Stability and Water Accessibility of the Trimeric Membrane Anchors of the HIV-1 Envelope Spikes

Abstract

HIV-1 envelope spike (Env) is a type I membrane protein that mediates viral entry. Recent studies showed that the transmembrane domain (TMD) of the Env forms a trimer in lipid bilayer and that disruption of the TMD could significantly alter the antigenic properties of the Env. The TMD structure has several peculiar features that remain difficult to explain. One is the presence of an arginine R696 (three in the trimer) in the middle of the TM helix. Additionally, the N- and C-terminal halves of the TM helix form trimeric cores of opposite nature (hydrophobic for the N half and hydrophilic for the C half). Here we determined the membrane partition and solvent accessibility of the TMD in bicelles that mimic a lipid bilayer. Solvent paramagnetic relaxation enhancement analysis showed that the R696 is indeed positioned close to the center of the bilayer, but, surprisingly, can exchange rapidly with water as indicated by hydrogen-deuterium exchange measurements. The solvent accessibility of R696 is likely mediated by the hydrophilic core, which also showed fast water exchange. In contrast, the N-terminal hydrophobic core showed extremely slow solvent exchange, suggesting the trimer formed by this region is extraordinarily stable. Our data explain how R696 is accommodated in the middle of the membrane while reporting the overall stability of the Env TMD trimer in lipid bilayer.

HIV-1 Env gp41 Transmembrane Domain Dynamics are Modulated by Lipid, Water, and Ion Interactions

“…To gain a greater understanding of the interplay between the HIV gp41 TMD and a viral membrane, we performed extensive MD simulations of the TMD trimer in a heterogeneous, asymmetric membrane. While it has been recently observed that a highly conserved arginine residue (R696) is a mediator of water exchange in the TMD trimer (24), we sought to further analyze the dynamics and movement of water, and potentially ions, into the trimer core. Additionally, we sought to provide further insight into the role of the highly conserved, positively charged residues that sit at the membrane-water interfaced based on the recent NMR structure (PDB ID: 5JYN).…”

“…The highly conserved gp41 TMD transmembrane domain (TMD) of Env ( Figure 1) is anchored in a cholesterol-rich lipid bilayer (22) that is flanked by the membrane proximal external region (MPER) on the exofacial leaflet and the cytoplasmic tail (CT) on the cytofacial leaflet. In the prefusion state, either one (23) or three (21,24,25) single-pass α-helices span the membrane ( Figure 1A). Previous studies have proposed that either oligomeric contacts (21,24) or head group snorkeling (23) stabilizes an internal arginine, R696, which is buried in the membrane (21).…”

“…In the prefusion state, either one (23) or three (21,24,25) single-pass α-helices span the membrane ( Figure 1A). Previous studies have proposed that either oligomeric contacts (21,24) or head group snorkeling (23) stabilizes an internal arginine, R696, which is buried in the membrane (21). Additionally, studies purport that a GxxxG motif in the TMD either stabilizes the trimer interface or acts as a signaling domain (26,27).…”

“…Prior TMD MD models were either derived by homology modeling using the HIV transmembrane viral protein U (Vpu) as template (28)(29)(30) or from a 20amino acid transmembrane de novo model (31) rather than the recently published trimeric NMR TMD structure (21) which was used in this study. Previous simulations that used monomeric transmembrane models (28)(29)(30) lack quaternary contacts that are believed to stabilize the internal arginine (R696) (21,24,25). Furthermore, previous MD simulations were limited in sampling and membrane composition and were therefore unable to reveal the complex properties of the membrane components and their effect on TMD dynamics (28)(29)(30)(31).…”

“…Furthermore, previous MD simulations were limited in sampling and membrane composition and were therefore unable to reveal the complex properties of the membrane components and their effect on TMD dynamics (28)(29)(30)(31). More recently, discussion has formed around the ability of the TMD to exist in reconstituted bicelles as a monomer (23) and the potential role of key arginine residues in stabilizing either a monomeric or a trimeric transmembrane structure (24). The positioning, dynamics, and relationship between lipid type and protein conformational change are of current interest given these recent findings and can be further explored in more atomistic detail with MD simulations.…”