We previously reported that truncation of the terminal 146 amino acids of the macaque simian immunodeficiency virus SIVmac239 envelope glycoprotein enhanced envelope-specific syncytium formation in HeLa T4, CEM X 174, and HUT 78 cell lines and caused a change in the conformation of the transmembrane subunit of the envelope complex on the surface of these cells [Ritter et al. (1993) Virology 197, 255-264; Spies et al. (1994) J. Virol. 68, 585-591]. To investigate the effects of different lengths of the cytoplasmic domain on syncytium formation and cell surface expression, we have compared the expression and cytopathic effects induced by five SIVmac239 envelope constructs which vary in the lengths of their cytoplasmic domains. In contrast to the envelope protein truncated by 146 amino acids, the ability of proteins truncated by 98 or 161 amino acids to form syncytia was substantially reduced in CEM X 174 and HUT 78 cells, while syncytium formation by a protein truncated by 53 amino acids was only slightly reduced compared to the full-length protein. Furthermore, only the glycoprotein which was truncated by 146 amino acids induced syncytium formation in HeLa T4 cells. When examining the expression of the truncated proteins on the surface of HeLa T4 cells, we found that, in contrast to the full-length SIVmac239 protein, each of the truncated transmembrane subunits could be efficiently biotinylated with the membrane-impermeable reagent NHS-SS-biotin. Furthermore, using cell surface iodination, we found stable oligomeric forms of both the transmembrane subunits and the uncleaved precursor proteins of each mutant protein on the surface of HeLa T4 cells. Using pulse-chase analysis, we also found that the precursor of the protein truncated by 98 residues was degraded more rapidly than the wild-type and the other mutant proteins. Finally, we constructed two mutants which expressed a full-length TM protein or a TM protein with a 146 amino acid C-terminal deletion and had most of the coding sequences of their SU subunits deleted. Neither of these two proteins was found to cause syncytium formation in HeLa T4, CEM X 174, or HUT 78 cell lines even though we could detect both proteins on the surfaces of HeLa T4 cells using iodination. These results could explain why the selection of truncated variants of SIV which emerge after prolonged passage in human cell lines is restricted to truncations which remove close to 146 amino acids in the cytoplasmic domain of the TM protein.