title: The truncated ghrelin receptor polypeptide (GHS-R1b) is localized in the endoplasmic reticulum where it forms heterodimers with ghrelin receptors (GHS-R1a) to attenuate their cell surface expression.
abstract: The ghrelin receptor (GHS-R1a) is remarkable amongst G-protein-coupled receptors for its high degree of constitutive activity, and this agonist-independent activity may be important for its physiological function in the control of food intake and body weight. Ghrelin receptors form heterodimers with the truncated ghrelin receptor polypeptide (GHS-R1b), which has a dominant-negative effect on ghrelin receptor function. Here we show that GHS-R1b has an intracellular localization distinct from ghrelin receptors, being primarily localized in the endoplasmic reticulum. Immunocytochemical studies suggest that GHS-R1b decreases the plasma membrane expression of ghrelin receptors, but the overall distribution profile of ghrelin receptors in isolated subcellular fractions is unaffected by GHS-R1b. Using bioluminescence resonance energy transfer methods, we have shown that while ghrelin receptor homodimers are evenly distributed in all subcellular fractions, GHS-R1a/GHS-R1b heterodimers are concentrated within the endoplasmic reticulum and these results suggest that GHS-R1b traps ghrelin receptors within the endoplasmic reticulum by the process of oligomerization. Furthermore, ghrelin receptors constitutively activated extracellular signal-regulated kinases 1/2 in the endoplasmic reticulum, but this small response was not affected by GHS-R1b and its physiological relevance is uncertain. Taken together, these results suggest that ghrelin receptors can be retained in the endoplasmic reticulum by heterodimerization with GHS-R1b, and constitutive activation of phospholipase C is attenuated due to decreased cell surface expression of ghrelin receptors. However, sufficient ghrelin receptor homodimers can still be expressed on the cell surface for maximal responses to agonist stimulation.
authors: School of Biomedical Sciences, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China.
authors: Kevin B S
authors: KB
authors: Chow
authors: Jingxin
authors: J
authors: Sun
authors: Kit Man
authors: KM
authors: Chu
authors: Wing
authors: W
authors: Tai Cheung
authors: Christopher H K
authors: CH
authors: Cheng
authors: Helen
authors: H
authors: Wise
mesh_terms: D002458:Cell Fractionation; D002462:Cell Membrane / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D049410:Luciferases, Renilla / Q000378:metabolism; D010766:Phosphorylation; D055503:Protein Multimerization; D021381:Protein Transport; D054440:Receptors, Ghrelin / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism
chemical_list: D054440:Receptors, Ghrelin; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D049410:Luciferases, Renilla; D048049:Extracellular Signal-Regulated MAP Kinases