Cohesin acetylation promotes sister chromatid cohesion only in association with the replication machinery.

Acetylation of the Smc3 subunit of cohesin is essential to establish functional cohesion between sister chromatids. Smc3 acetylation is catalyzed by members of the Eco family of acetyltransferases, although the mechanism by which acetylation is regulated and how it promotes cohesion are largely unknown. In vertebrates, the cohesin complex binds to chromatin during mitotic exit and is converted to a functional form during or shortly after DNA replication. The conserved proliferating cell nuclear antigen-interacting protein box motif in yeast Eco1 is required for function, and cohesin is acetylated during the S phase. This has led to the notion that acetylation of cohesin is stimulated by interaction of Eco1 with the replication machinery. Here we show that in vertebrates Smc3 acetylation occurs independently of DNA replication. Smc3 is readily acetylated before replication is initiated and after DNA replication is complete. However, we also show that functional acetylation occurs only in association with the replication machinery: disruption of the interaction between XEco2 and proliferating cell nuclear antigen prevents cohesion establishment while having little impact on the overall levels of Smc3 acetylation. These results demonstrate that Smc3 acetylation can occur throughout interphase but that only acetylation in association with the replication fork promotes sister chromatid cohesion. These data reveal how the generation of cohesion is limited to the appropriate time and place during the cell cycle and provide insight into the mechanism by which acetylation ensures cohesion.

A conserved motif at the C terminus of sororin is required for sister chromatid cohesion.

Sororin is a positive regulator of sister chromatid cohesion that interacts with the cohesin complex. Sororin is required for the increased stability of the cohesin complex on chromatin following DNA replication and sister chromatid cohesion during G(2). The mechanism by which sororin ensures cohesion is currently unknown. Because the primary sequence of sororin does not contain any previously characterized structural or functional motifs, we have undertaken a structure-function analysis of the sororin protein. Using a series of mutant derivatives of sororin, we show that the ability of sororin to bind to chromatin is separable from both its role in sister chromatid cohesion and its interaction with the cohesin complex. We also show that derivatives of sororin with deletions or mutations in the conserved C terminus fail to rescue the loss-of-cohesion phenotype caused by sororin RNAi and that these mutations also abrogate the association of sororin with the cohesin complex. Our data suggest that the interaction of the highly conserved motif at the C terminus of sororin with the cohesin complex is critical to its ability to mediate sister chromatid cohesion.