The Molecular Architecture of the Arachidonate-regulated Ca2+-selective ARC Channel is a Pentameric Assembly of Orai1 and Orai3 Subunits

The activation of Ca(2+) entry is a critical component of agonist-induced cytosolic Ca(2+) signals in non-excitable cells. Although a variety of different channels may be involved in such entry, the recent identification of the STIM and Orai proteins has focused attention on the channels in which these proteins play a key role. To date, two distinct highly Ca(2+)-selective STIM1-regulated and Orai-based channels have been identified - the store-operated CRAC channels and the store-independent arachidonic acid activated ARC channels. In contrast to the CRAC channels, where the channel pore is composed of only Orai1 subunits, both Orai1 and Orai3 subunits are essential components of the ARC channel pore. Using an approach involving the co-expression of a dominant-negative Orai1 monomer along with different preassembled concatenated Orai1 constructs, we recently demonstrated that the functional CRAC channel pore is formed by a homotetrameric assembly of Orai1 subunits. Here, we use a similar approach to demonstrate that the functional ARC channel pore is a heteropentameric assembly of three Orai1 subunits and two Orai3 subunits. Expression of concatenated pentameric constructs with this stoichiometry results in the appearance of large currents that display all the key biophysical and pharmacological features of the endogenous ARC channels. They also replicate the essential regulatory characteristics of native ARC channels including specific activation by low concentrations of arachidonic acid, complete independence of store depletion, and an absolute requirement for the pool of STIM1 that constitutively resides in the plasma membrane.