Generation and structural analysis of soluble oligomeric gp140 envelope proteins derived from neutralization-resistant and neutralization-susceptible primary HIV type 1 isolates

We generated DNA constructs expressing soluble truncated forms of the envelope of SF162, a neutralization-resistant primary human immunodeficiency virus type 1 isolate, and SF162 Delta V2, a neutralization-susceptible virus derived from SF162 after the deletion of 30 amino acids from the V2 loop. The constructs express the entire gp120 subunit and the extracellular region of the gp41 subunit, with either the presence ("cleaved" forms, designated gp140C) or the absence ("fused" forms, designated gp140F) of the gp120-gp41 cleavage site. Both gp140 forms derived from SF162 and SF162 Delta V2 are secreted in the cell medium and are recognized by the oligomer-specific anti-gp41 MAb T4. As is the case for the corresponding virion-associated envelope molecules, the CD4-binding region is occluded within both gp140F and gp140C forms. However, structural differences exist between these two forms. The gp140F proteins are less efficiently recognized than the gp140C proteins by antibodies present in the sera of HIV-infected patients with neutralizing activities against SF162 and SF162 Delta V2. Also, the V3 loop is more exposed on gp140F than gp140C. As is the case for intact virions, on CD4 binding both the gp140F and gp140C proteins undergo conformational changes that result in the exposure of the epitope recognized by MAb 17b, which has been implicated in coreceptor binding. In contrast, during these structural changes the exposure of specific V3 loop epitopes is not increased on either gp140C or gp140F. Taken together, our data indicate that although these gp140 forms differ structurally from the native envelope, their similarities, in particular that of gp140C, outweigh their differences.