STIM1L traps and gates Orai1 channels without remodeling the cortical ER

Abstract

STIM proteins populate and expand cortical endoplasmic reticulum (ER) sheets to mediate store-operated Ca ²⁺ entry (SOCE) by trapping and gating Orai channels in ER-plasma membrane clusters. A longer splice variant, STIM1L, forms permanent ER-plasma membrane clusters and mediates rapid Ca ²⁺ influx in muscle. Here, we used electron microscopy, total internal reflection fluorescence (TIRF) microscopy and Ca ²⁺ imaging to establish the trafficking and signaling properties of the two STIM1 isoforms in Stim1 ^−/−/Stim2 ^−/− fibroblasts. Unlike STIM1, STIM1L was poorly recruited into ER-plasma membrane clusters and did not mediate store-dependent expansion of cortical ER cisternae. Removal of the STIM1 lysine-rich tail prevented store-dependent cluster enlargement, whereas inhibition of cytosolic Ca ²⁺ elevations or removal of the STIM1L actin-binding domain had no impact on cluster expansion. Finally, STIM1L restored robust but not accelerated SOCE and clustered with Orai1 channels more slowly than STIM1 following store depletion. These results indicate that STIM1L does not mediate rapid SOCE but can trap and gate Orai1 channels efficiently without remodeling cortical ER cisternae. The ability of STIM proteins to induce cortical ER formation is dispensable for SOCE and requires the lysine-rich tail of STIM1 involved in binding to phosphoinositides.