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

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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 theSaccharomyces cerevisiaevacuolar Ca2+/H+ exchanger (Vcx1) at 2.3Å 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.

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