FIG. 1.

sCD4 activation of Env-mediated, coreceptor-dependent cell fusion. Effector HeLa cells were transfected with indicator plasmid pG1NT7β-gal and infected with recombinant vaccinia viruses encoding the designated Envs (Unc, uncleaved control). Target NIH 3T3 cells were transfected with either pGA9-CKR5 (CCR5: +) or pSC59 (CCR5: −) and infected with vP11T7gene1 encoding T7 RNA polymerase. Four-domain sCD4 (final concentration = 300 nM; sCD4: +) or an equivalent volume of buffer (sCD4: −) was added upon mixing of effector and target cells. Cell fusion was quantitated as described in Materials and Methods. β-gal, β-galactosidase; OD, optical density.