The GAPs, GEFs, GDIs and…now, GEMs: New kids on the heterotrimeric G protein signaling block

Abstract

The canonical process of activation of heterotrimeric G proteins by G protein coupled receptors (GPCRs) is well studied. Recently, a rapidly emerging paradigm has revealed the existence of a new, non-canonical set of cytosolic G protein modulators, guanine exchange modulators (GEMs). Among G proteins regulators, GEMs are uniquely capable of initiating pleiotropic signals: these bifunctional modulators can activate cAMP inhibitory (Gi) proteins and inhibit cAMP-stimulatory (Gs) proteins through a single short evolutionarily conserved module. A prototypical member of the GEM family, GIV/Girdin, integrates signals downstream of a myriad of cell surface receptors, e.g., growth factor RTKs, integrins, cytokine, GPCRs, etc., and translates these signals into G protein activation or inhibition. By their pleiotropic action, GIV and other GEMs modulate several key pathways within downstream signaling network. Unlike canonical G protein signaling that is finite and is triggered directly and exclusively by GPCRs, the temporal and spatial features of non-canonical activation of G protein via GIV-family of cytosolic GEMs are unusually relaxed. GIV uses this relaxed circuitry to integrate, reinforce and compartmentalize signals downstream of both growth factors and G proteins in a way that enables it to orchestrate cellular phenotypes in a sustained manner. Mounting evidence suggests the importance of GIV and other GEMs as disease modulators and their potential to serve as therapeutic targets; however, a lot remains unknown within the layers of the proverbial onion that must be systematically peeled. This perspective summarizes the key concepts of the GEM-dependent G protein signaling paradigm and discusses the multidisciplinary approaches that are likely to revolutionize our understanding of this paradigm from the atomic level to systems biology.

Keywords: G protein -coupled receptors; GIV/Girdin; growth factor receptor tyrosine kinases; guanine nucleotide dissociation inhibitor (GDI); guanine nucleotide exchange factor (GEF); guanine nucleotide exchange modulator (GEM); heterotrimeric G proteins.

Figure 1.

Mechanism of action of GEMs. Schematic comparing and contrasting key features of canonical (GPCR-triggered, left) and non-canonical (GEM-triggered, right) heterotrimeric G protein signaling. Left: Canonical activation of heterotrimeric G proteins is spatially and temporally restricted, i.e., triggered exclusively by agonist activated G-protein-coupled receptors (GPCRs) (signal input), may either lead to inhibition or stimulation of adenylate cyclase and cAMP production (signal output) depending on whether inhibitory G_iαβγ or stimulatory G_sαβγ heterotrimers are activated, respectively. Such activation is triggered primarily at the plasma membrane (PM) via a finite process that is rapid and completes within a few hundred milliseconds, and may continue on PM-contiguous endocytic compartments. Right: Non-canonical G protein signaling that is triggered by GEMs are characterized by 4 key differences (from top to bottom) – (A) GEMs can coordinately modulate heterotrimeric G protein signaling downstream of diverse classes of receptors (signal input). (B) GEMs can activate monomeric G_iα (as a GEF) or inhibit stimulatory G_sα (as a GDI) using the same short motif, thereby coordinately reducing cellular cAMP. The monomeric Gα substrate could either be a byproduct of GPCR signaling or actively released from G_i/sαβγ trimers by displacement of Gβγ heterodimers or GoLoco/GPR motif containing GDIs. (C) Being cytosolic in localization, GEMs can act upon heterotrimeric G proteins on both PM and internal membranes that are non-contiguous with the PM, e.g., Golgi and autophagosomes. (D) Signaling that is triggered by GEMs appears to be delayed (initiated after several minutes) and prolonged, lasting several minutes to hours (reviewed in¹⁴). Note: The spatiotemporal features listed here exclusively refer to the nature of heterotrimeric G protein activation within each paradigm, and do not refer to other downstream signals initiated within each pathway (like MAPK or Akt), or downstream gene transcription/translation responses that are typically delayed, or effects on the cytoskeleton or organelles that are brought about directly or indirectly via downstream intermediates within the signaling cascades.

Figure 2.

Key phosphoevents regulate the spatiotemporally separated GEF and GDI functions of GIV. Schematic showing the spatially separated GEF and GDI actions of GIV-GEM on Gαi and Gαs, respectively. Upon EGF stimulation, activated CDK5 kinase phosphorylates GIV on S1674, thereby turning ‘on’ its GEF function toward Gαi. Subsequently, GIV is phosphorylated also at S1689 by PKCθ; this phosphoevent turns ‘off’ GIV's GEF function, but turns ‘on’ its GDI function toward Gαs. Such coordinated activation of Gαi first at the PM within 5 min after EGF stimulation and subsequent inhibition of Gαs on endosomes at ∼15–30 min after EGF stimulation ensures that cAMP levels are suppressed both early and late in cells responding to EGF.