Molecular and topological characterization of the rat parotid Na+-K+-2Cl- cotransporter1.

Na+-K+-2Cl- cotransporters play a central role in driving salt and water movements across secretory and absorptive epithelia. We report the cloning of the rat parotid secretory Na+-K+-2Cl- cotransporter, rtNKCC1. The predicted amino acid sequence of this protein is highly homologous to a previously cloned NKCC1 from the shark rectal gland and to mammalian NKCC1s cloned from several cultured cell lines, confirming the presence of the NKCC1 isoform in a naturally occurring mammalian secretory epithelium. In contrast to previously published NKCC1 clones, our sequence also includes an apparently complete 2680 bp 3'-UTR. Hydropathy analyses of rtNKCC1 predicts that this protein consists of large hydrophilic N and C termini (approx. 30 kDa and 50 kDa, respectively) flanking a central hydrophobic transmembrane region consisting of ten to 12 membrane spanning domains. In addition, we report the results of confocal immunofluorescent microscopic studies using rat parotid acini and antibodies directed against specific regions of the predicted N- and C-terminal portions of rtNKCC1. These studies demonstrate that the epitopes recognized by these antibodies are exposed in permeabilized but not in unpermeabilized cells, indicating that the predicted N and C termini of rtNKCC1 are intracellular.