Evidence that direct binding of G beta gamma to the GIRK1 G protein-gated inwardly rectifying K+ channel is important for channel activation

Evidence for association of GABA(B) receptors with Kir3 channels and regulators of G protein signalling (RGS4) proteins

Many neurotransmitters and hormones signal by stimulating G protein-coupled neurotransmitter receptors (GPCRs), which activate G proteins and their downstream effectors. Whether these signalling proteins diffuse freely within the plasma membrane is not well understood. Recent studies have suggested that direct protein-protein interactions exist between GPCRs, G proteins and G protein-gated inwardly rectifying potassium (GIRK or Kir3) channels. Here, we used fluorescence resonance energy transfer (FRET) combined with total internal reflection fluorescence microscopy to investigate whether proteins within this signalling pathway move within 100 A of each other in the plasma membrane of living cells. GABA(B) R1 and R2 receptors, Kir3 channels, Galphao subunits and regulators of G protein signalling (RGS4) proteins were each fused to cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP) and first assessed for functional expression in HEK293 cells. The presence of the fluorophore did not significantly alter the signalling properties of these proteins. Possible FRET was then investigated for different protein pair combinations. As a positive control, FRET was measured between tagged GABA(B) R1 and R2 subunits ( approximately 12% FRET), which are known to form heterodimers. We measured significant FRET between tagged RGS4 and GABA(B) R1 or R2 subunits ( approximately 13% FRET), and between Galphao and GABA(B) R1 or R2 subunits ( approximately 10% FRET). Surprisingly, FRET also occurred between tagged Kir3.2a/Kir3.4 channels and GABA(B) R1 or R2 subunits ( approximately 10% FRET). FRET was not detected between Kir3.2a and RGS4 nor between Kir3.2a and Galphao. These data are discussed in terms of a model in which GABA(B) receptors, G proteins, RGS4 proteins and Kir3 channels are closely associated in a signalling complex.

Rapid activation of inwardly rectifying potassium channels by immobile G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) mediate slow synaptic transmission and many other effects of small molecule and peptide neurotransmitters. In the standard model of GPCR signaling, receptors and G-proteins diffuse laterally within the plane of the plasma membrane and encounter each other by random collision. This model predicts that signaling will be most efficient if both GPCRs and G-proteins are free to diffuse, thus maximizing collision frequency. However, neuronal GPCRs are often recruited to and enriched at specific synaptic locations, suggesting receptor mobility is restricted in these cells. Here, we test the hypothesis that restricting GPCR mobility impairs signaling in neurons by limiting the frequency of collisions between receptors and G-proteins. Mu-opioid receptors (MORs) were immobilized on the surface of cerebellar granule neurons by avidin-mediated cross-linking, and inwardly rectifying potassium (GIRK) channels were used as rapid indicators of G-protein activation. Mobile and immobile MORs activated GIRK channels with the same onset kinetics and agonist sensitivity in these neurons. In a heterologous expression system, GFP (green fluorescent protein)-tagged G alpha(oA) subunits remained mobile after cross-linking, but their mobility was reduced in the presence of immobile MORs, suggesting that these receptors and subunits were transiently precoupled. In addition, channel activation could be reconstituted with immobile GPCRs, G-protein heterotrimers, and GIRK channels. These results show that collision frequency is not rate-limiting for G-protein activation in CNS neurons, and are consistent with the idea that signaling components are compartmentalized or preassembled.

Interactions between GABA-B1 receptors and Kir 3 inwardly rectifying potassium channels

gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the mammalian brain. It acts via both ionotropic GABA-A and metabotropic GABA-B receptors. We evaluated the interaction of receptors with members of the inwardly rectifying potassium (Kir 3) channel family, which also play an important role in neuronal transmission and membrane excitability. These channels are functionally regulated by GABA-B receptors. Possible physical interactions between GABA-B receptor and Kir 3 channels expressed in HEK cells were evaluated using Bioluminescence Resonance Energy Transfer (BRET) experiments, co-immunoprecipitation and confocal microscopy. Our data indicate that Kir 3 channels and Gbetagamma subunits can interact with the GABA-B(1) subunits independently of the GABA-B(2) subunit or Kir 3.4 which are ultimately responsible for their targetting to the cell surface. Thus signalling complexes containing GABA-B receptors, G proteins and Kir channels are formed shortly after biosynthesis most likely in the endoplasmic reticulum.

RGS3 and RGS4 Differentially Associate with G Protein-coupled Receptor-Kir3 Channel Signaling Complexes Revealing Two Modes of RGS Modulation

RGS3 and RGS4 are GTPase-activating proteins expressed in the brain and heart that accelerate the termination of G(i/o)- and G(q)-mediated signaling. We report here the determinants mediating selective association of RGS4 with several G protein-coupled receptors (GPCRs) that form macromolecular complexes with neuronal G protein-gated inwardly rectifying potassium (Kir3 or GIRK) channels. Kir3 channels are instrumental in regulating neuronal firing in the central and peripheral nervous system and pacemaker activity in the heart. By using an epitope-tagged degradation-resistant RGS4 mutant, RGS4(C2V), immunoprecipitation of several hemagglutinin-tagged G(i/o)-coupled and G(q)-coupled receptors expressed in Chinese hamster ovary (CHO-K1) cells readily co-precipitated both Kir3.1/Kir3.2a channels and RGS4(C2V). In contrast to RGS4(C2V), the closely related and functionally active RGS3 "short" isoform (RGS3s) did not interact with any of the GPCR-Kir3 channel complexes examined. Deletion and chimeric RGS constructs indicate both the N-terminal domain and the RGS domain of RGS4(C2V) are necessary for association with m2 receptor-Kir3.1/Kir3.2a channel complexes, where the GPCR was found to be the major target for RGS4(C2V) interaction. The functional impact of RGS4(C2V) "precoupling" to the GPCR-Kir3 channel complex versus RGS3s "collision coupling" was a 100-fold greater potency in the acceleration of G protein-dependent Kir3 channel-gating kinetics with no attenuation in current amplitude. These findings demonstrate that RGS4, a highly regulated modulator and susceptibility gene for schizophrenia, can directly associate with multiple GPCR-Kir3 channel complexes and may affect a wide range of neurotransmitter-mediated inhibitory and excitatory events in the nervous and cardiovascular systems.

Molecular determinants for G protein betagamma modulation of ionotropic glycine receptors

The ligand-gated ion channel superfamily plays a critical role in neuronal excitability. The functions of glycine receptor (GlyR) and nicotinic acetylcholine receptor are modulated by G protein betagamma subunits. The molecular determinants for this functional modulation, however, are still unknown. Studying mutant receptors, we identified two basic amino acid motifs within the large intracellular loop of the GlyR alpha(1) subunit that are critical for binding and functional modulation by Gbetagamma. Mutations within these sequences demonstrated that all of the residues detected are important for Gbetagamma modulation, although both motifs are necessary for full binding. Molecular modeling predicts that these sites are alpha-helixes near transmembrane domains 3 and 4, near to the lipid bilayer and highly electropositive. Our results demonstrate for the first time the sites for G protein betagamma subunit modulation on GlyRs and provide a new framework regarding the ligand-gated ion channel superfamily regulation by intracellular signaling.

GIRK Channel Activation Involves a Local Rearrangement of a Preformed G Protein Channel Complex

G protein-coupled signaling is one of the major mechanisms for controlling cellular excitability. One of the main targets for this control at postsynaptic membranes is the G protein-coupled potassium channels (GIRK/Kir3), which generate slow inhibitory postsynaptic potentials following the activation of Pertussis toxin-sensitive G protein-coupled receptors. Using total internal reflection fluorescence (TIRF) microscopy combined with fluorescence resonance energy transfer (FRET), in intact cells, we provide evidence for the existence of a trimeric G protein-channel complex at rest. We show that activation of the channel via the receptor induces a local conformational switch of the G protein to induce channel opening. The presence of such a complex thus provides the means for a precise temporal and highly selective activation of the channel, which is required for fine tuning of neuronal excitability.

