FIG. 3.

Capacity of S mutant proteins to mediate entry of SARSpp (A) and cell-cell fusion (B). (A). Infectivity of SARSpp was assayed as described before (2), by fluorescence-activated cell sorting. The infectivity of the mutant SARSpp is relative to WT SARSpp infectivity (set to 100%). Two experiments were carried out. (B) Fusion of cells expressing S (red [DsRed, Discosoma red]) and ACE-2 (green [eGFP, enhanced green fluorescent protein]), resulting in double fluorescence (yellow). Representative samples are presented, showing cell fusion activity of all S mutant proteins. Magnification, ×40.