The F-actin crosslinker α-actinin-4 forms catch bonds through a two-state binding mechanism in which wild-type protein accesses both a weak binding state (with both calponin-homology domains contacting F-actin via electrostatic interactions) and a strong binding state (with only CH1 engaging F-actin), while mechanical force promotes a population-level transition from weak to strong states that stabilizes the interaction; the FSGS-causing K255E mutation disrupts this mechanosensing by constitutively locking the protein in the strong binding state, providing a structural explanation for disease pathogenesis.

Catch bonds, noncovalent supramolecular interactions whose lifetimes are increased by force, are ubiquitous in mechanical signaling pathways. The structural mechanisms of catch-bonding proteins remain unclear, hampering efforts to decipher how they are dysregulated in disease and exploit them therapeutically. The crosslinker α-actinin-4 (ACTN4) forms catch bonds with actin filaments (F-actin) to support the function of kidney podocytes, and its force-insensitive K255E variant causes autosomal dominant focal segmental glomerulosclerosis (FSGS). Using cryo-electron microscopy (cryo-EM), we find that wild-type ACTN4 engages F-actin in two modes, which biochemical experiments and molecular dynamics simulations assign as strong- and weak-binding states, while K255E ACTN4 only populates the strong binding state. By implementing a cryo-EM platform for applying tension across crosslinker–F-actin interfaces using myosin motors, we find that force promotes a weak-to-strong binding transition for wild-type ACTN4, consistent with a two-state catch bond model. Beyond providing mechanistic insight into how the K255E mutation disrupts ACTN4 F-actin catch-bonding in FSGS, this approach enables structural dissection of force-sensitive actin-binding proteins.