Identification of a structural element in phospholipase C beta2 that interacts with G protein betagamma subunits

Gbeta 5gamma 2 is a highly selective activator of phospholipid-dependent enzymes

In this study, Gbeta specificity in the regulation of Gbetagamma-sensitive phosphoinositide 3-kinases (PI3Ks) and phospholipase Cbeta (PLCbeta) isozymes was examined. Recombinant mammalian Gbeta(1-3)gamma(2) complexes purified from Sf9 membranes stimulated PI3Kgamma lipid kinase activity with similar potency (10-30 nm) and efficacy, whereas transducin Gbetagamma was less potent. Functionally active Gbeta(5)gamma(2) dimers were purified from Sf9 cell membranes following coexpression of Gbeta(5) and Ggamma(2-His). This preparation as well as Gbeta(1)gamma(2-His) supported pertussis toxin-mediated ADP-ribosylation of Galpha(i1). Gbeta(1)gamma(2-His) stimulated PI3Kgamma lipid and protein kinase activities at nanomolar concentrations, whereas Gbeta(5)gamma(2-His) had no effect. Accordingly, Gbeta(1)gamma(2-His), but not Gbeta(5)gamma(2-His), significantly stimulated the lipid kinase activity of PI3Kbeta in the presence or absence of tyrosine-phosphorylated peptides derived from the p85-binding domain of the platelet derived-growth factor receptor. Conversely, both preparations were able to stimulate PLCbeta(2) and PLCbeta(1). However, Gbeta(1)gamma(2-His), but not Gbeta(5)gamma(2-His), activated PLCbeta(3). Experimental evidence suggests that the mechanism of Gbeta(5)-dependent effector selectivity may differ between PI3K and PLCbeta. In conclusion, these data indicate that Gbeta subunits are able to discriminate among effectors independently of Galpha due to selective protein-protein interaction.

Dissecting G protein-coupled receptor signaling pathways with membrane-permeable blocking peptides. Endogenous 5-HT(2C) receptors in choroid plexus epithelial cells

To determine the intracellular signaling mechanism of the 5-HT(2C) receptor endogenously expressed in choroid plexus epithelial cells, we implemented a strategy of targeted disruption of protein-protein interactions. This strategy entails the delivery of conjugated membrane-permeable peptides that disrupt domain interaction at specific steps in the signaling cascade. As proof of concept, two peptides targeted against receptor-G protein interaction domains were examined. Only G(q)CT, which targets the receptor-G(q) protein interacting domain, disrupted 5-HT(2C) receptor-mediated phosphatidylinositide hydrolysis. G(s)CT, targeting the receptor-G(s) protein, disrupted beta2 adrenergic receptor-mediated activation of cAMP but not 5-HT(2C) receptor-mediated phosphatidylinositide hydrolysis. The peptide MPS-PLCbeta1M, mimicking the domain of phospholipase Cbeta1 (PLCbeta1) interacting with active Galpha(q), also blocked 5-HT(2C) receptor activation. In contrast, peptides PLCbeta2M and Phos that bind to and sequester free Gbetagamma subunits were ineffective at blocking 5-HT(2C) receptor-mediated phosphoinositol turnover. However, both peptides disrupted Gbetagamma-mediated alpha(2A) adrenergic receptor activation of mitogen-activated protein kinase. These results provide the first direct demonstration that active Galpha(q) subunits mediate endogenous 5-HT(2C) receptor activation of PLCbeta and that Gbetagamma subunits released from Galpha(q) heterotrimeric proteins are not involved. Comparable results were obtained with metabotropic glutamate receptor 5 expressed in astrocytes. Thus, conjugated, membrane-permeable peptides are effective tools for the dissection of intracellular signals.

Identification of a region at the N-terminus of phospholipase C-beta 3 that interacts with G protein beta gamma subunits

