A rigid anchor and a flexible blade: calcium activates gelsolin through an asymmetric conformational ensemble

Gelsolin is the founding member of a superfamily of multidomain proteins that remodel the actin cytoskeleton in response to calcium. How the full-length protein is activated remains unclear because its active form has escaped structural characterization. Here, we combine small-angle X-ray scattering, pulsed EPR spectroscopy and molecular simulations to follow its transition from the compact inactive state to the active form. Activation proceeds through sequential, domain-specific rearrangements and culminates not in a single structure but in a heterogeneous ensemble, in which a comparatively rigid half implicated in filament capping coexists with a highly dynamic half implicated in severing. These states are populated progressively across the nanomolar-to-millimolar calcium concentrations that gelsolin encounters inside and outside cells, linking local calcium availability to the protein's functional repertoire. Gelsolin activation thus emerges as a graded population shift rather than a switch between discrete conformations, providing a structural framework for its diverse functions and, more generally, for the regulation of dynamic multidomain proteins.