文件:细胞17-23.png

Cohesin. Cohesin is a 

protein complex with four subunits. (A) Two subunits of cohesin are members of a large family of proteins called SMC proteins (for structural maintenance of chromosomes). These subunits, Smc1 and Smc3, are coiled-coil proteins whose N- and C-terminal regions fold together to form a globular ATPase domain. (B) The Scc1 subunit contains two globular domains separated by a long disordered region that binds an additional subunit called Scc3. Binding of the globular domains of Scc1 to the Smc1 and Smc3 subunits results in a ring structure that encircles the sister chromatids as shown in (C). ATP binding promotes the interaction of the ATPase domains as shown here, while ATP hydrolysis dissociates the ATPase domains. Loading and unloading of cohesin on DNA requires ATP hydrolysis and is expected to involve the opening of one of the three intersubunit gates, but this process remains poorly understood. During S phase, acetylation of the ATPase domain of Smc3 (not shown) prevents the opening of the ring, thereby locking the cohesin ring around the sister chromatids. In metaphase, proteolytic cleavage of sites in the disordered region of Scc1 triggers sister-chromatid separation, as discussed later.