Differential role of mannose and glucose trimming in the ER degradation of asialoglycoprotein receptor subunits.
To gain insight into how sugar chain processing events modulate endoplasmic reticulum (ER)/proteasomal degradation we looked at human asialoglycoprotein receptor polypeptides H2a and H2b, variants which differ only by an extra pentapeptide (EGHRG) present in H2a. Membrane-bound H2a is a precursor of a soluble secreted form while H2b reaches the plasma membrane. Uncleaved precursor H2a molecules are completely retained in the ER and degraded as well as a portion of H2b. Inhibition of N-linked sugar chain mannose trimming stabilized both variants. In contrast, inhibition of glucose trimming with castanospermine greatly enhanced the degradation rate of H2a but not that of H2b. We studied a possible involvement of the ER chaperone calnexin, as inhibitors of glucose trimming are known to prevent calnexin binding. Incubation of cells with low concentrations of castanospermine (30 microg/ml) did not interfere with calnexin binding to H2a while causing the same accelerated degradation as high concentrations (>100 microg/ml) which did inhibit the association. Castanospermine treatment after calnexin binding blocked the dissociation of the chaperone but still caused accelerated degradation. The increased degradation could be blocked by a specific proteasome inhibitor, ZL(3)VS. Our results suggest that extensive mannose trimming or retention of glucose residues due to lack of glucose trimming are signals for ER/proteasomal degradation independent of interaction with calnexin.
Ayalon-Soffer
M
M
Department of Cell Research and Immunology, George Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel, 69978.
Shenkman
M
M
Lederkremer
G Z
GZ
D016475:3T3 Cells / Q000737:chemistry / Q000201:enzymology; D000818:Animals; D000906:Antibodies; D037263:Asialoglycoprotein Receptor; D002135:Calcium-Binding Proteins / Q000378:metabolism; D037281:Calnexin; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D007212:Indolizines / Q000494:pharmacology; D008358:Mannose / Q000378:metabolism*; D051379:Mice; D018832:Molecular Chaperones / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D054836:Indolizidines
D000906:Antibodies; D037263:Asialoglycoprotein Receptor; D002135:Calcium-Binding Proteins; D037281:Calnexin; D003546:Cysteine Endopeptidases; D004791:Enzyme Inhibitors; D005947:Glucose; D007212:Indolizines; D008358:Mannose; D018832:Molecular Chaperones; D009097:Multienzyme Complexes; D046988:Proteasome Endopeptidase Complex; D011956:Receptors, Cell Surface; C037806:castanospermine; D054836:Indolizidines