Members of the phospholipase C-beta (PLC-beta) family of proteins are activated either by G alpha or G beta gamma subunits of heterotrimeric G proteins. To define specific regions of PLC-beta 3 that are involved in binding and activation by G beta gamma, a series of fragments of PLC-beta 3 as glutathione-S-transferase (GST) fusion proteins were produced. A fragment encompassing the N-terminal pleckstrin homology (PH) domain and downstream sequence (GST-N) bound to G protein beta 1 gamma 2 in an in vitro binding assay, and binding was inhibited by G protein alpha subunit, G alpha i1. This PLC-beta 3 fragment also inhibited G beta gamma-stimulated PLC-beta activity in a reconstitution system, while having no significant effect on G alpha q-stimulated PLC-beta 3 activity. The N-terminal G beta gamma binding region was delineated further to the first 180 amino acids, and the sequence Asn150-Ser180, just distal to the PH domain, was found to be required for the interaction. Mutation of basic residues 154Arg, 155Lys, 159Lys, and 161Lys to Glu within this region reduced G beta gamma binding affinity and specifically reduced the EC50 for G beta gamma-dependent activation of the mutant enzyme 3-fold. Basal activity and G alpha q-dependent activation of the enzyme were unaffected by the mutations. While these basic residues may not directly mediate the interaction with G beta gamma, the data provide evidence for an N-terminal G beta gamma binding region of PLC-beta 3 that is involved in activation of the enzyme.

Association of the proto-oncogene product dbl with G protein betagamma subunits

The Rho family of GTP-binding proteins has been implicated in the regulation of various cellular functions including actin cytoskeleton-dependent morphological change. Its activity is directed by intracellular signals mediated by various types of receptors such as G protein-coupled receptors. However, the mechanisms underlying receptor-dependent regulation of Rho family members remain incompletely understood. The guanine nucleotide exchange factor (GEF) Dbl targets Rho family proteins thereby stimulating their GDP/GTP exchange, and thus is believed to be involved in receptor-mediated regulation of the proteins. Here, we show the association of Dbl with G protein betagamma subunits (Gbetagamma) in transient co-expression and cell-free systems. An amino-terminal portion conserved among a subset of Dbl family proteins is sufficient for the binding of Gbetagamma. In fact, Ost and Kalirin, which contain this Gbetagamma-binding motif, also associate with Gbetagamma. c-Jun N-terminal kinase was synergistically activated upon co-expression of Dbl and Gbeta in a dominant-negative Rho-sensitive manner. However, GEF activity of Dbl toward Rho as measured by in vitro GDP binding assays remained unaffected following Gbetagamma binding, suggesting that additional signals may be required for the regulation of Dbl.

Binding of Gbetagamma subunits to cRaf1 downregulates G-protein-coupled receptor signalling

Receptors of the seven transmembrane domain family are coupled to heterotrimeric G proteins [1]. Binding of ligand to these receptors induces dissociation of the heterotrimeric complex into free GTP-Galpha and Gbetagamma subunits, which then interact with their respective effector molecules to stimulate specific cellular responses. In some cases, these cellular responses involve mitogenic signalling [2]. The mitogen-activated protein (MAP) kinase cascade is initiated by the protein kinase cRaf1 and links growth factor receptor signalling to cell growth and differentiation [3]. The main activator of cRaf1 is the small GTP-binding protein Ras [4], and the binding of cRaf1 to GTP-Ras translocates cRaf1 to the plasma membrane, where it is activated [5]. It has been reported that cRaf1 associates directly with the beta subunit of heterotrimeric G proteins in vitro, and with the betagamma subunit complex in vivo [6], but the role of this association is not yet understood. Here, we show that cRaf1 associates with Gbeta1gamma2, and that this association in mammalian cells is significantly enhanced when active p21(Ras) is present or when cRaf1 is otherwise targeted to the membrane. Association with Gbeta1gamma2 has no effect on the kinase activity of cRaf1, but cRaf1 can affect Gbetagamma-mediated signalling events. Thus, membrane-localised cRaf1 inhibits G-protein-coupled receptor (GPCR)-stimulated activation of phospholipase Cbeta (PLCbeta) by sequestration of Gbetagamma subunits, an effect also observed with endogenous levels of cRaf1. Our data suggest that cRaf1 may be an important regulator of signalling by Gbetagamma, particularly in those GPCR systems that stimulate the MAP kinase cascade through the activation of p21(Ras).

The ordered visual transduction complex of the squid photoreceptor membrane

The study of visual transduction has given invaluable insight into the mechanisms of signal transduction by heptahelical receptors that act via guanine nucleotide binding proteins (G-proteins). However, the cyclic-GMP second messenger system seen in vertebrate photoreceptor cells is not widely used in other cell types. In contrast, the retina of higher invertebrates, such as squid, offers an equally accessible transduction system, which uses the widespread second messenger chemistry of an increase in cytosolic calcium caused by the production of inositol-(1,4,5)-trisphosphate (InsP3) by the enzyme phospholipase C, and which may be a model for store-operated calcium influx. In this article, we highlight some key aspects of invertebrate visual transduction as elucidated from the combination of biochemical techniques applied to cephalopods, genetic techniques applied to flies, and electrophysiology applied to the horseshoe crab. We discuss the importance and applicability of ideas drawn from these model systems to the understanding of some general processes in signal transduction, such as the integration of the cytoskeleton into the signal transduction process and the possible modes of regulation of store-operated calcium influx.

