Previous reports of Na/Ca exchanger gene 1 (NCX1) expression have revealed a major RNA transcript of 7 kilobase pairs (kb), minor transcripts of ϳ13 and ϳ4 kb, and a relatively abundant 1.8-kb RNA band. In the present report we demonstrate that the 1.8-kb message, which has a tissue and subcellular distribution matching that of full-length NCX1 but is not polyadenylated, corresponds to a perfectly circularized exon 2 species. The circular transcript contained the normal NCX1 start codon, a new

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... top codon introduced as a consequence of circularization, and encoded a protein corresponding to the NH 2 -terminal portion of NCX1, terminating just after amino acid 600 in the cytoplasmic loop. A linear version of the circular transcript was prepared and transfected into HEK-293 cells. A protein, matching the predicted size of ϳ70 kDa, was expressed, and the transfected cells possessed Na/Ca exchange activity. Although in native tissue we could not detect a protein corresponding exactly to that predicted from the circular transcript, a prominent band of slightly shorter size, possibly representing further proteolytic processing of circular transcript protein, was observed in membranes from LLC-MK2 cells and rat kidney.