Conformational Changes in Cell Surface HIV-1 Envelope Glycoproteins Are Triggered by Cooperation between Cell Surface CD4 and Co-receptors*

We have continuously measured CD4-induced conformational changes of cell surface-expressed human immunodeficiency virus type-1 envelope glycoprotein gp120-gp41 in situ using 4,4′-dianilino-1,1′-binaphthyl-5,5′-disulfonic acid, a fluorescent probe that binds to hydrophobic groups. CD4-expressing human T cell lines induced significant and rapid conformational changes (<1 min delay) in gp120-gp41 from T cell-tropic strains, and little conformational changes in gp120-gp41 from macrophage-tropic strains, with equivalent levels of envelope expression. Conversely, CD4-expressing human macrophages induced significant and rapid conformational changes in gp120-gp41 from macrophage-tropic strains, and little conformational changes in gp120-gp41 from T cell-tropic strains. Thus, the conformational changes undergone by gp120-gp41, which lead to membrane fusion, are highly cooperative and require both receptor and co-receptor. We used a dye transfer assay to show that neither membrane lipid fusion or fusion pore formation can occur with host cells having different tropism from the envelope.

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