Identification and characterization of SA/Scc3p subunits in the Xenopus and human cohesin complexes

A multisubunit protein complex, termed cohesin. plays an essential role in sister chromatid cohesion in yeast and in Xenopus laevis cell-free extracts . We report here that two distinct cohesin complexes exist in Xenopus egg e xtracts. A 14S complex (x-cohesin(SA1)) contains XSMC1, XSMC3, XRAD21, and a newly identified subunit, XSA1, In a second 12.5S complex (x-cohesin(SA2) ), XSMC1, XSMC3, and XRAD21 associate with a different subunit, XSA2. Both XSA1 and XSA2 belong to the SA family of mammalian proteins and exhibit sim ilarity to Scc3p, a recently identified component of yeast cohesin. In Xeno pus egg extracts, x-cohesin(SA1) is predominant, whereas x-cohesinSA2 const itutes only a very minor population. Human cells have a similar pair of coh esin complexes, but the SA2-type is the dominant form in somatic tissue cul ture cells. Immunolocalization experiments suggest that chromatin associati on of cohesin(SA1) and cohesin(SA2) may be differentially regulated. Dissoc iation of x-cohesin(SA1) from chromatin correlates with phosphorylation of XSA1 in the cell-free extracts. Purified cdc2-cyclin B can phosphorylate XS A1 in vitro and reduce the ability of x-cohesin(SA1) to bind to DNA or chro matin, These results shed light on the mechanism by which sister chromatid cohesion is partially dissolved in early mitosis, far before the onset of a naphase, in vertebrate cells.