Abstract
The store-operated calcium (Ca2+) channel Orai governs Ca2+ influx through the plasma membrane of many non-excitable cells in metazoans. The channel opens in response to the depletion of Ca2+ stored in 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 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 Ca2+ selectivity. Opening of the pore requires the release of cytosolic latches. Together with additional X-ray structures of an unlatched-but-closed conformation, we propose a sequence for store-operated activation.
Highlights
The dearth of calcium (Ca2+) ions in the cytosol of non-excitable metazoan cells under ‘resting’ conditions allows transient increases in the cytosolic calcium concentration to relay internal messages and enables cells to respond to external stimuli
While the most pronounced structural differences between the closed and open pores are within the basic region, we find that removal of the basic region by mutation of these residues to serine does not form a constitutively open channel
We cannot discern the conformations of amino acid side chains in the open structure, and there is a high degree of uncertainty when discussing the dimensions of the pore, the repositioning of the M1 helices in the opened pore indicates that the hydrophobic region widens markedly (Figure 11)
Summary
The dearth of calcium (Ca2+) ions in the cytosol of non-excitable metazoan cells under ‘resting’ conditions allows transient increases in the cytosolic calcium concentration to relay internal messages and enables cells to respond to external stimuli. These cytosolic calcium signals regulate a plethora of processes including gene transcription, cell motility, exocytosis, and cell metabolism (Clapham, 2007). Calcium influx through Orai is necessary for activation of immune response genes in T cells and a range of other physiological processes
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