Structural basis for broad neutralization of HIV-1 through the molecular recognition of 10E8 helical epitope at the membrane interface

Abstract

The mechanism by which the HIV-1 MPER epitope is recognized by the potent neutralizing antibody 10E8 at membrane interfaces remains poorly understood. To solve this problem, we have optimized a 10E8 peptide epitope and analyzed the structure and binding activities of the antibody in membrane and membrane-like environments. The X-ray crystal structure of the Fab-peptide complex in detergents revealed for the first time that the epitope of 10E8 comprises a continuous helix spanning the gp41 MPER/transmembrane domain junction (MPER-N-TMD; Env residues 671–687). The MPER-N-TMD helix projects beyond the tip of the heavy-chain complementarity determining region 3 loop, indicating that the antibody sits parallel to the plane of the membrane in binding the native epitope. Biophysical, biochemical and mutational analyses demonstrated that strengthening the affinity of 10E8 for the TMD helix in a membrane environment, correlated with its neutralizing potency. Our research clarifies the molecular mechanisms underlying broad neutralization of HIV-1 by 10E8, and the structure of its natural epitope. The conclusions of our research will guide future vaccine-design strategies targeting MPER.

Functional Contacts between MPER and the Anti-HIV-1 Broadly Neutralizing Antibody 4E10 Extend into the Core of the Membrane

“…By using the Wimley-White scale, the hydrophobicity can be quantified in terms of the free energy of transfer from bilayer interface to water (Δ G iwu , also termed free energy of partitioning). The neutralization potency of the antibody was determined by using pseudovirus particles (PsVs) [11,14]. …”

“…We have recently identified the existence of non-covalent interactions between the apex residue Trp100b HC of the 10E8 antibody, and residues of gp41 fully embedded in the membrane [14]. These interactions strengthen the affinity of the antigen-antibody complex when the epitope is embedded in the membrane, increasing the inhibitory capabilities of the antibody.…”

“…6d,e). To that purpose, and prior to each assay, the MPER(671–693) was fully anchored to the liposomes following the reported methodology [14]. The binding of the Fab to vesicles decorated with MPER was estimated from the relative abundance of antibody co-localized with the vesicles (fraction 4) in a flotation assay.…”

“…The data indicate that Trp100 HC must contribute to the overall activity of the antibody by stabilizing the antibody-peptide complex in a more indirect manner. In contrast, specific interactions involving Trp100b HC with this region of gp41 might occur in a manner analogous to that of the 10E8 antibody [14]. …”

“…These questions are more relevant in view that the hydrophobic residues of the apex region of the CDRH3 remain in a conformation exposed towards the solvent, and not oriented towards the bound peptide as it could be expected from their critical role in the virus neutralization by the antibody [7–12]. In contrast, the functionally similar bNAb 10E8 positions the apex of the CDRH3 loop in close proximity to the MPER peptide [13,14] thus rationalizing its neutralization potency in terms of binding affinity to the epitope in membranes.…”