Role of isoprenoid lipids on the heterotrimeric G protein gamma subunit in determining effector activation

Post-translational prenylation of heterotrimeric G protein gamma subunits is essential for high affinity alpha-beta gamma and alpha-beta gamma-receptor interactions, suggesting that the prenyl group is an important domain in the beta gamma dimer. To determine the role of the prenyl modification in the interaction of beta gamma dimers with effectors, the CAAX (where A indicates alipathic amino acid) motifs in the gamma1, gamma2, and gamma11 subunits were altered to direct modification with different prenyl groups. Six recombinant beta gamma dimers were overexpressed in baculovirus-infected Sf9 insect cells, purified, and examined for their ability to stimulate three phospholipase C-beta isozymes and type II adenylyl cyclase. The native beta1 gamma2 dimer (gamma subunit modified with geranylgeranyl) is more potent and effective in activating phospholipase C-beta than either the beta1 gamma1 (farnesyl) or the beta1 gamma11 (farnesyl) dimers. However, farnesyl modification of the gamma subunit in the beta1 gamma2 dimer (beta1 gamma2-L71S) caused a decrement in its ability to activate phospholipase C-beta. In contrast, both the beta1 gamma1-S74L (geranylgeranyl) and the beta1 gamma11-S73L (geranylgeranyl) dimers were more active than the native forms. The beta1 gamma2 dimer activates type II adenylyl cyclase about 12-fold; however, neither the beta1 gamma1 nor the beta1 gamma11 dimers activate the enzyme. As was the case with phospholipase C-beta, the beta1gamma2-L71S dimer was less able to activate adenylyl cyclase than the native beta1 gamma2 dimer. Interestingly, neither the beta1 gamma1-S74L nor the beta1 gamma11-S73L dimers stimulated adenylyl cyclase. The results suggest that both the amino acid sequence of the gamma subunit and its prenyl group play a role in determining the activity of the beta gamma-effector complex.

Determination of the affinities between heterotrimeric G protein subunits and their phospholipase C-beta effectors

Phosphatidylinositide-specific phospholipase C-betas play a key role in Ca2+ signaling and are specifically activated by the alphaq family of heterotrimeric G proteins and as well as betagamma subunits. We have determined the affinity between Gbetagamma subunits and GTPgammaS and GDP-liganded Galphaq subunits on membrane surfaces, and their respective affinities to PLC-beta1, -beta2 and -beta3 effectors by fluorescence spectroscopy. We find that activation of Galphaq by GTPgammaS decreases its affinity for Gbetagamma subunits at least 36-fold compared to the GDP-liganded form, but increases its affinity for PLC-betas at least 40-200-fold depending on the PLC-beta isoform. The affinity of Galphaq(GTPgammaS) is similar for PLC-beta1 and -beta3 and 10-fold stronger for PLC-beta2, which corresponds to the reported relationship between the concentration of Galphaq(GTPgammaS) and PLC-beta activation on lipid bilayers. We find that a large portion of the PLC-beta-Galphaq association energy lies within the 400 residue C-terminal region of PLC-beta1 since truncating this region reduces its Galphaq affinity. In contrast, the isolated N-terminal region does not interact with Galphaq. Gbetagamma subunits interact with all three PLC-beta isotypes, but only showed strong binding to PLC-beta2, and activation of the three PLC-betas by Gbetagamma subunits parallels this behavior. We also tested the possibility that both Galphaq and Gbetagamma can simultaneously bind PLC-beta2. Our data argue against simultaneous binding and show that Galphaq and Gbetagamma independently regulate this effector.

