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Essential role of N-terminal SAM regions in STIM1 multimerization and function.

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Stromal Interaction Molecule 1 (STIM1) multimerization in the endoplasmic reticulum is crucial for calcium channel activation. This study identifies key structural regions in STIM1

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Area of Science:

  • Cellular Biology
  • Molecular Biology
  • Biophysics

Background:

  • Stromal Interaction Molecule 1 (STIM1) regulates calcium (Ca2+) homeostasis by sensing ER Ca2+ levels.
  • STIM1 activates Orai1 calcium channels upon Ca2+ store depletion.
  • The structural basis for STIM1 luminal multimerization remained unclear.

Purpose of the Study:

  • To elucidate the structural mechanisms underlying STIM1 luminal di/multimerization.
  • To identify specific regions within the STIM1 sterile alpha motif (SAM) domain involved in multimerization.
  • To investigate the functional consequences of impaired STIM1 multimerization on calcium signaling.

Main Methods:

  • Molecular dynamics (MD) simulations to predict multimerization sites.
  • Site-directed mutagenesis of identified STIM1 SAM domain regions.
  • Biochemical assays and live-cell imaging to assess STIM1 multimerization and Orai1 channel activity.
  • Analysis of calcium influx and oscillation frequency.

Main Results:

  • MD simulations identified two key multimerization segments (α7 and near α9) in the STIM1 SAM domain.
  • Point mutations in these segments disrupted STIM1 higher-order multimerization and puncta formation.
  • Impaired STIM1 multimerization significantly reduced store-operated calcium entry and oscillation frequency.
  • Functional defects were partially rescued by combining mutations with gain-of-function mutations or Orai1 coexpression.

Conclusions:

  • Hydrophobicity-driven interactions within the STIM1 SAM domain are critical for its switch between monomeric and multimeric states.
  • STIM1 SAM domain interactions are essential for STIM1 multimerization and subsequent Orai1 channel activation.
  • This study reveals the structural basis for STIM1-mediated calcium signaling regulation.