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Accepted · first revision · record

Title: Stepping mechanism of SMC loop-extruding motors
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
Status note: Cryo-EM structures constrain the cycle, but several competing translocation models remain in 2026, to this survey's knowledge.
Posed since: (empty)