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			PubMed Journals: J Biol Chem

  Source:		PMID: 12171918


    		J Biol Chem. 2002 Oct 25;277(43):40844-52.
     		Epub 2002 Aug 8.

			Nonpalmitoylated human asialoglycoprotein
			receptors recycle constitutively but are
			defective in coated pit-mediated endocytosis,
			dissociation, and delivery of ligand to
			lysosomes.

			Yik JH(1), Saxena A, Weigel JA, Weigel PH.

			Author Information
			(1) Department of Biochemistry & Molecular
			Biology, The Oklahoma Center for Medical
			Glycobiology, University of Oklahoma Health
			Sciences Center, Oklahoma City, OK 73190,
			USA.

			The hepatic asialoglycoprotein receptor
			(ASGP-R) internalizes desialylated glycoproteins
			via the clathrin-coated pit pathway and
			mediates their delivery to lysosomes for
			degradation. The human ASGP-R contains two
			subunits, H1 and H2. Cytoplasmic residues
			Cys(36) in H1, as well as Cys(54) and Cys(58)
			in H2 are palmitoylated (Zeng, F.-Y., and
			Weigel, P. H. (1996) J. Biol. Chem. 271,
			32454). In order to study the function(s)
			of ASGP-R palmitoylation, we mutated these
			Cys residues to Ser and generated stably
			transfected SK-Hep-1 cell lines expressing
			either wild-type or nonpalmitoylated ASGP-Rs.
			Compared with wild-type ASGP-Rs,
			palmitoylation-defective ASGP-Rs showed
			normal ligand binding, intracellular distribution
			and trafficking patterns, and pH-induced
			dissociation profiles in vitro. However,
			continuous ASOR uptake, and the uptake of
			prebound cell surface ASOR were slower in
			cells expressing palmitoylation-defective
			ASGP-Rs than in cells expressing wild-type
			ASGP-Rs. Unlike native ASGP-Rs in hepatocytes
			or hepatoma cells, which mediate endocytosis
			via the clathrin-coated pit pathway and
			are almost completely inhibited by hypertonic
			medium, only approximately 40% of the ASOR
			uptake in SK-Hep-1 cells expressing wild-type
			ASGP-Rs was inhibited by hyperosmolarity.
			This result suggests the existence of an
			alternate nonclathrin-mediated internalization
			pathway, such as transcytosis, for the entry
			of ASGP-R.ASOR complexes into these cells.
			In contrast, ASOR uptake mediated by cells
			expressing palmitoylation-defective ASGP-Rs
			showed only a marginal difference under
			hypertonic conditions, indicating that most
			of the nonpalmitoylated ASGP-Rs were not
			internalized and processed normally through
			the clathrin-coated pit pathway. Furthermore,
			cells expressing wild-type ASGP-Rs were
			able to degrade the internalized ASOR, whereas
			ASOR dissociation was impaired and degradation
			was barely detectable in cells expressing
			nonpalmitoylated ASGP-Rs. We conclude that
			palmitoylation of the ASGP-R is required
			for its efficient endocytosis of ligand
			by the clathrin-dependent endocytic pathway
			and, in particular, for the proper dissociation
			and delivery of ligand to lysosomes.

			DOI: 10.1074/jbc.M204780200 PMID: 12171918
			[Indexed for MEDLINE]

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