胰管细胞HCO3-分泌:囊性纤维化跨膜电导调节体和SLC26转运体

胰管细胞以至少6倍浓度差逆向分泌HCO3^-（人体浓度约140mmol／L）。HCO3^-跨顶膜转运的可能机制包括SLC26阴离子转运体的Cl-HCO3^-交换和囊性纤维化跨膜电导调节体（cystic fibrosis transmembrane conductance regulator,cFrR）对HCO3^-的传导扩散。SLC26家族成员介导上皮顶膜Cl^--HCO3^-交换,胰管中检测到SLC26A6和SLC26A3。共表达研究揭示,鼠类slc26a6和slc26a3通过slc26的STAS结构域与CFTR的R结构域相互作用,导致活性互相增强。研究显示这些交换体是产电的：slc26a6介导1Cl^--2HCO3^-交换,slc26a3介导2Cl^--1HCO3^-交换。近期slc26a6^-／-小鼠离体胰管研究显示,slc26a6介导大部分Cl^-依赖的HCO3^-跨顶膜分泌,与slc26a6的产电性一致。然而,因为人体能分泌非常高浓度的HCO3^-,SLC26A6在胰管HCO3^-分泌中的作用并不十分清楚。SLC26A6的作用只能在与人类似能分泌约140mmol／LHCO3^-的物种,如豚鼠中研究。现有的豚鼠研究数据显示,像slc26a6介导的1Cl^--2HCO3^-交换不可能完成这种高浓度差的HCO3^-分泌。另一方面,CFTR的HCO3^-电导性可以在理论上支持HCO3^-逆向分泌。所以,在豚鼠和人胰腺HCO3^-的分泌中,CFTR可能比SLC26A6发挥更大作用。     

Gating of CFTR by the STAS domain of SLC26 transporters

Chloride absorption and bicarbonate secretion are vital functions of epithelia, as highlighted by cystic fibrosis and diseases associated with mutations in members of the SLC26 chloride-bicarbonate exchangers. Many SLC26 transporters (SLC26T) are expressed in the luminal membrane together with CFTR, which activates electrogenic chloride-bicarbonate exchange by SLC26T. However, the ability of SLC26T to regulate CFTR and the molecular mechanism of their interaction are not known. We report here a reciprocal regulatory interaction between the SLC26T DRA, SLC26A6 and CFTR. DRA markedly activates CFTR by increasing its overall open probablity (NP(o)) sixfold. Activation of CFTR by DRA was facilitated by their PDZ ligands and binding of the SLC26T STAS domain to the CFTR R domain. Binding of the STAS and R domains is regulated by PKA-mediated phosphorylation of the R domain. Notably, CFTR and SLC26T co-localize in the luminal membrane and recombinant STAS domain activates CFTR in native duct cells. These findings provide a new understanding of epithelial chloride and bicarbonate transport and may have important implications for both cystic fibrosis and diseases associated with SLC26T.     

The role of the STAS domain in the function and biogenesis of a sulfate transporter as probed by random mutagenesis

Sulfate transporters in plants represent a family of proteins containing transmembrane domains that constitute the catalytic part of the protein and a short linking region that joins this catalytic moiety with a C-terminal STAS domain. The STAS domain resembles an anti-sigma factor antagonist of Bacillus subtilis, which is one distinguishing feature of the SLC26 transporter family; this family includes transporters for sulfate and other anions such as iodide and carbonate. Recent work has demonstrated that this domain is critical for the activity of Arabidopsis thaliana sulfate transporters, and specific lesions in this domain, or the exchange of STAS domains between different sulfate transporters, can severely impair transport activity. In this work we generated a Saccharomyces cerevisiae expression library of the A. thaliana Sultr1;2 gene with random mutations in the linking region-STAS domain and identified STAS domain lesions that altered Sultr1;2 biogenesis and/or function. A number of mutations in the beta-sheet that forms the core of the STAS domain prevented intracellular accumulation of Sultr1;2. In contrast, the linking region and one surface of the STAS domain containing N termini of the first and second alpha-helices have a number of amino acids critical for the function of the protein; mutations in these regions still allow protein accumulation in the plasma membrane, but the protein is no longer capable of efficiently transporting sulfate into cells. These results suggest that the STAS domain is critical for both the activity and biosynthesis/stability of the transporter, and that STAS sub-domains correlate with these specific functions.     

