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Lipid Polarity And Sorting In Epithelial Cells
Published 2005 ·
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The human asialogiycoprotein (ASGP) receptor is a hetero-oligomeric complex composed of two highly homologous subunits H1 and H2 which are both required for ligand binding and internalization. We have previously shown that the major subunit H1 alone, although not capable of binding ligand, undergoes constitutive endocytosis and recycling and that its efficient clustering into coated pits depends on a tyrosine residue in its cytoplasmic domain 9 In contrast, we show now that H2, when expressed alone in fibroblasts, is endocytosed only slowly and that mutation of a cytoplasmic Phe residue at the homologous position of the critical Tyr in H1 9 does not influence internalization. Furthermore we analyzed the relative importance of the H1 and H2 tails within the hetero-oligomeric complex by csexpressing Tyrmutated H1 with wild-type H2, wild-type H1 with Phe-rnutated H2 or both mutants together 9 Internalization and degradation of ligand proceeded at normal rates when H2-Phe was mutated, but was reduced to the same low level either when only H1Tyr or both were mutated. Thus, H2 cannot compensate for mutant H1 in the oligomeric ASGP receptor and appears to lack an active internalization signal in its cytoplasmic domain 9 4