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

In the functional ASGP receptor, H1 is part of a heterooligomeric complex with subunit H2, which has a cytoplasmic domain of 58 amino acids. To test whether association with H2 influences the localization of Hl(A4-33A), a stable MDCK cell line expressing mutant H1 and wild-type H2 was created. Localization of Hl(A4-33A) and H2 was assayed separately by immunofluorescence using subunitspecific antisera. In nonpermeabilized cells, both subunits could be visualized on the surface (Fig. 10, A and C) . Furthermore, functional receptors could be detected on the cell surface by specific binding of 125I-iodinated asialoorosomucoid (not shown) which is indicative of correct hetero-oligomer formation. Thus, association with H2 rescued Hl(A4-33A) transport to the cell surface. In permeabilized cells, however, both subunits were also detected in typical Golgi structures (Fig. 10, B and D) , indicating that hetero-oligomers were partially retained in the Golgi.