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

Two previously prepared mutants, HI(A4-11) (lacking the signal for basolateral sorting from the TGN) and HI(h12-33), were both transported to the cell surface in stable expressing MDCK cell lines (Geffen et al., 1993) . This is in contrast to Hl(A4-33A) with the combined deletion. Mutation of tyrosine-5 of HI(A12-33) to alanine in the construct Hl(A12-33/5A) also resulted in transport to the cell surface, arguing against a role of the tyrosine signal in surface transport. In a series of amino-terminal deletions, HI(A2-19) and HI(A2-24) were expressed on the cell surface, whereas HI(A2-28), HI(A2-33), and HI(A2-37) accumulated intracellularly in the typical tubular structures. HI(A2-24) and HI(A2-28) differ only by the four consecutive prolines 25-28. Separate deletion of the proline-rich segment 25-30 in HI(APro), however, did not significantly affect the wildtype distribution. Furthermore, deletion of the juxtamembranous segment of residues 26-40 did not prevent surface transport. These results show that no single sequence element in the cytoplasmic domain can be made responsible either for retention in the Golgi or for surface transport.