Calcium store refilling and STIM activation in STIM- and Orai-deficient cell lines

Mediated through the combined action of STIM proteins and Orai channels, store-operated Ca ²⁺ entry (SOCE) functions ubiquitously among different cell types. The existence of multiple STIM and Orai genes has made it difficult to assign specific roles of each STIM and Orai homolog in mediating Ca ²⁺ signals. Using CRISPR/Cas9 gene editing tools, we generated cells with both STIM or all three Orai homologs deleted and directly monitored store Ca ²⁺ and Ca ²⁺ signals. We found that unstimulated, SOCE null KO cells still retain 50~70% of ER Ca ²⁺ stores of wildtype (wt) cells. After brief exposure to store-emptying conditions, acute refilling of ER Ca ²⁺ stores was totally blocked in KO cells. However, after 24 h in culture, stores were eventually refilled. Thus, SOCE is critical for immediate refilling of ER Ca ²⁺ but is dispensable for the maintenance of long-term ER Ca ²⁺ homeostasis. Using the Orai null background triple Orai-KO cells, we examined the plasma membrane translocation properties of a series of truncated STIM1 variants. FRET analysis reveals that, even though PM tethering of STIM1 expedites the activation of STIM1 by facilitating its oligomerization, migration, and accumulation in ER-PM junctions, it is not required for the conformational switch, oligomerization, and clustering of STIM1. Even without overt puncta formation at ER-PM junctions, STIM1 _1–491 and STIM1 _1–666 could still rescue SOCE when expressed in STIM KO cells. Thus, ER-PM trapping and clustering of STIM molecules only facilitates the process of SOCE activation, but is not essential for the activation of Orai channels.