Divergent Transducer-specific Molecular Efficacies Generate Biased Agonism at a G Protein-coupled Receptor (GPCR)

Biased agonism, or functional selectivity, describes the phenomenon where a single GPCR ligand preferentially activates one signaling pathway (e.g., G protein) over another (e.g., beta-arrestin). While this concept is therapeutically promising, the molecular mechanisms driving this selectivity have remained elusive due to the difficulty of isolating ligand efficacy from complex cellular environments. This study aims to quantify the thermodynamic driving forces of this bias by measuring transducer-specific molecular efficacies in a cell-free system.

The researchers developed a cell-free assay using the angiotensin II type 1 receptor (AT1R) as a model. They created fusion proteins where the AT1R was covalently linked to either Gq or beta-arrestin2. By analyzing the shift in ligand binding affinity (KLo/KHi) when the receptor is coupled to these transducers, the authors calculated the allosteric coupling constant, which serves as a direct, thermodynamic measure of molecular efficacy. This approach allowed them to compare the efficacy of various balanced and biased ligands across different signaling pathways without the confounding variables of cellular signal amplification.

The study found that transducer-specific molecular efficacies are strongly correlated with cellular signaling outcomes. Specifically, ligands identified as Gq-biased in cellular assays showed significantly higher molecular efficacy at the AT1R-Gq fusion protein, while beta-arrestin-biased ligands showed higher efficacy at the AT1R-beta-arrestin2 fusion protein. The researchers also identified two distinct mechanisms for bias: beta-arrestin bias often resulted from a reciprocal loss of Gq efficacy and gain in arrestin efficacy, whereas Gq bias appeared to result from a selective increase in Gq efficacy without a corresponding loss in arrestin efficacy.