Different beta-subunits determine G-protein interaction with transmembrane receptors

Potentiation of Gi-mediated phospholipase C activation by retinoic acid in HL-60 cells. Possible role of G gamma 2

Differentiated HL-60 cells acquire responsiveness to fMet-Leu-Phe (fMLP), which activates phospholipase C and O2- generation in a pertussis toxin-sensitive manner. Addition of retinoic acid (RA) for the last 24 h during dimethyl sulfoxide (Me2SO)-induced differentiation enhanced fMLP-dependent signals and interaction between fMLP receptor and G(i). RA modifies both the function and subunit composition of G(i)2, the predominant G(i) of HL-60 membranes, as shown by comparing purified G(i)2 from membranes of Me2SO-treated cells (D-G(i)2) to G(i)2 from membranes of cells treated with both Me2SO and RA (DR-G(i)2). As compared to D-G(i)2, DR-G(i)2 induced more fMLP binding when added to membranes of pertussis toxin-treated HL-60 cells and, in the presence of GTP gamma S, stimulated beta gamma-sensitive phospholipase C in extracts of HL-60 cells to a much greater extent at a lower concentrations. Immunoblasts revealed that RA induced expression of the gamma 2 subunit, which was otherwise undetectable in G(i)2 purified from HL-60 cells or in HL-60 membranes. Possibly by inducing expression of gamma 2, RA alters two functions of the G(i) beta gamma subunit, modulation of fMLP receptor-G(i)2 coupling and activation of the effector, Phospholipase C.

A segment of the C-terminal half of the G-protein beta 1 subunit specifies its interaction with the gamma 1 subunit

The beta and gamma subunits of the heterotrimeric guanine nucleotide binding (G protein) act as a dimer and directly regulate various signal transduction pathways. By using cotransfection assays, we tested the ability of several beta gamma combinations to activate inositol phospholipid-specific phospholipase C (PI-PLC)-beta 2. Our findings indicate that only beta gamma combinations that form dimers will activate PI-PLC-beta 2. Since G beta 1 interacts with G gamma 1, while G beta 2 cannot, chimeras between G beta 1 and G beta 2 were used to identify the regions in beta 1 that determine its specific association with gamma 1. Our evidence demonstrates that a chimera between beta 2 and beta 1 that contains the C-terminal 173 amino acids of beta 1 can interact and activate PI-PLC-beta 2 with gamma 1. Chimeras that contain portions of the beta 1 C-terminal region display a weaker association with gamma 1. Furthermore, the contribution of each of these regions depends on the sequence context of each chimeric protein. However, the segment between residues 210 and 293 of beta 1 consistently plays a critical role in specifying association with gamma 1.

Molecular pharmacology of somatostatin receptors

Somatostatin was discovered for its ability to inhibit growth hormone (GH) secretion. Later, it was found to be widely distributed in other brain regions, in which it fulfills a neuromodulatory role, and in several organs of the gastrointestinal tract where it can act as a paracrine factor or as a true circulating factor. In mammals, two molecules of 14 (somatostatin 14) and 28 (somatostatin 28) amino acids are the only biologically active members of the family. They originate from a single gene which gives rise to a single propeptide alternately cleaved in different tissues. In 1992, a major breakthrough in our understanding of somatostatin functions was made with the cloning of five different receptor genes (sstr1 to sstr5) which belong to the seven transmembrane domain receptor family. Their closer relatives are opioid receptors. In first approximation, the tissular expression of the sstrs matches quite well with the distribution of somatostatin binding sites in the "classical" targets of the peptide ie brain, pituitary pancreatic islets and adrenals. The pharmacology of GH inhibition is very close to sstr2 binding but other actions of somatostatins have not yet been attributed clearly to a single receptor subtype. All clinically relevant agonists tested so far (octreotide, lanreotide and vapreotide) are selective of sstr2 being less potent on sstr3 and inactive for sstr1 and sstr4. Surprisingly, rat sstr5 displays nanomolar affinities for octreotide and vapreotide while these agonists are only active at much higher concentrations on human sstr5. All five receptors can be more or less efficiently coupled to inhibition of adenylate cyclase activity in transfected cell systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine brain GO isoforms have distinct gamma subunit compositions

