Background

Several membrane-perturbing agents extract lipids from membranes; these include proteins, peptides, and detergents. Several of these lipid extractions are fundamental processes in biology. In some cases, the process is lethal; this is the case for some antibacterials that extract lipids from bacterial membranes leading to the death of the cells (Bechinger, 2014; Schaefer et al., 2014). In other systems, this process is vital. For example, ApoA1 is a protein that binds to cells and extracts phospholipids and cholesterol to form nascent high density lipoproteins (HDL), a process critical for the reverse cholesterol transport and therefore playing a pivotal role in the control of atherosclerosis (Strömstedt et al., 2010). It has been shown that some of these lipid extractions are lipid specific; in other words, the lipid composition of the extracted fraction is different than that of the original membranes. It is expected that such specificity will be reported more often considering the recent progress of lipidomics. In general, the lipid specificity of the induced lipid efflux is poorly characterized despite the pivotal role it plays in biological processes. We have recently shown that selective lipid efflux depends on specific interactions with membranes (Therrien et al., 2013; 2016). In order to gain a better description of this emerging phenomenon and to detail the intermolecular interactions that are involved, a method has been developed to characterize the extent and the specificity of membrane-lipid extraction by perturbing agents using model membranes. A perturbing agent is incubated with model membranes existing as multilamellar vesicles (MLVs). The classes of perturbing agents include proteins (e.g., binder-of-sperm proteins BSPA1, ApoA1), peptides (e.g., melittin, antibacterial peptides), and detergents (e.g., Triton X-100). The use of model membranes allows a complete control of the lipid composition, modulating in a precise manner molecular features such as the charge and the H-bond capacity, and defining how these factors contribute to the lipid specificity of the extraction. Subsequently, the remaining MLVs, which may include some perturbing agent, are separated by centrifugation from the small lipid/perturbing agent complexes resulting from the lipid extraction. The lipid compositions of the pellet (representative of intact membranes) and of the supernatant (representative of the lipid efflux) are analysed in order to assess the extent and the specificity of the lipid extraction.