Close association of the alpha subunits of Gq and G11 G proteins with actin filaments in WRK1 cells: relation to G protein-mediated phospholipase C activation

Kaposi sarcoma-associated herpesvirus promotes tumorigenesis by modulating the Hippo pathway

Kaposi sarcoma-associated herpesvirus (KSHV) is an oncogenic virus and the culprit behind the human disease Kaposi sarcoma (KS), an AIDS-defining malignancy. KSHV encodes a viral G-protein-coupled receptor (vGPCR) critical for the initiation and progression of KS. In this study, we identified that YAP/TAZ, two homologous oncoproteins inhibited by the Hippo tumor suppressor pathway, are activated in KSHV-infected cells in vitro, KS-like mouse tumors and clinical human KS specimens. The KSHV-encoded vGPCR acts through Gq/11 and G12/13 to inhibit the Hippo pathway kinases Lats1/2, promoting the activation of YAP/TAZ. Furthermore, depletion of YAP/TAZ blocks vGPCR-induced cell proliferation and tumorigenesis in a xenograft mouse model. The vGPCR-transformed cells are sensitive to pharmacologic inhibition of YAP. Our study establishes a pivotal role of the Hippo pathway in mediating the oncogenic activity of KSHV and development of KS, and also suggests a potential of using YAP inhibitors for KS intervention.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Preserved cardiomyocyte function and altered desmin pattern in transgenic mouse model of dilated cardiomyopathy

Taking advantage of the unique model of slowly developing dilated cardiomyopathy in mice with cardiomyocyte-specific transgenic overexpression of activated Gαq protein (Tgαq*44 mice) we analyzed the contribution of the cardiomyocyte malfunction, fibrosis and cytoskeleton remodeling to the development of heart failure in this model. Left ventricular (LV) in vivo function, myocardial fibrosis, cytoskeletal proteins expression and distribution, Ca(2+) handling and contractile function of isolated cardiomyocytes were evaluated at the stages of the early, compensated, and late, decompensated heart failure in 4-, 12- and 14-month-old Tgαq*44 mice, respectively, and compared to age-matched wild-type FVB mice. In the 4-month-old Tgαq*44 mice significant myocardial fibrosis, moderate myocyte hypertrophy and increased expression of regularly arranged and homogenously distributed desmin accompanied by increased phosphorylation of desmin chaperone protein, αB-crystallin, were found. Cardiomyocyte shortening, Ca(2+) handling and LV function were not altered. At 12 and 14 months of age, Tgαq*44 mice displayed progressive deterioration of the LV function. The contractile performance of isolated myocytes was still preserved, and the amplitude of Ca(2+) transients was even increased probably due to impairment of Na(+)/Ca(2+) exchanger function, while fibrosis was more extensive than in younger mice. Moreover, substantial disarrangement of desmin distribution accompanied by decreasing phosphorylation of αB-crystallin appeared. In Tgαq*44 mice disarrangement of desmin, at least partly related to inadequate phosphorylation of αB-crystallin seems to be importantly involved in the progressive deterioration of contractile heart function.

Non-canonical signaling and localizations of heterotrimeric G proteins

Heterotrimeric G proteins typically transduce signals from G protein-coupled receptors (GPCRs) to effector proteins. In the conventional G protein signaling paradigm, the G protein is located at the cytoplasmic surface of the plasma membrane, where, after activation by an agonist-bound GPCR, the GTP-bound Gα and free Gβγ bind to and regulate a number of well-studied effectors, including adenylyl cyclase, phospholipase Cβ, RhoGEFs and ion channels. However, research over the past decade or more has established that G proteins serve non-canonical roles in the cell, whereby they regulate novel effectors, undergo activation independently of a GPCR, and/or function at subcellular locations other than the plasma membrane. This review will highlight some of these non-canonical aspects of G protein signaling, focusing on direct interactions of G protein subunits with cytoskeletal and cell adhesion proteins, the role of G proteins in cell division, and G protein signaling at diverse organelles.

Vasopressin-induced morphological changes in polarized rat hepatocyte multiplets: dual calcium-dependent effects

Calcium-mobilizing hormones and neurotransmitters are known to affect cell morphology and function including cell differentiation or division. In this study, we examined vasopressin (AVP)-induced morphological changes in a polarized system of rat hepatocytes. Light and electron microscope observations showed that AVP induced microvilli formation and a remodeling of the isolated hepatocyte F-actin submembrane cytoskeleton, these two events being correlated. We showed that these effects were rapid, reversible, observed at nanomolar AVP concentration and mediated by the V(1a) receptor. On polarized multicellular systems of hepatocytes, we observed a rapid reduction of the bile canaliculi lumen at the apical pole and micovilli formation at the basolateral domain with an enlarged F-actin cytoskeleton. Neither activation of protein kinase C nor A via phorbol ester or dibutyryl cAMP induced such rapid morphological changes, at variance with ionomycin, suggesting that AVP-induced intracellular calcium rise plays a crucial role in those effects. By using spectrofluorimetry and cytochemistry, we showed that calcium release from intracellular stores was involved in bile canaliculus contraction, while calcium entry from the extracellular space controlled microvilli formation. Taken together, AVP and calcium-mobilizing agonists differentially regulate physiological hepatocyte plasma membrane events at the basal and the apical domains via topographically specialized calcium-dependent mechanisms.

Stimulation of Gαq-coupled M1 muscarinic receptor causes reversible spectrin redistribution mediated by PLC, PKC and ROCK

Spectrin is a cytoskeletal protein that plays a role in formation of the specialized plasma membrane domains. However, little is known of the molecular mechanism that regulates responses of spectrin to extracellular stimuli, such as activation of G-protein-coupled receptor (GPCR). We have found that αII spectrin is a component of the Gαq/11-associated protein complex in CHO cells stably expressing the M1 muscarinic receptor, and investigated the effect of activation of GPCR on the cellular localization of yellow-fluorescent-protein-tagged αII spectrin. Stimulation of Gαq/11-coupled M1 muscarinic receptor triggered reversible redistribution of αII spectrin following a rise in intracellular Ca2+ concentration. This redistribution, accompanied by non-apoptotic membrane blebbing, required an intact actin cytoskeleton and was dependent on activation of phospholipase C, protein kinase C, and Rho-associated kinase ROCK. Muscarinic-agonist-induced spectrin remodeling appeared particularly active at localized domains, which is clear contrast to that caused by constitutive activation of ROCK and to global rearrangement of the spectrin lattice caused by changes in osmotic pressure. These results suggest a role for spectrin in providing a dynamic and reversible signaling platform to the specific domains of the plasma membrane in response to stimulation of GPCR.

The role of the cytoskeleton in the morphogenesis and function of stomatal complexes

Microtubules (MTs) and actin filaments (AFs) form highly organized arrays in stomatal cells that play key roles in the morphogenesis of stomatal complexes. The cortical MTs controlling the orientation of the depositing cellulose microfibrils (CMs) and affecting the pattern of local wall thickenings define the mechanical properties of the walls of stomatal cells, thus regulating accurately their shape. Besides, they are involved in determination of the cell division plane. Substomatal cavity and stomatal pore formation are also MT-dependent processes. Among the cortical MT arrays, the radial ones lining the periclinal walls are of particular morphogenetic importance. Putative MT organizing centers (MTOCs) function at their focal regions, at least in guard cells (GCs), or alternatively, these regions either organize or nucleate cortical MTs. AFs are involved in cell polarization preceding asymmetrical divisions, in determination of the cell division plane and final cell plate alignment and probably in transduction of stimuli implicated in stomatal complex morphogenesis. Mature kidney-shaped GCs display radial AF arrays, undergoing definite organization cycles during stomatal movement. They are involved in stomatal movement, probably by controlling plasmalemma ion-channel activities. Radial MT arrays also persist in mature GCs, but a role in stomatal function cannot yet be attributed to them. Contents Summary 613 I. Introduction 614 II. Cytoskeleton and development of the stomatal complexes 614 III. Cytoskeleton and stomatal cell shaping 620 IV. Stomatal pore formation 624 V. Substomatal cavity formation 625 VI. Stomatal complex morphogenesis in mutants 626 VII. Cytoskeleton dynamics in functioning stomata 628 VIII. Mechanisms of microtubule organization in stomatal cells 631 IX. Conclusions-perspectives 634 References 635.

