Fig. 4. Most of the enlarged vesicles are acidic lysosomes in RabD^Q67L-expressing cells. In A, both wild-type () and RabDQ67L () cells were pulsed with FITC-dextran, washed and chased in fresh HL-5. At the indicated time points, cells were harvested, washed and resuspended in cold 50 mM MES buffer, pH 6.5. The ratio of fluorescence of FITC-dextran at 525 nm emission upon excitation at 495 nm/450 nm were measured against an in vitro standard curve from which the pH was extrapolated (n=3). Normal size lysosomes in the wild-type Ax4 cells (B,C) and the enlarged lysosomes in the RabD^Q67L cells (D,E) were observed when live cells were incubated with the acidophilic dye LysoSensor DND-189 (Molecular Probes). Bar, 2 µm. Arrows reveal enlarged vesicles in D and E.