Sun T T, Zhao H, Provet J, Aebi U, Wu X R
Department of Dermatology, Kaplan Cancer Center, New York University Medical School, NY 10016, USA.
Mol Biol Rep. 1996;23(1):3-11. doi: 10.1007/BF00357068.
Mammalian urothelium undergoes unique membrane specialization during terminal differentiation making numerous rigid-looking membrane plaques (0.3-0.5 micron 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.
哺乳动物的尿路上皮在终末分化过程中经历独特的膜特化,形成许多外观坚硬的膜斑(直径0.3 - 0.5微米),覆盖在顶端细胞表面。这些膜斑的外小叶几乎是内小叶厚度的两倍,因此被称为不对称单位膜(AUM)。超微结构研究表明,AUM的外小叶由形成二维晶体的16纳米颗粒组成,且每个颗粒形成“扭曲带”结构。我们最近发现,高度纯化的牛AUM含有四种主要的整合膜蛋白:uroplakins Ia(27 kD)、Ib(28 kD)、II(15 kD)和III(47 kD)。对uroplakins不同亚结构域的蛋白酶敏感性研究及其他考虑因素表明,UPIa和UPIb有4个跨膜结构域,而UPII和UPIII只有1个跨膜结构域。化学交联研究表明,氨基酸序列有39%相同的UPIa和UPIb在拓扑结构上分别与UPII和UPIII相邻,从而增加了存在两种生物化学性质不同的AUM颗粒的可能性,即含有UPIa/UPII的颗粒与含有UPIb/UPIII的颗粒。在3T3饲养细胞存在的情况下生长的牛膀胱上皮细胞进行克隆生长,形成能够合成和加工所有已知uroplakins的分层菌落。转基因小鼠研究表明,小鼠uroplakin II基因的3.6 kb 5'侧翼序列可驱动细菌LacZ基因在尿路上皮中表达。对uroplakins生物合成、组装和靶向的进一步研究将为更好地理解AUM的结构和功能以及哺乳动物尿路上皮的生物学特性提供独特的机会。