Endosomes and Endocytic Pathways

Abstract

Different endocytic portals and pathways are known in mammalian cells (see diagram), roughly classified as clathrin-dependent (I, cf. Fig. 46) and clathrin-independent ones. The latter include endocytosis by budding of “smooth” vesicles lacking a distinct morphologically visible coat (2, cf. Fig. 52), uptake via pits containing lipid rafts (3), traffic via caveolae (4, cf. Fig. 51) and caveosomes (C), phagocytosis (5, cf. Fig. 52) and uptake via macropinocytosis (6) leading to the formation of large endocytic vacuoles, termed macropinosomes (MP, cf. Fig. 46). The routes travelled by endocytosed molecules involve complex endosomal compartments with mosaics of specialised structural and functional domains. Endosomes are highly dynamic and transform in response to endocytosis-connected signals. Early endosomes, classified in early sorting endosomes (SE) and recycling endosomes (RE), represent first stations from where proteins and lipids are sorted to different routes. These can be either routes to late endosomes (LE) and lysosomes (LY) for degradation, or recycling pathways to the plasma membrane (PM), or routes to other destinations involving trans Golgi networks (TGN), and the Golgi apparatus (GA).