Figure 4.

Recycling endosomes in MDCK cells are less acidic than early endosomes. Tf-FITC was prebound at 4°C to m-hTfR cells and then internalized for 5 min at 20°C (upper panels) or internalized continuously for 20 min at 37°C (lower panels). Typical examples of the respective labels are shown. Because of technical constraints, we used subconfluent cells grown on glass coverslips rather than on filters. The pH of individual organelles was measured by fluorescence ratio imaging of internalized Tf-FITC. The histograms show the pH distribution of 767 and 206 endosomes for the upper and lower panels, respectively, with the mean ± SD given in the left panels.