Heterotrimeric G-proteins associate with microtubules during differentiation in PC12 pheochromocytoma cells

Tubulin modifies G-protein signaling and heterotrimeric G-proteins regulate microtubule assembly. Here we report an interplay among G-protein-coupled receptor and receptor tyrosine kinase (such as nerve growth factor-NGF) signaling systems in PC12 pheochromocytoma cells that resulted in a translocation of Galpha(s), Galpha(i1), and Galpha(o) from cell bodies to cellular processes where they appear to localize with tubulin-containing structures. This relocation appeared to depend on the integrity of microtubules, as it was blocked and reversed by nocodazole. Latrunculin, which promotes actin filament depolymerization, had no effect. Both deconvolution microscopy and immunoprecipitation showed a significant increase of Galpha association with microtubules that was coincident with the extension of "neurites." There were distinctions among the Galpha subtypes, with Galpha(s) showing the most profound NGF-induced colocalization with tubulin. Translocation of Galpha was blocked by agents that inhibit the MAP kinases required for neuronal differentiation, suggesting that G-protein relocation is triggered by the intracellular signals for differentiation. Consistent with this, Galpha in Neuro-2A cells, which spontaneously differentiate, showed a similar translocation coincident with differentiation. Thus, diverse signals that promote neuronal differentiation and changes in cell morphology may use specific G-proteins to evoke cytoskeletal rearrangement.

A specific domain of Gialpha required for the transactivation of Gialpha by tubulin is implicated in the organization of cellular microtubules

G(s)alpha, G(i)alpha(1), and G(q)alpha subunits bind tubulin with high affinity, whereas transducin (G(t)alpha) does not. The interaction between tubulin and Galpha, which also involves the direct transfer of GTP from tubulin to Galpha (transactivation), is not yet fully understood. This study, using chimeras of G(i)alpha and G(t)alpha, showed that the G(i)alpha (215-295) segment converted G(t)alpha to bind to tubulin and this chimera (chimera 1) could be transactivated by tubulin. Insertion of G(t)alpha (237-270) into chimera 1 to form chimera 2 resulted in a protein that, like G(t)alpha, did not bind tubulin. Thus, it was thought that the G(i)alpha (237-270) domain was essential to modulate the binding of G(i)alpha(1) to tubulin. Surprisingly, when domain (237-270) of G(i)alpha was replaced by G(t)alpha (237-270) to form chimera 3, the chimera bound to tubulin with a similar affinity (K(D) congruent with 120 nm) as wild-type G(i)alpha(1). However, even though chimera 3 displayed normal GTP binding, it was not transactivated by GTP-tubulin. Furthermore, when these chimeras were expressed in COS-1 cells, cellular processes in cells overexpressing G(i)alpha(1) or chimera 1 were more abundant and longer than those in native cells. Galpha was seen throughout the length of the process. Morphology of cells expressing chimera 2 was identical to controls. Consistent with the role of Chimera 3 as a "dominant negative" Galpha, cells transfected with chimera 3 had only few truncated processes. This study demonstrates that although G(i)alpha (237-270) is not obligatory for the binding of G(i)alpha to tubulin, it is crucial for the transactivation of Galpha by tubulin. These results also suggest that the transactivation of Galpha by tubulin may play an important role in modulating microtubule organization and cell morphology.

Inducible nitric-oxide synthase attenuates vasopressin-dependent Ca2+ signaling in rat hepatocytes

Increases in both Ca(2+) and nitric oxide levels are vital for a variety of cellular processes; however, the interaction between these two crucial messengers is not fully understood. Here, we demonstrate that expression of inducible nitric-oxide synthase in hepatocytes, in response to inflammatory mediators, dramatically attenuates Ca(2+) signaling by the inositol 1,4,5-trisphosphate-forming hormone, vasopressin. The inhibitory effects of induction were reversed by nitric oxide inhibitors and mimicked by prolonged cyclic GMP elevation. Induction was without effect on Ca(2+) signals in response to AlF(4)(-) or inositol 1,4,5-trisphosphate, indicating that phospholipase C activation and release of Ca(2+) from inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores were not targets for nitric oxide inhibition. Vasopressin receptor levels, however, were dramatically reduced in induced cultures. Our data provide a possible mechanism for hepatocyte dysfunction during chronic inflammation.

Phosphatidylinositol 4,5-bisphosphate modifies tubulin participation in phospholipase Cbeta1 signaling

Tubulin forms the microtubule and regulates certain G-protein-mediated signaling pathways. Both functions rely on the GTP-binding properties of tubulin. Signal transduction through Galpha(q)-regulated phospholipase Cbeta1 (PLCbeta1) is activated by tubulin through a direct transfer of GTP from tubulin to Galpha(q). However, at high tubulin concentrations, inhibition of PLCbeta1 is observed. This report demonstrates that tubulin inhibits PLCbeta1 by binding the PLCbeta1 substrate phosphatidylinositol 4,5-bisphosphate (PIP2). Tubulin binding of PIP2 was specific, because PIP2 but not phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-phosphate, phosphatidylinositol, phosphatidylcholine, phosphatidylethanolamine, or inositol 1,4,5-trisphosphate inhibited microtubule assembly. PIP2 did not affect GTP binding or GTP hydrolysis by tubulin. Muscarinic agonists promoted microtubule depolymerization and translocation of tubulin to the plasma membrane. PIP2 augmented this process in both Sf9 cells, containing a recombinant PLCbeta1 pathway, and SK-N-SH neuroblastoma cells. Colocalization of tubulin and PIP2 at the plasma membrane was demonstrated with confocal laser immunofluorescence microscopy. Although tubulin bound to both Galpha(q) and PLCbeta1, PIP2 facilitated the interaction between tubulin and PLCbeta1 but not that between tubulin and Galpha(q). However, PIP2 did augment formation of tubulin--Galpha(q)-PLCbeta1 complexes. Subsequent to potentiating PLCbeta1 activation, sustained agonist-independent membrane binding of tubulin at PIP2- and PLCbeta1-rich sites appeared to inhibit Galpha(q) coupling to PLCbeta1. Furthermore, colchicine increased membrane-associated tubulin and also inhibited PLCbeta1 activity in SK-N-SH cells. Thus, tubulin, depending on local membrane concentration, may serve as a positive or negative regulator of phosphoinositide hydrolysis. Rapid changes in membrane lipid composition or in the cytoskeleton might modify neuronal signaling through such a mechanism.

Evidence for a direct negative coupling between dopamine-D2 receptors and PLC by heterotrimeric Gi1/2 proteins in rat anterior pituitary cell membranes

Dopamine (DA) is known to inhibit basal and hormone TRH- or angiotensin II (AngII)-stimulated PRL secretion and inositol phosphate accumulation in rat pituitary cells in primary culture. This inhibition persists when cells are incubated in a calcium-free medium (a condition in which DA could not inhibit PLC activities by blocking calcium influx) and is abolished by a Pertussis toxin treatment. These data suggest that DA receptor could be negatively coupled to PLC by a direct mechanism involving a Pertussis toxin-sensitive G protein. To demonstrate this hypothesis, we measured PLC activities on crude plasma membranes obtained from rat pituitary cells in primary culture grown in the presence of tritiated myo-inositol. We showed that 1) DA and quinpirole or RU24926 (specific D2 agonists) inhibited both basal and TRH- or AngII-stimulated membrane PLC activities. 2) Such inhibitions were completely prevented by sulpiride (specific D2 antagonist). 3) Heterotrimeric Gi1/2 proteins coupled the DA receptors to PLC because DA inhibitions were completely reversed by preincubation either with Pertussis toxin or with a specific G(alpha)i1/(alpha)i2 antibody. Such data are in favor of the existence of a direct negative coupling between DA-D2 receptor and PLC on a native physiological plasma membrane model.

