The remarkable diversity, glycosylation and conformational flexibility of the human immunodeficiency virus type 1 (HIV-1) envelope (Env), including substantial rearrangement of the gp120 glycoprotein upon binding the CD4 receptor, allow it to evade antibody-mediated neutralization. Despite this complexity, the HIV-1 Env must retain conserved determinants that mediate CD4 binding. To evaluate how these determinants might provide opportunities for antibody recognition, we created variants of gp120 stabilized in the CD4-bound state, assessed binding of CD4 and of receptor-binding-site antibodies, and determined the structure at 2.3 Å resolution of the broadly neutralizing antibody b12 in complex with gp120. b12 binds to a conformationally invariant surface that overlaps a distinct subset of the CD4-binding site. This surface is involved in the metastable attachment of CD4, before the gp120 rearrangement required for stable engagement. A site of vulnerability, related to a functional requirement for efficient association with CD4, can therefore be targeted by antibody to neutralize HIV-1.
Human immunodeficiency virus type 1 (HIV-1) evades host defences thanks to its protective layers of immune camouflage. But b12 antibody pierces these barriers, so its interactions with HIV's gp120 envelope glycoprotein are of interest to vaccine and drug designers. The conformational flexibility of gp120 complicates analysis, but this obstacle has been overcome by using gp120 molecules stabilized in the CD4-bound state. Biophysical analysis with these 'frozen' gp120s shows receptor binding to occur in two steps: an initial 'handshake' followed by a shape-shifting 'bear hug'. The antibody by-passes gp120's protective shape shifting by exploiting an unexpected vulnerability of the handshake: the need for a substantial 'onrate' to efficiently engage CD4. The cover X-ray crystallographic image catches b12 antibody (green) as it grabs onto this newly identified site of vulnerability (yellow).