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Store-dependent and -independent modes regulating Ca2+ release-activated Ca2+ channel activity of human Orai1 and

Shenyuan L Zhang1, J Ashot Kozak, Weihua Jiang

  • 1Department of Physiology and Biophysics, University of California, Irvine, California 92697, USA.

The Journal of Biological Chemistry
|April 19, 2008
PubMed

Insights

Researchers studied Orai and STIM1 proteins, finding Orai1 forms calcium-selective channels. Orai3, however, generates a nonselective current with 2-APB, suggesting a distinct activation pathway for calcium release-activated channels.

Area of Science:

  • Molecular biology
  • Cell physiology
  • Ion channel function

Background:

  • Orai and STIM proteins regulate store-operated calcium entry (SOCE).
  • CRAC channels, formed by Orai and STIM1, are crucial for calcium homeostasis.
  • Understanding Orai homolog function and activation mechanisms is vital.

Purpose of the Study:

  • To investigate currents induced by co-expression of human Orai homologs and STIM1.
  • To characterize the properties and activation of Orai1 and Orai3 channels.
  • To identify structural determinants of 2-APB-induced currents in Orai proteins.

Main Methods:

  • Co-expression of human Orai1, Orai3, and STIM1 in HEK cells.
  • Electrophysiological recordings to measure ion currents.
  • Site-directed mutagenesis and chimera analysis of Orai proteins.

Main Results:

  • Orai1/STIM1 co-expression induced a Ca(2+)-selective, inwardly rectifying current.
  • A mutation in Orai1 altered ion selectivity but maintained inward rectification.
  • Orai3/STIM1 co-expression with 2-APB generated a nonselective current, distinct from SOCE.
  • Orai3 alone, with 2-APB, induced a mixed Ca(2+) and monovalent cation current.
  • Structural analysis identified transmembrane segments 2-3 of Orai3 as critical for 2-APB-induced currents.

Conclusions:

  • Orai1 forms canonical CRAC channels, while Orai3 exhibits unique activation and ion selectivity.
  • 2-APB can activate Orai3 independently of store depletion, suggesting alternative channel gating.
  • The C-terminal STIM1 portion is sufficient for Orai1 CRAC current generation.
  • Orai3's distinct properties may represent a store-independent mode of CRAC channel activation.

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