Glycosylation of the external domain of SIV gp41 is influenced by the cytoplasmic tail

Evidence against extracellular exposure of a highly immunogenic region in the C-terminal domain of the simian immunodeficiency virus gp41 transmembrane protein

The generally accepted model for human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein topology includes a single membrane-spanning domain. An alternate model has been proposed which features multiple membrane-spanning domains. Consistent with the alternate model, a high percentage of HIV-1-infected individuals produce unusually robust antibody responses to a region of envelope, the so-called "Kennedy epitope," that in the conventional model should be in the cytoplasm. Here we show analogous, robust antibody responses in simian immunodeficiency virus SIVmac239-infected rhesus macaques to a region of SIVmac239 envelope located in the C-terminal domain, which in the conventional model should be inside the cell. Sera from SIV-infected rhesus macaques consistently reacted with overlapping oligopeptides corresponding to a region located within the cytoplasmic domain of gp41 by the generally accepted model, at intensities comparable to those observed for immunodominant areas of the surface component gp120. Rabbit serum raised against this highly immunogenic region (HIR) reacted with SIV envelope in cell surface-staining experiments, as did monoclonal anti-HIR antibodies isolated from an SIVmac239-infected rhesus macaque. However, control experiments demonstrated that this surface staining could be explained in whole or in part by the release of envelope protein from expressing cells into the supernatant and the subsequent attachment to the surfaces of cells in the culture. Serum and monoclonal antibodies directed against the HIR failed to neutralize even the highly neutralization-sensitive strain SIVmac316. Furthermore, a potential N-linked glycosylation site located close to the HIR and postulated to be outside the cell in the alternate model was not glycosylated. An artificially introduced glycosylation site within the HIR was also not utilized for glycosylation. Together, these data support the conventional model of SIV envelope as a type Ia transmembrane protein with a single membrane-spanning domain and without any extracellular loops.

Domains in the simian immunodeficiency virus gp41 cytoplasmic tail required for envelope incorporation into particles

The mechanism by which lentivirus envelope (Env) glycoproteins are packaged into budding virions is poorly understood. Simian immunodeficiency virus (SIV) contains an Env protein with an unusually long cytoplasmic tail. To investigate the role of this domain in the incorporation of the SIV Env into virions, we generated a series of SIV Env mutants carrying small in-frame deletions within the cytoplasmic domain. The effects of these mutations on Env synthesis, processing, and association with Gag particles were analyzed by means of the vaccinia virus expression system. All of the mutant Env glycoproteins were synthesized and processed in a manner similar to that of the wild-type Env. However, deletions affecting domains C-terminal to residue 832 in the SIV Env protein significantly impaired Env incorporation into particles. Cell surface biotinylation assays showed that this phenotype could not be attributed to inefficient cell surface expression of the Env mutants. Furthermore, when the Env deletion mutants were tested for their ability to mediate virus entry in single-cycle infectivity assays, those mutations that impaired Env incorporation also caused a severe defect in virus infectivity. Our results suggest that domains in the C-terminal third of the SIV Env protein are required for Env incorporation into particles and Env-mediated virus entry.

Features of the SIVmac transmembrane glycoprotein cytoplasmic domain that are important for Env functions

The cytoplasmic domain (CD) of the SIVmac transmembrane protein (TM) can affect viral infectivity by modulating several Env functions, notably fusogenic capacity and incorporation into virions. In addition, envelopes with a truncated CD are counterselected in primary cells in culture and in vivo in rhesus macaques, suggesting a role for this domain in viral persistence. Here, we have used mutagenesis to examine specific features of the SIVmac TM CD, including the conserved C-terminal alpha helix and the overall length of the CD. Several mutations dramatically reduced and/or delayed virus infectivity in lymphoid cell lines. Detailed analysis of mutants revealed defects in envelope stability, fusogenic capacity, and virion incorporation. The primary defect associated with an envelope containing a 64-residue CD was rapid degradation. A mutant Env lacking the C-terminal alpha helix but encoding an exceptionally long CD (373 residues) was highly fusogenic but inefficiently incorporated into virions. A third mutant, containing amino acid substitutions designed to alter the charge density of the C-terminal helix, retained cytopathic properties and showed enhanced fusogenic capacity but replicated with delayed kinetics. Taken together, these results demonstrate that CD sequence variation entails functional "tradeoffs" that can involve optimization of certain Env functions at the expense of others.

Human immunodeficiency virus type 2 glycoprotein enhancement of particle budding: role of the cytoplasmic domain

Previous studies have shown that the glycoprotein cytoplasmic domains of human immunodeficiency virus type 2 (HIV-2) or simian immunodeficiency virus of macaques modulate biological activities of the viral glycoprotein complex, including syncytium formation, exterior glycoprotein conformation, and glycoprotein incorporation into budding virus particles. We have now utilized a recombinant expression system to study interactions of full-length or truncated HIV-2 glycoproteins with coexpressed HIV-2 Gag proteins which self-assemble and bud as virus-like particles. Interestingly, budding of HIV-2 virus-like particles from cells was enhanced 5- to 24-fold when Gag was coexpressed with the full-length HIV-2 glycoprotein, compared with Gag expressed either alone or with a truncated HIV-2 glycoprotein. The results obtained in this model system indicate that an additional effect of the lengthy cytoplasmic domain of the glycoprotein of HIV-2 is enhancement of particle budding. We speculate that the cytoplasmic domain of the viral glycoprotein of HIV-2 enhances budding by (i) potentiation of Gag structure or function or (ii) membrane modulation.

Attachment of an oligopeptide epitope to the C-terminus of recombinant SIV gp160 facilitates the construction of SMAA complexes while preserving CD4 binding

A small 14 amino acid oligopeptide tag (termed SV5-Pk) was fused onto the carboxy-terminus of simian immunodeficiency virus gp160 expressed from a recombinant baculovirus. The presence of the Pk tag had no obvious effect on the expression and glycosylation of gp160 and did not interfere either with CD4 binding or with cleavage at its maturation site by the protease furin. The presence of the Pk tag did, however, facilitate the simplified purification of full-length gp160 and its incorporation into immunogenic solid matrix-antibody-antigen (SMAA) complexes.