Forced unfolding of the fibronectin type III module reveals a tensile molecular recognition switch

The 10th type III module of fibronectin possesses a β-sandwich structure consisting of seven β-strands (A-G) that are arranged in two antiparallel sheets. It mediates cell adhesion to surfaces via its integrin binding motif, Arg⁷⁸, Gly⁷⁹, and Asp⁸⁰ (RGD), which is placed at the apex of the loop connecting β-strands F and G. Steered molecular dynamics simulations in which tension is applied to the protein's terminal ends reveal that the β-strand G is the first to break away from the module on forced unfolding whereas the remaining fold maintains its structural integrity. The separation of strand G from the remaining fold results in a gradual shortening of the distance between the apex of the RGD-containing loop and the module surface, which potentially reduces the loop's accessibility to surface-bound integrins. The shortening is followed by a straightening of the RGD-loop from a tight β-turn into a linear conformation, which suggests a further decrease of affinity and selectivity to integrins. The RGD-loop therefore is located strategically to undergo strong conformational changes in the early stretching stages of the module and thus constitutes a mechanosensitive control of ligand recognition.