Fusion Activity of HIV gp41 Fusion Domain Is Related to Its Secondary Structure and Depth of Membrane Insertion in a Cholesterol-Dependent Fashion

The human immunodeficiency virus (HIV) gp41 fusion domain plays a critical role in membrane fusion during viral entry. A thorough understanding of the relationship between the structure and the activity of the fusion domain in different lipid environments helps to formulate mechanistic models on how it might function in mediating membrane fusion. The secondary structure of the fusion domain in small liposomes composed of different lipid mixtures was investigated by circular dichroism spectroscopy. The fusion domain formed an α-helix in membranes containing less than 30 mol% cholesterol and formed β-sheet secondary structure in membranes containing ≥ 30 mol% cholesterol. EPR spectra of spin-labeled fusion domains also indicated different conformations in membranes with and without cholesterol. Power saturation EPR data were further used to determine the orientation and depth of α-helical fusion domains in lipid bilayers. Fusion and membrane perturbation activities of the gp41 fusion domain were measured by lipid mixing and contents leakage. The fusion domain fused membranes in both its helical form and its β-sheet form. High cholesterol, which induced β-sheets, promoted fusion; however, acidic lipids, which promoted relatively deep membrane insertion as an α-helix, also induced fusion. The results indicate that the structure of the HIV gp41 fusion domain is plastic and depends critically on the lipid environment. Provided that their membrane insertion is deep, α-helical and β-sheet conformations contribute to membrane fusion.