The gamma subunit composition of the major bovine brain Go and Gi proteins (GOA, GOB, GOC, Gi1, and Gi2) was characterized using antibodies against specific gamma isoforms. Each of the purified G protein heterotrimers contained a heterogeneous population of gamma subunits, and the profiles of the gamma subunits found with Gi1, Gi2, and GOA were similar. In contrast, each GO isoform had a distinct pattern of associated gamma subunits. These differences were surprising given that all three alpha O isoforms are thought to share a common amino-terminal sequence important for the binding of beta gamma dimers and that the alpha OA and alpha OC proteins may come from the same alpha O1 mRNA. The free alpha OA and alpha OC subunits had unique elution behaviors during MonoQ chromatography, compatible with differences in their post-translational processing. These results indicate that both the alpha and gamma subunit compositions of heterotrimers define the structure of an intact G protein. Furthermore, the exact subunit composition of G protein heterotrimers may depend upon regulated expression of different subunit isoforms or upon cellular processing of alpha subunits.

Receptors and G proteins as primary components of transmembrane signal transduction. Part 2. G proteins: structure and function

Seven-transmembrane receptors signal through nucleotide-binding proteins (G proteins) into the cell. G proteins are membrane-associated proteins composed of three subunits termed alpha, beta and gamma, of which the G alpha subunit classifies the heterotrimer. So far, 23 different mammalian G alpha subunits are known, which are grouped in four subfamilies (Gs, Gi, Gq, G12) on the basis of their amino acid similarity. They carry an endogenous GTPase activity allowing reversible functional coupling between ligand-bound receptors and effectors such as enzymes and ion channels. In addition, five G beta and seven G gamma subunits have been identified which form tightly associated beta gamma heterodimers. Upon activation by a ligand-bound receptor the G protein dissociates into G alpha and G beta gamma, which both transmit signal by interacting with effectors. On the G protein level, specificity and selectivity of the incoming signal is accomplished by G protein trimers composed of distinct subunits. On the other hand, many receptors have been shown to activate different G proteins, thereby regulating diverse signal transduction pathways.

Specificity of G protein alpha-gamma subunit interactions. N-terminal 15 amino acids of gamma subunit specifies interaction with alpha subunit

The existence of multiple alpha, beta, and gamma subunits raises questions regarding the assembly of particular G proteins. Based on the results of a previous study (Rahmatullah, M., and Robishaw, J. D. (1994) J. Biol. Chem. 269, 3574-3580), we hypothesized that the assembly of G proteins may be determined by the interactions of the more structurally diverse alpha and gamma subunits. This hypothesis was confirmed in the present study by showing striking differences in the abilities of the gamma 1 and gamma 2 subunits to interact with the alpha o subunit. Chimeras of the gamma 1 and gamma 2 subunits were used to delineate which region is responsible. Support for the importance of the N-terminal region of the gamma subunit comes from our observations that 1) the gamma 2 subunit and the gamma 211 chimera bound strongly to the alpha o-agarose matrix, but the gamma 1 subunit and the gamma 112 chimera bound weakly, if at all; 2) an N-terminal peptide made to the gamma 2 subunit blocked the binding of the gamma 211 chimera to the alpha o-agarose matrix; 3) both the gamma 211 chimera and the N-terminal peptide were able to partially protect the alpha o subunit against tryptic cleavage; and 4) the gamma 211 chimera, but not the gamma 112 chimera, supported ADP-ribosylation of the alpha o subunit.

