In order to stabilize the GPCR-G protein complex, an agonist must be bound to the receptor and the alpha subunit of the heterotrimer must be in a nucleotide-free state. Ground-breaking work by expert crystallographers made use of so-called mini G (mG) proteins to stabilize the active conformation of the adenosine A2A receptor in the presence of agonist and guanine nucleotides, but in the absence of Gβγ . These engineered G proteins behave in a way that mimics the nucleotide-free state despite being bound to GDP; thus, they can be seen as conformational sensors of the active receptor state. This work paved the way for another study recently published in the Journal of Biological Chemistry led by Nevin A. Lambert that looked to build on this minimalistic approach to see if representative mG proteins from the four subclasses (Gs, Gi/o, Gq/11 and G12/13) could 1) detect active GPCRs and 2) retain coupling specificity . Using bioluminescence resonance energy transfer (BRET) assays, the interaction between mGs, mGsi, mGsq or mG12 with prototypical GPCRs was quantified to examine whether these tools could reveal ligand efficacy/potency and G protein specificity. This was not only confirmed through exhaustive validation, but surprisingly uncovered secondary coupling interactions that might be of potential interest for follow-up studies. The GPCR superfamily comprises more than 800 GPCRs – most of which we know very little about. These elegant tools should prove valuable in increasing our knowledge about the lesser known GPCRs as well as allow for the discovery of G protein subtype-biased ligands and for unravelling receptor coupling complexity.
Comments by Shane C. Wright and Gunnar Schulte, Karolinska Institute
- Carpenter B, Nehme R, Warne T, Leslie AG and Tate CG. (2016) Structure of the adenosine A(2A) receptor bound to an engineered G protein. Nature, 536 (7614): 104-107. [PMID:27462812]
- Wan Q, Okashah N, Inoue A, Nehme R, Carpenter B, Tate CG and Lambert NA. (2018) Mini G protein probes for active G protein-coupled receptors (GPCRs) in live cells. J Biol Chem. pii: jbc.RA118.001975. doi: 10.1074/jbc.RA118.001975. [Epub ahead of print] [PMID:29523687]