Differential association of the pleckstrin homology domains of phospholipases C-beta 1, C-beta 2, and C-delta 1 with lipid bilayers and the beta gamma subunits of heterotrimeric G proteins

Pleckstrin homology (PH) domains are recognized in more than 100 different proteins, including mammalian phosphoinositide-specific phospholipase C (PLC) isozymes (isotypes beta, gamma, and delta). These structural motifs are thought to function as tethering devices linking their host proteins to membranes containing phosphoinositides or beta gamma subunits of heterotrimeric GTP binding (G) proteins. Although the PH domains of PLC-delta and PLC-gamma have been studied, the comparable domains of the beta isotypes have not. Here, we have measured the affinities of the isolated PH domains of PLC-beta 1 and -beta 2 (PH-beta 1 and PH-beta 2, respectively) for lipid bilayers and G-beta gamma subunits. Like the intact enzymes, these PH domains bind to membrane surfaces composed of zwitterionic phosphatidylcholine with moderate affinity. Inclusion of the anionic lipid phosphatidylserine or phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and inclusion of G-beta gamma subunits had little affect on their membrane affinity. In contrast, binding of PLC-delta 1 or its PH domain was highly dependent on PI(4,5)P2. We also determined whether these domains laterally associate with G-beta gamma subunits bound to membrane surfaces using fluorescence resonance energy transfer. Affinities for G-beta gamma were in the following order: PH-beta 2 >/= PH-beta 1 > PH-delta 1; the affinities of the native enzyme were as follows: PLC-beta 2 >> PLC-delta 1 > PLC-beta 1. Thus, the PH domain of PLC-beta 1 interacts with G-beta gamma in isolation, but not in the context of the native enzyme. By contrast, docking of the PH domain of PLC-beta2 with G-beta gamma is comparable to that of the full-length protein and may play a key role in G-beta gamma recognition.

Families of phosphoinositide-specific phospholipase C: structure and function

A large number of extracellular signals stimulate hydrolysis of phosphatidylinositol 4,5-bisphosphate by phosphoinositide-specific phospholipase C (PI-PLC). PI-PLC isozymes have been found in a broad spectrum of organisms and although they have common catalytic properties, their regulation involves different signalling pathways. A number of recent studies provided an insight into domain organisation of PI-PLC isozymes and contributed towards better understanding of the structural basis for catalysis, cellular localisation and molecular changes that could underlie the process of their activation.

Identification of a phospholipase C beta subtype in rat taste cells

From rat circumvallate papillae a novel phospholipase C (PLC) subtype has been cloned and identified as most closely related to human PLC beta2. The corresponding mRNA was only detected in sensory lingual tissue but not in non-taste lingual tissue or any other tissues examined by Northern blot analysis. In situ hybridization revealed that a subset of taste receptor cells of circumvallate papillae was specifically labeled. A functional involvement of this PLC beta subtype in taste signal transduction emerged from biochemical analysis monitoring the second messenger response in circumvallate preparations induced by denatonium benzoate. This bitter agent elicited a rapid and transient increase of the inositol 1,4,5-trisphosphate level; this response was blocked by U73122, a potent inhibitor of PLC, and by PLC beta2-specific antibodies. These data indicate that a phospholipase C beta2 isoform mediates a denatonium benzoate-induced second messenger response of taste sensory cells in the circumvallate papillae.

Phosphorylation of the G protein gamma12 subunit regulates effector specificity

Although the G protein betagamma dimer is an important mediator in cell signaling, the mechanisms regulating its activity have not been widely investigated. The gamma12 subunit is a known substrate for protein kinase C, suggesting phosphorylation as a potential regulatory mechanism. Therefore, recombinant beta1 gamma12 dimers were overexpressed using the baculovirus/Sf9 insect cell system, purified, and phosphorylated stoichiometrically with protein kinase C alpha. Their ability to support coupling of the Gi1 alpha subunit to the A1 adenosine receptor and to activate type II adenylyl cyclase or phospholipase C-beta was examined. Phosphorylation of the beta1 gamma12 dimer increased its potency in the receptor coupling assay from 6.4 to 1 nM, changed the Kact for stimulation of type II adenylyl cyclase from 14 to 37 nM, and decreased its maximal efficacy by 50%. In contrast, phosphorylation of the dimer had no effect on its ability to activate phospholipase C-beta. The native beta1gamma10 dimer, which has 4 similar amino acids in the phosphorylation site at the N terminus, was not phosphorylated by protein kinase C alpha. Creation of a phosphorylation site in the N terminus of the protein (Gly4 --> Lys) resulted in a beta1 gamma10G4K dimer which could be phosphorylated. The activities of this beta gamma dimer were similar to those of the phosphorylated beta1 gamma12 dimer. Thus, phosphorylation of the beta1 gamma12 dimer on the gamma subunit with protein kinase C alpha regulates its activity in an effector-specific fashion. Because the gamma12 subunit is widely expressed, phosphorylation may be an important mechanism for integration of the multiple signals generated by receptor activation.