Morphogenesis of post-Golgi transport carriers

Abstract

The trans-Golgi network (TGN) is one of the main, if not the main, sorting stations in the process of intracellular protein trafficking. It is therefore of central importance to understand how the key players in the TGN-based sorting and delivery process, the post-Golgi carriers (PGCs), form and function. Over the last few years, modern morphological approaches have generated new insights into the questions of PGC biogenesis, structure and dynamics. Here, we present a view by which the "lifecycle" of a PGC consists of several distinct stages: the formation of TGN tubular export domains (where different cargoes are segregated from each other and from the Golgi enzymes); the docking of these tubular domains onto molecular motors and their extrusion towards the cell periphery along microtubules; the fission of the forming PGC from the donor membrane; and the delivery of the newly formed PGC to its specific acceptor organelle. It is now important to add the many molecular machineries that have been described as operating at the TGN to this "morphofunctional map" of the TGN export process.

Fig. 1

Formation of post-Golgi transport carriers. Subsequent frames extracted from a time-lapse sequence illustrating the different stages of PGC biogenesis (arrows): a formation of the tubular domain containing the cargo VSVG-YFP and devoid of the Golgi-resident protein galatosyltransferase-CFP. b Extrusion of this tubular domain from the Golgi complex. c, d Fission of the domain from the parental Golgi membranes. e Thin section of a cell expressing the TGN38-HRP construct. Arrows indicate tubular PGC precursor that has been pulled out of the TGN area of the Golgi complex

Fig. 2

Fission of post-Golgi transport carriers. a TGN precursors of post-Golgi carriers are pulled along microtubules by kinesin. The fission (red line) of the carriers occurs at the thinnest parts of the PGC precursor, which correspond to thin tubular segments of the TGN membrane at the electron microscopy level. In contrast, fission does not take place at the TGN regions with a complex morphology (i.e., containing tubular networks and fenestrae, or in thick vacuolar regions). If fission occurs close to the tip of a PGC precursor, the carrier will be smaller in size (1). In contrast, larger PGCs can be formed by cleavage at the bottom of a PGC precursor ( 2). b PGCs directed to endosomes detach from the TGN as simple clathrin-coated vesicles if fission (red line) occur at the neck of the clathrin-coated bud (1). Alternatively, entire chunks of the TGN membrane containing 2-3 clathrin-coated buds can be cleaved from the Golgi complex