CONFORMATIONAL TRANSITIONS IN THE CELL-BINDING DOMAIN OF FIBRONECTIN

Plasma fibronectin readily changes shape in response to environmental conditions which may, in turn, lead to differential expression of its multiple functional sites. To test this possibility, the expression of two of the type III modules within cell binding domain of fibronectin was assessed with monoclonal antibodies (mAb). Utilizing proteolytic and recombinant fragments of plasma fibronectin, the epitopes recogniz ed by mAbIII-9 and mAbIII-10 were localized to the ninth and tenth (RG D-containing) type III repeats of fibronectin, respectively. Both mAb inhibited the adhesion of platelets to immobilized fibronectin, sugges ting that the recognized epitopes resided in close spatial proximity t o the cell binding sites. Radioimmunoassay and Scatchard analyses show ed that, in solution, each dimeric fibronectin molecule bound two mAbI II-9 but only one mAbIII-10 molecule (ionic strength 0.15, pH 7.4). Th e binding of a single mAbIII-10 per fibronectin molecule was verified by electron microscopy. Heparin, heparan sulfate, gangliosides (but no t chondroitin sulfates A and B and hyaluronic acid), and self-associat ion increased the apparent affinity of mAbIII-10 for soluble fibronect in. Adsorption of fibronectin onto a polystyrene surface resulted in t he appearance of an additional binding site for mAbIII-10. MAbIII-9 bi nding also was altered by fibronectin immobilization. These results su ggest that the deposition of fibronectin and its interaction with comp onents of the extracellular matrix can modulate the expression of the cell binding domains including the RGDS-containing type III repeat. Ex posure of the second tenth type III repeat within the fibronectin dime r, as a result of unfolding on a surface, could contribute to the enha nced adhesiveness of adsorbed fibronectin.