HIV Fusion Peptide Perturbs Membrane Structure in a Cholesterol Dependent Fashion

Abstract

The fusion between viral envelopes with host cell membranes is required for viral entry and infection, which is mediated by special glycoproteins anchored on the viral membrane. Fusion peptide (FP) is the domain that initiates membrane fusion. However, the mechanism of membrane fusion is still unclear. We previously found that the influenza hemagglutinin FP increases the order of DMPC lipid bilayer. We hypothesize that inducing lipid ordering might be a critical step in fusion caused by a variety of fusion proteins. HIV gp41 FP plays a similar role as influenza HA FP. However, gp41 FP is polymorphic and changes from alpha helix to beta aggregation as cholesterol concentration in lipid increases. We used PC spin labels on the lipid head group and different positions on the acyl chain to detect the perturbation by gp41 FP to POPC/POPG lipid bilayers with different cholesterol concentration (0% to 30%) by CW-ESR. Our data show that 1) gp41 FP affects the lipid order in the same pattern as HA FP does, i.e., a cooperative effect vs. lipid/peptide ratio, thus supporting our hypothesis; 2) gp41 FP induces membrane ordering in all tested lipid compositions, consistent with promoting membrane fusion in these compositions; 3) in the high cholesterol containing lipid bilayers, whereas gp41 FP is in the beta aggregation conformation, its effect on the lipid ordering reaches deeper into the bilayer, consistent with deeper membrane insertion for gp41 FP in this conformation. We are extending the ESR studies to look for coexisting membrane microdomains induced by different conformations of FPs and the FP partitioning between them and for precise separation of the effects of ordering and molecular motion.