INTERACTION OF PARTIALLY STRUCTURED STATES OF ACIDIC FIBROBLAST GROWTH-FACTOR WITH PHOSPHOLIPID-MEMBRANES

Although acidic fibroblast growth factor (aFGF) lacks a conventional s ignal sequence, it is often found complexed to sulfated proteoglycans on the external surface of cells. The protein also forms a ''molten gl obule''-like state at neutral pH and physiological temperatures as wel l as at acidic pH in the presence of physiological ionic strength or m oderate quantities of polyanions. These states display a marked tenden cy to aggregate. Such observations suggest that related partially stru ctured states might be involved in the membrane translocation of aFGF. To explore this hypothesis, we examined the interaction of this growt h factor with lipid vesicles as well as the effect of such surfaces on the structure of the protein. We find that these states interact with negatively charged but not neutral phosholipid unilammelar vesicles a t acidic pH, inducing bilayer disruption. The rate of leakage of a lip osome-entrapped fluorescent probe is proportional to the logarithm of the aFGF concentration, suggesting competition between protein self-as sociation and membrane binding. Liposome leakage can be also induced a t neutral pH by partial unfolding of aFGF at or above physiological te mperature in contrast to most control proteins. The importance of part ially folded hydrophobic surfaces in aFGF self-association and membran e binding is further suggested by the fact that thermally unfolded aFG F does not aggregate, in contrast to states observed at intermediate t emperatures or transiently during unfolding at high temperatures. In c ontrast to heparin, a polyanion which stabilizes the native structure of aFGF, negatively charged phospholipid membranes appear to enhance t he disruption of aFGF tertiary structure at submicellar concentrations of sodium dodecyl sulfate but stabilize the remaining secondary struc ture. Thus negatively charged lipid bilayers appear to interact with p artially structured states of aFGF by preferential binding of both its apolar and charged surfaces to complementary regions of the lipid bil ayer. Such interactions may play a role in the membrane translocation of this growth factor.