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

There is, however, a correlation between the size of the cytoplasmic domain and the fate of the mutant proteins (see Fig. 1 ). Constructs with cytoplasmic domains of 17 or more residues were all transported to the plasma membrane; those with domains of 13 or fewer amino acids were retained in the Golgi. To test this correlation, the cytoplas-mic sequence of Hl(A4-33A) was extended by insertion of an unrelated sequence of 10 residues, the human c-myc epitope, in place of the initial deletion. The cytoplasmic domain of the resulting mutant Hl(A4-33m) thus consists of 21 amino acids. Upon expression in transfected MDCK cells, the protein did not accumulate intracellularly but was detected on the cell surface (Fig. 8 D) . In addition, the cytoplasmic domain of H1 was replaced with that of the transferrin receptor. The resulting fusion protein H1T, with a cytoplasmic tail of 65 residues, was efficiently transported to the plasma membrane (Fig. 8 E) . However, upon truncation of this domain to only 11 residues, the fusion protein H1TA was completely retained in the typical Golgi structures (Fig. 8 F) . These results confirm that the localization of H1 does not depend on the sequence of the cytoplasmic domain, but rather on its size.