title: Palmitoylation-defective asialoglycoprotein receptors are normal in their cellular distribution and ability to bind ligand, but are defective in ligand uptake and degradation.
abstract: The hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that mediates the endocytosis of desialylated glycoproteins. The human ASGP-R is composed of two homologous subunits, H1 and H2, and the cytoplasmic Cys residues in both subunits are palmitoylated. To study the effects of palmitoylation on ASGP-R activity and function, we generated four types of stably transfected cell lines in SK-Hep-1 hepatoma cells, expressing wild-type, or partially or completely palmitoylation-defective ASGP-Rs containing Cys-to-Ser mutations in either one or both subunits. Scatchard analysis showed that all four stable cell lines expressed a similar number of binding sites for asialo-orosomucoid, with comparable dissociation constants of approximately 1-3nM. Immunofluorescence confocal microscopy indicated a normal distribution of the palmitoylation-defective H1 and H2 subunits compared to the wild-type. However, cell lines expressing palmitoylation-defective ASGP-Rs had markedly reduced rates of ligand uptake and degradation compared to cells expressing wild-type ASGP-Rs. We conclude that failure to palmitoylate Cys residues in either or both subunits of human ASGP-Rs results in very inefficient uptake and degradation of ligands.
mesh_terms: D037263:Asialoglycoprotein Receptor / Q000235:genetics / Q000378:metabolism*; D001212:Asialoglycoproteins / Q000378:metabolism*; D001665:Binding Sites; D001692:Biological Transport; D002460:Cell Line; D006801:Humans; D007457:Iodine Radioisotopes; D007700:Kinetics; D008024:Ligands; D018613:Microscopy, Confocal; D016297:Mutagenesis, Site-Directed; D009994:Osmolar Concentration; D019308:Palmitic Acid / Q000378:metabolism*; D011865:Radioisotope Dilution Technique; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins; D007457:Iodine Radioisotopes; D008024:Ligands; D011994:Recombinant Proteins; D019308:Palmitic Acid