Hu Guiqing, Liu Jun, Roux Kenneth H, Taylor Kenneth A
Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, USA.
The University of Texas-Houston Medical School, Department of Pathology & Laboratory Medicine, Houston, Texas, USA.
J Virol. 2017 Jul 27;91(16). doi: 10.1128/JVI.00134-17. Print 2017 Aug 15.
The human immunodeficiency virus type 1 (HIV-1)/simian immunodeficiency virus (SIV) envelope spike (Env) mediates viral entry into host cells. The V3 loop of the gp120 component of the Env trimer contributes to the coreceptor binding site and is a target for neutralizing antibodies. We used cryo-electron tomography to visualize the binding of CD4 and the V3 loop monoclonal antibody (MAb) 36D5 to gp120 of the SIV Env trimer. Our results show that 36D5 binds gp120 at the base of the V3 loop and suggest that the antibody exerts its neutralization effect by blocking the coreceptor binding site. The antibody does this without altering the dynamics of the spike motion between closed and open states when CD4 is bound. The interaction between 36D5 and SIV gp120 is similar to the interaction between some broadly neutralizing anti-V3 loop antibodies and HIV-1 gp120. Two conformations of gp120 bound with CD4 are revealed, suggesting an intrinsic dynamic nature of the liganded Env trimer. CD4 binding substantially increases the binding of 36D5 to gp120 in the intact Env trimer, consistent with CD4-induced changes in the conformation of gp120 and the antibody binding site. Binding by MAb 36D5 does not substantially alter the proportions of the two CD4-bound conformations. The position of MAb 36D5 at the V3 base changes little between conformations, indicating that the V3 base serves as a pivot point during the transition between these two states. Glycoprotein spikes on the surfaces of SIV and HIV are the sole targets available to the immune system for antibody neutralization. Spikes evade the immune system by a combination of a thick layer of polysaccharide on the surface (the glycan shield) and movement between spike domains that masks the epitope conformation. Using SIV virions whose spikes were "decorated" with the primary cellular receptor (CD4) and an antibody (36D5) at part of the coreceptor binding site, we visualized multiple conformations trapped by the rapid freezing step, which were separated using statistical analysis. Our results show that the CD4-induced conformational dynamics of the spike enhances binding of the antibody.
1型人类免疫缺陷病毒（HIV-1）/猴免疫缺陷病毒（SIV）包膜刺突（Env）介导病毒进入宿主细胞。Env三聚体的gp120组分的V3环有助于共受体结合位点，并且是中和抗体的靶标。我们使用冷冻电子断层扫描来观察CD4和V3环单克隆抗体（MAb）36D5与SIV Env三聚体的gp120的结合。我们的结果表明，36D5在V3环的基部结合gp120，并表明该抗体通过阻断共受体结合位点发挥其中和作用。该抗体在结合CD4时不会改变刺突在闭合和开放状态之间运动的动力学。36D5与SIV gp120之间的相互作用类似于一些广泛中和的抗V3环抗体与HIV-1 gp120之间的相互作用。揭示了与CD4结合的gp120的两种构象，表明配体Env三聚体具有内在的动态性质。在完整的Env三聚体中，CD4结合显著增加36D5与gp120的结合，这与CD4诱导的gp120构象和抗体结合位点的变化一致。MAb 36D5的结合不会显著改变两种与CD4结合的构象的比例。在不同构象之间，MAb 36D5在V3基部的位置变化很小，表明V3基部在这两种状态之间的转变过程中作为一个枢轴点。SIV和HIV表面的糖蛋白刺突是免疫系统进行抗体中和的唯一可用靶标。刺突通过表面上一层厚厚的多糖（聚糖屏蔽）以及刺突结构域之间的运动来逃避免疫系统，这些运动掩盖了表位构象。使用其刺突在部分共受体结合位点被主要细胞受体（CD4）和抗体（36D5）“修饰”的SIV病毒粒子，我们观察到了快速冷冻步骤捕获的多种构象，并通过统计分析将它们分开。我们的结果表明，刺突的CD4诱导的构象动力学增强了抗体的结合。
