Phosphoregulation of STIM1 leads to exclusion of the endoplasmic reticulum from the mitotic spindle

The endoplasmic reticulum (ER) undergoes significant reorganization between interphase and mitosis, but the underlying mechanisms are unknown [1]. Stromal interaction molecule 1 (STIM1) is an ER Ca²⁺ sensor that activates store-operated Ca²⁺ entry (SOCE) [2, 3] and also functions in ER morphogenesis through its interaction with the microtubule +TIP protein end binding 1 (EB1) [4]. We previously demonstrated that phosphorylation of STIM1 during mitosis suppresses SOCE [5]. We now show that STIM1 phosphorylation is a major regulatory mechanism that excludes ER from the mitotic spindle. In mitotic HeLa cells, the ER forms concentric sheets largely excluded from the mitotic spindle. We show that STIM1 dissociates from EB1 in mitosis and localizes to the concentric ER sheets. However, a nonphosphorylatable STIM1 mutant (STIM1 ^10A) colocalized extensively with EB1 and drove ER mislocalization by pulling ER tubules into the spindle. This effect was rescued by mutating the EB1 interaction site of STIM1^10A, demonstrating that aberrant association of STIM1^10A with EB1 is responsible for the ER mislocalization. A STIM1 phosphomimetic exhibited significantly impaired +TIP tracking in interphase but was ineffective at inhibiting SOCE, suggesting different mechanisms of regulation of these two STIM1 functions by phosphorylation. Thus, ER spindle exclusion and ER-dependent Ca²⁺ signaling during mitosis require multimodal STIM1 regulation by phosphorylation.