The Cartilage-specific Fibronectin Isoform Has a High Affinity Binding Site for the Small Proteoglycan Decorin

Binding of fibronectin to the small proteoglycan decorin plays an important role in cell differentiation and cell migration. The cartilage-specific (V+C) − fibronectin isoform, in which nucleotides that normally encode the protein segments V, III 15 , and I 10 are spliced out, is one of the major splice variants present in cartilage matrices. Full-length and truncated cDNA constructs were used to express recombinant versions of fibronectin. Results demonstrated that the (V+C) − isoform has a higher affinity for decorin. Dissociation constants for decorin and fibronectin interaction were calculated to be 93 n m for the V + C + isoform and 24 n m and 223 n m for (V+C) − fibronectin. Because heparin competed with decorin competitively, binding of decorin to fibronectin likely occurs at a heparin-binding region. We propose that alternative splicing of the V and C regions changes the global conformation of fibronectin in such a way that it opens an additional decorin-binding site. This conformational change is responsible for the higher affinity of the (V+C) − fibronectin isoform for decorin.