Structure and functional significance of mechanically unfolded fibronectin type III₁ intermediates

Fibronectin (FN) forms fibrillar networks coupling cells to the extracellular matrix. The formation of FN fibrils, fibrillogenesis, is a tightly regulated process involving the exposure of cryptic binding sites in individual FN type III (FN-III) repeats presumably exposed by mechanical tension. The FN-III₁ module has been previously proposed to contain such cryptic sites that promote the assembly of extracellular matrix FN fibrils. We have combined NMR and steered molecular dynamics simulations to study the structure and mechanical unfolding pathway of FN-III₁. This study finds that FN-III₁ consists of a β-sandwich structure that unfolds to a mechanically stable intermediate about four times the length of the native folded state. Considering previous experimental findings, our studies provide a structural model by which mechanical stretching of FN-III ₁ may induce fibrillogenesis through this partially unfolded intermediate.