NMR Contacts between the HIV Fusion Peptide and Lipid Support a Beta-Bowl Membrane Topology of the Peptide with Thermodynamic Preference for Peptide/Cholesterol Contact

Abstract

The HIV fusion peptide (FP) is the ∼25-residue N-terminal region of the gp41 protein and plays a critical role in joining virus and host cell membranes. Segments of gp41 that include the FP catalyze fusion of membrane vesicles, which supports a FP contribution to viral fusion. The FP adopts intermolecular antiparallel β sheet structure in membrane that contains ∼30 mole% cholesterol, which is typical for HIV host cells. NMR measurements support distinct populations of membrane locations of the FP β sheet. The dominant population has <5 Å contacts between residues in the sheet interior and –CH3 groups of phosphatidylcholine (PC) and cholesterol lipids near the membrane center. Longer >10 Å distances are detected to lipid headgroups. By contrast, close headgroup contacts are observed for residues at the sheet edges, with longer distances to the –CH3 groups. There is a smaller FP population with contacts between interior strand residues and the membrane surface. A FP β-bowl inserted in a single membrane leaflet is consistent with many of these contact data. The middle sections of interior strands of the bowl contact the membrane center, whereas strand termini contact lipid headgroups. Most of the bowl edge strands contact the membrane surface. Lipids contacting the bowl adopt its curvature, which may reduce activation energy needed to achieve the high-curvature transition state of fusion. The NMR data also support higher population of FP-cholesterol vs. FP-PC contacts. The thermodynamic preference of cholesterol for the FP bowl may be due to higher intrinsic curvature of cholesterol vs. PC.