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Several parts of TSHR are very important for the functioning of TSH signaling. The <scene name='95/952709/Hinge_region_real/2'>hinge region</scene> is a scaffold for the attachment of the ECD to the 7TMD. Also, this region, has been found to have impact on the binding potency of TSH as well as intracellular cyclic adenosine monophosphate (cAMP) levels, which are partially mediated by the activation of the GPCR. Several features of this region have been found to be crucial to the potent activation of of the TSHR by TSH.<ref name="Mizutori et al.">Yumiko Mizutori, Chun-Rong Chen, Sandra M. McLachlan, Basil Rapoport, The Thyrotropin Receptor Hinge Region Is Not Simply a Scaffold for the Leucine-Rich Domain but Contributes to Ligand Binding and Signal Transduction, Molecular Endocrinology, Volume 22, Issue 5, 1 May 2008, Pages 1171–1182, https://doi.org/10.1210/me.2007-0407</ref>. The most important feature of the hinge region is the interaction of the <scene name='95/952709/Hinge_helix_rotation/1'>hinge helix</scene> with the <scene name='95/952709/P10_peptide_region/2'>p10 peptide</scene> through disulfides. The p10 peptide is a conserved sequence that spans from the last beta sheet of the LRRD to the first transmembrane helix (TM1)<ref name="Faust et al.">Faust, B., Billesbølle, C.B., Suomivuori, CM. et al. Autoantibody mimicry of hormone action at the thyrotropin receptor. Nature 609, 846–853 (2022). https://doi.org/10.1038/s41586-022-</ref>. These disulfides are critical as they link the ECD, where thyrotropin binds, to the TMD, whose conformational changes directly mediate the activation of the receptor's complementary G-protein. Movement of the ECD, caused by TSH binding, will cause rotation of the hinge helix and subsequent movement of the p10 peptide leading to movement of the transmembrane helices which will cause activation of the G-protein. In addition to activation, the hinge region plays an important role in tightly binding TSH. Residues 382-390 of the hinge region adopt a short helix containing Y385 and D386. Y385 is buried into a hydrophobic region of TSH while D386 forms a salt bride with R386 of the hormone. Together, <scene name='95/952709/Binding_interactions_hinge/1'>these interactions</scene> assist in facilitating the stable binding of TSH to the TSHR <ref name="Duan et al.">PMID:35940204</ref>. This being said, it is important to acknowledge, the hinge region itself is not required for the activation of the receptor. In many of the aforementioned misregulations of thyroid hormone release, auto-antibodies are responsible. These auto-antibodies bind to the receptor through alternative interactions which cause conformational changes to the 7TMD without any need for a conformational change or extensive interactions with the hinge region<ref name="Faust et al.">Faust, B., Billesbølle, C.B., Suomivuori, CM. et al. Autoantibody mimicry of hormone action at the thyrotropin receptor. Nature 609, 846–853 (2022). https://doi.org/10.1038/s41586-022-</ref>. | Several parts of TSHR are very important for the functioning of TSH signaling. The <scene name='95/952709/Hinge_region_real/2'>hinge region</scene> is a scaffold for the attachment of the ECD to the 7TMD. Also, this region, has been found to have impact on the binding potency of TSH as well as intracellular cyclic adenosine monophosphate (cAMP) levels, which are partially mediated by the activation of the GPCR. Several features of this region have been found to be crucial to the potent activation of of the TSHR by TSH.<ref name="Mizutori et al.">Yumiko Mizutori, Chun-Rong Chen, Sandra M. McLachlan, Basil Rapoport, The Thyrotropin Receptor Hinge Region Is Not Simply a Scaffold for the Leucine-Rich Domain but Contributes to Ligand Binding and Signal Transduction, Molecular Endocrinology, Volume 22, Issue 5, 1 May 2008, Pages 1171–1182, https://doi.org/10.1210/me.2007-0407</ref>. The most important feature of the hinge region is the interaction of the <scene name='95/952709/Hinge_helix_rotation/1'>hinge helix</scene> with the <scene name='95/952709/P10_peptide_region/2'>p10 peptide</scene> through disulfides. The p10 peptide is a conserved sequence that spans from the last beta sheet of the LRRD to the first transmembrane helix (TM1)<ref name="Faust et al.">Faust, B., Billesbølle, C.B., Suomivuori, CM. et al. Autoantibody mimicry of hormone action at the thyrotropin receptor. Nature 609, 846–853 (2022). https://doi.org/10.1038/s41586-022-</ref>. These disulfides are critical as they link the ECD, where thyrotropin binds, to the TMD, whose conformational changes directly mediate the activation of the receptor's complementary G-protein. Movement of the ECD, caused by TSH binding, will cause rotation of the hinge helix and subsequent movement of the p10 peptide leading to movement of the transmembrane helices which will cause activation of the G-protein. In addition to activation, the hinge region plays an important role in tightly binding TSH. Residues 382-390 of the hinge region adopt a short helix containing Y385 and D386. Y385 is buried into a hydrophobic region of TSH while D386 forms a salt bride