Ric-8B is a GTP-dependent G protein alphas guanine nucleotide exchange factor

Neomorphic Gαo mutations gain interaction with Ric8 proteins in GNAO1 encephalopathies

GNAO1 mutated in pediatric encephalopathies encodes the major neuronal G protein Gαo. Of the more than 80 pathogenic mutations, most are single amino acid substitutions spreading across the Gαo sequence. We performed extensive characterization of Gαo mutants, showing abnormal GTP uptake and hydrolysis and deficiencies in binding Gβγ and RGS19. Plasma membrane localization of Gαo was decreased for a subset of mutations that leads to epilepsy; dominant interactions with GPCRs also emerged for the more severe mutants. Pathogenic mutants massively gained interaction with Ric8A and, surprisingly, Ric8B proteins, relocalizing them from cytoplasm to Golgi. Of these 2 mandatory Gα-subunit chaperones, Ric8A is normally responsible for the Gαi/Gαo, Gαq, and Gα12/Gα13 subfamilies, and Ric8B solely responsible for Gαs/Gαolf. Ric8 mediates the disease dominance when engaging in neomorphic interactions with pathogenic Gαo through imbalance of the neuronal G protein signaling networks. As the strength of Gαo-Ric8B interactions correlates with disease severity, our study further identifies an efficient biomarker and predictor for clinical manifestations in GNAO1 encephalopathies. Our work uncovers the neomorphic molecular mechanism of mutations underlying pediatric encephalopathies and offers insights into other maladies caused by G protein malfunctioning and further genetic diseases.

Heterotrimeric G protein signaling without GPCRs: The Gα-binding-and-activating (GBA) motif

Heterotrimeric G proteins (Gαβγ) are molecular switches that relay signals from 7-transmembrane receptors located at the cell surface to the cytoplasm. The function of these receptors is so intimately linked to heterotrimeric G proteins that they are named G protein-coupled receptors (GPCRs), showcasing the interdependent nature of this archetypical receptor-transducer axis of transmembrane signaling in eukaryotes. It is generally assumed that activation of heterotrimeric G protein signaling occurs exclusively by the action of GPCRs, but this idea has been challenged by the discovery of alternative mechanisms by which G proteins can propagate signals in the cell. This review will focus on a general principle of G protein signaling that operates without the direct involvement of GPCRs. The mechanism of G protein signaling reviewed here is mediated by a class of G protein regulators defined by containing an evolutionarily conserved sequence named the Gα-binding-and-activating (GBA) motif. Using the best characterized proteins with a GBA motif as examples, Gα-interacting vesicle-associated protein (GIV)/Girdin and dishevelled-associating protein with a high frequency of leucine residues (DAPLE), this review will cover (i) the mechanisms by which extracellular cues not relayed by GPCRs promote the coupling of GBA motif-containing regulators with G proteins, (ii) the structural and molecular basis for how GBA motifs interact with Gα subunits to facilitate signaling, (iii) the relevance of this mechanism in different cellular and pathological processes, including cancer and birth defects, and (iv) strategies to manipulate GBA-G protein coupling for experimental therapeutics purposes, including the development of rationally engineered proteins and chemical probes.

The neuronal calcium sensor NCS-1 regulates the phosphorylation state and activity of the Gα chaperone and GEF Ric-8A

The neuronal calcium sensor 1 (NCS-1), an EF-hand Ca2+ binding protein, and Ric-8A coregulate synapse number and probability of neurotransmitter release. Recently, the structures of Ric-8A bound to Gα have revealed how Ric-8A phosphorylation promotes Gα recognition and activity as a chaperone and guanine nucleotide exchange factor. However, the molecular mechanism by which NCS-1 regulates Ric-8A activity and its interaction with Gα subunits is not well understood. Given the interest in the NCS-1/Ric-8A complex as a therapeutic target in nervous system disorders, it is necessary to shed light on this molecular mechanism of action at atomic level. We have reconstituted NCS-1/Ric-8A complexes to conduct a multimodal approach and determine the sequence of Ca2+ signals and phosphorylation events that promote the interaction of Ric-8A with Gα. Our data show that the binding of NCS-1 and Gα to Ric-8A are mutually exclusive. Importantly, NCS-1 induces a structural rearrangement in Ric-8A that traps the protein in a conformational state that is inaccessible to casein kinase II-mediated phosphorylation, demonstrating one aspect of its negative regulation of Ric-8A-mediated G-protein signaling. Functional experiments indicate a loss of Ric-8A guanine nucleotide exchange factor (GEF) activity toward Gα when complexed with NCS-1, and restoration of nucleotide exchange activity upon increasing Ca2+ concentration. Finally, the high-resolution crystallographic data reported here define the NCS-1/Ric-8A interface and will allow the development of therapeutic synapse function regulators with improved activity and selectivity.

The G protein alpha chaperone and guanine-nucleotide exchange factor RIC-8 regulates cilia morphogenesis in Caenorhabditis elegans sensory neurons

Heterotrimeric G (αβγ) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for Gα protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and Gα-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 Gα client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple Gα ODR-3 functions in ciliogenesis and olfaction.

The G protein alpha Chaperone and Guanine-Nucleotide Exchange Factor RIC-8 Regulates Cilia Morphogenesis in Caenorhabditis elegans Sensory Neurons

Heterotrimeric G (αβγ) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for Gα protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and Gα-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 Gα client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite severe defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple Gα ODR-3 functions in ciliogenesis and olfaction.

Structures of Ric-8B in complex with Gα protein folding clients reveal isoform specificity mechanisms

Mammalian Ric-8 proteins act as chaperones to regulate the cellular abundance of heterotrimeric G protein α subunits. The Ric-8A isoform chaperones Gαi/o, Gα12/13, and Gαq/11 subunits, while Ric-8B acts on Gαs/olf subunits. Here, we determined cryoelectron microscopy (cryo-EM) structures of Ric-8B in complex with Gαs and Gαolf, revealing isoform differences in the relative positioning and contacts between the C-terminal α5 helix of Gα within the concave pocket formed by Ric-8 α-helical repeat elements. Despite the overall architectural similarity with our earlier structures of Ric-8A complexed to Gαq and Gαi1, Ric-8B distinctly accommodates an extended loop found only in Gαs/olf proteins. The structures, along with results from Ric-8 protein thermal stability assays and cell-based Gαolf folding assays, support a requirement for the Gα C-terminal region for binding specificity, and highlight that multiple structural elements impart specificity for Ric-8/G protein binding.

The role of resistance to inhibitors of cholinesterase 8b in the control of heart rate

We have assessed the role of ric-b8 in the control of heart rate after the gene was implicated in a recent genome-wide association study of resting heart rate. We developed a novel murine model in which it was possible to conditionally delete ric-8b in the sinoatrial (SA) node after the addition of tamoxifen. Despite this, we were unable to obtain homozygotes and thus studied heterozygotes. Haploinsufficiency of ric-8b in the sinoatrial node induced by the addition of tamoxifen in adult animals leads to mice with a reduced heart rate. However, other electrocardiographic intervals (e.g., PR and QRS) were normal, and there was no apparent arrhythmia such as heart block. The positive chronotropic response to isoprenaline was abrogated, whereas the response to carbachol was unchanged. The pacemaker current I_f (funny current) has an important role in regulating heart rate, and its function is modulated by both isoprenaline and carbachol. Using a heterologous system expressing HCN4, we show that ric-8b can modulate the HCN4 current. Overexpression of ric-8b led to larger HCN4 currents, whereas silencing ric-8b led to smaller currents. Ric-8b modulates heart rate responses in vivo likely via its actions on the stimulatory G-protein.

