Molecular Cloning and Functional Expression of the Cardiac Sarcolemmal Na ⁺ -Ca ²⁺ Exchanger

Abstract

The Na(+)-Ca2+ exchanger of the cardiac sarcolemma can rapidly transport Ca2+ during excitation-contraction coupling. To begin molecular studies of this transporter, polyclonal antibodies were used to identify a complementary DNA (cDNA) clone encoding the Na(+)-Ca2+ exchanger protein. The cDNA hybridizes with a 7-kilobase RNA on a Northern blot and has an open reading frame of 970 amino acids. Hydropathy analysis suggests that the protein has multiple transmembrane helices, and a small region of the sequence is similar to that of the Na(+)- and K(+)-dependent adenosine triphosphatase. Polyclonal antibodies to a synthetic peptide from the deduced amino acid sequence react with sarcolemmal proteins of 70, 120, and 160 kilodaltons on immunoblots. RNA, synthesized from the cDNA clone, induces expression of Na(+)-Ca2+ exchange activity when injected into Xenopus oocytes.