Phosphatidylinositol 4,5-bisphosphate modifies tubulin participation in phospholipase Cbeta1 signaling

Tubulin forms the microtubule and regulates certain G-protein-mediated signaling pathways. Both functions rely on the GTP-binding properties of tubulin. Signal transduction through Galpha(q)-regulated phospholipase Cbeta1 (PLCbeta1) is activated by tubulin through a direct transfer of GTP from tubulin to Galpha(q). However, at high tubulin concentrations, inhibition of PLCbeta1 is observed. This report demonstrates that tubulin inhibits PLCbeta1 by binding the PLCbeta1 substrate phosphatidylinositol 4,5-bisphosphate (PIP2). Tubulin binding of PIP2 was specific, because PIP2 but not phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-phosphate, phosphatidylinositol, phosphatidylcholine, phosphatidylethanolamine, or inositol 1,4,5-trisphosphate inhibited microtubule assembly. PIP2 did not affect GTP binding or GTP hydrolysis by tubulin. Muscarinic agonists promoted microtubule depolymerization and translocation of tubulin to the plasma membrane. PIP2 augmented this process in both Sf9 cells, containing a recombinant PLCbeta1 pathway, and SK-N-SH neuroblastoma cells. Colocalization of tubulin and PIP2 at the plasma membrane was demonstrated with confocal laser immunofluorescence microscopy. Although tubulin bound to both Galpha(q) and PLCbeta1, PIP2 facilitated the interaction between tubulin and PLCbeta1 but not that between tubulin and Galpha(q). However, PIP2 did augment formation of tubulin--Galpha(q)-PLCbeta1 complexes. Subsequent to potentiating PLCbeta1 activation, sustained agonist-independent membrane binding of tubulin at PIP2- and PLCbeta1-rich sites appeared to inhibit Galpha(q) coupling to PLCbeta1. Furthermore, colchicine increased membrane-associated tubulin and also inhibited PLCbeta1 activity in SK-N-SH cells. Thus, tubulin, depending on local membrane concentration, may serve as a positive or negative regulator of phosphoinositide hydrolysis. Rapid changes in membrane lipid composition or in the cytoskeleton might modify neuronal signaling through such a mechanism.

Real-time visualization of a fluorescent G(alpha)(s): dissociation of the activated G protein from plasma membrane

To study behavior of activated G(alpha)(s) in living cells, green fluorescent protein (GFP) was inserted within the internal amino acid sequence of G(alpha)(s) to generate a G(alpha)(s)-GFP fusion protein. The fusion protein maintained a bright green fluorescence and was identified by immunoblotting with antibodies against G(alpha)(s) or GFP. The cellular distribution of G(alpha)(s)-GFP was similar to that of endogenous G(alpha)(s). G(alpha)(s)-GFP was tightly coupled to the beta adrenergic receptor to activate the G(alpha)(s) effector, adenylyl cyclase. Activation of G(alpha)(s)-GFP by cholera toxin caused a gradual displacement of the fusion protein from the plasma membrane throughout the cytoplasm in living cells. Unlike the slow release of G(alpha)(s)-GFP from the membrane induced by cholera toxin, the beta-adrenergic agonist isoproterenol caused a rapid partial release of the fusion protein into the cytoplasm. At 1 min after treatment with isoproterenol, the extent of G(alpha)(s)-GFP release from plasma membrane sites was maximal; however, insertion of G(alpha)(s)-GFP at other membrane sites occurred during the same time period. Translocation of G(alpha)(s)-GFP fusion protein induced by isoproterenol suggested that the internalization of G(alpha)(s) might play a role in signal transduction by interacting with effector molecules and cytoskeletal elements at multiple cellular sites.

HIV type 1 Gag and nucleocapsid proteins: cytoskeletal localization and effects on cell motility

Cell motility is likely to play a pivotal role in HIV infection by promoting the dissemination of infected cells. On the basis of observations indicating an interaction between HIV-1 Gag and target cell filamentous actin, we hypothesized that these interactions would promote cell motility of HIV-infected cells. Indeed, we have found that HIV-1 infection enhances the chemotactic response of macrophages. To specifically investigate the significance of the interactions between Gag and cellular actin, we transfected NIH 3T3 fibroblasts and HeLa cells with a construct that permits the expression of HIV-1 Gag in the absence of any other viral protein. Fractionation experiments showed that Gag was present in cytoskeletal fraction containing long actin filaments and in a high-speed postcytoskeletal fraction with short actin filaments. We have also localized HIV-1 Gag to the lamellipodia of chemoattractant-stimulated cells. Significantly, the motility of Gag-expressing cells was enhanced in chemotaxis assays. In vitro mutagenesis experiments showed that HIV-1 Gag binds filamentous actin through the nucleocapsid domain (NC). An NC-green fluorescent protein fusion had the same cellular distribution as the complete protein, and its expression increased cell motility. These data suggest that interactions between HIV-1 Gag and actin in infected cells enhance cell motility. Ultimately this enhanced motility of infected cells could promote the dissemination of virus into the brain and other tissues.

Disruption of cytoskeletal integrity impairs Gi-mediated signaling due to displacement of Gi proteins

beta1 integrins play a crucial role as cytoskeletal anchorage proteins. In this study, the coupling of the cytoskeleton and intracellular signaling pathways was investigated in beta1 integrin deficient (-/-) embryonic stem cells. Muscarinic inhibition of the L-type Ca2+ current (ICa) and activation of the acetylcholine-activated K+ current (IK,ACh) was found to be absent in beta1 integrin-/- cardiomyocytes. Conversely, beta adrenoceptor-mediated modulation of ICa was unaffected by the absence of beta1 integrins. This defect in muscarinic signaling was due to defective G protein coupling. This was supported by deconvolution microscopy, which demonstrated that Gi exhibited an atypical subcellular distribution in the beta1 integrin-/- cardiomyocytes. A critical role of the cytoskeleton was further demonstrated using cytochalasin D, which displaced Gi and impaired muscarinic signaling. We conclude that cytoskeletal integrity is required for correct localization and function of Gi-associated signaling microdomains.

ATP induces intracellular calcium increases and actin cytoskeleton disaggregation via P2x receptors

The consequences of purinoceptor activation on calcium signalling, inositol phosphate metabolism, protein secretion and the actin cytoskeleton were demonstrated in the WRK-1 cell line. Extracellular ATP was used as a secretagogue to induce a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)), acting via P2x purinergic receptors, which causes actin skeleton disaggregation and protein secretion. ATP bound specifically to purinergic receptors, with Ki of 0.8 microM. The magnitude order for binding of different nucleotides was alpha beta-Met-ATP >or= dATPalphaS > ATP >or= ADP > UTP > AMP > suramin. No increase in inositol phosphates (IPs) was observed after ATP application suggesting that the purinergic sites in WRK-1 cells are not of a P2y type. ATP (1-100 microM) caused a concentration-dependent increase in [Ca(2+)](i)(EC(50)= 30 microM). The responses were reproducible without any desensitization over several applications. The response to ATP was abolished when extracellular calcium ([Ca(2+)](e)) was reduced to 100 nM. A non-specific purinergic antagonist, suramin, reversibly inhibited the ATP-response suggesting that ATP is able to bind to P2x purinergic sites to trigger Ca(2+) entry and increase of [Ca(2+)](i). ATP induced a concentration-dependent disaggregation of actin and exocytotic release of proteins both, which were dependent upon [Ca(2+)](e). Similarly, alpha,beta-Met-ATP, a potent P2x agonist also stimulated Ca(2+) mobilization, actin network destructuration, and protein release. In the isolated rat neurohypophysial nerve terminals, ATP was shown to act as a physiological stimulus for vasopressin release via Ca(2+) entry through a P2x receptor [6]. Here, we show that in these nerve terminals, ATP is also able to induce actin disaggregation by a Ca(2+) dependent mechanism. Thus, actin cytoskeleton alterations induced by ATP through activation of P2x receptors could be a prelude to exocytosis.