Heterotrimeric G proteins form stable complexes with adenylyl cyclase and Kir3.1 channels in living cells

Bioluminescence resonance energy transfer (BRET) and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several G-protein subunits (Gαs, Gαi, Gβ1 and Gγ2) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically Gβ1-YFP1-158 and Gγ2-YFP159-238, which heterodimerize to produce fluorescent YFP-Gβ1γ2). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-Gγ2, GFP-Gβ1 or YFP-Gβ1γ2. Gα subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and Gβγ was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.

Dominant negative effects of a Gβ mutant on G-protein coupled inward rectifier K+channel

HEK293 cells were transfected with cDNAs for Gbeta1(W332A) [a mutant Gbeta1], Ggamma2, and inward rectifier K+ channels (Kir3.1/Kir3.2). Application of Gbeta1gamma2 protein to these cells activated the K+ channels only slightly. When mu-opioid receptors and Kir3.1/Kir3.2 were transfected, application of a mu-opioid agonist induced a Kir3 current. However, co-expression of Gbeta1(W332A) suppressed this current. Most likely, Gbeta1(W332A) inhibited the action of the endogenous Gbeta. Such a dominant negative effect of Gbeta1(W332A) was also observed in neuronal Kir3 channels in locus coeruleus. The mutant, Gbeta1(W332A) protein, although inactive, retains its ability to bind Kir3 and prevents the wild type Gbeta from activating the channel.

Compartment-dependent colocalization of Kir3.2-containing K+ channels and GABAB receptors in hippocampal pyramidal cells

G-protein-coupled inwardly rectifying K+ channels (Kir3 channels) coupled to metabotropic GABAB receptors are essential for the control of neuronal excitation. To determine the distribution of Kir3 channels and their spatial relationship to GABAB receptors on hippocampal pyramidal cells, we used a high-resolution immunocytochemical approach. Immunoreactivity for the Kir3.2 subunit was most abundant postsynaptically and localized to the extrasynaptic plasma membrane of dendritic shafts and spines of principal cells. Quantitative analysis of immunogold particles for Kir3.2 revealed an enrichment of the protein around putative glutamatergic synapses on dendritic spines, similar to that of GABA(B1). Consistent with this observation, a high degree of coclustering of Kir3.2 and GABA(B1) was revealed around excitatory synapses by the highly sensitive SDS-digested freeze-fracture replica immunolabeling. In contrast, in dendritic shafts receptors and channels were found to be mainly segregated. These results suggest that Kir3.2-containing K+ channels on dendritic spines preferentially mediate the effect of GABA, whereas channels on dendritic shafts are likely to be activated by other neurotransmitters as well. Thus, Kir3 channels, localized to different subcellular compartments of hippocampal principal cells, appear to be differentially involved in synaptic integration in pyramidal cell dendrites.

A specialized subclass of interneurons mediates dopaminergic facilitation of amygdala function

The amygdala is under inhibitory control from the cortex through the activation of local GABAergic interneurons. This inhibition is greatly diminished during heightened emotional states due to dopamine release. However, dopamine excites most amygdala interneurons, suggesting that this dopaminergic gate may be mediated by an unknown subpopulation of interneurons. We hypothesized that this gate is mediated by paracapsular intercalated cells, a subset of interneurons that are innervated by both cortical and mesolimbic dopaminergic afferents. Using transgenic mice that express GFP in GABAergic interneurons, we show that paracapsular cells form a network surrounding the basolateral complex of the amygdala. We found that they provide feedforward inhibition into the basolateral and the central amygdala. Dopamine hyperpolarized paracapsular cells through D1 receptors and substantially suppressed their excitability, resulting in a disinhibition of the basolateral and central nuclei. Suppression of the paracapsular system by dopamine provides a compelling neural mechanism for the increased affective behavior observed during stress or other hyperdopaminergic states.

Gbetagamma inhibits Galpha GTPase-activating proteins by inhibition of Galpha-GTP binding during stimulation by receptor

Gbetagamma subunits modulate several distinct molecular events involved with G protein signaling. In addition to regulating several effector proteins, Gbetagamma subunits help anchor Galpha subunits to the plasma membrane, promote interaction of Galpha with receptors, stabilize the binding of GDP to Galpha to suppress spurious activation, and provide membrane contact points for G protein-coupled receptor kinases. Gbetagamma subunits have also been shown to inhibit the activities of GTPase-activating proteins (GAPs), both phospholipase C (PLC)-betas and RGS proteins, when assayed in solution under single turnover conditions. We show here that Gbetagamma subunits inhibit G protein GAP activity during receptor-stimulated, steady-state GTPase turnover. GDP/GTP exchange catalyzed by receptor requires Gbetagamma in amounts approximately equimolar to Galpha, but GAP inhibition was observed with superstoichiometric Gbetagamma. The potency of inhibition varied with the GAP and the Galpha subunit, but half-maximal inhibition of the GAP activity of PLC-beta1 was observed with 5-10 nM Gbetagamma, which is at or below the concentrations of Gbetagamma needed for regulation of physiologically relevant effector proteins. The kinetics of GAP inhibition of both receptor-stimulated GTPase activity and single turnover, solution-based GAP assays suggested a competitive mechanism in which Gbetagamma competes with GAPs for binding to the activated, GTP-bound Galpha subunit. An N-terminal truncation mutant of PLC-beta1 that cannot be directly regulated by Gbetagamma remained sensitive to inhibition of its GAP activity, suggesting that the Gbetagamma binding site relevant for GAP inhibition is on the Galpha subunit rather than on the GAP. Using fluorescence resonance energy transfer between cyan or yellow fluorescent protein-labeled G protein subunits and Alexa532-labeled RGS4, we found that Gbetagamma directly competes with RGS4 for high-affinity binding to Galpha(i)-GDP-AlF4.

Heterotrimeric G proteins precouple with G protein-coupled receptors in living cells

Using fluorescence resonance energy transfer (FRET) microscopy, we investigate how heterotrimeric G proteins interact with G protein-coupled receptors (GPCRs). In the absence of receptor activation, the alpha2A adrenergic and muscarinic M4 receptors are present on the cell membrane as dimers. Furthermore, there is an interaction between the G protein subunits alpha o, beta1, and gamma2 and a number of GPCRs including M4, alpha2A, the adenosine A1 receptor, and the dopamine D2 receptor under resting conditions. The interaction between GPCRs and Galpha proteins shows specificity: there is interaction between the alpha2A receptor and Go, but little interaction between the alpha2A receptor and Gs. In contrast, the predominantly Gs-coupled prostacyclin receptor interacted with Gs, but there was little interaction between the prostacyclin receptor and Go. Inverse agonists did not change the FRET ratio, whereas the addition of agonist resulted in a modest fall. Our work suggests that GPCR dimers and the G protein heterotrimer are present at the cell membrane in the resting state in a pentameric complex.

Psychostimulants, madness, memory... and RGS proteins?

The ingestion of psychostimulant drugs by humans imparts a profound sense of alertness and well-being. However, repeated use of these drugs in some individuals will induce a physiological state of dependence, characterized by compulsive behavior directed toward the acquisition and ingestion of the drug, at the expense of customary social obligations. Drugs of abuse and many other types of experiences share the ability to alter the morphology and density of neuronal dendrites and spines. Dopaminergic modulation of corticostriatal synaptic plasticity is necessary for these morphological changes. Changes in the density of dendritic spines on striatal neurons may underlie the development of this pathological pattern of drug-seeking behavior. Identifying proteins that regulate dopaminergic signaling are of value. A family of proteins, the regulators of G protein signaling (RGS) proteins, which regulate signaling from G protein-coupled receptors, such as dopamine and glutamate, may be important in this regard. By regulating corticostriatal synaptic plasticity, RGS proteins can influence presynaptic activity, neurotransmitter release, and postsynaptic depolarization and thereby play a key role in the development of this plasticity. Pharmacological agents that modify RGS activity in humans could be efficacious in ameliorating the dependence on psychostimulant drugs.

