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

In contrast to the growing amount of information available on structural determinants for retention in the Golgi stacks, very little is known about signals for localization to the TGN. Two isoforms of a rat type I integral membrane protein, known as TGN38 and TGN41, have recently been shown to be specifically localized to a membrane-bound compartment with the characteristics of the TGN (Luzio et al., 1990; Reaves et al., 1992) . Mature TGN38 has an apparent molecular weight of 85,000-95,000, of which 38,000 are contributed by the core polypeptide and the rest by N-and O-linked oligosaccharide chains. TGN38 is organized into a 286amino acid NH2-terminal lumenal domain, a 21-amino acid membrane-spanning sequence and a 33-amino acid COOHterminal cytoplasmic tail (Luzio et al., 1990) . TGN41 is identical to TGN38, except that the last three amino acids of the TGN38 cytoplasmic tail are replaced by an additional 23-amino acid sequence (Reaves et al., 1992) . Since both TGN38 and TGN41 have been localized to the TGN by morphological studies (Luzio et al., 1990; Reaves et al., 1992) , the COOH-terminal polypeptide extension in TGN41 is not likely to play any role in TGN localization. Deletion of the cytoplasmic tail of TGN38 results in increased expression of the truncated protein at the cell surface, suggesting that this topologic domain contains information necessary for intracellular localization of the protein (Luzio et al., 1990) . The exact nature of the structural information contained within the TGN38 cytoplasmic tail and the possible contribution of other topologic domains of the protein to TGN localization remain to be established.