Intermolecular DNA interactions stimulated by the cohesin complex in vitro: Implications for sister chromatid cohesion

The establishment of sister chromatid cohesion during S phase and its disso lution at the metaphase-anaphase transition are essential for the faithful segregation of chromosomes in mitosis [1-4], Recent studies in yeast geneti cs and Xenopus biochemistry have identified a large protein complex, cohesi n, that plays a key role in sister chromatid cohesion [5-10]. The cohesin c omplex consists of a heterodimeric pair of SMC (structural maintenance of c hromosomes) subunits and at least two non-SMC subunits. This structural org anization is reminiscent of that of condensin, another major SMC protein co mplex that drives chromosome condensation in eukaryotic cells [11], Condens in has been shown to reconfigure and compact DNA in vitro by utilizing the energy of ATP hydrolysis [12], Very little is known, however, about how coh esin works at a mechanistic level. Here we report the first set of biochemi cal activities associated with an intact cohesin complex purified from HeLa cell extracts. The cohesin complex binds directly to double-stranded DNA a nd induces the formation of large protein-DNA aggregates. In the presence o f topoisomerase II, cohesin stimulates intermolecular catenation of circula r DNA molecules. This activity is in striking contrast to intramolecular kn otting directed by condensin [13], Cohesin also increases the probability o f intermolecular ligation of linear DNA molecules in the presence of DNA li gase, Our results are consistent with a model in which cohesin functions as an intermolecular DNA crosslinker and is part of the molecular "glue" that holds sister chromatids together [14].