FIG. 1.

pH dependencies of the initial rate of fusion of virosomes containing X-47 HA (⧫), A/Shangdong HA (■), or a 1:1 mixture of both (▴). Erythrocyte ghosts were equilibrated in fusion buffer at the indicated pH for 3 min, at 37°C, before the injection of N-NBD-PE- and N-Rh-PE-labeled virosomes into the cuvette. Fusion was measured as described in Materials and Methods. Final concentrations were 50 μg of ghost protein per ml and 5 μM virosomes (phospholipid phosphate). The results shown were obtained with the same batch of ghosts and virosomes, and the fusion rates are representative of repeated experiments, although the rates may vary between ghost preparations.