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First published online December 11, 2006
doi: 10.1242/10.1242/jcs.03285

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Integrity of all four transmembrane domains of the tetraspanin uroplakin Ib is required for its exit from the ER

¹ Department of Cell Biology The Ronald O. Perelman Department of Dermatology and Departments of Pharmacology and Urology, NYU Cancer Institute, New York University School of Medicine, New York, NY 10016, USA
² Department of Biochemistry The Ronald O. Perelman Department of Dermatology and Departments of Pharmacology and Urology, NYU Cancer Institute, New York University School of Medicine, New York, NY 10016, USA
³ Department of Epithelial Biology Unit, The Ronald O. Perelman Department of Dermatology and Departments of Pharmacology and Urology, NYU Cancer Institute, New York University School of Medicine, New York, NY 10016, USA

^* Author for correspondence (e-mail: kreibg01{at}med.nyu.edu)

Accepted 28 September 2006

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.

Key words: Uroplakins, Tetraspanins, Endoplasmic reticulum, Membrane proteins

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