LAUSR

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are one of the two types of tetrameric ion channels that release calcium ion (Ca2+) from the endoplasmic reticulum (ER) into the cytosol. Ca2+ released via IP3Rs is a fundamental second messenger for numerous cell functions. Disturbances in the intracellular redox environment resulting from various diseases and aging interfere with proper calcium signaling, however, the details are unclear. Here, we elucidated the regulatory mechanisms of IP3Rs by protein disulfide isomerase family proteins localized in the ER by focusing on four cysteine residues residing in the ER lumen of IP3Rs. First, we revealed that two of the cysteine residues are essential for functional tetramer formation of IP3Rs. Two other cysteine residues, on the contrary, were revealed to be involved in the regulation of IP3Rs activity; its oxidation by ERp46 and the reduction by ERdj5 caused the activation and the inactivation of IP3Rs activity, respectively. We previously reported that ERdj5 can activate the sarco/endoplasmic reticulum Ca2+-ATPase isoform 2b (SERCA2b) using its reducing activity [Ushioda et al., Proc. Natl. Acad. Sci. U.S.A. 113, E6055-E6063 (2016)]. Thus, we here established that ERdj5 exerts the reciprocal regulatory function for IP3Rs and SERCA2b by sensing the ER luminal Ca2+ concentration, which contributes to the calcium homeostasis in the ER.