First published online September 12, 2003
Journal of Cell Science 116, e2004 (2003)
Copyright © 2003 The Company of Biologists Limited
Uroplakin - how the umbrella works
The mammalian urothelium is one of the most effective permeability barriers evident in nature, maintaining an incredibly steep concentration gradient between the urine and plasma. Plaques comprising the transmembrane protein uroplakin on the apical surface of urothelial umbrella cells constitute the permeability barrier, and these can also act as docking sites for bacteria that cause urinary tract infections. Xiang-Peng Kong and coworkers have examined the structural basis for barrier function by performing cryoelectron microscopy of uroplakin particles, which naturally form hexagonally packed, 2D crystals (see p. 4087). This approach has allowed them to generate an 
7-Å-resolution projection map and a 20-Å-resolution 3D image of the 16-nm uroplakin particle, which reveal numerous novel structural aspects. For example, they show that each particle has a central, 6 nm, lipid-filled hole surrounded by six inverted U-shaped subunits. Uroplakin could therefore severely restrict the movement of membrane lipids, which might contribute to its barrier function. The structures also reveal that there is little contact between neighbouring subunits and thus the potential for significant conformational change, which could be important for urothelial signal transduction following bladder stretching and bacterial binding.
Related articles in JCS:
- Structural basis of urothelial permeability barrier function as revealed by Cryo-EM studies of the 16 nm uroplakin particle
- Guangwei Min, Ge Zhou, Matthieu Schapira, Tung-Tien Sun, and Xiang-Peng Kong
JCS 2003 116: 4087-4094.
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