Role of Orai Proteins in Activation of Endogenous Trpc1-Composed Channels

Depletion of intracellular calcium stores activates store-operated channels. This process induces numerous intracellular signaling events. The most studied store-operated channels are CRAC channels. Other channels are believed to consist of TRPC and Orai proteins. However, their role in molecular composition of endogenous store-operated non-CRAC channels remains obscure. One of the main questions to ask is how TRPC channels are activated after store depletion. Most studies have used whole-cell patch clamp or calcium imaging techniques. To discriminate different types of store-operated channels, we used single-channel patch-clamp recordings in HEK293 cells. We showed that in experiments with dominant-negative mutant ORAI E106Q endogenous TRPC1-composed channels were not sensitive to store depletion, but they were activated by other pathways. In cells overexpressing STIM2 proteins endogenous TRPC1 channels were activated with a delay (similarly to ORAI channels activated by STIM2). In summary, we propose that (i) ORAI does not serve as a pore forming subunit of endogenous TRPC1 channels, but it is necessary to maintain sensitivity to calcium stores and (ii) TRPC1 channels are activated downstream to ORAI channels after store depletion. This study was supported by the Russian Scientific Foundation, Project 14-14-00720 (to E. K., D.K. and A.S.); and the Russian Foundation for Basic Research Project 16-04-01792 (A.S. and L.G.).