Structural basis for alternating access of a eukaryotic calcium/proton exchanger

The X-ray crystal structure of a member of the Ca2+/H+ (CAX) antiporter family from Saccharomyces cerevisiae in a cytosol-facing, substrate-bound conformation is solved; using the structure, a mechanism by which members of the Ca2+:cation (CaCA) superfamily facilitate Ca2+ transport across cellular membranes is proposed. Robert Stroud and colleagues report the first inward, cytosol-facing crystal structure of a calcium exchanger, a member of the Ca2+/H+ (CAX) antiporter family from Saccharomyces cerevisiae. On the basis of this structure the authors propose a mechanism by which members of the CaCA superfamily facilitate Ca2+ transport across cellular membranes. Release of intracellular Ca2+ stores into the cytosol is central to many eukaryotic signal transduction processes, and it is the CaCA superfamily membrane protein antiporters that remove the unwanted Ca2+ after such events. Eukaryotic Ca2+ regulation involves sequestration into intracellular organelles, and expeditious Ca2+ release into the cytosol is a hallmark of key signalling transduction pathways. Bulk removal of Ca2+ after such signalling events is accomplished by members of the Ca2+:cation (CaCA) superfamily1,2,3,4,5. The CaCA superfamily includes the Na+/Ca2+ (NCX) and Ca2+/H+ (CAX) antiporters, and in mammals the NCX and related proteins constitute families SLC8 and SLC24, and are responsible for the re-establishment of Ca2+ resting potential in muscle cells, neuronal signalling and Ca2+ reabsorption in the kidney1,6. The CAX family members maintain cytosolic Ca2+ homeostasis in plants and fungi during steep rises in intracellular Ca2+ due to environmental changes, or following signal transduction caused by events such as hyperosmotic shock, hormone response and response to mating pheromones7,8,9,10,11,12,13. The cytosol-facing conformations within the CaCA superfamily are unknown, and the transport mechanism remains speculative. Here we determine a crystal structure of the Saccharomyces cerevisiae vacuolar Ca2+/H+ exchanger (Vcx1) at 2.3 Å resolution in a cytosol-facing, substrate-bound conformation. Vcx1 is the first structure, to our knowledge, within the CAX family, and it describes the key cytosol-facing conformation of the CaCA superfamily, providing the structural basis for a novel alternating access mechanism by which the CaCA superfamily performs high-throughput Ca2+ transport across membranes.