A low molecular weight agonist signals by binding to the transmembrane domain of thyroid-stimulating hormone receptor (TSHR) and luteinizing hormone/chorionic gonadotropin receptor (LHCGR).
Many cognate low molecular weight (LMW) agonists bind to seven transmembrane-spanning receptors within their transmembrane helices (TMHs). The thienopyrimidine org41841 was identified previously as an agonist for the luteinizing hormone/chorionic gonadotropin receptor (LHCGR) and suggested to bind within its TMHs because it did not compete for LH binding to the LHCGR ectodomain. Because of its high homology with LHCGR, we predicted that thyroid-stimulating hormone receptor (TSHR) might be activated by org41841 also. We show that org41841 is a partial agonist for TSHR but with lower potency than for LHCGR. Analysis of three-dimensional molecular models of TSHR and LHCGR predicted a binding pocket for org41841 in common clefts between TMHs 3, 4, 5, 6, and 7 and extracellular loop 2 in both receptors. Evidence for this binding pocket was obtained in signaling studies with chimeric receptors that exhibited improved responses to org41841. Furthermore, a key receptor-ligand interaction between the highly conserved negatively charged E3.37 and the amino group of org41841 predicted by docking of the ligand into the three-dimensional TSHR model was experimentally confirmed. These findings provide the first evidence that, in contrast to the ectodomain binding of cognate ligands, a LMW agonist can bind to and activate glycoprotein hormone receptors via interaction with their transmembrane domain.
Clinical Endocrinology Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, 50 South Drive, Bethesda, MD 20892, USA.
Holger
H
Jäschke
Susanne
S
Neumann
Susanna
S
Moore
Craig J
CJ
Thomas
Anny-Odile
AO
Colson
Stefano
S
Costanzi
Gunnar
G
Kleinau
Jian-Kang
JK
Jiang
Ralf
R
Paschke
Bruce M
BM
Raaka
Gerd
G
Krause
Marvin C
MC
Gershengorn
D000595:Amino Acid Sequence; D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism; D005822:Genetic Vectors; D006801:Humans; D006860:Hydrogen Bonding; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011743:Pyrimidines / Q000737:chemistry / Q000494:pharmacology*; D011974:Receptors, LH / Q000737:chemistry*; D011989:Receptors, Thyrotropin / Q000737:chemistry*; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D015398:Signal Transduction; D013876:Thiophenes / Q000737:chemistry / Q000494:pharmacology*; D014162:Transfection
D008024:Ligands; C510603:Org 41841; D011743:Pyrimidines; D011974:Receptors, LH; D011989:Receptors, Thyrotropin; D011993:Recombinant Fusion Proteins; D013876:Thiophenes; C476003:thienopyrimidine; D000242:Cyclic AMP