The Mechanics of STIM-ORAI Communication

Abstract

The ER-resident regulatory protein STIM1 triggers store-operated calcium entry by direct interaction with the plasma membrane calcium channel ORAI1. Cell-based research combining the expression of engineered proteins, electrophysiological recording, and advanced light-microscopic techniques has provided considerable insight into STIM-ORAI signalling, but has left tantalizing unanswered questions regarding the gating, modulation, and inactivation of ORAI-family channels. We have taken two complementary paths to investigate these questions. First, in order to dissect the basic mechanisms intrinsic to STIM, ORAI, and the STIM-ORAI complex, we have reconstituted channel gating in vitro using the purified recombinant STIM and ORAI proteins. This approach has begun to yield direct insight into ORAI channel gating. Second, in order to identify additional mechanisms that control STIM-ORAI signalling in cells, we have carried out a genome-wide RNAi screen for modulators of store-operated calcium entry. The screen led to the identification of septins and other protein modulators of STIM-ORAI signalling, and to the discovery that there is a striking rearrangement of the plasma membrane microdomain surrounding STIM-ORAI complexes during the onset of store-operated calcium entry. Continued parallel investigations in vitro and in cells will be needed for a full understanding of this key calcium entry pathway.