Fig. 1. Defective secretion of post lysosomes in lvsB mutant cells. (A) Schematic representation of the endocytic pathway in Dictyostelium. Phagocytosis and macropinocytosis allow internalization of extracellular material into acidic lysosomes. These compartments then mature into neutral post lysosomes, which are eventually exocytosed. The contractile vacuole (CV) is a non-endocytic specialized organelle involved in osmoregulation. The localization of the different markers (p80, H⁺-ATPase and actin) used in this study is shown. (B) Confocal images of a typical exocytic p80 patch at the cell surface. Cells were fixed and stained for p80 (green) and PM4C4 (red). The PM4C4 protein is evenly distributed at the plasma membrane whereas p80 is highly concentrated in exocytic patches at the cell surface. Bar, 5 µm. (C) Fewer p80 patches in lvsB mutant cells. The percentage of cells exhibiting a surface p80 patch was determined for each cell line. The mean and s.e.m. of three independent experiments are indicated. At least 300 cells were analyzed for each cell type in each experiment. (D) The p80 patches found at the surface of lvsB mutant cells are similar in size and intensity to those found in WT cells. Exocytic p80 patches were identified on random confocal images, and for each patch the size and the intensity of p80 fluorescence were determined. Twenty patches were analyzed for WT and lvsB mutant cells. Each dot represents one patch and colored crosses indicate the average size and intensity as well as the s.d. of the results (green, WT; red, lvsB).