Intracellular digestion is a basic function of cells. The membranes enclosing the digestive compartments protect the cell from the toxic chemicals, microorganisms, and extreme acidity, and are involved in transport of nutrients into the cell's cytosol. In Paramecia, these functions are carried out efficiently in the digestive vacuoles by the sequential addition and/or retrieval of several morphologically and functionally distinct pools of membrane vesicles: discoidal vesicles, which provide membranes for vacuole formation; acidosomes, which are involved in acidification of the digestive vacuoles;and lysosomes, which contain the acid hydrolases responsible for macromolecular digestion. The degraded products are transported from within the digestive vacuoles to the cytosol, to be used for the cell's growth. These various intracellular membrane-bound compartments will be separately isolated and biochemically characterized, using a variety of approaches including monoclonal antibodies, free-flow electrophoresis, gradient density centrifugation, magnetic separation, in vitro fusion studies, pulse-chase experiments, and lysosomotropic drugs. The results of these studies will not only provide us with a better understanding of how intracellular digestion is carried out and regulated in phagocytic protozoa, but will also provide important new insights into the fundamental problem of eukaryotic intracellular membrane trafficking and compartmentalization in general.
