Talin Impacts Force-Induced Vinculin Activation through ‘Loosening’ the Vinculin Inactive State

Abstract

Focal Adhesions (FA) are large, multi-protein complexes connecting the cytoskeleton to the extracellular matrix. Their adhesive functionality is tightly regulated by mechanical stress. A key component of FA-associated mechanosensing is vinculin. It consists of a globular head, a proline-rich neck region, and a rod-like tail domain that contains binding sites for many other cytoplasmic proteins. Vinculin can assume either a closed ("inactive") or open (“active”) conformation. The underlying activation mechanism, however, remains yet to be fully understood. Here we employ molecular dynamics (MD) simulation to demonstrate that vinculin activation is greatly facilitated by the binding of vinculin on talin's vinculin binding site. Steered MD simulations reveal that the force required for vinculin activation is drastically reduced, by more than 50%, upon formation of the vinculin-talin complex. To explore this observation further, we use dynamic network fluctuation analysis showing how force propagation through vinculin changes upon complex formation. Interestingly, after talin dissociation, vinculin returns to its native conformation on a submicrosecond time scale, with 60% of its native contacts restored. Our results suggest a rapid dynamic equilibrium between 'tight' and 'loosened' inactive vinculin, which depends on talin and determines the level of mechanical stress required for activation. Our study has important implications for our understanding of mechano-sensing mechanisms at FAs.