Importance of the G protein gamma subunit in activating G protein-coupled inward rectifier K(+) channels

G protein αq exerts expression level-dependent distinct signaling paradigms

G protein αq-coupled receptors (Gq-GPCRs) primarily signal through GαqGTP mediated phospholipase Cβ (PLCβ) stimulation and the subsequent hydrolysis of phosphatidylinositol 4, 5 bisphosphate (PIP2). Though Gq-heterotrimer activation results in both GαqGTP and Gβγ, unlike Gi/o-receptors, it is unclear if Gq-coupled receptors employ Gβγ as a major signal transducer. Compared to Gi/o- and Gs-coupled receptors, we observed that most cell types exhibit a limited free Gβγ generation upon Gq-pathway and Gαq/11 heterotrimer activation. We show that cells transfected with Gαq or endogenously expressing more than average-levels of Gαq/11 compared to Gαs and Gαi exhibit a distinct signaling regime primarily characterized by recovery-resistant PIP2 hydrolysis. Interestingly, the elevated Gq-expression is also associated with enhanced free Gβγ generation and signaling. Furthermore, the gene GNAQ, which encodes for Gαq, has recently been identified as a cancer driver gene. We also show that GNAQ is overexpressed in tumor samples of patients with Kidney Chromophobe (KICH) and Kidney renal papillary (KIRP) cell carcinomas in a matched tumor-normal sample analysis, which demonstrates the clinical significance of Gαq expression. Overall, our data indicates that cells usually express low Gαq levels, likely safeguarding cells from excessive calcium as wells as from Gβγ signaling.

Mutual action by Gγ and Gβ for optimal activation of GIRK channels in a channel subunit-specific manner

The tetrameric G protein-gated K⁺ channels (GIRKs) mediate inhibitory effects of neurotransmitters that activate G_i/o-coupled receptors. GIRKs are activated by binding of the Gβγ dimer, via contacts with Gβ. Gγ underlies membrane targeting of Gβγ, but has not been implicated in channel gating. We observed that, in Xenopus oocytes, expression of Gγ alone activated homotetrameric GIRK1* and heterotetrameric GIRK1/3 channels, without affecting the surface expression of GIRK or Gβ. Gγ and Gβ acted interdependently: the effect of Gγ required the presence of ambient Gβ and was enhanced by low doses of coexpressed Gβ, whereas excess of either Gβ or Gγ imparted suboptimal activation, possibly by sequestering the other subunit “away” from the channel. The unique distal C-terminus of GIRK1, G1-dCT, was important but insufficient for Gγ action. Notably, GIRK2 and GIRK1/2 were not activated by Gγ. Our results suggest that Gγ regulates GIRK1* and GIRK1/3 channel’s gating, aiding Gβ to trigger the channel’s opening. We hypothesize that Gγ helps to relax the inhibitory effect of a gating element (“lock”) encompassed, in part, by the G1-dCT; GIRK2 acts to occlude the effect of Gγ, either by setting in motion the same mechanism as Gγ, or by triggering an opposing gating effect.

Melanopsin (Opn4) utilizes Gαi and Gβγ as major signal transducers

Melanopsin (Opn4), a ubiquitously expressed photoreceptor in all classes of vertebrates, is crucial for both visual and non-visual signaling. Opn4 supports visual functions of the eye by sensing radiance levels and discriminating contrast and brightness. Non-image-forming functions of Opn4 not only regulate circadian behavior, but also control growth and development processes of the retina. It is unclear how a single photoreceptor could govern such a diverse range of physiological functions; a role in genetic hardwiring could be one explanation, but molecular and mechanistic evidence is lacking. In addition to its role in canonical G_q pathway activation, here we demonstrate that Opn4 efficiently activates G_i heterotrimers and signals through the G protein βγ. Compared with the low levels of G_i pathway activation observed for several G_q-coupled receptors, the robust Gα_i and Gβγ signaling of Opn4 led to both generation of PIP₃ and directional migration of RAW264.7 macrophages. We propose that the ability of Opn4 to signal through Gα_i and Gβγ subunits is a major contributor to its functional diversity.

