SECONDARY STRUCTURAL ELEMENTS AS A BASIS FOR ANTIBODY RECOGNITION IN THE IMMUNODOMINANT REGION OF HUMAN IMMUNODEFICIENCY VIRUS-1 AND VIRUS-2

Synthetic peptide antigens corresponding to the entire third variable region V3, the principal neutralizing determinant of the human immunod eficiency virus (HIV) envelope glycoprotein of HIV-1 subtype B (1), HI V-2 subtype A (5), and HIV-2 subtype B (7) were synthesized by solid-p hase peptide synthesis (Table 1). 1 and 5 were also prepared as their GlcNAc-glycosylated forms at the natural N-glycosylation site NXT (pos itions 6-8; peptides 4 and 6). Additionally, the proposed beta-turn re gion of 1 (GPGR; positions 15-18) was altered by introducing D-Ala17 ( 2) and D-Pro16 (3). All compounds have been studied by two-dimensional NMR techniques. Interproton distances and (3)J(NH/H) coupling constan ts derived from NMR data are used as restraints in distance geometry a nd ENSEMBLE-Distance and angle-bound driven dynamics calculations. The simulations led to disordered conformations except for a high propens ity of a beta(II)-turn in the region GPXR (positions 15-18) in 1, 2, a nd 4. In 3 (G-D-ProGR, positions 15-18), a type beta(I)-turn was mainl y found instead. For peptide 7, the consensus sequence of HIV-2 subtyp e B, a type beta(II)-turn was also found although the primary structur e (VSGL; positions 15-18) differs grossly from the HIV-1 peptide 1. Wi th the exception of 2, all beta(II)-turns were able to form a canonica lly opened beta-turn by a 180 degrees rotation of phi(G17). Surprising ly, compounds 5 and 6 that are highly similar to 7 showed no beta(II)- type turn within MSGL (positions 15-18). They form a type beta(VIII)-t urn across the tetrapeptide SGLV (positions 16-19) together with a non -canonical turn conformation across LMSG (positions 14-17) leading to an S-conformation. The reaction of the peptides with HIV-positive sera from patients infected with different subtypes of HIV-1 and HIV-2 was tested in enzyme-linked immunosorbent assays (ELISA reactions). No HI V-2 sera reacted with peptide 1 and no HIV-1 sera showed reactivity to peptide 5. We propose that certain amino acid exchanges within the V3 domain lead to altered conformations of the V3 loop resulting in anti bodies that show altered binding properties to the peptide antigens us ed in the ELISA reactions.