Structure and domain organization of the trans-Golgi network

Abstract

The trans-Golgi network (TGN) is a unique compartment located at the exit face of the Golgi stack (Griffiths and Simons 1986) typically associated with a large amount of vesicular and tubular membranes (Ladinsky et al. 1994; Mollenhauer and Morre 1998; Roth and Taatjes 1998). The TGN varies morphologically between different cell types, considered to be a consequence of the differences in secretory activity (Clermont et al. 1995; Gu et al. 2001). Like other Golgi cisternae, the TGN contains a number of resident enzymes involved in the processing of cargo molecules, such as glycosyltransferases involved in the addition of terminal sugars (Rabouille et al. 1995), several proprotein convertases including furin (Thomas 2002) and tyrosine sulphation enzymes. However, unique functions and characteristics clearly distinguish the TGN from the other Golgi cisterna. Firstly, the TGN sorts various mature cargo proteins and lipids into membrane carriers destined for the plasma membrane, endosomes, secretory granules, or earlier Golgi cisternae or the ER (Keller and Simons 1997; Traub and Kornfeld 1997; Opat et al. 2001; Sannerud et al. 2003; Young et al. 2005). Secondly, the TGN receives cargo from various endosomal locations (Bos et al. 1993; Pavelka et al. 1998; Medigeshi and Schu 2003; Shewan et al. 2003). Thus the TGN presents a trafficking hub where the secretory and endocytic pathways merge. Thirdly, the TGN responds differently to treatments which block membrane transport. For example, treatment with brefeldin-A results in the membranes of the Golgi stack rapidly fusing into the ER, while the TGN tubulates and merges with the recycling endosomal system (Chege and Pfeffer 1990; Lippincott-Schwartz et al. 1991).