Fusion Activity of HIV GP41 Fusion Domain is Related to its Secondary Structure in a Cholesterol-Dependent Fashion

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. We examined the secondary structure of the fusion domain in small unilamellar vesicles composed of different lipid compositions by circular dichroism spectroscopy. The results show that the secondary structure switches from alpha helix below approx. 30% cholesterol to beta structure above this threshold. The lipid and content mixing activities of the fusion domain were examined by standard FRET fusion assays. Although the fusion activities increase with higher percentages of cholesterol, the helical conformation supports fusion in the absence of cholesterol and the beta-sheet conformation supports fusion in the presence of >30% cholesterol. Our results also show that the fusion domain has a higher activity in more negatively charged membranes, which can be explained by its higher binding affinity to such membranes.