Membrane (octadecylrhodamine B [R18], left
column) and aqueous (CF, right column) dye redistribution between
HA-expressing cells and double-labeled RBCs. (A and B) G520L cell–RBC
complexes were exposed to pH 4.8 for 2 min at 23°C, followed by
incubating at neutral pH at 23°C for 10 min to achieve the GL23
intermediate. Neither dye spread. (C and D) Upon increasing temperature
to 37°C, both aqueous and lipid dye spread, signifying fusion. (E and
F) HA cell–RBC complexes were exposed to pH 4.8 for 2 min at 4°C,
followed by reneutralization at 4°C for 10 min. The temperature was
then increased to 23°C (i.e., it is HA4-23). Neither lipids nor
contents mixed. (G and H) The cells were subsequently incubated at
37°C for 3 min, which induced extensive fusion, as seen by the
aqueous and membrane dye transfer.