Three members of the ghrelin receptor family were characterized in parallel: the ghrelin receptor, the neurotensin receptor 2 and the orphan receptor GPR39. In transiently transfected COS-7 and human embryonic kidney 293 cells, all three receptors displayed a high degree of ligand-independent signaling activity. The structurally homologous motilin receptor served as a constitutively silent control; upon agonist stimulation, however, it signaled with a similar efficacy to the three related receptors. The constitutive activity of the ghrelin receptor and of neurotensin receptor 2 through the G(q), phospholipase C pathway was approximately 50% of their maximal capacity as determined through inositol phosphate accumulation. These two receptors also showed very high constitutive activity in activation of cAMP response element-driven transcription. GPR39 displayed a clear but lower degree of constitutive activity through the inositol phosphate and cAMP response element pathways. In contrast, GPR39 signaled with the highest constitutive activity in respect of activation of serum response element-dependent transcription, in part, possibly, through G(12/13) and Rho kinase. Antibody feeding experiments demonstrated that the epitope-tagged ghrelin receptor was constitutively internalized but could be trapped at the cell surface by an inverse agonist, whereas GPR39 remained at the cell surface. Mutational analysis showed that the constitutive activity of both the ghrelin receptor and GPR39 could systematically be tuned up and down depending on the size and hydrophobicity of the side chain in position VI:16 in the context of an aromatic residue at VII:09 and a large hydrophobic residue at VII:06. It is concluded that the three ghrelin-like receptors display an unusually high degree of constitutive activity, the structural basis for which is determined by an aromatic cluster on the inner face of the extracellular ends of TMs VI and VII.
Keywords: Amino Acid Sequence; Animals; COS Cells; Cell Line; Cyclic AMP; DNA Mutational Analysis; DNA, Complementary; Dose-Response Relationship, Drug; Enzyme-Linked Immunosorbent Assay; GTP-Binding Protein alpha Subunits, G12-G13; Humans; Inositol Phosphates; Ligands; MAP Kinase Signaling System; Microscopy; Models, Molecular; Molecular Sequence Data; Phosphatidylinositols; Phylogeny; Protein Conformation; Protein Structure, Secondary; Protein Structure, Tertiary; Receptors, G-Protein-Coupled; Receptors, Gastrointestinal Hormone; Receptors, Ghrelin; Receptors, Neuropeptide; Receptors, Neurotensin; Signal Transduction; Transcription, Genetic; Transfection; Type C Phospholipases