To understand the functional significance of the carboxy-terminal half of the intracellular region of the human TSH receptor (hTSHR), a mutant hTSHR lacking amino acids from the carboxyterminal to His726 was constructed.
Wild type hTSHR cDNA and truncated hTSHR cDNA were subcloned into a eukaryotic expression vector, pRc/CMV, and transfected into Chinese hamster ovary cells to obtain cell lines which stably expressed hTSHRs at high levels. This allowed us to observe highly efficient coupling of hTSHR and adenylyl cyclase as well as desensitization and internalization of hTSHR.
Despite the differences in potential phosphorylation sites and internalization signals, dose-dependent stimulation of adenylyl cyclase by TSH, TSH-dependent desensitization and the rate of hTSHR internalization were similar for wild type and truncated hTSHRs. We conclude that the carboxy-terminal half of the intracellular region of hTSHR does not have a major functional role in TSH-dependent signal transduction.