Phosphorylation of STIM1 at ERK/CDK sites is dispensable for cell migration and ER partitioning in mitosis

ABSTRACT Store-operated Ca 2+ entry (SOCE) is a ubiquitous Ca 2+ influx required for multiple physiological functions including cell motility. SOCE is activated in response to depletion of intracellular Ca 2+ stores following the activation of endoplasmic reticulum (ER) Ca 2+ sensor STIM1 which recruits the plasma membrane (PM) Ca 2+ channel Orai1 at ER-PM junctions to induce Ca 2+ influx. STIM1 is phosphorylated dynamically and this phosphorylation has been implicated in several processes including SOCE inactivation during M-phase, maximal SOCE activation, ER segregation during mitosis, and cell migration. Human STIM1 has 10 Ser/Thr residues in its cytosolic domain that match the ERK/CDK consensus phosphorylation. We recently generated a mouse knock-in line where wild-type STIM1 was replaced by a non-phosphorylatable STIM1 with all 10 S/T mutated to Ala (STIM1-10A). Here, we generate mouse embryonic fibroblasts (MEF) the STIM1-10A mouse line and a control MEF line (WT) that express wild-type STIM1 from a congenic mouse strain. These lines offer a unique model to address the role of STIM1 phosphorylation at endogenous expression and modulation levels in contrast to previous studies that relied mostly on overexpression. We show that STIM1 phosphorylation at ERK/CDK sites is not required for SOCE activation, cell migration, or ER partitioning during mitosis. These results rule out STIM1 phosphorylation as a regulator of SOCE, migration and ER distribution in mitosis.