Multi-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.

中文翻译：

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Smc臂在原核凝聚素中逐渐开放

多亚基SMC ATPases通过催化DNA环-挤出反应明显控制染色体超结构。SMC蛋白带有一个ABC型ATPase头和一个由卷曲螺旋臂连接的铰链二聚结构域。SMC二聚体中的两个臂可以相互对齐，从而形成棒状颗粒。ATP结合后，SMC头部接合，并且认为手臂分开了。在这里，我们通过电子自旋共振光谱研究了枯​​草芽孢杆菌Smc-ScpAB的形状。在没有配体的情况下，很容易在头部附近检测到手臂分离，而铰链附近的分离很大程度上取决于ATP和DNA。手臂开放的人为堵塞消除了ATP水解的DNA刺激，但并未阻止基础ATPase活性。我们确定手臂之间的接触对于控制分子转化很重要。该手臂界面上的点突变消除了Smc功能。我们建议通过结合合适的DNA底物，部分开放的中间构象为SMC DNA易位提供方向性。