Analysis of the fine structure of the antigenic determinants of the transmembrane protein in the HIV coat with chemically modified synthetic peptides.

The amino acids involved in IgG reactivity to four HIV-1 gp41 overlapping synthetic peptides from the sequence 584-624 have been determined by a method based on the chemical modification of trifunctional amino acids, especially the acetylation of the amino groups of the lysine residues at pH 8-9. The reactivities of the sera from HIV-infected individuals and gp41-specific human Mab were studied with the overlapping peptides and their modified forms in indirect and competitive ELISA. Peptides 584-602 and 609-624 (CN-185) reacted with 88% of HIV-positive sera; the highest diagnostic significance (100%) was found with peptides 584-611 (AS-551) and 603-624 (CN-191). Acetylation resulted in a 10%-15% decrease in peptide reactivity. Moreover the concentration at which 50% inhibition occurred was 1.5 x 10(-6) M for unmodified AS-551 compared with 1.5 x 10(-5) M for the modified peptide. Circular dichroism spectra showed that acetylation did not alter the conformation of these peptides. Coupling of peptide AS-551 to a protein carrier at pH 6.5-7.0 did not affect the immunoreactivity of this peptide. Mab against human gp41 reacted with peptide 603-624 (CN-191). The concentration of this peptide necessary for 50% inhibition of Mab binding was 5.2 x 10(-6) M. It is concluded from the epitope mapping of the Mab that the antigenic determinant lies within the 603-609 fragment. Lys-608 appears to play a crucial role in the interaction with human HIVc-Mab.