Structure of the Cytosolic Portion of the Motor Protein Prestin and Functional Role of the STAS Domain in SLC26/SulP Anion Transporters

Prestin is the motor protein responsible for the somatic electromotility of cochlear outer hair cells and is essential for normal hearing sensitivity and frequency selectivity of mammals. Prestin is a member of mammalian solute-linked carrier 26 (SLC26) anion exchangers, a family of membrane proteins capable of transporting a wide variety of monovalent and divalent anions. SLC26 transporters play important roles in normal human physiology in different tissues, and many of them are involved in genetic diseases. SLC26 and related SulP transporters carry a hydrophobic membrane core and a C-terminal cytosolic portion that is essential in plasma membrane targeting and protein function. This C-terminal portion is mainly composed of a STAS (sulfate transporters and anti-sigma factor antagonist) domain, whose name is due to a remote but significant sequence similarity with bacterial ASA (anti-sigma factor antagonist) proteins. Here we present the crystal structure at 1.57 Å resolution of the cytosolic portion of prestin, the first structure of a SulP transporter STAS domain, and its characterization in solution by heteronuclear multidimensional NMR spectroscopy. Prestin STAS significantly deviates from the related bacterial ASA proteins, especially in the N-terminal region, which—although previously considered merely as a generic linker between the domain and the last transmembrane helix—is indeed fully part of the domain. Hence, unexpectedly, our data reveal that the STAS domain starts immediately after the last transmembrane segment and lies beneath the lipid bilayer. A structure-function analysis suggests that this model can be a general template for most SLC26 and SulP anion transporters and supports the notion that STAS domains are involved in functionally important intramolecular and intermolecular interactions. Mapping of disease-associated or functionally harmful mutations on STAS structure indicates that they can be divided into two categories: those causing significant misfolding of the domain and those altering its interaction properties.     

SLC26A7 is a Cl- channel regulated by intracellular pH

Members of the SLC26 transporter family play an essential role in several epithelial functions, as revealed by diseases associated with mutations in members of the family. Several members were shown to function as Cl(-) and HCO(3)(-) transporters that likely play an important role in epithelial Cl(-) absorption and HCO(3)(-) secretion. However, the mechanism of most transporters is not well understood. SLC26A7 is a member of the SLC26 transporter family reported to be expressed in the basolateral membrane of the cortical collecting duct and parietal cells and functions as a coupled Cl(-)/HCO(3)(-) exchanger. In the present work we examined the transport properties of SLC26A7 to determine its transport characteristics and electrogenicity. We found that when expressed in Xenopus oocytes or HEK293 cells SLC26A7 functions as a pH(i)-regulated Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability. Expression of SLC26A7 in oocytes or HEK293 cells generated a Cl(-) current with linear I/V and an instantaneous current that was voltage- and time-independent. Based on measurement of reversal potential the selectivity of SLC26A7 is NO(3)(-)>Cl(-)=Br(-)=I(-)>SO(4)(2-)=Glu(-), although I(-) partially inhibited the current. Incubating the cells with HCO(3)(-) or butyrate acidified the cytosol and increased the selectivity of SLC26A7 for Cl(-). Measurement of membrane potential and pH(i) showed minimal OH(-) and HCO(3)(-) transport by SLC26A7 when the cells were incubated in Cl(-)-containing or Cl(-)-free media. The activity of SLC26A7 was inhibited by all inhibitors of anion transporters tested, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, diphenylamine-2-carboxylic acid, and glybenclamide. These findings reveal that SLC26A7 functions as a unique Cl(-) channel that is regulated by intracellular H(+).     