Visualization of a functional Galpha q-green fluorescent protein fusion in living cells. Association with the plasma membrane is disrupted by mutational activation and by elimination of palmitoylation sites, but not be activation mediated by receptors or AlF4-

To investigate how G protein alpha subunit localization is regulated under basal and activated conditions, we inserted green fluorescent protein (GFP) into an internal loop of Galpha(q). alpha(q)-GFP stimulates phospholipase C in response to activated receptors and inhibits betagamma-dependent activation of basal G protein-gated inwardly rectifying K(+) currents as effectively as alpha(q) does. Association of alpha(q)-GFP with the plasma membrane is reduced by mutational activation and eliminated by mutation of the alpha(q) palmitoylation sites, suggesting that alpha(q) must be in the inactive, palmitoylated state to be targeted to this location. We tested the effects of activation by receptors and by AlF(4)(-) on the localization of alpha(q)-GFP in cells expressing both alpha(q)-GFP and a protein kinase Cgamma-red fluorescent protein fusion that translocates to the plasma membrane in response to activation of G(q). In cells that clearly exhibit protein kinase Cgamma-red fluorescent protein translocation responses, relocalization of alpha(q)-GFP is not observed. Thus, under conditions associated with palmitate turnover and betagamma dissociation, alpha(q)-GFP remains associated with the plasma membrane. These results suggest that upon reaching the plasma membrane alpha(q) receives an anchoring signal in addition to palmitoylation and association with betagamma, or that during activation, one or both of these factors continues to retain alpha(q) in this location.

Hormone-stimulated calcium release is inhibited by cytoskeleton-disrupting toxins in AR4-2J cells

We have studied the role of the actin cytoskeleton in bombesin-induced inositol 1,4,5-trisphosphate (IP(3))-production and Ca(2+)release in the pancreatic acinar tumour cell line AR4-2J. Intracellular and extracellular free Ca(2+)concentrations were measured in cell suspensions, using Fura-2. Disruption of the actin cytoskeleton by pretreatment of the cells with latrunculin B (10 microM), cytochalasin D (10 microM) or toxin B from Clostridium difficile (20 ng/ml) for 5-29 h led to inhibition of both, bombesin-stimulated IP(3)-production and Ca(2+)release. The toxins had no effect on binding of bombesin to its receptor, on Ca(2+)uptake into intracellular stores and on resting Ca(2+)levels. Ca(2+)mobilization from intracellular stores, induced by thapsigargin (100 nM) or IP(3)(1 microM) was not impaired by latrunculin B. In latrunculin B-pretreated cells inhibition of both, bombesin-stimulated IP(3)- production and Ca(2+)release was partly suspended in the presence of aluminum fluoride, an activator of G-proteins. Aluminum fluoride had no effect on basal IP(3)and Ca(2+)levels of control and toxin-pretreated cells. We conclude that disruption of the actin cytoskeleton impairs coupling of the bombesin receptor to its G-protein, resulting in inhibition of phospholipase C-activity with subsequent decreases in IP(3)-production and Ca(2+)release.

Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

Heterotrimeric GTP-binding proteins (G-proteins) are involved in a diverse array of signalling pathways. They are generally thought to be membrane-bound proteins, which disassociate on receptor activation and binding of GTP. A model to explain this has been proposed, which is often described as 'the G-protein cycle'. The 'G-protein cycle' is discussed in the present paper in relation to evidence that now exists regarding the non- membranous localization of G-proteins. Specifically, the experimental evidence demonstrating association of G-proteins with the cytoskeleton and the nucleus, and the mechanisms by which G-proteins translocate to these sites are reviewed. Furthermore, the possible effector pathways and the physiological function of G-proteins at these sites are discussed.

Mutation of Asn-391 within the conserved NPXXY motif of the cholecystokinin B receptor abolishes Gq protein activation without affecting its association with the receptor

Among the most conserved regions in the G-protein-coupled receptors is the (N/D)PX(2-3)Y motif of the seventh transmembrane domain (X represents any amino acid). The mutation of the Asn/Asp residue of this motif in different G-protein-coupled receptors was shown to affect the activation of either adenylyl cyclase or phospholipase C. We have mutated the Asn residue (Asn-391) of the NPXXY motif in the CCKBR to Ala and determined the effects of the mutation on binding, signaling, and G-proteins coupling after expression of the mutated receptor in COS cells. The mutated receptor displayed similar expression levels and high affinity CCK binding compared with the wild type CCKBR. However, unlike the wild type CCKBR, the mutated receptor was completely unable to mediate activation of either phospholipase C and protein kinase C-dependent and -independent mitogen-activated protein kinase pathways, indicating an essential role of Asn-391 in CCKBR signaling. Coimmunoprecipitation experiments allowed us to show that the inactive mutant retains an intact capacity to form stable complexes with G(q)alpha subunits in response to CCK. These results indicate that the formation of high affinity CCK-receptor-G(q) protein complexes is not sufficient to activate G(q) and suggest that Asn-391 is specifically involved in G(q) proteins activation.

The effects of agonist stimulation and beta(2)-adrenergic receptor level on cellular distribution of gs(alpha) protein

This study examines the effects of adrenergic ligands, cholera toxin, forskolin, and varying levels of beta(2) adrenergic receptors (beta(2)AR) on the cellular distribution of Gs(alpha) subunits in CHO cells. Localization of Gs(alpha) was evaluated by confocal microscopy and beta(2)AR-mediated signalling was assessed by adenylyl cyclase (AC) activity. In cells expressing 0.2 pmol/mg protein beta(2)ARs (WT18), the localization of Gs(alpha) subunit was restricted to the plasma membrane region. Isoproterenol (ISO), cholera toxin or forskolin elicited redistribution of cellular Gs(alpha) so that Gs(alpha) appeared as intense spots throughout the plasma membrane as well as the cytoplasm. Exposure to a neutral beta(2)AR antagonist, alprenolol, prevented the ISO-stimulated Gs(alpha) translocation from peripheral to inner cytoplasm. In cells expressing high level of beta(2)ARs (8.2 pmol/mg) (WT4), basal and ISO-stimulated AC activities were significantly elevated when compared to the values detected in WT18 clone, suggesting a positive correlation between receptor expression and receptor-mediated signalling. Basal Gs(alpha) distribution in this group was similar to that observed in ISO-, cholera toxin-, or forskolin-stimulated WT18 clone. ISO, cholera toxin, or forskolin did not change the distribution of Gs(alpha) significantly when tested in WT4 clone. No difference in the cellular level of Gs(alpha) protein between WT18 and WT4 clones was detected. Alprenolol did not affect the distribution of Gs(alpha) in WT4 clone. ICI 118,551, a negative beta(2)AR antagonist, altered Gs(alpha) distribution from a dispersed basal pattern to a membrane-confined pattern. The latter appearance was similar to that observed in unstimulated WT18 clone. Taken together, these data suggest that: (1) enhanced beta(2)AR-Gs(alpha) coupling induced by agonist stimulation or by increased expression of beta(2)ARs remodel the cellular distribution of Gs(alpha); (2) the alteration in Gs(alpha) distribution induced by beta(2)AR overexpression provides evidence for agonist-independent interaction of beta(2)AR and Gs(alpha), that can be inhibited by a negative antagonist but not by a neutral antagonist; and (3) forskolin influences the activity state of Gs(alpha) that displays a Gs(alpha) distribution pattern comparable to that observed when Gs(alpha) is activated via beta(2)AR stimulation or directly by cholera toxin.

