FIG. 6.
Simultaneous downregulation of CXCR4 and CD4 by Nef correlates with maximum inhibition of X4 HIV Env-mediated fusion at the plasma membrane. (A) Assay used for quantification of Env-mediated cell-cell fusion. (B, C, and D) Fusion efficiencies of TZM target cells expressing the indicated GFP or Nef.GFP proteins after they were mixed with CHO Tat cells expressing the indicated HIV-1 Env proteins (HXB-2, NL4-3GIA, and NL4-3) were determined via β-galactosidase enzyme activity in relation to the protein concentration. (E and F) Marburg virus GP and VSV-G served as control envelopes that fuse independently of CD4 and CXCR4. The relative fusion efficiencies of GFP-expressing cells were set to 100%, and the arithmetic means + standard deviations (n = 8) are given. The experiment shown is representative of three independent experiments. *, P < 0.05 by Student's t test.