Structural determinants for G-protein activation and specificity in the third intracellular loop of the thyroid-stimulating hormone receptor.
The selectivity of G-protein recognition is determined by the intracellular loops (ICLs) of seven-transmembrane-spanning receptors. In a previous study, we have shown that the N-terminal and central portions of ICL2 from F525 to D530 participate in dual Galphas-/Galphaq-protein activation by the thyroid-stimulating hormone receptor (TSHR). ICL3 is another major determinant for G-protein activation. Therefore, the aim of our study was to identify important amino acids within ICL3 of the TSHR to gain insight in more detail about its specific function for Galphas- and Galphaq-protein activation and selectivity. Single-alanine substitutions of residues in the N-terminal, middle, and C-terminal region of ICL3 were generated. N-terminal residues Y605 and V608 and C-terminal positions K618, K621, and I622 were identified as selectively important for Galphaq activation, whereas mutations in the center of ICL3 had no effect on TSHR signaling. Our findings provide evidence for an amino acid pattern in the N- and C-terminal part of ICL3, which is involved in Galphaq-mediated signaling. Furthermore, molecular modeling of interaction of TSHR ICL2 and 3 with Galphaq suggests three potential contact sites: TSHR C-terminal ICL3 with beta5-6 loop of Galphaq, TSHR ICL2 residues I523-R531 with beta2-3 loop and N-terminal helix of Galphaq, and TSHR ICL2/transmembrane helix (TMH) 3+ICL3/TMH6 with C-terminal tail of Galphaq.
Claus
Maren
M
III. Medical Department, University of Leipzig, Philipp-Rosenthal-Str. 27, 04103, Leipzig, Germany.
Neumann
Susanne
S
Kleinau
Gunnar
G
Krause
Gerd
G
Paschke
Ralf
R
D000409:Alanine; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002522:Chlorocebus aethiops; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000737:chemistry / Q000378:metabolism*; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011989:Receptors, Thyrotropin / Q000819:agonists / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D013972:Thyrotropin / Q000378:metabolism* / Q000494:pharmacology; D014162:Transfection
D011989:Receptors, Thyrotropin; D013972:Thyrotropin; D000242:Cyclic AMP; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D019205:GTP-Binding Protein alpha Subunits, Gs; D000409:Alanine