title: Calnexin and other factors that alter translocation affect the rapid binding of ubiquitin to apoB in the Sec61 complex.
abstract: Several secretory proteins, including apolipoprotein B, have been shown to undergo degradation by proteasomes. We found that the rapid degradation of nascent apolipoprotein B in HepG2 cells was diminished but not abolished by the addition of any of three different inhibitors of proteasomes. Ubiquitin is conjugated to apolipoprotein B that is not assembled with sufficient lipids either during or soon after synthesis. In addition, we found that apolipoprotein B that has entered the endoplasmic reticulum sufficiently to become glycosylated can be degraded by proteasomes. Furthermore, we detected ubiquitin-apolipoprotein B that is associated with the Sec61 complex, the major constituent of the translocational channel. Treatment of cells with monomethylethanolamine or dithiothreitol decreased the translocation of apolipoprotein B and increased the proportion of ubiquitin-conjugated molecules associated with Sec61. Conversely, treatment of cells with oleic acid, which increased the proportion of translocated apolipoprotein B, decreased the amount of ubiquitin-apolipoprotein B in the Sec61 complex. Finally, we found that inhibition of the interaction between calnexin and apolipoprotein B decreases the translocation of apolipoprotein B, increases the ubiquitin-apolipoprotein B in the Sec61 complex, and increases the proteasomal degradation of glycosylated apolipoprotein B. Thus, ubiquitin can be attached to unassembled apolipoprotein B in the Sec61 complex, and this process is affected by factors including calnexin that alter the translocation of apolipoprotein B.
authors.affiliation: Molecular Medicine Unit, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Chen
authors.forename: F
authors.initials: F
authors.lastname: Le Cahérec
authors.forename: S L
authors.initials: SL
authors.lastname: Chuck
mesh_terms: D001055:Apolipoproteins B / Q000378:metabolism* / Q000648:ultrastructure; D001692:Biological Transport; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D037281:Calnexin; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004229:Dithiothreitol / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D007212:Indolizines / Q000494:pharmacology; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D017510:Protein Folding; D000069816:SEC Translocation Channels; D014407:Tumor Cells, Cultured; D014452:Ubiquitins / Q000378:metabolism*; D054836:Indolizidines
chemical_list: D001055:Apolipoproteins B; D002135:Calcium-Binding Proteins; D037281:Calnexin; D003546:Cysteine Endopeptidases; D004229:Dithiothreitol; D007212:Indolizines; D046911:Macromolecular Substances; D008565:Membrane Proteins; D009097:Multienzyme Complexes; D046988:Proteasome Endopeptidase Complex; D000069816:SEC Translocation Channels; D014452:Ubiquitins; C037806:castanospermine; D054836:Indolizidines