Phosphorylation of STIM1 underlies suppression of store-operated calcium entry during mitosis

Store-operated Ca2+ entry (SOCE) and Ca2+ release-activated Ca2+ currents (I-crac) are strongly suppressed during mitosis, the only known physiological situation in which Ca2+ store depletion is uncoupled from the activation of Ca2+ influx. We found that the endoplasmic reticulum (ER) Ca2+ sensor STIM1 failed to rearrange into near-plasma membrane puncta in mitotic cells, a critical step in the SOCE-activation pathway. We also found that STIM1 from mitotic cells is recognized by the phospho-specific MPM-2 antibody, suggesting that STIM1 is phosphorylated during mitosis. Removal of ten MPM-2 recognition sites by truncation at amino acid 482 abolished MPM-2 recognition of mitotic STIM1, and significantly rescued STIM1 rearrangement and SOCE response in mitosis. We identified Ser 486 and Ser 668 as mitosis-specific phosphorylation sites, and STIM1 containing mutations of these sites to alanine also significantly rescued mitotic SOCE. Therefore, phosphorylation of STIM1 at Ser 486 and Ser 668, and possibly other sites, underlies suppression of SOCE during mitosis.