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 glycoprotein 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‐terminal 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 gp120HXB2 (rgp120) was found to be similar to that for the synthetic gp120 peptide (502–516). Circular dichroism studies indicate that the synthetic peptide largely 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 region 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 change 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 suggest either that the 502–504 region does not change conformation upon CD4 binding, 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.