LOW CONCENTRATIONS OF DIACYLGLYCEROL PROMOTE THE BINDING OF APOLIPOPHORIN-III TO A PHOSPHOLIPID-BILAYER - A SURFACE-PLASMON RESONANCE SPECTROSCOPY STUDY

The binding of the exchangeable apolipoprotein apolipophorin III (apoLp-III) to an egg phosphatidylcholine bilayer as a function of the concentration of diacylglycerol (DG) in the bilayer was studied by surface plasmon resonance spectroscopy. At a DG concentration of 2 mol % in the bilayer, the binding of apoLp-III reached saturation, Under saturating conditions, apoLp-III forms a closely packed monolayer approximate to 55 Angstrom thick in which each molecule of protein occupies approximate to 500 Angstrom(2) at the membrane surface. These dimensions are consistent with the molecular size of the apoLp-III molecule determined by x-ray crystallography, if apoLp-III binds to the bilayer with the long axis of the apoLp-III normal to the membrane surface. In the absence of protein, the overall structure of the lipid bilayer was not significantly changed up to 2.5 mol % DG. However, at 4 and 6 mol % DG, the presence of nonbilayer structures was observed, The addition of apoLp-III to a membrane containing 6 mol Ci DG promoted the formation of large lipid-protein complexes., These data support a two-step sequential binding mechanism for binding of apoLp-III to a lipid surface. The first step is a recognition process, consisting of the adsorption of apoLp-III to a nascent hydrophobic defect in the phospholipid bilayer caused by the presence of DG, This recognition process might depend on the presence of a hydrophobic sensor located at one of the ends of the long asis of the apoLp-III molecule but would be consolidated through H-bond and electrostatic interactions, Once primary binding is achieved, subsequent enlargement of the hydrophobic defect in the lipid surface would trigger the unfolding of the apolipoprotein and binding via the amphipathic alpha-helices. This two-step sequential binding mechanism could be a general mechanism for all exchangeable apolipoproteins. A possible physiological role of the ability of apoLp-III to bind to lipid structures in two orientations is also proposed.