Relationship of the human immunodeficiency virus type 1 gp120 third variable loop to a component of the CD4 binding site in the fourth conserved region.

Neutralizing antibodies that recognize the human immunodeficiency virus gp120 exterior envelope glycoprotein and are directed against either the third variable (V3) loop or conserved, discontinuous epitopes overlapping the CD4 binding region have been described. Here we report several observations that suggest a structural relationship between the V3 loop and amino acids in the fourth conserved (C4) gp120 region that constitute part of the CD4 binding site and the conserved neutralization epitopes. Treatment of the gp120 glycoprotein with ionic detergents resulted in a V3 loop-dependent masking of both linear C4 epitopes and discontinuous neutralization epitopes overlapping the CD4 binding site. Increased recognition of the native gp120 glycoprotein by an anti-V3 loop monoclonal antibody, 9284, resulted from from single amino acid changes either in the base of the V3 loop or in the gp120 C4 region. These amino acid changes also resulted in increased exposure of conserved epitopes overlapping the CD4 binding region. The replication-competent subset of these mutants exhibited increased sensitivity to neutralization by antibody 9284 and anti-CD4 binding site antibodies. The implied relationship of the V3 loop, which mediates post-receptor binding steps in virus entry, and components of the CD4 binding region may be important for the interaction of these functional gp120 domains and for the observed cooperativity of neutralizing antibodies directed against these regions.