Local and global structural properties of the HIV-MN V3 loop

Studies of the feasibility of a subunit vaccine to protect against human immunodeficiency virus (HIV) infection have principally focused on the third variable (V3) loop. The principal neutralizing determinant (PND) of HIV-1 is located inside the V3 loop of the surface envelope glycoprotein, gp120. However, progress toward a PND-based vaccine has been impeded by the amino acid sequence variability in the V3 loops of different HIV isolates. Theoretical studies revealed that the variability in sequence and structure of the V3 loop is confined to the N- and C-terminal sides of the conserved GPG crest. This leaves three regions of the V3 loop conserved both in sequence and secondary structure. We present the results of NMR studies that test the validity of our theoretical predictions. Structural studies are reported for the HIV-V3 loop (HIV-MN) in the linear and cyclic (S-S-bridged) forms. For the V3 loop sequence of the HIV-MN isolate, the three conserved secondary structural elements are as underlined below: turns turn helix CTRPNYNKRKRIHIGPGRAFYTTKNIIGTIROAHC Finally, the conformational requirement of the PND in the V3 loop-antibody interaction is tested by monitoring the monoclonal antibody binding to the HIV-MN V3 loop in the linear and cyclic forms by enzyme-linked immunosorbent assay. The binding data reveal that the cyclic V3 loop is a better ligand for the monoclonal antibodies than the linear form although the latter has the same sequence. This means that the monoclonal antibodies recognize the PNDs as conformational epitopes.