The physical chemistry of interphase loop extrusion

Cohesin drives genome organization via loop extrusion, orchestrated by the dynamic exchange of multiple essential accessory proteins. Although these regulators bind the core cohesin complex only transiently, their disruption can dramatically alter loop-extrusion dynamics and chromosome morphology. Still, a quantitative theory of cohesin regulation and its interplay with genome folding is still elusive. Here, we derive a chemical-reaction network model of loop-extrusion regulation from first principles that is fully specified by available in vivo measurements. This "bursty extrusion model" untangles the distinct roles of regulators, whose exchange coincides with intermittent periods of motor activity. By incorporating bursty extrusion in polymer simulations, we reveal how variations in regulatory protein abundance can alter chromatin architecture across length and timescales. Our results are corroborated by in vivo and in vitro observations, bridging the gap between cohesin-regulator dynamics at the molecular scale and their genome-wide consequences on chromosome organization.