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A key step in the infection of T cells by HIV-1 is the interaction between the cellular receptor CD4 and the viral envelope protein gp120. Various compound of potential pharmacological value have been tested for their ability to interfere with this interaction with aim of identifying drugs that reduce virus infectivity. In particular, monoclonal antibodies directed agianst the gp120-binding domain of CD4 and gp120 derived synthetic peptides which bind to CD4 can be used to modulate the immunoresponse and block HIV-1 infectivity. We report an affinity chromatography assay, which, in combination with epitope-specific anti-CD4 mAbs, may allow the rapid identification of ligands recognized by CD4 and their cognate binding sites. This method should facilitate the design of analogues exhibiting a higher affinity or selectivity for the protein thus restricting the number of potentially useful compounds to be tested in more sophisticated assays.

The interaction between the viral envelope protein gpl 20 and the cellular surface antigen CD4 is a key event in HIV-1 infection. Reciprocal high affinity binding sites have been located in the first domain of CD4 and in the carboxy-terminal region of gpl20, respectively. Upon infection, the membranes of the target cells fuse; sites of CD4 and gp120, distinct from their high affinity binding sites, play a role in the post-binding events leading to syncytia formation. We have studied the interactions of CD4 with gp120 and gp120-derived peptides using an in vitro assay based on immobilized recombinant soluble CD4 (sCD4). In this system CD4 binds to recombinant soluble gpl 20 and to anti-receptor peptides derived from the high affinity CD4-binding site of gp120, as well as to peptides corresponding to the principal neutralizing domain (PND) of the envelope protein, i.e., to the domain required for HIV-1-mediated syncytium formation. Competition experiments performed using epitope-specific mAbs and a variety of peptides indicated that PND-derived peptides are specifically recognized by a CD4 site adjacent to, but distinct from, the high affinity gp120-binding site of CD4. Synthetic peptides patterned on the PND of different viral isolates were retained onto sCD4-based affinity columns at different extent; some of the structural requirements for binding were analyzed. Studies performed on CD4+ T-cells showed that PND-derived peptides also interact with CD4 in its native membrane-bound conformation. These results indicate that a direct contact takes place between CD4 and the gp120 domain participating in HIV-induced syncytia formation.

The proteins secreted by the rat seminal vesicle can be separated into five denaturing conditions. Two polymorphic proteins, svp-1 and svp-2, also present in the mouse, are produced by the seminal vesicle as well, but the procedure used for their identification makes it impossible to ascertain whether they correspond to any of the major fractions mentioned above. We show here that, on the basis of molecular weight measurements and of amino acid composition determinations, svp-1 and RSV-V are indeed the same protein. We also show that svp-2 is strictly related to another major secretory protein, RSV-IV, whose amino acid composition is almost identical, but for a few amino acid residues, to that of svp-2. We thus conclude that the latter protein is a variant of RSV-IV that can be expressed only in rats homozygous for a given allele at the svp-2 locus. This paper thus brings together published information on the genetics of the loci coding for svp-1 and for svp-2 and on the molecular biology of RSV-IV and RSV-V and of their corresponding gene.