FIG 7.

Efficiency of polykaryon formation (as a measure of fusion activity) of mutant HA proteins. HeLa cells were transfected with plasmids expressing the indicated wild-type or mutant HA proteins and were then exposed to acidic buffers over a pH range (5.0 to 5.8) to induce cell-cell fusion. (A) The efficiency of polykaryon formation was calculated from the number of nuclei in the polykaryons divided by the total number of nuclei in the same field. Shown are the means and standard deviations determined from five randomly selected fields. pCAGGS, vector control. (B) Representative photographs of polykaryon formation at the indicated pH values. Images were taken at ×20 magnification. A “+” indicates that the HA protein encodes the respective amino acid change(s) in the background of H5HA-N224K+Q226L.