Among the different families of G-protein-coupled receptors (GPCRs), adhesion GPCRs (aGPCRs) represent the second most abundant in humans. Structurally they characterize by a long extracellular region of adhesion-like domains which modulate protein-protein interactions; and also by the presence of the GPCR-Autoproteolysis INducing (GAIN) domain located upstream of the first transmembrane pass. As if its structure were not already complex enough, during their synthesis in the endoplasmic reticulum, many of these receptors are cleaved at the GPCR Proteolysis Site motif of the GAIN domain through an auto-catalysis process generating two peptides that are held together by non-covalent bonds during their transport to the membrane1. When I started studying aGPCRs, the structural conformation of the GAIN domain of ADGRL1/Lphn1 and ADGRB3/BAI3 was already known. These crystal structures showed how the Stalk region, which is a short peptide released from the GAIN during auto-proteolysis, has a β-lamin conformation and is held within the GAIN surrounded by numerous hydrophobic interactions1. However, at that time the structural conformation of the transmembrane (TM) region was not yet known.
Fortunately, the scientific community has become increasingly interested in studying these proteins and this year the transmembrane structures of the aGPCRs ADGRL3/Lphn3, ADGRG1/GPR56, ADGRG5/GPR114, ADGRD1/GPR133, ADGRF1/GPR110, ADGRG2/GPR64, and ADGRG4/GPR112 were reported in their self-activating state, i.e. a unique activation model of aGPCRs where in the absence of the extracellular region the Stalk peptide functioning as an agonist (or also known as a tethered agonist) and adopts a hooked alpha-helix conformation within the transmembrane domains leading to receptor activation. The resolution of these Cryo-EM structures provided the basis for the mechanism of self-activation of aGPCRs supporting the encrypted ligand hypothesis that was put forward by the community before these structures were known2-6.
As occurs with other GPCRs in an active state, in aGPCRs the rearrangements induced by the interaction of the Stalk sequence with the transmembrane regions promote their coupling with G-proteins. Different studies have shown that aGPCRs such as Lphns have promiscuous behavior, where in a constitutive state Lphns signal through different types of G proteins, so knowing the factors that determine the selectivity of coupling represents a very interesting area of research.
Last month a very detailed paper analyzed the molecular mechanisms of the aGPCRs self-activation as well the selectivity of G-protein coupling using a mouse ADGRL3 receptor without extracellular region as a study model7. This paper proposes a series of molecular mechanisms that would be occurring during the self-activation of aGPCRs, some of which differ from those of GPCRs belonging other families, and I will tell you about some of their findings below.
Through cell-based assays and Cryo-EM of high quality, it was possible to know that ADGRL3 can activate and form stable complexes with Gs, Gi, Gq, and G12, where like other GPCRs, the distal αH5 region of the G protein was needed to maintain an interface with the receptor core.
Similar to Barros's 2022 report, which describes the Cryo-EM structure of human ADGRL3-G13, this paper reports that the stalk peptide of ADGRL3 adopts a hook conformation when binds to the binding pocket formed by TM1-3,5-7 and extracellular loop (ECL) 1,2,3. Furthermore, when an alignment analysis of the Stalk peptide of the aGPCR family was performed highlighted a new hydrophobic conserved motif composed of phenylalanine (F)/leucine (L) and methionine (M) which adopted a similar conformation in the ligand binding pocket and helps to stabilize the tethered ligand-receptor.
Comparison between a predicted model of inactive receptor structure and self-activated Cryo-EM highlighted that outward movement of TM6 (a signature of GPCR activation), sharp bending of TM6, and tilting of TM7 and TM1 are characteristic of ADGRL3 self-activation. Interestingly, it was also reported that rearrangements of the TMs lead to a group of five hydrophobic amino acids present in TM1, 3, 5 and 7 forming a horizontal plane that helps to stabilize the self-active conformation of ADGRL3.
Finally a comparative close analysis of the different Cryo-EM receptor structures with Gq, Gs, Gi and G12 showed that Gq/Gs have a similar mode of receptor binding, while Gi/G12 use a different engagement mechanism. Overall the count of interactions between the last eight αH5 residues of each G protein with the receptor showed that there are more polar interactions in Gq/Gs engagements than in G1/G12 engagements; where Gi had the fewest hydrophobic and polar interactions with the receptor and the αH5 tilt of G12 towards TM3/TM5 of the receptor is more pronounced compared to the rest of the G proteins. From a structural perspective, the -4 position of αH5 was key for the selectivity of G-protein coupling, since the change of amino acids in positions close to this position favored signaling toward a specific G-protein, which is interesting in biased signaling purposes.
In view of the above, these new findings clearly demonstrate part of the molecular mechanisms involved in the self-activation of aGPCRs and open new perspectives for the community to formulate strategies to help modulate the activation and signaling of these receptors, particularly with a pharmacological approach. Indeed, despite their broad physiological importance in normal and pathological processes, so far no drugs have been approved that target any aGPCR.
Check the original article at https://pubmed.ncbi.nlm.nih.gov/36309016/
1. Araç, D., et al., A novel evolutionarily conserved domain of cell-adhesion GPCRs mediates autoproteolysis. Embo j, 2012. 31(6): p. 1364-78.
2. Barros-Álvarez, X., et al., The tethered peptide activation mechanism of adhesion
GPCRs. Nature, 2022.
3. Ping, Y.-Q., et al., Structural basis for the tethered peptide activation of adhesion GPCRs. Nature, 2022.
4. Qu, X., et al., Structural basis of tethered agonism of the adhesion GPCRs ADGRD1 and ADGRF1. Nature, 2022.
5. Xiao, P., et al., Tethered peptide activation mechanism of the adhesion GPCRs
ADGRG2 and ADGRG4. Nature, 2022.
6. Boucard, A.A., Self-activated adhesion receptor proteins visualized. Nature, 2022.
604(7907): p. 628-630.
7. Qian, Y., Ma, Z., Liu, C., Li, X., Zhu, X., Wang, N., Xu, Z., Xia, R., Liang, J., Duan, Y., Yin, H., Xiong, Y., Zhang, A., Guo, C., Chen, Z., Huang, Z., & He, Y. (2022). Structural insights into adhesion GPCR ADGRL3 activation and Gq, Gs, Gi, and G12 coupling. Molecular cell, 82(22), 4340–4352.e6.