The orphan receptor GPR97 is a member of the adhesion GPCR (aGPCR) family. As one of the evolutionarily ancient families in the GPCR superfamily, aGPCRs are crucial molecular switches that regulate many physiological processes, including brain development, ion-water homeostasis, inflammation, and cell-fate determination. Compared with other GPCR families, aGPCRs are well-known for the presence of a large ectodomain that contains the GAIN domain, which functions with the seven-transmembrane (7TM) bundle as a pair, and subsequent activation of the receptor through tethered agonism or other mechanisms such as mechanical force. However, the structural basis for aGPCR activation is unclear and whether the 7TM bundle of aGPCR constitutes a typical pocket to recognize a small chemical ligand is uncertain.
On January 6th, 2021, SUN Jin-Peng from Shandong University, Dr. H. Eric XU from Shanghai Institute of Materia Medica (SIMM), Chinese Academy of Sciences, Dr. ZHANG Yan from Zhejiang University and their colleagues, reported two high-resolution cryo-electron microscopy (Cryo-EM) structures of the adhesion GPCR GPR97–Go complexes bound to the anti-inflammatory drug beclomethasone (BCM) and the glucocorticoid hormone cortisol. This research has been published on Nature, entitled Structures of the glucocorticoid-bound adhesion receptor GPR97–Go complex.
Glucocorticoids (GCs) are steroid hormones regulating development, metabolism, and immune response. When GCs bind to the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily, the activated GC-GR complex enters the nucleus and regulates gene transcription. This classic genomic regulation of GR takes hours since it needs many intermediate steps from GR activation to protein expression. Dr. H. Eric Xu has solved the crystal structures of GR bound to anti-inflammatory drug dexamethasone (DEX) and natural agonist hydrocortisone in 2002 (Cell, 110: 93-105) and 2014 (Cell Research, 24: 713–726), respectively, to reveal this classic genomic regulation mechanism. However, a rapid cell response occurs within minutes after GC stimulation, eliciting a hypothesis that membrane receptors may mediate these rapid effects instead of nuclear receptors. It has been reported that pertussis toxin (PTX), an inhibitor of inhibitory G protein Gi, can reverse the rapid effects of GCs, indicating that GPCR is one of the putative membrane receptors of GCs. The research groups of Dr. YI Fan and SUN Jin-Peng from Shandong University have clarified the physiological role of GPR97 and mining its natural ligand (J. Am. Soc. Nephrol. 2018. 29, 1475–1489). The research group found that several endogenous GCs, such as hydrocortisone, cortisol, and 11-deoxycortisol, etc., can activate GPR97, of which DEX displays the most potent activation effect. They further found that GCs inhibited cAMP accumulation and identified Go protein as the downstream transducer of GPR97.
The three research groups jointly determined the cryo-EM structures of active GPR97 in complex with the Go heterotrimer and two GCs, the anti-inflammatory drug BCM and cortisol at the resolution of 3.1 Angstrom and 2.9 Angstrom , respectively (Figure 1a and 1b).
Figure: Cryo-EM structures of the GPR97–Go complexes
These structures unexpectedly highlighted that the small steroid hormone could bind to the 7TM bundle of aGPCR for receptor activation. The GCs bound at the center of this pocket with their D rings packed against the toggle switch residue W4906.53 (Figure 1c). Furthermore, these structures revealed that GPR97 has a unique 7TM architecture with an extended ECL2 β sheet and a relatively short helix TM6. Although GPR97 has neither the traditional “PIF” core triad nor the NPxxY motif that commonly mediates class A GPCR activation, it senses ligand binding by the conserved toggle switch W6.53 and tethers TM3-TM5-TM6 by the newly identified and sequence-conserved "upper quaternary core" (UQC) for the active receptor conformation. In addition, in the aGPCR family, the conserved D/ERY motif is replaced by HLY, which mediates Go protein coupling. Unexpectedly, a palmitoylation at the C-tail of the Gαo subunit was found essential for efficient engagement with GPR97 but it is not observed in other solved GPCR complex structures. Together, this work provides a structural basis for ligand binding to the 7TM domain of an adhesion GPCR and subsequent G protein coupling.
Prof. H. Eric XU, Ph.D Principal Investigator