The first cytoplasmic loop of the thyrotropin receptor is important for phosphoinositide signaling but not for agonist‐induced adenylate cyclase activation

We investigated the role of the 1st cytoplasmic loop of the thyrotropin receptor (TSHR) on signal transduction using mutants as we did the 2nd and 3rd cytoplasmic loops [Kosugi S. et al. (1994) Mol. Endocrinol., in press; and 7, 1009-1020]. Five substitution mutants involving the first cytoplasmic loop showed a TSH- or Graves' IgG-stimulated cAMP response despite the low TSH binding Bmax. All the mutants completely lost or markedly decreased the TSH- or Graves' IgG-stimulated inositol phosphate increase. These findings suggest that the 1st cytoplasmic loop of the TSHR does not play a crucial role in agonist-induced adenylate cyclase activation but that it is important for phosphoinositide signaling.