FSH and TSH binding to their respective receptors: similarities, differences and implication for glycoprotein hormone specificity

Abstract

The crystal structures of the leucine-rich repeat domain (LRD) of the FSH receptor (FSHR) in complex with FSH and the TSH receptor (TSHR) LRD in complex with the thyroid-stimulating autoantibody (M22) provide opportunities to assess the molecular basis of the specificity of glycoprotein hormone-receptor binding. A comparative model of the TSH-TSHR complex was built using the two solved crystal structures and verified using studies on receptor affinity and activation. Analysis of the FSH-FSHR and TSH-TSHR complexes allowed identification of receptor residues that may be important in hormone-binding specificity. These residues are in leucine-rich repeats at the two ends of the FSHR and the TSHR LRD structures but not in their central repeats. Interactions in the interfaces are consistent with a higher FSH-binding affinity for the FSHR compared with the binding affinity of TSH for the TSHR. The higher binding affinity of porcine (p)TSH and bovine (b)TSH for human (h)TSHR compared with hTSH appears not to be dependent on interactions with the TSHR LRD as none of the residues that differ among hTSH, pTSH or bTSH interact with the LRD. This suggests that TSHs are likely to interact with other parts of the receptors in addition to the LRD with these non-LRD interactions being responsible for affinity differences. Analysis of interactions in the FSH-FSHR and TSH-TSHR complexes suggests that the a-chains of both hormones tend to be involved in the receptor activation process while the b-chains are more involved in defining binding specificity.

Functional Differences of Invariant and Highly Conserved Residues in the Extracellular Domain of the Glycoprotein Hormone Receptors

“…However, these hormones share the same ␣-subunit, suggesting that this subunit mainly contributes to receptor activation (58). The results of this study revealed that some of the identical or highly conserved amino acid residues, present in different ␤-strands of the LRRs of the GpHRs, do not have the same functions in the three receptors.…”

“…Our models of the CG⅐LHR and TSH⅐TSHR ECD complexes (Fig. 7) (58). Thus, disruption of the ionic bridges in the LHR ECD impacts more adversely on signaling than similar changes in the other two GpHRs.…”

Similarities and differences in interactions of thyroid stimulating and blocking autoantibodies with the TSH receptor

“…3). In the predicted binding arrangements, K1-18 interacts across the whole concave surface of the TSHR LRD including the C-terminal part similar to M22 and TSH binding (Sanders et al 2007a, Nú ñez Miguel et al 2008. (Fig.…”

“…The predicted binding arrangements were validated by the effects of the TSHR amino acid mutations on the biological activity of the MAbs, and the binding arrangements of the TSHR LRD with two stimulating (M22 and K1-18) and two blocking MAbs (K1-70 and RSR-B2) were compared. The limitation of this study is that it is based on a combination of evidence from high-resolution crystal structures (Sanders et al 2007a(Sanders et al , 2011), a comparative model (Nú ñez Miguel et al 2008), mutational analysis and more robust chargecharge interaction mapping. However, these studies showed that the binding sites for the TSHR antibodies with different biological activities overlap on the concave surface of the TSHR; however, stimulating antibodies show interactions with both the N-and C-terminus of the TSHR LRD while the blocking antibodies do not interact with the C-terminus.…”

“…9 Å ; Sanders et al 2011) provided molecular details of the interactions of M22 and K1-70 with the receptor. To study the interactions of TSH with the TSHR, a comparative model of a TSH-TSHR LRD complex has been produced using the crystal structures of M22-TSHR LRD (Sanders et al 2007a) and FSH-FSH receptor (FSHR;Fan & Hendrickson 2005) complexes to model the binding arrangements between TSH and the TSHR (Nú ñez Miguel et al 2008).…”