Phorbol ester-induced redistribution of the ASGP receptor is independent of receptor phosphorylation.
Like virtually all endocytic receptors, the human asialoglycoprotein (ASGP) receptor is phosphorylated by protein kinase C at serine residues within the cytoplasmic domains of its two subunits H1 and H2. Activation of protein kinase C by phorbol esters results in hyperphosphorylation and in a concomitant net redistribution of receptors to intracellular compartments (down-regulation) in HepG2 cells. To test whether there is a causal relationship between receptor hyperphosphorylation and redistribution, we examined the effect of phorbol ester treatment on the ASGP receptor composed of either wild-type subunits or of mutant subunits lacking any cytoplasmic serine residues in transfected NIH3T3 fibroblast and COS-7 cells. Although the wild-type subunits were hyperphosphorylated in fibroblast cells, the distribution of neither the wild-type nor the mutant receptors was affected. In contrast, phorbol ester treatment of transfected COS-7 cells induced down-regulation of both wild-type and mutant receptors. These findings indicate that redistribution of the receptor is independent of its cytoplasmic serines and is not caused by receptor phosphorylation.
Geffen
I
I
Department of Biochemistry, University of Basel, Switzerland.
Spiess
M
M
D016475:3T3 Cells; D000818:Animals; D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins / Q000378:metabolism*; D002461:Cell Line, Transformed; D002522:Chlorocebus aethiops; D015536:Down-Regulation; D006801:Humans; D051379:Mice; D009154:Mutation; D009961:Orosomucoid / Q000031:analogs & derivatives* / Q000378:metabolism; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology*; D010766:Phosphorylation; D011971:Receptors, Immunologic / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins; D009961:Orosomucoid; D011971:Receptors, Immunologic; C017030:asialoorosomucoid; D015240:Phorbol 12,13-Dibutyrate