Role of the isoprenyl pocket of the G protein beta gamma subunit complex in the binding of phosducin and phosducin-like protein

A model for how Gβγ couples Gα to GPCR

Representing ∼5% of the human genome, G-protein-coupled receptors (GPCRs) are a primary target for drug discovery; however, the molecular details of how they couple to heterotrimeric G protein subunits are incompletely understood. Here, I propose a hypothetical initial docking model for the encounter between GPCR and Gβγ that is defined by transient interactions between the cytosolic surface of the GPCR and the prenyl moiety and the tripeptide motif, asparagine-proline-phenylalanine (NPF), in the C-terminus of the Gγ subunit. Analysis of class A GPCRs reveals a conserved NPF binding site formed by the interaction of the TM1 and H8. Functional studies using differentially prenylated proteins and peptides further suggest that the intracellular hydrophobic core of the GPCR is a prenyl binding site. Upon binding TM1 and H8 of GPCRs, the propensity of the C-terminal region of Gγ to convert into an α helix allows it to extend into the hydrophobic core of the GPCR, facilitating the GPCR active state. Conservation of the NPF motif in Gγ isoforms and interacting residues in TM1 and H8 suggest that this is a general mechanism of GPCR-G protein signaling. Analysis of the rhodopsin dimer also suggests that Gγ-rhodopsin interactions may facilitate GPCR dimer transactivation.

A Collision Coupling Model Governs the Activation of Neuronal GIRK1/2 Channels by Muscarinic-2 Receptors

The G protein-activated Inwardly Rectifying K⁺-channel (GIRK) modulates heart rate and neuronal excitability. Following G-Protein Coupled Receptor (GPCR)-mediated activation of heterotrimeric G proteins (Gαβγ), opening of the channel is obtained by direct binding of Gβγ subunits. Interestingly, GIRKs are solely activated by Gβγ subunits released from Gα_i/o-coupled GPCRs, despite the fact that all receptor types, for instance Gα_q-coupled, are also able to provide Gβγ subunits. It is proposed that this specificity and fast kinetics of activation stem from pre-coupling (or pre-assembly) of proteins within this signaling cascade. However, many studies, including our own, point towards a diffusion-limited mechanism, namely collision coupling. Here, we set out to address this long-standing question by combining electrophysiology, imaging, and mathematical modeling. Muscarinic-2 receptors (M2R) and neuronal GIRK1/2 channels were coexpressed in Xenopus laevis oocytes, where we monitored protein surface expression, current amplitude, and activation kinetics. Densities of expressed M2R were assessed using a fluorescently labeled GIRK channel as a molecular ruler. We then incorporated our results, along with available kinetic data reported for the G-protein cycle and for GIRK1/2 activation, to generate a comprehensive mathematical model for the M2R-G-protein-GIRK1/2 signaling cascade. We find that, without assuming any irreversible interactions, our collision coupling kinetic model faithfully reproduces the rate of channel activation, the changes in agonist-evoked currents and the acceleration of channel activation by increased receptor densities.

Heterotrimeric G-Protein Signaling in Plants: Conserved and Novel Mechanisms

Heterotrimeric GTP-binding proteins are key regulators of a multitude of signaling pathways in all eukaryotes. Although the core G-protein components and their basic biochemistries are broadly conserved throughout evolution, the regulatory mechanisms of G proteins seem to have been rewired in plants to meet specific needs. These proteins are currently the focus of intense research in plants due to their involvement in many agronomically important traits, such as seed yield, organ size regulation, biotic and abiotic stress responses, symbiosis, and nitrogen use efficiency. The availability of massive sequence information from a variety of plant species, extensive biochemical data generated over decades, and impressive genetic resources for plant G proteins have made it possible to examine their role, unique properties, and novel regulation. This review focuses on some recent advances in our understanding of the mechanistic details of this critical signaling pathway to enable the precise manipulation and generation of plants to meet future needs.

