A Circularized Sodium-Calcium Exchanger Exon 2 Transcript

Abstract

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 stop 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.

Expression and function of Na+/Ca2+ exchangers 1 and 3 in human macrophages and monocytes

“…Otherwise, it may be synthesized starting from a circularized NCX1 exon 2 transcript generated by alternative splicing [11]. In all cases, the truncated forms retain Na 1 /Ca 21 exchange activity despite the absence of several transmembrane segments [10,11,48]. It is not yet known whether these truncated forms have any pathophysiological function.…”

“…This truncated protein may represent a proteolytic fragment of either the COOH-terminal [48] or the NH 2 -terminal [10] domain of the full-length NCX1. Otherwise, it may be synthesized starting from a circularized NCX1 exon 2 transcript generated by alternative splicing [11]. In all cases, the truncated forms retain Na 1 /Ca 21 exchange activity despite the absence of several transmembrane segments [10,11,48].…”

“…Two other immunoreactive bands of approximately 70 and 60 kDa were detected only in macrophages. The first may represent the truncated form of NCX1 and the second a proteolytic fragment [11]. Finally, in monocytes and BHK-NCX1 a large immunoreactive area of high molecular weight (between 140 and 200 kDa) was detected.…”

“…NCX is modeled with nine transmembrane segments involved in ion transport and a long central cytoplasmic loop that has regulatory functions [8]. However, truncated forms lacking certain transmembrane segments, but still possessing NCX activity, have also been described as the result of alternative splicing of the three genes [9][10][11]. NCX1 is largely predominant in mammalian tissues such as heart, brain, and kidney while the expression of NCX2 and NCX3 appears to be limited to the brain and skeletal muscle [7].…”

“…2009. 39: 1405-1418 DOI 10.1002 Leukocyte signaling 1405 described as the result of alternative splicing of the three genes [9][10][11]. NCX1 is largely predominant in mammalian tissues such as heart, brain, and kidney while the expression of NCX2 and NCX3 appears to be limited to the brain and skeletal muscle [7].…”