Structural and functional analysis of the C-terminal STAS (sulfate transporter and anti-sigma antagonist) domain of the Arabidopsis thaliana sulfate transporter SULTR1.2

The C-terminal region of sulfate transporters from plants and animals belonging to the SLC26 family members shares a weak but significant similarity with the Bacillus sp. anti-anti-sigma protein SpoIIAA, thus defining the STAS domain (sulfate transporter and anti-sigma antagonist). The present study is a structure/function analysis of the STAS domain of SULTR1.2, an Arabidopsis thaliana sulfate transporter. A three-dimensional model of the SULTR1.2 STAS domain was built which indicated that it shares the SpoIIAA folds. Moreover, the phosphorylation site, which is necessary for SpoIIAA activity, is conserved in the SULTR1.2 STAS domain. The model was used to direct mutagenesis studies using a yeast mutant defective for sulfate transport. Truncation of the whole SULTR1.2 STAS domain resulted in the loss of sulfate transport function. Analyses of small deletions and mutations showed that the C-terminal tail of the SULTR1.2 STAS domain and particularly two cysteine residues plays an important role in sulfate transport by SULTR1.2. All the substitutions made at the putative phosphorylation site Thr-587 led to a complete loss of the sulfate transport function of SULTR1.2. The reduction or suppression of sulfate transport of the SULTR1.2 mutants in yeast was not due to an incorrect targeting to the plasma membrane. Both our three-dimensional modeling and mutational analyses strengthen the hypothesis that the SULTR1.2 STAS domain is involved in protein-protein interactions that could control sulfate transport.     

Structure of a SLC26 anion transporter STAS domain in complex with acyl carrier protein: implications for E. coli YchM in fatty acid metabolism

Escherichia coli YchM is a member of the SLC26 (SulP) family of anion transporters with an N-terminal membrane domain and a C-terminal cytoplasmic STAS domain. Mutations in human members of the SLC26 family, including their STAS domain, are linked to a number of inherited diseases. Herein, we describe the high-resolution crystal structure of the STAS domain from E. coli YchM isolated in complex with acyl-carrier protein (ACP), an essential component of the fatty acid biosynthesis (FAB) pathway. A genome-wide genetic interaction screen showed that a ychM null mutation is synthetically lethal with mutant alleles of genes (fabBDHGAI) involved in FAB. Endogenous YchM also copurified with proteins involved in fatty acid metabolism. Furthermore, a deletion strain lacking ychM showed altered cellular bicarbonate incorporation in the presence of NaCl and impaired growth at alkaline pH. Thus, identification of the STAS-ACP complex suggests that YchM sequesters ACP to the bacterial membrane linking bicarbonate transport with fatty acid metabolism.     

Solution structure of the guanine nucleotide-binding STAS domain of SLC26-related SulP protein Rv1739c from Mycobacterium tuberculosis

The structure and intrinsic activities of conserved STAS domains of the ubiquitous SulP/SLC26 anion transporter superfamily have until recently remained unknown. Here we report the heteronuclear, multidimensional NMR spectroscopy solution structure of the STAS domain from the SulP/SLC26 putative anion transporter Rv1739c of Mycobacterium tuberculosis. The 0.87-Å root mean square deviation structure revealed a four-stranded β-sheet with five interspersed α-helices, resembling the anti-σ factor antagonist fold. Rv1739c STAS was shown to be a guanine nucleotide-binding protein, as revealed by nucleotide-dependent quench of intrinsic STAS fluorescence and photoaffinity labeling. NMR chemical shift perturbation analysis partnered with in silico docking calculations identified solvent-exposed STAS residues involved in nucleotide binding. Rv1739c STAS was not an in vitro substrate of mycobacterial kinases or anti-σ factors. These results demonstrate that Rv1739c STAS binds guanine nucleotides at physiological concentrations and undergoes a ligand-induced conformational change but, unlike anti-σ factor antagonists, may not mediate signals via phosphorylation.     

Low resolution structure of a bacterial SLC26 transporter reveals dimeric stoichiometry and mobile intracellular domains

The SLC26/SulP (solute carrier/sulfate transporter) proteins are a superfamily of anion transporters conserved from bacteria to man, of which four have been identified in human diseases. Proteins within the SLC26/SulP family exhibit a wide variety of functions, transporting anions from halides to carboxylic acids. The proteins comprise a transmembrane domain containing between 10-12 transmembrane helices followed a by C-terminal cytoplasmic sulfate transporter and anti-sigma factor antagonist (STAS) domain. These proteins are expected to undergo conformational changes during the transport cycle; however, structural information for this family remains sparse, particularly for the full-length proteins. To address this issue, we conducted an expression and detergent screen on bacterial Slc26 proteins. The screen identified a Yersinia enterocolitica Slc26A protein as the ideal candidate for further structural studies as it can be purified to homogeneity. Partial proteolysis, co-purification, and analytical size exclusion chromatography demonstrate that the protein purifies as stable oligomers. Using small angle neutron scattering combined with contrast variation, we have determined the first low resolution structure of a bacterial Slc26 protein without spectral contribution from the detergent. The structure confirms that the protein forms a dimer stabilized via its transmembrane core; the cytoplasmic STAS domain projects away from the transmembrane domain and is not involved in dimerization. Supported by additional biochemical data, the structure suggests that large movements of the STAS domain underlie the conformational changes that occur during transport.     