Biochemical approaches to examine the specificity of interactions between receptors and guanine nuclotide binding proteins

It is now appreciated both that G-protein-linked receptors and signal transducing heterotrimeric G-proteins consist of large multi-member superfamilies and that regulation of a signal transduction cascade can be produced by a variety of means following activation of a G-protein by a receptor. To begin to unravel the complexities of this regulation it is clearly important to be able to define the molecular identity of the G-protein or G-proteins activated by a receptor and to assess the quantitative importance of such interactions for the integration of signals produced by a receptor agonist. Substantial progress has been made towards these goals in recent years and the purpose of this short review will be to discuss the use and potential limitations of some of the currently most widely used approaches.

Signal transduction by G proteins: 1994 edition

Findings from the last two years in signal transduction research, including the elucidation of the crystal structure of alpha1, the uncovering of multiple roles for lipidation, the mimicry of receptor action with peptides, and both the in vitro reconstitution of inhibition of adenylyl cyclase and the in cell reconstitution of receptor-G protein coupling in transient and stable expression studies, are integrated into a "current" view of the receptor --> G protein --> effector pathway. The question is raised whether receptor or betagamma is the nucleotide exchange factor, and the central participation of Mg2+ in G protein activation and the change in affinity of the G protein for Mg2+ during receptor-stimulated activation are stressed.

Subunit expression of signal transducing G proteins in cardiac tissue: implications for phospholipase C-beta regulation

In the heart, alpha-adrenergic, angiotensin II and endothelin signaling pathways modulate short-term changes in chronotropy and inotropy, and participate in the long-term control of cardiac growth. A shared feature of these signaling pathways is the stimulation of phosphatidylinositol (PI) turnover, which is thought to occur via G protein-mediated regulation of phospholipase C (PLC) activity. However, G protein subunits capable of regulating PLC activity have not been identified in different regions and cell types of the heart and members of the G protein-regulated PLC-beta isozyme family have not been documented in the heart. Using a battery of antipeptide specific antisera directed against the G protein alpha q, beta and gamma subunit families and against members of the PLC-beta, PLC-gamma and PLC-delta families, we demonstrated that heart tissues express the G protein alpha subunits alpha q and alpha 11, multiple G protein beta and gamma subunits, and PLC-beta 3, a phospholipase C isozyme regulated by either G protein alpha or beta gamma subunits. The degree of expression and distribution of these subunits differed between regions of the heart (atria versus ventricle) and changed with development. These data lay the ground work for future studies to determine the functional coupling of specific subsets of these components involved in receptor activation of PI turnover in the heart.

Somatostatin receptor activation of cellular effector systems

Somatostatin (SRIF) induces its multiple biological actions by interacting with a family of receptors, referred to as SSTR1-SSTR5. These receptors are capable of associating with particular guanine nucleotide binding proteins to couple the receptors to distinct cellular effector systems. Therefore, G proteins have an important role in directing SRIF signalling and may provide the molecular basis for the diverse cellular actions of SRIF.

GTP-binding proteins in plants: new members of an old family

Regulatory guanine nucleotide-binding proteins (G proteins) have been studied extensively in animal and microbial organisms, and they are divided into the heterotrimeric and the small (monomeric) classes. Heterotrimeric G proteins are known to mediate signal responses in a variety of pathways in animals and simple eukaryotes, while small G proteins perform diverse functions including signal transduction, secretion, and regulation of cytoskeleton. In recent years, biochemical analyses have produced a large amount of information on the presence and possible functions of G proteins in plants. Further, molecular cloning has clearly demonstrated that plants have both heterotrimeric and small G proteins. Although the functions of the plant heterotrimeric G proteins are yet to be determined, expression analysis of an Arabidopsis G alpha protein suggests that it may be involved in the regulation of cell division and differentiation. In contrast to the very few genes cloned thus far that encode heterotrimeric G proteins in plants, a large number of small G proteins have been identified by molecular cloning from various plants. In addition, several plant small G proteins have been shown to be functional homologues of their counterparts in animals and yeasts. Future studies using a number of approaches are likely to yield insights into the role plant G proteins play.

