Mechanical forces play a pivotal role in cellular adhesions, where a vast array of proteins interacts with the cell cytoskeleton, affecting focal adhesions and adherens junctions, and regulating cell behavior and fate. To directly observe such events, we developed an experimental assay that combines several advanced single molecule techniques. Here, ultrafast force-clamp spectroscopy is employed to directly probe the force-dependence of molecular interactions between a single actin filament and a binding protein with sub-ms time resolution. Stabilization of the microscope through local gradient localization enables the resolution of protein conformational changes and binding position with sub-nm accuracy. An experimental arrangement, termed oriented dumbbell, allows us to determine the actin filament orientation and, thus, asymmetries in the force response of the interacting proteins. We applied our methodology to the interaction between α-catenin and F-actin, revealing that α-catenin switches between a slip and an asymmetric cooperative catch-bond with F-actin. This mechanism may underlie fluid-to-solid phase transitions that occur at the membrane-cytoskeleton interface.
