MAPPING THE IG SUPERANTIGEN-BINDING SITE OF HIV-1 GP120

The envelope glycoprotein, gp120, of HIV-1 has recently been identifie d as a member of the new family of Ig superantigens (Ig-SAg). This cla ssification is based on the selective binding of gp120 to an unusually high proportion of endogenous, nonimmune Ig, and the selective activa tion of nonimmune B cells by gp120 in vitro. Many, if not al of the no nimmune Ig that bind to gp120 are members of the VH3 Ig gene family. T he aim of this study was to determine the epitope on gp120 that was re sponsible for its Ig-Sag binding activity. To do this, we utilized a p anel of 30 peptides derived from gp160 in a competition-binding assay. For five Igs that were tested, as well as for polyclonal serum IgM, t wo overlapping peptides (each 20 amino acids in length) were identifie d that were potent inhibitors of gp120 binding. Similarly, the 10 amin o acid overlap region of these two peptides had inhibitory activity. T hus; this decamer sequence represented the optimal Ig-SAg epitope or m imotope. The amino acid residue at position 1 of the decamer, and to a lesser extent at position 10, was critical for peptide binding. In ad dition to this decamer peptide, other peptides that shared modest sequ ence homology were also selectively inhibitory for specific Ig samples . These findings provide the first definition of an Ig-SAg ligand at t he peptide level and will facilitate further structural and biologic c haracterization of this new class of pathogenic Ags.