The binding of a glycoprotein 120 V3 loop peptide to HIV-1 neutralizing antibodies - Structural implications

The structural and antigenic properties of a peptide ("CRK") derived from t he V3 loop of HIV-1 gp120 protein were studied using NMR and SPR techniques . The sequence of CRK corresponds to the central portion of the V3 loop con taining the highly conserved ''GPGR'' residue sequence. Although the biolog ical significance of this conserved sequence is unknown, the adoption of co nserved secondary structure (type II beta -turn) in this region has been pr oposed. The tendency of CRK (while free or conjugated to protein), to adopt such structure and the influence of such structure upon CRK antigenicity w ere investigated by NMR and SPR, respectively. Regardless of conjugation, C RK is conformationally averaged in solution but a weak tendency of the CRK "GPGR'' residues to adopt a beta -turn conformation was observed after conj ugation. The influence of GPGR structure upon CRK antigenicity was investig ated by measuring the affinities of two cognate antibodies: "5023A" and "50 25A," for CRK, protein-conjugated CRK and gp120 protein. Each antibody boun d to all the antigens with nearly the same affinity. From these data, it ap pears that: (a) antibody binding most likely involves an induced fit of the peptide and (b) the gp120 V3 loop is probably conformationally heterogeneo us. Since 5023A and 5025A are HIV-1 neutralizing antibodies, neutralization in these cases appears to be independent of adopted GPGR beta -turn struct ure.