Bi-directional trafficking between the trans-Golgi network and the endosomal/lysosomal system.

Protein transport in the secretory and endocytic pathways of eukaryotic cells is mediated by vesicular transport intermediates. Their formation is a tightly controlled multistep process in which coat components are recruited onto specific membranes, and cargo, as well as targeting molecules, become segregated into nascent vesicles. At the trans-Golgi network, two transport systems deliver cargo molecules to the endosomal system. They can be distinguished with regard to coat components that select cargo molecules. AP-1 assembly proteins mediate transport of MPRs and furin, whereas AP-3 adaptors mediate transport of lysosomal membrane glycoproteins to the endosomal/lysosomal system. The molecular basis for protein-specific sorting lies within sorting signals that are present in the cytoplasmic tails of cargo proteins and allow specific interactions with individual coat components. In order to maintain cellular homeostasis, some proteins are retrieved from endosomal compartments and transported back to the trans-Golgi network. Distinct points for protein retrieval exist within the endosomal system, retrieval occurring from either early or late endosomes. Whereas significant progress has been made in recent years in identifying anterograde and retrograde transport pathways, the molecular mechanisms underlying protein sorting and retrieval are only poorly defined. Recently, however, novel vesicle coats (e.g. AP-4) and proteins that might be involved in sorting (e.g. PACS-1 and TIP47) have been described, and the interactions between assembly proteins and sorting signals are becoming increasingly well defined.