SLC26A9 is expressed in gastric surface epithelial cells, mediates Cl-/HCO3- exchange, and is inhibited by NH4+

HCO3- secretion by gastric mucous cells is essential for protection against acidic injury and peptic ulcer. Herein we report the identification of an apical HCO3- transporter in gastric surface epithelial cells. Northern hybridization and RT-PCR demonstrate the expression of this transporter, also known as SLC26A9, in mouse and rat stomach and trachea (but not kidney). In situ hybridization in mouse stomach showed abundant expression of SLC26A9 in surface epithelial cells with apical localization on immunofluorescence labeling. Functional studies in HEK-293 cells demonstrated that SLC26A9 mediates Cl-/HCO3- exchange and is also capable of Cl--independent HCO3- extrusion. Unlike other anion exchangers or transport proteins reported to date, SLC26A9 activity is inhibited by ammonium (NH4+). The inhibitory effect of NH4+ on gastric HCO3- secretion was also indicated by reduced gastric juxtamucosal pH (pHjm) in rat stomach in vivo. This report is the first to describe the inhibition of HCO3- transport in vitro and the reduction of pHjm in stomach in vivo by NH4+. Given its critical localization on the apical membrane of surface epithelial cells, its ability to transport HCO3-, and its inhibition by NH4+, we propose that SLC26A9 mediates HCO3- secretion in surface epithelial cells and is essential for protection against acidic injury in the stomach. Disease states that are associated with increased ammonia (NH3)/NH4+ generation (e.g., Helicobacter pylori) may impair gastric HCO3- secretion and therefore predispose patients to peptic ulcer by inhibiting SLC26A9.     

Characterization of the L683P mutation of SLC26A9 in Xenopus oocytes

In the present study, we characterized a STAS-domain amino acid mutation of SLC26A9 having a significant impact on ion transport. We focused on the sole conserved L- leucine residue of the STAS domain identified among SLC26 members. We therefore characterized the L683P mutation of SLC26A9 in Xenopus oocytes by monitoring the protein functional expression (two-electrode technique for voltage-clamp analysis) and its presence at the cell membrane (surface protein biotinylation technique). This mutation was found to reduce Cl(-) transport through SLC26A9 as well as the positive interaction exerted by SLC26A9 on CFTR ion transport activity. The origin of this effect is discussed in the light of the presence of the SLC26A9-L683P mutant at the plasma membrane.     

Metabolon disruption: a mechanism that regulates bicarbonate transport

Carbonic anhydrases (CA) catalyze the reversible conversion of CO2 to HCO3-. Some bicarbonate transporters bind CA, forming a complex called a transport metabolon, to maximize the coupled catalytic/transport flux. SLC26A6, a plasma membrane Cl-/HCO3- exchanger with a suggested role in pancreatic HCO3- secretion, was found to bind the cytoplasmic enzyme CAII. Mutation of the identified CAII binding (CAB) site greatly reduced SLC26A6 activity, demonstrating the importance of the interaction. Regulation of SLC26A6 bicarbonate transport by protein kinase C (PKC) was investigated. Angiotensin II (AngII), which activates PKC, decreased Cl-/HCO3- exchange in cells coexpressing SLC26A6 and AT1a-AngII receptor. Activation of PKC reduced SLC26A6/CAII association in immunoprecipitates. Similarly, PKC activation displaced CAII from the plasma membrane, as monitored by immunofluorescence. Finally, mutation of a PKC site adjacent to the SLC26A6 CAB site rendered the transporter unresponsive to PKC. PKC therefore reduces CAII/SLC26A6 interaction, reducing bicarbonate transport rate. Taken together, our data support a mechanism for acute regulation of membrane transport: metabolon disruption.     

