Intracellular Ca2+ release via the ER translocon activates store-operated calcium entry

Store-operated Ca2+ entry (SOCE) is activated in response to depletion of intracellular Ca2+ from the endoplasmic reticulum (ER). A variety of agonists stimulate SOCE via IP3-dependent Ca2+ depletion. SOCE is also activated by thapsigargin, an inhibitor of Ca2+ reuptake into the ER that induces a net Ca2+ loss from the ER by unmasking a Ca2+ leak pathway. The molecular identity of this Ca2+ leak channel and the physiological conditions under which such agonist-independent Ca2+ depletion might occur remain poorly characterized. In this study, we report that inhibition of the initiation step of protein synthesis (with pactamycin) resulted in detectable Ca2+ depletion in ER and activation of SOCE. This was completely prevented if the ribosome-nascent chain complexes were first stabilized with an irreversible inhibitor of translational elongation (emetine), suggesting that ER Ca2+ depletion had occurred through open translocons at the ER. Notably, emetine pretreatment also attenuated thapsigargin-mediated Ca2+ release and SOCE. Furthermore, both pactamycin and thapsigargin stimulated translocation of STIM1, a protein required for activation of SOCE, to the subplasma membrane region and activated the SOCE-associated current, I-SOC. In aggregate, these data reveal an agonist-independent mechanism for internal Ca2+ store depletion and activation of SOCE. We suggest that the functional coupling between SOCE and protein synthesis is likely to be critical for maintaining [Ca2+](ER) within a range that is required to prevent ER stress during changes in cellular translational activity.