DISSOCIATION OF THE CD4 AND CXCR4 BINDING-PROPERTIES OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 GP120 BY DELETION OF THE FIRST PUTATIVE ALPHA-HELICAL CONSERVED STRUCTURE

To evaluate conserved structures of the surface gp120 subunit (SU) of the human immunodeficiency virus type 1 (HIV-1) envelope in gp120-cell interactions, we designed and produced an HIV-1 IIIB (HXB2R) gp120 ca rrying a deletion of amino acids E61 to S85. This sequence corresponds to a highly conserved predicted amphipathic alpha-helical structure l ocated in the gp120 C1 region. The resultant soluble mutant with a del eted alpha helix 1 (gp120 Delta alpha HX1) exhibited a strong interact ion with CXCR4, although CD4 binding was undetectable. The former inte raction was specific since it inhibited the binding of the anti-CXCR4 monoclonal antibody (12G5), as well as SDF1 alpha, the natural ligand of CXCR4. Additionally, the mutant gp120 was able to bind to CXCR4(+)/ CD4(-) cells but not to CXCR4(-)/CD4(-) cells. Although efficiently ex pressed on cell surface, HIV envelope harboring the deleted gp120 Delt a alpha HX1 associated,vith wild-type transmembrane gp41 was unable to induce cell-to-cell fusion with HeLa CD4(+) cells. Nevertheless, the soluble gp120 Delta alpha HX1 efficiently inhibited a single round of HIV-1 LAI infection in HeLa P4 cells, with a 50% inhibitory concentrat ion of 100 nM. Our data demonstrate that interaction with the CXCR4 co receptor was maintained in a SUgp120 HIV envelope lacking alpha HX1. M oreover, in the absence of CD4 binding, the interaction of gp120 Delta alpha HX1 with CXCR4 was sufficient to inhibit HIV-1 infection.