Calcium sensing receptor mutations implicated in pancreatitis and idiopathic epilepsy syndrome disrupt an arginine-rich retention motif.

[familial hypocalciuric hypercalcemia]

Calcium sensing receptor (CaSR ) mutations implicated in familial hypocalciuric hypercalcemia , pancreatitis and idiopathic epilepsy syndrome map to an extended arginine-rich region in the proximal carboxyl terminus . Arginine-rich motifs mediate endoplasmic reticulum retention and /or retrieval of multisubunit proteins so we asked whether these mutations , R 886 P , R 896 H or R 898 Q , altered CaSR targeting to the plasma membrane . Targeting was enhanced by all three mutations , and Ca (2 +)-stimulated ERK 1 /2 phosphorylation was increased for R 896 H and R 898 Q . To define the role of the extended arginine-rich region in CaSR trafficking , we independently determined the contributions of R 890 /R 891 and /or R 896 /K 897 /R 898 motifs by mutation to alanine . Disruption of the motif (s ) significantly increased surface expression and function relative to wt CaSR . The arginine-rich region is flanked by phosphorylation sites at S 892 (protein kinase C ) and S 899 (protein kinase A ). The phosphorylation state of S 899 regulated recognition of the arginine-rich region ; S 899 D showed increased surface localization . CaSR assembles in the endoplasmic reticulum as a covalent disulfide-linked dimer and we determined whether retention requires the presence of arginine-rich regions in both subunits . A single arginine-rich region within the dimer was sufficient to confer intracellular retention comparable to wt CaSR . We have identified an extended arginine-rich region in the proximal carboxyl terminus of CaSR (residues R 890 - R 898 ) which fosters intracellular retention of CaSR and is regulated by phosphorylation . Mutation (s ) identified in chronic pancreatitis and idiopathic epilepsy syndrome therefore increase plasma membrane targeting of CaSR , likely contributing to the altered Ca (2 +) signaling characteristic of these diseases .