Development of fluorescent peptide G Protein Coupled Receptor activation biosensors for NanoBRET characterisation of intracellular allosteric modulators

G protein coupled receptors (GPCRs) are widely therapeutically targeted, and recent advances in allosteric modulator development at this class of receptors offer further potential for exploitation. In particular GPCR intracellular allosteric modulators (IAM) represent a class of ligands that bind to the receptor-effector interface (e.g. G protein) and so inhibit agonist responses non-competitively. This potentially offers a tailored mode of action and greater selectivity between conserved receptor subtypes compared to classical orthosteric ligands. However, while specific examples of the IAM class of ligands are well described (particularly for chemokine receptors), a more general methodology for assessing compound interactions at the GPCR IAM site is lacking. Here fluorescent labelled peptides based on the G peptide C terminus are developed as novel binding and activation biosensors for the GPCR IAM binding site. In TR-FRET binding studies, unlabelled peptides derived from the GS subunit C-terminus were first characterised for their ability to positively modulate agonist affinity at the {beta}2-adrenoceptor. On this basis, a tetramethylrhodamine (TMR) labelled tracer was synthesized based on the 19 amino acid C terminal GS peptide (TMR-GS19cha18, where cha=cyclohexylalanine). Using NanoBRET technology to detect binding, TMR-GS19cha18 was recruited to Gs coupled {beta}2-adrenoceptor and EP2 receptors in an agonist dependent manner (correlated with ligand efficacy), but not to the Gi coupled CXCR2 receptor. Moreover, NanoBRET competition binding assays using TMR-GS19cha18 enabled direct assessment of the affinity of unlabelled ligands for {beta}2-adrenoceptor IAM site. Thus the NanoBRET platform using fluorescent-labelled G protein peptide mimetics offers novel potential for medium-throughput affinity screens to identify new IAMs, applicable across GPCRs coupled to a G protein class. Using the same platform, Gs peptide biosensors also represent useful tools to probe orthosteric agonist efficacy and the dynamics of receptor activation.