Dual signaling of human Mel1a melatonin receptors via G(i2), G(i3), and G(q/11) proteins

Mel 1a melatonin receptors belong to the super-family of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. So far, interest in Mel 1a receptor signaling has focused mainly on the modulation of the adenylyl cyclase pathway via pertussis toxin (PTX)-sensitive G proteins. To further investigate signaling of the human Mel 1a receptor, we have developed an antibody directed against the C terminus of this receptor. This antibody detected the Mel 1a receptor as a protein with an apparent molecular mass of approximately 60 kDa in immunoblots after separation by SDS-PAGE. It also specifically precipitated the 2-[125I]iodomelatonin (125I-Mel)-labeled receptor from Mel 1a-transfected HEK 293 cells. Coprecipitation experiments showed that G(i2), G(i3), and G(q/11) proteins couple to the Mel 1a receptor in an agonist-dependent and guanine nucleotide-sensitive manner. Coupling was selective since other G proteins present in HEK 293 cells, (G(i1), G(o), G(s), G(z), and G12) were not detected in receptor complexes. Coupling of the Mel 1a receptor to G(i) and G(q) was confirmed by inhibition of high-affinity 125I-Mel binding to receptors with subtype-selective G protein alpha-subunit antibodies. G(i2) and/or G(i3) mediated adenylyl cyclase inhibition while G(q/11) induced a transient elevation in cytosolic calcium concentrations in HEK 293 cells stably expressing Mel 1a receptors. Melatonin-induced cytosolic calcium mobilization via PTX-insensitive G proteins was confirmed in primary cultures of ovine pars tuberalis cells endogenously expressing Mel 1a receptors. In conclusion, we report the development of the first antibody recognizing the cloned human Mel 1a melatonin receptor protein. We show that Mel 1a receptors functionally couple to both PTX-sensitive and PTX-insensitive G proteins. The previously unknown signaling of Mel 1a receptors through G(q/11) widens the spectrum of potential targets for melatonin.

Visualization of distinct patterns of subcellular redistribution of the thyrotropin-releasing hormone receptor-1 and gqalpha /G11alpha induced by agonist stimulation

The rat thyrotropin-releasing hormone receptor-1 (TRHR-1) was modified by the addition of green fluorescent protein (GFP) and expressed stably in HEK293 cells. Extensive overlap of plasma membrane distribution of autofluorescent TRHR-1-GFP with that of the phosphoinositidase C-linked G-proteins Gqalpha/G11alpha, identified by indirect immunofluorescence, was monitored concurrently. Addition of thyrotropin-releasing hormone resulted in rapid separation of TRHR-1-GFP and Gqalpha/G11alpha signals as the receptor was internalized. This situation persisted for more than an hour. At longer time periods a fraction of the cellular Gqalpha/G11alpha was also internalized, although much of the Gqalpha/G11alpha immunoreactivity remained associated with the plasma membrane. Parallel experiments, in which the cellular distribution of TRHR-1-GFP and Gqalpha/G11alpha immunoreactivity were monitored in sucrose-gradient fractions following cell disruption, also demonstrated a rapid, agonist-induced movement of TRHR-1-GFP away from the plasma membrane to low-density vesicular fractions. At later time points, a fraction of the cellular Gqalpha/G11alpha immunoreactivity was also redistributed to overlapping, but non-identical, low-density-vesicle-containing fractions. Pretreatment of the cells with cytochalasin D or nocodazole prevented agonist-induced redistribution of G-protein but not TRHR-1-GFP, further indicating resolution of the mechanics of these two processes. The combination of a GFP-modified receptor and immunostaining of the G-proteins activated by that receptor allows, for the first time, concurrent analysis of the varying dynamics and bases of internalization and redistribution of two elements of the same signal-transduction cascade.

Heterotrimetric Gi/o proteins control cyclic AMP oscillations and cytoskeletal structure assembly in primary human granulosa-lutein cells

In granulosa cells, the luteinising hormone (LH) and the follicle-stimulating hormone (FSH) receptors are coupled to the adenylyl cyclase-cAMP pathway. We identified at least eight different G proteins belonging to three families--Gs, Gq, and Gi/o--in primary human granulosa-lutein cells. By exploring the function of Gi/o by time-lapse and digital-imaging microscopy of live cells, we found that the reversible actin stress fibre-dependent cytoplasmic retraction of pre-luteinised cells in primary culture is a highly sensitive and quite rapid system allowing detection of an intracellular cAMP surge. This morphology was characterised by maintenance of connexin43-dependent cell-cell contacts and that of microtubule-directed cell processes attached to the substrate and to neighbouring cells. Inhibitors of cyclic nucleotide phosphodiesterase subfamily type 4 (PDE-4), hLH and hFSH provoked this reversible cAMP-dependent phenotype in a temporal-, spatial- and dose-dependent manner. Gi/o inhibited adenylyl cyclase in membranes, and cell treatment with islet-activating protein (IAP) caused the cAMP-dependent retracted phenotype. It is concluded that the basal intracellular cAMP level is kept within a narrow range of concentrations, below the threshold for disassembly of stress fibres, through Gs, Gi/o, adenylyl cyclases and phosphodiesterase-4. This work supports the paradigm that switching of the agonist-occupied receptors to Gs and Gi/o would control both the intracellular bursts of cAMP (through the gonadotropin-catalysed activation of Gs) and the basal cAMP (through a Gi/o-mediated braking effect).

Association of heterotrimeric G-proteins with bovine aortic phospholipase C gamma

The widely expressed phospholipase C gamma1 (PLCgamma1) isoform has been implicated in the signalling of cell growth through its ability to hydrolyse phosphatidylinositol 4,5-bisphosphate to give inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. Stimulation of PLCgamma1 activity occurs upon phosphorylation of specific tyrosine residues, although it is unclear how this phosphorylation actually stimulates catalytic activity. Indeed recent reports suggest that accessory factors such as GTP-binding proteins may also be required for complete activation of PLCgamma1 in some cells. This may be of importance in vascular smooth muscle where traditionally G-protein linked PLCbeta isoforms are often absent. Here, we show that bovine aortic PLCgamma1 activity is substantially enhanced by both GTPgammaS and sodium fluoride. Similarly, immunoprecipitated PLCgamma1 is associated with an approximately 40kDa GTPgammaS-binding protein and both Galphai and Galphaq were detected in this immunoprecipitate. This data suggests that bovine aortic PLCgamma1 is both associated with, and may be activated by, heterotrimeric G-proteins.

Distribution of signaling molecules involved in vasopressin-induced Ca2+ mobilization in rat hepatocyte multiplets

In freshly isolated rat hepatocyte multiplets, Ca2+ signals in response to vasopressin are highly organized. In this study we used specific probes to visualize, by fluorescence and confocal microscopy, the main signaling molecules involved in vasopressin-mediated Ca2+ responses. V1a receptors were detected with a novel fluorescent antagonist, Rhm8-PVA. The Galphaq/Galpha11, PLCbeta3, PIP2, and InsP3 receptors were detected with specific antibodies. V1a vasopressin receptors and PIP2 were associated with the basolateral membrane and were not detected in the bile canalicular domain. Galphaq/Galpha11, PLCbeta3, and InsP3 receptors were associated with the basolateral membrane and also with other intracellular structures. We used double labeling, Western blotting, and drugs (cytochalasin D, colchicine) known to disorganize the cytoskeleton to demonstrate the partial co-localization of Galphaq/Galpha11 with F-actin.