Mapping of five new putative anion transporter genes in human and characterization of SLC26A6, a candidate gene for pancreatic anion exchanger

A second distinct family of anion transporters, in addition to the classical SLC4 (or AE) family, has recently been delineated. Members of the SLC26 family are structurally well conserved and can mediate the electroneutral exchange of Cl(-) for HCO(-)(3) across the plasma membrane of mammalian cells like members of the SLC4 family. Three human transporter proteins have been functionally characterized: SLC26A2 (DTDST), SLC26A3 (CLD or DRA), and SLC26A4 (PDS) can transport with different specificities the chloride, iodine, bicarbonate, oxalate, and hydroxyl anions, whereas SLC26A5 (prestin) was suggested to act as the motor protein of the cochlear outer hair cell. We report the expansion of the SLC26 family with five new members in chromosomes 3, 6, 8, 12, and 17 and mapping of SLC26A1 to 4p16.3. We have characterized one of them, SLC26A6, in more detail. It maps to chromosome 3p21.3, encodes a predicted 738-amino-acid transmembrane protein, and is most abundantly expressed in the kidney and pancreas. Pancreatic ductal cell lines Capan-1 and Capan-2 express SLC26A6, and immunohistochemistry localizes SLC26A6 protein to the apical surface of pancreatic ductal cells, suggesting it as a candidate for a luminal anion exchanger. The functional characterization of the novel members of this tissue-specific gene family may provide new insights into anion transport physiology in different parts of the body.     

The cyanobacterial bicarbonate transporter BicA: its physiological role and the implications of structural similarities with human SLC26 transporters

The cyanobacterial Na+-dependent HCO3- transporter BicA is a member of the ubiquitous and important SulP/SLC26 family of anion transporters found in eukaryotes and prokaryotes. BicA is an important component of the cyanobacterial CO2 concentrating mechanism, an adaptation that contributes to cyanobacteria being able to achieve an estimated 25% of global primary productivity, largely in the oceans. The human SLC26 members are involved in a range of key cellular functions involving a diverse range of anion transport activities including Cl-/HCO3-, I-/HCO3-, and SO42-/HCO3- exchange; mutations in SLC26 members are known to be associated with debilitating diseases such as Pendred syndrome, chondrodysplasias, and congenital chloride diarrhoea. We have recently experimentally determined the membrane topology of BicA using the phoA-lacZ reporter system and here consider some of the extrapolated implications for topology of the human SLC26 family and the Sultr plant sulphate transporters.     

Characterization of the L683P mutation of SLC26A9 in Xenopus oocytes

In the present study, we characterized a STAS-domain amino acid mutation of SLC26A9 having a significant impact on ion transport. We focused on the sole conserved L- leucine residue of the STAS domain identified among SLC26 members. We therefore characterized the L683P mutation of SLC26A9 in Xenopus oocytes by monitoring the protein functional expression (two-electrode technique for voltage-clamp analysis) and its presence at the cell membrane (surface protein biotinylation technique). This mutation was found to reduce Cl⁻ transport through SLC26A9 as well as the positive interaction exerted by SLC26A9 on CFTR ion transport activity. The origin of this effect is discussed in the light of the presence of the SLC26A9–L683P mutant at the plasma membrane.     

Genotype-dependency of butyrate efficacy in children with congenital chloride diarrhea

Background

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder characterized by life-long, severe diarrhea with intestinal Cl^- malabsorption. It results from a reduced activity of the down regulated in adenoma exchanger (DRA), due to mutations in the solute carrier family 26, member 3 (SLC26A3) gene. Currently available therapies are not able to limit the severity of diarrhea in CLD. Conflicting results have been reported on the therapeutic efficacy of oral butyrate.

Methods

We investigated the effect of oral butyrate (100 mg/kg/day) in seven CLD children with different SLC26A3 genotypes. Nasal epithelial cells were obtained to assess the effect of butyrate on the expression of the two main Cl^- transporters: DRA and putative anion transporter-1 (PAT-1).

Results

A variable clinical response to butyrate was observed regarding the stool pattern and fecal ion loss. The best response was observed in subjects with missense and deletion mutations. Variable response to butyrate was also observed on SLC26A3 (DRA) and SLC26A6 (PAT1) gene expression in nasal epithelial cells of CLD patients.

