Tuusa Jussi T, Markkanen Piia M H, Apaja Pirjo M, Hakalahti Anna E, Petäjä-Repo Ulla E
Biocenter Oulu and Department of Anatomy and Cell Biology, University of Oulu, P.O.Box 5000, FI-90014, Oulu, Finland.
J Mol Biol. 2007 Aug 17;371(3):622-38. doi: 10.1016/j.jmb.2007.02.108. Epub 2007 Mar 15.
Calcium (Ca(2+)) 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 Ca(2+) ATPase 2b (SERCA2b), which maintains high Ca(2+) concentration in the lumen of the endoplasmic reticulum, interacts specifically with the human delta 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 delta 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 Ca(2+) homeostasis was found to be essential for delta opioid receptor biogenesis, as inhibition of its Ca(2+) pumping activity by thapsigargin reduced the interaction and impaired the efficiency of receptor maturation, two phenomena that were not affected by a Ca(2+) 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.
钙（Ca(2+)）在细胞信号传导和蛋白质合成中都起着关键作用。然而，钙代谢与分泌型和膜结合型蛋白质合成之间的关系尚未完全清楚。在这里，我们证明了肌浆网/内质网Ca(2+) ATP酶2b（SERCA2b），它能维持内质网腔中的高Ca(2+)浓度，在人胚肾293细胞中，在受体生物合成的早期步骤中与人δ阿片受体特异性相互作用。这种相互作用涉及新合成的未完全折叠的受体前体，因为δ阿片受体与SERCA2b之间的关联（i）是短暂的，且在受体翻译后不久就发生了，（ii）不受受体错误折叠的影响，并且（iii）如果通过阿片受体药理学伴侣增强受体折叠，则这种关联会减少。发现与SERCA2b的物理关联是A类G蛋白偶联受体中的一个普遍特征，并且还显示在大鼠卵巢中内源性表达的促黄体生成素受体与SERCA2b之间也会发生这种关联。重要的是，发现活性SERCA2b而非未受干扰的Ca(2+)稳态对于δ阿片受体生物合成至关重要，因为毒胡萝卜素对其Ca(2+)泵浦活性的抑制会减少相互作用并损害受体成熟效率，这两种现象不受Ca(2+)离子载体A23187的影响。然而，对SERCA2b的抑制并不损害能够成熟的受体的功能。因此，我们提出与SERCA2b的关联是G蛋白偶联受体有效折叠和/或膜整合所必需的。
