title: Integrity of all four transmembrane domains of the tetraspanin uroplakin Ib is required for its exit from the ER.
abstract: The surface of the mammalian urinary bladder is covered by a crystalline, asymmetric unit membrane (AUM) structure that contains the four major uroplakins (UPs): Ia, Ib, II and IIIa. UPIa and UPIb belong to the family of tetraspanins. Although UPIa and UPIb are structurally conserved, only UPIb could exit from the endoplasmic reticulum (ER) and reach the cell surface when expressed alone in 293T cells. Modifications of the large extracellular loop of UPIb, such as mutation of the N-glycosylation site or the cysteines involved in the formation of three disulfide bridges, or exchanging the large luminal loop of UPIb with that of UPIa did not affect the ability of UPIb to reach the cell surface. However, modifications of any of the four transmembrane domains of UPIb led to ER retention, suggesting that the proper formation of helical bundles consisting of the tetraspanin transmembrane domains is a prerequisite for UPIb to exit from the ER. Results of sedimentation analysis suggested that aggregate formation is a mechanism for ER retention.
authors.affiliation: Department of Cell Biology, The Ronald O Perelman, NYU Cancer Institute, New York University School of Medicine, New York, NY 10016, USA.
authors.forename: Liyu
authors.initials: L
authors.lastname: Tu
authors.forename: Xiang-Peng
authors.initials: XP
authors.lastname: Kong
authors.forename: Tung-Tien
authors.initials: TT
authors.lastname: Sun
authors.forename: Gert
authors.initials: G
authors.lastname: Kreibich
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D015153:Blotting, Western; D019556:COS Cells; D002460:Cell Line; D002522:Chlorocebus aethiops; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D008969:Molecular Sequence Data; D021381:Protein Transport / Q000502:physiology; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D008164:Luminescent Proteins; D008562:Membrane Glycoproteins; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins