Condensin and cohesin: more than chromosome compactor and glue

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Abstract
Although distinct in structure and function, cohesin and condensin both contain conserved chromosomal ATPases of the structural maintenance of chromosomes (SMC) protein family. Present models indicate that cohesin might encircle two sister chromatids to promote their cohesion, whereas condensin might stabilize supercoiled DNA loops on a single chromatid to promote its condensation. Whether the diversity of functions exhibited by each complex during interphase and mitosis reflects a common molecular mechanism remains unknown. Cohesin and condensin affect gene expression through influences on control mechanisms that operate globally at a chromosome-wide level, regionally over a subchromosomal domain, or locally on an individual gene. As well as a mitotic condensin complex, C. elegans contains a second condensin-like complex that mediates the global twofold repression of genes along hermaphrodite X chromosomes to achieve dosage compensation, a vital process that equalizes X-linked gene products between males (one X chromosome) and hermaphrodites (two X chromosomes). The complex also mediates the local 20-fold repression of a single autosomal gene. A condensin subunit might help maintain repression over regulatory domains of the Drosophila body-patterning gene Abd-B. Cohesin might need to be removed during mitosis to establish transcriptional silencing of the S. cerevisiae mating-type locus, whereas condensin might facilitate silencing over this domain. In S. cerevisiae, cohesin localizes to several DNA segments that border regions of active and silent chromatin and might be required for their chromatin insulation function. A cohesin regulatory protein might facilitate contact between a distant enhancer and the promoter of the Drosophila wing gene cut. In several organisms, cohesin subunits associate with DNA-repair proteins to form new complexes with roles in DNA repair. Accordingly, mutations in several cohesin subunits show hypersensitivity to DNA damage. Condensin and some cohesin subunits have been implicated in the DNA-damage checkpoint-signalling pathway. Cohesin, and possibly condensin, might help organize the structure and orientation of the centromere, so that sister chromatids are properly captured by microtubules from opposite poles of the spindle. Mutations in cohesin subunits trigger the spindle checkpoint.