| During each round of division, the cell must ensure that each daughter cell receives an exact copy of the genome. In order for this to be accomplished, sister chromatids are cohesed until they align along the metaphase plate during mitosis and segregate to opposite poles of the cell. The cohesin complex is responsible for linking sister chromatids after replication has occurred in S phase until their segregation during mitosis. Defects in cohesin can lead to chromosome mis-segregation at anaphase, sensitivity to DNA damage, and human syndromes collectively termed cohesinopathies. For the cohesin complex to effectively link sister chromatids, it must become established, or more stably associated with chromatin. Establishment involves the acetylation of the cohesin subunit Smc3 by the acetyltransferases Escol and Esco2. Through the use of an antibody that specifically detects the acetylated form of Smc3, we describe the timing, localization, and regulation of cohesin acetylation. Acetylated Smc3 localizes to nuclear foci that begin to form in early G1 phase, but maximum acetylation requires replication in S phase. Depletion based studies in HeLa cells suggest that cohesin acetylation depends on the activity of both Escol and Esco2, but their specific contributions are unknown. By constructing conditional knockouts of each acetyltransferase in hTERT-RPE cells, we are able to more clearly define the roles of both Escol and Esco2. Surprisingly, the phenotypes of the two knockouts differ greatly. The Escol knockout displays a milder phenotype, with decreased levels of Smc3 acetylation but normal growth and cohesion. The Esco2 knockout has only a slight reduction in Smc3 acetylation, but these cells exhibit a severe growth phenotype, a sensitivity to replication stress, and defects in mitosis. These results support the model that Escol and Esco2 have non-redundant roles in cellular maintenance and also suggest that Esco2 may have an important function besides Smc3 acetylation. |
