Molecular cloning and functional characterization of KCC3, a new K-Cl cotransporter.

We isolated and characterized a novel K-Cl cotransporter, KCC3, from human placenta. The deduced protein contains 1,150 amino acids. KCC3 shares 75-76% identity at the amino acid level with human, pig, rat, and rabbit KCC1 and 67% identity with rat KCC2. KCC3 is 40 and 33% identical to two Caenorhabditis elegans K-Cl cotransporters and approximately 20% identical to other members of the cation-chloride cotransporter family (CCC), two Na-K-Cl cotransporters (NKCC1, NKCC2), and the Na-Cl cotransporter (NCC). Hydropathy analysis indicates a typical KCC topology with 12 transmembrane domains, a large extracellular loop between transmembrane domains 5 and 6 (unique to KCCs), and large NH(2) and COOH termini. KCC3 is predominantly expressed in kidney, heart, and brain, and is also expressed in skeletal muscle, placenta, lung, liver, and pancreas. KCC3 was localized to chromosome 15. KCC3 transiently expressed in human embryonic kidney (HEK)-293 cells fulfilled three criteria for increased expression of K-Cl cotransport: stimulation of cotransport by swelling, treatment with N-ethylmaleimide, or treatment with staurosporine.

Cloning, characterization, and gene organization of K-Cl cotransporter from pig and human kidney and C. elegans.

We isolated and characterized the cDNAs for the human, pig, and Caenorhabditis elegans K-Cl cotransporters. The pig and human homologs are 94% identical and contain 1,085 and 1,086 amino acids, respectively. The deduced protein of the C. elegans K-Cl cotransporter clone (CE-KCC1) contains 1,003 amino acids. The mammalian K-Cl cotransporters share approximately 45% similarity with CE-KCC1. Hydropathy analyses of the three clones indicate typical KCC topology patterns with 12 transmembrane segments, large extracellular loops between transmembrane domains 5 and 6 (unique to KCC), and large COOH-terminal domains. Human KCC1 is widely expressed among various tissues. This KCC1 gene spans 23 kb and is organized in 24 exons, whereas the CE-KCC1 gene spans 3.5 kb and contains 10 exons. Transiently and stably transfected human embryonic kidney cells (HEK-293) expressing the human, pig, and C. elegans K-Cl cotransporter fulfilled two (pig) or five (human and C. elegans) criteria for increased expression of the K-Cl cotransporter. The criteria employed were basal K-Cl cotransport; stimulation of cotransport by swelling, N-ethylmaleimide, staurosporine, and reduced cell Mg concentration; and secondary stimulation of Na-K-Cl cotransport.

Swelling activation of K-Cl cotransport in LK sheep erythrocytes: a three-state process.

K-Cl cotransport in LK sheep erythrocytes is activated by osmotic swelling and inhibited by shrinkage. The mechanism by which changes in cell volume are transduced into changes in transport was investigated by measuring time courses of changes in transport after osmotic challenges in cells with normal and reduced Mg concentrations. When cells of normal volume and normal Mg are swollen, there is a delay of 10 min or more before the final steady-state flux is achieved, as there is for swelling activation of K-Cl cotransport in erythrocytes of other species. The delay was shown to be independent of the extent of swelling. There was also a delay after shrinkage inactivation of cotransport. Reducing cellular Mg concentration activates cotransport. Swelling of low-Mg cells activates cotransport further, but with no measurable delay. In contrast, there is a delay in shrinkage inactivation of cotransport in low-Mg cells. The results are interpreted in terms of a three-state model: [formula see text] in which A state, B state, and C state transporters have relatively slow, intermediate, and fast transport rates, respectively. Most transporters in shrunken cells with normal Mg are in the A state. Swelling converts transporters to the B state in the rate-limiting process, followed by rapid conversion to the C state. Reducing cell Mg also promotes the A-->B conversion. Swelling of low-Mg cells activates transport rapidly because of the initial predominance of B state transporters. The results support the following conclusions about the rate constants of the three-state model: k21 is the rate constant for a Mg-promoted process that is inhibited by swelling; k12 is not volume sensitive. Both k23 and k32 are increased by swelling and reduced by shrinkage; they are rate constants for a single process, whereas k12 and k21 are rate constants for separate processes. Finally, the A-->B conversion entails an increase in Jmax of the transporters, and the B-->C conversion entails an increase in the affinity of the transporters for K.

Potassium-chloride cotransport in resealed human red cell ghosts.

Furosemide-inhibitable K influx is threefold higher in resealed ghosts of human erythrocytes than in intact cells. The enhancement is specific for K in that furosemide-inhibitable Na influx is the same in resealed ghosts and intact cells. The enhanced K influx resembles K-Cl cotransport in intact cells in that it requires Cl but not Na. N-ethylmaleimide (NEM), which stimulates furosemide-inhibitable K influx in intact cells, is without effect (or slightly inhibitory) in resealed ghosts. The failure of NEM to enhance the flux was not due to low ATP in the ghosts. These findings suggest that enhancement of the K flux in ghosts occurs by oxidation of membrane protein sulfhydryl groups, known to occur with lysis, the same sulfhydryl groups at which NEM acts by alkylation. This conclusion is supported by two observations: dithiothreitol completely prevents the increase in K influx in ghosts; this agent inhibits both oxidation of sulfhydryl groups and alkylation of them by NEM; and K influx in resealed ghosts is sensitive to changes in cell volume, just as it is in NEM-treated intact cells.

Stimulation of the Na/K pump in LK sheep erythrocytes by immunoglobulin fragments.

Alloimmune antiserum against the L antigen of red cells from sheep of the LK phenotype is known to stimulate by several fold active Na/K transport in LK cells. We have shown that monomeric fragments, Fab1, of anti-L, as well as dimeric fragments, F(ab1)2, stimulate transport to the same extent as intact anti-L Ig. Special care was taken to obtain pure fragments. Two earlier reports on the effect of immunoglobulin fragments were contradictory.

Anion-dependent cation transport in erythrocytes.

A selective survey of the literature reveals at least three major anion-dependent cation transport systems, defined as Na+ + Cl-, K+ + Cl- and Na+ + K+ + Cl- respectively. In human red cells, kinetic data on the fraction of K+ and Na+ influx inhibitable by bumetanide are presented to indicate an Na+:K+ stoichiometry of 1:2. For LK sheep red cells the large Cl- -dependent K+ leak induced by swelling is shown to share many characteristics with that induced by N-ethylmaleimide (NEM) treatment. NEM has complex effects, both inhibiting and then activating Cl- -dependent K+ fluxes dependent on NEM concentration. The alloantibody anti-L can prevent the action of NEM. In human red cells NEM induces a large Cl- -dependent specific K+ flux, which shows saturation kinetics. Its anion preference is Cl- greater than Br- greater than SCN- greater than I- greater than NO3- greater than MeSO4-. This transport pathway is not inhibited by oligomycin or SITS, although phloretin and high concentrations of furosemide and bumetanide (over 0.3 mM) do inhibit. Quinine (0.5 mM) is also an inhibitor. It is concluded that at least two distinct Cl- -dependent transport pathways for K+ are inducible in mammalian red cells, although the evidence for their separation is not absolute.