Calcium promotes syntaxin 1 mesoscale domain formation through phosphatidylinositol 4,5‐bisphosphate

Phosphatidylinositiol 4,5‐bisphosphate (PI(4,5)P 2 ) is a minor component of total plasma membrane lipids, yet it has a substantial role in regulation of many cellular functions including exo‐ and endocytosis. Recently, it was shown that PI(4,5)P 2 and syntaxin 1, a protein that catalyzes regulated exocytosis, form domains in the plasma membrane that constitute recognition sites for vesicle docking. Also, calcium was shown to promote syntaxin 1 clustering in the plasma membrane, but the molecular mechanism was unknown. Here, using a combination of super‐resolution STED microscopy, Förster resonance energy transfer (FRET) and atomic force microscopy (AFM) we show that Ca 2+ acts as a charge bridge that specifically and reversibly connects multiple syntaxin 1/PI(4,5)P 2 complexes into larger mesoscale domains. This transient reorganization of the plasma membrane by physiological Ca 2+ concentrations may be important for Ca 2+ ‐regulated secretion. Support or Funding Information This work was supported by the Deutsche Forschungsgemeinschaft (SFB803) and the US National Institutes of Health (P01 GM072694) to RJ. DM is a graduate student at the International Max Planck Research School for Molecular Biology, Goettingen. Reversible clustering of syntaxin 1/PI(4,5)P 2 domains by Ca 2+A. Series of STED images of membranes composed of 97 mol% DOPC and 3 mol% PI(4,5)P 2 with sx‐1 TMD reconstituted and ceramide labeled with Atto590. The same membrane area is shown before and after addition of 1 mM Mg 2+ , 500 μM Ca 2+ , and 0.5 M EGTA, respectively. Scale bar, 4 μm. B. Size distribution of sx‐1 TMD domains in the presence of 500 μM Ca 2+ (purple: size distribution, green: Gaussian distribution fit).