Synthesis and assembly of G protein βγ dimers: comparison of in vitro and in vivo studies

The heterotrimeric GTP-binding proteins (G proteins) are the canonical cellular machinery used with the approximately 700 G protein-coupled receptors (GPCRs) in the human genome to transduce extracellular signals across the plasma membrane. The synthesis of the constituent G protein subunits, and their assembly into Gβγ dimers and G protein heterotrimers, determines the signaling repertoire for G-protein/GPCR signaling in cells. These synthesis/assembly -processes are intimately related to two other overlapping events in the intricate pathway leading to formation of G protein signaling complexes, posttranslational modification and intracellular trafficking of G proteins. The assembly of the Gβγ dimer is a complex process involving multiple accessory proteins and organelles. The mechanisms involved are becoming increasingly appreciated, but are still incompletely understood. In vitro and in vivo (cellular) studies provide different perspectives of these processes, and a comparison of them can provide insight into both our current level of understanding and directions to be taken in future investigations.

Steric volume exclusion sets soluble protein concentrations in photoreceptor sensory cilia

Proteins segregate into discrete subcellular compartments via a variety of mechanisms, including motor protein transport, local binding, and diffusion barriers. This physical separation of cell functions serves, in part, as a mechanism for controlling compartment activity by allowing regulation of local protein concentrations. In this study we explored how soluble protein size impacts access to the confined space within the retinal photoreceptor outer segment signaling compartment and discovered a strikingly steep relationship. We find that GFP monomers, dimers, and trimers expressed transgenically in frog rods are present in the outer segment at 1.8-, 2.9-, and 6.8-fold lower abundances, relative to the cell body, than the small soluble fluorescent marker, calcein. Theoretical analysis, based on statistical-mechanical models of molecular access to polymer meshes, shows that these observations can be explained by the steric hindrance of molecules occupying the highly constrained spaces between outer segment disc membranes. This mechanism may answer a long-standing question of how the soluble regulatory protein, arrestin, is effectively excluded from the outer segments of dark-adapted rods and cones. Generally, our results suggest an alternate mode for the control of protein access to cell domains based on dynamic, size-dependent compartmental partitioning that does not require diffusion barriers, active transport, or large numbers of immobile binding sites.

NMR analysis of G-protein betagamma subunit complexes reveals a dynamic G(alpha)-Gbetagamma subunit interface and multiple protein recognition modes

G-protein betagamma (Gbetagamma) subunits interact with a wide range of molecular partners including: G(alpha) subunits, effectors, peptides, and small molecule inhibitors. The molecular mechanisms underlying the ability to accommodate this wide range of structurally distinct binding partners are not well understood. To uncover the role of protein flexibility and alterations in protein conformation in molecular recognition by Gbetagamma, a method for site-specific (15)N-labeling of Gbeta-Trp residue backbone and indole amines in insect cells was developed. Transverse Relaxation Optimized Spectroscopy-Heteronuclear Single-Quantum Coherence Nuclear Magnetic Resonance (TROSY-HSQC NMR) analysis of (15)N-Trp Gbetagamma identified well-dispersed signals for the individual Trp residue side chain and amide positions. Surprisingly, a wide range of signal intensities was observed in the spectrum, likely representing a range of backbone and side chain mobilities. The signal for GbetaW99 indole was very intense, suggesting a high level of mobility on the protein surface and molecular dynamics simulations indicate that GbetaW99 is highly mobile on the nanosecond timescale in comparison with other Gbeta tryptophans. Binding of peptides and phosducin dramatically altered the mobility of GbetaW99 and GbetaW332 in the binding site and the chemical shifts at sites distant from the direct binding surface in distinct ways. In contrast, binding of G(alpha)(i1)-GDP to Gbetagamma had relatively little effect on the spectrum and, most surprisingly, did not significantly alter Trp mobility at the subunit interface. This suggests the inactive heterotrimer in solution adopts a conformation with an open subunit interface a large percentage of the time. Overall, these data show that Gbetagamma subunits explore a range of conformations that can be exploited during molecular recognition by diverse binding partners.

