is characterized by having an unusual long amino-terminal domain (590 amino acids), which includes a Venus flytrap domain and a cysteine-rich domain containing nine conserved cysteine residues, and a 7-transmembrane (7TM) domain containing three extra-and three intracellular loops and an intracellular C-terminal ( Figure 1). 1 The receptor forms homodimers through a large hydrophobic interface and a disulphide bridge in the Venus flytrap domain, and the receptor is N-glycosylated at seven extracellular asparagine residues, which modulates surface expression and function. 2 The amino acids are highlighted in the snakeplot in Figure 1. The receptor is constitutively internalized and suggested to be recycled via the Rab11-slow recycling pathway to ensure a steady pool of receptors in the plasma membrane despite a chronic exposure to the omnipresent L-amino acids and divalent cation agonists. 3
AbstractThe G protein-coupled receptor GPRC6A (GPCR, Class C, group 6, subtype A) is a G q/11 -coupled receptor widely expressed in human and rodent tissues. The proposed endogenous ligands are L-amino acids, divalent cations, osteocalcin and testosterone. This MiniReview provides an updated overview of the literature including the latest in vitro and in vivo studies. GPRC6A forms homodimers, it undergoes constitutive internalization, and very interestingly, the reason for the intracellular retention of the human receptor has been revealed. Multiple physiological functions of GPRC6A have been suggested based on studies using three different global GPRC6A knockout (KO) mouse models where exon II, exon VI or the full locus has been deleted. The newest studies on the full locus GPRC6A KO model show intact glucose and bone homoeostasis with a minor reduction in serum osteocalcin levels. Unfortunately, the physiological function of the receptor remains elusive due to a general lack of consensus/validation of reported phenotypes of the different KO models, and more research is thus warranted to uncover the physiological function. Recent discoveries of human genetic variants that cause either a premature stop codon or an intracellular retention of the receptor point towards human population studies as the preferred approach to continue studies on the function of GPRC6A. 78 | JØRGENSEN aNd BRÄUNER-OSBORNE How to cite this article: Jørgensen CV, Bräuner-Osborne H. Pharmacology and physiological function of the orphan GPRC6A receptor.