Exiting the Golgi complex

The trans-Golgi network (TGN) is an assembly of pleiomorphic tubular membranes that emanates from the trans-Golgi pole. It has multiple crucial roles in intracellular transport as a sorting node for secretory cargo, a biosynthetic centre for sphingolipids and the interface between the exocytic and endocytic pathways.The TGN is an extremely dynamic structure, and its extent (size and number of tubules) largely depends on the amount of cargo and membrane flowing through it. We therefore propose that this organelle is an assembly of cargo-sorting domains under extrusion as tubules for export out of the Golgi complex.The main destinations of TGN-derived carriers are the apical and basolateral plasma membrane, the early and late endosomes, the secretory granules and other specialized compartments in specialized cells. Each of these destinations corresponds to at least one carrier type, although different carriers might ferry specific cargo proteins to the same acceptor organelle.The final destination of each specific cargo type is determined by the sorting signals in the cargo molecule. These signals are decoded by a complex cytosolic machinery, within which coat proteins and adaptors have a major role.The formation of these pleiomorphic tubular TGN carriers can be divided into three main stages: cargo sorting into a forming tubule; extrusion of the tubular carrier along microtubules by molecular motors; and fission of the elongated tubule into a free carrier. Each of these stages involves a multi-component machinery, only parts of which have been identified to date.Future research in the TGN area will require the unravelling of the further components and their assigning to the appropriate carrier formation stage within specific Golgi export pathways. Another key challenge will be to determine the regulatory mechanisms by which the functions of the TGN are controlled and coordinated both at the organelle level within the secretory pathway, and between the TGN and other global cellular functions and responses.