Prenylation-deficient G protein gamma subunits disrupt GPCR signaling in the zebrafish

Prenylation of G protein gamma (gamma) subunits is necessary for the membrane localization of heterotrimeric G proteins and for functional heterotrimeric G protein coupled receptor (GPCR) signaling. To evaluate GPCR signaling pathways during development, we injected zebrafish embryos with mRNAs encoding Ggamma subunits mutated so that they can no longer be prenylated. Low-level expression of these prenylation-deficient Ggamma subunits driven either ubiquitously or specifically in the primordial germ cells (PGCs) disrupts GPCR signaling and manifests as a PGC migration defect. This disruption results in a reduction of calcium accumulation in the protrusions of migrating PGCs and a failure of PGCs to directionally migrate. When co-expressed with a prenylation-deficient Ggamma, 8 of the 17 wildtype Ggamma isoforms individually confer the ability to restore calcium accumulation and directional migration. These results suggest that while the Ggamma subunits possess the ability to interact with G Beta (beta) proteins, only a subset of wildtype Ggamma proteins are stable within PGCs and can interact with key signaling components necessary for PGC migration. This in vivo study highlights the functional redundancy of these signaling components and demonstrates that prenylation-deficient Ggamma subunits are an effective tool to investigate the roles of GPCR signaling events during vertebrate development.

Shuttling and translocation of heterotrimeric G proteins and Ras

Heterotrimeric G proteins (alphabetagamma) and Ras proteins are activated by cell-surface receptors that sense extracellular signals. Both sets of proteins were traditionally thought to be constrained to the plasma membrane and some intracellular membranes. Live-cell-imaging experiments have now shown that these proteins are mobile inside a cell, shuttling continually between the plasma membrane and intracellular membranes in the basal state, maintaining these proteins in dynamic equilibrium in different membrane compartments. Furthermore, on receptor activation, a family of G protein betagamma subunits translocates rapidly and reversibly to the Golgi and endoplasmic reticulum enabling direct communication between the extracellular signal and intracellular membranes. A member of the Ras family has similarly been shown to translocate on activation. Although the impact of this unexpected intracellular movement of signaling proteins on cell physiology is likely to be distinct, there are striking similarities in the properties of these two families of signal-transducing proteins.

Transducin gamma-subunit sets expression levels of alpha- and beta-subunits and is crucial for rod viability

Transducin is a prototypic heterotrimeric G-protein mediating visual signaling in vertebrate photoreceptor cells. Despite its central role in phototransduction, little is known about the mechanisms that regulate its expression and maintain approximately stoichiometric levels of the alpha- and betagamma-subunits. Here we demonstrate that the knock-out of transducin gamma-subunit leads to a major downregulation of both alpha- and beta-subunit proteins, despite nearly normal levels of the corresponding transcripts, and fairly rapid photoreceptor degeneration. Significant fractions of the remaining alpha- and beta-subunits were mislocalized from the light-sensitive outer segment compartment of the rod. Yet, the tiny amount of the alpha-subunit present in the outer segments of knock-out rods was sufficient to support light signaling, although with a markedly reduced sensitivity. These data indicate that the gamma-subunit controls the expression level of the entire transducin heterotrimer and that heterotrimer formation is essential for normal transducin localization. They further suggest that the production of transducin beta-subunit without its constitutive gamma-subunit partner sufficiently stresses the cellular biosynthetic and/or chaperone machinery to induce cell death.

Function of phosducin-like proteins in G protein signaling and chaperone-assisted protein folding

Members of the phosducin gene family were initially proposed to act as down-regulators of G protein signaling by binding G protein betagamma dimers (Gbetagamma) and inhibiting their ability to interact with G protein alpha subunits (Galpha) and effectors. However, recent findings have over-turned this hypothesis by showing that most members of the phosducin family act as co-chaperones with the cytosolic chaperonin complex (CCT) to assist in the folding of a variety of proteins from their nascent polypeptides. In fact rather than inhibiting G protein pathways, phosducin-like protein 1 (PhLP1) has been shown to be essential for G protein signaling by catalyzing the folding and assembly of the Gbetagamma dimer. PhLP2 and PhLP3 have no role in G protein signaling, but they appear to assist in the folding of proteins essential in regulating cell cycle progression as well as actin and tubulin. Phosducin itself is the only family member that does not participate with CCT in protein folding, but it is believed to have a specific role in visual signal transduction to chaperone Gbetagamma subunits as they translocate to and from the outer and inner segments of photoreceptor cells during light-adaptation.

