TRPC3 Regulates Agonist-stimulated Ca2+ Mobilization by Mediating the Interaction between Type I Inositol 1,4,5-Trisphosphate Receptor, RACK1, and Orai1

There is a body of evidence suggesting that Ca(2+) handling proteins assemble into signaling complexes required for a fine regulation of Ca(2+) signals, events that regulate a variety of critical cellular processes. Canonical transient receptor potential (TRPC) and Orai proteins have both been proposed to form Ca(2+)-permeable channels mediating Ca(2+) entry upon agonist stimulation. A number of studies have demonstrated that inositol 1,4,5-trisphosphate receptors (IP(3)Rs) interact with plasma membrane TRPC channels; however, at present there is no evidence supporting the interaction between Orai proteins and IP(3)Rs. Here we report that treatment with thapsigargin or cellular agonists results in association of Orai1 with types I and II IP(3)Rs. In addition, we have found that TRPC3, RACK1 (receptor for activated protein kinase C-1), and STIM1 (stromal interaction molecule 1) interact with Orai1 upon stimulation with agonists. TRPC3 expression silencing prevented both the interaction of Orai1 with TRPC3 and, more interestingly, the association of Orai1 with the type I IP(3)R, but not with the type II IP(3)R, thus suggesting that TRPC3 selectively mediates interaction between Orai1 and type I IP(3)R. In addition, TRPC3 expression silencing attenuated ATP- and CCh-stimulated interaction between RACK1 and the type I IP(3)R, as well as Ca(2+) release and entry. In conclusion, our results indicate that agonist stimulation results in the formation of an Orai1-STIM1-TRPC3-RACK1-type I IP(3)R complex, where TRPC3 plays a central role. This Ca(2+) signaling complex might be important for both agonist-induced Ca(2+) release and entry.