We have isolated two cDNA clones, NaPi-2 and NaPi-3, by screening rat kidney cortex and human kidney cortex cDNA libraries, respectively, for expression of sodiumdependent phosphate transport in Xenopus laevis oocytes.

A second distinct family of anion exchangers, SLC26, in addition to the classical SLC4 (or anion exchanger) family, has recently been delineated. Particular interest in this gene family is stimulated by the fact that the SLC26A2, SLC26A3, and SLC26A4 genes have been recognized as the disease genes mutated in diastrophic dysplasia, congenital chloride diarrhea, and Pendred syndrome, respectively. We report the expansion of the SLC26 gene family by characterizing three novel tissuespecific members, named SLC26A7, SLC26A8, and SLC26A9, on chromosomes 8, 6, and 1, respectively. The SLC26A7-A9 proteins are structurally very similar at the amino acid level to the previous family members and show tissue-specific expression in kidney, testis, and lung, respectively. More detailed characterization by immunohistochemistry and/or in situ hybridization localized SLC26A7 to distal segments of nephrons, SLC26A8 to developing spermatocytes, and SLC26A9 to the lumenal side of the bronchiolar and alveolar epithelium of lung. Expression of SLC26A7-A9 proteins in Xenopus oocytes demonstrated chloride, sulfate, and oxalate transport activity, suggesting that they encode functional anion exchangers. The functional characterization of the novel tissue-specific members may provide new insights to anion transport physiology in different parts of body.The systematic characterization of gene families using full genome sequences provides a rich source for expanding our physiological understanding of body functions. Recently, a second distinct family of anion exchangers, SLC26, has been delineated. The members of the SLC26 1 family are structurally well conserved across different species and can mediate the electroneutral exchange of Cl Ϫ for HCO 3 Ϫ across the plasma membrane of mammalian cells like members of the classical SLC4 (anion exchanger) family (1-3). Specific interest in the SLC26 gene family is stimulated by the fact that the first three human genes are associated with phenotypically distinct recessive diseases. The SLC26A2, SLC26A3, and SLC26A4 genes have been recognized as disease genes mutated in diastrophic dysplasia, congenital chloride diarrhea, and Pendred syndrome, respectively (4 -6). Thus, the three closely related but highly tissue-specific human anion transporters play central roles in the etiology of phenotypically very different recessive diseases.In human, six tissue-specific genes of the SLC26 family have been cloned so far, namely SLC26A1-A4 (previously known as SAT-1, DTDST, CLD or DRA, and PDS, respectively), SLC26A6, and TAT1. The SLC26A2-A4 members have been shown to transport, with different specificities, the chloride, iodide, bicarbonate, oxalate, and hydroxyl anions (7-12). SLC26A5 has been cloned from gerbil and rat and shown to act as a motor protein of cochlear outer hair cell; it is sensitive to intracellular anions but has not been found to act as a transporter (13,14). The SLC26A6 protein is expressed at highest levels in the kidney and the pancreas and suggested SLC26A6 as a candidate for a yet u...

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

In renal collecting ducts, a vasopressin-induced cAMP increase results in the phosphorylation of aquaporin-2 (AQP2) water channels at Ser-256 and its redistribution from intracellular vesicles to the apical membrane. Hormones that activate protein kinase C (PKC) proteins counteract this process. To determine the role of the putative kinase sites in the trafficking and hormonal regulation of human AQP2, three putative casein kinase II (Ser-148, Ser-229, Thr-244), one PKC (Ser-231), and one protein kinase A (Ser-256) site were altered to mimic a constitutively non-phosphorylated/phosphorylated state and were expressed in Madin-Darby canine kidney cells. Except for Ser-256 mutants, seven correctly folded AQP2 kinase mutants trafficked as wild-type AQP2 to the apical membrane via forskolin-sensitive intracellular vesicles. With or without forskolin, AQP2-Ser-256A was localized in intracellular vesicles, whereas AQP2-S256D was localized in the apical membrane. Phorbol 12-myristate 13-acetate-induced PKC activation following forskolin treatment resulted in vesicular distribution of all AQP2 kinase mutants, while all were still phosphorylated at Ser-256. Our data indicate that in collecting duct cells, AQP2 trafficking to vasopressinsensitive vesicles is phosphorylation-independent, that phosphorylation of Ser-256 is necessary and sufficient for expression of AQP2 in the apical membrane, and that PMA-induced PKC-mediated endocytosis of AQP2 is independent of the AQP2 phosphorylation state.In humans, the kidney is the prime organ for regulation of body fluid osmolarity, which is maintained within strict boundaries. To fine-tune this balance, principal cells of the renal collecting duct reabsorb water from pro-urine, which is under control of the anti-diuretic hormone arginine vasopressin (AVP).1 Upon hypovolemia or hypernatremia, pituitary-derived AVP binds its V2 receptor in the basolateral membrane of these cells and initiates an intracellular cAMP signaling cascade that causes a transient increase in cytosolic calcium (1) and the activation of protein kinase A (PKA), which in turn phosphorylates homotetrameric aquaporin-2 (AQP2) water channels and possibly other proteins. Consequently, AQP2-containing vesicles fuse with the apical membrane, rendering the principal cells water-permeable (2, 3). Driven by an osmotic gradient, water will then enter these cells via AQP2 and will exit the cells via AQP3 and AQP4, located in the basolateral membrane, a process in which urine is concentrated. By using antibodies that recognize Ser-256-phosphorylated AQP2 (p-AQP2), Nishimoto et al. (4) were able to show that in vivo AVP-induced redistribution of AQP2 from vesicles to the apical membrane coincides with phosphorylation of Ser-256. By using similar antibodies, Christensen et al. (5) demonstrated that p-AQP2 is, besides the apical membrane, also present in intracellular vesicles of principal cells and that the intracellular distribution of AQP2 is regulated via V2 receptors by altering the phosphorylation state of Ser-256 in AQ...

We have isolated a cDNA clone by screening a rabbit kidney cortex cDNA library for expression of sodiumindependent transport of L-arginine and L-alanine in Xenopus laevis oocytes. Expressed uptake relates to a single component of sodium-independent transport for dibasic and neutral amino acids. This transport activity resembles the functionally defined system b0'+ and carries cystine and dibasic amino acids with high affmity. The rBAT (bO'+ amino acid transporterrelated) mRNA is found mainly in kidney and intestinal mucosa. It encodes a predicted 77.8-kDa protein with only one putative transmembrane domain and seven potential N-glycosylation sites. This protein could either be a constitutive element or a specific activator of system b°'+.