Quantitative assessment of constitutive G protein–coupled receptor activity with BRET-based G protein biosensors

Heterotrimeric G proteins constitute the primary transducers of G protein–coupled receptor (GPCR) signaling. In addition to mediating ligand-induced GPCR activation, G proteins transduce basal signaling in various physiological and pathophysiological settings evoked by constitutively active, native GPCRs or disease-related receptor mutants. Optical biosensors have been developed and optimized to monitor GPCR ligand–induced activation of G proteins, but these biosensors cannot be used to detect constitutively active GPCRs. Here, we designed and validated eight bioluminescence resonance energy transfer (BRET)–based G protein sensors that can measure the activity of all four major families of G proteins. We also established a protocol to identify constitutive GPCR or G protein signaling in live cells. These G protein–based, tricistronic activity sensors (G-CASE) rely on the encoding of all three G protein subunits by a single plasmid, enabling their expression at the desired relative amounts and resulting in reduced signal variability in mammalian cells. We also present an experimental protocol to use the G-CASE sensor toolbox to quantify constitutive signaling of native and mutated GPCRs through these heterotrimeric transducers. This approach will help to characterize constitutively active GPCRs and their role in health and disease.