Cytosolic Ca²⁺ prevents the subplasmalemmal clustering of STIM1: an intrinsic mechanism to avoid Ca²⁺ overload

The stromal interacting molecule (STIM1) is pivotal for store-operated Ca²⁺ entry (SOC). STIM1 proteins sense the Ca²⁺ concentration within the lumen of the endoplasmic reticulum (ER) via an EF-hand domain. Dissociation of Ca²⁺ from this domain allows fast oligomerization of STIM1 and the formation of spatially discrete clusters close to the plasma membrane. By lifetime-imaging of STIM1 interaction, the rearrangement of STIM1, ER Ca²⁺ concentration ([Ca²⁺]_ER) and cytosolic Ca²⁺ signals ([Ca²⁺]_cyto) we show that [Ca²⁺]_cyto affects the subcellular distribution of STIM1 oligomers and prevents subplasmalemmal STIM clustering, even if the ER is depleted. These data indicate that [Ca²⁺]_cyto, independently of the ER Ca²⁺ filling state, crucially tunes the formation and disassembly of subplasmalemmal STIM1 clusters, and, thus, protects cells against Ca²⁺ overload resulting from excessive SOC activity.