Localization of TGN38 to the trans-Golgi network: involvement of a cytoplasmic tyrosine-containing sequence

T HE Golgi complex plays a central role in the intracellular transport, processing and sorting of proteins in the secretory pathway. Morphological and functional studies have established that the Golgi complex of higher eukaryotes is organized into at least three contiguous but distinct regions: the cis-Golgi network (CGN) t, the Golgi stack and the TGN (reviewed by Mellman and Simons, 1992; Rothman and Orci, 1992) . The CGN is a collection of tubules connected to the cis-Golgi cisternae which serves as the entry site for proteins transiting from the ER into the Golgi complex. Itinerant proteins entering the CGN are either recycled back to the ER or routed forward into the Golgi stack (Huttner and Tooze, 1989; Hsu et al., 1991; Pelham, 1991) . The Golgi stack is composed of a variable number of flattened cisternae, in which an array of resident carbohydrate-modifying enzymes sequentially process the oligosaccharide chains of glycoproteins (Farquhar and Palade, 1981; Paulson and CoUey, 1989) . Immediately apposed to the transmost aspect of the Golgi stack is the TGN (Roth et al., 1985; Geuze et al., 1985) , a tubulo-reticular structure where proteins undergo further carbohydrate modifications and, in some cases, proteolytic processing (Duncan and Kornfeld, 1988) . In addition to having a role in biosynthetic protein Dr. Peters is on sabbatical leave from the Department of Cell Biology, Medical School, University of Utrecht, The Netherlands. processing, the TGN is a major sorting compaaia~ient for proteins destined to more distal organelles of the secretory pathway, such as lysosomes, endosomes, secretory granules, and different domains of the plasma membrane (Griffiths and Simons, 1986) .