Binding of fibronectin to phospholipid vesicles

Human plasma fibronectin, a membrane-associated protein involved in cell-cell adhesion and growth control, binds tightly to phospholipid vesicles of various compositions, as shown by density gradient centrifugation. The binding occurs in the absence of other proteins, sugars, or divalent cations and results in extensive aggregation of the vesicles. Circular dichroism spectroscopy demonstrated that the protein when associated with vesicles adopts a different conformation from that which it adopts in aqueous solution. The vesicle conformation results from a specific interaction and not merely a hydrophobic effect since it also differs substantially from the conformation of the protein in several detergents. Thus, fibronectin is not only capable of binding to lipid vesicles, but the binding induces both a conformational change in the protein and a structural change (aggregation) in the vesicles, which could be related to its role in cell-cell interactions. While these studies do not constitute proof that lipid molecules are the physiological sites of attachment for fibronectin, they do demonstrate that an intermediate receptor need not necessarily be involved to account for the binding properties.