Stepping mechanism of SMC loop-extruding motors
In plain words
Cohesin and condensin are ring-shaped protein machines that pull DNA into loops by burning ATP. Single-molecule experiments show steps of hundreds of base pairs, comparable to the size of the protein, and how the structure achieves this is unknown.
Precise statement
Determine the conformational cycle of cohesin or condensin that couples ATP binding and hydrolysis to DNA translocation, explaining the measured step-size distribution (of order hundreds of base pairs per step, approximate), the force-velocity relation with stall at DNA tensions of order $10^{-8}\text{ to }10^{-7}\,\mathrm{dyn}$ depending on the complex (approximate), and one-sided versus two-sided extrusion. Answer: a structural-kinetic model that reproduces these measurements.
What would settle it
Time-resolved structures or single-molecule FRET of the motor cycle combined with force spectroscopy that match one model's predicted step and force-velocity statistics.
Status in the literature
Unverified note
Cryo-EM structures constrain the cycle, but several competing translocation models remain in 2026, to this survey's knowledge.