Mapping the B cell superantigen binding site for HIV-1 gp120 on a V(H)3 Ig

The emerging class of B cell superantigens includes HIV-1 gp120, which bind s to many members of the V(H)3 Ig gene family. The present study addresses the structural features of V(H)3 antibodies conferring gp120 binding activi ty using a panel of recombinant full-length and Fab Ig proteins. Binding ac tivity was fully conferred by the Fab portion of the Ig molecule. The V-H r egion was the major determinant of binding; diverse light chains were permi ssive for gp120 binding. A series of recombinant V(H)3-V(H)1 chimeric molec ules was created to analyze the contribution of different subregions of V(H )3 to gp120 binding. Hypervariable loop 1 (H1) substitution alone caused a 10-fold reduction in binding activity. The framework subregions (FR1, FR2 a nd FR3) and H2 also influenced binding, since substitutions of various comb inations of these subregions conferred 10- to 100-fold binding reductions. We conclude that gp120 binding occurs through a nonconventional interaction involving multiple discontinuously arrayed residues spanning the V-H, and including roles in gp120 contact and favorable conformation of the V-H.