Structures reveal opening of the store-operated calcium channel Orai

Abstract

The store-operated calcium (Ca 2+ ) channel Orai governs Ca 2+ influx through the plasma membrane of many non-excitable cells in metazoans. The channel opens in response to depletion of Ca 2+ within the endoplasmic reticulum (ER). Loss-and gain-of-function mutants of Orai cause disease. Our previous work revealed the structure of Orai with a closed pore. Here, using a gain-of-function mutation that constitutively activates the channel, we present an X-ray structure of Drosophila melanogaster Orai in an open conformation. Well-defined electron density maps reveal that the open pore is dramatically dilated on its cytosolic side in comparison to the slender closed pore. Cations and anions bind in different regions of the open pore, informing mechanisms for ion permeation and the exquisite selectivity of the channel for Ca 2+ . Opening of the pore requires the release of cytosolic latches.Together with additional X-ray structures of an unlatched-but-closed intermediate, we propose a sequence for store-operated activation.

Pore properties of Orai1 calcium channel dimers and their activation by the STIM1 ER calcium sensor

“…This conclusion agrees with studies on cross-linking of the M4-ext helices by Tirado-Lee et al (27). Recent structural analysis has suggested that the interacting pairs of M4-ext helices act as latches on the outside of the channel that must be opened to allow the M1-ext helices to expand outward thus permitting the cytosolic mouth of the pore to open (36). This model was based on a crystallized dOrai structure that stabilized the unlatched channel with straightened M4-ext helices.…”

“…First, our studies provide some important new understanding on the properties of the pore-selectivity filter that normally functions with six charged Glu residues. We speculate that these residues may have sufficient flexibility to allow formation of two Ca 2ϩ -binding sites within the selectivity filter, as suggested recently (36). The symmetrical and somewhat conservative replacement of three Glu residues with three intervening Gln residues (formed with OQ or QO dimers) may prevent formation of the putative second site and render the pore as a nonselective cation channel.…”

“…F, I/V relationship of currents mediated by OQ collected in different external solutions as indicated by the arrows in E. G, HEK-O1 ko S1 ϩ cells expressing the QO dimer were exposed to external solutions as shown in E. H, I/V relationship of currents mediated by QO collected in the external solutions indicated in G. the ion-selectivity mechanism for the Orai channel. Thus, in an interesting new crystallography study on dOrai channel structure, Hou et al (36) suggest that glutamates comprising the selectivity filter have side-chain flexibility that allows two Ca 2ϩ ions to be positioned one above the other. Repulsion between the two Ca 2ϩ ions allows one Ca 2ϩ to be displaced and move through the pore and to be replaced by a new Ca 2ϩ ion.…”

A Heuristic Derived from Analysis of the Ion Channel Structural Proteome Permits the Rapid Identification of Hydrophobic Gates

“…We illustrate the effectiveness of this heuristic approach using two recent structures ( Fig. 6): one of the TRPV3 channel in a putative sensitised (but non-conductive) conformation (PDB ID: 6MHS) and one of the CRAC channel (Orai) in an open conformation (PDB ID: 6BBF) (35). For the openstate Orai channel, none of the (hydrophobicity, radius) points fall below the SVM classification line and so the structure is predicted to be fully wetted and correspond to a functionally open state of the channel.…”

Conformational surveillance of Orai1 by a rhomboid intramembrane protease prevents inappropriate CRAC channel activation

“…The consequent reduction of ER Ca 2+ is sensed by the ER-resident membrane 75 protein Stim1 (Hogan and Rao, 2015), resulting in its interaction with Orai1 at PM-ER contact sites, 76 CRAC channel opening, and the influx of extracellular Ca 2+ . Channel opening relies both on Orai1 77 multimerisation into a pore-forming hexameric unit, and a concerted set of conformational changes 78 (Hou et al, 2012;Cai et al, 2016;Hou et al, 2018). The main interaction with the cytoplasmic 79 domain of Stim1 occurs via the C-terminal cytoplasmic domain of Orai1, which is anchored to the 80 membrane by its fourth transmembrane domain (TMD) (Park et al, 2009).…”