Maintaining intracellular calcium (Ca2+) homeostasis is critical for cellular survival and the ability to respond to external signals. How do cells know that the Ca2+ in the endoplasmic reticulum (ER) is low and needs to be restored? Ushioda et al. found that the disulfide reductase ERdj5 associated with and reduced inhibitory disulfide bonds in the Ca2+-ATPase SERCA2b in response to reduced ER calcium in a manner that was enhanced by the ER chaperone BiP, thereby connecting redox signals to Ca2+ and protein homeostasis in the ER. SERCA2b and ERdj5 coimmunoprecipitated and ran at a higher molecular weight when analyzed under nonreducing conditions, indicating the formation of disulfide linked bonds between the two proteins. A mutated form of ERdj5 lacking the cysteine motifs in the thioredoxin (Trx) domains that could form the disulfide bonds exhibited reduced coimmunoprecipitation with SERCA2b. ERdj5 knockout cells had an increased proportion of oxidized SERCA2b, reduced Ca2+ uptake into the ER, and released less Ca2+ from the ER. The reduced form of SERCA2b had greater ATPase activity than the oxidized form, and incubating SERCA2b with ERdj5 increased ATPase activity in a manner dependent on the cysteine residues in the Trx domains, indicating that the reducing activity of ERdj5 was necessary. Exposing cells to compounds that reduce ER Ca2+—but not those that cause ER stress by inhibiting N-glycosylation—increased the amount of reduced ERdj5, which is the active form that can reduce SERCA2b. In vitro ERdj5 formed oligomers as Ca2+ was increased, and ERdj5 isolated from cells that were exposed to different concentrations was monomeric at submillimolar concentrations of Ca2+ and formed oligomers at higher Ca2+ concentrations. A mutant form of ERdj5 that could not interact with BiP coimmunoprecipitated less with SERCA2b than did wild-type ERdj5, and overexpression of BiP increased the interaction between wild-type ERdj5 and SERCA2b and increased the amount of monomeric ERdj5 even under high Ca2+ concentrations. Thus, cells rely on redox-mediated regulation of SERCA2b activity to adaptively control ER Ca2+ uptake.
R. Ushioda, A. Miyamoto, M. Inoue, S. Watanabe, M. Okumura, K.-i. Maegawa, K. Uegaki, S. Fujii, Y. Fukuda, M. Umitsu, J. Takagi, K. Inaba, K. Mikoshiba, K. Nagata, Redox-assisted regulation of Ca2+ homeostasis in the endoplasmic reticulum by disulfide reductase ERdj5. Proc. Natl. Acad. Sci. U.S.A. 113, E6055–E6063 (2016). [PubMed]