pairs of DNA that are packaged in chromatin fibres. At selected gene loci, cohesin complexes have been proposed to arrange these fibres into higher-order structures, but how important this function is for determining overall chromosome architecture and how the process is regulated are not well understood. Using conditional mutagenesis in the mouse, here we show that depletion of the cohesin-associated protein Wapl stably locks cohesin on DNA, leads to clustering of cohesin in axial structures, and causes chromatin condensation in interphase chromosomes. These findings reveal that the stability of cohesin-DNA interactions is an important determinant of chromatin structure, and indicate that cohesin has an architectural role in interphase chromosome territories. Furthermore, we show that regulation of cohesin-DNA interactions by Wapl is important for embryonic development, expression of genes such as c-myc (also known as Myc), and cell cycle progression. In mitosis, Wapl-mediated release of cohesin from DNA is essential for proper chromosome segregation and protects cohesin from cleavage by the protease separase, thus enabling mitotic exit in the presence of functional cohesin complexes.

The GTPase RAN has an established role in spindle assembly and in mitotic progression, although not all mechanisms are fully understood in somatic cells. Here, we have downregulated RAN-binding protein 1 (RANBP1), a RAN partner that has highest abundance in G2 and mitosis, in human cells. RANBP1-depleted cells underwent prolonged prometaphase delay often followed by apoptosis. Cells that remained viable assembled morphologically normal spindles; these spindles, however, were hyperstable and failed to recruit cyclin B1 or to restrict the localization of HURP (DLG7), a microtubule-stabilizing factor, to plus-ends. RANBP1 depletion did not increase the frequency of unattached chromosomes; however, RANBP1-depleted cells frequently showed lagging chromosomes in anaphase, suggesting that merotelic attachments form and are not efficiently resolved. These data indicate that RANBP1 activity is required for the proper localization of specific factors that regulate microtubule function; loss of this activity contributes to the generation of aneuploidy in a microtubule-dependent manner.