Regulation of G Protein βγ Signaling

Heterotrimeric guanine nucleotide-binding proteins (G proteins) deliver external signals to the cell interior, upon activation by the external signal stimulated G protein-coupled receptors (GPCRs).While the activated GPCRs control several pathways independently, activated G proteins control the vast majority of cellular and physiological functions, ranging from vision to cardiovascular homeostasis. Activated GPCRs dissociate GαGDPβγ heterotrimer into GαGTP and free Gβγ. Earlier, GαGTP was recognized as the primary signal transducer of the pathway and Gβγ as a passive signaling modality that facilitates the activity of Gα. However, Gβγ later found to regulate more number of pathways than GαGTP does. Once liberated from the heterotrimer, free Gβγ interacts and activates a diverse range of signaling regulators including kinases, lipases, GTPases, and ion channels, and it does not require any posttranslation modifications. Gβγ family consists of 48 members, which show cell- and tissue-specific expressions, and recent reports show that cells employ the subtype diversity in Gβγ to achieve desired signaling outcomes. In addition to activated GPCRs, which induce free Gβγ generation and the rate of GTP hydrolysis in Gα, which sequester Gβγ in the heterotrimer, terminating Gβγ signaling, additional regulatory mechanisms exist to regulate Gβγ activity. In this chapter, we discuss structure and function, subtype diversity and its significance in signaling regulation, effector activation, regulatory mechanisms as well as the disease relevance of Gβγ in eukaryotes.

Gγ identity dictates efficacy of Gβγ signaling and macrophage migration

G protein βγ subunit (Gβγ) is a major signal transducer and controls processes ranging from cell migration to gene transcription. Despite having significant subtype heterogeneity and exhibiting diverse cell- and tissue-specific expression levels, Gβγ is often considered a unified signaling entity with a defined functionality. However, the molecular and mechanistic basis of Gβγ's signaling specificity is unknown. Here, we demonstrate that Gγ subunits, bearing the sole plasma membrane (PM)-anchoring motif, control the PM affinity of Gβγ and thereby differentially modulate Gβγ effector signaling in a Gγ-specific manner. Both Gβγ signaling activity and the migration rate of macrophages are strongly dependent on the PM affinity of Gγ. We also found that the type of C-terminal prenylation and five to six pre-CaaX motif residues at the PM-interacting region of Gγ control the PM affinity of Gβγ. We further show that the overall PM affinity of the Gβγ pool of a cell type is a strong predictor of its Gβγ signaling-activation efficacy. A kinetic model encompassing multiple Gγ types and parameterized for empirical Gβγ behaviors not only recapitulated experimentally observed signaling of Gβγ, but also suggested a Gγ-dependent, active-inactive conformational switch for the PM-bound Gβγ, regulating effector signaling. Overall, our results unveil crucial aspects of signaling and cell migration regulation by Gγ type-specific PM affinities of Gβγ.

Signal transduction pathway for the substance P-induced inhibition of rat Kir3 (GIRK) channel

Certain transmitters inhibit Kir3 (GIRK) channels, resulting in neuronal excitation. We analysed signalling mechanisms for substance P (SP)-induced Kir3 inhibition in relation to the role of phosphatidylinositol 4,5-bisphosphate (PIP(2)). SP rapidly - with a half-time of approximately 10 s with intracellular GTPgammaS and approximately 14 s with intracellular GTP - inhibits a robustly activated Kir3.1/Kir3.2 current. A mutant Kir3 channel, Kir3.1(M223L)/Kir3.2(I234L), which has a stronger binding to PIP(2) than does the wild type Kir3.1/Kir3.2, is inhibited by SP as rapidly as the wild type Kir3.1/Kir3.2. This result contradicts the idea that Kir3 inhibition originates from the depletion of PIP(2). A Kir2.1 (IRK1) mutant, Kir2.1(R218Q), despite having a weaker binding to PIP(2) than wild type Kir3.1/Kir3.2, shows a SP-induced inhibition slower than the wild type Kir3.1/Kir3.2 channel, again conflicting with the PIP(2) theory of channel inhibition. Co-immunoprecipitation reveals that Galpha(q) binds with Kir3.2, but not with Kir2.2 or Kir2.1. These functional results and co-immunoprecipitation data suggest that G(q) activation rapidly inhibits Kir3 (but not Kir2), possibly by direct binding of Galpha(q) to the channel.

