Gated regulation of CRAC channel ion selectivity by STIM1

Two defining functional features of ion channels are ion selectivity and channel gating. Ion selectivity is generally considered an immutable property of the open channel structure, whereas gating involves transitions between open and closed channel states typically without changes in ion selectivity (1). In store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels, the molecular mechanism of channel gating by the CRAC channel activator, STIM1 (stromal interaction molecule 1) remains unknown. CRAC channels are distinguished by an extraordinarily high Ca(2+) selectivity and are instrumental in generating sustained [Ca(2+)](i) elevations necessary for gene expression and effector function in many eukaryotic cells (2). Here, we probed the central features of the STIM1 gating mechanism in the CRAC channel protein, Orai1, and identified V102, a residue located in the extracellular region of the pore, as a candidate for the channel gate. Mutations at V102 produced constitutively active CRAC channels that were open even in the absence of STIM1. Unexpectedly, although STIM1-free V102 mutant channels were not Ca(2+)-selective, their Ca(2+) selectivity was dose-dependently boosted by interactions with STIM1. Similar enhancement of Ca(2+) selectivity also occurred in wild-type (WT) Orai1 channels by increasing the number of STIM1 activation domains directly tethered to Orai1 channels. Thus, exquisite Ca(2+) selectivity is not an intrinsic property of CRAC channels, but rather a tunable feature bestowed on otherwise non-selective Orai1 channels by STIM1. Our results demonstrate that STIM1-mediated gating of CRAC channels occurs through an unusual mechanism wherein permeation and gating are closely coupled.