Structural definition of CD4-induced HIV-1 Env conformational changes required for infection

Duke University, Durham NC

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Abstract

The HIV-1 envelope glycoprotein (Env), a homotrimer of gp120-gp41 heterodimers, mediates viral entry into host cells. The gp120 subunit engages host receptors, while the gp41 subunit contains a fusion peptide (FP) that is inserted into the host membrane to induce host and virus membrane fusion. Prior to its binding to host receptors, the HIV-1 Env is characterized by a closed configuration with gp120 protomers packed against each other and the gp41 subunit, while the highly conserved and immunodominant coreceptor-binding region at the Env trimer apex remains occluded by packing of the first and second (V1V2) as well as the third (V3) variable loops. At the trimer base, FP comprises a hydrophobic stretch of about 20 amino acids at the gp41 N terminus that is accessible for antibody binding in the closed configuration of Env. FP is a site of vulnerability to broadly neutralizing antibodies (bnAbs) and thus of vaccine focus. Until recently, two distinct structural configurations of the FP were defined in the literature, one that is antibody accessible in the pre- fusion closed Env and a second that is sequestered within a gp41-gp120 pocket in a partially or fully open CD4-induced Env. We recently described a functional intermediate state in atomic level details where the FP remains accessible to antibody binding despite substantial receptor binding mediated Env opening. In the study, Aim 1 will focus on the early stages of Env opening, including the role that the unique FP configuration in HIV-1 plays in the metastability of the pre-fusion, pre-receptor, closed Env and how this impacts the Env conformational landscape. Aim 2 will focus on detailed characterization of a newly defined functional intermediate state. Aim 3 will focus on later stages of Env opening where the FP gets buried within a hydrophobic core and is no longer available for antibody binding. These studies will advance our understanding of the HIV-1 entry mechanism and inform structure-based immunogen and drug design. The scientific premise of this grant is that the FP is a critical component of the HIV-1 entry machinery that determines virus attachment. Understanding the mechanistic basis of the movement of the FP during the entry process is thus critical to our understanding of HIV-1 biology. A mechanistic understanding of subsequent conformational changes and the intermediate states uncovered in the process will facilitate the development of intervention strategies that include immunogen design for HIV-1 vaccine efforts and drug design for novel cure AIDS strategies to eliminate the latent pool of HIV-1-infected CD4 T cells. The innovation in this grant derives from our discovery of a novel receptor-bound Env intermediate that shows substantial opening yet can bind a FP-directed antibody. The proposed studies will improve our understanding of HIV-1 entry and will inform vaccine and therapeutics development.