Extended and Structurally Supported Insights into Extracellular Hormone Binding, Signal Transduction and Organization of the Thyrotropin Receptor
Figure 5
Scheme of initial TSHR activation.
New functional-structural TSHR features at the extracellular site are described for the hinge region and the LRRD ain a schematic overview. Repeat 11 with the pivotal helix (green) is part of the LRRD that ends at Asn288 and therefore includes Cys283 and Cys284 of cysteine-box 2 (C-b2). These cysteines are linked by disulfide bridges to Cys398 and Cys408 of cysteine-box 3, respectively. Constitutively activating mutations at Pro280 and Ser281 (magenta) influence the conformation of this helix and underline the importance of this region for receptor function. Further positions of CAMs are located at the C-terminal part of C-b3, in close spatial proximity to Cb-2. Mutations at both fragments probably cause conformational changes at this region which induce receptor activation. Thus, these wild type amino acids constitute an activation-sensitive intramolecular agonistic unit (magenta, boxed). This unit is adjacent to TMH1 and is probably located between the ECLs as an interface between the extracellular and transmembrane regions. The transition between the extracellular region and TMH1 is made up of negatively charged amino acids, in which side chain substitutions lead to impaired signaling capacity. These are key to interactions with the serpentine domain spanning the membrane. Several residues identified as sensitive to hormone binding are enclosed in a red box (Glu297, Glu303, Asp382 and Asp386) and represent a spatial cluster that is linked by a disulfide bridge between Cys301 and Cys390. Asp382 (and Glu303) are important for super-agonistic effects of bTSH but are not important for hTSH. Sulfation at Tyr385 is obligatory for hormone binding and is accompanied by negatively charged amino acids close to Tyr385 (Asp386, Asp382). Two cleavage sites define the so-called cleavable peptide (C-peptide, ∼50 amino acids). Deletions at the transition between the C-peptide and the Cb-3 are reported to activate the TSHR (and also the FSHR) constitutively (magenta trapezoid). In contrast, single point mutations lead to partial inactivation of TSHR signaling at this region. The dashed arrows indicate the potential signal processing path for activation upon ligand binding from the hinge region and/or potentially via the LRRD. In either case, the extracellular modifications converge at the pivotal helix that links the intramolecular agonistic unit together (dashed box).