CDNA CLONING OF THE HUMAN RETINAL ROD NA-CA- COMPARISON WITH A REVISED BOVINE SEQUENCE(K EXCHANGER )

PURPOSE. TO clone the complementary DNA of the human retinal rod Na-Ca +K exchanger. METHODS. A human retinal cDNA library was screened initi ally with a radiolabeled probe representing the entire bovine rod Na-C a+K exchanger cDNA and subsequently with probes from polymerase chain reaction fragments of the human retinal rod Na-Ca+K exchanger obtained after the initial screen. Twelve positive clones were used to obtain the entire coding sequence of the human retinal rod Na-Ca+K exchanger. RESULTS. The cDNA of the human retinal rod Na-Ca+K exchanger codes fo r a protein of 1081 amino acids, which shows 64.3% overall identity wi th the bovine retinal rod Na-Ca+K exchanger at the amino acid level. T he two sets of putative transmembrane-spanning domains and their short connecting loops showed the highest degree of identity (94%-95%), whe reas the extracellular loop at the N terminus showed a 59% identity. T he large cytosolic loop that bisects the two sets of transmembrane-spa nning domains contained two large deletions in the human exchanger; th e first deletion contains 18 amino acids, whereas the second deletion involves a series of repeats that are dominated by acidic amino acid r esidues observed in the bovine, but not in the human, sequence. The au thors observed that the bovine sequence contains a ninth repeat in add ition to the eight repeats of the published sequence. CONCLUSIONS. The authors cloned the cDNA of the human retinal rod Na-Ca+K exchanger as a first step in examining the possibility that this gene could be the locus of disease causing mutations.