Structural and functional characterization of an epitope in the conserved C-terminal region of HIV-1 gp120

Through an integrated study of the reactivity of a monoclonal antibody, 803 -15.6, with synthetic peptides and native recombinant HIV-1 envelope glycop rotein gp120, we have obtained structure-functional information on a region of rgp120 not yet elucidated by X-ray crystallography. mAb 803-15.6 binds with high affinity and broad cross-clade specificity to the conserved C-ter minal region (amino acids 502-516) of HIV-1 rgp120. Phage display selection from a random peptide library identified the core binding motif as AXXKXRH , homologous to residues 502-508. Using quantitative binding analyses, the affinity of mAb 803-15.6 for native, monomeric recombinant gp120(HXB2), (rg p120) was found to be similar to that for the synthetic gp120 peptide (502- 516). Circular dichroism studies indicate that the synthetic peptide largel y has a random coil conformation in solution. The results therefore suggest that the 803-15.6 epitope is fully accessible on rgp120 and that this regi on of rgp120 is as flexible as the synthetic peptide. Residues 502-504 are on the edge of a putative gp41 binding site that has been postulated to cha nge conformation on CD4 binding, However, the affinity of mAb 803-15.6 for rgp120 is not affected by binding of CD4 and vice-versa. These results sugg est either that the 502-504 region does not change conformation upon CD4 bi nding, or that recombinant gp120 does not undergo the same changes as occur in the native viral gp120-gp41 oligomer. The detailed characterization of the 803-15.6 epitope may be useful for further study of the role of the C5 region of gp120 in the viral attachment and fusion process.