Conclusions

We demonstrate a genotype-dependency for butyrate therapeutic efficacy in CLD. The effect of butyrate is related in part on a different modulation of the expression of the two main apical membrane Cl^- exchangers of epithelial cells, members of the SLC26 anion family.

Trial registration

Australian New Zealand Clinical trial Registry ACTRN12613000450718.

     

Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants: differences in anion selectivity, regulation, and electrogenicity

The unusually low 78% amino acid identity between the orthologous human SLC26A6 and mouse slc26a6 polypeptides prompted systematic comparison of their anion transport functions in Xenopus oocytes. Multiple human SLC26A6 variant polypeptides were also functionally compared. Transport was studied as unidirectional fluxes of (36)Cl(-), [(14)C]oxalate, and [(35)S]sulfate; as net fluxes of HCO(3)(-) by fluorescence ratio measurement of intracellular pH; as current by two-electrode voltage clamp; and as net Cl(-) flux by fluorescence intensity measurement of relative changes in extracellular and intracellular [Cl(-)]. Four human SLC26A6 polypeptide variants each exhibited rates of bidirectional [(14)C]oxalate flux, Cl(-)/HCO(3)(-) exchange, and Cl(-)/OH(-) exchange nearly equivalent to those of mouse slc26a6. Cl(-)/HCO(3)(-) exchange by both orthologs was cAMP-sensitive, further enhanced by coexpressed wild type cystic fibrosis transmembrane regulator but inhibited by cystic fibrosis transmembrane regulator DeltaF508. However, the very low rates of (36)Cl(-) and [(35)S]sulfate transport by all active human SLC26A6 isoforms contrasted with the high rates of the mouse ortholog. Human and mouse orthologs also differed in patterns of acute regulation. Studies of human-mouse chimeras revealed cosegregation of the high (36)Cl(-) transport phenotype with the transmembrane domain of mouse slc26a6. Mouse slc26a6 and human SLC26A6 each mediated electroneutral Cl(-)/HCO(3)(-) and Cl(-)/OH(-) exchange. In contrast, whereas Cl(-)/oxalate exchange by mouse slc26a6 was electrogenic, that mediated by human SLC26A6 appeared electroneutral. The increased currents observed in oocytes expressing either mouse or human ortholog were pharmacologically distinct from the accompanying monovalent anion exchange activities. The human SLC26A6 polypeptide variants SLC26A6c and SLC26A6d were inactive as transporters of oxalate, sulfate, and chloride. Thus, the orthologous mouse and human SLC26A6 proteins differ in anion selectivity, transport mechanism, and acute regulation, but both mediate electroneutral Cl(-)/HCO(3)(-) exchange.     

Pendrin function and regulation in Xenopus oocytes

SLC26A4/PDS mutations cause Pendred Syndrome and non-syndromic deafness. but some aspects of function and regulation of the SLC26A4 polypeptide gene product, pendrin, remain controversial or incompletely understood. We have therefore extended the functional analysis of wildtype and mutant pendrin in Xenopus oocytes, with studies of isotopic flux, electrophysiology, and protein localization. Pendrin mediated electroneutral, pH-insensitive, DIDS-insensitive anion exchange, with extracellular K((1/2)) (in mM) of 1.9 (Cl(-)), 1.8 (I(-)), and 0.9 (Br(-)). The unusual phenotype of Pendred Syndrome mutation E303Q (loss-of-function with normal surface expression) prompted systematic mutagenesis at position 303. Only mutant E303K exhibited loss-of-function unrescued by forced overexpression. Mutant E303C was insensitive to charge modification by methanethiosulfonates. The corresponding mutants SLC26A2 E336Q, SLC26A3 E293Q, and SLC26A6 E298Q exhibited similar loss-of-function phenotypes, with wildtype surface expression also documented for SLC26A2 E336Q. The strong inhibition of wildtype SLC26A2, SLC26A3, and SLC26A6 by phorbol ester contrasts with its modest inhibition of pendrin. Phorbol ester inhibition of SLC26A2, SLC26A3, and SLC26A6 was blocked by coexpressed kinase-dead PKCδ but was without effect on pendrin. Mutation of SLC26A2 serine residues conserved in PKCδ -sensitive SLC26 proteins but absent from pendrin failed to reduce PKCδ sensitivity of SLC26A2 (190).