Retrograde endosome-to-TGN transport

Abstract

Eukaryotic cells are characterized by the existence of many membranebound organelles with each having a unique structural composition and carrying out specific cellular functions. Many of these organelles are parts of the secretory and endocytic pathways and are integrated by dynamic interorganellar membrane trafficking events. Endocytosis is the process of uptaking of molecules from the cell surface into intracellular compartments, through which cargos at the plasma membrane (PM), such as solutes, lipids, receptors (and their bound ligands), toxins and viruses, are internalized by vesicles, which either fuse with existing endosomes or undergo homotypic fusion to form endosomes. Recent advances in this area have established the existence of multiple routes/modes of endocytosis and the resulting diverse types of endosomes. Thus, aside from the classic early endosome (EE), which is mostly derived from clathrin-dependent endocytosis and is characterized by the presence of Rab5 GTPase and PtdIns(3)P, there are endosomes generated from non-clathrin mediated endocytic routes such as ARF6 endosome and caveolin endosome (Pelkmans et al. 2001) from ARF6- and caveolae-dependent endocytosis, respectively. Endosomes of non-clathrin endocytic origin are less understood currently. Different types of endosomes can undergo heterotypic fusion to deliver cargos derived from non-clathrin-dependent endocytosis to the classic endocytic pathway (Naslavsky et al. 2003).