How many Orai’s does it take to make a CRAC channel?

Department of Pharmacology and Physiology, University of Rochester Medical Center, Rochester, NY 14642, USA.
Scientific Reports (Impact Factor: 5.58). 06/2013; 3:1961. DOI: 10.1038/srep01961
Source: PubMed


CRAC (Calcium Release-Activated Calcium) channels represent the primary pathway for so-called "store-operated calcium entry" - the cellular entry of calcium induced by depletion of intracellular calcium stores. These channels play a key role in diverse cellular activities, most noticeably in the differentiation and activation of Tcells, and in the response of mast cells to inflammatory signals. CRAC channels are formed by members of the recently discovered Orai protein family, with previous studies indicating that the functional channel is formed by a tetramer of Orai subunits. However, a recent report has shown that crystals obtained from the purified Drosophila Orai protein display a hexameric channel structure. Here, by comparing the biophysical properties of concatenated hexameric and tetrameric human Orai1 channels expressed in HEK293 cells, we show that the tetrameric channel displays the highly calcium-selective conductance properties consistent with endogenous CRAC channels, whilst the hexameric construct forms an essentially non-selective cation channel.

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    ABSTRACT: STIM1 and Orai1 are two essential components of the calcium release-activated calcium (CRAC) channel. Recently, distinct stoichiometries of STIM1 to Orai1 in assembling CRAC channels are proposed based on different techniques, such as single-molecule bleaching, biochemistry, crystallography, and concatenated constructs for electrophysiological experiments. Here, we review in detail these experiments as well as the strength and weakness of methods used. We propose that the tetrameric Orai1 is the pore for the resting and activated CRAC channel, where from two to eight STIM1 proteins open the channel in a graded manner.
    Current Topics in Membranes 07/2013; 71:95-108. DOI:10.1016/B978-0-12-407870-3.00004-4 · 3.30 Impact Factor
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    ABSTRACT: The discovery of the Orai proteins, and the identification of STIM1 as the molecule that regulates them, was based on their role in the agonist-activated store-operated entry of calcium via the CRAC channels. However, these same proteins are also essential components of the ARC channels responsible for a similar agonist-activated, but store-independent, arachidonic acid-regulated entry of calcium. The fact that these 2 biophysically similar calcium entry pathways frequently co-exist in the same cells suggests that they must each possess different features that allow them function in distinct ways to regulate specific cellular activities. This review begins to address this question by describing recent findings characterizing the unique features of the ARC channels-their molecular composition, STIM1-dependent activation, and physiological activities-and the importance of defining such features for the accurate therapeutic targeting of these 2 Orai channel subtypes.
    Channels (Austin, Tex.) 09/2013; 7(5). DOI:10.4161/chan.26156 · 2.32 Impact Factor
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    ABSTRACT: Ca (2+) influx via store-operated Ca (2+) release activated Ca (2+) (CRAC) channels represents a main signaling pathway for T-cell activation as well as mast-cell degranulation. The ER-located Ca (2+)-sensor, STIM1 and the Ca (2+)-selective ion pore, Orai1 in the membrane are sufficient to fully reconstitute CRAC currents. Their identification, but even more the recent structural resolution of both proteins by X-ray crystallography has substantially advanced the understanding of the activation mechanism of CRAC channels. In this review, we provide a detailed description of the STIM1/Orai1 signaling pathway thereby focusing on the critical domains mediating both, intra- as well as intermolecular interactions and on the ion permeation pathway. Based on the results of functional studies as well as the recently published crystal structures, we portray a mechanistic view of the steps in the CRAC channel signaling cascade ranging from STIM1 oligomerization over STIM1-Orai1 coupling to the ultimate Orai1 channel activation and permeation.
    Channels (Austin, Tex.) 10/2013; 7(5). DOI:10.4161/chan.26742 · 2.32 Impact Factor
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