Multi-subunit SMC ATPases control chromosome superstructure and DNA topology, presumably by DNA translocation and loop extrusion. Chromosomal DNA is entrapped within the tripartite SMCkleisin ring. Juxtaposed SMC heads (''J heads'') or engaged SMC heads (''E heads'') split the SMCkleisin ring into ''S'' and ''K'' sub-compartments. Here, we map a DNA-binding interface to the S compartment of E heads SmcScpAB and show that head-DNA association is essential for efficient DNA translocation and for traversing highly transcribed genes in Bacillus subtilis. We demonstrate that in J heads, SmcScpAB chromosomal DNA resides in the K compartment but is absent from the S compartment. Our results imply that the DNA occupancy of the S compartment changes during the ATP hydrolysis cycle. We propose that DNA translocation involves DNA entry into and exit out of the S compartment, possibly by DNA transfer between compartments and DNA segment capture.

SummarySMC proteins support vital cellular processes in all domains of life by organizing chromosomal DNA. They are composed of ATPase “head” and “hinge“ dimerization domains and a connecting coiled-coil “arm.” Binding to a kleisin subunit creates a closed tripartite ring, whose ∼47-nm-long SMC arms act as barrier for DNA entrapment. Here, we uncover another, more active function of the bacterial Smc arm. Using high-throughput genetic engineering, we resized the arm in the range of 6–60 nm and found that it was functional only in specific length regimes following a periodic pattern. Natural SMC sequences reflect these length constraints. Mutants with improper arm length or peptide insertions in the arm efficiently target chromosomal loading sites and hydrolyze ATP but fail to use ATP hydrolysis for relocation onto flanking DNA. We propose that SMC arms implement force transmission upon nucleotide hydrolysis to mediate DNA capture or loop extrusion.

Integrin α5β1 is a major fibronectin receptor critical for cell migration. Upon complex formation, fibronectin and α5β1 undergo conformational changes. While this is key for cell-tissue connections, its mechanism is unknown. Here, we report cryo–electron microscopy structures of native human α5β1 with fibronectin to 3.1-angstrom resolution, and in its resting state to 4.6-angstrom resolution. The α5β1-fibronectin complex revealed simultaneous interactions at the arginine-glycine-aspartate loop, the synergy site, and a newly identified binding site proximal to adjacent to metal ion–dependent adhesion site, inducing the translocation of helix α1 to secure integrin opening. Resting α5β1 adopts an incompletely bent conformation, challenging the model of integrin sharp bending inhibiting ligand binding. Our biochemical and structural analyses showed that affinity of α5β1 for fibronectin is increased with manganese ions (Mn2+) while adopting the half-bent conformation, indicating that ligand-binding affinity does not depend on conformation, and α5β1 opening is induced by ligand-binding.

Focal adhesions (FA) are large macromolecular assemblies relevant for various cellular and pathological events such as migration, polarization and metastatic cancer formation. At FA sites at the migrating periphery of a cell, hundreds of players gather and form a network to respond to extra cellular stimuli transmitted by the integrin receptor, the most upstream component within a cell, initiating the FA signaling pathway. Numerous cellular experiments have been performed to understand the FA architecture and functions, however, their intricate network formation hampers unraveling the precise molecular actions of individual players. Here, in vitro bottom-up reconstitution presents an advantageous approach for elucidating the FA machinery and the hierarchical crosstalk of involved cellular players.