Depletion of Ric-8B leads to reduced mTORC2 activity

mTOR, a serine/threonine protein kinase that is involved in a series of critical cellular processes, can be found in two functionally distinct complexes, mTORC1 and mTORC2. In contrast to mTORC1, little is known about the mechanisms that regulate mTORC2. Here we show that mTORC2 activity is reduced in mice with a hypomorphic mutation of the Ric-8B gene. Ric-8B is a highly conserved protein that acts as a non-canonical guanine nucleotide exchange factor (GEF) for heterotrimeric Gαs/olf type subunits. We found that Ric-8B hypomorph embryos are smaller than their wild type littermates, fail to close the neural tube in the cephalic region and die during mid-embryogenesis. Comparative transcriptome analysis revealed that signaling pathways involving GPCRs and G proteins are dysregulated in the Ric-8B mutant embryos. Interestingly, this analysis also revealed an unexpected impairment of the mTOR signaling pathway. Phosphorylation of Akt at Ser473 is downregulated in the Ric-8B mutant embryos, indicating a decreased activity of mTORC2. Knockdown of the endogenous Ric-8B gene in cultured cell lines leads to reduced phosphorylation levels of Akt (Ser473), further supporting the involvement of Ric-8B in mTORC2 activity. Our results reveal a crucial role for Ric-8B in development and provide novel insights into the signals that regulate mTORC2.

Gene inactivation in mice can be used to identify genes that are involved in important biological processes and that may contribute to disease. We used this approach to study the Ric-8B gene, which is highly conserved in mammals, including humans. We found that Ric-8B is essential for embryogenesis and for the proper development of the nervous system. Ric-8B mutant mouse embryos are smaller than their wild type littermates and show neural tube defects at the cranial region. This approach also allowed us to identify the biological pathways that potentially contribute to the observed phenotypes, and uncover a novel role for Ric-8B in the mTORC2 signaling pathway. mTORC2 plays particular important roles in the adult brain, and has been implicated in neurological disorders. Our mutant mice provide a model to study the complex molecular and cellular processes underlying the interplay between Ric-8B and mTORC2 in neuronal function.

Structures of Gα Proteins in Complex with Their Chaperone Reveal Quality Control Mechanisms

Many chaperones promote nascent polypeptide folding followed by substrate release through ATP-dependent conformational changes. Here we show cryoEM structures of Gα subunit folding intermediates in complex with full-length Ric-8A, a unique chaperone-client system in which substrate release is facilitated by guanine nucleotide binding to the client G protein. The structures of Ric-8A-Gαi and Ric-8A-Gαq complexes reveal that the chaperone employs its extended C-terminal region to cradle the Ras-like domain of Gα, positioning the Ras core in contact with the Ric-8A core while engaging its switch2 nucleotide binding region. The C-terminal α5 helix of Gα is held away from the Ras-like domain through Ric-8A core domain interactions, which critically depend on recognition of the Gα C terminus by the chaperone. The structures, complemented with biochemical and cellular chaperoning data, support a folding quality control mechanism that ensures proper formation of the C-terminal α5 helix before allowing GTP-gated release of Gα from Ric-8A.

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

Resistance to inhibitors of cholinesterase 8A (Ric-8A) is a prominent non-receptor GEF and a chaperone of G protein α-subunits (Gα). Recent studies shed light on the structure of Ric-8A, providing insights into the mechanisms underlying its interaction with Gα. Ric-8A is composed of a core armadillo-like domain and a flexible C-terminal tail. Interaction of a conserved concave surface of its core domain with the Gα C-terminus appears to mediate formation of the initial Ric-8A/GαGDP intermediate, followed by the formation of a stable nucleotide-free complex. The latter event involves a large-scale dislocation of the Gα α5-helix that produces an extensive primary interface and disrupts the nucleotide-binding site of Gα. The distal portion of the C-terminal tail of Ric-8A forms a smaller secondary interface, which ostensibly binds the switch II region of Gα, facilitating binding of GTP. The two-site Gα interface of Ric-8A is distinct from that of GPCRs, and might have evolved to support the chaperone function of Ric-8A.

Structure of the G protein chaperone and guanine nucleotide exchange factor Ric-8A bound to Gαi1

Ric-8A is a cytosolic Guanine Nucleotide exchange Factor (GEF) that activates heterotrimeric G protein alpha subunits (Gα) and serves as an essential Gα chaperone. Mechanisms by which Ric-8A catalyzes these activities, which are stimulated by Casein Kinase II phosphorylation, are unknown. We report the structure of the nanobody-stabilized complex of nucleotide-free Gα bound to phosphorylated Ric-8A at near atomic resolution by cryo-electron microscopy and X-ray crystallography. The mechanism of Ric-8A GEF activity differs considerably from that employed by G protein-coupled receptors at the plasma membrane. Ric-8A engages a specific conformation of Gα at multiple interfaces to form a complex that is stabilized by phosphorylation within a Ric-8A segment that connects two Gα binding sites. The C-terminus of Gα is ejected from its beta sheet core, thereby dismantling the GDP binding site. Ric-8A binds to the exposed Gα beta sheet and switch II to stabilize the nucleotide-free state of Gα.

DAPLE protein inhibits nucleotide exchange on Gαs and Gαq via the same motif that activates Gαi

Besides being regulated by G-protein-coupled receptors, the activity of heterotrimeric G proteins is modulated by many cytoplasmic proteins. GIV/Girdin and DAPLE (Dvl-associating protein with a high frequency of leucine) are the best-characterized members of a group of cytoplasmic regulators that contain a Gα-binding and -activating (GBA) motif and whose dysregulation underlies human diseases, including cancer and birth defects. GBA motif-containing proteins were originally reported to modulate G proteins by binding Gα subunits of the G_i/o family (Gα_i) over other families (such as G_s, G_q/11, or G_12/13), and promoting nucleotide exchange in vitro However, some evidence suggests that this is not always the case, as phosphorylation of the GBA motif of GIV promotes its binding to Gα_s and inhibits nucleotide exchange. The G-protein specificity of DAPLE and how it might affect nucleotide exchange on G proteins besides Gα_i remain to be investigated. Here, we show that DAPLE's GBA motif, in addition to Gα_i, binds efficiently to members of the G_s and G_q/11 families (Gα_s and Gα_q, respectively), but not of the G_12/13 family (Gα₁₂) in the absence of post-translational phosphorylation. We pinpointed Met-1669 as the residue in the GBA motif of DAPLE that diverges from that in GIV and enables better binding to Gα_s and Gα_q Unlike the nucleotide-exchange acceleration observed for Gα_i, DAPLE inhibited nucleotide exchange on Gα_s and Gα_q These findings indicate that GBA motifs have versatility in their G-protein-modulating effect, i.e. they can bind to Gα subunits of different classes and either stimulate or inhibit nucleotide exchange depending on the G-protein subtype.

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

Neurotransmitters signal via G protein coupled receptors (GPCRs) to modulate activity of neurons and muscles. C. elegans has ∼150 G protein coupled neuropeptide receptor homologs and 28 additional GPCRs for small-molecule neurotransmitters. Genetic studies in C. elegans demonstrate that neurotransmitters diffuse far from their release sites to activate GPCRs on distant cells. Individual receptor types are expressed on limited numbers of cells and thus can provide very specific regulation of an individual neural circuit and behavior. G protein coupled neurotransmitter receptors signal principally via the three types of heterotrimeric G proteins defined by the G alpha subunits Gαo, Gαq, and Gαs. Each of these G alpha proteins is found in all neurons plus some muscles. Gαo and Gαq signaling inhibit and activate neurotransmitter release, respectively. Gαs signaling, like Gαq signaling, promotes neurotransmitter release. Many details of the signaling mechanisms downstream of Gαq and Gαs have been delineated and are consistent with those of their mammalian orthologs. The details of the signaling mechanism downstream of Gαo remain a mystery. Forward genetic screens in C. elegans have identified new molecular components of neural G protein signaling mechanisms, including Regulators of G protein Signaling (RGS proteins) that inhibit signaling, a new Gαq effector (the Trio RhoGEF domain), and the RIC-8 protein that is required for neuronal Gα signaling. A model is presented in which G proteins sum up the variety of neuromodulator signals that impinge on a neuron to calculate its appropriate output level.

