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 nm for the V(+)C(+) isoform and 24 nm and 223 nm 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.