Lysosome-endosome fusion and lysosome biogenesis

Recent data both from cell-free experiments and from cultured cells have sh own that lysosomes can fuse directly with late endosomes to form a hybrid o rganelle. This has a led to a hypothesis that dense core lysosomes are in e ssence storage granules for acid hydrolases and that, when the former fuse with late endosomes, a hybrid organelle for digestion of endocytosed macrom olecules is created. Lysosomes are then re-formed from hybrid organelles by a process involving condensation of contents. In this Commentary we review the evidence for formation of the hybrid organelles and discuss the curren t status of our understanding of the mechanisms of fusion and lysosome re-f ormation. We also review lysosome biosynthesis, showing how recent studies of lysosome-like organelles including the yeast vacuole, Drosophila eye pig ment granules and mammalian secretory lysosomes have identified novel prote ins involved in this process.