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SummaryMulti-subunit SMC complexes control chromosome superstructure and promote chromosome disjunction, conceivably by actively translocating along DNA double helices. SMC subunits comprise an ABC ATPase “head” and a “hinge” dimerization domain connected by a 49 nm coiled-coil “arm.” The heads undergo ATP-dependent engagement and disengagement to drive SMC action on the chromosome. Here, we elucidate the architecture of prokaryotic Smc dimers by high-throughput cysteine cross-linking and crystallography. Co-alignment of the Smc arms tightly closes the interarm space and misaligns the Smc head domains at the end of the rod by close apposition of their ABC signature motifs. Sandwiching of ATP molecules between Smc heads requires them to substantially tilt and translate relative to each other, thereby opening up the Smc arms. We show that this mechanochemical gating reaction regulates chromosome targeting and propose a mechanism for DNA translocation based on the merging of DNA loops upon closure of Smc arms.

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SummarySMC condensin complexes are central modulators of chromosome superstructure in all branches of life. Their SMC subunits form a long intramolecular coiled coil, which connects a constitutive “hinge” dimerization domain with an ATP-regulated “head” dimerization module. Here, we address the structural arrangement of the long coiled coils in SMC complexes. We unequivocally show that prokaryotic Smc-ScpAB, eukaryotic condensin, and possibly also cohesin form rod-like structures, with their coiled coils being closely juxtaposed and accurately anchored to the hinge. Upon ATP-induced binding of DNA to the hinge, however, Smc switches to a more open configuration. Our data suggest that a long-distance structural transition is transmitted from the Smc head domains to regulate Smc-ScpAB’s association with DNA. These findings uncover a conserved architectural theme in SMC complexes, provide a mechanistic basis for Smc’s dynamic engagement with chromosomes, and offer a molecular explanation for defects in Cornelia de Lange syndrome.