LENGTH POLYMORPHISM WITHIN THE 2ND VARIABLE REGION OF THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ENVELOPE GLYCOPROTEIN AFFECTS ACCESSIBILITY OF THE RECEPTOR-BINDING SITE

Sequential mutations were introduced into the V2 region of human immun odeficiency virus (HN) type 1 HXB2, affecting the length, charge, and number of potential glycosylation sites. The insertions had no effect on cytopathicity or on the ability of virus to replicate in peripheral blood mononuclear cells and established T-cell lines. However, deleti on of amino acids 186 to 188, encoding a conserved glycosylation site, resulted in a nonviable virus, suggesting a minimal length requiremen t of 40 amino acids for a functional V2 loop. However, all amino acid insertions affected the sensitivity of the variants to neutralization by soluble CD4 and monoclonal antibodies specific for epitopes in the V3 and CD4 binding site regions. Furthermore, these mutant viruses sho wed resistance to neutralization by HIV-positive human sera. Soluble g p120 mutant glycoproteins showed increased affinities for soluble CD4 and monoclonal antibodies specific for a number of epitopes overlappin g the CD4 binding site, confirming that length increases in V2 affect exposure of the CD4 binding site. In summary, these data demonstrate t hat differences in V2 length modulate immunoreactivity of the envelope glycoprotein and support an association between the V2 and CD4 bindin g site regions.