Induced fit in HIV-neutralizing antibody complexes: Evidence for alternative conformations of the gp120 V3 loop and the molecular basis for broad neutralization

Human monoclonal antibody (mAb) 447-52D neutralizes a broad spectrum of HIV-1 isolates, whereas murine mAb 0.5 beta, raised against gp120 of the X4 isolate HIV-1(IIIB), neutralizes this strain specifically. Two distinct gp120 V3 peptides, V3(MN) and V3,B, adopt alternative beta-hairpin conformations when bound to 447-52D and 0.5 beta, respectively, suggesting that the alternative conformations of this loop play a key role in determining the coreceptor specificity of HIV-1. To test this hypothesis and to better understand the molecular basis underlying an antibody's breadth of neutralization, the solution structure of the V3(IIIB) peptide bound to 447-52D was determined by NMR. V3(IIIB) and V3(MN) peptides bound to 447-52D exhibited the same N-terminal strand conformation, while the V3(IIIB) peptide revealed alternative N-terminal conformations when bound to 447-52D and 0.5 beta. Comparison of the three known V3 structures leads to a model in which a 180 degrees change in the orientation of the side chains and the resulting one-residue shift in hydrogen bonding patterns in the N-terminal strand of the beta-hairpins markedly alter the topology of the surface that interacts with antibodies and that can potentially interact with the HIV-1 coreceptors. Predominant interactions of 447-52D with three conserved residues of the N-terminal side of the V3 loop, K312 I314, and I316, can account for its broad cross reactivity, whereas the predominant interactions of 0.5 beta with variable residues underlie its strain specificity.