Residues in the gp41 Ectodomain Regulate HIV-1 Envelope Glycoprotein Conformational Transitions Induced by gp120-Directed Inhibitors

Abstract

Interactions between the gp120 and gp41 subunits of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer maintain the metastable unliganded form of the viral spike. Binding of gp120 to the receptor, CD4, changes the Env conformation to promote gp120 interaction with the second receptor, CCR5 or CXCR4. CD4 binding also induces the transformation of Env into the prehairpin intermediate, in which the gp41 heptad repeat 1 (HR1) coiled coil is assembled at the trimer axis. In nature, HIV-1 Envs must balance the requirements to maintain the noncovalent association of gp120 with gp41 and to evade the host antibody response with the need to respond to CD4 binding. Here we show that the gp41 HR1 region contributes to gp120 association with the unliganded Env trimer. Changes in particular amino acid residues in the gp41 HR1 region decreased the efficiency with which Env moved from the unliganded state. Thus, these gp41 changes decreased the sensitivity of HIV-1 to cold inactivation and ligands that require Env conformational changes to bind efficiently. Conversely, these gp41 changes increased HIV-1 sensitivity to small-molecule entry inhibitors that block Env conformational changes induced by CD4. Changes in particular gp41 HR1 amino acid residues can apparently affect the relative stability of the unliganded state and CD4-induced conformations. Thus, the gp41 HR1 region contributes to the association with gp120 and regulates Env transitions from the unliganded state to downstream conformations.IMPORTANCE The development of an efficient vaccine able to prevent HIV infection is a worldwide priority. Knowledge of the envelope glycoprotein structure and the conformational changes that occur after receptor engagement will help researchers to develop an immunogen able to elicit antibodies that block HIV-1 transmission. Here we identify residues in the HIV-1 transmembrane envelope glycoprotein that stabilize the unliganded state by modulating the transitions from the unliganded state to the CD4-bound state.

Disruption of the HIV-1 Envelope allosteric network blocks CD4-induced rearrangements

“…55 Further, mutation of H66 has been demonstrated in the soluble gp140 SOSIP and membrane anchored gp160 contexts to prevent CD4 induced rearrangements. 10,55,56 Together, with the mechanistic insights provided by the observed structural rearrangements in the SOSIP Env, the results from this study indicate direct manipulation of the Env allosteric network allows for fine-tuned conformational control of Env structure that can assist in the development and refinement of the next generation of Env vaccine immunogens.…”

Identification of HIV-1 Envelope Mutations that Enhance Entry Using Macaque CD4 and CCR5

“…We note that although the NHR and CHR mutations identified in this work generally enhance usage of both macaque and human receptors, they occur at sites that are highly conserved among circulating HIV-1 strains and lentiviruses (43, 45). Selective pressures exerted by the human immune system could explain the discrepancy between the enrichment of these mutations in our experiments and their lack of representation in natural isolates.…”

“…Mutations in both regions are located at sites that make inter- and intra-protomer contacts with adjacent gp120 and gp41 subunits (Figure 7). Given that previous reports have described N/CHR mutations that modulate trimer reactivity through alterations of inter-protomer contacts (37, 42, 43), it is possible that the mutations identified here promote a state 2 phenotype by disrupting the stabilizing interactions between protomors within the unliganded trimer. If N/CHR identified here do indeed promote a state 2-like conformation, this would lower the thermodynamic barrier between unliganded and CD4 bound Env states, which could allow relatively weak interactions between Env and macaque CD4 to initiate the conformational changes leading to exposure of the co-receptor binding site.…”

Identification of HIV-1 Envelope Mutations that Enhance Entry Using Macaque CD4 and CCR5

“…Mutations in both regions are located at sites that make inter-and intra-protomer contacts with adjacent gp120 and gp41 subunits (Figure 7). Given that previous reports have described N/CHR mutations that modulate trimer reactivity through alterations of inter-protomer contacts [43,49,50], it is possible that the mutations identified here promote a state 2 phenotype by disrupting the stabilizing interactions between protomers within the unliganded trimer. If N/CHR identified here do indeed promote a state 2-like conformation, this would lower the thermodynamic barrier between unliganded and CD4 bound Env states, which could allow relatively weak interactions between Env and macaque CD4 to initiate the conformational changes leading to exposure of the co-receptor binding site.…”