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Human δ opioid receptor biogenesis is regulated via interactions with SERCA2b and calnexin

Tuusa, Jussi T., Leskelä, Tarja T., Petäjä-Repo, Ulla E.
FEBS journal 2010 v.277 no.13 pp. 2815-2829
Ca2-transporting ATPase, G-protein coupled receptors, adenosine triphosphate, biogenesis, biosynthesis, calcium, calnexin, cytosol, endoplasmic reticulum, homeostasis, humans, molecular chaperones, mutants, protein folding, protein-protein interactions, quality control, reticulum
Sarco(endo)plasmic reticulum calcium ATPase (SERCA)2b maintains the cellular Ca²⁺ homeostasis by transferring Ca²⁺ from the cytosol to the lumen of the endoplasmic reticulum (ER). Recently, SERCA2b has also been shown to be involved in the biosynthesis of secreted and membrane proteins via direct protein-protein interactions, involving components of the ER folding and quality-control machinery, as well as newly synthesized G protein-coupled receptors. Here we demonstrate that the human delta opioid receptor (hδOR) exists in a ternary complex with SERCA2b and the ER molecular chaperone calnexin. The interaction between SERCA2b and hδOR in vivo did not require calnexin as it was independent of the C-terminal calnexin-interacting domain of SERCA2b. However, the receptor was able to mediate co-immunoprecipitation of calnexin with the C-terminally truncated SERCA2b. The association of SERCA2b with hδOR was regulated in vitro by Ca²⁺ and ATP in a manner that was opposite to the calnexin-hδOR interaction. Importantly, co-expression of the catalytically inactive SERCA2b(D351A) or calnexin binding-compromised SERCA2bΔC mutants with the receptor decreased the expression of mature receptors in a manner that did not directly relate to changes in the ER Ca²⁺ concentration. We conclude that dynamic interactions among SERCA2b, calnexin and the hδOR precursor orchestrate receptor biogenesis and are regulated by Ca²⁺ and ATP. We further hypothesize that the primary role of SERCA2b in this process is to act as a Ca²⁺ sensor in the vicinity of active translocons, integrating protein folding with local fluctuations of ER Ca²⁺ levels.