Apolipophorin III induced DMPC vesicle transformation

Abstract

Apolipophorin III (apoLp-III) is a model exchangeable apolipoprotein. The protein consists of a bundle of five £\-helices, organized in a simple up-and-down topology. The protein is able to transform phospholipid vesicles, made of dimyristoylphosphatidylcholine (DMPC), into small disc-like structures. Previous studies demonstrated that four apoLp-III molecules align as an extended £\-helix on the periphery of the disc bilayer. The vesicles will be used as a model membrane to study the interaction of apoLp-III with various lipids. To gain a detailed insight into discoidal particle formation, we started to investigate the ability of apoLp-III to transform DMPC vesicles into discs at the lipid transition temperature. The absorbance of phospholipid vesicle suspensions, measured at 325 nm, decreased rapidly upon addition of apoLp-III. This indicates formation of the much smaller discoidal particles. Multilamellar vesicles were cleared at a slower rate compared to small unilamellar vesicles (SUV, made by extrusion). The rate of clearance was enhanced with increasing apoLp-III concentrations, as well as increasing the total amount of both apoLp-III and DMPC. The larger SUVs (200–500 nm diameter), were cleared at a faster rate compared to smaller SUVs (50–100 nm). Incorporation of cholesterol into SUV also increased vesicle transformation rates. Further studies will be employed to investigate the effect of other lipid additions to phospholipid vesicles, thereby gaining understanding about the lipid factors that trigger apolipoprotein binding.