Article

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

ABSTRACT

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.

    • "Further studies analyzing the permeation properties of the Orai1 channel reported that the hexameric hOrai1 is permeable to Ca 2+ as well as to monovalent cations in the presence of divalent cations, while the tetrameric channel displays the highly Ca 2+ -selective conductance characteristic of endogenous CRAC channels [42]. Further crystallographic studies are needed to determine the stoichiometry of native hOrai1. "
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    ABSTRACT: The ion Ca(2+) is a ubiquitous second messenger that mediates a variety of cellular functions. Dysfunction of the mechanisms involved in Ca(2+) homeostasis underlies a number of pathological processes, including cancer. Store-operated Ca(2+) entry (SOCE) is a major mechanism for Ca(2+) entry modulated by the intracellular Ca(2+) stores. The Ca(2+)-selective store-operated current (ICRAC) is mediated by the endoplasmic reticulum (ER) Ca(2+) sensor STIM1 and the store-operated Ca(2+) (SOC) channel Orai1, while other non-selective cation currents (ISOC) involves the participation of members of the canonical transient receptor potential (TRPC) channel family, including TRPC1. Distinct isoforms of the key components of SOCE have been described in mammalian cells, STIM1 and 2, Orai1-3 and TRPC1-7. In cancer cells, SOCE has been reported to play an important role in cell cycle progression and proliferation, migration, metastasis and evasion of apoptosis. Changes in the expression of the key elements of SOCE and Ca(2+) homeostasis remodelling have been account to play important roles in the phenotypic changes observed in transformed cells. Despite there are differences in the expression level of the molecular components of SOCE, as well as in the relevance of the STIM, Orai and TRPC isoforms in SOCE and tumorigenesis among cancer cell types, there is a body of evidence supporting an important role for SOCE underlying the phenotypic modifications of cancer cells that propose STIM and the SOC channels as suitable candidate targets for future prognostic or therapeutic strategies.
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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.
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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.
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