Rescue of Calcium-sensing Receptor Mutants by Allosteric Modulators Reveals a Conformational Checkpoint in Receptor Biogenesis

• Published in: The Journal of biological chemistry Vol. 282; no. 13; pp. 9517 - 9525
• Main Authors: Huang, Ying, Breitwieser, Gerda E.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 30.03.2007; American Society for Biochemistry and Molecular Biology
• Subjects: Allosteric Regulation - genetics; Allosteric Regulation - physiology; Cell Line; Humans; Mutagenesis, Site-Directed; Point Mutation; Proteasome Endopeptidase Complex - metabolism; Proteasome Endopeptidase Complex - physiology; Protein Conformation; Protein Folding; Receptors, Calcium-Sensing - biosynthesis; Receptors, Calcium-Sensing - genetics
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M609045200

The calcium-sensing receptor (CaR), a member of G protein-coupled receptor family C, regulates systemic calcium homeostasis by activating Gq- and Gi-linked signaling in the parathyroid, kidney, and intestine. CaR is ubiquitinated by the E3 ligase dorfin and degraded via the endoplasmic reticulum-associated degradation pathway (Huang, Y., Niwa, J., Sobue, G., and Breitwieser, G. E. (2006) J. Biol. Chem. 281, 11610–11617). Here we provide evidence for a conformational or functional checkpoint in CaR biogenesis using two complementary approaches. First we characterized the sensitivity of loss- or gain-of-function CaR mutants to proteasome inhibition by MG132. The stabilization of loss-of-function mutants and insensitivity of gain-of-function mutants to MG132 suggests that receptor sensitivity to calcium influences susceptibility to proteasomal degradation. Second, we used the allosteric activator NPS R-568 and antagonist NPS 2143 to promote the active and inactive conformations of wild type CaR, respectively. Overnight culture in NPS R-568 increased expression of CaR, whereas NPS 2143 had the opposite effect. NPS R-568 and NPS 2143 differentially regulated maturation and cell surface expression of wild type CaR, directly affecting maximal signaling responses. NPS R-568 rescued expression of loss-of-function CaR mutants, increasing plasma membrane expression and ERK1/2 phosphorylation in response to 5 mm Ca2+. Disorders of calcium homeostasis caused by CaR mutations may therefore result from altered receptor biogenesis independent of receptor function, i.e. a protein folding disorder. The allosteric modulators NPS R-568 and NPS 2143 not only alter CaR sensitivity to calcium and hence signaling but also modulate receptor expression.