Disseminating the HIV Fusion Mechanism: gp41 Structural Details in Membranes

Abstract

Membrane fusion is the initial step of HIV infection, and is carried out by the viral gp41 protein through direct interaction with target cells at physiologic pH and conformational change. The N-terminal fusion peptide (FP) region of gp41 is involved in both target membrane insertion and perturbation, and its trimeric assembly and depth of membrane insertion have been correlated with fusion fitness. Key gp41 fusion conformations include early and late, and are characterized by trimeric coiled-coil and low energy six-helix bundle structure, respectively. Recent findings point to early gp41 as fusion active, with late gp41 involved in membrane bilayer stabilization. We present solid-state NMR structural analysis of constructs modeling early and late gp41 fusion conformations in membranes which suggests that specific FP structure is not correlated with gp41 fusion action or inaction, and that fusion arrest may be due to withdrawal of the FP from membranes for the late gp41 six-helix bundle conformation. Specifically, we observe populations of helical and β-sheet FP structure for both early and late gp41. β-sheet FP structure in early and late gp41 is predominant and analogous to structure observed in minimal FP fragments, with antiparallel strand arrangement crossing at approximately Leucine-7. Helical structure in coiled-coil and six-helix bundle regions is preserved upon membrane interaction, and induces partial helical structure in downstream FP for early and late gp41, respectively. FP structure is sensitive to membrane cholesterol only in early fusion active gp41, which suggests FP insertion into membranes as shown for minimal FP fragments, and lack of FP insertion for late, fusion inactive gp41. Additional solid-state NMR findings are presented examining the membrane location of the gp41 apolar FP region. Our structural analysis provides new insight into the mechanism of membrane fusion induced by HIV gp41.