G protein Gi1 exhibits basal coupling but not preassembly with G protein-coupled receptors [Cell Biology]

The Gi/o protein family transduces signals from a diverse group of G protein-coupled receptors (GPCRs). The observed specificity of Gi/o-GPCR coupling and high rate of Gi/o signal transduction have been hypothesized to be enabled by existence of stable associates between Gi/o proteins and their cognate GPCRs in the inactive state (Gi/o-GPCR preassembly). To test this hypothesis, we applied the recently developed technique of two-photon polarization microscopy (2PPM) to Gαi1 subunits labeled with fluorescent proteins and four GPCRs (the α2A-adrenergic receptor (α2A-AR), γ-aminobutyric acid receptor B (GABAB), cannabinoid receptor type 1 (CB1R) and dopamine receptor type 2 (D2R)). Our experiments with non-dissociating mutants of fluorescently labeled Gαi1 subunits (exhibiting impaired dissociation from activated GPCRs) showed that 2PPM is capable of detecting GPCR-G protein interactions. 2PPM experiments with non-mutated fluorescently labeled Gαi1 subunits and α2A-AR, GABAB or D2R receptors did not reveal any interaction between the Gi1 protein and the non-stimulated GPCRs. In contrast, non-stimulated CB1R shows interaction with the Gi1 protein. Further experiments revealed that this interaction is caused solely by CB1R basal activity; no preassembly between CB1R and the Gi1 protein could be observed. Our results demonstrate that four diverse GPCRs do not preassemble with non-active Gi1. However, we also show that basal GPCR activity allows interactions between non-stimulated GPCRs and Gi1 (basal coupling). These findings suggest that Gi1 interacts only with active GPCRs, and that the well-known high speed of GPCR signal transduction does not require preassembly between G proteins and GPCRs.

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