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Calcium ions are released from intracellular stores in response to agonist-stimulated production of inositol 1,4,5-trisphosphate (InsP₃), a second messenger generated at the cell membrane. Depletion of Ca²⁺ from internal stores triggers a capacitative influx of extracellular Ca²⁺ across the plasma membrane,. The influx of Ca²⁺ can be recorded as store-operated channels (SOC) in the plasma membrane or as a current known as the Ca²⁺-release-activated current (I_crac)^,,. A critical question in cell signalling is how SOC and I_crac sense and respond to Ca²⁺-store depletion: in one model, a messenger molecule is generated that activates Ca²⁺ entry in response to store depletion,; in an alternative model, InsP₃ receptors in the stores are coupled to SOC and I_crac. The mammalian Htrp3 protein forms a well defined store-operated channel, and so provides a suitable system for studying the effect of Ca²⁺-store …