Ca²⁺-dependent redox modulation of SERCA 2b by ERp57

We demonstrated previously that calreticulin (CRT) interacts with the lumenal COOH-terminal sequence of sarco endoplasmic reticulum (ER) calcium ATPase (SERCA) 2b to inhibit Ca²⁺ oscillations. Work from other laboratories demonstrated that CRT also interacts with the ER oxidoreductase, ER protein 57 (also known as ER-60, GRP58; ERp57) during folding of nascent glycoproteins. In this paper, we demonstrate that ERp57 overexpression reduces the frequency of Ca²⁺ oscillations enhanced by SERCA 2b. In contrast, overexpression of SERCA 2b mutants defective in cysteines located in intralumenal loop 4 (L4) increase Ca²⁺ oscillation frequency. In vitro, we demonstrate a Ca²⁺-dependent and -specific interaction between ERp57 and L4. Interestingly, ERp57 does not affect the activity of SERCA 2a or SERCA 2b mutants lacking the CRT binding site. Overexpression of CRT domains that disrupt the interaction of CRT with ERp57 behave as dominant negatives in the Ca²⁺ oscillation assay. Our results suggest that ERp57 modulates the redox state of ER facing thiols in SERCA 2b in a Ca²⁺-dependent manner, providing dynamic control of ER Ca²⁺ homeostasis.