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 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.