Abstract
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 Å or 96 Å reconstituted high density lipoproteins, rHDL) on its ability to bind to, and abstract 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 exposure 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 significant amount of lipid nor perturb the vesicle structure. The 96 Å 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 Å rHDL have a region of apoA-I, corresponding to a few amphipathic 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 particles of larger diameters, including 96 Å rHDL. We conclude that the conformation of apoA-I regulates its binding affinity for phospholipid membranes and its ability to abstract 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 conformations with palmitoyl oleoyl phosphatidylcholine vesicles.
Highlights
Interactions of apolipoprotein A-I with cell membranes appear to be important in the initial steps of reverse cholesterol transport
It is well known that the conformation of apolipoprotein A-I (apoA-I) is distinct in the lipid-free state and in these two species of reconstituted high density lipoprotein (rHDL) particles [21, 37]
In this study we demonstrated that three different conformations of apoA-I regulate their binding to phospholipid vesicle membranes and the abstraction of lipids from them
Summary
Interactions of apolipoprotein A-I (apoA-I) with cell membranes appear to be important in the initial steps of reverse cholesterol transport. There is increasing evidence that apoA-I can adopt many distinct conformations, and that its ability to rearrange in response to changes in HDL lipids due to the action of plasma factors, play a key role in modulating the functions of the different HDL subpopulations [11, 12]. Because of their participation in reverse cholesterol transport, attention has turned to nascent subspecies of HDL containing apoA-I as its sole protein moiety. Several mechanisms could be contributing to the process: 1) spontaneous aqueous diffusion of cholesterol from cell membranes to the HDL particles, 2) interaction of apoA-I with membrane lipid domains and microsolubization of phospholipids and cholesterol, and 3) specific apolipoprotein interactions with the plasma membrane transporter ABC1 [8] or SR-BI receptors, which facilitate lipid efflux from cells [17, 18]
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