Mutation of critical GIRK subunit residues disrupts N- and C-termini association and channel function

The subfamily of G-protein-linked inwardly rectifying potassium channels (GIRKs) is coupled to G-protein receptors throughout the CNS and in the heart. We used mutational analysis to address the role of a specific hydrophobic region of the GIRK1 subunit. Deletion of the GIRK1 C-terminal residues 330-384, as well as the point mutation I331R, resulted in a decrease in channel function when coexpressed with GIRK4 in oocytes and in COS-7 cells. Surface protein expression of GIRK1 I331R coexpressed with GIRK4 was comparable with wild type, indicating that subunits assemble and are correctly localized to the membrane. Subsequent mutation of homologous residues in both the GIRK4 subunit and Kir2.1 (Gbetagamma-independent inward rectifier) also resulted in a decrease in channel function. Intracellular domain associations resulted in the coimmunoprecipitation of the GIRK1 N and C termini and GIRK4 N and C termini. The point mutation I331R in the GIRK1 C terminus or L337R in the GIRK4 C terminus decreased the association between the N and C termini. Mutation of a GIRK1 N-terminal hydrophobic residue, predicted structurally to interact with the C-terminal domain, also resulted in a decrease in channel function and termini association. We hypothesize that the hydrophobic nature of this GIRK1 subunit region is critical for interaction between adjacent termini and is permissive for channel gating. In addition, the homologous mutation in cytoplasmic domains of Kir2.1 (L330R) did not disrupt association, suggesting that the overall structural integrity of this region is critical for inward rectifier function.

Pertussis-toxin-sensitive Galpha subunits selectively bind to C-terminal domain of neuronal GIRK channels: evidence for a heterotrimeric G-protein-channel complex

Neuronal G-protein-gated inwardly rectifying potassium (Kir3; GIRK) channels are activated by G-protein-coupled receptors that selectively interact with PTX-sensitive (Galphai/o) G proteins. Although the Gbetagamma dimer is known to activate GIRK channels, the role of the Galphai/o subunit remains unclear. Here, we established that Galphao subunits co-immunoprecipitate with neuronal GIRK channels. In vitro binding studies led to the identification of six amino acids in the GIRK2 C-terminal domain essential for Galphao binding. Further studies suggested that the Galphai/obetagamma heterotrimer binds to the GIRK2 C-terminal domain via Galpha and not Gbetagamma. Galphai/o binding-impaired GIRK2 channels exhibited reduced receptor-activated currents, but retained normal ethanol- and Gbetagamma-activated currents. Finally, PTX-insensitive Galphaq or Galphas subunits did not bind to the GIRK2 C-terminus. Together, these results suggest that the interaction of PTX-sensitive Galphai/o subunit with the GIRK2 C-terminal domain regulates G-protein receptor coupling, and may be important for establishing specific Galphai/o signaling pathways.

G-protein signaling: back to the future

Heterotrimeric G-proteins are intracellular partners of G-protein-coupled receptors (GPCRs). GPCRs act on inactive Galpha.GDP/Gbetagamma heterotrimers to promote GDP release and GTP binding, resulting in liberation of Galpha from Gbetagamma. Galpha.GTP and Gbetagamma target effectors including adenylyl cyclases, phospholipases and ion channels. Signaling is terminated by intrinsic GTPase activity of Galpha and heterotrimer reformation - a cycle accelerated by 'regulators of G-protein signaling' (RGS proteins). Recent studies have identified several unconventional G-protein signaling pathways that diverge from this standard model. Whereas phospholipase C (PLC) beta is activated by Galpha(q) and Gbetagamma, novel PLC isoforms are regulated by both heterotrimeric and Ras-superfamily G-proteins. An Arabidopsis protein has been discovered containing both GPCR and RGS domains within the same protein. Most surprisingly, a receptor-independent Galpha nucleotide cycle that regulates cell division has been delineated in both Caenorhabditis elegans and Drosophila melanogaster. Here, we revisit classical heterotrimeric G-protein signaling and explore these new, non-canonical G-protein signaling pathways.

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.

Gbetagamma-dependent and Gbetagamma-independent basal activity of G protein-activated K+ channels

Cardiac and neuronal G protein-activated K+ channels (GIRK; Kir3) open following the binding of Gbetagamma subunits, released from Gi/o proteins activated by neurotransmitters. GIRKs also possess basal activity contributing to the resting potential in neurons. It appears to depend largely on free Gbetagamma, but a Gbetagamma-independent component has also been envisaged. We investigated Gbetagamma dependence of the basal GIRK activity (A(GIRK,basal)) quantitatively, by titrated expression of Gbetagamma scavengers, in Xenopus oocytes expressing GIRK1/2 channels and muscarinic m2 receptors. The widely used Gbetagamma scavenger, myristoylated C terminus of beta-adrenergic kinase (m-cbetaARK), reduced A(GIRK,basal) by 70-80% and eliminated the acetylcholine-evoked current (I(ACh)). However, we found that m-cbetaARK directly binds to GIRK, complicating the interpretation of physiological data. Among several newly constructed Gbetagamma scavengers, phosducin with an added myristoylation signal (m-phosducin) was most efficient in reducing GIRK currents. m-phosducin relocated to the membrane fraction and did not bind GIRK. Titrated expression of m-phosducin caused a reduction of A(GIRK,basal) by up to 90%. Expression of GIRK was accompanied by an increase in the level of Gbetagamma and Galpha in the plasma membrane, supporting the existence of preformed complexes of GIRK with G protein subunits. Increased expression of Gbetagamma and its constitutive association with GIRK may underlie the excessively high A(GIRK,basal) observed at high expression levels of GIRK. Only 10-15% of A(GIRK,basal) persisted upon expression of both m-phosducin and cbetaARK. These results demonstrate that a major part of Ibasal is Gbetagamma-dependent at all levels of channel expression, and only a small fraction (<10%) may be Gbetagamma-independent.

Cytoplasmic domain structures of Kir2.1 and Kir3.1 show sites for modulating gating and rectification

N- and C-terminal cytoplasmic domains of inwardly rectifying K (Kir) channels control the ion-permeation pathway through diverse interactions with small molecules and protein ligands in the cytoplasm. Two new crystal structures of the cytoplasmic domains of Kir2.1 (Kir2.1(L)) and the G protein-sensitive Kir3.1 (Kir3.1(S)) channels in the absence of PIP(2) show the cytoplasmic ion-permeation pathways occluded by four cytoplasmic loops that form a girdle around the central pore (G-loop). Significant flexibility of the pore-facing G-loop of Kir2.1(L) and Kir3.1(S) suggests a possible role as a diffusion barrier between cytoplasmic and transmembrane pores. Consistent with this, mutations of the G-loop disrupted gating or inward rectification. Structural comparison shows a di-aspartate cluster on the distal end of the cytoplasmic pore of Kir2.1(L) that is important for modulating inward rectification. Taken together, these results suggest the cytoplasmic domains of Kir channels undergo structural changes to modulate gating and inward rectification.

Regulators of G-protein signaling form a quaternary complex with the agonist, receptor, and G-protein. A novel explanation for the acceleration of signaling activation kinetics

Regulators of G-protein signaling (RGS) proteins modulate signaling through heterotrimeric G-proteins. They act to enhance the intrinsic GTPase activity of the Galpha subunit but paradoxically have also been shown to enhance receptor-stimulated activation. To study this paradox, we used a G-protein gated K+ channel to report the dynamics of the G-protein cycle and fluorescence resonance energy transfer techniques with cyan and yellow fluorescent protein-tagged proteins to report physical interaction. Our data show that the acceleration of the activation kinetics is dissociated from deactivation kinetics and dependent on receptor and RGS type, G-protein isoform, and RGS expression levels. By using fluorescently tagged proteins, fluorescence resonance energy transfer microscopy showed a stable physical interaction between the G-protein alpha subunit and RGS (RGS8 and RGS7) that is independent of the functional state of the G-protein. RGS8 does not directly interact with G-protein-coupled receptors. Our data show participation of the RGS in the ternary complex between agonist-receptor and G-protein to form a "quaternary complex." Thus we propose a novel model for the action of RGS proteins in the G-protein cycle in which the RGS protein appears to enhance the "kinetic efficacy" of the ternary complex, by direct association with the G-protein alpha subunit.

