Fusion Activity of HIV Gp41 Fusion Domain Is Related to its Secondary Structure

Abstract

The HIV gp41 fusion domain plays a critical role in membrane fusion during the viral entry process. A thorough understanding of the relationship between the structure and activity of the fusion domain in different environments helps to formulate mechanistic models on how it might function in mediating membrane fusion. The secondary structure of the fusion domain in small unilamellar vesicles composed of different lipid compositions was investigated by circular dichroism spectroscopy. In membranes containing less than 30% cholesterol, the fusion domain forms an alpha helix, and in membranes containing more than 30% cholesterol, the fusion domain forms a beta structure. EPR spectroscopy of spin-labeled fusion domains is consistent with two different conformations in membranes with and without cholesterol and further determines the orientation and depth of the fusion domain in membranes. The fusion and membrane penetration activity of this domain in different membranes was measured by fluorescent lipid mixing and contents leakage assays. Interestingly, the fusion domain fuses membranes in both its helical and beta-structured forms. A high percentage of cholesterol, which promotes beta structure, promotes fusion, but a high concentration of acidic lipids, which in the absence of cholesterol leads to membrane insertion as an alpha helix, also promotes fusion. The results indicate that the HIV gp41 fusion domain is plastic in different membrane environments and that both the alpha helical and beta structured forms may contribute to fusion. Supported by NIH grant AI030557