Labeling of a major fibroblast surface protein (fibronectin) catalyzed by blood coagulation factor XIIa

Incubation of cultured human fibroblasts with blood coagulation factor XIIIa (plasma transglutaminase, fibrinoligase) and the fluorescent primary amine, N-(5-aminopentyl)-5-dimethylaminonaphthalene-1-sulfonamide, resulted in fluorescent labeling of three cellular polypeptides. The molecular weights of the labeled polypeptides, estimated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate after reduction, were: greater than 1.2-10(6), 2.2-10(5), and 1.3-10(5). The labeled 2.2-10(5) dalton polypeptide was susceptible to mild trypsinization and not present in cultures of SV-40 transformed fibroblasts, indicating that it is the subunit of cell-surface fibronectin and identical with the external transformation-sensitive polypeptide of similar molecular weight described by others. Upon coelectrophoresis, the labeled 2.2-10(5) dalton polypeptide migrated slightly behind the subunit of plasma fibronectin (cold-insoluble globulin), indicating that the immunologically cross-reactive forms of fibronectin in human plasma and cultured human fibroblasts differ slightly in molecular weight. The identities of the labeled greater than 1.2-10(6) and 1.3-10(5) dalton polypeptides are not known. The XIIa-reactive glutamine residues of fibroblast cell-surface proteins are potential sites for intermolecular cross-linking (by xi-(gamma-glutamyl)lysyl linkages) to other proteins of connective tissue.