trans-Golgi retention of a plasma membrane protein: mutations in the cytoplasmic domain of the asialoglycoprotein receptor subunit H1 result in trans-Golgi retention

The situation of wild-type and truncated forms of H1 with respect to intracellular localization is reminiscent of that of 13-1,4-galactosyltransferase (Shur, 1993) . Two mRNAs are generated by alternative promoters which encode two forms of the enzyme (Lopez et al., 1991) . A short form with 11 cytoplasmic residues is localized specifically in the trans-Golgi. A long form with 13 additional amino-terminal residues is less efficiently retained and also found on the cell surface, where it acts as a cell adhesion molecule. Modulation of Golgi retention by the cytoplasmic domain may thus be a general mechanism for fine tuning the intracellular distribution of membrane proteins. Whether in the case of galactosyltransferase this is a specific effect of the additional sequence (e.g., by interaction with the cytoskeleton) or whether, as in the case of H1, simply the additional size of the cytoplasmic domain is responsible for reduced retention remains to be tested.