Figure 4.

In vitro endosome–endosome and lysosome–lysosome in alveolar macrophages. Cells were incubated at 37°C with 0.5 mg/ml b-HRP or avidin for either 5 min (endosome) or 45 min, followed by a 90 minute chase period (lysosome). Cells were homogenized, PNS was obtained and layered over separate 27% Percoll gradients, and then subcellular fractionation was performed as described in Fig. 3. Endosomes (fractions 11–16) and lysosomes (fractions 3–7) were isolated and Percoll removed before in vitro vesicle fusion as described in Materials and Methods.