The Endoplasmic Reticulum Ca²⁺-pump SERCA2b Interacts with G Protein-coupled Receptors and Enhances their Expression at the Cell Surface

Calcium (Ca²⁺) plays a pivotal role in both cellular signaling and protein synthesis. However, it is not well understood how calcium metabolism and synthesis of secreted and membrane-bound proteins are related. Here we demonstrate that the sarco(endo)plasmic reticulum Ca²⁺ ATPase 2b (SERCA2b), which maintains high Ca²⁺ concentration in the lumen of the endoplasmic reticulum, interacts specifically with the human δ opioid receptor during early steps of receptor biogenesis in human embryonic kidney 293 cells. The interaction involves newly synthesized incompletely folded receptor precursors, because the association between the δ opioid receptor and SERCA2b (i) was short-lived and took place soon after receptor translation, (ii) was not affected by misfolding of the receptor, and (iii) decreased if receptor folding was enhanced by opioid receptor pharmacological chaperone. The physical association with SERCA2b was found to be a universal feature among G protein-coupled receptors within family A and was shown to occur also between the endogenously expressed luteinizing hormone receptor and SERCA2b in rat ovaries. Importantly, active SERCA2b rather than undisturbed Ca²⁺ homeostasis was found to be essential for δ opioid receptor biogenesis, as inhibition of its Ca²⁺ pumping activity by thapsigargin reduced the interaction and impaired the efficiency of receptor maturation, two phenomena that were not affected by a Ca²⁺ ionophore A23187. Nevertheless, inhibition of SERCA2b did not compromise the functionality of receptors that were able to mature. Thus, we propose that the association with SERCA2b is required for efficient folding and/or membrane integration of G protein-coupled receptors.