Truncation of the Cytoplasmic Domain of the Simian Immunodeficiency Virus Envelope Glycoprotein Alters the Conformation of the External Domain

We previously reported that truncation of the cytoplasmic domain of the macaque simian immunodeficiency virus SIVmac239 envelope glycoprotein enhanced its ability to induce cell fusion in a variety of cell lines. In the present study, we examined the expression of the full-length and truncated SIVmac239 envelope glycoprotein complex on cell surfaces. Using a membrane-impermeable reagent to biotinylate proteins on cell surfaces followed by immunoprecipitation, we found that under conditions in which the full-length TM protein could not be detected on the surfaces of CD4-positive or CD4-negative cell lines, the truncated TM protein was detected efficiently. In contrast, using a membrane-impermeable iodination reagent to label proteins on cell surfaces, we could detect both the full-length and truncated TM proteins. No difference between the full-length and truncated proteins was observed in the detection of the SU proteins in the biotinylation assay. Additionally, we used an assay in which SIV-specific antibodies are prebound to the native envelope proteins expressed on the cell surface and then the proteins are immunoprecipitated. Using this assay, we could not detect the truncated or full-length TM protein on the cell surface, whereas we could detect the SU subunits of both proteins. We also observed that the truncated TM protein formed more stable sodium dodecyl sulfate-resistant oligomers than the full-length TM protein did. These results indicate that truncation of the cytoplasmic domain of the SIVmac239 envelope glycoprotein affects the conformation of the external domain of the TM protein on the cell surface, even though the two proteins have no differences in the amino acid sequences of their external domains. This altered conformation could play a role in the enhanced fusion activity of the truncated SIV glycoprotein.