cDNA Cloning and Functional Expression of the Dolphin Retinal Rod Na−Ca+K Exchanger NCKX1: Comparison with the Functionally Silent Bovine NCKX1

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Abstract
CDNAs of human and bovine retinal rod Na+−Ca2++K+ exchanger (NCKX1) have previously been cloned, but potassium-dependent Na−Ca exchange activity upon heterologous expression has not been demonstrated. We have cloned NCKX1 cDNA from dolphin, examined function upon transfection in HEK293 cells, and compared the dolphin sequence encoded by the cDNA with those of human and bovine. The dolphin NCKX1 cDNA encodes 1013 amino acid residues. Comparison to bovine and human NCKX1 revealed strong conservation in the transmembrane domains (>95%), but relatively low conservation in the large extracellular (∼50%) and cytosolic (95% of the experiments, whereas transfection with bovine NCKX1 yielded no function. The bovine NCKX1 phenotype was imparted on dolphin NCKX1 when the dolphin cytosolic loop was replaced by that from bovine. Conversely, deletion of 114 amino acids from the bovine sequence to match the dolphin sequence resulted in a mutant bovine NCKX1 which performed K+-dependent Na−Ca exchange. These results suggest that domains within the large cytosolic loop of NCKX1 control functional activity when expressed in heterologous systems.