sentences:  The structures of three related proton-calcium exhangers (CAX) have been solved from yeast and bacteria.While structurally and functionally homologus, these structures illustrate novel oligomeric structures, substrate coupling, and regulation.In 1968, H Reuter and N Seitz published findings that, when Na+ is removed from the medium surrounding a cell, the efflux of Ca2+ is inhibited, and they proposed that there might be a mechanism for exchanging the two ions.