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

The differentiated structure and function of the various Golgi regions is likely the result of the ordered localization of specific proteins to successive compartments of the system. Maintenance of such an organized arrangement of resident proteins requires mechanisms that retain certain proteins at defined locations while allowing other proteins to flow through the Golgi complex. Additional mechanisms must also exist to sort itinerant proteins along the different routes emanating from the Golgi complex. Both protein residence and sorting in the Golgi complex are thought to be mediated by specific structural signals. Recent studies have investigated the nature of retention signals of proteins that are specifically localized to the Golgi stack, including the avian coronavirus E1 glycoprotein and several glycosyltransferases (reviewed by Machamer, 1991) . For all of these proteins, information necessary for Golgi localization was contained within the membrane-spanning domains. No obvious homology was noticed among the different transmembrane sequences involved in Golgi localization, suggesting a retention process that relies on a physical property of the membrane-spanning domains rather than on receptor-mediated recognition of a specific sequence motif. This lack of homology and the inability to saturate the retention mechanism by overexpression of these proteins have suggested that localization to the Golgi stacks may be caused by formation of transport-incompetent assemblies involving transmembrane domain interactions (Machamer, 1991; Swift and Machamer, 1991; Munro, 1991; Nilsson et al., 1991) .