Cytoskeletal deformation at high strains and the role of cross-link unfolding or unbinding

Actin cytoskeleton has long been a focus of attention due to its biological significance and unique rheological properties. Although F-actin networks have been extensively studied experimentally and several theoretical models proposed, the detailed molecular interactions between actin binding proteins (ABPs) and actin filaments that regulate network behavior remain unclear. Here, using an in vitro assay that allows direct measurements on the bond between one actin cross-linking protein and two actin filaments, we demonstrate force-induced unbinding and unfolding of filamin. The critical forces prove to be similar, 70 ± 23 pN for unbinding and 57 ± 19 pN for unfolding, suggesting that both are important mechanisms governing cytoskeletal rheology. We also obtain the mechanical response of a cross-linked F-actin network to an optically trapped microbead and observe abrupt transitions implying rupture or unfolding of cross-links. These measurements are interpreted with the aid of a computational simulation of the experiment to provide greater insight into physical mechanisms.