Proinflammatory chemokines, such as C-C chemokine ligand 3, desensitize mu-opioid receptors on dorsal root ganglia neurons

Pain is one of the hallmarks of inflammation. Opioid receptors mediate antipain responses in both the peripheral nervous system and CNS. In the present study, pretreatment of CCR1: mu-opioid receptor/HEK293 cells with CCL3 (MIP-1alpha) induced internalization of mu-opioid receptors and severely impaired the mu-opioid receptor-mediated inhibition of cAMP accumulation. Immunohistochemical staining showed that CCR1 and mu-opioid receptors were coexpressed on small to medium diameter neurons in rat dorsal root ganglion. Analysis of ligand-induced calcium flux showed that both types of receptors were functional. Pretreatment of neurons with CCL3 exhibited an impaired [D-Ala(2),N-MePhe(4),Gly-o15]enkephalin-elicited calcium response, indicative of the heterologous desensitization of mu-opioid receptors. Other chemokines, such as CCL2, CCL5, and CXCL8, exhibited similar inhibitory effects. Our data indicate that proinflammatory chemokines are capable of desensitizing mu-opioid receptors on peripheral sensory neurons, providing a novel potential mechanism for peripheral inflammation-induced hyperalgesia.

Coordination of Membrane Excitability through a GIRK1 Signaling Complex in the Atria

Control of heart rate is a complex process that integrates the function of multiple G protein-coupled receptors and ion channels. Among them, the G protein-regulated inwardly rectifying K+ (GIRK or KACh) channels of sinoatrial node and atria play a major role in beat-to-beat regulation of the heart rate. The atrial KACh channels are heterotetrameric proteins that consist of two pore-forming subunits, GIRK1 and GIRK4. Following m2-muscarinic acetylcholine receptor (M2R) stimulation, KACh channel activation is conferred by the direct binding of G protein betagamma subunits (Gbetagamma) to the channel. Here we show that atrial KACh channels are assembled in a signaling complex with Gbetagamma, G protein-coupled receptor kinase, cyclic adenosine monophosphate-dependent protein kinase, two protein phosphatases, PP1 and PP2A, receptor for activated C kinase 1, and actin. This complex would enable the KACh channels to rapidly integrate beta-adrenergic and M2R signaling in the membrane, and it provides insight into general principles governing spatial integration of different transduction pathways. Furthermore, the same complex might recruit protein kinase C (PKC) to the KACh channel following alpha-adrenergic receptor stimulation. Our electro-physiological recordings from single atrial KACh channels revealed a potent inhibition of Gbetagamma-induced channel activity by PKC, thus validating the physiological significance of the observed complex as interconnecting site where signaling molecules congregate to execute a coordinated control of membrane excitability.

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channel activation kinetics via Galphai and Galphao-coupled receptors are determined by Galpha-specific interdomain interactions that affect GDP release rates

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channels have distinct gating properties when activated by receptors coupled specifically to Galpha(o) versus Galpha(i) subunit isoforms, with Galpha(o)-coupled currents having approximately 3-fold faster agonist-evoked activation kinetics. To identify the molecular determinants in Galpha subunits mediating these kinetic differences, chimeras were constructed using pertussis toxin (PTX)-insensitive Galpha(oA) and Galpha(i2) mutant subunits (Galpha(oA(C351G)) and Galpha(i2(C352G))) and examined in PTX-treated Xenopus oocytes expressing muscarinic m2 receptors and Kir3.1/3.2a channels. These experiments revealed that the alpha-helical N-terminal region (amino acids 1-161) and the switch regions of Galpha(i2) (amino acids 162-262) both partially contribute to slowing the GIRK activation time course when compared with the Galpha(oA(C351G))-coupled response. When present together, they fully reproduce Galpha(i2(C352G))-coupled GIRK kinetics. The Galpha(i2) C-terminal region (amino acids 263-355) had no significant effect on GIRK kinetics. Complementary responses were observed with chimeras substituting the Galpha(o) switch regions into the Galpha(i2(C352G)) subunit, which partially accelerated the GIRK activation rate. The Galpha(oA)/Galpha(i2) chimera results led us to examine an interaction between the alpha-helical domain and the Ras-like domain previously implicated in mediating a 4-fold slower in vitro basal GDP release rate in Galpha(i1) compared with Galpha(o). Mutations disrupting the interdomain contact in Galpha(i2(C352G)) at either the alphaD-alphaE loop (R145A) or the switch III loop (L233Q/A236H/E240T/M241T), significantly accelerated the GIRK activation kinetics consistent with the Galpha(i2) interdomain interface regulating receptor-catalyzed GDP release rates in vivo. We propose that differences in Galpha(i) versus Galpha(o)-coupled GIRK activation kinetics are due to intrinsic differences in receptor-catalyzed GDP release that rate-limit Gbetagamma production and is attributed to heterogeneity in Galpha(i) and Galpha(o) interdomain contacts.

Gαi1 and Gαi3 Differentially Interact with, and Regulate, the G Protein-activated K+ Channel

G protein-activated K(+) channels (GIRKs; Kir3) are activated by direct binding of Gbetagamma subunits released from heterotrimeric G proteins. In native tissues, only pertussis toxin-sensitive G proteins of the G(i/o) family, preferably Galpha(i3) and Galpha(i2), are donors of Gbetagamma for GIRK. How this specificity is achieved is not known. Here, using a pull-down method, we confirmed the presence of Galpha(i3-GDP) binding site in the N terminus of GIRK1 and identified novel binding sites in the N terminus of GIRK2 and in the C termini of GIRK1 and GIRK2. The non-hydrolyzable GTP analog, guanosine 5'-3-O-(thio)triphosphate, reduced the binding of Galpha(i3) by a factor of 2-4. Galpha(i1-GDP) bound to GIRK1 and GIRK2 much weaker than Galpha(i3-GDP). Titrated expression of components of signaling pathway in Xenopus oocytes and their activation by m2 muscarinic receptors revealed that G(i3) activates GIRK more efficiently than G(i1), as indicated by larger and faster agonist-evoked currents. Activation of GIRK by purified Gbetagamma in excised membrane patches was strongly augmented by coexpression of Galpha(i3) and less by Galpha(i1). Differences in physical interactions of GIRK with GDP-bound Galpha subunits, or Galphabetagamma heterotrimers, may dictate different extents of Galphabetagamma anchoring, influence the efficiency of GIRK activation by Gbetagamma, and play a role in determining signaling specificity.

Molecular basis for the inhibition of G protein-coupled inward rectifier K(+) channels by protein kinase C

G protein-coupled inward rectifier K(+) (GIRK) channels regulate cellular excitability and neurotransmission. The GIRK channels are activated by a number of inhibitory neurotransmitters through the G protein betagamma subunit (G(betagamma)) after activation of G protein-coupled receptors and inhibited by several excitatory neurotransmitters through activation of phospholipase C. If the inhibition is produced by PKC, there should be PKC phosphorylation sites in GIRK channel proteins. To identify the PKC phosphorylation sites, we performed systematic mutagenesis analysis on GIRK4 and GIRK1 subunits expressed in Xenopus oocytes. Our data showed that the heteromeric GIRK1/GIRK4 channels were inhibited by a PKC activator phorbol 12-myristate 13-acetate (PMA) through reduction of single channel open-state probability. Direct application of the catalytic subunit of PKC to excised patches had a similar inhibitory effect. This inhibition was greatly eliminated by mutation of Ser-185 in GIRK1 and Ser-191 in GIRK4 that remained G protein sensitive. The PKC-dependent phosphorylation seems to mediate the channel inhibition by the excitatory neurotransmitter substance P (SP) as specific PKC inhibitors and mutation of these PKC phosphorylation sites abolished the SP-induced inhibition of GIRK1/GIRK4 channels. Thus, these results indicate that the PKC-dependent phosphorylation underscores the inhibition of GIRK channels by SP, and Ser-185 in GIRK1 and Ser-191 in GIRK4 are the PKC phosphorylation sites.

