Gelatin-binding domain-specific anti-human plasma fibronectin Fab' inhibits fibronectin-mediated gelatin binding but not cell spreading.

Antisera against a Mr = 60,000 peptide containing the gelatin-binding domain of human plasma fibronectin (McDonald, J. A., and Kelley, D. G. (1980) J. Biol. Chem. 255, 8848-8858) bound the Mr = 60,000 peptide and intact fibronectin but not three other fragments released by leukocyte elastase proteinolysis (the Mr = 25,000 amino-terminal sequence, Mr = 140,000 sequence containing cell adhesive activity, and a Mr = 31,000 fragment). Affinity-purified Fab' blocked Mr = 60,000 peptide binding to gelatin and inhibited plasma and cellular fibronectin gelatin binding without affecting fibronectin-mediated cell spreading. In contrast, antifibronectin Fab' absorbed with the gelatin-binding fragment completely blocked fibronectin-mediated cell spreading. These data indicate that the gelatin-binding domain of fibronectin is immunogenic, and antisera against this domain recognize cellular fibronectin gelatin-binding sites. Inhibition of gelatin binding but not cell spreading by anti-gelatin binding domain Fab' confirms the hypothesis that fibronectin has separate sites mediating these activities. Selective inhibition of fibronectin-collagen binding by domain-specific antisera may help elucidate the role of fibronectin in organization of the extracellular matrix.