Structural basis of tethered agonism of the adhesion GPCRs ADGRD1 and ADGRF1

Abstract

Adhesion G protein-coupled receptors (aGPCRs) are essential for a variety of physiological processes such as immune responses, organ development, cellular communication, proliferation and homeostasis1–7. An intrinsic manner of activation that involves a tethered agonist in the N-terminal region of the receptor has been proposed for the aGPCRs8,9, but its molecular mechanism remains elusive. Here we report the G protein-bound structures of ADGRD1 and ADGRF1, which exhibit many unique features with regard to the tethered agonism. The stalk region that proceeds the first transmembrane helix acts as the tethered agonist by forming extensive interactions with the transmembrane domain; these interactions are mostly conserved in ADGRD1 and ADGRF1, suggesting that a common stalk–transmembrane domain interaction pattern is shared by members of the aGPCR family. A similar stalk binding mode is observed in the structure of autoproteolysis-deficient ADGRF1, supporting a cleavage-independent manner of receptor activation. The stalk-induced activation is facilitated by a cascade of inter-helix interaction cores that are conserved in positions but show sequence variability in these two aGPCRs. Furthermore, the intracellular region of ADGRF1 contains a specific lipid-binding site, which proves to be functionally important and may serve as the recognition site for the previously discovered endogenous ADGRF1 ligand synaptamide. These findings highlight the diversity and complexity of the signal transduction mechanisms of the aGPCRs.

Stachel-mediated activation of adhesion G protein-coupled receptors: insights from cryo-EM studies

“… 1 However, the same effect was observed in a cleavage-deficient mutant, 2 indicating that these mutations promote the activation state even without NTF removal. Support for the second hypothesis comes from a cryo-EM structure solved by Qu et al 3 Although the NTF of full-length structures is not solved most likely due to high flexibility, analysis of a cleavage-deficient full-length ADGRF1/GPR110 showed the Stachel sequence already in the 7TMD binding pocket as found in the CTF structures. 3 Such a prebound tethered agonist isomerising upon extracellular ligand binding was identified as the activation mechanism for another group of GPCRs with large N termini - the glycoprotein hormone receptors.…”

“…Support for the second hypothesis comes from a cryo-EM structure solved by Qu et al 3 Although the NTF of full-length structures is not solved most likely due to high flexibility, analysis of a cleavage-deficient full-length ADGRF1/GPR110 showed the Stachel sequence already in the 7TMD binding pocket as found in the CTF structures. 3 Such a prebound tethered agonist isomerising upon extracellular ligand binding was identified as the activation mechanism for another group of GPCRs with large N termini - the glycoprotein hormone receptors. To decide which of these two models – or perhaps both – represents the actual mode of activation will require high-resolution structures of full-length aGPCRs in their inactive and active stages.…”

“…Recently, a set of cryo-electron microscopy (cryo-EM) structures of different adhesion G protein-coupled receptors (aGPCRs) has been published by Xiao et al 1 , Ping et al 2 , Qu et al 3 , and Barros-Álvarez et al 4 shedding light on the activation of the seven-helix transmembrane domain (7TMD) via their tethered peptide agonist.…”

“…Most interestingly, the cryo-EM structure of a cleavage-deficient full-length construct of ADGRF1/GPR110 showed the Stachel sequence already positioned at the 7TMD binding pocket challenging the current model of NTF removal for receptor activation. 3 Indeed, several studies have shown that GPS cleavage is not required for aGPCR function in vitro and in vivo. Furthermore, the new cryo-EM structures of aGPCRs provide characterisations and comparisons of the 7TMD-G protein contact interfaces, thereby offering molecular insights into G-protein selectivity.…”

Thwarting of Lphn3 Functions in Cell Motility and Signaling by Cancer-Related GAIN Domain Somatic Mutations

“…A cleavage motif known as the GPCR Proteolysis Site (GPS) is embedded into subdomain B before the last β-strand, generating two protomers that remain non-covalently bound to each other during their trafficking from the endoplasmic reticulum to the cell-membrane [ 15 , 16 ]. The last membrane-proximal β-strand acts as a tethered agonist by inserting itself into the GPCR region, whether cleavage at the GPS occurs or not, thus suggesting that the GAIN domain might bear crucial activation determinants [ 17 , 18 , 19 , 20 ].…”