Molecular interactions of fibronectin

Fibronectin mediates attachment of cells to surfaces in vitro, and its abundance in basement membrane structures suggests that it serves a similar function in vivo. The cell attachment promoting activity of fibronectin depends on its capacity to interact with a number of other macromolecules. These include collagen, fibrin(ogen), glycosaminoglycans, and as yet unidentified receptors on the surfaces of various mammalian and avian cells and on the surfaces of Staphylococci. Denatured collagens bind more avidly to fibronectin than the native forms. Of the latter, type III is the most active one. The collagen-binding is inhibited by fibrinogen and vice versa, indicating that the same (or overlapping) binding site are responsible for the binding of fibronectin to collagen and fibrinogen. The binding site for collagen in fibronectin can be localized in a homogeneous fragment with a molecular weight of 30,000. This binding site is distinct from the one interacting with cell surfaces, since the cell attachment promoting activity is found associated with collagen-nonbinding fragments of fibronectin. These same fragments also bind to Staphylococci, suggesting that the cell surface receptor in these bacteria may be similar to that of eukaryotic cells. Glycosaminoglycans bind to collagen-fibronectin complexes resulting in a complex more stable to disruption with urea than complexes of any two of the components. This phenomenon may be important in the formation of the fibronectin-containing, adhesive, extracellular matrix, and its significance to the apparent disturbances that many malignant cells have in forming such a matrix in particular, warrants further study.