Binding of tenascin-C to soluble fibronectin and matrix fibrils

The small splice variant of tenascin-C (TN) has eight fibronectin type III (FN3) domains. The major large splice variant has three (in chicken) or seven (in human) additional FN3 domains inserted between domains five and six. Chiquet-Ehrismann et al. (Chiquet-Ehrismann, R., Matsuoka, Y., Hofer, U., Spring, J., Bernasconi, C., and Chiquet, M. (1991, Cell Regul. 2, 927-938) demonstrated that the small variant bound preferentially to fibronectin in enzyme-linked immunosorbent assay, and only the small variant was incorporated into the matrix by cultures of chicken fibroblasts. Here we have studied human TN, and confirmed that the small variant binds preferentially to purified fibronectin and to fibronectin-containing extracellular matrix. Thus this differential binding appears to be conserved across vertebrate species. Using bacterial expression proteins, we mapped the major binding site to the third FN3 domain of TN. Consistent with this mapping, a monoclonal antibody against an epitope in this domain did not stain TN segments bound to cell culture matrix fibrils. The enhanced binding of the small TN variant suggests the existence of another, weak binding site probably in FN3 domains 6-8, which is only positioned to bind fibronectin in the small splice variant. This binding of domains 6-8 may involve a third molecule present in matrix fibrils, as the enhanced binding of small TN was much more prominent to matrix fibrils than to purified fibronectin.