Structural Mechanism of Trimeric HIV-1 Envelope Glycoprotein Activation

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.

Molecular architecture of the uncleaved HIV-1 envelope glycoprotein trimer

“…This correlation supports the relevance of the Env(-)ΔCT trimer structure to that of the functional virion Env spike. Our results provide a structural explanation for experimental observations suggesting that the ability to bind the Env trimer in its unliganded state represents a major factor in determining the neutralization potency of CD4BS antibodies (29,(46)(47)(48).…”

Conformation-Dependent Recognition of HIV gp120 by Designed Ankyrin Repeat Proteins Provides Access to Novel HIV Entry Inhibitors

“…With the increasing understanding of the architecture of the viral spike (21,76,77), the possibility to generate stable soluble trimers that closely resemble the native spike (78,79), and the means to generate structurally arrested peptide mimetics of gp120 microdomains (31), a number of tools have become available which have promise to tailor future DARPin selections to specific domains of interest. As discussed above, based on our current data, this holds particular promise to improve envelope-specific DARPin identification and to harness the distinctive binding properties of DARPins for HIV inhibitor development.…”

Cryo-EM structure of a CD4-bound open HIV-1 envelope trimer reveals structural rearrangements of the gp120 V1V2 loop

“…The binding of soluble CD4 (sCD4) produces little to no changes in the structures of gp120 cores (gp120 monomers with truncations in the N and C termini and variable V1V2 and V3 loops) (3), but results in rotation of the gp120 protomers within virion-bound Env trimers to create an open conformation distinct from the closed conformation of unliganded virion-bound trimers (4). Single-particle electron microscopy (EM) structures of recombinant native-like soluble Env gp140 trimers (SOSIPs) confirmed that they can adopt the same closed and open architectures as virion-bound Env trimers (5)(6)(7), thus the SOSIP substitutions (introduction of a disulfide bond linking gp120 to gp41 and an Ile→Pro mutation in gp41; ref. 6) do not appear to prevent transition to the open state.…”

“…6) do not appear to prevent transition to the open state. Despite the plethora of recent crystal and EM structures at atomic and nearatomic resolutions of closed Env trimers, most in complex with broadly neutralizing antibodies (bNAbs) (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), only low-resolution structures derived from cryo-electron tomography of HIV-1 virions have been available for sCD4-bound open Env trimers (4)(5)(6)(7).…”