The ring consists of cohesin subunits Smc1, Smc3, and Scc1. Previous results indicated that Smc heterodimers form a V-shape, with the two proteins joined at the hinge. Now, immunoprecipitation experiments demonstrate that the ends of the V are joined by Scc1 (or by its meiotic replacement, Rec8), forming a ring of cohesin on chromatin in vivo.
Cleavage of the ring by separase released the it from chromatin in vitro and in vivo. “The big question now,” says Nasmyth, “is what is the pathway of the DNA. Is it trapped inside the ring?” Although he has not yet seen sister chromatids within a single ring, the ring size is estimated to be about four times the size of a chromatin fiber, large enough to accommodate both chromatids. Ring assembly may be coordinated with DNA replication to ensure that cohesin traps sister strands.
Scc1- and Smc-like proteins are also found in complexes involved in condensing and organizing individual chromosomes in bacteria and eukaryotes. Rings may therefore be one of the most ancient structures used to regulate DNA structure, certainly older than eukaryote-specific nucleosomes. ▪