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, ISOC. 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.