The Ric-8A/Gα13/FAK signalling cascade controls focal adhesion formation during neural crest cell migration in Xenopus

Ric-8A is a pleiotropic guanine nucleotide exchange factor involved in the activation of various heterotrimeric G-protein pathways during adulthood and early development. Here, we sought to determine the downstream effectors of Ric-8A during the migration of the vertebrate cranial neural crest (NC) cells. We show that the Gα13 knockdown phenocopies the Ric-8A morphant condition, causing actin cytoskeleton alteration, protrusion instability, and a strong reduction in the number and dynamics of focal adhesions. In addition, the overexpression of Gα13 is sufficient to rescue Ric-8A-depleted cells. Ric-8A and Gα13 physically interact and colocalize in protrusions of the cells leading edge. The focal adhesion kinase FAK colocalizes and interacts with the endogenous Gα13, and a constitutively active form of Src efficiently rescues the Gα13 morphant phenotype in NC cells. We propose that Ric-8A-mediated Gα13 signalling is required for proper cranial NC cell migration by regulating focal adhesion dynamics and protrusion formation.

Production of Phosphorylated Ric-8A proteins using protein kinase CK2

Resistance to Inhibitors of Cholinesterase-8 (Ric-8) proteins are molecular chaperones that fold heterotrimeric G protein α subunits shortly after biosynthesis. Ric-8 proteins also act as test tube guanine nucleotide exchange factors (GEF) that promote Gα subunit GDP for GTP exchange. The GEF and chaperoning activities of Ric-8A are regulated by phosphorylation of five serine and threonine residues within protein kinase CK2 consensus sites. The traditional way that Ric-8A proteins have been purified is from Spodoptera frugiperda (Sf9) or Trichoplusia ni (Tni) insect cells. Endogenous insect cell kinases do phosphorylate the critical regulatory sites of recombinant Ric-8A reasonably well, but there is batch-to-batch variability among recombinant Ric-8A preparations. Additionally, insect cell-production of some Ric-8 proteins with phosphosite alanine substitution mutations is proscribed as there seems to be interdependency of multi-site phosphorylation for functional protein production. Here, we present a method to produce wild type and phosphosite mutant Ric-8A proteins that are fully occupied with bound phosphate at each of the regulatory positions. Ric-8A proteins were expressed and purified from E. coli. Purified Ric-8A was phosphorylated in vitro with protein kinase CK2 and then re-isolated to remove kinase. The phosphorylated Ric-8A proteins were ∼99% pure and the completeness of phosphorylation was verified by chromatography, phos-tag SDS-PAGE mobility shifts, immunoblotting using phospho-site specific antibodies, and mass spectrometry analysis. E. coli-produced Ric-8A that was phosphorylated using this method promoted a faster rate of Gα subunit guanine nucleotide exchange than Ric-8A that was variably phosphorylated during production in insect cells.

A biochemical and genetic discovery pipeline identifies PLCδ4b as a nonreceptor activator of heterotrimeric G-proteins

Recent evidence has revealed that heterotrimeric G-proteins can be activated by cytoplasmic proteins that share an evolutionarily conserved sequence called the Gα-binding-and-activating (GBA) motif. This mechanism provides an alternative to canonical activation by G-protein-coupled receptors (GPCRs) and plays important roles in cell function, and its dysregulation is linked to diseases such as cancer. Here, we describe a discovery pipeline that uses biochemical and genetic approaches to validate GBA candidates identified by sequence similarity. First, putative GBA motifs discovered in bioinformatics searches were synthesized on peptide arrays and probed in batch for Gα_i3 binding. Then, cDNAs encoding proteins with Gα_i3-binding sequences were expressed in a genetically-modified yeast strain that reports mammalian G-protein activity in the absence of GPCRs. The resulting GBA motif candidates were characterized by comparison of their biochemical, structural, and signaling properties with those of all previously described GBA motifs in mammals (GIV/Girdin, DAPLE, Calnuc, and NUCB2). We found that the phospholipase Cδ4 (PLCδ4) GBA motif binds G-proteins with high affinity, has guanine nucleotide exchange factor activity in vitro, and activates G-protein signaling in cells, as indicated by bioluminescence resonance energy transfer (BRET)-based biosensors of G-protein activity. Interestingly, the PLCδ4 isoform b (PLCδ4b), which lacks the domains required for PLC activity, bound and activated G-proteins more efficiently than the full-length isoform a, suggesting that PLCδ4b functions as a G-protein regulator rather than as a PLC. In summary, we have identified PLCδ4 as a nonreceptor activator of G-proteins and established an experimental pipeline to discover and characterize GBA motif-containing proteins.

Dual phosphorylation of Ric-8A enhances its ability to mediate G protein α subunit folding and to stimulate guanine nucleotide exchange

Resistance to inhibitors of cholinesterase-8A (Ric-8A) and Ric-8B are essential biosynthetic chaperones for heterotrimeric G protein α subunits. We provide evidence for the direct regulation of Ric-8A cellular activity by dual phosphorylation. Using proteomics, Western blotting, and mutational analyses, we determined that Ric-8A was constitutively phosphorylated at five serines and threonines by the protein kinase CK2. Phosphorylation of Ser⁴³⁵ and Thr⁴⁴⁰ in rat Ric-8A (corresponding to Ser⁴³⁶ and Thr⁴⁴¹ in human Ric-8A) was required for high-affinity binding to Gα subunits, efficient stimulation of Gα subunit guanine nucleotide exchange, and mediation of Gα subunit folding. The CK2 consensus sites that contain Ser⁴³⁵ and Thr⁴⁴⁰ are conserved in Ric-8 homologs from worms to mammals. We found that the homologous residues in mouse Ric-8B, Ser⁴⁶⁸ and Ser⁴⁷³, were also phosphorylated. Mutation of the genomic copy of ric-8 in Caenorhabditis elegans to encode alanine in the homologous sites resulted in characteristic ric-8 reduction-of-function phenotypes that are associated with defective G_q and G_s signaling, including reduced locomotion and defective egg laying. The C. elegans ric-8 phosphorylation site mutant phenotypes were partially rescued by chemical stimulation of G_q signaling. These results indicate that dual phosphorylation represents a critical form of conserved Ric-8 regulation and demonstrate that Ric-8 proteins are needed for effective Gα signaling. The position of the CK2-phosphorylated sites within a structural model of Ric-8A reveals that these sites contribute to a key acidic and negatively charged surface that may be important for its interactions with Gα subunits.

Conditional Deletion of Ric-8b in Olfactory Sensory Neurons Leads to Olfactory Impairment

The olfactory system can discriminate a vast number of odorants. This ability derives from the existence of a large family of odorant receptors expressed in the cilia of the olfactory sensory neurons. Odorant receptors signal through the olfactory-specific G-protein subunit, Gαolf. Ric-8b, a guanine nucleotide exchange factor, interacts with Gαolf and can amplify odorant receptor signal transduction in vitro To explore the function of Ric-8b in vivo, we generated a tissue specific knock-out mouse by crossing OMP-Cre transgenic mice to Ric-8b floxed mice. We found that olfactory-specific Ric-8b knock-out mice of mixed sex do not express the Gαolf protein in the olfactory epithelium. We also found that in these mice, the mature olfactory sensory neuron layer is reduced, and that olfactory sensory neurons show increased rate of cell death compared with wild-type mice. Finally, behavioral tests showed that the olfactory-specific Ric-8b knock-out mice show an impaired sense of smell, even though their motivation and mobility behaviors remain normal.SIGNIFICANCE STATEMENT Ric-8b is a guanine nucleotide exchange factor (GEF) expressed in the olfactory epithelium and in the striatum. Ric-8b interacts with the olfactory Gαolf subunit, and can amplify odorant signaling through odorant receptors in vitro However, the functional significance of this GEF in the olfactory neurons in vivo remains unknown. We report that deletion of Ric-8b in olfactory sensory neurons prevents stable expression of Gαolf. In addition, we demonstrate that olfactory neurons lacking Ric-8b (and consequently Gαolf) are more susceptible to cell death. Ric-8b conditional knock-out mice display impaired olfactory guided behavior. Our results reveal that Ric-8b is essential for olfactory function, and suggest that it may also be essential for Gαolf-dependent functions in the brain.

