Sodium/calcium exchanger protein <p>Na⁺/Ca²⁺ exchange proteins are involved in maintaining Ca²⁺ homeostasis ina wide variety of cell types. They are found in both the plasma membraneand intracellular organellar membranes, where they exchange Na⁺ for Ca²⁺ inan electrogenic manner. When located in the plasma membrane, they generallyutilise the transmembrane (TM) Na⁺ concentration gradient in order toextrude Ca²⁺ from cells. Three mammalian isoforms have been cloned to date(NCX1-3), which consist of 920-970 amino acid residues that are predictedto possess 11 or 12 TM domains. Interestingly, they possess a short motif(~30 residues) that is similar to the Na⁺/K⁺-ATPase, although its functionis unknown [<cite idref="PUB00005133"/>, <cite idref="PUB00002973"/>].</p><p>NCX1 has been found to be predominantly expressed in the heart, where itplays an important role in excitation-contraction coupling, but it is alsoabundant in a variety of other tissues [<cite idref="PUB00005133"/>]. NCX2 and NCX3 transcripts havebeen detected in the brain and skeletal muscle [<cite idref="PUB00002973"/>, <cite idref="PUB00006595"/>]. Homologous Na⁺/Ca²⁺exchange proteins have also been found in <taxon tax_id="6239">Caenorhabditis elegans</taxon>, <taxon tax_id="7227">Drosophila melanogaster</taxon> and<taxon tax_id="31211">Loligo opalescens</taxon> (California market squid).</p>