FIG. 5.

239-ST1.2-32 determinants of CXCR4 use in cell-cell fusion assays. (A) Fusion activities of 239-ST1.2-32 Envs with individual mutations reverted back to the corresponding residues in SIVmac239. (B) Fusion activities of SIVmac239 Envs containing 239-ST1.2-32 mutations alone and in combination. The 239-ST1.2-32 Env with a full-length CT (Long CT) was included for comparison with the SIVmac239 mutant Envs, which contain full-length tails. Percent fusion was calculated by using luciferase activity normalized to 239-ST1.2-32 fusion with CD4⁺ CCR5⁺ or CD4⁺ CXCR4⁺ QT6 cells. Background fusion levels with QT6 cells expressing only CD4 were subtracted prior to normalization. The data shown are means of three experiments plus SEM.