Cell signal control of the G protein-gated potassium channel and its subcellular localization

G protein-gated inward rectifier K(+) (K(G)) channels are directly activated by the betagamma subunits released from pertussis toxin-sensitive G proteins, and contribute to neurotransmitter-induced deceleration of heart beat, formation of slow inhibitory postsynaptic potentials in neurones and inhibition of hormone release in endocrine cells. The physiological roles of K(G) channels are critically determined by mechanisms which regulate their activity and their subcellular localization. K(G) channels are tetramers of inward rectifier K(+) (Kir) channel subunits, Kir3.x. The combination of Kir3.x subunits in each K(G) channel varies among tissues and cell types. Each subunit of the channel possesses one Gbetagamma binding site. The binding of Gbetagamma increases the number of functional K(G) channels via a mechanism that can be described by the Monod-Wyman-Changeux allosteric model. During voltage pulses K(G) channel current alters time dependently. The K(G) current exhibits inward rectification due to blockade of outward-going current by intracellular Mg(2+) and polyamines. Upon repolarization, this blockade is relieved practically instantaneously and then the current slowly increases further. This slow current alteration is called 'relaxation'. Relaxation is caused by the voltage-dependent behaviour of regulators of G protein signalling (RGS proteins), which accelerate intrinsic GTP hydrolysis mediated by the Galpha subunit. Thus, the relaxation behaviour of K(G) channels reflects the time course with which the G protein cycle is altered by RGS protein activity at each membrane potential. Subcellular localization of K(G) channels is controlled by several distinct mechanisms, some of which have been recently clarified. The neuronal K(G) channel, which contains Kir3.2c, is localized in the postsynaptic density (PSD) of various neurones including dopaminergic neurones in substantia nigra. Its localization at PSD may be controlled by PDZ domain-containing anchoring proteins. The K(G) channel in thyrotrophs is localized exclusively on secretary vesicles, which upon stimulation are rapidly inserted into the plasma membrane and causes hyperpolarization of the cell. This mechanism indicates a novel negative feedback regulation of exocytosis. In conclusion, K(G) channels are under the control of a variety of signalling molecules which regulate channel activity, subcellular localization and thus their physiological roles in myocytes, neurones and endocrine cells.

betaL-betaM loop in the C-terminal domain of G protein-activated inwardly rectifying K(+) channels is important for G(betagamma) subunit activation

The activity of G protein-activated inwardly rectifying K(+) channels (GIRK or Kir3) is important for regulating membrane excitability in neuronal, cardiac and endocrine cells. Although G(betagamma) subunits are known to bind the N- and C-termini of GIRK channels, the mechanism underlying G(betagamma) activation of GIRK is not well understood. Here, we used chimeras and point mutants constructed from GIRK2 and IRK1, a G protein-insensitive inward rectifier, to determine the region within GIRK2 important for G(betagamma) binding and activation. An analysis of mutant channels expressed in Xenopus oocytes revealed two amino acid substitutions in the C-terminal domain of GIRK2, GIRK2(L344E) and GIRK2(G347H), that exhibited decreased carbachol-activated currents but significantly enhanced basal currents with coexpression of G(betagamma) subunits. Combining the two mutations (GIRK2(EH)) led to a more severe reduction in carbachol-activated and G(betagamma)-stimulated currents. Ethanol-activated currents were normal, however, suggesting that G protein-independent gating was unaffected by the mutations. Both GIRK2(L344E) and GIRK2(EH) also showed reduced carbachol activation and normal ethanol activation when expressed in HEK-293T cells. Using epitope-tagged channels expressed in HEK-293T cells, immunocytochemistry showed that G(betagamma)-impaired mutants were expressed on the plasma membrane, although to varying extents, and could not account completely for the reduced G(betagamma) activation. In vitro G(betagamma) binding assays revealed an approximately 60% decrease in G(betagamma) binding to the C-terminal domain of GIRK2(L344E) but no statistical change with GIRK2(EH) or GIRK2(G347H), though both mutants exhibited G(betagamma)-impaired activation. Together, these results suggest that L344, and to a lesser extent, G347 play an important functional role in G(betagamma) activation of GIRK2 channels. Based on the 1.8 A structure of GIRK1 cytoplasmic domains, L344 and G347 are positioned in the betaL-betaM loop, which is situated away from the pore and near the N-terminal domain. The results are discussed in terms of a model for activation in which G(betagamma) alters the interaction between the betaL-betaM loop and the N-terminal domain.

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.

Merging functional studies with structures of inward-rectifier K+ channels

Inwardly rectifying K(+) (Kir) channels have a wide range of functions including the control of neuronal signalling, heart rate, blood flow and insulin release. Because of the physiological importance of these channels, considerable effort has been invested in understanding the structural basis of their physiology. In this review, we use two recent, high-resolution structures as foundations for examining our current understanding of the fundamental functions that are shared by all K(+) channels, such as K(+) selectivity and channel gating, as well as characteristic features of Kir channel family members, such as inward rectification and their regulation by intracellular factors.

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.

Properties and modulation of the G protein-coupled K+ channel in rat cerebellar granule neurons: ATP versus phosphatidylinositol 4,5-bisphosphate

Cerebellar granule (CG) neurons express a G protein-gated K+ current (GIRK) that is involved in the neurotransmitter regulation of the excitatory input to the Purkinje fibres of the cerebellum. Here, we characterized the single-channel behaviour of GIRK in CG neurons, and examined the effects of several known modulators of GIRK and their putative physiological roles. Whole-cell GIRKs were activated by baclofen, a GABAB receptor agonist. In cell-attached patches, baclofen activated GIRK with a single-channel conductance of 34 pS and a mean open time of 0.5 ms. In inside-out patches, application of GTPgammaS to the cytoplasmic side activated GIRK with similar kinetic properties. Addition of 2 mM ATP resulted in a marked increase in GIRK activity and induced longer-lived openings with a mean open time of 2.3 ms (ATP-dependent gating). Brain cytosolic fraction or free fatty acids inhibited this effect of ATP, and this was reversed by addition of purified recombinant brain fatty acid binding protein. Applying phosphatidylinositol 4,5-bisphosphate (PIP2) to inside-out patches in place of ATP also increased GIRK activity; however, only an increase in the frequency of opening was observed. The stimulatory effect of PIP2 on GIRK activity was not inhibited by the cytosolic fraction. Following maximal activation by PIP2, ATP caused an additional 2.2-fold increase in GIRK activity. These results show that GIRKs in CG neurons are regulated by positive and negative modulators that affect frequency as well as open time duration. The net effect is that the ligand-activated GIRK is in the 'low activity' state associated with short-lived openings, mainly due to strong action of the cytosolic inhibitor of ATP-dependent gating. Our results also show that intracellular ATP modulates GIRK via pathways different from that of PIP2 in CG neurons.

Diadenosine-5-phosphate exerts A1-receptor-mediated proarrhythmic effects in rabbit atrial myocardium

(1) Diadenosine polyphosphates have been described to be present in the myocardium and exert purinergic- and nonreceptor-mediated effects. Since the electrophysiological properties of atrial myocardium are effectively regulated by A(1) receptors, we investigated the effect of diadenosine pentaphosphate (Ap(5)A) in rabbit myocardium. (2) Parameters of supraventricular electrophysiology and atrial vulnerability were measured in Langendorff-perfused rabbit hearts. Muscarinic potassium current (I(K(ACh/Ado))) and ATP-sensitive potassium current (I(K(ATP))) were measured by using the whole-cell voltage clamp method. (3) Ap(5)A prolonged the cycle length of spontaneously beating Langendorff perfused hearts from 225+/-14 (control) to 1823+/-400 ms (Ap(5)A 50 micro M; n=6; P<0.05). This effect was paralleled by higher degree of atrio-ventricular block. Atrial effective refractory period (AERP) in control hearts was 84+/-14 ms (n=6). Ap(5)A>/=1 micro M reduced AERP (100 micro M, 58+/-11 ms; n=6). (4) Extrastimuli delivered to hearts perfused with Ap(5)A- or adenosine (>/= micro M)-induced atrial fibrillation, the incidence of which correlated to the concentration added to the perfusate. The selective A(1)-receptor antagonist CPX (20 micro M) inhibited the Ap(5)A- and adenosine-induced decrease of AERP. Atrial fibrillation was no longer observed in the presence of CPX. (5) The described Ap(5)A-induced effects in the multicellular preparation were enhanced by dipyridamole (10 micro M), which is a cellular adenosine uptake inhibitor. Dipyridamole-induced enhancement was inhibited by CPX. (6) Ap(5)A (</=1 mM) did neither induce I(K(Ado)) nor I(K(ATP)). No effect on activated I(K(Ado/ATP)) was observed in myocytes superfused with Ap(5)A. However, effluents from Langendorff hearts perfused with Ap(5)A 100 micro M activated I(K(Ado)) by using A(1) receptors. (7) Ap(5)A did not activate A(1) receptors in rabbit atrial myocytes. The Ap(5)A induced A(1)-receptor-mediated effects on supraventricular electrophysiology and vulnerability suggest that in the multicellular preparation Ap(5)A is hydrolyzed to yield adenosine, which acts via A(1) receptors. An influence on atrial electrophysiology or a facilitation of atrial fibrillation under conditions resulting in increased interstitial Ap(5)A concentrations might be of physiological/pathophysiological relevance.

