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

The participation of a cytoplasmic sequence in localization of a protein to TGN-associated structures has also been suggested for the insulin-regulatable glucose transporter, GLUT-4 (Piper et al., 1992) . Immunoelectron microscopy studies have localized GLUT-4 and chimeric proteins having the GLUT-4 NH2-terminal cytoplasmic segment to tubulovesicular structures associated with the TGN (Slot et al., 1991; Piper et al., 1992) . The intracellular distribution of these proteins, however, seemed to be less restricted to the TGN than in our studies. Although the identity of residues critical for this localization of GLUT-4 has not been reported, it is noteworthy that the NH2-terminal sequence contains a potential aromatic amino acid-based tetrapeptide motif (FQQI; Piper et al., 1992) .