The N-terminal domain of Orai3 determines selectivity for activation of the store-independent ARC channel by arachidonic acid.

Although highly selective Ca²(+) entry pathways play a critical role in agonist-activated Ca²(+) signals in non-excitable cells, only with the recent discovery of the Orai proteins have the first insights into the molecular nature of these pathways been possible. To date, just two such highly Ca²(+)-selective "Orai channels" have been identified in native cells - the store-operated CRAC channels and the store-independent, arachidonic acid-activated ARC channels. Studies have shown that the functional CRAC channel pore is formed by a tetrameric arrangement of Orai1 subunits, whilst a heteropentamer of three Orai1 subunits and two Orai3 subunits forms the functional ARC channel pore. Importantly, this inclusion of Orai3 subunits in the ARC channel structure has been shown to play a specific role in determining the selectivity of these channels for activation by arachidonic acid. Using an approach based on the expression of various concatenated constructs, we examined the basis for this Orai3-dependent effect on selectivity for arachidonic acid. We show that, whilst heteropentamers containing only one Orai3 subunit are sensitive to arachidonic acid, specific selectivity for activation by this fatty acid is only achieved on inclusion of the second Orai3 subunit in the pentamer. Further studies identified the cytosolic N-terminal domain of Orai3 as the region specifically responsible for this switch in selectivity. Substitution of just this domain into an otherwise complete single Orai1 subunit within a concatenated 31111 pentamer is sufficient to change the resulting channel from one that is predominantly store-operated, to one that is exclusively activated by arachidonic acid.