Mapping the Gbetagamma-binding sites in GIRK1 and GIRK2 subunits of the G protein-activated K+ channel

G protein-activated K+ channels (Kir3 or GIRK) are activated by direct binding of Gbetagamma. The binding sites of Gbetagamma in the ubiquitous GIRK1 (Kir3.1) subunit have not been unequivocally charted, and in the neuronal GIRK2 (Kir3.2) subunit the binding of Gbetagamma has not been studied. We verified and extended the map of Gbetagamma-binding sites in GIRK1 by using two approaches: direct binding of Gbetagamma to fragments of GIRK subunits (pull down), and competition of these fragments with the Galphai1 subunit for binding to Gbetagamma. We also mapped the Gbetagamma-binding sites in GIRK2. In both subunits, the N terminus binds Gbetagamma. In the C terminus, the Gbetagamma-binding sites in the two subunits are not identical; GIRK1, but not GIRK2, has a previously unrecognized Gbetagamma-interacting segments in the first half of the C terminus. The main C-terminal Gbetagamma-binding segment found in both subunits is located approximately between amino acids 320 and 409 (by GIRK1 count). Mutation of C-terminal leucines 262 or 333 in GIRK1, recognized previously as crucial for Gbetagamma regulation of the channel, and of the corresponding leucines 273 and 344 in GIRK2 dramatically altered the properties of K+ currents via GIRK1/GIRK2 channels expressed in Xenopus oocytes but did not appreciably reduce the binding of Gbetagamma to the corresponding fusion proteins, indicating that these residues are mainly important for the regulation of Gbetagamma-induced changes in channel gating rather than Gbetagamma binding.

Altered sinus nodal and atrioventricular nodal function in freely moving mice overexpressing the A1 adenosine receptor

To investigate whether altered function of adenosine receptors could contribute to sinus node or atrioventricular (AV) nodal dysfunction in conscious mammals, we studied transgenic (TG) mice with cardiac-specific overexpression of the A1 adenosine receptor (A1AR). A Holter ECG was recorded in seven freely moving littermate pairs of mice during normal activity, exercise (5 min of swimming), and 1 h after exercise. TG mice had lower maximal heart rates (HR) than wild-type (WT) mice (normal activity: 437 +/- 18 vs. 522 +/- 24 beats/min, P < 0.05; exercise: 650 +/- 13 vs. 765 +/- 28 beats/min, P < 0.05; 1 h after exercise: 588 +/- 18 vs. 720 +/- 12 beats/min, P < 0.05; all values are means +/- SE). Mean HR was lower during exercise (589 +/- 16 vs. 698 +/- 34 beats/min, P < 0.05) and after exercise (495 +/- 16 vs. 592 +/- 27 beats/min, P < 0.05). Minimal HR was not different between genotypes. HR variability (SD of RR intervals) was reduced by 30% (P < 0.05) in TG compared with WT mice. Pertussis toxin (n = 4 pairs, 150 microg/kg ip) reversed bradycardia after 48 h. TG mice showed first-degree AV nodal block (PQ interval: 42 +/- 2 vs. 37 +/- 2 ms, P < 0.05), which was diminished but not abolished by pertussis toxin. Isolated Langendorff-perfused TG hearts developed spontaneous atrial arrhythmias (3 of 6 TG mice vs. 0 of 9 WT mice, P < 0.05). In conclusion, A1AR regulate sinus nodal and AV nodal function in the mammalian heart in vivo. Enhanced expression of A1AR causes sinus nodal and AV nodal dysfunction and supraventricular arrhythmias.

Agonist unbinding from receptor dictates the nature of deactivation kinetics of G protein-gated K+ channels

G protein-gated inwardly rectifying K(+) (Kir) channels are found in neurones, atrial myocytes, and endocrine cells and are involved in generating late inhibitory postsynaptic potentials, slowing the heart rate and inhibiting hormone release. They are activated by G protein-coupled receptors (GPCRs) via the inhibitory family of G protein, G(i/o), in a membrane-delimited fashion by the direct binding of Gbetagamma dimers to the channel complex. In this study we are concerned with the kinetics of deactivation of the cloned neuronal G protein-gated K(+) channel, Kir3.1 + 3.2A, after stimulation of a number of GPCRs. Termination of the channel activity on agonist removal is thought to solely depend on the intrinsic hydrolysis rate of the G protein alpha subunit. In this study we present data that illustrate a more complex behavior. We hypothesize that there are two processes that account for channel deactivation: agonist unbinding from the GPCR and GTP hydrolysis by the G protein alpha subunit. With some combinations of agonist/GPCR, the rate of agonist unbinding is slow and rate-limiting, and deactivation kinetics are not modulated by regulators of G protein-signaling proteins. In another group, channel deactivation is generally faster and limited by the hydrolysis rate of the G protein alpha subunit. G protein isoform and interaction with G protein-signaling proteins play a significant role with this group of GPCRs.

Conformational Rearrangements Associated with the Gating of the G Protein-Coupled Potassium Channel Revealed by FRET Microscopy

G protein-coupled potassium channels (GIRK/Kir3.x) are key determinants that translate inhibitory chemical neurotransmission into changes in cellular excitability. To understand the mechanism of channel activation by G proteins, it is necessary to define the structural rearrangements in the channel that result from interaction with Gbetagamma subunits. In this study we used a combination of fluorescence spectroscopy and through-the-objective total internal reflection microscopy to monitor the conformational rearrangements associated with the activation of GIRK channels in single intact cells. We detect activation-induced changes in FRET consistent with a rotation and expansion of the termini along the central axis of the channel. We propose that this rotation and expansion of the termini drives the channel to open by bending and possibly rotating the second transmembrane segment.

Gating dependence of inner pore access in inward rectifier K(+) channels

Cation channel gating may occur either at or below the inner vestibule entrance or at the selectivity filter. To differentiate these possibilities in inward rectifier (Kir) channels, we examined cysteine accessibility in the ATP-gated Kir6.2 channel. MTSEA and MTSET both block channels and modify M2 cysteines with identical voltage dependence. If entry is restricted to open channels, modification rates will slow in ATP-closed channels, but because the reagent can be trapped in the pore following brief openings, this may not be apparent until open probability is extremely low (<0.01). When these conditions are met, modification does slow significantly, indicating gated access and highlighting an important caveat for interpretation of MTS-accessibility measurements: reagent "trapping" in nominally "closed" channels may obscure gated access.

The dynamics of formation and action of the ternary complex revealed in living cells using a G-protein-gated K+ channel as a biosensor

Traditionally the consequences of activation of G-protein-coupled receptors (GPCRs) by an agonist are studied using biochemical assays. In this study we use live cells and take advantage of a G-protein-gated inwardly rectifying potassium channel (Kir3.1+3.2A) that is activated by the direct binding of Gbetagamma subunit to the channel complex to report, in real-time, using the patch clamp technique the activity of the "ternary complex" of agonist/receptor/G-protein. This analysis is further facilitated by the use of pertussis toxin-resistant fluorescent and non-fluorescent Galpha(i/o) subunits and a series of HEK293 cell lines stably expressing both channel and receptors (including the adenosine A(1) receptor, the adrenergic alpha(2A) receptor, the dopamine D(2S) receptor, the M4 muscarinic receptor, and the dimeric GABA-B(1b/2) receptor). We systematically analyzed the contribution of the various inputs to the observed kinetic response of channel activation. Our studies indicate that the combination of agonist, GPCR, and G-protein isoform uniquely specify the behavior of these channels and thus support the importance of the whole ternary complex at a kinetic level.

