The external glycoproteins of human immunodeficiency virus type 1 (HIV-1) (gp120) and HIV-2 (gp105) are responsible for binding the cellular receptor CD4. The proteins are functionally identical although their affinity for CD4 varies, with gp120 binding 10- to 20- fold more efficiently than gp105. To investigate the structural requirements for CD4 binding in each molecule we have constructed a number of hybrid glycoproteins in which sequences are exchanged between the two molecules via conserved residues and subsequently tested for their ability to bind to CD4. We found that two constructs in which the V1/V2 or V3 loops of gp105 are exchanged for those of gp120 continue to bind to CD4. Surprisingly, however, all other domain exchange mutants failed to bind to CD4 suggesting that long-range interactions within the molecule are sequence-specific. Mixing mutant molecules did not rescue CD4 binding. However, coexpression of a number of mutant glycoprotein pairs within the same cell produced complementation of CD4 binding ability; complementing molecules were shown to be heteromeric in structure. Alignment of the molecules within each complementation group allowed the interactive sequences necessary for receptor binding to be determined. These sequences constitute a novel target for the disruption of gp120 function.


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