Coproteins [1]. Binding of gp120 to the receptor, CD4, on the target cell surface induces major conformational changes in the envelope glycoproteins [2]. These changes allow gp120 to bind the viral coreceptor, either CXCR4 or CCR5 [3?]. CD4 binding also induces the formation of a gp41 pre-hairpin intermediate, in which three hydrophobic grooves on the surface of a coiled coil formed by the heptad