Quaternary contact in the initial interaction of CD4 with the HIV-1 envelope trimer

Abstract

Binding of the gp120 envelope glycoprotein to the CD4 receptor is the first step in the HIV-1 infectious cycle. Although the CD4-binding site has been extensively characterized, the initial receptor interaction has been difficult to study because of major CD4-induced structural rearrangements. Here, we use cryogenic electron microscopy to visualize the initial contact of CD4 with the HIV-1-envelope trimer at 6.8-Å resolution. A single CD4 molecule is embraced by a quaternary HIV-1-Env surface formed by coalescence of the previously defined CD4-contact region with a second CD4-binding site (CD4-BS2) in the inner domain of a neighboring gp120 protomer. Disruption of CD4-BS2 destabilized CD4-trimer interaction and abrogated HIV-1 infectivity by preventing acquisition of coreceptor-binding competence. A corresponding reduction in HIV-1 infectivity occurred upon mutation of CD4 residues interacting with CD4-BS2. These results document the critical role of quaternary interactions in the initial HIV-1 envelope-receptor contact, with implications for treatment and vaccine design.

Asymmetric recognition of HIV-1 Envelope trimer by V1V2 loop-targeting antibodies

“…Quaternary interactions visualized thus far for V1V2 bNAbs involve the binding of a single Fab to the apex of Env trimer (Julien et al, 2013b; Liu et al, 2017), but here we demonstrate that the stoichiometry of binding for the new V1V2 bNAb BG1 (Freund et al, 2017) is two Fabs per Env trimer, with a minor population of 3:1 BG1-Env complexes (Figures 2 and 3; Figure 3—figure supplement 1). …”

“…None of the four tyrosines within the BG1 CDRH3 (Tyr100A CDRH3 , Tyr100C CDRH3 , Tyr100J CDRH3 and Tyr100L CDRH3 ) or within other CDRs show electron density for a sulfate group (Figure 1—figure supplement 1). By contrast, the CRH3s of other V1V2 bNAbs form stable subdomains involving regions of two-stranded β-sheet (Figure 1E), sometimes containing sulfated tyrosine sidechains (e.g., PG9’s Tyr100G CDRH3 and Tyr100H CDRH3 ) that interact with basic residues within V1V2 (McLellan et al, 2011; Pancera et al, 2013; Liu et al, 2017).

10.7554/eLife.27389.002Figure 1.Comparison of BG1 and other V1V2 bNAbs.( A ) Sequences of the HCs of BG1 and representative other V1V2 bNAbs.

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Open and closed structures reveal allostery and pliability in the HIV-1 envelope spike

“…2b). The functional role of α0 that we describe here supports a different conclusion than one based on a recently published low-resolution structure of CD4 bound to an Env trimer in a closed conformation claiming that CD4 makes specific contacts with C1 in the adjacent protomer 21 . In our structure, all of the highly conserved residues claimed to interact with CD4 during its initial pose form intra-gp120 stabilizing interactions in the open CD4-bound structure presented here (Fig.…”

“…3d). Mutation of these residues lead to a loss of infectivity 21 ; in light of the present work, likely due to destabilization of the conformation competent for co-receptor binding.…”

How HIV-1 entry mechanism and broadly neutralizing antibodies guide structure-based vaccine design

“…In all prefusion forms, gp41 interactions remain largely intact while gp120 subunits undergo varying degrees of movement upon CD4 binding. Recent studies suggest a single CD4-bound Env trimer represents an obligatory intermediate along the Env entry pathway [10,46••,101,102]. Cryo-EM structures of trimers bound to CD4 alone [103], CD4 in the presence of 17b [104,105] or 17b and 8ANC195 [12••,68] reveal how binding by multiple CD4 molecules ultimately results in an outward rotation of gp120 domains that rearrange V1V2 and V3 loop regions to open up the trimer, thereby exposing the V3 loop and leading to the formation of bridging sheet and the coreceptor binding site.…”