Location of the cell-attachment site in fibronectin with monoclonal antibodies and proteolytic fragments of the molecule.

Proteolytic fragments of human plasma fibronectin were used to identify monoclonal antibodies reacting with the various domains of fibronectin. One of these antibodies, which reacts with cell-attachment-promoting fragments of fibronectin, inhibits attachment of cells to fibronectin-coated surfaces. A cell-attachment-promoting, chymotryptic, 120 kilodalton fragment was cleaved further with pepsin into three main fragments. The smallest, 15 kilodalton fragment was purified by affinity chromatography on the cell-attachment-inhibiting antibody insolubilized on Sepharose. This fragment is active in promoting cell attachment but lacks the other known binding activities of fibronectin. It can be localized between the collagen-binding and heparin-binding domains, about 127 to 197 kilodaltons from the NH2 terminus of the polypeptide. These results show that the interaction of fibronectin with cells is restricted to a defined portion of the molecule and is independent of the direct involvement of the known affinities toward other macromolecules.