The prokaryotic Na/Ca exchanger NCX_Mj transports Na and Ca in a 3:1 stoichiometry.

Intracellular Ca signals control a wide array of cellular processes. These signals require spatial and temporal regulation of the intracellular Ca concentration, which is achieved in part by a class of ubiquitous membrane proteins known as sodium-calcium exchangers (NCXs). NCXs are secondary-active antiporters that power the translocation of Ca across the cell membrane by coupling it to the flux of Na in the opposite direction, down an electrochemical gradient. Na and Ca are translocated in separate steps of the antiport cycle, each of which is thought to entail a mechanism whereby ion-binding sites within the protein become alternately exposed to either side of the membrane. The prokaryotic exchanger NCX_Mj, the only member of this family with known structure, has been proposed to be a good functional and structural model of mammalian NCXs; yet our understanding of the functional properties of this protein remains incomplete. Here, we study purified NCX_Mj reconstituted into liposomes under well-controlled experimental conditions and demonstrate that this homologue indeed shares key functional features of the NCX family. Transport assays and reversal-potential measurements enable us to delineate the essential characteristics of this antiporter and establish that its ion-exchange stoichiometry is 3Na:1Ca Together with previous studies, this work confirms that NCX_Mj is a valid model system to investigate the mechanism of ion recognition and membrane transport in sodium-calcium exchangers.