Nanosecond Dynamics of Gαi1 Bound to Nucleotides or Ric-8A, a Gα Chaperone with GEF Activity

Resistance to Inhibitors of Cholinesterase A (Ric-8A) is a 60-kDa cytosolic protein that has chaperone and guanine nucleotide exchange (GEF) activity toward heterotrimeric G protein α subunits of the i, q, and 12/13 classes, catalyzing the release of GDP from Gα and subsequent binding of GTP. In the absence of GTP or GTP analogs, and subsequent to GDP release, Gα forms a stable nucleotide-free complex with Ric-8A. In this study, time-resolved fluorescence anisotropy measurements were employed to detect local motions of Gαi1 labeled at selected sites with Alexa 488 (C5) fluorescent dye (Ax) in the GDP, GTPγS (collectively, GXP), and Ric-8A-bound states. Sites selected for Alexa 488 (C5) derivatization were in the α-helical domain (residue 106), the α-helical domain-Ras-like domain hinge (residue 63), Switch I (residue 180), Switch II (residue 209), Switch III (residue 238), the α4 helix (residue 305), and at the junction between the purine-binding subsite in the β6-α5 loop and the C-terminal α helix (residue 330). In the GXP-bound states, the Alexa fluorophore reports local motions with correlation times ranging from 1.0 to 1.8 ns. The dynamics at Ax180 is slower in Gαi1•GDP than in Gαi1•GTPγS. The reverse is true at Ax209. The order parameters, S(2), for Alexa probes at switch residues are high (0.78-0.88) in Gαi1•GDP and lower (0.67-0.75) in Gαi1•GTPγS, although in crystal structures, switch segments are more ordered in the latter. Local motions at Ax63, Ax180, Ax209, and Ax330 are all markedly slower (2.3-2.8 ns) in Gαi1:Ric-8A than in Gαi1•GXP, and only modest (± 0.1) differences in S(2) are observed at most sites in Gαi1:Ric-8A relative to Gαi1•GXP. The slow dynamics suggests long-range correlated transitions within an ensemble of states and, particularly in the hinge and switch segments that make direct contact with Ric-8A. Induction of Gαi1 structural heterogeneity by Ric-8A provides a mechanism for nucleotide release.

Xenopus as a model organism to study heterotrimeric G-protein pathway during collective cell migration of neural crest

Collective cell migration is essential in many fundamental aspects of normal development, like morphogenesis, organ formation, wound healing, and immune responses, as well as in the etiology of severe pathologies, like cancer metastasis. In spite of the huge amount of data accumulated on cell migration, such a complex process involves many molecular actors, some of which still remain to be functionally characterized. One of these signals is the heterotrimeric G-protein pathway that has been studied mainly in gastrulation movements. Recently we have reported that Ric-8A, a GEF for Gα proteins, plays an important role in neural crest migration in Xenopus development. Xenopus neural crest cells, a highly migratory embryonic cell population induced at the border of the neural plate that migrates extensively in order to differentiate in other tissues during development, have become a good model to understand the dynamics that regulate cell migration. In this review, we aim to provide sufficient evidence supporting how useful Xenopus model with its different tools, such as explants and transplants, paired with improved in vivo imaging techniques, will allow us to tackle the multiple signaling mechanisms involved in neural crest cell migration.

Regulation, Signaling, and Physiological Functions of G-Proteins

Heterotrimeric guanine-nucleotide-binding regulatory proteins (G-proteins) mainly relay the information from G-protein-coupled receptors (GPCRs) on the plasma membrane to the inside of cells to regulate various biochemical functions. Depending on the targeted cell types, tissues, and organs, these signals modulate diverse physiological functions. The basic schemes of heterotrimeric G-proteins have been outlined. In this review, we briefly summarize what is known about the regulation, signaling, and physiological functions of G-proteins. We then focus on a few less explored areas such as the regulation of G-proteins by non-GPCRs and the physiological functions of G-proteins that cannot be easily explained by the known G-protein signaling pathways. There are new signaling pathways and physiological functions for G-proteins to be discovered and further interrogated. With the advancements in structural and computational biological techniques, we are closer to having a better understanding of how G-proteins are regulated and of the specificity of G-protein interactions with their regulators.

ARMS/Kidins220 and synembryn-B levels regulate NGF-mediated secretion

Proper development of the nervous system requires a temporally and spatially orchestrated set of events including differentiation, synapse formation and neurotransmission. Nerve growth factor (NGF) acting through the TrkA neurotrophin receptor (also known as NTRK1) regulates many of these events. However, the molecular mechanisms responsible for NGF-regulated secretion are not completely understood. Here, we describe a new signaling pathway involving TrkA, ARMS (also known as Kidins220), synembryn-B and Rac1 in NGF-mediated secretion in PC12 cells. Whereas overexpression of ARMS blocked NGF-mediated secretion, without affecting basal secretion, a decrease in ARMS resulted in potentiation. Similar effects were observed with synembryn-B, a protein that interacts directly with ARMS. Downstream of ARMS and synembryn-B are Gαq and Trio proteins, which modulate the activity of Rac1 in response to NGF. Expression of dominant-negative Rac1 rescued the secretion defects of cells overexpressing ARMS or synembryn-B. Thus, this neurotrophin pathway represents a new mechanism responsible for NGF-regulated secretion.

Heterotrimeric G protein signaling via GIV/Girdin: Breaking the rules of engagement, space, and time

Canonical signal transduction via heterotrimeric G proteins is spatially and temporally restricted, that is, triggered exclusively at the plasma membrane (PM), only by agonist activation of G protein-coupled receptors (GPCRs) via a process that completes within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a non-canonical pathway for activation of heterotrimeric G proteins by the non-receptor guanidine-nucleotide exchange factor (GEF), GIV/Girdin. This pathway has distinctive temporal and spatial features and an unusual profile of receptor engagement: diverse classes of receptors, not just GPCRs can engage with GIV to trigger such activation. Such activation is spatially and temporally unrestricted, that is, can occur both at the PM and on internal membranes discontinuous with the PM, and can continue for prolonged periods of time. Here, we provide the most complete up-to-date review of the molecular mechanisms that govern the unique spatiotemporal aspects of non-canonical G protein activation by GIV and the relevance of this new paradigm in health and disease.

The CREB Transcription Factor Controls Transcriptional Activity of the Human RIC8B Gene

Proper regulation of gene expression is essential for normal development, cellular growth, and differentiation. Differential expression profiles of mRNA coding for vertebrate Ric-8B during embryo and adult stages have been observed. In addition, Ric-8B is expressed in few cerebral nuclei subareas. These facts point to a dynamic control of RIC8B gene expression. In order to understand the transcriptional regulation of this gene, we searched for cis-elements in the sequence of the human RIC8B promoter region, identifying binding sites for the basic/leucine zipper (bZip) CREB transcription factor family (CRE sites) and C/EBP transcription factor family (C/EBP sites). CRE sites were found clustered near the transcription start site, while the C/EBP sites were found clustered at around 300 bp upstream the CRE sites. Here, we demonstrate the ability of CREB1 and C/EBPβ to bind their respective elements identified in the RIC8B promoter. Comparative protein-DNA interaction analyses revealed only the proximal elements as high affinity sites for CREB1 and only the distal elements as high affinity sites for C/EBPβ. Chromatin immunoprecipitation analyses, carried out using a human neuroblastoma cell line, confirmed the preferential association of CREB to the proximal region of the RIC8B promoter. By performing luciferase reporter assays, we found the CRE sites as the most relevant elements for its transcriptional activity. Taken together, these data show the existence of functional CREB and C/EBP binding sites in the human RIC8B gene promoter, a particular distribution of these sites and demonstrate a relevant role of CREB in stimulating transcriptional activity of this gene. J. Cell. Biochem. 117: 1797-1805, 2016. © 2016 Wiley Periodicals, Inc.

