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

Both fusion proteins (constructs AN2 and AN2A, illustrated in Fig. 9) were expressed on the cell surface of transiently transfected MDCK cells and did not accumulate in the Golgi (Fig. 9, A and B ). Conversely, we tested whether the exoplasmic domain of H1 is sufficient for Golgi localization if anchored in the membrane by an artificial transmembrane segment. In the construct H1ALeUl9 (Fig. 9) , the lumenal domain of H1 was fused to a generic hydrophobic sequence of 19 leucine residues preceded by the positively charged sequence MGPR, to warrant efficient membrane insertion. For a control, this short cytoplasmic tail was extended by the cytoplasmic sequence of wild-type H1 in the construct H1Leu19. Whereas H1Leu w was efficiently expressed on the cell surface of transfected MDCK cells, H1ALeuw accumulated in Golgi structures like Hl(A4-33A) (Fig. 9 , C and D). The particular sequence of the transmembrane segment of HI is thus not necessary for Golgi localization, whereas the exoplasmic portion of H1 is sufficient for Golgi retention in combination with a membrane anchor that lacks a sizable cytoplasmic domain.