Many proteins, including prions and fibronectin, have physiological characteristics in the aggregated state that are greatly different from those of their non-aggregated and water soluble counterparts. Nature evolved a series of mechanisms to control and regulate molecular recognition processes. These include conformational changes induced by ligand binding, or the burial of recognition sites through intra- or intermolecular self-assembly. Understanding how alterations of the functional states of proteins are regulated through molecular assembly is particularly interesting for large multidomain proteins that carry multiple recognition sites. The model protein discussed here will be fibronectin, which mediates cell adhesion to surfaces. We found that the application of mechanical force is crucial to facilitate its assembly into fibers, as well as to regulate the accessibility of its cell binding site.