REGIONS REQUIRED FOR CD4 BINDING IN THE EXTERNAL GLYCOPROTEIN GP120 OF SIMIAN IMMUNODEFICIENCY VIRUS

The external domain of the envelope glycoprotein, gp120, of simian imm unodeficiency virus (SIV has been expressed as a mature secreted produ ct using recombinant baculoviruses and the expressed protein, which ha s an observed molecular mass of 110 kDa, was purified by monoclonal an tibody (MAb) affinity chromatography. N-terminal sequence analysis sho wed a signal sequence cleavage identity similar to that of the gp120s of both human immunodeficiency virus type 1 (HIV-1) and HIV type 2. Th e expressed molecule hound to soluble CD4 with an affinity that was ap proximately 10-fold lower than that of gp120 from HIV-1. A screening o f the ability of SIV envelope MAbs to inhibit CD4 binding revealed two groups of inhibitory MAbs. One group is dependent on conformation, wh ile the second group maps to a discrete epitope near the amino terminu s. The particular role of the V3 loop region of the molecule in CD4 bi nding was investigated by the construction of an SIV-HIV hybrid in whi ch the V3 loop of SIV was precisely replaced with the equivalent domai n from HIV-1 MN. The hybrid glycoprotein bound HIV-1 V3 loop MAbs and not SIV V3 MAbs but continued to bind conformational SIV MAbs and solu ble CD4 as well as the parent molecule.