Lipid–protein interactions in GPCR-associated signaling

Signal transduction via G-protein-coupled receptors (GPCRs) is a fundamental pathway through which the functions of an individual cell can be integrated within the demands of a multicellular organism. Since this family of receptors first discovered, the proteins that constitute this signaling cascade and their interactions with one another have been studied intensely. In parallel, the pivotal role of lipids in the correct and efficient propagation of extracellular signals has attracted ever increasing attention. This is not surprising given that most of the signal transduction machinery is membrane-associated and therefore lipid-related. Hence, lipid-protein interactions exert a considerable influence on the activity of these proteins. This review focuses on the post-translational lipid modifications of GPCRs and G proteins (palmitoylation, myristoylation, and isoprenylation) and their significance for membrane binding, trafficking and signaling. Moreover, we address how the particular biophysical properties of different membrane structures may regulate the localization of these proteins and the potential functional consequences of this phenomenon in signal transduction. Finally, the interactions that occur between membrane lipids and GPCR effector enzymes such as PLC and PKC are also considered.

Mechanism of assembly of G protein betagamma subunits by protein kinase CK2-phosphorylated phosducin-like protein and the cytosolic chaperonin complex

Phosducin-like protein (PhLP) is a widely expressed binding partner of the G protein betagamma subunit complex (Gbetagamma) that has been recently shown to catalyze the formation of the Gbetagamma dimer from its nascent polypeptides. Phosphorylation of PhLP at one or more of three consecutive serines (Ser-18, Ser-19, and Ser-20) is necessary for Gbetagamma dimer formation and is believed to be mediated by the protein kinase CK2. Moreover, several lines of evidence suggest that the cytosolic chaperonin complex (CCT) may work in concert with PhLP in the Gbetagamma-assembly process. The results reported here delineate a mechanism for Gbetagamma assembly in which a stable ternary complex is formed between PhLP, the nascent Gbeta subunit, and CCT that does not include Ggamma. PhLP phosphorylation permits the release of a PhLP x Gbeta intermediate from CCT, allowing Ggamma to associate with Gbeta in this intermediate complex. Subsequent interaction of Gbetagamma with membranes releases PhLP for another round of assembly.

Phosducin and Phosducin-like Protein Attenuate G-Protein-Coupled Receptor-Mediated Inhibition of Voltage-Gated Calcium Channels in Rat Sympathetic Neurons

Phosducin (PDC) has been shown in structural and biochemical experiments to bind the Gbetagamma subunit of heterotrimeric G-proteins. A proposed function of PDC and phosducin-like protein (PDCL) is the sequestration of "free" Gbetagamma from the plasma membrane, thereby terminating signaling by Gbetagamma. The functional impact of heterologously expressed PDC and PDCL on N-type calcium channel (CaV2.2) modulation was examined in sympathetic neurons, isolated from rat superior cervical ganglia, using whole-cell voltage clamp. Expression of PDC and PDCL attenuated voltage-dependent inhibition of N-type calcium channels, a Gbetagamma-dependent process, in a time-dependent fashion. Calcium current inhibition after short-term exposure to norepinephrine was minimally altered by PDC or PDCL expression. However, in the continued presence of norepinephrine, PDC or PDCL relieved calcium channel inhibition compared with control neurons. We observed similar results after activation of heterologously expressed metabotropic glutamate receptors with 100 microM L-glutamate. Neurons expressing PDC or PDCL maintained suppression of inhibition after re-exposure to agonist. Unlike other Gbetagamma sequestering proteins that abolish the short-term inhibition of Ca2+ channels, PDC and PDCL require prolonged agonist exposure before effects on modulation are realized.

