Chemical crosslinking studies with the mouse Kcc1 K-Cl cotransporter.

Oligomerization, function, and regulation of unmodified mouse Kcc1 K-Cl cotransporter were studied by chemical crosslinking. Treatment of Xenopus oocytes and 293T cells expressing K-Cl cotransporter Kcc1 with several types of chemical cross-linkers shifted Kcc1 polypeptide to higher molecular weight forms. More extensive studies were performed with the amine-reactive disuccinyl suberate (DSS) and with the sulfhydryl-reactive bis-maleimidohexane (BMH). Kcc1 cross-linking was time-dependent in intact oocytes, and was independent of protein concentration in detergent lysates from oocytes or 293T cells. Kcc1 cross-linking by the cleavable cross-linker DTME was reversible. The N-terminal and C-terminal cytoplasmic tails of Kcc1 were not essential for Kcc1 crosslinking. PFO-PAGE and gel filtration revealed oligomeric states of uncrosslinked KCC1 corresponding in mobility to that of cross-linked protein. DSS and BMH each inhibited KCC1-mediated (86)Rb(+) uptake stimulated by hypotonicity or by N-ethylmaleimide (NEM) without reduction in nominal surface abundance of KCC1. These data add to evidence supporting the oligomeric state of KCC polypeptides.

Increased sulfate uptake by E. coli overexpressing the SLC26-related SulP protein Rv1739c from Mycobacterium tuberculosis.

Growth and virulence of mycobacteria requires sulfur uptake. The Mycobacterium tuberculosis genome contains, in addition to the ABC sulfate permease cysTWA, three SLC26-related SulP genes of unknown function. We report that induction of Rv1739c expression in E. coli increased bacterial uptake of sulfate, but not Cl(-), formate, or oxalate. Uptake was time-dependent, maximal at pH 6.0, and exhibited a K(1/2) for sulfate of 4.0 muM. Na(+)-independent sulfate uptake was not reduced by bicarbonate, nitrate, or phosphate, but was inhibited by sulfite, selenate, thiosulfate, N-ethylmaleimide and carbonyl cyanide 3-chloro-phenylhydrazone. Sulfate uptake was also increased by overexpression of the Rv1739c transmembrane domain, but not of the cytoplasmic C-terminal STAS domain. Mutation to serine of the three cysteine residues of Rv1739c did not affect magnitude, pH-dependence, or pharmacology of sulfate uptake. Expression of Rv1739c in a M. bovis BCG strain lacking the ABC sulfate permease subunit CysA could not complement sulfate auxotrophy. Moreover, inducible expression of Rv1739c in an E. coli strain lacking CysA did not increase sulfate uptake by intact cells. Our data show that facilitation of bacterial sulfate uptake by Rv1739c requires CysA and its associated sulfate permease activity, and suggest that Rv1739c may be a CysTWA-dependent sulfate transporter.

Anion exchangers in flux: functional differences between human and mouse SLC26A6 polypeptides.

The SLC26 anion transporter polypeptides exhibit considerably greater sequence diversity among near-species orthologues than is found among the SLC4 bicarbonate transporters, and among SLC26 transporters is most marked among SLC26A6 orthologues. This observation prompted systematic functional comparison in Xenopus oocytes of mouse Slc26a6 with several human SLC26A6 polypeptide variants. Mouse and human polypeptides exhibited similar rates of bidirectional [14C]oxalate flux, Cl-/HCO3- exchange, and Cl-/OH- exchange, and similar cAMP-stimulation and enhancement of that stimulation by wild-type but not delta F508 CFTR. However, high rates of 36Cl- and 35S-sulfate transport by mouse Slc26a6 contrasted with low transport rates of the human proteins. The high 36Cl- transport phenotype cosegregated with the transmembrane domain of mouse Slc26a6 in chimera studies. Mouse Slc26a6 and human SLC26A6 each mediated electroneutral Cl-/HCO3- and Cl-/OH- exchange. But, whereas Cl-/oxalate exchange by mouse Slc26a6 was electrogenic, that mediated by human SLC26A6 appeared electroneutral. Oocyte expression of either mouse or human orthologue elicited currents that were pharmacologically distinct from the monovalent anion exchange activities measured in the same lots of oocytes. The human SLC26A6 polypeptide variants SLC26A6c and SLC26A6d were inactive in isotopic flux assays. Understanding of SLC26 transport mechanisms and pathophysiology will benefit from recognition of substantial differences in transport properties among orthologues.

NMR assignment and secondary structure of the STAS domain of Rv1739c, a putative sulfate transporter of Mycobacterium tuberculosis.

We report (1)H(N), (15)N, and (13)C resonance assignments for the 15.6 kDa STAS domain of the putative sulfate transporter of Mycobacterium tuberculosis, Rv1739c, using heteronuclear, multidimensional NMR spectroscopy. Rv1739c is a SulP anion permease, related in structure to the SLC26 gene family of metazoan anion exchangers and anion channels.

AE2 isoforms in rat kidney: immunohistochemical localization and regulation in response to chronic NH4Cl loading.

Three splice variants of anion exchanger (AE)2 (AE2a, b, and c) have been described in the rat, but their relative distribution in rat kidney is not known. The purpose of this study was to describe the segmental and cellular distribution of the AE2 isoforms in the rat kidney and to evaluate whether the expression levels of these AE2 isoforms are regulated independently in response to chronic NH(4)Cl loading. Two polyclonal antibodies were generated, respectively, recognizing a NH(2)-terminal peptide unique to AE2a and an amino acid sequence common to AE2a and AE2b. Antibody specificities were tested using cells transfected separately with the AE2a, AE2b, and AE2c isoforms. Immunohistochemistry on sections of paraffin-embedded rat kidneys showed a distribution of AE2a/AE2b labeling in the kidney similar to the distribution of AE2 in the rat kidney reported previously. AE2 is highly expressed in the medullary thick ascending limb, cortical thick ascending limb (cTAL), and macula densa. The pattern of AE2a-specific labeling differed from the pattern of AE2a/AE2b labeling in that relatively more of the total immunolabel was observed in the terminal inner medullary collecting duct. NH(4)Cl loading (0.033 mmol NH(4)Cl/g body wt for 7 days) did not change the labeling of AE2 isoforms in the medulla, whereas the labeling in the cortex was intensified and included more distal parts of the cTAL. Immunoblotting confirmed upregulation of AE2a/b expression in the cortex. These results indicate that AE2a and AE2b are differentially expressed and regulated in the rat kidney. The regulation following NH(4)Cl loading of AE2b in the cTAL suggests a role for AE2 in transepithelial bicarbonate reabsorption in this segment.