Inhibition of G-protein-coupled inward rectifying K+ channels by intracellular acidosis

G-protein-coupled inward rectification K(+) (GIRK) channels play an important role in modulation of synaptic transmission and cellular excitability. The GIRK channels are regulated by diverse intra- and extracellular signaling molecules. Previously, we have shown that GIRK1/GIRK4 channels are activated by extracellular protons. The channel activation depends on a histidine residue in the M1-H5 linker and may play a role in neurotransmission. Here, we show evidence that the heteromeric GIRK1/GIRK4 channels are inhibited by intracellular acidification. This inhibition was produced by selective decrease in the channel open probability with a modest drop in the single-channel conductance. The inhibition does not seem to require G-proteins as it was seen in two G-protein coupling-defective GIRK mutants and in excised patches in the absence of exogenous G-proteins. Three histidine residues in intracellular domains were critical for the inhibition. Individual mutation of His-64, His-228, or His-352 in GIRK4 abolished or greatly diminished the inhibition in homomeric GIRK4. Mutations of any of these histidine residues in GIRK4 or their counterparts in GIRK1 were sufficient to eliminate the pH(i) sensitivity of the heteromeric GIRK1/GIRK4 channels. Thus, the molecular and biophysical bases for the inhibition of GIRK channels by intracellular protons are illustrated. Because of the inequality of the pH(i) and pH(o) in most cells and their relatively independent controls by cellular versus systemic mechanisms, such pH(i) sensitivity may allow these channels to regulate cellular excitability in certain physiological and pathophysiological conditions when intracellular acidosis occurs.

Basal activity of GIRK5 isoforms

G protein-coupled inwardly rectifying K(+) channels (GIRK or Kir3) form functional heterotetramers gated by Gbetagamma subunits. GIRK channels are critical for functions as diverse as heart rate modulation and neuronal post-synaptic inhibition. GIRK5 (Kir3.5) is the oocyte homologue of the mammalian GIRK subunits that conform the K(ACh) channel. It has been claimed that even when the oocytes express GIRK5 proteins they do not form functional channels. However, the GIRK5 gene shows three initiation sites that suggest the existence of three isoforms. In a previous work we demonstrated the functionality of homomultimers of the shortest isoform overexpressed in the own oocytes. Remarkably, the basal GIRK5-Delta25 inward currents were not coupled to the activation of a G-protein receptor in the oocytes. These results encouraged us to study this channel in another expression system. In this work we show that Sf21 insect cells can be successfully transfected with this channel. GIRK5-Delta25 homomultimers produce time-dependent inward currents only with GTPgammaS in the recording pipette. Therefore, alternative modes of stimulus input to heterotrimeric G-proteins should be present in the oocytes to account for these results.

A role for the middle C terminus of G-protein-activated inward rectifier potassium channels in regulating gating

We have used sulfhydryl-modifying reagents to investigate the regulation of G-protein-activated inward rectifier potassium (GIRK) channels via their cytoplasmic domains. Modification of either the conserved N-terminal cysteines (GIRK1C53 and GIRK2C65) or the middle C-terminal cysteines (GIRK1C310 and GIRK2C321) independently inhibited GIRK1/GIRK2 heteromeric channels. With the exception of GIRK2C65, these cysteines were relatively inaccessible to large modifying reagents. The accessibility was further reduced by a mutation at the end of the second transmembrane domain that stabilized the open state of the channel. Thus it is unlikely that these cysteines line the permeation pathway of the open pore. Cysteines introduced 3 and 6 amino acids upstream of GIRK2C321 (G318C and E315C) were considerably more accessible. The effect of modification was dependent on the charge of the reagent. Modification of E315C in GIRK2 and E304C in GIRK1 by sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES(-)) increased the current by approximately 17-fold, whereas modification by 2-aminoethyl methanethiosulfonate hydrochloride (MTSEA(+)), abolished the current. There was no effect on single-channel conductance. Thus a switch in charge at this middle C-terminal position was sufficient to gate the channel open and closed. This glutamate is conserved in all members of the Kir family. The E303K mutation in Kir2.1 inhibits channel function and causes Andersen's syndrome in humans (Plaster, N. M., Tawil, R., Tristani-Firouzi, M., Canun, S., Bendahhou, S., Tsunoda, A., Donaldson, M. R., Iannaccone, S. T., Brunt, E., Barohn, R., Clark, J., Deymeer, F., George, A. L., Jr., Fish, F. A., Hahn, A., Nitu, A., Ozdemir, C., Serdaroglu, P., Subramony, S. H., Wolfe, G., Fu, Y. H., and Ptacek, L. J. (2001) Cell 105, 511-519 and Preisig-Muller, R., Schlichthorl, G., Goerge, T., Heinen, S., Bruggemann, A., Rajan, S., Derst, C., Veh, R. W., and Daut, J. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 7774-7779). Our results suggest that this residue regulates channel gating through an electrostatic mechanism.

Gating properties of GIRK channels activated by Galpha(o)- and Galpha(i)-coupled muscarinic m2 receptors in Xenopus oocytes: the role of receptor precoupling in RGS modulation

'Regulators of G protein Signalling' (RGSs) accelerate the activation and deactivation kinetics of G protein-gated inwardly rectifying K(+) (GIRK) channels. In an apparent paradox, RGSs do not reduce steady-state GIRK current amplitudes as expected from the accelerated rate of deactivation when reconstituted in Xenopus oocytes. We present evidence here that this kinetic anomaly is dependent on the degree of G protein-coupled receptor (GPCR) precoupling, which varies with different Galpha(i/o)-RGS complexes. The gating properties of GIRK channels (Kir3.1/Kir3.2a) activated by muscarinic m2 receptors at varying levels of G protein expression were examined with or without the co-expression of either RGS4 or RGS7 in Xenopus oocytes. Different levels of specific m2 receptor-Galpha coupling were established by uncoupling endogenous pertussis toxin (PTX)-sensitive Galpha(i/o) subunits with PTX, while expressing varying amounts of a single PTX-insensitive subunit (Galpha(i1(C351G)), Galpha(i2(C352G)), Galpha(i3(C351G)), Galpha(oA(C351G)), or Galpha(oB(C351G))). Co-expression of each of the PTX-insensitive Galpha(i/o) subunits rescued acetylcholine (ACh)-elicited GIRK currents (I(K,ACh)) in a concentration-dependent manner, with Galpha(o) isoforms being more effective than Galpha(i) isoforms. Receptor-independent 'basal' GIRK currents (I(K,basal)) were reduced with increasing expression of PTX-insensitive Galpha subunits and were accompanied by a parallel rise in I(K,ACh). These effects together are indicative of increased Gbetagamma scavenging by the expressed Galpha subunit and the subsequent formation of functionally coupled m2 receptor-G protein heterotrimers (Galpha((GDP))betagamma). Co-expression of RGS4 accelerated all the PTX-insensitive Galpha(i/o)-coupled GIRK currents to a similar extent, yet reduced I(K,ACh) amplitudes 60-90 % under conditions of low Galpha(i/o) coupling. Kinetic analysis indicated the RGS4-dependent reduction in steady-state GIRK current was fully explained by the accelerated deactivation rate. Thus kinetic inconsistencies associated with RGS4-accelerated GIRK currents occur at a critical threshold of G protein coupling. In contrast to RGS4, RGS7 selectively accelerated Galpha(o)-coupled GIRK currents. Co-expression of Gbeta5, in addition to enhancing the kinetic effects of RGS7, caused a significant reduction (70-85 %) in steady-state GIRK currents indicating RGS7-Gbeta5 complexes disrupt Galpha(o) coupling. Altogether these results provide further evidence for a GPCR-Galphabetagamma-GIRK signalling complex that is revealed by the modulatory affects of RGS proteins on GIRK channel gating. Our functional experiments demonstrate that the formation of this signalling complex is markedly dependent on the concentration and composition of G protein-RGS complexes.

