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 nature of the alteration in H1 resulting in Golgi accumulation was analyzed by a series of mutations in the cytoplasmic domain. The results cannot be explained by the accidental generation of a retention motif or the deletion of a hypothetical export signal. However, the localization of H1 mutants correlates with the size of their cytoplasmic domains. Deletion of more than two-thirds of the 40 cytoplasmic residues caused Golgi retention. Extension of the truncated tail of Hl(A4-33A) by insertion of 10 unrelated residues of c-myc again resulted in surface transport. Similarly, the 65-residue cytoplasmic domain of the transferrin receptor fused to the transmembrane and exoplasmic portion of H1 allowed surface transport, whereas truncation of the fusion protein's tail to only 11 residues yielded efficient Golgi retention. In contrast, tailless transferrin receptors have been reported to be transported to the cell surface (Giron~s et al., 1991; Kundu and Nayak, 1994) . Cytoplasmic sequences have been shown by mutational analysis to be necessary for endocytosis, basolateral transport, and sorting to lysosomes (summarized by Sandoval and Bakke, 1994) . Mutant proteins lacking cytoplasmic sequences may be missorted to inappropriate post-Golgi compartments, but are generally not blocked in exit from the Golgi. Golgi retention of truncated H1 therefore appears to be due to a specific interaction of the trans-membrane and/or exoplasmic portions of H1 with Golgi resident components. This interaction is impeded by large cytoplasmic domains. Coexpression of Hl(A4-33A) with wild-type H2 resulted in the formation of functional ligand-binding hetero-oligomers detectable on the cell surface. However, since both subunits could also be detected by immunofluorescence microscopy in the typical Golgi pattern, association with H2 only weakened the interaction of Hl(A4-33A) with the trans-Golgi.