Formation of asymmetric unit membrane during urothelial differentiation

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Abstract
Mammalian urothelium undergoes unique membrane specialization during terminal differentiation making numerous rigid-looking membrane plaques (0.3–0.5 μm diameter) that cover the apical cell surface. The outer leaflet of these membrane plaques is almost twice as thick as the inner leaflet hence the name asymmetric unit membrane (AUM). Ultrastructural studies established that the outer leaflet of AUM is composed of 16 nm particles forming two dimensional crystals, and that each particle forms a ‘twisted ribbon’ structure. We showed recently that highly purified bovine AUMs contain four major integral membrane proteins: uroplakins Ia (27 kD), Ib (28 kD), II (15 kD) and III (47 kD). Studies of the protease sensitivity of the different subdomains of uroplakins and other considerations suggest that UPIa and UPIb have 4 transmembrane domains, while UPII and UPIII have only one transmembrane domain. Chemical Crosslinking studies showed that UPIa and UPIb, which share 39% amino acid sequence, are topologically adjacent to UPII and UPIII, respectively, thus raising the possibility that there exist two biochemically distinct AUM particles, i.e., those containing UPIa/UPII vs. UPIb/UPIII. Bovine urothelial cells grown in the presence of 3T3 feeder cells undergo clonal growth forming stratified colonies capable of synthesizing and processing all known uroplakins. Transgenic mouse studies showed that a 3.6 kb 5′-flanking sequence of mouse uroplakin II gene can drive the expression of bacterial LacZ gene to express in the urothelium. Further studies on the biosynthesis, assembly and targeting of uroplakins will offer unique opportunities for better understanding the structure and function of AUM as well as the biology of mammalian urothelium.