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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 selectivity1. In store-operated Ca2+ release-activated Ca2+ (CRAC) channels, the molecular mechanism of channel gating by the CRAC channel activator, stromal interaction molecule 1 (STIM1), remains unknown. CRAC channels are distinguished by a very high Ca2+ selectivity and are instrumental in generating sustained intracellular calcium concentration elevations that are necessary for gene expression and effector function in many eukaryotic cells2. Here we probe the central features of the STIM1 gating mechanism in the human CRAC channel protein, ORAI1, and identify V102, a residue located in the extracellular region of the pore, as a candidate for the channel gate. Mutations at V102 produce constitutively active CRAC channels that are open even in the absence of STIM1. Unexpectedly, although STIM1-free V102 mutant channels are not Ca2+-selective, their Ca2+ selectivity is dose-dependently boosted by interactions with STIM1. Similar enhancement of Ca2+ selectivity is also seen in wild-type ORAI1 channels by increasing the number of STIM1 activation domains that are directly tethered to ORAI1 channels, or by increasing the relative expression of full-length STIM1. Thus, exquisite Ca2+ selectivity is not an intrinsic property of CRAC channels but rather a tuneable feature that is bestowed on otherwise non-selective ORAI1 channels by STIM1. Our results demonstrate that STIM1-mediated gating of CRAC channels occurs through an unusual mechanism in which permeation and gating are closely coupled.

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Figure 1: State-dependent accessibility of pore-lining residues localizes the activation gate to the extracellular TM1 region.
Figure 2: Mutations at V102 cause STIM1-independent constitutive ORAI1 activation.
Figure 3: STIM1 regulates ion selectivity of constitutively active V102C ORAI1 channels.
Figure 4: STIM1 dose-dependently modulates the ion selectivity of ORAI1 channels.


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We thank C. Lingle, R. Lewis, K. Swartz, J. Sack, A. Gross, L. Tirado-Lee and T. Hornell for discussions and comments on the manuscript. This work was supported by NIH grant NS057499 to M.P. An American Heart Association predoctoral fellowship supported A.S.

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B.A.M. generated, expressed and functionally characterized the ORAI mutants by patch-clamp electrophysiology with help from M.Y. The Ca2+ imaging, FRET imaging, TIRF microscopy and western blot analysis of ORAI1 mutants were performed by A.S. B.A.M, A.S. and M.P. contributed to the writing and editing of the paper.

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Correspondence to Murali Prakriya.

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The authors declare no competing financial interests.

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McNally, B., Somasundaram, A., Yamashita, M. et al. Gated regulation of CRAC channel ion selectivity by STIM1. Nature 482, 241–245 (2012).

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