Effects of an amino acid substitution(s) and deletion(s) on HIV-1 gp120-elicited cell-cell fusion. CD4+ MAGI cells which expressed sufficient numbers of wild-type or mutated CCR5 were cultured with HIV-1 env+, tat+ 293T cells for 6 hours, and the fusion efficiency was determined with the luciferase activity (luminescence levels) using the reporter gene activation assay. The magnitude of luminescence levels with mutant CCR5 is expressed as % fusion (% control compared to the luminescence level with wild-type CCR5). Note that three single amino acid substitutions (E172A, L174A, and C178A), which resulted in a substantial reduction in the fusion level (> 70%), are located in ECL2 that has an antiparallel β-hairpin structure (Fig. 2 and Fig. 3), while C101A and G163R are in TM3 and TM4, respectively, and W248A and Y251A are in TM6.