Structural Plasticity in the Topology of Membrane-Spanning Domain of HIV-1 gp41

Abstract

We use a number of computational and experimental approaches to investigate the mem-brane topology of the Membrane-Spanning Domain of the HIV-1 gp41 ectodomain. Several new putative transmembrane regions are identified using hydrophobicity analysis based on Wimley-White scales, including the membrane-proximal external region (MPER). The MPER region is an important target for neutralizing anti-HIV monoclonal antibodies and is believed to have an interfacial topology in the membrane. To assess the possibility of a transmembrane topology of MPER we examined the membrane interactions of a peptide corresponding to a 22 residue long stretch of MPER sequence (residues 662-683) using fluo-rescence spectroscopy and oriented circular dichroism. In addition to the previously report-ed interfacial location, we identify a stable transmembrane conformation of the peptide in synthetic lipid bilayers. All-atom molecular dynamics simulations of the MPER-derived peptide in a lipid bilayer demonstrate a stable helical structure with an average tilt of 24 de-grees with the five tryptophan residues sampling different environments inside bilayer's hy-drocarbon core, consistent with spectral properties of intrinsic fluorescence. The degree of lipid bilayer penetration obtained by computer simulation was verified using depth-dependent fluorescence quenching of selectively attached fluorescence probe. Our overall data indicate that the MPER sequence can have at least two stable conformations in the lipid bilayer, interfacial and transmembrane, and suggest a possibility that external perturbations can switch the topology during physiological functioning. Supported by NIH GM069783.