FIG. 3.
Fusion of IBV with host cells is low pH dependent. R18-labeled IBV strain Beaudette or Sendai virus strain Cantell was bound to BHK cells at 4°C for 60 min and then injected into a spectrofluorimeter cuvette containing 1 ml of pH 7.0 buffer at 37°C (t = 100 s) (A). Samples were monitored for fluorescence dequenching at 37°C for 300 seconds before addition of 1% Triton X-100 (final concentration) to obtain complete (100%) dequenching. (B) Similarly, R18-labeled IBV strain Beaudette or influenza virus strain A/WSN/33 was bound to BHK cells at 4°C, but samples were added to pH 7.0 buffer at 37°C (t = 0 s). At t = 100 s, the buffer pH was reduced to 5.0 and samples were monitored for fluorescence dequenching at 37°C. At t = 400 s, the final concentration of 1% Triton X-100 was added to obtain 100% dequenching. (C) Samples were treated as described for panel B under various pH conditions, and dequenching activities are shown in terms of actual fusion units. The initial rate of fusion obtained from panel C was analyzed by four-parameter exponential decay and is plotted against various pHs (D). The pH which gave the half-maximal initial rate of IBV fusion (pH1/2) is indicated.