Critical determinants of the G protein gamma subunits in the Gbetagamma stimulation of G protein-activated inwardly rectifying potassium (GIRK) channel activity

The betagamma subunits of G proteins modulate inwardly rectifying potassium (GIRK) channels through direct interactions. Although GIRK currents are stimulated by mammalian Gbetagamma subunits, we show that they were inhibited by the yeast Gbetagamma (Ste4/Ste18) subunits. A chimera between the yeast and the mammalian Gbeta1 subunits (ymbeta) stimulated or inhibited GIRK currents, depending on whether it was co-expressed with mammalian or yeast Ggamma subunits, respectively. This result underscores the critical functional influence of the Ggamma subunits on the effectiveness of the Gbetagamma complex. A series of chimeras between Ggamma2 and the yeast Ggamma revealed that the C-terminal half of the Ggamma2 subunit is required for channel activation by the Gbetagamma complex. Point mutations of Ggamma2 to the corresponding yeast Ggamma residues identified several amino acids that reduced significantly the ability of Gbetagamma to stimulate channel activity, an effect that was not due to improper association with Gbeta. Most of the identified critical Ggamma residues clustered together, forming an intricate network of interactions with the Gbeta subunit, defining an interaction surface of the Gbetagamma complex with GIRK channels. These results show for the first time a functional role for Ggamma in the effector role of Gbetagamma.

Interaction of G protein beta subunit with inward rectifier K(+) channel Kir3

G protein betagamma subunits bind and activate G protein-coupled inward rectifier K+ (GIRK) channels. This protein-protein interaction is crucial for slow hyperpolarizations of cardiac myocytes and neurons. The crystal structure of Gbeta shows a seven-bladed propeller with four beta strands in each blade. The Gbeta/Galpha interacting surface contains sites for activating GIRK channels. Furthermore, our recent investigation using chimeras between Gbeta1 and yeast beta (STE4) suggested that the outer strands of blades 1 and 2 of Gbeta1 could be an interaction area between Gbeta1 and GIRK. In this study, we made point mutations on suspected residues on these outer strands and investigated their ability to activate GIRK1/GIRK2 channels. Mutations at Thr-86, Thr-87, and Gly-131, all located on the loops between beta-strands, substantially reduced GIRK channel activation, suggesting that these residues are Gbeta/GIRK interaction sites. These mutations did not affect the expression of Gbeta1 or its ability to stimulate PLCbeta2. These residues are surface-accessible and located outside Gbeta/Galpha interaction sites. These results suggest that the residues on the outer surface of blades 1 and 2 are involved in the interaction of Gbetagamma with GIRK channels. Our study suggests a mechanism by which different effectors use different blades to achieve divergence of signaling. We also observed that substitution of alanine for Trp-332 of Gbeta1 impaired the functional interaction of Gbeta1 with GIRK, in agreement with the data on native neuronal GIRK channels. Trp-332 plays a critical role in the interaction of Gbeta1 with Galpha as well as all effectors so far tested.

A glutamate residue at the C terminus regulates activity of inward rectifier K+ channels: implication for Andersen's syndrome

G protein-coupled inward rectifiers (GIRKs) are activated directly by G protein betagamma subunits, whereas classical inward rectifiers (IRKs) are constitutively active. We found that a glutamate residue of GIRK2 (E315), located on a hydrophobic domain of the C terminus, is crucial for the channel activation. This glutamate (or aspartate) residue is conserved in all members of the Kir family. Substitution of alanine for the glutamate on GIRK1, GIRK2, and IRK2, expressed in HEK293 cells, greatly reduced the whole-cell currents. The whole-cell current of GIRK channels with a constitutively active gate, GIRK2(V188A), [Yi, B. A., Lin, Y. F., Jan, Y. N. & Jan, L. Y. (2001) Neuron 29, 657-667] was also reduced by the same glutamate mutation. Mean open time and conductance of single channels in GIRK2 and IRK2 were not affected by the mutation, indicating that the reduced whole-cell current resulted from a lowered probability of channel activation. The mutated GIRK and IRK showed normal trafficking to the cell membrane. The mutated GIRK2 retained the ability to interact with G protein betagamma subunits, and it showed almost the same inwardly rectifying property as the wild type. The mutated GIRK1 and GIRK2 retained ion selectivity to K(+) ions. This glutamate residue corresponds to one of the residues causing Andersen's syndrome [Plaster, N. M., Tawil, R., Tristani-Firouzi, M., Canun, S., Bendahhou, S., Tsunoda, A., Donaldson, M. R., Iannaccone, S. T., Brunt, E., Barohn, R., et al. (2001) Cell 105, 511-519]. Our interpretation is that this region of the glutamate residue is crucial in relaying the activating message from the ligand sensor region to the gate.