Phosphorylation of F-actin-associating G protein gamma12 subunit enhances fibroblast motility

Eleven isoforms of G protein gamma subunit have been found thus far, but the precise roles of individual gamma subunits are not known. The gamma12 subunit has two unique properties: phosphorylation by protein kinase C and association with F-actin. To elucidate the role of gamma12, we overexpressed gamma12 and other gamma subunits in NIH 3T3 cells together with the beta1 subunit. The overexpressed gamma12 as well as endogenous gamma12, but not gamma2, gamma5, and gamma7 subunits, associated with cytoskeletal components. Expression of gamma12 induced remarkable changes including cell rounding, disruption of stress fibers, and enhancement of cell migration, but expression of other gamma subunits did not induce significant changes. Deletion of the N-terminal region of gamma12 decreased the abilities of gamma12 to associate with cytoskeletal fractions, to induce cell rounding, and to increase cell motility. Replacement by alanine of Ser2 of gamma12 (Ser1 of a mature gamma12 protein), a phosphorylation site for protein kinase C, eliminated these effects of gamma12, whereas a mutant in which Ser2 was replaced with glutamic acid showed effects equivalent to wild-type gamma12. These results indicate that phosphorylation of gamma12 at Ser2 enhances the motility of cells.

Agonist-induced internalization of the G protein G11alpha and thyrotropin-releasing hormone receptors proceed on different time scales

Using a combination of confocal immunofluorescence microscopy and subcellular fractionation, we demonstrate for the first time active internalization, trafficking, and down-regulation of a G protein alpha subunit subsequent to agonist occupation of a receptor. This proceeds on a much slower time scale than internalization of the corresponding receptor. In intact E2M11 HEK293 cells that express high levels of murine G11alpha and the rat thyrotropin-releasing hormone (TRH) receptor, the immunofluorescence signal of G11alpha was restricted almost exclusively to the plasma membrane. Exposure to TRH (10 microM) resulted first in partial relocation of G11alpha to discrete, segregated patches within the plasma membrane (10-60 min). Further exposure to TRH caused internalization of G11alpha to discrete, punctate, intracellular bodies (2-4 h) and subsequently to a virtually complete loss of G11alpha from plasma membranes and the cells (8-16 h). Short-term treatment with TRH followed by wash-out of the ligand allowed G11alpha immunofluorescence to be restored to the plasma membrane within 12 h. In subcellular membrane fractions, G11alpha was centered on plasma membranes, and this was not altered by up to 1-2 h of incubation with TRH. Further exposure to TRH (2-4 h) resulted in transfer of a significant portion of G11alpha to light-vesicular and cytosol fractions. At longer time intervals (4-16 h), an overall decrease in G11alpha content was observed.

Receptor-oriented intercellular calcium waves evoked by vasopressin in rat hepatocytes

Agonist-induced intracellular calcium signals may propagate as intercellular Ca2+ waves in multicellular systems as well as in intact organs. The mechanisms initiating intercellular Ca2+ waves in one cell and determining their direction are unknown. We investigated these mechanisms directly on fura2-loaded multicellular systems of rat hepatocytes and on cell populations issued from peripheral (periportal) and central (perivenous) parts of the hepatic lobule. There was a gradient in vasopressin sensitivity along connected cells as demonstrated by low vasopressin concentration challenge. Interestingly, the intercellular sensitivity gradient was abolished either when D-myo-inositol 1,4, 5-trisphosphate (InsP3) receptor was directly stimulated after flash photolysis of caged InsP3 or when G proteins were directly stimulated with AlF4-. The gradient in vasopressin sensitivity in multiplets was correlated with a heterogeneity of vasopressin sensitivity in the hepatic lobule. There were more vasopressin-binding sites, vasopressin-induced InsP3 production and V1a vasopressin receptor mRNAs in perivenous than in periportal cells. Therefore, we propose that hormone receptor density determines the cellular sensitivity gradient from the peripheral to the central zones of the liver cell plate, thus the starting cell and the direction of intercellular Ca2+ waves, leading to directional activation of Ca2+-dependent processes.

Involvement of tyrosine phosphorylation and MAPK activation in the mechanism of action of ACTH, angiotensin II and vasopressin

ACTH, Angiotensin II (Ang II) and Vasopressin (AVP) are among the well known regulators of aldosterone secretion and also have a trophic action on the adrenal gland. According to classic studies, Ang II and AVP activate phospholipase C (PLC), diacylglycerol (DAG) and inositol phosphate (InsPs) production whereas ACTH activates cAMP production. However, our data indicate that the three peptides are able to induce a time-dependent increase in the level of Tyr-phosphorylation of several proteins. Western Blot analysis indicates a biphasic activation of Tyr-phosphorylation by AVP, with a peak at 30 s and a second one at 15 min incubation. Ang II induced a rapid (2 min) and sustained activation of Tyr-phosphorylation, while ACTH induced a progressive time course with a plateau reached at 15 min. Ang II and AVP also increased phosphorylation of p42mapk and p44mapk, while ACTH did not affect MAPK activity. Moreover, pre-incubation of the cells with genistein (Tyr-kinase inhibitor) and PD 098059 (a MAPK inhibitor) did not affect InsPs production or aldosterone secretion induced by Ang II or AVP. These results suggest that the MAPK pathway is involved in the control of cell growth rather than aldosterone secretion.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

Phospholipase C-beta1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex

Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-beta1 (PLC-beta1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-beta1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-beta1 expression in the developing rat cerebral cortex. PLC-beta1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4-7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-beta1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-beta1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.

Functional rescue of a constitutively desensitized beta2AR through receptor dimerization

We have recently demonstrated that wild-type beta2-adrenergic receptors (beta2AR) form homodimers and that disruption of receptor dimerization inhibits signalling via Gs [Hebert, Moffett, Morello, Loisel, Bichet, Barret and Bouvier (1996) J. Biol. Chem. 271, 16384-16392]. Here taking advantage of the altered functional properties of a non-palmitoylated, constitutively desensitized mutant beta2AR (C341Gbeta2AR), we sought to study whether physical interactions between mutant and wild-type beta2AR expressed in Sf9 cells could occur and have functional consequences. Using metabolic labelling with [3H]palmitate and co-immunoprecipitation we demonstrated the existence of heterodimerization between wild-type and C341Gbeta2AR. Furthermore, we show that, in co-expression experiments, wild-type receptors have a dominant positive effect resulting in the functional complementation of C341Gbeta2AR. Indeed, when expressed alone, the mutant C341G receptor displays altered functional characteristics in that (1) the response of the receptor to agonist is reduced as compared to the wild-type receptor and (2) the desensitization of the receptor in response to prolonged exposure to agonist is minimal. In contrast, when C341G and the wild-type beta2AR were expressed together, both the response to agonist and subsequent desensitization (at a constant level of total receptor) were equivalent to the wild-type beta2AR expressed alone. This dominant positive effect was also seen when C341G was co-expressed with a second receptor mutant in which the two protein kinase A phosphorylation sites (S261, 262, 345, 346A beta2AR) were mutated. Taken together these data suggest that intermolecular interactions between receptors may have both functional and structural implications for G-protein-mediated signalling.

Measurement of agonist efficacy using an alpha2A-adrenoceptor-Gi1alpha fusion protein

A fusion protein was constructed between the porcine alpha2A-adrenoceptor and a pertussis toxin-insensitive (Cys351Gly) form of the alpha subunit of the G protein Gi1. Addition of agonist ligands to membranes of COS-7 cells transiently transfected to express this construct, and treated with pertussis toxin prior to cell harvest, resulted in stimulation of both high affinity GTPase activity and enhanced binding of [35S]GTPgammaS. By considering the fusion protein as an agonist-activated enzyme and measuring Vmax of GTP hydrolysis a range of agonist ligands displayed varying efficacy in their capacity to activate the receptor-associated G protein with adrenaline = noradrenaline = alpha-methylnoradrenaline > UK14304 > BHT933 > or = xylazine = clonidine. A similar rank order was observed following independent co-expression of the alpha2A-adrenoceptor and Cys351Gly-Gi1alpha. These data demonstrate the utility and applicability of using a receptor-G protein fusion protein approach, in which the stoichiometry of receptor and G protein is fixed at 1:1, to measure and further understand the nature of agonist efficacy.

