Yu J, Lin J H, Wu X R, Sun T T
Ronald O. Perelman Department of Dermatology, New York University Medical School, New York 10016.
J Cell Biol. 1994 Apr;125(1):171-82. doi: 10.1083/jcb.125.1.171.
The mammalian bladder epithelium elaborates, as a terminal differentiation product, a specialized plasma membrane called asymmetric unit membrane (AUM) which is believed to play a role in strengthening and stabilizing the urothelial apical surface through its interactions with an underlying cytoskeleton. Previous studies indicate that the outer leaflet of AUM is composed of crystalline patches of 12-nm protein particles, and that bovine AUMs contain three major proteins: the 27- to 28-kD uroplakin I, the 15-kD uroplakin II and the 47-kD uroplakin III. As a step towards elucidating the AUM structure and function, we have cloned the cDNAs of bovine uroplakin I (UPI). Our results established the existence of two isoforms of bovine uroplakin I: a 27-kD uroplakin Ia and a 28-kD uroplakin Ib. These two glycoproteins are closely related with 39% identity in their amino acid sequences. Hydropathy plot revealed that both have four potential transmembrane domains (TMDs) with connecting loops of similar length. Proteolytic digestion of UPIa inserted in vitro into microsomal vesicles suggested that its two main hydrophilic loops are exposed to the luminal space, possibly involved in interacting with the luminal domains of other uroplakins to form the 12-nm protein particles. The larger loop connecting TMD3 and TMD4 of both UPIa and UPIb contains six highly conserved cysteine residues; at least one centrally located cysteine doublet in UPIa is involved in forming intramolecular disulfide bridges. The sequences of UPIa and UPIb (the latter is almost identical to a hypothetical, TGF beta-inducible, TI-1 protein of mink lung epithelial cells) are homologous to members of a recently described family all possessing four transmembrane domains (the "4TM family"); members of this family include many important leukocyte differentiation markers such as CD9, CD37, CD53, and CD63. The tissue-specific and differentiation-dependent expression as well as the naturally occurring crystalline state of uroplakin I molecules make them uniquely suitable, as prototype members of the 4TM family, for studying the structure and function of these integral membrane proteins.
哺乳动物膀胱上皮细胞可产生一种特殊的质膜，作为终末分化产物，称为不对称单位膜（AUM），据信它通过与下层细胞骨架相互作用，在强化和稳定尿路上皮细胞顶端表面方面发挥作用。先前的研究表明，AUM的外小叶由12纳米蛋白质颗粒的晶体斑块组成，牛的AUM含有三种主要蛋白质：27至28千道尔顿的uroplakin I、15千道尔顿的uroplakin II和47千道尔顿的uroplakin III。作为阐明AUM结构和功能的第一步，我们克隆了牛uroplakin I（UPI）的cDNA。我们的结果证实了牛uroplakin I存在两种异构体：27千道尔顿的uroplakin Ia和28千道尔顿的uroplakin Ib。这两种糖蛋白在氨基酸序列上有39%的同一性，关系密切。亲水性图谱显示，两者都有四个潜在的跨膜结构域（TMD），其连接环长度相似。对体外插入微粒体囊泡中的UPIa进行蛋白水解消化表明，其两个主要的亲水性环暴露于腔隙空间，可能参与与其他uroplakin的腔隙结构域相互作用，以形成12纳米的蛋白质颗粒。连接UPIa和UPIb的TMD3和TMD4的较大环含有六个高度保守的半胱氨酸残基；UPIa中至少一个位于中央的半胱氨酸双联体参与形成分子内二硫键。UPIa和UPIb的序列（后者几乎与水貂肺上皮细胞中一种假设的、TGFβ诱导的TI-1蛋白相同）与最近描述的一个家族的成员同源，该家族所有成员都拥有四个跨膜结构域（“4TM家族”）；这个家族的成员包括许多重要的白细胞分化标志物，如CD9、CD37、CD53和CD63。uroplakin I分子的组织特异性和分化依赖性表达以及天然存在的晶体状态，使其作为4TM家族的原型成员，特别适合用于研究这些整合膜蛋白的结构和功能。
