Separate Compartments for Chromosome Entrapment and DNA Binding during SMC translocation

Multi-subunit SMC ATPase complexes translocate on chromosomal DNA. They control chromosome structure and DNA topology, presumably by acting as DNA extrusion motors. The SMC-kleisin ring entraps chromosomal DNA. The ring lumen is strongly reduced in size by alignment of the SMC arms and upon ATP binding is divided in two by engagement of SMC head domains. Here, we provide evidence for DNA binding in the SMC compartment and chromosome entrapment in the Kleisin compartment of B. subtilis Smc/ScpAB. We show that DNA binding at the Smc hinge is dispensable and identify an essential DNA binding site at engaged heads which faces the S compartment. Mutations interfering with DNA binding do not prevent ATP hydrolysis but block DNA translocation by Smc/ScpAB. Our findings are consistent with the notion that Smc/DNA contacts stabilize looped DNA segments in the S compartment, while the base of a chromosomal DNA loop is enclosed in the K compartment. Transfer of DNA double helices between S and K compartments may support DNA translocation.