Uroplakins Ia and Ib, two major differentiation products of bladder epithelium, belong to a family of four transmembrane domain (4TM) proteins.

Abstract

Abstract. 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 H and the 47-kD uroplakin Ill. 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/3-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.T HE asymmetric unit membrane (AUM) ~ is a unique plasma membrane elaborated as a major differentiation product of the mammalian urothelium which lines the surfaces of renal pelvis, ureter, bladder, and urethra (Porter and Bonneville, 1963;Porter et al., 1967;Hicks, 1975). It forms numerous plaques covering the apical surface of the superficial cells of the multi-layered urothelium. The name AUM reflects the fact that its outer leaflet is almost twice as Drs. Yu and Lin contributed equally to this research.

De Novo Uroplakin IIIa Heterozygous Mutations Cause Human Renal Adysplasia Leading to Severe Kidney Failure

“…Urothelial cells undergo endo/exocytosis to modulate urothelial surface area in response to changes in bladder pressure (36), and binding of the Escherichia coli FimH receptor to UPIa mediates urothelial invasion of this bacterium (37); unlike UPIa, UPIb, or UPII, UPIIIa has a significant cytoplasmic tail, and it has been suggested that this region interacts with cytoplasmic proteins to mediate membrane/cytoplasmic interactions (25). We showed in COS-1 cells that wild-type UPIIIa protein can escape the endoplasmic reticulum and reach the cell surface only in the presence of UPIb, as was shown previously in 293T cells (23).…”

“…All have large luminal/extracellular domains, but only UPIIIa and UPIIIb have significant cytoplasmic portions in their C-termini (22). UP heterodimers (UPIb/UPIII and UPIa/UPII) are major components of plaques, also known as the "asymmetric unit membrane," covering almost the entire urothelial apical cell surface (25). Indeed, these specific heterodimers must form as a prerequisite for their transport to the apical membrane of urothelial cells, and in the absence of the appropriate partner, UPIa/ UPII/UPIIIa cannot escape the endoplasmic reticulum, whereas UPIb becomes mistargeted to the basolateral membrane (23,26).…”

Assembly of Urothelial Plaques: Tetraspanin Function in Membrane Protein Trafficking

“…(Figure 1B, sequence 1), which was almost identical to that of the prosequence of UPII (sequence 2; Lin et al, 1994). A similar sequence (sequence 5 or X) was obtained by subtracting the N-terminal sequence of UPIb (sequence 4; Yu et al, 1994) from the mixed N-terminal sequences of the 27-32-kDa "uroplakin I" protein band in AUM (sequence 3). These results raised the possibility that the AE31 antigen was the pro-UPII, instead of being related to uroplakin I as we previously thought .…”

“…(B) N-terminal sequence of the bovine AE31 antigen. Listed are the N-terminal sequences of (1) the 32-kDa AE31 antigen immunoaffinity-purified from the Triton X-100 solubilized total urothelial plaque proteins; (2) the cDNA-deduced prosequence of pro-UPII ; (3) the SDS-PAGE-purified, 27-32-kDa proteins of the bovine AUM (double-asterisked region in lane 1 of A); (4) the cDNA-deduced UPIb sequence ; and (5) a hypothetical protein (X)-deduced by subtracting the UPIb sequence shown in (4) from the mixed sequence shown in (3) (described as "UPIc" in Yu et al, 1994). Note that the N-terminal sequence of the AE31 antigen was almost identical to that of pro-UPII and X.…”

High Expression of Human Uroplakin Ia in Urinary Bladder Transitional Cell Carcinoma

“…3,7,8,[14][15][16] However, only a small number of antibodies binding to UP Ia have been reported, and all were generated against bovine UP Ia. 3,14,15) Therefore, to our knowledge, this is the first report of the establishment of an anti-human UP Ia polyclonal antibody. We chose amino acid residues 141 to 152 (DQGQELTRL-WDR) as the immunogen because this sequence is located in the extracellular loop domain and has high specificity for UP Ia.…”