STIM proteins,Orai1, and gene expression

Cytoplasmic Ca²⁺ is an universal intracellular messenger that activates cellular responses over a broad temporal range, from neurotransmitter release to cell growth and proliferation.^1,2 Inherent to the use of the multifarious Ca²⁺ signal is the question of specificity: how can some Ca²⁺-dependent responses be activated in a cell and not others? A rise in cytoplasmic Ca²⁺ can evoke a response either by binding directly to the target (as occurs with certain Ca²⁺-activated K⁺ and Cl⁻ channels, for example) or through recruitment of intermediary proteins, such as calmodulin and troponin C. A substantial body of evidence has now established that Ca²⁺-binding proteins differ both in their affinities for Ca²⁺ and in their on- and off-rates for Ca²⁺ binding/unbinding. Furthermore, different Ca²⁺-binding proteins often occupy distinct locations within the cell. Therefore, the size, kinetics and spatial profile of a cytoplasmic Ca²⁺ signal are all important in determining which Ca²⁺-dependent response will be activated, when and for how long.³