FIG. 4.

HIV-1 Env utilizes multiple regions of the coreceptor CXCR4 for viral fusion. HIV-luciferase reporter viruses pseudotyped with the X4 Envs of HXB2 and NL4-3 were used to infect U87-MG cells transiently transfected with the constructs indicated. All cells were transfected with pT4, and the vector control (CD4) was cotransfected with plasmid vector alone (pcDNA3) instead of vector expressing a chemokine receptor. Pertussis toxin (PTX) was added 8 to 16 h prior to infection of cells expressing CXCR4 and either removed at the time of infection or maintained in culture during infection, with identical results. The results of SDF-1 binding (Fig. 3) and Ca²⁺ mobilization data (Fig. 1 and 2) are summarized below (+, near wild-type activity; +/−, <50% of wild-type activity; −, no significant activity detectable). Chimeras 2442 and 2242 did not respond to SDF-1 by Ca²⁺ mobilization but have been shown to be on the cell surface by FACS at near wild-type levels (data not shown). Chimera 2244 is expressed on the surface, but at <10% of the wild-type level (data not shown). Data shown are the average and standard error of independent experiments repeated at least three times. RLU, relative light units.