Synthetic peptides spanning specific sequences of HIV-1 gp120 principal neutralizing domain, and T-lymphocyte CD4 were characterized and utilized to study the complex interactions between these two important proteins, at the molecular level. CD4 interactions with gp120 and gp120-derived peptides were studied using an in vitro assay based on immobilized recombinant soluble CD4. In this system, CD4 binds to peptides corresponding to the principal neutralizing domain of the envelope protein. Competition experiments indicated that our principal neutralizing domain-related synthetic peptides were specifically recognized by a CD4 site adjacent to, but distinct from, the high affinity gp120-binding site of CD4. Interestingly, we also found that a peptide designed from the principal neutralizing domain of HIV-1 MN virus greatly enhanced viral infection: the observed effect occurred in the early steps of viral infection and was not strain-restricted. The biological assays performed on point-mutated and/or shortened analogues of the active peptide, in addition to conformational spectroscopic measurements and theoretical calculations, allowed the formulation of a model that was utilized to interpret the experimental data. Two CD4-derived peptides, corresponding to residues 37-53 and 37-55 of the V1 domain of CD4, which were recently proposed as the most likely binding site of HIV-1 gp120 on CD4, were also synthesized chemically by solid-phase techniques. Their ability to inhibit the infectivity of different HIV-1 strains in different cell lines was monitored by several biological assays and by electron microscopy. Preliminary findings suggest that the peptides inhibit HIV-1 infectivity only slightly, without any detectable cytotoxicity.
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