The C- and the N-terminal Regions of Glycoprotein 41 Ectodomain Fuse Membranes Enriched and Not Enriched with Cholesterol, Respectively

Abstract

To infect target cells, HIV-1 employs a virally encoded transmembrane protein (gp41) to fuse its viral envelope with the target cell plasma membrane. We describe the gp41 ectodomain as comprised of N-and C-terminal subdomains, each containing a heptad repeat as well as a fusogenic region, whose organization is mirrored by the intervening loop region. Recent evidence indicates that the gp41 directed fusion reaction proceeds to initial pore formation prior to gp41 folding into its low energy hairpin conformation. This implies that exposed regions of the gp41 ectodomain are responsible for the bulk of the fusion work, probably through direct protein-membrane interactions. Prevalent fusion models contend that the gp41 ectodomain initially interacts with the target cell surface through its highly hydrophobic N terminus, which is believed to insert into the target membrane, thereby linking the virus to the target cell. This arrangement allows the N-terminal subdomain to interact with the target cell surface, whereas the Cterminal subdomain remains proximal to the virion, allowing interaction with the viral envelope. The composition of the viral envelope and the target cell surface differ due to the virus budding from raft microdomains. We show here that constructs corresponding to the Cterminal subdomain specifically destabilize ordered and cholesterol rich membranes (33 molar %), whereas the N-terminal subdomain is more effective in fusing both unordered cholesterol-free membranes and those containing lower amounts of cholesterol (10 molar %). Moreover we show that, in the context of the C-terminal subdomain, the heptad repeat contributes helical structure, which may describe the enhanced inhibitory effect of the C-terminal subdomain relative to the C-terminal heptad repeat (C34) alone. Our results are discussed in light of recent findings that showcase the role of exposed gp41 regions in effecting membrane fusion.

The Aromatic Domain of the Coronavirus Class I Viral Fusion Protein Induces Membrane Permeabilization: Putative Role during Viral Entry

“…This region is proximal to a predicted TM domain (Figure 2A), which was identified using the WW octanol hydrophobicity scale that allows for the identification of TM helices of membrane proteins with very high accuracy (73). On the basis of these observations, it is unlikely that the aromatic domain is part of the TM anchor as previously predicted by Rota et al (6), rather, as described for the aromatic domains of HIV-1 and EboV (10,55,56,58), this region is most likely an independent domain proximal to the TM anchor of the S2 subunit.…”

“…The resulting structure is believed to align the fusion peptide, aromatic domain, and TM anchor (30), resulting in viral/ cell membrane fusion. On the basis of the observations that (1) the aromatic peptides of coronaviruses and other viruses (10,55,56,58) strongly interact with and disrupt membranes and (2) the overall conservation of these sequences in many viruses, we hypothesize that the aromatic domain of class I viral fusion proteins plays a critical role in viral/cell membrane fusion. Specifically, we propose that the aromatic domain, in conjunction with the fusion peptide and the TM anchor, forms a continuous track of hydrophobic, membraneinteracting surfaces that provide a low-energy (low-barrier) path for lipid flow and membrane fusion during virion/cell fusion ( Figure 7).…”

Conformational Flexibility and Strand Arrangements of the Membrane-Associated HIV Fusion Peptide Trimer Probed by Solid-State NMR Spectroscopy

“…Peptides composed of the HFP sequence induce fusion between large unilamellar vesicles (LUVs) and between erythrocytes (5,6). There are similar mutation/fusion activity relationships for HFP-induced fusion and HIV-induced fusion which suggests that HFP is a useful model system to understand some aspects of viral/target cell fusion (7-10).There is also evidence for participation of other regions of gp41 in membrane fusion (11)(12)(13)(14).There are atomic-resolution structures of the "soluble ectodomain" of gp41 which does not contain the ∼30 N-terminal residues of gp41 (including the HFP) (2,(15)(16)(17)(18)(19). These structures are believed to correspond to the conformation after fusion has occurred and perhaps during some fusion steps (20).…”

“…8a). The two-spin (three-spin) model has two (three) adjacent in-register parallel β strands and the simulations are based on two (three) 13 COs, each of which is at the same residue position in its respective strand. The interstrand distance and corresponding d CC value were input parameters for the simulations.…”

HIV-1 Glycoprotein Immunogenicity

“…This pattern plays an important role in both the early and late stages of the membrane fusion (Chan and Kim, 1998;Johnson et al, 2001;Liu et al, 2005;Lu et al, 1995;Munoz-Barroso et al, 1998). This process can also participate in the formation of the fusion pore through direct proteinmembrane interactions (Munoz-Barroso et al, 1998;Shnaper et al, 2004). Different studies have shown the immunogenicity of the fusogenic complex and describe Abs direct against this structure (Gustchina et al, 2010;Opalka et al, 2004;Vincent et al, 2008).…”