with R386 of the hormone. Together, <scene name='95/952709/Binding_interactions_hinge/1'>these interactions</scene> assist in facilitating the stable binding of TSH to the TSHR <ref name="Duan et al.">PMID:35940204</ref>. This being said, it is important to acknowledge, the hinge region itself is not required for the activation of the receptor. In many of the aforementioned misregulations of thyroid hormone release, auto-antibodies are responsible. These auto-antibodies bind to the receptor through alternative interactions which cause conformational changes to the 7TMD without any need for a conformational change or extensive interactions with the hinge region<ref name="Faust et al.">Faust, B., Billesbølle, C.B., Suomivuori, CM. et al. Autoantibody mimicry of hormone action at the thyrotropin receptor. Nature 609, 846–853 (2022). https://doi.org/10.1038/s41586-022-</ref>. | ||
===7 Transmembrane Helices=== | ===7 Transmembrane Helices=== | ||
The thyrotropin receptor is anchored to the membrane through seven transmembrane helices which is characteristic of GPCRs. Conformational changes in this region of the receptor are responsible for the activation of associated G protein<ref name= "Keinau et al.">Kleinau, G., Worth, C. L., Kreuchwig, A., Biebermann, H., Marcinkowski, P., Scheerer, P., & Krause, G. (2017). Structural–functional features of the thyrotropin receptor: A class A G-protein-coupled receptor at work. Frontiers in Endocrinology, 8. https://doi.org/10.3389/fendo.2017.00086</ref>. In thyrotropin binding, changes to this region are mediated by movements in the afforementioned p10 peptide. When the hinge helix rotates, it causes p10 peptide displacement that allows the <scene name='95/952709/Helix_7_of_7tmd/2'>seventh helix</scene> of the transmembrane domain to migrate towards the center of the 7TMD to increase van der waals contacts. In addition, lysine 660 of TM7 forms an <scene name='95/952709/Helix_7_and__p10_interaction_2/ | The thyrotropin receptor is anchored to the membrane through seven transmembrane helices which is characteristic of GPCRs. Conformational changes in this region of the receptor are responsible for the activation of associated G protein<ref name= "Keinau et al.">Kleinau, G., Worth, C. L., Kreuchwig, A., Biebermann, H., Marcinkowski, P., Scheerer, P., & Krause, G. (2017). Structural–functional features of the thyrotropin receptor: A class A G-protein-coupled receptor at work. Frontiers in Endocrinology, 8. https://doi.org/10.3389/fendo.2017.00086</ref>. In thyrotropin binding, changes to this region are mediated by movements in the afforementioned p10 peptide. When the hinge helix rotates, it causes p10 peptide displacement that allows the <scene name='95/952709/Helix_7_of_7tmd/2'>seventh helix</scene> of the transmembrane domain to migrate towards the center of the 7TMD to increase van der waals contacts. In addition, lysine 660 of TM7 forms an <scene name='95/952709/Helix_7_and__p10_interaction_2/6'>ionic interaction</scene> with glutamate 409 of the p10 region. This interaction assists in the stabilization of the active state of 7TMD. In addition, the movement of the hinge helix has been found to bee associated with movement of a tyrosine residue relative to and isoleucine residue on the neighboring extracellular loop 1 (ECL1) helix. The identity of these residues has been found to be an important predictor in the activation of the thyrotropin receptor. For instance, structurally guided mutagenic studies have shown that the mutation of the isoleucine to a more sizeable phenylalanine decreases TSH signaling potency<ref name="Faust et al.">Faust, B., Billesbølle, C.B., Suomivuori, CM. et al. Autoantibody mimicry of hormone action at the thyrotropin receptor. Nature 609, 846–853 (2022). https://doi.org/10.1038/s41586-022-</ref><ref name="Vlaeminck-Guillem et al.">Virginie Vlaeminck-Guillem, Su-Chin Ho, Patrice Rodien, Gilbert Vassart, Sabine Costagliola, Activation of the cAMP Pathway by the TSH Receptor Involves Switching of the Ectodomain from a Tethered Inverse Agonist to an Agonist, Molecular Endocrinology, Volume 16, Issue 4, 1 April 2002, Pages 736–746, https://doi.org/10.1210/mend.16.4.0816</ref>.In addition to these conformational changes, the sixth transmembrane helix of TSHR is also moved outward from the center of the 7TMD to facilitate alpha helix 5 of the alpha subunit of the G protein, which is the domain that becomes activated<ref name="Faust et al.">Faust, B., Billesbølle, C.B., Suomivuori, CM. et al. Autoantibody mimicry of hormone action at the thyrotropin receptor. Nature 609, 846–853 (2022). https://doi.org/10.1038/s41586-022-</ref><ref name="Goricanec et al.">Goricanec, D., Stehle, R., Egloff, P., Grigoriu, S., Plückthun, A., Wagner, G., & Hagn, F. (2016). Conformational dynamics of a G-protein α subunit is tightly regulated by nucleotide binding. Proceedings of the National Academy of Sciences, 113(26). https://doi.org/10.1073/pnas.1604125113 </ref>. | ||
== Relevance == | == Relevance == | ||
<scene name='95/952709/Interactions_with_thyrotropin/1'>Glu 98 and Asp 91</scene> | <scene name='95/952709/Interactions_with_thyrotropin/1'>Glu 98 and Asp 91</scene> | ||