title: Evidence for cooperative signal triggering at the extracellular loops of the TSH receptor.
abstract: The mechanisms governing transition of the thyroid stimulating hormone (TSH) receptor (TSHR) from basal to active conformations are poorly understood. Considering that constitutively activating mutations (CAMs) and inactivating mutations in each of the extracellular loops (ECLs) trigger only partial TSHR activation or inactivation, respectively, we hypothesized that full signaling occurs via multiple extracellular signal propagation events. Therefore, individual CAMs in the extracellular region were combined to create double and triple mutants. In support of our hypothesis, combinations of mutants in the ECLs are in some cases additive, while in others they are even synergistic, with triple mutant I486A/I568V/V656F exhibiting a 70-fold increase in TSH-independent signaling. The proximity but likely different spatial orientation of the residues of activating and inactivating mutations in each ECL supports a dual functionality to facilitate signal induction and conduction, respectively. This is the first report for G-protein coupled receptors, suggesting that multiple and cooperative signal propagating events at all three ECLs are required for full receptor activation. Our findings provide new insights concerning molecular signal transmission from extracellular domains toward the transmembrane helix bundle of the glycoprotein hormone receptors.
authors.affiliation: Leibniz-Institut für Molekulare Pharmakologie, Robert-Rössle-Str.10, D-13125 Berlin, Germany.
authors.forename: Gunnar
authors.initials: G
authors.lastname: Kleinau
authors.forename: Holger
authors.initials: H
authors.lastname: Jaeschke
authors.forename: Sandra
authors.initials: S
authors.lastname: Mueller
authors.forename: Bruce M
authors.initials: BM
authors.lastname: Raaka
authors.forename: Susanne
authors.initials: S
authors.lastname: Neumann
authors.forename: Ralf
authors.initials: R
authors.lastname: Paschke
authors.forename: Gerd
authors.initials: G
authors.lastname: Krause
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D019556:COS Cells; D002522:Chlorocebus aethiops; D000242:Cyclic AMP / Q000096:biosynthesis; D006801:Humans; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011989:Receptors, Thyrotropin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D011989:Receptors, Thyrotropin; D011994:Recombinant Proteins; D000242:Cyclic AMP