Two different inward rectifier K+ channels are effectors for transmitter-induced slow excitation in brain neurons

Substance P (SP) excites large neurons of the nucleus basalis (NB) by inhibiting an inward rectifier K(+) channel (Kir). The properties of the Kir in NB (KirNB) in comparison with the G protein-coupled Kir (GIRK) were investigated. Single-channel recordings with the cell-attached mode showed constitutively active KirNB channels, which were inhibited by SP. When the recording method was changed from the on-cell to the inside-out mode, the channel activity of KirNB remained intact with its constitutive activity unaltered. Application of Gbeta(1gamma2) to inside-out patches induced activity of a second type of Kir (GIRK). Application of Gbeta(1gamma2), however, did not change the KirNB activity. Sequestering Gbeta(1gamma2) with Galpha(i2) abolished the GIRK activity, whereas the KirNB activity was not affected. The mean open time of KirNB channels (1.1 ms) was almost the same as that of GIRKs. The unitary conductance of KirNB was 23 pS (155 mM [K(+)](o)), whereas that of the GIRK was larger (32-39 pS). The results indicate that KirNB is different from GIRKs and from any of the classical Kirs (IRKs). Whole-cell current recordings revealed that application of muscarine to NB neurons induced a GIRK current, and this GIRK current was also inhibited by SP. Thus, SP inhibits both KirNB and GIRKs. We conclude that the excitatory transmitter SP has two types of Kirs as its effectors: the constitutively active, Gbetagamma-independent KirNB channel and the Gbetagamma-dependent GIRK.

N-terminal tyrosine residues within the potassium channel Kir3 modulate GTPase activity of Galphai

trkB activation results in tyrosine phosphorylation of N-terminal Kir3 residues, decreasing channel activation. To determine the mechanism of this effect, we reconstituted Kir3, trkB, and the mu opioid receptor in Xenopus oocytes. Activation of trkB by BDNF (brain-derived neurotrophic factor) accelerated Kir3 deactivation following termination of mu opioid receptor signaling. Similarly, overexpression of RGS4, a GTPase-activating protein (GAP), accelerated Kir3 deactivation. Blocking GTPase activity with GTPgammaS also prevented Kir3 deactivation, and the GTPgammaS effect was not reversed by BDNF treatment. These results suggest that BDNF treatment did not reduce Kir3 affinity for Gbetagamma but rather acted to accelerate GTPase activity, like RGS4. Tyrosine phosphatase inhibition by peroxyvanadate pretreatment reversibly mimicked the BDNF/trkB effect, indicating that tyrosine phosphorylation of Kir3 may have caused the GTPase acceleration. Tyrosine to phenylalanine substitution in the N-terminal domain of Kir3.4 blocked the BDNF effect, supporting the hypothesis that phosphorylation of these tyrosines was responsible. Like other GAPs, Kir3.4 contains a tyrosine-arginine-glutamine motif that is thought to function by interacting with G protein catalytic domains to facilitate GTP hydrolysis. These data suggest that the N-terminal tyrosine hydroxyls in Kir3 normally mask the GAP activity and that modification by phosphorylation or phenylalanine substitution reveals the GAP domain. Thus, BDNF activation of trkB could inhibit Kir3 by facilitating channel deactivation.

The (beta)gamma subunits of G proteins gate a K(+) channel by pivoted bending of a transmembrane segment

The molecular mechanism of ion channel gating remains unclear. Using approaches such as proline scanning mutagenesis and homology modeling, we localize the gate of the K(+) channels controlled by the (beta)gamma subunits of G proteins at the pore-lining bundle crossing of the second transmembrane (TM2) helices. We show that the flexibility afforded by a highly conserved glycine residue in the middle of TM2 is crucial for channel gating. In contrast, flexibility introduced immediately below the gate disrupts gating. We propose that the force produced by channel-G(beta)gamma interactions is transduced through the rigid region below the helix bundle crossing to bend TM2 at the glycine that serves as a hinge and open the gate.

Graded contribution of the Gbeta gamma binding domains to GIRK channel activation

G protein coupled inwardly rectifying K(+) channels (GIRK/Kir3.x) are mainly activated by a direct interaction with Gbetagamma subunits, released upon the activation of inhibitory neurotransmitter receptors. Although Gbetagamma binding domains on all four subunits have been found, the relative contribution of each of these binding sites to channel gating has not yet been defined. It is also not known whether GIRK channels open once all Gbetagamma sites are occupied, or whether gating is a graded process. We used a tandem tetrameric approach to enable the selective elimination of specific Gbetagamma binding domains in the tetrameric context. Here, we show that tandem tetramers are fully operational. Tetramers with only one wild-type channel subunit showed receptor-independent high constitutive activity. The presence of two or three wild-type subunits reconstituted receptor activation gradually. Furthermore, a tetramer with no GIRK1 Gbetagamma binding domain displayed slower kinetics of activation. The slowdown in activation was found to be independent of regulator of G protein signaling or receptor coupling, but this slowdown could be reversed once only one Gbetagamma binding domain of GIRK1 was added. These results suggest that partial activation can occur under low Gbetagamma occupancy and that full activation can be accomplished by the interaction with three Gbetagamma binding subunits.

Classic D1 dopamine receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390) directly inhibits G protein-coupled inwardly rectifying potassium channels

R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390) is a widely used, highly selective antagonist of D1 dopamine receptors. While investigating the crosstalk between D1 and D3 dopamine receptor signaling pathways, we discovered that in addition to being a D1 receptor antagonist, SCH23390 and related compounds inhibit G protein-coupled inwardly rectifying potassium (GIRK) channels. We present evidence that SCH23390 blocks endogenous GIRK currents induced by either somatostatin or D3 dopamine receptors in AtT-20 cells (IC50, 268 nM). The inhibition is receptor-independent because constitutive GIRK currents in Chinese hamster ovary cells expressing only GIRK channels are also blocked by SCH23390. The inhibition of GIRK channels is somewhat selective because members of the closely related Kir2.0 family of inwardly rectifying potassium channels, as well as various endogenous cationic currents present in AtT-20 cells, are not affected. In addition, in current clamp recordings, SCH23390 can depolarize the membrane potential and induce AtT-20 cells to fire action potentials, indicating potential physiological significance of the GIRK channel inhibition. To identify the chemical features that contribute to GIRK channel block, we tested several structurally related compounds [SKF38393, R-(+)-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (nor-methyl-SCH23390), and R-(+)-2,3,4,5-tetrahydro-8-iodo-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrochloride (iodo-SCH23390)], and our results indicate that the halide atom is critical for blocking GIRK channels. Taken together, our results suggest that SCH23390 and related compounds might provide the basis for designing novel GIRK channel-selective blockers. Perhaps more importantly, some studies that have exclusively used SCH23390 to probe D1 receptor function or as a diagnostic of D1 receptor involvement may need to be reevaluated in light of these results.

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.

Structural and functional determinants of conserved lipid interaction domains of inward rectifying Kir6.2 channels

All members of the inward rectifiier K(+) (Kir) channel family are activated by phosphoinositides and other amphiphilic lipids. To further elucidate the mechanistic basis, we examined the membrane association of Kir6.2 fragments of K(ATP) channels, and the effects of site-directed mutations of these fragments and full-length Kir6.2 on membrane association and K(ATP) channel activity, respectively. GFP-tagged Kir6.2 COOH terminus and GFP-tagged pleckstrin homology domain from phospholipase C delta1 both associate with isolated membranes, and association of each is specifically reduced by muscarinic m1 receptor-mediated phospholipid depletion. Kir COOH termini are predicted to contain multiple beta-strands and a conserved alpha-helix (residues approximately 306-311 in Kir6.2). Systematic mutagenesis of D307-F315 reveals a critical role of E308, I309, W311 and F315, consistent with residues lying on one side of a alpha-helix. Together with systematic mutation of conserved charges, the results define critical determinants of a conserved domain that underlies phospholipid interaction in Kir channels.