The hinge region of glycoprotein hormone receptors is influenced by its adjacent domains particularly the transmembrane domain

The hinge region (HinR) as part of the extracellular domain (ECD) is a variable structure of glycoprotein hormone receptors and connects the hormone binding leucine-rich repeat domain (LRRD) with the transmembrane domain (TMD). The structure of the FSHR HinR has been largely resolved, showing how the conserved sulfotyrosine interacts with the ligand. However, the central question how extracellular ligand-binding triggers the conformational changes in the TMD for G-protein activation remains unanswered due to missing information about the interplay between the ECD and TMD. To gain further insights into the interplay of the three receptor components (LRRD, HinR and TMD) we generated chimeras between the TSHR and FSHR and explored their functionality in terms of cell surface expression (CSE), Gs signaling and ligand-binding by transient expression in COS-7 cells. Results show that the HinRs of GPHRs are not completely interchangeable. The FSHR TMD is incompatible with the TSHR HinR regarding folding and CSE, which is contrary to TSHR TMD and the FSHR HinR. Differences in charge distributions at the C-terminal part of the LRR domain of FSHR and TSHR effect ligand-binding and signaling. In conclusion, the HinR of the respective GPHR is highly influenced by adjacent domains especially the TMD. In particular combination of the TSHR HinR and FSHR TMD results in impaired CSE, which is most likely caused by missing specific interactions or structural clashes between these particular structures. Small changes regarding charge distribution in the HinR can contribute to different signaling profiles and/or binding modes. TSH analogs are more sensitive to changes in the HinR and have stronger interactions with the C-terminus of the HinR whereas FSH analogs are much less affected by the same changes. The HinRs of GPHRs do not only share similar characteristics but also behave as ligand specific structural and functional entities.