Interaction of apolipoprotein A-I in three different conformations with palmitoyl oleoyl phosphatidylcholine vesicles

Interactions of apolipoprotein A-I (apoA-I) with cell membranes appear to be important in the initial steps of reverse cholesterol transport. The objective of this work was to examine the effect of three distinct conformations of apoA-I (lipid-free and in 78 A or 96 A reconstituted high density lipoproteins, rHDL) on its ability to bind to, and ab- stract lipids from, palmitoyl oleoyl phosphatidylcholine membrane vesicles (small unilamellar vesicles, SUV, and giant unilamellar vesicles, GUV). The molecular interactions were observed by two-photon fluorescence microscopy, and the binding parameters were quantified by gel-permeation chromatography or isothermal titration microcalorimetry. Rearrangement of apoA-I-containing particles after expo- sure to SUVs was examined by native gel electrophoresis. The results indicate that lipid-free apoA-I binds reversibly, with high affinity, to the vesicles but does not abstract a sig- nificant amount of lipid nor perturb the vesicle structure. The 96 A rHDL, where all the amphipathic helices of apoA-I are saturated with lipid within the particles, do not bind to vesicles or perturb their structure. In contrast, the 78 A rHDL have a region of apoA-I, corresponding to a few am- phipathic helical segments, which is available for external or internal phospholipid binding. These particles bind to vesicles with measurable affinity (lower than lipid-free apoA-I), abstract lipids from the membranes, and form par- ticles of larger diameters, including 96 A rHDL. We con- clude that the conformation of apoA-I regulates its binding affinity for phospholipid membranes and its ability to ab- stract lipids from the membranes. —Tricerri, M. A., S. A. Sanchez, C. Arnulphi, D. M. Durbin, E. Gratton, and A. Jonas. Interaction of apolipoprotein A-I in three different confor- mations with palmitoyl oleoyl phosphatidylcholine vesicles. J. Lipid Res. 2002. 43: 187-197.