The untapped potential of tyrosine-based G protein signaling

Tyrosine-based and trimeric G protein-based signaling are the two most widely studied and distinct mechanisms for signal transduction in eukaryotes. How each of them relay signals across the plasma membrane independently of each other has been extensively characterized; however, an understanding of how they work together remained obscure. Recently, a rapidly emerging paradigm has revealed that tyrosine based signals are relayed via G proteins, and that the cross-talk between the two hubs are more robustly and sophisticatedly integrated than was previously imagined. More importantly, by straddling the two signaling hubs that are most frequently targeted for their therapeutic significance, the tyrosine-based G-protein signaling pathway has its own growing list of pathophysiologic importance, both as therapeutic target in a variety of disease states, and by paving the way for personalized medicine. The fundamental principles of this emerging paradigm and its pharmacologic potential are discussed.

Interdicting Gq Activation in Airway Disease by Receptor-Dependent and Receptor-Independent Mechanisms

Gαqβγ heterotrimer (Gq), an important mediator in the pathology of airway disease, plays a central role in bronchoconstriction and airway remodeling, including airway smooth muscle growth and inflammation. Current therapeutic strategies to treat airway disease include the use of muscarinic and leukotriene receptor antagonists; however, these pharmaceuticals demonstrate a limited clinical efficacy as multiple Gq-coupled receptor subtypes contribute to these pathologies. Thus, broadly inhibiting the activation of Gq may be an advantageous therapeutic approach. Here, we investigated the effects of broadly inhibiting Gq activation in vitro and ex vivo using receptor-dependent and receptor-independent strategies. P4pal-10 is a protease activated receptor 4-derived pepducin that exhibits efficacy toward multiple Gq-coupled receptors. Mechanistic studies demonstrated that P4pal-10 selectively inhibits all G protein coupling to several Gq-coupled receptors, including protease activated receptor 1, muscarinic acetylcholine M3, and histamine H1 receptors, while demonstrating no direct effect on Gq. We also evaluated the ability of FR900359, also known as UBO-QIC, to directly inhibit Gq activation. FR900359 inhibited spontaneous Gαq nucleotide exchange, while having little effect on Gαsβγ, Gαiβγ, or Gα12/13βγ heterotrimer activity. Both P4pal-10 and FR900359 inhibited Gq-mediated intracellular signaling and primary human airway smooth muscle growth, whereas only FR900359 effectively interdicted agonist-promoted airway contraction in human precision cut lung slices. These studies serve as a proof of concept that the broad-based inhibition of Gq activation may be a useful therapeutic approach to treat multiple common pathologies of airway disease.

Inhibition of Gαs/cAMP Signaling Decreases TCR-Stimulated IL-2 transcription in CD4(+) T Helper Cells

BACKGROUND

The role of cAMP in regulating T cell activation and function has been controversial. cAMP is generally known as an immunosuppressant, but it is also required for generating optimal immune responses. As the effect of cAMP is likely to depend on its cellular context, the current study investigated whether the mechanism of activation of Gαs and adenylyl cyclase influences their effect on T cell receptor (TCR)-stimulated interleukin-2 (IL-2) mRNA levels.

METHODS

The effect of blocking Gs-coupled receptor (GsPCR)-mediated Gs activation on TCR-stimulated IL-2 mRNA levels in CD4(+) T cells was compared with that of knocking down Gαs expression or inhibiting adenylyl cyclase activity. The effect of knocking down Gαs expression on TCR-stimulated cAMP accumulation was compared with that of blocking GsPCR signaling.

RESULTS

ZM-241385, an antagonist to the Gs-coupled A2A adenosine receptor (A2AR), enhanced TCR-stimulated IL-2 mRNA levels in primary human CD4(+) T helper cells and in Jurkat T cells. A dominant negative Gαs construct, GαsDN3, also enhanced TCR-stimulated IL-2 mRNA levels. Similar to GsPCR antagonists, GαsDN3 blocked GsPCR-dependent activation of both Gαs and Gβγ. In contrast, Gαs siRNA and 2',5'-dideoxyadenosine (ddA), an adenylyl cyclase inhibitor, decreased TCR-stimulated IL-2 mRNA levels. Gαs siRNA, but not GαsDN3, decreased TCR-stimulated cAMP synthesis. Potentiation of IL-2 mRNA levels by ZM-241385 required at least two days of TCR stimulation, and addition of ddA after three days of TCR stimulation enhanced IL-2 mRNA levels.

CONCLUSIONS

GsPCRs play an inhibitory role in the regulation of TCR-stimulated IL-2 mRNA levels whereas Gαs and cAMP can play a stimulatory one. Additionally, TCR-dependent activation of Gαs does not appear to involve GsPCRs. These results suggest that the context of Gαs/cAMP activation and the stage of T cell activation and differentiation determine the effect on TCR-stimulated IL-2 mRNA levels.

Low levels of Gαs and Ric8b in testicular sertoli cells may underlie restricted FSH action during infancy in primates

FSH acts via testicular Sertoli cells (Sc) bearing FSH receptor (FSH-R) for regulating male fertility. Despite an adult-like FSH milieu in infant boys and monkeys, spermatogenesis is not initiated until the onset of puberty. We used infant and pubertal monkey Sc to reveal the molecular basis underlying developmental differences of FSH-R signaling in them. Unlike pubertal Sc, increasing doses of FSH failed to augment cAMP production by infant Sc. The expression of Gαs subunit and Ric8b, which collectively activate adenylyl cyclase (AC) for augmenting cAMP production and gene transcription, were significantly low in infant Sc. However, forskolin, which acts directly on AC bypassing FSH-R, augmented cAMP production and gene transcription uniformly in both infant and pubertal Sc. FSH-induced Gαs mRNA expression was higher in pubertal Sc. However, Gαi-2 expression was down-regulated by FSH in pubertal Sc, unlike infant Sc. FSH failed, but forskolin or 8-Bromoadenosine 3',5'-cyclic monophosphate treatment to infant Sc significantly augmented the expression of transferrin, androgen binding protein, inhibin-β-B, stem cell factor, and glial-derived neurotropic factor, which are usually up-regulated by FSH in pubertal Sc during spermatogenic onset. This suggested that lack of FSH mediated down-regulation of Gαi-2 expression and limited expression of Gαs subunit as well as Ric8b may underlie limited FSH responsiveness of Sc during infancy. This study also divulged that intracellular signaling events downstream of FSH-R are in place and can be activated exogenously in infant Sc. Additionally, this information may help in the proper diagnosis and treatment of infertile individuals having abnormal G protein-coupled FSH-R.

Mammalian odorant receptors: functional evolution and variation

In mammals, the perception of smell starts with the activation of odorant receptors (ORs) by volatile molecules in the environment. The mammalian OR repertoire has been subject to rapid evolution, and is highly diverse within the human population. Recent advances in the functional expression and ligand identification of ORs allow for functional analysis of OR evolution, and reveal that changes in OR protein sequences translate into high degrees of functional variations. Moreover, in several cases the functional variation of a single OR affects the perception of its cognate odor ligand, providing clues as to how an odor is coded at the receptor level.

