Extracellular calcium alters calcium-sensing receptor network integrating intracellular calcium-signaling and related key pathway

G-protein-coupled receptors (GPCRs) are a target for over 34% of current drugs. The calcium-sensing receptor (CaSR), a family C GPCR, regulates systemic calcium (Ca(2+)) homeostasis that is critical for many physiological, calciotropical, and noncalciotropical outcomes in multiple organs. However, the mechanisms by which extracellular Ca(2+) (Ca(2+)(ex)) and the CaSR mediate networks of intracellular Ca(2+)-signaling and players involved throughout the life cycle of CaSR are largely unknown. Here we report the first CaSR protein–protein interactome with 94 novel putative and 8 previously published interactors using proteomics. Ca(2+)(ex) promotes enrichment of 66% of the identified CaSR interactors, pertaining to Ca(2+) dynamics, endocytosis, degradation, trafficking, and primarily to protein processing in the endoplasmic reticulum (ER). These enhanced ER-related processes are governed by Ca(2+)(ex)-activated CaSR which directly modulates ER-Ca(2+) (Ca(2+)(ER)), as monitored by a novel ER targeted Ca(2+)-sensor. Moreover, we validated the Ca(2+)(ex) dependent colocalizations and interactions of CaSR with ER-protein processing chaperone, 78-kDa glucose regulated protein (GRP78), and with trafficking-related protein. Live cell imaging results indicated that CaSR and vesicle-associated membrane protein-associated A (VAPA) are inter-dependent during Ca(2+)(ex) induced enhancement of near-cell membrane expression. This study significantly extends the repertoire of the CaSR interactome and reveals likely novel players and pathways of CaSR participating in Ca(2+)(ER) dynamics, agonist mediated ER-protein processing and surface expression.