Analysis of the N-terminal binding domain of Go alpha

Signalling from membrane receptors through heterotrimeric G-proteins (G alpha and G beta gamma) to intracellular effectors is a highly regulated process. Receptor activation causes exchange of GTP for GDP on G alpha and dissociation of G alpha from G beta gamma. Both subunits remain membrane-associated and interact with a series of other molecules throughout the cycle of activation. The N-terminal binding domain of G alpha subunits interacts with the membrane by several partially defined mechanisms: the anchoring of G alpha to the more hydrophobic G beta gamma subunits, the interaction of N-terminal lipids (palmitate and/or myristate) with the membrane, and attachment of amino acid regions to the membrane {amino acids 11-14 of Go alpha (D[11-14]); Busconi, Boutin and Denker (1997) Biochem. J. 323, 239-244}. We characterized N-terminal mutants of Go alpha with known G beta gamma-binding properties for the ability to interact with phospholipid vesicles and membranes prepared from cultured cells (acceptor membranes). In vitro analysis allows membrane interactions that are important to the activated and depalmitoylated state of G alpha to be characterized. Subcellular localization was also determined in transiently transfected COS cells. All of the mutant proteins are myristoylated, and differences in myristoylation do not account for changes in membrane binding. Disrupting the N-terminal alpha-helix of Go alpha with a proline point mutation at Arg-9 (R9P) does not affect interactions with G beta gamma on sucrose-density gradients but significantly reduces acceptor membrane binding. Deletion of amino acids 6-15 (D[6-15]; reduced G beta gamma binding) or deletion of amino acids 3-21 (D[3-21]); no detectable G beta gamma binding) further reduces acceptor membrane binding. When expressed in COS cells, R9P and D[6-15] are localized in the membrane similar to wild-type Go alpha as a result of the contribution from palmitoylation. In contrast, D[3-21] is completely soluble in COS cells, and no palmitoylation is detected. The binding of Go alpha and mutants translated in vitro to liposomes indicates that Go alpha preferentially binds to neutral phospholipids (phosphatidylcholine). R9P and D[11-14] bind to phosphatidylcholine liposomes like Go alpha, but D[6-15] exhibits no detectable binding. Taken together, these studies suggest that interactions of the N-terminus of G alpha subunits with the membrane may be affected by both membrane proteins and lipids. A detailed understanding of G alpha-membrane interactions may reveal unique mechanisms for regulating signal transduction.

Actin Filaments Modulate Both Stomatal Opening and Inward K+-Channel Activities in Guard Cells of Vicia faba L

Actin antagonists have previously been shown to alter responses of Commelina communis stomata to physiological stimuli, implicating actin filaments in the control of guard cell volume changes (M. Kim, P.K. Hepler, S.-O. Eun, K.S. Ha, Y. Lee [1995] Plant Physiol 109: 1077-1084). Since K+ channels in the guard cell play an important role in stomatal movements, we examined the possible regulation of K+-channel activities by the state of actin polymerization. Agents affecting actin polymerization altered light-induced stomatal opening and inward K+-channel activities measured by patch clamping in Vicia faba. Cytochalasin D, which induces depolymerization of actin filaments, promoted light-induced stomatal opening and potentiated the inward K+ current in guard cell protoplasts. Phalloidin, a stabilizer of filamentous actin, inhibited both light-induced stomatal opening and inward K+ current. Inward K+-channel activities in outside-out membrane patches showed responses to these agents that support results at the whole-cell current level, suggesting that cytochalasin D facilitates and phalloidin inhibits K+ influx in intact guard cells, thus resulting in enhancement and inhibition of stomatal opening, respectively. To our knowledge, this is the first report that provides evidence that actin filaments may regulate an important physiological process by modulating the activities of ion channels in plant cells.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

The heterotrimeric GTP-binding protein, GS, modulates the Cl- conductance of rat parotid acinar secretory granules

Gsalpha has been reported to be present in rat parotid acinar secretory granule membrane (SGM) fractions. In the present study, we evaluated epitope orientation of Gsalpha on the secretory granule (SG) and the ability of Gs to modulate the Cl- conductance of isolated granules by measuring granule lysis. Gsalpha was found to be associated with the cytoplasmic face of the SGM. Aluminum fluroide (AlF4-, 20 microM Al3+ and 10 mM F-) significantly increased granule lysis and this effect was blocked by GDPbetaS. Cholera toxin (5 microg/ml) mimicked the effects of AlF4- on granule lysis, whereas pertussis toxin (0.5 microg/ml) was without effect. GTPgammaS, however, reduced granule lysis in a concentration-dependent manner. The orientation of Gsalpha on the SGM as well as the effects of AlF4- and cholera toxin on granule lysis lends support for a role of Gs in the exocytotic process.

N-methyl-D-aspartate evokes rapid net depolymerization of filamentous actin in cultured rat cerebellar granule cells

Filamentous actin (F-actin) was measured in cultured rat cerebellum granule neurons with the use of fluorescently labeled phallotoxin as a site-specific probe for F-actin, and fluorescence microscopy. The averaged apparent intensity of soma-associated F-actin-derived fluorescence (F(app)) was measured from fixed cells after incubation in either 1) normal Krebs solution containing 2 mM extracellular calcium ([Ca2+]ex) or 2) normal Krebs solution plus N-methyl-D-aspartate (NMDA) for 2 min immediately before fixation. NMDA (10, 50, and 100 microM) decreased F(app) to 63 +/- 5% (mean +/- SE), 53 +/- 4%, and 47 +/- 2%, respectively, of that measured from control cells. This effect was mimicked by treatment of cells with ionomycin. The ability of NMDA to reduce the F(app) in the presence of [Ca2+]ex was abolished when cells were maintained in [Ca2+]ex-free medium. Cells first treated with NMDA for 2 min and then left in normal medium for 30 min before fixation gave F(app) fluorescence similar to control values (91 +/- 12%). However, if the F-actin polymerization inhibitor cytochalasin D was added to cells immediately after NMDA was removed, the F(app) did not recover with time (36 +/- 3%). Cells treated for 30 min with cytochalasin D alone showed a small reduction in staining (approximately 20%). It is concluded that the actin polymerization state of rat cerebellar granule neurons is sensitive to changes in intracellular calcium, and that NMDA receptor activation evokes an initial rapid depolymerization of F-actin.

Heterotrimeric G-protein Gq/11 localized on pancreatic zymogen granules is involved in calcium-regulated amylase secretion

The heterotrimeric G-protein Gq/11 was identified on pancreatic acinar zymogen granules and its function in calcium-regulated exocytosis was examined. Western blotting showed alphaq/11, but not alphas or alphao, to be localized to the zymogen granule membrane along with G-protein beta-subunit; all three alpha subunits were present in a plasma membrane fraction and the alphaq/11 signal was 30-fold more enriched in the plasma membrane as compared with granule membrane. Neither CCK receptors nor alpha subunits of the sodium pump, both plasma membrane markers were present on granule membranes. Immunohistochemistry of pancreatic lobules showed that alphaq/11 localized to the zymogen granule-rich apical region of acinar cells together with a much stronger signal at the basolateral plasma membrane. When the substance-P-related peptide GPAnt-2a, an antagonist of Gq/11, was introduced into streptolysin-O permeabilized acini to bypass the plasma membrane, the amylase release induced by 10 microM free calcium was potentiated in a concentration-dependent manner. By contrast, another substance-P-related peptide, GPAnt-1, an antagonist of Go and Gi, showed no effect on calcium-induced amylase release from permeabilized acini. GPAnt-2a peptide also exerted an inhibitory effect on the total GTPase activity of the purified zymogen granules and a larger inhibitory effect on the GTPase activity of the Gq/11 protein immunopurified from zymogen granules. GPAnt-1, however, did not inhibit GTPase activity of either zymogen granules or immunopurified Gq/11. These results suggest that GPAnt-2a peptide augmented calcium-induced amylase release from permeabilized acini by inhibiting GTPase activity of the Gq/11 protein on zymogen granules. We conclude that Gq/11 protein on zymogen granules plays a tonic inhibitory role in calcium-regulated amylase secretion from pancreatic acini.

