Calcium inhibition and calcium potentiation of Orai1, Orai2, and Orai3 calcium release-activated calcium channels.
The recent discoveries of Stim1 and Orai proteins have shed light on the molecular makeup of both the endoplasmic reticulum Ca(2+) sensor and the calcium release-activated calcium (CRAC) channel, respectively. In this study, we investigated the regulation of CRAC channel function by extracellular Ca(2+) for channels composed primarily of Orai1, Orai2, and Orai3, by co-expressing these proteins together with Stim1, as well as the endogenous channels in HEK293 cells. As reported previously, Orai1 or Orai2 resulted in a substantial increase in CRAC current (I(crac)), but Orai3 failed to produce any detectable Ca(2+)-selective currents. However, sodium currents measured in the Orai3-expressing HEK293 cells were significantly larger in current density than Stim1-expressing cells. Moreover, upon switching to divalent free external solutions, Orai3 currents were considerably more stable than Orai1 or Orai2, indicating that Orai3 channels undergo a lesser degree of depotentiation. Additionally, the difference between depotentiation from Ca(2+) and Ba(2+) or Mg(2+) solutions was significantly less for Orai3 than for Orai1 or -2. Nonetheless, the Na(+) currents through Orai1, Orai2, and Orai3, as well as the endogenous store-operated Na(+) currents in HEK293 cells, were all inhibited by extracellular Ca(2+) with a half-maximal concentration of approximately 20 mum. We conclude that Orai1, -2, and -3 channels are similarly inhibited by extracellular Ca(2+), indicating similar affinities for Ca(2+) within the selectivity filter. Orai3 channels appeared to differ from Orai1 and -2 in being somewhat resistant to the process of Ca(2+) depotentiation.
DeHaven
Wayne I
WI
Laboratory of Signal Transduction, NIEHS, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina 27709, USA.
Smyth
Jeremy T
JT
Boyles
Rebecca R
RR
Putney
James W
JW
D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000235:genetics / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism; D002460:Cell Line; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D000071740:ORAI1 Protein; D000071741:ORAI2 Protein; D018408:Patch-Clamp Techniques; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D000071737:Stromal Interaction Molecule 1
D015220:Calcium Channels; D002413:Cations, Divalent; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D000071740:ORAI1 Protein; C510746:ORAI1 protein, human; D000071741:ORAI2 Protein; C514506:ORAI2 protein, human; C522867:Orai3 protein, human; D034741:RNA, Small Interfering; C103530:STIM1 protein, human; D000071737:Stromal Interaction Molecule 1; D002118:Calcium