The previous finding that sister
The previous finding that sister DNAs are entrapped by rings cross-linked at hinge and both Smc-kleisin interfaces is consistent with several scenarios (Figure 6D). DNAs could be entrapped in open SK rings but never in ones whose heads are in either E or J mode. Both DNAs could be entrapped either in J-S or in E-S compartments. Both could be entrapped either in J-K or in E-K-compartments. Lastly, one DNA could be entrapped in an E-S or in a J-S compartment while the other is in the associated K compartment. Our new cross-linking studies imply that both sister DNAs are in fact frequently entrapped in J-K, but not E-K, compartments. The lack of entrapment either of individual or sister DNAs in either type of S compartment is inconsistent with the notion that both DNAs are entrapped in a S compartment or that one DNA is entrapped within a S compartment and the second within its associated K compartment. DNA entrapment in J-K compartments is also found in B. subtilis (Vazquez Nunez et al., 2019 [this issue of Molecular Cell]), and this mode of association may therefore be universal. In this regard, it is interesting that both cohesin and condensin depend on the ability of their Scc3 and Ycg1 regulatory subunits, respectively, to bind DNA when associated with their kleisin partners (Kschonsak et al., 2017, Li et al., 2018). Thus, DNAs trapped inside J-K compartments may bind to Hawk regulatory subunits. Crucially, such binding is insufficient to maintain association with L-α-Hydroxyglutaric Acid as kleisin cleavage triggers release of both cohesin and condensin from chromosomes (Cuylen et al., 2011, Gruber et al., 2003, Houlard et al., 2015). The finding that acetylation of Smc3 during S phase is more frequently associated with J-specific cross-linking suggests that entrapment of sister DNAs within J-K compartments may be a feature of cohesion throughout the genome and does not merely apply to small circular minichromosomes. The observation that sister DNAs are entrapped in J-K compartments refines our view of cohesion while the failure to observe entrapment exclusively by open SK rings, S compartments, E-K compartments, or entrapment of one DNA in an S and its sister in a K compartment disagrees with most previously proposed scenarios (Huber et al., 2016, Li et al., 2017, Murayama et al., 2018, Murayama and Uhlmann, 2015). Nevertheless, the insensitivity of our assay may have precluded rarer instances of entrapment in S compartments. It is also important to point out that detection of sister DNA entrapment by J-K compartments does not exclude the possibility that J-K compartments are in dynamic equilibrium with open SK rings. Though sister DNAs were never observed in E-K compartments, individual DNAs were, albeit infrequently. An explanation for this finding is that ATP-driven head engagement necessary for E-K compartment entrapment cannot occur when sister DNAs are present. To explain why engineered cleavage of Smc3's coiled coil alleviates the retardation of sister chromatid disjunction caused by hos1 mutations, it was suggested that sister DNAs are normally entrapped in an E-S compartment and that the Smc3 acetylation that persists in hos1 mutations blocks the head disengagement necessary for DNA escape via a gate created by kleisin cleavage (Li et al., 2017). Our failure to observe stable entrapment of DNAs in either J-S or E-S compartments is inconsistent with this hypothesis and demands an alternative interpretation for the hos1 effect. We suggest that Smc3 de-acetylation is instead required to facilitate the escape of DNAs from J-K compartments upon Scc1 cleavage, possibly by weakening an association of Pds5 with Scc3 or Smc1 heads that would otherwise hinder escape. Because formation of J-K compartments is likely to require ATP hydrolysis, the notion of cohesion being mediated by entrapment of sister DNAs within J-K compartments is hard to reconcile with the proposal that ATP hydrolysis is unnecessary for building sister chromatid cohesion. The argument that hydrolysis is not required is based on the behavior of Smc1D1164E mutants that can load onto chromosomes and build cohesion despite being defective in ATP hydrolysis (Çamdere et al., 2018). Though these mutations may reduce ATP hydrolysis, we suggest that their viability in fact depends on residual ATPase activity. Cohesin containing Smc1E1158Q Smc3E1155Q, which is completely defective in ATP hydrolysis (Petela et al., 2018), cannot load correctly onto chromosomes, let alone build cohesion (Arumugam et al., 2006, Hu et al., 2011, Hu et al., 2015).