Lipid Selectivity in Lipid Efflux Induced by Proteins and Peptides

Abstract

Some proteins and peptides have the ability to extract lipids from membranes. In some systems, such as some toxins, it leads to cell death. In other systems, this process is vital. For example, bovine seminal plasma contains phosphocholine-binding proteins that associate with sperm membranes upon ejaculation and cause lipid efflux; this effect modifies the membrane lipid composition, an essential step in the maturation process of fertile sperm. In general, the affinity of these peptides/proteins for membranes and their impact on membrane permeability are well characterized. However the induced lipid efflux and particularly its lipid specificity are much less investigated. Using model membranes with different compositions, we identified the lipid selectivity of the membrane lipid efflux of two systems. First, we characterized the lipid efflux induced by BSP1, the most abundant Binder-of-Sperm protein in bull seminal plasma. The protein binding displays a distinct affinity for phosphatidylcholine. However, even though the protein anchoring on membranes is PC-specific, we demonstrated that the lipid extraction is practically without any lipid specificity; BSP1 molecules solubilise a lipid patch to form small complexes with a stoichiometry of 10-15 lipids per protein. Second, we characterized the lipid specificity of lipid efflux induced by melittin, a helical peptide with a secondary amphipathic character. Phosphatidylethanolamine, and cholesterol, two molecular species ordering bilayers, reduce the membrane affinity of melittin. We determined that, in parallel, the lipid fraction extracted by melittin is depleted of these lipids. It is proposed that the peptide accumulates in regions of bilayers where the bilayers are less ordered and, when its concentration is sufficient, detaches this patch from the membrane. These findings illustrate the need to investigate globally lipid efflux processes, from the peptide/protein binding to the mixed lipid-peptide/protein self-assembly formation.