Introduction
G protein-coupled receptors (GPCRs) are encoded by approximately 800
different genes and form the largest family of membrane proteins in the
human genome. GPCRs mediate the effects of a variety of extracellular
cues, from neurotransmitters to photons, and consequently, are
fundamental regulators of physiological homeostasis (Calebiro and
Godbole, 2018). It is therefore unsurprising that around one third of
current pharmaceuticals target this receptor family (Calebiro and
Godbole, 2018). Despite this, several important aspects of GPCR
signalling remain insufficiently understood. In the classical view of
GPCR signalling, ligand-activated GPCRs signal at the plasma membrane
via heterotrimeric G proteins, often rapidly desensitise, and undergo
arrestin-mediated internalisation (Calebiro and Godbole, 2018).
Signalling competence can then be restored by GPCR re-sensitisation and
recycling back to the plasma membrane (Calebiro and Godbole, 2018).
However, evidence gathered over the last decade indicates that select
GPCRs can also signal through heterotrimeric G proteins at distinct
intracellular sites and that the resulting signals may be important for
physiological functions (Irannejad et al., 2013, Eichel and von Zastrow,
2018, Calebiro et al., 2009, Godbole et al., 2017, Yarwood et al.,
2017).
Evidence
of intracellular GPCR signalling has been largely obtained through the
application of genetically encoded fluorescence/bioluminescence
resonance energy transfer (FRET/BRET)-based sensors that measure dynamic
protein–protein and intramolecular interactions in real-time and in
live cells. Such methodologies have a much higher temporal resolution
than endpoint biochemical assays and have substantially improved our
understanding of GPCR signalling. Above all, they have revealed that
GPCR signalling is not limited to the plasma membrane, but instead is a
highly regulated event that works in combination with GPCR trafficking
to enhance the specificity of signalling in response to distinct
physiological cues.