Bottom: cartoons to designate in the previous models the different trimer components, and positions of the neutralizing epitopes for bnAbs used in this study PGT145 (V1/V2), VRC01 and b12 (CD4bs); and 10E8/4E10 (MPER) Some authors postulate that, due to steric occlusion, the MPER helix remains hidden within the native pre-fusion Env complexes on virions, becoming accessible for binding in the fusion-activated Env intermediates1620. importance for the design of therapeutic interventions. The Membrane-Proximal External Region (MPER) of the HIV Env gp41 subunit is a target for broadly neutralizing antibodies. Here, the authors apply super-resolution stimulated emission depletion (STED) microscopy on single virions and provide insights into how the MPER epitope is recognized. == Introduction == The envelope glycoprotein (Env) of the human immunodeficiency virus type-1 (HIV-1) embodies a common class I viral fusion machinery, but also configures diverse antigenic surfaces across the different viral clades, strains, and isolates1,2. Quaternary structure adjustments, heavy glycosylation, and intrinsic genetic variability of Env are thought to allow viral escape from neutralization by Env-specific antibodies3,4. Despite the effectiveness of these escape mechanisms, discovery in the 1990s KU 59403 of a handful of broadly neutralizing antibodies (bnAbs) proved the potential of humoral immunity to protect from HIV-1 infection3,4. Recent advances in the isolation of bnAbs with different specificities, together with their systematic structurefunction analyses, further support the existence of a number of sites of vulnerability on the native surface subunit gp120, or on the interface between this and the transmembrane subunit gp4158. Here, we focus on a distinct site of vulnerability existing on the transmembrane Env subunit gp41: the membrane proximal external region (MPER)5,7. Two reasons explain the special interest on solving the mechanisms underlying the molecular recognition of this Env site, namely, the exceptional degree of conservation of the MPER epitope sequence, and the fact that its engagement with the bnAbs 10E8 and 4E10 results in one of the broadest HIV neutralization STMN1 levels described so far (98% of viruses blocked in customary infectivity tests)5,9. The 10E8/4E10 epitope arranges onto a lateral face of the continuous helix connecting the Env ectodomain with the transmembrane domain (TMD) at the point where this structural element emerges from the lipid bilayer911(Fig.1). However, a mechanistic understanding of MPER recognition by bnAbs has been hampered by the limited information available on its native antigenic structure: a molecular surface that lies in contact with the viral KU 59403 membrane at the base of KU 59403 the Env complex (Fig.1). This structural complexity has proven challenging to reproduce by model systems amenable to biophysical and biochemical characterization, and precludes crystallization of MPER-containing Env specimens8,1214. Even for the case of a solubilized Env-detergent complex KU 59403 used in single-particle cryo-electron microscopy (Cryo-EM) studies15, it is unclear whether it would correctly recapitulate native Env MPER conformation in the viral lipid. Furthermore, in these studies, the Fab 10E8 was used to enable purification, which does not provide an opportunity to understand the native MPER epitope unliganded, and the mechanism of recognition; it only provides a still view post-binding albeit with sub-nanometer details. == Fig. 1. == Model for MPER accessibility within native Env complexes based on Cryo-EM reconstructions15. Top: Contours derived from detergent-solubilized Env trimers, without (left) or with 10E8 bound (right), have been docked into a viral lipid bilayer. A putative transition between these two states would result in exposure of the MPER helix in native Env (green). Bottom: cartoons to designate in the previous models the different trimer components, and positions of the neutralizing epitopes for bnAbs used in this study PGT145 (V1/V2), VRC01 and b12 (CD4bs); and 10E8/4E10 (MPER) Some authors postulate that, due to steric occlusion, the MPER helix remains hidden within the native pre-fusion Env complexes on virions, becoming accessible for binding in the fusion-activated Env intermediates1620. This assumption implies that, in contrast to epitopes within the solvent-accessible subunit gp120, the neutralization proficient structure of MPER would exist transiently, and its convenience be limited to the population of virions primed for fusion, greatly limiting antibody efficacy. According to this model, this limitation would be surpassed by a pre-attachment step to the viral membrane through antibodylipid relationships. This.