Structure and Dynamics of HIV-1 Env Trimer

Abstract

The human immunodeficiency virus-1 (HIV-1) infects CD4+ helper T cells by fusing its lipid membrane to the host cell plasma membrane. HIV-1 Envelope (Env) glycoprotein, a trimer of gp120-gp41 heterodimers, is the fusion machinery that mediates viral and host cell membrane fusion, which is initiated by interaction between the host cell receptor CD4 and viral glycoprotein gp120. Receptor binding induces conformational changes in the Env trimer such that coreceptor CCR5/CXCR4 can bind to gp120 and trigger subsequent membrane fusion steps. Previous cryo-electron tomography (cryoET) and spectroscopy studies have shown that CD4 binding induces changes in Env trimer from the closed, prefusion conformation to an open, CD4-bound conformation. However, the CD4-bound Env trimer is dynamic and flexible such that structural biology study of CD4-bound Env trimer is difficult. As the sole glycoprotein located on HIV-1 viral surface, Env trimer is the only target for neutralizing antibodies. Since HIV-1 mutates rapidly, most neutralizing antibodies are strain-specific. However, a small percent of HIV-1 infected patients can develop broadly neutralizing antibodies (bNAbs) that neutralize a wide spectrum of viruses. In 2011, our collaborator isolated a bNAb called 8ANC195. To understand how 8ANC195 recognizes Env trimer and prevents infection, we used a combination of structural biology and biochemistry tools. Interestingly, we found that 8ANC195 can recognize Env trimer in both its prefusion and CD4-bound conformations by targeting an epitope at gp120-gp41 interface. Furthermore, 8ANC195 stabilizes the Env-CD4 complex, allowing us to investigate the conformational changes induced by CD4 in atomic resolution. By solving and comparing single-particle cryo-EM structures of Env trimer in complex with CD4 and/or antibodies, we found that: 1) CD4 binding induces coreceptor binding site exposure through a β-sheet rearrangements in each gp120 monomer. Several key residues allosterically regulate this coreceptor binding site exposure. 2) CD4 binding induces Env trimer opening together with gp41 conformational changes, which represents an intermediate between prefusion gp41 and postfusion gp41. 3) Env trimer opening is necessary but not sufficient for coreceptor binding site exposure, while CD4 binding induces both, some antibodies can open Env trimer without exposing the coreceptor binding site. These conclusions further illuminate how Env trimer mediates membrane fusion and inform potential strategies blocking viral entry.