Figure 2.

Lowering the pH triggers virion-target vesicle lipid mixing detected by fluorescence dequenching. (A) Schematic diagram illustrating the single-virus lipid mixing experiment. A Texas-Red-labeled vesicle (self-quenched concentration) is tethered via DNA lipids to a surface-tethered influenza virion (see Fig. 1A). When the pH is lowered, lipid mixing occurs, and the lipids in the target vesicle are diluted upon mixing with the viral membrane. The schematic is not drawn to scale. (B) Example microscope images of Texas-Red-labeled vesicles that are bound to surface-tethered influenza virions. At pH 7.4 (left), the self-quenched target vesicle is dim but detectable. After the pH is lowered to 5.1 (right), particles display dequenching due to lipid mixing. (C) Example time trace (red) of the fluorescence intensity of a Texas-Red vesicle (shown in white box in B) that undergoes dequenching. The wait time is defined as the time from the pH drop to the lipid mixing event. To see this figure in color, go online.