G protein betagamma complex translocation from plasma membrane to Golgi complex is influenced by receptor gamma subunit interaction

On activation of a receptor the G protein betagamma complex translocates away from the receptor on the plasma membrane to the Golgi complex. The rate of translocation is influenced by the type of gamma subunit associated with the G protein. Complementary approaches--imaging living cells expressing fluorescent protein tagged G proteins and assaying reconstituted receptors and G proteins in vitro--were used to identify mechanisms at the basis of the translocation process. Translocation of Gbetagamma containing mutant gamma subunits with altered prenyl moieties showed that the differences in the prenyl moieties were not sufficient to explain the differential effects of geranylgeranylated gamma5 and farnesylated gamma11 on the translocation process. The translocation properties of Gbetagamma were altered dramatically by mutating the C terminal tail region of the gamma subunit. The translocation characteristics of these mutants suggest that after receptor activation, Gbetagamma retains contact with a receptor through the gamma subunit C terminal domain and that differential interaction of the activated receptor with this domain controls Gbetagamma translocation from the plasma membrane.

Phosducin-like protein acts as a molecular chaperone for G protein betagamma dimer assembly

Phosducin-like protein (PhLP) is a widely expressed binding partner of the G protein betagamma subunit dimer (Gbetagamma). However, its physiological role is poorly understood. To investigate PhLP function, its cellular expression was blocked using RNA interference, resulting in inhibition of Gbetagamma expression and G protein signaling. This inhibition was caused by an inability of nascent Gbetagamma to form dimers. Phosphorylation of PhLP at serines 18-20 by protein kinase CK2 was required for Gbetagamma formation, while a high-affinity interaction of PhLP with the cytosolic chaperonin complex appeared unnecessary. PhLP bound nascent Gbeta in the absence of Ggamma, and S18-20 phosphorylation was required for Ggamma to associate with the PhLP-Gbeta complex. Once Ggamma bound, PhLP was released. These results suggest a mechanism for Gbetagamma assembly in which PhLP stabilizes the nascent Gbeta polypeptide until Ggamma can associate, resulting in membrane binding of Gbetagamma and release of PhLP to catalyze another round of assembly.

Differential Sensitivity of Phosphatidylinositol 3-Kinase p110γ to Isoforms of G Protein βγ Dimers

The ability of G protein alpha and betagamma subunits to activate the p110gamma isoform of phosphatidylinositol 3-kinase (PtdIns 3-kinase) was examined using pure, recombinant G proteins and the p101/p110gamma form of PtdIns 3-kinase reconstituted into synthetic lipid vesicles. GTP-activated Gs, Gi, Gq, or Go alpha subunits were unable to activate PtdIns 3-kinase. Dimers containing Gbeta(1-4) complexed with gamma2-stimulated PtdIns 3-kinase activity about 26-fold with EC50 values ranging from 4 to 7 nm. Gbeta5gamma2 was not able to stimulate PtdIns 3-kinase despite producing a 10-fold activation of avian phospholipase Cbeta. A series of dimers with beta subunits containing point mutations in the amino acids that undergo a conformational change upon interaction of betagamma with phosducin (beta1H311Agamma2, beta1R314Agamma2, and beta1W332Agamma2) was tested, and only beta1W332Agamma2 inhibited the ability of the dimer to stimulate PtdIns 3-kinase. Dimers containing the beta1 subunit complexed with a panel of different Ggamma subunits displayed variation in their ability to stimulate PtdIns 3-kinase. The beta1gamma2, beta1gamma10, beta1gamma12, and beta1gamma13 dimers all activated PtdIns 3-kinase about 26-fold with 4-25 nm EC50 values. The beta1gamma11 dimer, which contains the farnesyl isoprenoid group and is highly expressed in tissues containing the p101/p110gamma form of PtdIns 3-kinase, was ineffective. The role of the prenyl group on the gamma subunit in determining the activation of PtdIns 3-kinase was examined using gamma subunits with altered CAAX boxes directing the addition of farnesyl to the gamma2 subunit and geranylgeranyl to the gamma1 and gamma11 subunits. Replacement of the geranylgeranyl group of the gamma2 subunit with farnesyl inhibited the activity of beta1gamma2 on PtdIns 3-kinase. Conversely, replacement of the farnesyl group on the gamma1 and gamma11 subunit with geranylgeranyl restored almost full activity. These findings suggest that all beta subunits, with the exception of beta5, interact equally well with PtdIns 3-kinase. In contrast, the composition of the gamma subunit and its prenyl group markedly affects the ability of the betagamma dimer to stimulate PtdIns 3-kinase.