Activators of G protein signaling in the kidney

Heterotrimeric G proteins play a crucial role in regulating signal processing to maintain normal cellular homeostasis, and subtle perturbations in its activity can potentially lead to the pathogenesis of renal disorders or diseases. Cell-surface receptors and accessory proteins, which normally modify and organize the coupling of individual G protein subunits, contribute to the regulation of heterotrimeric G protein activity and their convergence and/or divergence of downstream signaling initiated by effector systems. Activators of G protein signaling (AGS) are a family of accessory proteins that intervene at multiple distinct points during the activation-inactivation cycle of G proteins, even in the absence of receptor stimulation. Perturbations in the expression of individual AGS proteins have been reported to modulate signal transduction pathways in a wide array of diseases and disorders within the brain, heart, immune system, and more recently, the kidney. This review will provide an overview of the expression profile, localization, and putative biologic role of the AGS family in the context of normal and diseased states of the kidney.

GIV/Girdin transmits signals from multiple receptors by triggering trimeric G protein activation

Activation of trimeric G proteins has been traditionally viewed as the exclusive job of G protein-coupled receptors (GPCRs). This view has been challenged by the discovery of non-receptor activators of trimeric G proteins. Among them, GIV (a.k.a. Girdin) is the first for which a guanine nucleotide exchange factor (GEF) activity has been unequivocally associated with a well defined motif. Here we discuss how GIV assembles alternative signaling pathways by sensing cues from various classes of surface receptors and relaying them via G protein activation. We also describe the dysregulation of this mechanism in disease and how its targeting holds promise for novel therapeutics.

The guanine nucleotide exchange factor Ric-8A induces domain separation and Ras domain plasticity in Gαi1

Heterotrimeric G proteins are activated by exchange of GDP for GTP at the G protein alpha subunit (Gα), most notably by G protein-coupled transmembrane receptors. Ric-8A is a soluble cytoplasmic protein essential for embryonic development that acts as both a guanine nucleotide exchange factor (GEF) and a chaperone for Gα subunits of the i, q, and 12/13 classes. Previous studies demonstrated that Ric-8A stabilizes a dynamically disordered state of nucleotide-free Gα as the catalytic intermediate for nucleotide exchange, but no information was obtained on the structures involved or the magnitude of the structural fluctuations. In the present study, site-directed spin labeling (SDSL) together with double electron-electron resonance (DEER) spectroscopy is used to provide global distance constraints that identify discrete members of a conformational ensemble in the Gαi1:Ric-8A complex and the magnitude of structural differences between them. In the complex, the helical and Ras-like nucleotide-binding domains of Gαi1 pivot apart to occupy multiple resolved states with displacements as large as 25 Å. The domain displacement appears to be distinct from that observed in Gαs upon binding of Gs to the β2 adrenergic receptor. Moreover, the Ras-like domain exhibits structural plasticity within and around the nucleotide-binding cavity, and the switch I and switch II regions, which are known to adopt different conformations in the GDP- and GTP-bound states of Gα, undergo structural rearrangements. Collectively, the data show that Ric-8A induces a conformationally heterogeneous state of Gαi and provide insight into the mechanism of action of a nonreceptor Gα GEF.

The G protein α chaperone Ric-8 as a potential therapeutic target

Resistance to inhibitors of cholinesterase (Ric-8)A and Ric-8B are essential genes that encode positive regulators of heterotrimeric G protein α subunits. Controversy persists surrounding the precise way(s) that Ric-8 proteins affect G protein biology and signaling. Ric-8 proteins chaperone nucleotide-free Gα-subunit states during biosynthetic protein folding prior to G protein heterotrimer assembly. In organisms spanning the evolutionary window of Ric-8 expression, experimental perturbation of Ric-8 genes results in reduced functional abundances of G proteins because G protein α subunits are misfolded and degraded rapidly. Ric-8 proteins also act as Gα-subunit guanine nucleotide exchange factors (GEFs) in vitro. However, Ric-8 GEF activity could strictly be an in vitro phenomenon stemming from the ability of Ric-8 to induce partial Gα unfolding, thereby enhancing GDP release. Ric-8 GEF activity clearly differs from the GEF activity of G protein-coupled receptors (GPCRs). G protein βγ is inhibitory to Ric-8 action but obligate for receptors. It remains an open question whether Ric-8 has dual functions in cells and regulates G proteins as both a molecular chaperone and GEF. Clearly, Ric-8 has a profound influence on heterotrimeric G protein function. For this reason, we propose that Ric-8 proteins are as yet untested therapeutic targets in which pharmacological inhibition of the Ric-8/Gα protein-protein interface could serve to attenuate the effects of disease-causing G proteins (constitutively active mutants) and/or GPCR signaling. This minireview will chronicle the understanding of Ric-8 function, provide a comparative discussion of the Ric-8 molecular chaperoning and GEF activities, and support the case for why Ric-8 proteins should be considered potential targets for development of new therapies.

Implications of non-canonical G-protein signaling for the immune system

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which consist of three subunits α, β, and γ, function as molecular switches to control downstream effector molecules activated by G protein-coupled receptors (GPCRs). The GTP/GDP binding status of Gα transmits information about the ligand binding state of the GPCR to intended signal transduction pathways. In immune cells heterotrimeric G proteins impact signal transduction pathways that directly, or indirectly, regulate cell migration, activation, survival, proliferation, and differentiation. The cells of the innate and adaptive immune system abundantly express chemoattractant receptors and lesser amounts of many other types of GPCRs. But heterotrimeric G-proteins not only function in classical GPCR signaling, but also in non-canonical signaling. In these pathways the guanine exchange factor (GEF) exerted by a GPCR in the canonical pathway is replaced or supplemented by another protein such as Ric-8A. In addition, other proteins such as AGS3-6 can compete with Gβγ for binding to GDP bound Gα. This competition can promote Gβγ signaling by freeing Gβγ from rapidly rebinding GDP bound Gα. The proteins that participate in these non-canonical signaling pathways will be briefly described and their role, or potential one, in cells of the immune system will be highlighted.

Activators of G protein signaling exhibit broad functionality and define a distinct core signaling triad

Activators of G protein signaling (AGS), initially discovered in the search for receptor-independent activators of G protein signaling, define a broad panel of biologic regulators that influence signal transfer from receptor to G-protein, guanine nucleotide binding and hydrolysis, G protein subunit interactions, and/or serve as alternative binding partners for Gα and Gβγ independently of the classic heterotrimeric Gαβγ. AGS proteins generally fall into three groups based upon their interaction with and regulation of G protein subunits: group I, guanine nucleotide exchange factors (GEF); group II, guanine nucleotide dissociation inhibitors; and group III, entities that bind to Gβγ. Group I AGS proteins can engage all subclasses of G proteins, whereas group II AGS proteins primarily engage the Gi/Go/transducin family of G proteins. A fourth group of AGS proteins with selectivity for Gα16 may be defined by the Mitf-Tfe family of transcription factors. Groups I-III may act in concert, generating a core signaling triad analogous to the core triad for heterotrimeric G proteins (GEF + G proteins + effector). These two core triads may function independently of each other or actually cross-integrate for additional signal processing. AGS proteins have broad functional roles, and their discovery has advanced new concepts in signal processing, cell and tissue biology, receptor pharmacology, and system adaptation, providing unexpected platforms for therapeutic and diagnostic development.

