title: Molecular determinants for the differential coupling of Galpha(16) to the melatonin MT1, MT2 and Xenopus Mel1c receptors.
abstract: The pineal neurohormone melatonin modulates a variety of physiological processes through different receptors. It has recently been reported that the cloned melatonin receptors (MT1, MT2 and Mel1c) exhibit differential abilities to stimulate phospholipase C (PLC) via G(16). Here we examined the molecular basis of such differences in melatonin receptor signaling. Coexpression of MT1 or MT2 with the alpha subunit of G(16) (Galpha(16) ) allowed COS-7 cells to accumulate inositol phosphates in response to 2-iodomelatonin. In contrast, Mel1c did not activate Galpha(16) even though its expression was demonstrated by radioligand binding and agonist-induced inhibition of adenylyl cyclase. As Mel1c possesses an exceptionally large C-terminal tail, we further asked if this structural feature prevented productive coupling to Galpha(16). Eleven chimeric melatonin or mutant receptors were constructed by swapping all or part of the C-terminal tail between MT1, MT2 and Mel1c. All chimeras were fully capable of binding 2-[(125) I]iodomelatonin and inhibiting adenylyl cyclase. Chimeras containing the full-length Mel1c tail were incapable of activating Galpha(16), while those that contained the complete C-terminal region of either MT1 or MT2 stimulated PLC. Incorporation of the extra portion of the C-terminal tail of Mel1c to either MT1 or MT2 completely abolished the chimeras' ability to stimulate PLC via Galpha(16). In contrast, truncation of the C-terminal tail of Mel1c allowed interaction with Galpha(16). Our results suggest that Galpha(16) can discern structural differences amid the three melatonin receptors and provide evidence for functional distinction of Mel1c from MT1 and MT2 receptors.
authors.affiliation: Department of Biochemistry, the Molecular Neuroscience Center, and the Biotechnology Research Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
authors.forename: Frank P L
authors.initials: FP
authors.lastname: Lai
authors.forename: Sejal M
authors.initials: SM
authors.lastname: Mody
authors.forename: Lisa Y
authors.initials: LY
authors.lastname: Yung
authors.forename: Jason Y M
authors.initials: JY
authors.lastname: Kam
authors.forename: Celia S
authors.initials: CS
authors.lastname: Pang
authors.forename: Shiu F
authors.initials: SF
authors.lastname: Pang
authors.forename: Yung H
authors.initials: YH
authors.lastname: Wong
mesh_terms: D000067956:Adenylyl Cyclase Inhibitors; D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive / Q000502:physiology; D019556:COS Cells; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D020962:Heterotrimeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007457:Iodine Radioisotopes; D008550:Melatonin / Q000031:analogs & derivatives* / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D009842:Oligopeptides; D010455:Peptides; D011485:Protein Binding / Q000502:physiology; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D044094:Receptors, Melatonin; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D017384:Sequence Deletion; D013329:Structure-Activity Relationship; D014162:Transfection; D010738:Type C Phospholipases / Q000378:metabolism; D029867:Xenopus Proteins
chemical_list: D000067956:Adenylyl Cyclase Inhibitors; D007457:Iodine Radioisotopes; D009842:Oligopeptides; D010455:Peptides; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D044094:Receptors, Melatonin; D011993:Recombinant Fusion Proteins; D029867:Xenopus Proteins; C016832:xenopsin; C044556:2-iodomelatonin; D010738:Type C Phospholipases; C112631:G protein alpha 16; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D020962:Heterotrimeric GTP-Binding Proteins; D000262:Adenylyl Cyclases; D008550:Melatonin