MOLECULAR-CLONING AND FUNCTIONAL EXPRESSION OF THE K-CL COTRANSPORTERFROM RABBIT, RAT, AND HUMAN - A NEW MEMBER OF THE CATION-CHLORIDE COTRANSPORTER FAMILY

We report the cloning, sequence analysis, tissue distribution, and fun ctional expression of the K-Cl cotransport protein, KCC1, KCC1 was ide ntified by searching the human expressed sequence tag data base, based on the expectation that it would be distantly related to the Na-K-Cl cotransporter. Rabbit KCC1 (rbKCC1) and rat KCC1 (rtKCC1) were cloned by screening rabbit kidney and rat brain cDNA libraries using homologo us cDNA probes, Human KCC1 (hKCC1) was obtained from I.M.A.G.E. clones and in part by reverse transcription-polymerase chain reaction; it ex hibits 97% identity with rbKCC1, KCC1 encodes a 1085-residue polypepti de with substantial sequence homology (24-25% identity) to the bumetan ide-sensitive Na-K-Cl cotransporter (NKCC or BSC) and the thiazide-sen sitive Na-Cl cotransporter (NCC or TSC), Hydropathy analysis of KCC1 i ndicates structural homology to NKCC, including 12 transmembrane domai ns, a large extracellular loop with potential N linked glycosylation s ites, and cytoplasmic N- and C-terminal regions, Northern blot analysi s revealed a ubiquitously expressed 3,8-kilobase transcript. Much of t he genomic sequence of hKCC1 is in the data base, and the gene has bee n previously localized to 16q22.1 (Larsen, F., Solhein, J., Kristensen , T., Kolsto, A. B., and Prydz, H. (1993) Hum. Mel. Genet. 2, 1589-159 5), Epitope-tagged rbKCC1 was stably expressed in human embryonic kidn ey (HEK 293) cells, resulting in production of a similar to 150-kDa gl ycoprotein. The initial rate of Rb-86 efflux from cells expressing rbK CC1 was more than 7 times greater than efflux from control cells and w as inhibited by 2 mM furosemide; Rb-86 efflux was stimulated by cell s welling, Uptake of Rb-86 into rbKCC1 cells after a 15-min pretreatment with 1 mM N-ethylmaleimide was dependent on external chloride but not on external sodium, and was inhibited by furosemide with a K-i of sim ilar to 40 mu M and by bumetanide with a K-i of similar to 60 mu M. Th ese data demonstrate that the KCC1 cDNAs encode a widely expressed K-C l cotransporter with the characteristics of the K-Cl transporter that has been characterized in red cells.