The principal neutralizing determinant of HIV-1 located in V3 of gp120 forms a 12-residue loop by internal hydrophobic interactions

The interactions of the peptide RP135a (RKSIRIQRGPGRAFVT), corresponding to residues 311–326 of gp120 of HIV-1 IIIB, with the anti-gp120 HIV-1 IIIB neutralizing antibody 0.5β were studied by NMR. The NOESY difference spectra measured using specifically deuterated derivatives of the peptide show exclusively the interactions of the deuterated residues both within the bound peptide and with the Fab fragment of the antibody. These measurements reveal hydrophobic interactions within the bound peptide between Ile-4, Ile-6 and Val-15 that create a 12-residue loop with these residues at the base and the conserved GPGR sequence at its top.