Ric-8A, a GEF, and a Chaperone for G Protein α-Subunits: Evidence for the Two-Faced Interface

Non-canonical G protein signaling

The original paradigm of classical - also referred to as canonical - cellular signal transduction of heterotrimeric G proteins (G protein) is defined by a hierarchical, orthograde interaction of three players: the agonist-activated G protein-coupled receptor (GPCR), which activates the transducing G protein, that in turn regulates its intracellular effectors. This receptor-transducer-effector concept was extended by the identification of regulators and adapters such as the regulators of G protein signaling (RGS), receptor kinases like βARK, or GPCR-interacting arrestin adapters that are integrated into this canonical signaling process at different levels to enable fine-tuning. Finally, the identification of atypical signaling mechanisms of classical regulators, together with the discovery of novel modulators, added a new and fascinating dimension to the cellular G protein signal transduction. This heterogeneous group of accessory G protein modulators was coined "activators of G protein signaling" (AGS) proteins and plays distinct roles in canonical and non-canonical G protein signaling pathways. AGS proteins contribute to the control of essential cellular functions such as cell development and division, intracellular transport processes, secretion, autophagy or cell movements. As such, they are involved in numerous biological processes that are crucial for diseases, like diabetes mellitus, cancer, and stroke, which represent major health burdens. Although the identification of a large number of non-canonical G protein signaling pathways has broadened the spectrum of this cellular communication system, their underlying mechanisms, functions, and biological effects are poorly understood. In this review, we highlight and discuss atypical G protein-dependent signaling mechanisms with a focus on inhibitory G proteins (Gi) involved in canonical and non-canonical signal transduction, review recent developments and open questions, address the potential of new approaches for targeted pharmacological interventions.

Gαs slow conformational transition upon GTP binding and a novel Gαs regulator

G proteins are major signaling partners for G protein-coupled receptors (GPCRs). Although stepwise structural changes during GPCR-G protein complex formation and guanosine diphosphate (GDP) release have been reported, no information is available with regard to guanosine triphosphate (GTP) binding. Here, we used a novel Bayesian integrative modeling framework that combines data from hydrogen-deuterium exchange mass spectrometry, tryptophan-induced fluorescence quenching, and metadynamics simulations to derive a kinetic model and atomic-level characterization of stepwise conformational changes incurred by the β2-adrenergic receptor (β2AR)-Gs complex after GDP release and GTP binding. Our data suggest rapid GTP binding and GTP-induced dissociation of Gαs from β2AR and Gβγ, as opposed to a slow closing of the Gαs α-helical domain (AHD). Yeast-two-hybrid screening using Gαs AHD as bait identified melanoma-associated antigen D2 (MAGE D2) as a novel AHD-binding protein, which was also shown to accelerate the GTP-induced closing of the Gαs AHD.

Transducin Partners Outside the Phototransduction Pathway

Transducin mediates signal transduction in a classical G protein-coupled receptor (GPCR) phototransduction cascade. Interactions of transducin with the receptor and the effector molecules had been extensively investigated and are currently defined at the atomic level. However, partners and functions of rod transducin α (Gα_{t 1}) and βγ (Gβ₁γ₁) outside the visual pathway are not well-understood. In particular, light-induced redistribution of rod transducin from the outer segment to the inner segment and synaptic terminal (IS/ST) allows Gα_t1 and/or Gβ₁γ₁ to modulate synaptic transmission from rods to rod bipolar cells (RBCs). Protein-protein interactions underlying this modulation are largely unknown. We discuss known interactors of transducin in the rod IS/ST compartment and potential pathways leading to the synaptic effects of light-dispersed Gα_t1 and Gβ₁γ₁. Furthermore, we show that a prominent non-GPCR guanine nucleotide exchange factor (GEF) and a chaperone of Gα subunits, resistance to inhibitors of cholinesterase 8A (Ric-8A) protein, is expressed throughout the retina including photoreceptor cells. Recent structures of Ric-8A alone and in complexes with Gα subunits have illuminated the structural underpinnings of the Ric-8A activities. We generated a mouse model with conditional knockout of Ric-8A in rods in order to begin defining the functional roles of the protein in rod photoreceptors and the retina. Our analysis suggests that Ric-8A is not an obligate chaperone of Gα_t1. Further research is needed to investigate probable roles of Ric-8A as a GEF, trafficking chaperone, or a mediator of the synaptic effects of Gα_t1.