Structural insights into differences in G protein activation by family A and family B GPCRs

Abstract

Family B heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCRs) play important roles in carbohydrate metabolism. Recent structures of family B GPCR-Gs protein complexes reveal a disruption in the α-helix of transmembrane segment 6 (TM6) not observed in family A GPCRs. To investigate the functional impact of this structural difference, we compared the structure and function of the glucagon receptor (GCGR; family B) with the β2 adrenergic receptor (β2AR; family A). We determined the structure of the GCGR-Gs complex by means of cryo–electron microscopy at 3.1-angstrom resolution. This structure shows the distinct break in TM6. Guanosine triphosphate (GTP) turnover, guanosine diphosphate release, GTP binding, and G protein dissociation studies revealed much slower rates for G protein activation by the GCGR compared with the β2AR. Fluorescence and double electron-electron resonance studies suggest that this difference is due to the inability of agonist alone to induce a detectable outward movement of the cytoplasmic end of TM6.

Dynamics of GLP-1R peptide agonist engagement are correlated with kinetics of G protein activation

“…With the exception of His1, the largest difference in the interaction of GLP-1 and exendin-4 occurred within the mid region of the peptides where their sequences differ substantially (E 16 EAVRL 21 for exendin-4 vs G 22 QAAKE 27 for GLP-1). While the mid regions of both peptides interact with the top of TM1/ECD stalk, the ECD, ECL1 and the top of TM2, exendin-4 exhibits more persistent interactions in the simulations, particularly with the TM1/stalk and ECD (Supplemental Tables 1 and 3).…”

“…Class B1 GPCRs bind their peptide agonists via a two-domain model, whereby the C-terminus of the peptide interacts with the receptor extracellular N-terminal domain (ECD) promoting an “affinity trap” that enables the engagement of the N-terminus of the peptide with the receptor transmembrane domain (TMD), with interactions with the TMD required for receptor activation 9 . In recent years, advances in cryo-electron microscopy (cryo-EM) have enabled structural determination of a large number of class B1 GPCRs bound to their endogenous agonists, and coupled to G s , including that of the GLP-1R, which confirm engagement of these peptides with both the ECD and the TMD 10–21 .…”

The role of structural dynamics in GPCR‐mediated signaling

“…Recent structural characterization of family B receptors in the inactive and active G protein-coupled state has shown that the conformational changes associated with receptor activation are relatively similar to the ones seen for family A GPCRs [30,196,197]. However, the conserved sequence motifs found in family A GPCRs are lacking in other GPCR families, suggesting that the details of their activation mechanism will likely be different [196,198]. Future NMR, DEER, or fluorescence studies to characterize the conformational ensemble and dynamics of receptors from other GPCR subfamilies will be required to gain insight into the differences and similarities of the receptor activation processes across the entire GPCR superfamily.…”

Structural insights into hormone recognition by the human glucose-dependent insulinotropic polypeptide receptor

“…GIPR shares ~50% sequence similarity with GCGR, especially in the TMD region (75%), thus GCGR structures published previously provide a good template for the present study (Fig. S4) ( 14, 22–25 ). It was found the TMD of activated GIPR exhibits a conformation similar to that of GCGR activated by glucagon or ZP3780 (Cα RMSD = 1.2 and 0.7 Å, respectively) ( 14, 22 ) and distinct from that of GCGR bound by the negative allosteric modulator NNC0640 or partial agonist NNC1702 (Cα RMSD = 4.0 and 3.9 Å, respectively) ( 26 ).…”

“…S4) ( 14, 22–25 ). It was found the TMD of activated GIPR exhibits a conformation similar to that of GCGR activated by glucagon or ZP3780 (Cα RMSD = 1.2 and 0.7 Å, respectively) ( 14, 22 ) and distinct from that of GCGR bound by the negative allosteric modulator NNC0640 or partial agonist NNC1702 (Cα RMSD = 4.0 and 3.9 Å, respectively) ( 26 ). Facilitated by Gly 7.50b located in the middle of TM7, the extracellular half of TM7 bends towards TM6 by 8.0 Å (measured by Cα atom of Gly 7.32b )(Fig.…”