Protein transport from the trans-Golgi network to endosomes

After traversing the endoplasmic reticulum (ER), the ER-Golgi intermediate compartment (ERGIC) and the cisternae of the Golgi complex, newly synthesized proteins reach the trans-Golgi network (TGN), from where they can be sorted to several destinations: the extracellular space, different domains of the plasma membrane, regulated secretory granules and lysosome-related organelles. In addition, various types of protein have been shown to undergo direct transport from the TGN to endosomes. These include a subpopulation of lysosomal membrane proteins (Harter and Mellman 1992), endocytic receptors (Futter et al. 1995; Leitinger et al. 1995), processing peptidases (Molloy et al. 1999), and intracellular sorting receptors with their cargos (Le Borgne and Hof lack 1998; Ghosh et al. 2003). The latter are by far the best characterized and will be the main subject of this chapter.