Journal of virology 2017 05 24() pii 10.1128/JVI.00134-17
The HIV-1/SIV envelope spike (Env) mediates the viral entry into host cells. The V3 loop of the gp120 component of the Env trimer contributes to the co-receptor binding site and is a target for neutralizing antibodies. We have used cryoelectron tomography to visualize the binding of CD4 and the V3 loop monoclonal antibody 36D5 to gp120 of the SIV Env. Our results show that 36D5 binds gp120 at the base of the V3 loop and suggest the antibody exerts its neutralization effect by blocking the co-receptor 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 with 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 for gp120 in the intact Env consistent with CD4 induced changes in the conformation of gp120 and the antibody-binding site. Binding by MAb 36D5 does not alter substantially 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.IMPORTANCE Glycoprotein spikes located on the surface of SIV and HIV are the sole targets available to the immune system for antibody neutralization. Spikes evade the immune system by combination of a thick layer of polysaccharide on the surface (the glycan shield) and movement between spike domains that mask the epitope conformation. Using SIV virions whose spikes had been "decorated" with the primary cellular receptor, CD4, and an antibody, 36D5, to part of the co-receptor-binding site, we visualize multiple conformations trapped by the rapid freezing step which were separated using statistical analysis. Our results show that the CD4 induced conformational dynamics in the spike enhances binding of the antibody.