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

Although acidic fibroblast growth factor (aFGF) lacks a conventional signal sequence, it is often found complexed to sulfated proteoglycans on the external surface of cells. The protein also forms a ''molten globule''-like state at neutral pH and physiological temperatures as well as at acidic pH in the presence of physiological ionic strength or moderate quantities of polyanions. These states display a marked tendency to aggregate. Such observations suggest that related partially structured states might be involved in the membrane translocation of aFGF. To explore this hypothesis, we examined the interaction of this growth 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 at acidic pH, inducing bilayer disruption. The rate of leakage of a liposome-entrapped fluorescent probe is proportional to the logarithm of the aFGF concentration, suggesting competition between protein self-association and membrane binding. Liposome leakage can be also induced at neutral pH by partial unfolding of aFGF at or above physiological temperature in contrast to most control proteins. The importance of partially folded hydrophobic surfaces in aFGF self-association and membrane binding is further suggested by the fact that thermally unfolded aFGF does not aggregate, in contrast to states observed at intermediate temperatures or transiently during unfolding at high temperatures. In contrast to heparin, a polyanion which stabilizes the native structure of aFGF, negatively charged phospholipid membranes appear to enhance the disruption of aFGF tertiary structure at submicellar concentrations of sodium dodecyl sulfate but stabilize the remaining secondary structure. Thus negatively charged lipid bilayers appear to interact with partially structured states of aFGF by preferential binding of both its apolar and charged surfaces to complementary regions of the lipid bilayer. Such interactions may play a role in the membrane translocation of this growth factor.