Structural Basis for the Counter-Transport Mechanism of a H + /Ca 2+ Exchanger

Inward-Facing Antiporter Calcium/cation antiporters play a role in regulating the cytosolic calcium concentration by using the electrochemical gradient of other cations to catalyze Ca 2+ transport across cell membranes. The structure of a Na + /Ca 2+ exchanger in an outward-facing conformation was recently determined. Nishizawa et al. (p. 168 , published online 23 May) now report the crystal structure of a H + /Ca 2+ exchanger in an inward-facing conformation. Comparison of the structures shows how structural changes create hydrophilic cavities to alternate between the intra- and extracellular sides of the protein, facilitating cation transport.