Nongenomic actions of thyroid hormone on the heart

Extranuclear or nongenomic actions of thyroid hormone do not require formation of a nuclear complex between the hormone and its traditional 3,5,3'-triiodo-L-thyronine (T3) receptor (TR). Among nongenomic actions of iodothyronines that are relevant to the heart are those on membrane ion channels or pumps. These include stimulation of the sarcolemmal Na+ channel, inward-rectifying K+ channel, voltage-activated potassium channels, and calcium pump (Ca2+-adenosine triphosphatases [ATPases]) and have been shown in intact cells or isolated membranes. Because circulating levels of thyroid hormone are relatively stable, actions on channels or pumps may contribute to setting of basal activity of these transport functions. The mechanism of certain of these membrane effects may involve actions of the hormone on signal transducing protein kinases that modulate levels of activity of plasma membrane channels. Thyroid hormone nongenomically enhances myocardial contractility in isolated myocardial cells, in the isolated perfused rat heart and in human subjects. Iodothyronines also decrease vasomotor tone in a variety of models and in man by a mechanism independent of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), or nitric oxide generation. Acutely increased myocardial mitochondrial respiration has been demonstrated in isolated organelles exposed to thyroid hormone. Genomic and nongenomic actions of thyroid hormone can interface, e.g., at the level of sarcoplasmic reticulum Ca2+-ATPase, where gene expression is regulated by the TR-T3 complex and activity of the enzyme can be modulated nongenomically. The relevance of nongenomic actions of thyroid hormone on the heart has been demonstrated in acute effects of the hormone on cardiac output and systemic vascular resistance in human subjects.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

Gbetagamma subunit combinations differentially modulate receptor and effector coupling in vivo

In vitro, little specificity is seen for modulation of effectors by different combinations of Gbetagamma subunits from heterotrimeric G proteins. Here, we demonstrate that the coupling of specific combinations of Gbetagamma subunits to different receptors leads to a differential ability to modulate effectors in vivo. We have shown that the beta(1)AR and beta(2)AR can activate homomultimers of the human inwardly rectifying potassium channel Kir 3.2 when coexpressed in Xenopus oocytes, and that this requires a functional mammalian Gs heterotrimer. Modulation was independent of cAMP production, suggesting a membrane-delimited mechanism. To analyze further the importance of different Gbetagamma combinations, we have tested the facilitation of Kir 3.2 activation by betaAR mediated by different Gbetagamma subunits. The subunits tested were Gbeta(1,5) and Ggamma(1,2,7,11). These experiments demonstrated significant variation between the ability of the Gbetagamma combinations to activate the channels after receptor stimulation. This was in marked contrast to the situation in vitro where little specificity for binding of a Kir 3.1 C-terminal GST fusion protein by different Gbetagamma combinations was detected. More importantly, neither receptor, although homologous both structurally and functionally, shared the same preference for Gbetagamma subunits. In the presence of beta(1)AR, Gbeta(5)gamma(1) and Gbeta(5)gamma(11) activated Kir 3.2 to the greatest extent, while for the beta(2)AR, Gbeta(1)gamma(7), Gbeta(1)gamma(11,) and Gbeta(5)gamma(2) produced the greatest responses. Interestingly, no preference was seen in the ability of different Gbetagamma subunits to facilitate receptor-stimulated GTPase activity of the Gsalpha. These results suggest that it is not the receptor/G protein alpha subunit interaction or the Gbetagamma/effector interaction that is altered by Gbetagamma, but rather that the ability of the receptor to interact productively with the Gbetagamma subunit directly and/or the G protein/effector complex is dependent on the specific G protein heterotrimer associated with the receptor.