E-syt1 Re-arranges STIM1 Clusters to Stabilize Ring-shaped ER-PM Contact Sites and Accelerate Ca²⁺ Store Replenishment.

In many non-excitable cells, the depletion of endoplasmic reticulum (ER) Ca²⁺ stores leads to the dynamic formation of membrane contact sites (MCSs) between the ER and the plasma membrane (PM), which activates the store-operated Ca²⁺ entry (SOCE) to refill the ER store. Two different Ca²⁺-sensitive proteins, STIM1 and extended synaptotagmin-1 (E-syt1), are activated during this process. Due to the lack of live cell super-resolution imaging, how MCSs are dynamically regulated by STIM1 and E-syt1 coordinately during ER Ca²⁺ store depletion and replenishment remain unknown. With home-built super-resolution microscopes that provide superior axial and lateral resolution in live cells, we revealed that extracellular Ca²⁺ influx via SOCE activated E-syt1s to move towards the PM by ~12 nm. Unexpectedly, activated E-syt1s did not constitute the MCSs per se, but re-arranged neighboring ER structures into ring-shaped MCSs (230~280 nm in diameter) enclosing E-syt1 puncta, which helped to stabilize MCSs and accelerate local ER Ca²⁺ replenishment. Overall, we have demonstrated different roles of STIM1 and E-syt1 in MCS formation regulation, SOCE activation and ER Ca²⁺ store replenishment.

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