Structural Mechanism of Trimeric HIV-1 Envelope Glycoprotein Activation

Oleg Kuybeda,Erin E H Tran,Mario J Borgnia,Gabriel A Frank,David M Schauder,Sriram Subramaniam,Félix A Rey,Jacqueline L S Milne,Alberto Bartesaghi,Guillermo Sapiro

doi:10.1371/journal.ppat.1002797

Oleg Kuybeda, Erin E H Tran + Show 8 more

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https://doi.org/10.1371/journal.ppat.1002797

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Abstract

HIV-1 infection begins with the binding of trimeric viral envelope glycoproteins (Env) to CD4 and a co-receptor on target T-cells. Understanding how these ligands influence the structure of Env is of fundamental interest for HIV vaccine development. Using cryo-electron microscopy, we describe the contrasting structural outcomes of trimeric Env binding to soluble CD4, to the broadly neutralizing, CD4-binding site antibodies VRC01, VRC03 and b12, or to the monoclonal antibody 17b, a co-receptor mimic. Binding of trimeric HIV-1 BaL Env to either soluble CD4 or 17b alone, is sufficient to trigger formation of the open quaternary conformation of Env. In contrast, VRC01 locks Env in the closed state, while b12 binding requires a partial opening in the quaternary structure of trimeric Env. Our results show that, despite general similarities in regions of the HIV-1 gp120 polypeptide that contact CD4, VRC01, VRC03 and b12, there are important differences in quaternary structures of the complexes these ligands form on native trimeric Env, and potentially explain differences in the neutralizing breadth and potency of antibodies with similar specificities. From cryo-electron microscopic analysis at ∼9 Å resolution of a cleaved, soluble version of trimeric Env, we show that a structural signature of the open Env conformation is a three-helix motif composed of α-helical segments derived from highly conserved, non-glycosylated N-terminal regions of the gp41 trimer. The three N-terminal gp41 helices in this novel, activated Env conformation are held apart by their interactions with the rest of Env, and are less compactly packed than in the post-fusion, six-helix bundle state. These findings suggest a new structural template for designing immunogens that can elicit antibodies targeting HIV at a vulnerable, pre-entry stage.

Highlights

human immunodeficiency virus (HIV) infection occurs following the binding of viral envelope glycoproteins (Env) to receptors on target cell surfaces
We use cryo-electron tomography to characterize the conformational changes that occur in Env at individual steps in the entry process, revealing the unexpected finding that binding to a co-receptor mimic alone induces the same conformational changes as binding to CD4
In previous cryo-electron tomographic studies [21], we determined that native trimeric Env undergoes large quaternary changes upon simultaneous binding of soluble CD4 (sCD4) and the co-receptor mimic, 17b [42], to gp120 on intact HIV-1 BaL virions

Summary

Introduction

In the ‘‘pre-hairpin intermediate’’, the gp molecules rearrange so that the N-terminal peptides form a trimer of helices that present the fusion peptide to the target cell, while the C-terminal helices remain attached to the viral membrane [2]. This stage is vulnerable to a number of neutralizing antibodies [6] and peptides [7,8,9] capable of binding either the N- or C-terminal peptides. A detailed structural understanding of these molecular rearrangements in the context of native trimeric Env remains elusive

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