Modulation of interprotomer relationships is important for activation of dimeric calcium-sensing receptor

The extracellular calcium-sensing receptor (CaR) forms a disulfide-linked dimer through cysteine residues within its N-terminal extracellular domain (ECD). However, these disulfide linkages are dispensable for the formation of the dimeric CaR and for the functional reconstitution of two inactive CaRs. In this study, using molecular modeling, mutagenesis, and biochemical and biophysical analyses, we examined the importance of two leucine residues, Leu-112 and Leu-156, in the ECD of the CaR for the non-covalent dimerization and functional reconstitution. We found that the mutant receptor carrying L112S and L156S still exists mostly as a covalently linked dimer and has a significantly higher apparent affinity for calcium than the wild-type receptor. However, a combination of four mutations, L112S, L156S, C129S, and C131S, significantly reduces receptor dimerization and markedly inactivates the CaR. We also found that L112S and L156S mediate the non-covalent intermolecular interactions important for functional reconstitution. Because mutating either the two cysteines or the two leucines enhances the apparent ligand affinity of the CaR, it is likely that the changes in intermolecular relationships between two receptor protomers linked by these leucines and cysteines are essential for receptor activation. Moreover, these mutations are unlikely to have negative effects on the secondary structure of each protomer of the dimeric receptor. Thus, the detrimental effects of the combined mutations on the function of the CaR further suggest that CaR dimerization through its ECD is essential for the formation of a functional tertiary structure of the CaR.