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Published on: June 2, 2023
Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges
Shumin Ma1, Xueliang Yin1, Jean-Philippe Pin2
1Cellular Signaling Laboratory, International Research Center for Sensory Biology and Technology of MOST, Key Laboratory of Molecular Biophysics of MOE, and College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan, Hubei, China.
Abstract:
G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.
Insights
Inter-subunit disulfide bridges maintain the inactive state of the calcium sensing receptor (CaSR), preventing constitutive activity. Disrupting these bridges, as seen in hypocalcemia mutations, causes significant CaSR constitutive activity.
Area of Science:
- Biochemistry
- Molecular Biology
- Endocrinology
Background:
- G protein-coupled receptors (GPCRs) exhibit dynamic conformational changes, cycling between inactive and active states.
- Constitutive activity in GPCRs can have significant physiological implications.
- Class C GPCRs, including the calcium sensing receptor (CaSR), are crucial for sensing amino acids and regulating physiological processes.
Purpose of the Study:
- To investigate the role of inter-subunit disulfide bridges in the inactive state of the calcium sensing receptor (CaSR).
- To determine the impact of these disulfide bridges on CaSR constitutive activity.
- To correlate genetic mutations affecting these bridges with clinical conditions like hypocalcemia.
Main Methods:
- Comparative analysis of wild-type CaSR and CaSR mutants lacking inter-subunit disulfide bridges.
- Assessment of CaSR constitutive activity through functional assays.
- Evaluation of the effect of amino acid binding mutations on disulfide bridge-deleted CaSR.
Main Results:
- Inter-subunit disulfide bridges are essential for maintaining the inactive state of CaSR, resulting in undetectable constitutive activity.
- Deletion of these disulfide bridges leads to significant CaSR constitutive activity.
- This constitutive activity is dependent on amino acid binding, indicating the bridges' role in limiting agonist effects.
- Human genetic mutations disrupting these bridges are associated with hypocalcemia and elevated CaSR constitutive activity.
Conclusions:
- Inter-subunit disulfide bridges are critical for stabilizing the inactive conformation of CaSR.
- Fine-tuning CaSR constitutive activity via these bridges is physiologically important for calcium homeostasis.
- Disruption of CaSR disulfide bridges provides a molecular basis for certain forms of hypocalcemia.
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