The two subunits of the asialoglycoprotein receptor contain different sorting information.
The human asialoglycoprotein receptor, an endocytic transport receptor of the basolateral surface of hepatocytes, is a hetero-oligomer of two homologous subunits H1 and H2. The cytoplasmic domain of H1 has been shown previously to contain a tyrosine-based signal for endocytosis and basolateral sorting. Here, we have investigated sorting determinants within subunit H2 and their contribution to the targeting of the hetero-oligomeric receptor complex. Despite extensive sequence homology, H2 expressed separately in fibroblast cells was endocytosed poorly, and mutation of phenylalanine 5 (corresponding to the critical tyrosine in H1) did not further reduce internalization. Consistent with this observation, ligand uptake by receptors composed of H1 lacking tyrosine 5 and H2 was inefficient. With respect to polarized transport in Madin-Darby canine kidney cells, H2 could not be analyzed separately, because in the absence of H1 subunit H2 was completely degraded intracellularly. Coexpression of both subunits yielded ligand-binding receptors located specifically on the basolateral surface. The mutant H1(5A) (tyrosine 5 replaced by alanine) is approximately 55% apical in the absence of H2. In cells expressing H1(5A) together with H2, however, subunit H2 directed receptor complexes exclusively to the basolateral domain. Phenylalanine 5 is not essential for basolateral transport. Thus, whereas the endocytosis signal of the hetero-oligomeric asialoglycoprotein receptor resides exclusively in subunit H1, polarized transport to the basolateral domain of Madin-Darby canine kidney cells may involve two signals, only one of which is active for endocytosis.
Fuhrer
C
C
Department of Biochemistry, University of Basel, Switzerland.
Geffen
I
I
Huggel
K
K
Spiess
M
M
D000595:Amino Acid Sequence; D000818:Animals; D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins / Q000378:metabolism*; D001692:Biological Transport; D002460:Cell Line; D004285:Dogs; D006801:Humans; D007668:Kidney; D007700:Kinetics; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009838:Oligodeoxyribonucleotides; D010649:Phenylalanine; D017354:Point Mutation; D011956:Receptors, Cell Surface / Q000096:biosynthesis / Q000378:metabolism*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000378:metabolism; D014162:Transfection
D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins; D046911:Macromolecular Substances; D009838:Oligodeoxyribonucleotides; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; D010649:Phenylalanine