Recognition of Lipid Domain Boundaries by the HIV Fusion Peptide is an Essential Step for HIV Membrane Fusion

Abstract

“Lipid rafts” which are cholesterol-enriched regions of the plasma membrane have been recognized as possible platforms for HIV entry. However, due to their highly dynamic and nanoscopic nature in biological membranes, it is challenging to investigate the role of lipid rafts in HIV-host interactions. In this study, we have used model systems with microscopic raft-like domains (Lo phase) in supported lipid bilayers and giant unilamellar vesicles mimicking HIV envelopes and T-cell membranes. We show that the phase separation of HIV or T-cell lipid mixtures is cholesterol-dependent, and membrane binding and lipid mixing are much more efficient in vesicles with coexisting Ld and Lo phases than those with single Ld or Lo phase, indicating that lipid phase separation is necessary and sufficient for efficient membrane fusion. Interestingly, time-resolved TIRF microscopy demonstrates that the HIV fusion peptide preferentially targets Lo/Ld boundary regions and promotes membrane fusion at the interface between Lo and Ld phases. Analysis of individual fusion events shows that pure Ld phase vesicles proceed to hemifusion and only vesicles with Lo/Ld phase boundaries fuse fully. Based on our designed minimal systems for understanding lipid raft-dependent HIV-host interactions, we propose that recognition of domain boundaries by the HIV fusion peptide is an essential step for HIV entry.