Tu Liyu, Sun Tung-Tien, Kreibich Gert
Department of Cell Biology, New York University, School of Medicine, New York, New York 10016, USA.
Mol Biol Cell. 2002 Dec;13(12):4221-30. doi: 10.1091/mbc.e02-04-0211.
Much of the lower urinary tract, including the bladder, is lined by a stratified urothelium forming a highly differentiated, superficial umbrella cell layer. The apical plasma membrane as well as abundant cytoplasmic fusiform vesicles of the umbrella cells is covered by two-dimensional crystals that are formed by four membrane proteins named uroplakins (UPs) Ia, Ib, II, and III. UPs are synthesized on membrane-bound polysomes, and after several co- and posttranslational modifications they assemble into planar crystals in a post-Golgi vesicular compartment. Distension of the bladder may cause fusiform vesicles to fuse with the apical plasma membrane. We have investigated the early stages of uroplakin assembly by expressing the four uroplakins in 293T cells. Transfection experiments showed that, when expressed individually, only UPIb can exit from the endoplasmic reticulum (ER) and move to the plasma membrane, whereas UPII and UPIII reach the plasma membrane only when they form heterodimeric complexes with UPIa and UPIb, respectively. Heterodimer formation in the ER was confirmed by pulse-chase experiment followed by coimmunoprecipitation. Our results indicate that the initial building blocks for the assembly of crystalline uroplakin plaques are heterodimeric uroplakin complexes that form in the ER.
包括膀胱在内的大部分下尿路由复层尿路上皮衬里，形成高度分化的表层伞细胞层。伞细胞的顶端质膜以及丰富的细胞质梭形囊泡被由四种名为uroplakins（UPs）Ia、Ib、II和III的膜蛋白形成的二维晶体覆盖。UPs在膜结合多核糖体上合成，经过几次共翻译和翻译后修饰后，它们在高尔基体后囊泡区室组装成平面晶体。膀胱扩张可能导致梭形囊泡与顶端质膜融合。我们通过在293T细胞中表达四种uroplakins来研究uroplakin组装的早期阶段。转染实验表明，当单独表达时，只有UPIb能够从内质网（ER）中出来并移动到质膜，而UPII和UPIII只有在分别与UPIa和UPIb形成异二聚体复合物时才能到达质膜。通过脉冲追踪实验和共免疫沉淀证实了内质网中异二聚体的形成。我们的结果表明，结晶uroplakin斑块组装的初始构建块是在内质网中形成的异二聚体uroplakin复合物。