Increased ubiquitination and the crosstalk of G protein signaling in cardiac myocytes: involvement of Ric-8B in Gs suppression by Gq signal

Hyperactivation of Gq signaling causes cardiac hypertrophy, and β-adrenergic receptor-mediated Gs signaling is attenuated in hypertrophic cardiomyocytes. Here, we found the increase in a global ubiquitination in hypertrophic mouse heart. The activation of Gq signaling resulted in the ubiquitination of Gαs in neonatal rat cardiomyocytes, reduced Gαs expression, and suppressed cAMP response to β-adrenergic receptor stimulation. Ectopic expression of Gαq induced a similar suppression, which is due to the degradation of Gαs by a ubiquitin-proteasome pathway. Co-expression of Ric-8B, a positive regulator of Gαs, effectively canceled the Gαq-induced ubiquitination of Gαs and recovered the cAMP accumulation. In vitro, Gαq competes for the binding of Gαs to Ric-8B. These data show a new role of Ric-8B in the crosstalk of two distinct G protein signaling pathways, which are possibly involved in a part of mechanisms of chronic heart failure.

Drosophila Ric-8 interacts with the Gα12/13 subunit, Concertina, during activation of the Folded gastrulation pathway

A novel tissue culture model for studying cellular constriction is introduced and used to show that activation of the Fog signaling pathway depends on Ric-8 and that Ric-8 preferentially binds and localizes inactive Cta. Conserved residues are identified within Ric-8 that are important for the binding and function of Cta.

Heterotrimeric G proteins, composed of α, β, and γ subunits, are activated by exchange of GDP for GTP on the Gα subunit. Canonically, Gα is stimulated by the guanine-nucleotide exchange factor (GEF) activity of ligand-bound G protein–coupled receptors. However, Gα subunits may also be activated in a noncanonical manner by members of the Ric-8 family, cytoplasmic proteins that also act as GEFs for Gα subunits. We used a signaling pathway active during Drosophila gastrulation as a model system to study Ric-8/Gα interactions. A component of this pathway, the Drosophila Gα_12/13 subunit, Concertina (Cta), is necessary to trigger actomyosin contractility during gastrulation events. Ric-8 mutants exhibit similar gastrulation defects to Cta mutants. Here we use a novel tissue culture system to study a signaling pathway that controls cytoskeletal rearrangements necessary for cellular morphogenesis. We show that Ric-8 regulates this pathway through physical interaction with Cta and preferentially interacts with inactive Cta and directs its localization within the cell. We also use this system to conduct a structure–function analysis of Ric-8 and identify key residues required for both Cta interaction and cellular contractility.

Ablation of RIC8A function in mouse neurons leads to a severe neuromuscular phenotype and postnatal death

Resistance to inhibitors of cholinesterase 8 (RIC8) is a guanine nucleotide exchange factor required for the intracellular regulation of G protein signalling. RIC8 activates different Gα subunits via non-canonical pathway, thereby amplifying and prolonging the G protein mediated signal. In order to circumvent the embryonic lethality associated with the absence of RIC8A and to study its role in the nervous system, we constructed Ric8a conditional knockout mice using Cre/loxP technology. Introduction of a synapsin I promoter driven Cre transgenic mouse strain (SynCre) into the floxed Ric8a (Ric8a (F/F) ) background ablated RIC8A function in most differentiated neuron populations. Mutant SynCre (+/-) Ric8 (lacZ/F) mice were born at expected Mendelian ratio, but they died in early postnatal age (P4-P6). The mutants exhibited major developmental defects, like growth retardation and muscular weakness, impaired coordination and balance, muscular spasms and abnormal heart beat. Histological analysis revealed that the deficiency of RIC8A in neurons caused skeletal muscle atrophy and heart muscle hypoplasia, in addition, the sinoatrial node was misplaced and its size reduced. However, we did not observe gross morphological changes in brains of SynCre (+/-) Ric8a (lacZ/F) mutants. Our results demonstrate that in mice the activity of RIC8A in neurons is essential for survival and its deficiency causes a severe neuromuscular phenotype.

Ric-8 regulation of heterotrimeric G proteins

Resistance to inhibitors of cholinesterase 8 proteins (Ric-8A and Ric-8B) collectively bind the four classes of heterotrimeric G protein α subunits. Ric-8A and Ric-8B act as non-receptor guanine nucleotide exchange factors (GEFs) toward the Gα subunits that each binds in vitro and seemingly regulate diverse G protein signaling systems in cells. Combined evidence from worm, fly and mammalian systems has shown that Ric-8 proteins are required to maintain proper cellular abundances of G proteins. Ric-8 proteins support G protein levels by serving as molecular chaperones that promote Gα subunit biosynthesis. In this review, the evidence that Ric-8 proteins act as non-receptor GEF activators of G proteins in signal transduction contexts will be weighed against the evidence supporting the molecular chaperoning function of Ric-8 in promoting G protein abundance. I will conclude by suggesting that Ric-8 proteins may act in either capacity in specific contexts. The field awaits additional experimentation to delineate the putative multi-functionality of Ric-8 towards G proteins in cells.

Eukaryotic G protein signaling evolved to require G protein-coupled receptors for activation

Although bioinformatic analysis of the increasing numbers of diverse genome sequences and amount of functional data has provided insight into the evolution of signaling networks, bioinformatics approaches have limited application for understanding the evolution of highly divergent protein families. We used biochemical analyses to determine the in vitro properties of selected divergent components of the heterotrimeric guanine nucleotide-binding protein (G protein) signaling network to investigate signaling network evolution. In animals, G proteins are activated by cell-surface seven-transmembrane (7TM) receptors, which are named G protein-coupled receptors (GPCRs) and function as guanine nucleotide exchange factors (GEFs). In contrast, the plant G protein is intrinsically active, and a 7TM protein terminates G protein activity by functioning as a guanosine triphosphatase-activating protein (GAP). We showed that ancient regulation of the G protein active state is GPCR-independent and "self-activating," a property that is maintained in Bikonts, one of the two fundamental evolutionary clades containing eukaryotes, whereas G proteins of the other clade, the Unikonts, evolved from being GEF-independent to being GEF-dependent. Self-activating G proteins near the base of the Eukaryota are controlled by 7TM-GAPs, suggesting that the ancestral regulator of G protein activation was a GAP-functioning receptor, not a GEF-functioning GPCR. Our findings indicate that the GPCR paradigm describes a recently evolved network architecture found in a relatively small group of Eukaryota and suggest that the evolution of signaling network architecture is constrained by the availability of molecules that control the activation state of nexus proteins.

Dictyostelium Ric8 is a nonreceptor guanine exchange factor for heterotrimeric G proteins and is important for development and chemotaxis

Heterotrimeric G proteins couple external signals to the activation of intracellular signal transduction pathways. Agonist-stimulated guanine nucleotide exchange activity of G-protein-coupled receptors results in the exchange of G-protein-bound GDP to GTP and the dissociation and activation of the complex into Gα-GTP and a Gβγ dimer. In Dictyostelium, a basal chemotaxis pathway consisting of heterotrimeric and monomeric G proteins is sufficient for chemotaxis. Symmetry breaking and amplification of chemoattractant sensing occurs between heterotrimeric G protein signaling and Ras activation. In a pull-down screen coupled to mass spectrometry, with Gα proteins as bait, we have identified resistant to inhibitors of cholinesterase 8 (Ric8) as a nonreceptor guanine nucleotide exchange factor for Gα-protein. Ric8 is not essential for the initial activation of heterotrimeric G proteins or Ras by uniform chemoattractant; however, it amplifies Gα signaling, which is essential for Ras-mediated symmetry breaking during chemotaxis and development.

