Gradual opening of Smc arms in prokaryotic condensin

Abstract

SummaryMulti-subunit SMC ATPases control chromosome superstructure apparently by catalyzing a DNA-loop-extrusion reaction. SMC proteins harbor an ABC-type ATPase ‘head’ and a ‘hinge’ dimerization domain connected by a coiled coil ‘arm’. Two arms in a SMC dimer can co-align, thereby forming a rod-shaped particle. Upon ATP binding, SMC heads engage, and arms are thought to separate. Here, we studied the shape of B. subtilis Smc-ScpAB by electron-spin resonance spectroscopy. Arm separation was readily detected proximal to the heads in the absence of ligands, while separation near the hinge largely depended on ATP and DNA. Artificial blockage of arm opening eliminated DNA stimulation of ATP hydrolysis, but did not prevent basal ATPase activity. We identified an arm-to-arm contact as being important for controlling the molecular transformations. Point mutations at this arm interface eliminate Smc function. We propose that partially open, intermediary conformations provide directionality to SMC DNA translocation by binding suitable DNA substrates.

Cryo-EM structure of MukBEF reveals DNA loop entrapment at chromosomal unloading sites

“…Sub-classification of the cryo-EM dataset revealed additional forms of MukBEF-MatP with arms in different states of openness (Figure 4A). This suggests that the arms can gradually 'zip up', similar to what has been proposed for Smc-ScpAB based on distance measurements by electron paramagnetic resonance (Nunez et al, 2021). The most open class has arms unzipped up to the elbow, and the most closed one is the apo state.…”