Smooth muscle sarcolemma-associated phospholipase C-beta 2; agonist-evoked translocation

About 25% of the total cellular PLC beta 2 content was found to be associated with a sarcolemmal fraction (SARC) isolated from unstimulated porcine trachealis smooth muscle. SARC-associated PLC beta 2 was located within two compartments, a detergent-extractable compartment and a nondetergent extractable compartment. SARC PLC beta 2 was measured after extraction with 0.6 M KCI; therefore, PLC beta 2 was not bound solely by electrostatic forces within either of these compartments. PLC beta 2 was shown to translocate from cytosol to SARC during a 20-sec activation of intact muscle with a muscarinic agonist, carbachol (CARB); i.e., cytosolic total PLC beta 2 content decreased significantly to 73 +/- 7% of control and SARC total PLC beta 2 content increased to 180 +/- 15% of control value. This translocation was maintained at 5 min of CARB. CARB-evoked translocation occurred into the detergent-extractable SARC fraction, and PLC beta 2 content in this fraction increased 300% compared with that in unstimulated muscle. After CARB, SARC PLC beta 2 content accounted for > 50% of total cellular PLC beta 2 content. CARB-evoked increase in PLC activity in SARC paralleled the increase in PLC beta 2 content. CARB-induced translocations of PLC beta 2 from the cytosol to SARC were of a similar magnitude as occurred with phorbol ester-induced translocations of PKC alpha.

Identifying the G protein, Gz alpha, and its associated proteins in nervous tissue using mass spectrometry and microsequencing techniques

The signaling pathway associated with pertussis and cholera toxin sensitive G proteins have been extensively investigated. In contrast, the function and associated signal transduction cascade for the pertussis toxin insensitive G protein, Gz alpha have remained elusive. Therefore, the aim of this study was to identify the signal transduction pathway associated with Gz alpha by using the protein identification techniques of matrix assisted laser desorption ionization-time of flight mass spectroscopy and N-terminal Edman sequencing. We have chosen this technique to identify proteins that Gz alpha associates with and to gain insights into the potential role this G protein plays in cells. As Gz alpha is predominantly localized in neuronal tissues, homogenates of whole brain tissue were used. Gz alpha and its associated proteins were immunoprecipitated from brain tissue and identified. The immunoprecipitation of four proteins (140, 46, 41 and 36 kDa) was shown to be inhibited in the presence of the Gz alpha peptide. These proteins were subsequently identified as phospholipase C (PLC)-gamma, beta or gamma-actin, Gz alpha and G beta, the beta subunit of heterotrimeric G proteins, respectively. These results suggest that Gz alpha exists in a protein complex with the actin cytoskeleton and an important intracellular signalling enzyme, PLC-gamma. These methods are powerful techniques for determining protein-protein interactions, and provide the first step to the identification of signalling proteins that Gz alpha associates with. However further experimentation will be required to determine the biological relevance of these protein interactions.

N-terminal binding domain of Galpha subunits: involvement of amino acids 11-14 of Galphao in membrane attachment

Heterotrimeric guanine nucleotide binding proteins (G-proteins) transmit signals from membrane receptors to a variety of intracellular effectors. G-proteins reversibly associate with components of the signal transduction system, yet remain membrane attached throughout the cycle of activation. The Galpha subunits remain attached to the plasma membrane through a combination of factors that are only partially defined. We now demonstrate that amino acids within the N-terminal domain of Galpha subunits are involved in membrane binding. We used in vitro translation, a technique widely utilized to characterize functional aspects of G-proteins, and interactions with donor-acceptor membranes to demonstrate that amino acids 11-14 of Galphao contribute to membrane binding. The membrane binding of Galphao lacking amino acids 11-14 (D[11-14]) was significantly reduced at all membrane concentrations in comparison with wild-type Galphao. Several other N-terminal mutants of Galphao were characterized as controls, and these results indicate that differences in myristoylation, palmitoylation and betagamma interactions do not account for the reduced membrane binding of D[11-14]. Furthermore, when membrane attachment of Galphao and mutants was characterized in transiently transfected 35S-labelled and [3H]myristate-labelled COS cells, amino acids 11-14 contributed to membrane binding. These studies reveal that membrane binding of Galpha subunits occurs by a combination of factors that include lipids and amino acid sequences. These regions may provide novel sites for interaction with membrane components and allow additional modulation of signal transduction.

Phospholipase C-beta1 is present in the botrysome, an intermediate compartment-like organelle, and Is regulated by visual experience in cat visual cortex

Monoclonal antibody Cat-307 identifies a 165 kDa neuronal protein expressed in the cat visual cortex during the period of sensitivity to alterations in visual experience (). Dark-rearing, which prolongs the sensitive period, also prolongs the expression of the Cat-307 protein. The Cat-307 protein localizes to an organelle, here called the botrysome (from the Greek botrys, cluster of grapes), that is located between the endoplasmic reticulum (ER) and Golgi apparatus. The botrysome is composed of small ring-shaped profiles with electron-dense coats. The size and morphology of the rings and their coats are similar to those described for ER to Golgi transport vesicles. Biochemically, the Cat-307 protein cofractionates with microsomes and partitions with subunits of the coatomer proteins that coat ER-to-Golgi transport vesicles. Partial amino acid sequencing reveals that the Cat-307 protein is phospholipase C-beta1, the G-protein-dependent phosphodiesterase that hydrolyses phosphatidylinositol 4,5 biphosphate into inositol 1,4,5 triphosphate and diacylglycerol after the stimulation of a variety of neurotransmitter receptors at the cell surface. These results suggest a role for phospholipase C-beta1 and the botrysome in developmental plasticity and provide a possible link between receptor activation at the cell surface and protein transport during neuronal development.

Organization of the receptor-mediated phosphoinositide cycle: relationship between receptor occupancy and accession of phosphatidylinositol

We have previously reported the existence of separate hormone-responsive and -unresponsive pools of inositol phospholipids in WRK-1 cells. In order to further explore this concept, we have performed experiments to examine the relationship between the plasma membrane receptor and the pool of phosphatidylinositol (Ptdlns) that is metabolized in response to hormonal stimulation. The results support the following conclusions. 1) The amount of Ptdlns metabolized in WRK-1 cells in response to vasopressin is proportional to the number of receptors occupied; neither prolonged activation with nor readdition of submaximal concentration of vasopressin induced the same degree of Ptdlns metabolism as maximal concentration of vasopressin. 2) Dissociation of cytoskeletal structures by incubation with cytochalasin D did not alter the amount of Ptdlns accessed during hormonal stimulation. 3) Accession of Ptdlns from internal membranes does not depend on internalization and recycling of the receptor; cells incubated in potassium-free medium failed to internalize receptor-ligand complexes, yet they accessed the same amount of Ptdlns in response to vasopressin as did control cells. 4) Golgi-mediated phosphatidylinositol transport is not involved in hormone-stimulated phosphoinositide turnover, since brefeldin A, which interferes with Golgi-mediated transport processes, had no effect on the amount of Ptdlns accessed during vasopressin stimulation. 5) Phosphoinositide breakdown and compensatory resynthesis is not a closed process; newly synthesized Ptdlns is not preferentially localized to a hormone-responsive pool but is generally redistributed between responsive and unresponsive pools.