A helical epitope in the C4 domain of HIV glycoprotein 120.

The fourth conserved domain of the human immunodeficiency virus type 1 (HIV-1) envelope, the C4 region of glycoprotein 120 (gp120), is an amphipathic stretch of amino acids that, based on mutational analyses, constitutes a major component of the CD4 binding region in gp120. In the absence of crystallographic and NMR data on C4 in intact gp120, we sought to gain insight into C4's conformation and accessibility in gp120 by taking an immunochemical approach. In this study, a peptomer composed of a repeat peptide of C4 amino acids 419-436 from gp120 of HIV-1MN was synthesized for use as a conformationally constrained immunogen. Circular dichroism studies disclosed that the polymerized peptide, peptomer-(419-436), in 0.01 M Na2HPO4 buffer, pH 7.4, at 25 degrees C contained a dominant alpha-helical structure (53 +/- 1%) compared with 2 +/- 4% alpha-helical content for the monomeric peptide-(419-436). The peptomer in Ribi's adjuvant induced the production of rabbit antibodies that recognized recombinant and native gp120 but, consistent with the literature, the C4 peptide having no conformational constraints did not. The experimental results show that only those antibodies formed against a helical immunogen from C4 will recognize recombinant and native gp120, and, therefore, the results support the notion that C4 is an alpha-helix in gp120.