title: Cell surface expression of calnexin, a molecular chaperone in the endoplasmic reticulum.
abstract: The folding and assembly of nascent proteins in the endoplasmic reticulum are assisted by the molecular chaperone calnexin, which is itself retained within the endoplasmic reticulum. It was up to now assumed that calnexin was selectively expressed on the surface of immature thymocytes because of a particular characteristic of the protein sorting machinery in these cells. We now report that a small fraction of calnexin is normally expressed on the surface of various cells such as mastocytoma cells, murine splenocytes, fibroblast cells, and human HeLa cells. Surface biotinylation followed by chase culture of living cells revealed that calnexin is continuously delivered to the cell surface and then internalized for lysosomal degradation. These results suggest that there is continuous exocytosis and endocytosis of calnexin, and the amount of calnexin on the plasma membrane results from the balance of the rates of these two events. To study the structural requirement of calnexin for surface expression, we created deletion mutants of calnexin and found that the luminal domain, particularly the glycoprotein binding domain, is necessary. These findings suggest that the surface expression of calnexin depends on the association with glycoproteins and that calnexin may play a certain role as a chaperone on the plasma membrane as well.
authors.affiliation: Department of Molecular Genetics, Chiba University Graduate School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Okazaki
authors.forename: H
authors.initials: H
authors.lastname: Ohno
authors.forename: K
authors.initials: K
authors.lastname: Takase
authors.forename: T
authors.initials: T
authors.lastname: Ochiai
authors.forename: T
authors.initials: T
authors.lastname: Saito
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D019921:Biotinylation; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D037281:Calnexin; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry* / Q000378:metabolism; D004705:Endocytosis; D004721:Endoplasmic Reticulum / Q000737:chemistry* / Q000378:metabolism; D005434:Flow Cytometry; D008562:Membrane Glycoproteins / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018832:Molecular Chaperones / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011993:Recombinant Fusion Proteins
chemical_list: D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D018832:Molecular Chaperones; D021382:Protein Sorting Signals; D011993:Recombinant Fusion Proteins; D037281:Calnexin