Molecular chaperoning function of Ric-8 is to fold nascent heterotrimeric G protein α subunits

We have shown that resistance to inhibitors of cholinesterase 8 (Ric-8) proteins regulate an early step of heterotrimeric G protein α (Gα) subunit biosynthesis. Here, mammalian and plant cell-free translation systems were used to study Ric-8A action during Gα subunit translation and protein folding. Gα translation rates and overall produced protein amounts were equivalent in mock and Ric-8A-immunodepleted rabbit reticulocyte lysate (RRL). GDP-AlF4(-)-bound Gαi, Gαq, Gα13, and Gαs produced in mock-depleted RRL had characteristic resistance to limited trypsinolysis, showing that these G proteins were folded properly. Gαi, Gαq, and Gα13, but not Gαs produced from Ric-8A-depleted RRL were not protected from trypsinization and therefore not folded correctly. Addition of recombinant Ric-8A to the Ric-8A-depleted RRL enhanced GDP-AlF4(-)-bound Gα subunit trypsin protection. Dramatic results were obtained in wheat germ extract (WGE) that has no endogenous Ric-8 component. WGE-translated Gαq was gel filtered and found to be an aggregate. Ric-8A supplementation of WGE allowed production of Gαq that gel filtered as a ∼100 kDa Ric-8A:Gαq heterodimer. Addition of GTPγS to Ric-8A-supplemented WGE Gαq translation resulted in dissociation of the Ric-8A:Gαq heterodimer and production of functional Gαq-GTPγS monomer. Excess Gβγ supplementation of WGE did not support functional Gαq production. The molecular chaperoning function of Ric-8 is to participate in the folding of nascent G protein α subunits.

β-Arrestin-1 directly interacts with Gαs and regulates its function

β-Arrestins function to mediate G protein-coupled receptor (GPCR) desensitization and internalization and to initiate G protein independent signaling of GPCRs. Elucidating how β-arrestin and G protein signal pathways coordinate with each other is important to fully understand GPCR signaling. Here we report that β-arrestin-1 directly interacts with Gα(s). Purified β-arrestin-1 binds to Gα(s) in a rapid association and dissociation manner. β-Arrestin-1 promotes the binding and the release of GTPγS from Gα(s) in vitro. β-Arrestin-1 L33K mutant shows reduced interaction with Gα(s) and has no detectable effects on Gα(s) function. Our study thus reveals a direct crosstalk of β-arrestin-1 with Gα(s).

Fine-tuning of GPCR signals by intracellular G protein modulators

Heterotrimeric G proteins convey receptor signals to intracellular effectors. Superimposed over the basic GPCR-G protein-effector scheme are three types of auxiliary proteins that also modulate Gα. Regulator of G protein signaling proteins and G protein signaling modifier proteins respectively promote GTPase activity and hinder GTP/GDP exchange to limit Gα activation. There are also diverse proteins that, like GPCRs, can promote nucleotide exchange and thus activation. Here we review the impact of these auxiliary proteins on GPCR signaling. Although their precise physiological functions are not yet clear, all of them can produce significant effects in experimental systems. These signaling changes are generally consistent with established effects on isolated Gα; however, the activation state of Gα is seldom verified and many such changes appear also to reflect the physical disruption of or indirect effects on interactions between Gα and its associated GPCR, Gβγ, and/or effector.

Ric-8: different cellular roles for a heterotrimeric G-protein GEF

Signaling via heterotrimeric G-proteins is evoked by agonist-mediated stimulation of seven transmembrane spanning receptors (GPCRs). During the last decade it has become apparent that Gα subunits can be activated by receptor-independent mechanisms. Ric-8 belongs to a highly conserved protein family that regulates heterotrimeric G-protein function, acting as a non-canonical guanine nucleotide exchange factors (GEF) over a subset of Gα subunits. In this review we discuss the roles of Ric-8 in the regulation of diverse cell functions, emphasizing the contribution of its multiple domain protein structure in these diverse functions.

Chaperone-mediated assembly of G protein complexes

G protein signaling depends on the ability of the individual subunits of the G protein heterotrimer to assemble into functional complexes. Formation of the G protein βγ (Gβγ) dimer is particularly challenging because it is an obligate dimer in which the individual subunits are unstable on their own. Recent studies have revealed an intricate chaperone system that brings the Gβ and Gγ subunits together. This system includes the cytosolic chaperonin containing TCP-1 (CCT) and its co-chaperone phosducin-like protein 1 (PhLP1). CCT assists Gβ in achieving its β-propeller structure, while PhLP1 releases Gβ from CCT and facilitates its interaction with Gγ. Once Gβγ is formed, PhLP1 remains bound until it is displaced by the Gα subunit and the G protein heterotrimer is brought together. Another obligate dimer is the complex between the G protein β(5) subunit and a regulator of G protein signaling protein (Gβ(5)-RGS). Gβ(5)-RGS also requires CCT for Gβ(5) folding, but PhLP1 plays a different role. It stabilizes the interaction between Gβ(5) and CCT, perhaps to increase folding efficiency. After Gβ(5) folding PhLP1 must subsequently release, allowing the RGS protein to bind and form the Gβ(5)-RGS dimer directly on CCT. Gβ(5)-RGS is then freed from CCT to interact with its membrane anchoring protein and form a stable complex that turns off the G protein signal by catalyzing GTP hydrolysis on Gα.

Ric-8 proteins are molecular chaperones that direct nascent G protein α subunit membrane association

Ric-8A (resistance to inhibitors of cholinesterase 8A) and Ric-8B are guanine nucleotide exchange factors that enhance different heterotrimeric guanine nucleotide-binding protein (G protein) signaling pathways by unknown mechanisms. Because transgenic disruption of Ric-8A or Ric-8B in mice caused early embryonic lethality, we derived viable Ric-8A- or Ric-8B-deleted embryonic stem (ES) cell lines from blastocysts of these mice. We observed pleiotropic G protein signaling defects in Ric-8A(-/-) ES cells, which resulted from reduced steady-state amounts of Gα(i), Gα(q), and Gα(13) proteins to <5% of those of wild-type cells. The amounts of Gα(s) and total Gβ protein were partially reduced in Ric-8A(-/-) cells compared to those in wild-type cells, and only the amount of Gα(s) was reduced substantially in Ric-8B(-/-) cells. The abundances of mRNAs encoding the G protein α subunits were largely unchanged by loss of Ric-8A or Ric-8B. The plasma membrane residence of G proteins persisted in the absence of Ric-8 but was markedly reduced compared to that in wild-type cells. Endogenous Gα(i) and Gα(q) were efficiently translated in Ric-8A(-/-) cells but integrated into endomembranes poorly; however, the reduced amounts of G protein α subunits that reached the membrane still bound to nascent Gβγ. Finally, Gα(i), Gα(q), and Gβ(1) proteins exhibited accelerated rates of degradation in Ric-8A(-/-) cells compared to those in wild-type cells. Together, these data suggest that Ric-8 proteins are molecular chaperones required for the initial association of nascent Gα subunits with cellular membranes.

Effects of integrin-mediated cell adhesion on plasma membrane lipid raft components and signaling

Anchorage dependence of cell growth, which is mediated by multiple integrin-regulated signaling pathways, is a key defense against cancer metastasis. Detachment of cells from the extracellular matrix triggers caveolin-1-dependent internalization of lipid raft components, which mediates suppression of Rho GTPases, Erk, and phosphatidylinositol 3-kinase in suspended cells. Elevation of cyclic adenosine monophosphate (cAMP) following cell detachment is also implicated in termination of growth signaling in suspended cells. Studies of integrins and lipid rafts, however, examined mainly ganglioside GM1 and glycosylphosphatidylinositol-linked proteins as lipid raft markers. In this study, we examine a wider range of lipid raft components. Whereas many raft components internalized with GM1 following cell detachment, flotillin2, connexin43, and Gα(s) remained in the plasma membrane. Loss of cell adhesion caused movement of many components from the lipid raft to the nonraft fractions on sucrose gradients, although flotillin2, connexin43, and H-Ras were resistant. Gα(s) lost its raft association, concomitant with cAMP production. Modification of the lipid tail of Gα(s) to increase its association with ordered domains blocked the detachment-induced increase in cAMP. These data define the effects of that integrin-mediated adhesion on the localization and behavior of a variety of lipid raft components and reveal the mechanism of the previously described elevation of cAMP after cell detachment.