Characterization of Stable, Soluble Trimers Containing Complete Ectodomains of Human Immunodeficiency Virus Type 1 Envelope Glycoproteins

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Abstract
The human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins function as a membrane-anchored trimer of three gp120 exterior glycoproteins and three gp41 transmembrane glycoproteins. Previously, we reported three approaches to stabilize soluble trimers containing parts of the gp41 ectodomains: addition of GCN4 trimeric helices, disruption of the cleavage site between gp120 and gp41, and introduction of cysteines in the gp41 coiled coil to form intersubunit disulfide bonds. Here, we applied similar approaches to stabilize soluble gp140 trimers including the complete gp120 and gp41 ectodomains. A combination of fusion with the GCN4 trimeric sequences and disruption of the gp120-gp41 cleavage site resulted in relatively homogeneous gp140 trimers with exceptional stability. The gp120 epitopes recognized by neutralizing antibodies are intact and exposed on these gp140 trimers. By contrast, the nonneutralizing antibody epitopes on the gp120 subunits of the soluble trimers are relatively occluded compared with those on monomeric gp120 preparations. This antigenic similarity to the functional HIV-1 envelope glycoproteins and the presence of the complete gp41 ectodomain should make the soluble gp140 trimers useful tools for structural and immunologic studies.