Shugoshin regulates cohesion by driving relocalization of PP2A in Xenopus extracts

Sister chromatid cohesion is mediated by cohesin. At the onset of mitosis, most cohesin dissociates from chromatin with the exception of a small population, present along chromosome arms and enriched at centromeres. A protein known as shugoshin (Sgo) is essential to maintain arm and centromeric cohesion until the onset of anaphase in transformed human cells, but not in other organisms like Drosophila or mouse. We have used Xenopus egg extracts to further explore this issue. Chromosomes assembled in extracts depleted of Sgo have little or no cohesin at centromeres and display centromeric cohesion defects. Unlike transformed human cells, however, arm cohesion is maintained in the absence of Sgo. Furthermore, Sgo depletion impairs the prophase dissociation of cohesin. This phenotype can be rescued by inhibition of PP2A. The protein phosphatase interacts with Sgo and accumulates at centromeres in mitosis in a Sgo-dependent manner. We propose that Sgo drives relocalization of PP2A from arms to centromeres and, in this way, coordinates release of arm cohesin with protection of centromeric cohesin in mitosis.