The C-terminal actin binding domain of talin forms an asymmetric catch bond with F-actin

Abstract

Focal adhesions (FAs) are large, integrin-based adhesion complexes that link cells to the extracellular matrix (ECM). Previous work demonstrates that FAs form only when and where they are necessary to transmit force between the cellular cytoskeleton and the ECM, but how this occurs remains poorly understood. Talin is a 270 kDa adapter protein that links integrins to filamentous (F)-actin and recruits additional components during FA assembly in a force-dependent manner. Cell biological and developmental data demonstrate that the third, and C-terminal, F-actin binding site (ABS3) of talin is required for normal FA formation. However, ABS3 binds F-actin only weakly in in vitro, biochemical assays. We used a single-molecule optical trap assay to examine how and whether ABS3 binds F-actin under physiologically relevant, pN mechanical loads. We find that ABS3 forms a directional catch bond with F-actin when force is applied towards the pointed end of the actin filament, with binding lifetimes more than 100-fold longer than when force is applied towards the barbed end. Long-lived bonds to F-actin under load require the ABS3 C-terminal dimerization domain, whose cleavage is known to regulate focal adhesion turnover. Our results support a mechanism in which talin ABS3 preferentially binds and orients actin filaments with barbed ends facing the cell periphery, thus nucleating long-range order in the actin cytoskeleton. We suggest that talin ABS3 may function as a molecular AND gate that allows FA growth only when sufficient integrin density, F-actin polarization, and mechanical tension are simultaneously present.