Somatostatin receptor subtype SSTR2 mediates the inhibition of high-voltage-activated calcium channels by somatostatin and its analogue SMS 201-995

Somatostatin and its analogue SMS 201-995 inhibit high voltage-activated (HVA) Ca2+ currents in the rat insulinoma cell line RINm5F which stably express cloned human somatostatin receptor subtype 2 (hSSTR2). In contrast, neither somatostatin nor SMS 201-995 suppresses the HVA Ca2+ currents in RINm5F which stably express cloned hSSTR1. These results suggest that somatostatin-induced inhibition of HVA Ca2+ currents is mediated by a specific receptor subtype and that inhibition of calcium influx through HVA Ca2+ channels is one of the mechanisms of SMS 201-995 action on inhibitory processes of hormone secretion and cell proliferation.

Molecular pharmacology of somatostatin receptors

The neuropeptide somatostatin (SRIF) is widely expressed in the brain and in the periphery in two main forms, SRIF-14 and SRIF-28. Similarly, the presence of SRIF receptors throughout the whole body has been reported. SRIF produces a variety of effects including modulation of hormone release (e.g. GH, glucagon, insulin), of neurotransmitter release (e.g. acetylcholine, dopamine, 5-HT), and its own release is modulated by many neurotransmitters. SRIF affects cognitive and behavioural processes, the endocrine system, the gastrointestinal tract and the cardiovascular system and also has tumor growth inhibiting effects. Initially, two classes of SRIF receptors have been proposed on the basis of biochemical and functional studies. However, the recent cloning of five putative SRIF receptor subtypes which belong to the G-protein coupled receptor superfamily suggests that SRIF mediates its various effects via a whole family of receptors. Here we review, in this new context, the molecular pharmacology of the SRIF receptor subtypes present in the brain and in the periphery, and address the question of nomenclature of SRIF receptors.

Roles of heterotrimeric and monomeric G proteins in sperm-induced activation of mouse eggs

Results of several lines of experimentation suggest that sperm-induced egg activation has several features in common with G protein-coupled receptor signal transduction mechanisms. We report that microinjection of GDP beta S into metaphase II-arrested mouse eggs blocks sperm-induced egg activation. Since GDP beta S inactivates both heterotrimeric and monomeric classes of G proteins, the involvement of members of each of these families in sperm-induced egg activation was evaluated. Neither pertussis toxin treatment of eggs nor microinjection of eggs with inhibitory antibodies toward G alpha q blocked sperm-induced egg activation. Nevertheless, microinjection of phosducin, a protein that binds tightly to free G protein beta gamma subunits, specifically inhibited second polar body emission, the fertilization evoked decrease of H1 kinase activity and pronucleus formation. Microinjection of phosducin, however, did not inhibit the fertilization-induced modifications of the zona pellucida and microinjection of beta gamma t did not result in egg activation in the absence of sperm. Inactivation of the monomeric Rho family of G proteins with C3 transferase from Clostridium botulinum inhibited emission of the second polar body and cleavage to the 2-cell stage, but did not affect the modifications of the zona pellucida or pronucleus formation. Microinjection of Rasval12, which is a constitutively active form of Ras, did not result in egg activation in the absence of sperm. Moreover, microinjection of either an anti-Ras neutralizing antibody (Y13-259) or a dominant negative form of Ras (RasT) did not affect events of sperm-induced egg activation. In contrast, microinjection of RasT inhibited embryo cleavage to the 2-cell stage. These results suggest that both heterotrimeric and monomeric G proteins are involved in various aspects of sperm-induced egg activation.

Peptides as probes for G protein signal transduction

Triggered by agonist binding to cell surface receptors, the heterotrimeric G proteins dissociate into alpha and beta gamma subunits, each activating distinct second messenger pathways. Peptides from the primary sequences of receptors, G proteins, and effectors have been used to study the molecular interactions between these proteins. Receptor-derived peptides from the second, third and fourth intracellular loops and certain naturally occurring peptides antagonize G protein interactions and can directly activate G protein. These peptides bind to G protein sites that include the N and C terminal regions of the alpha subunit and a yet to be identified region of the beta subunit. Peptides have also been useful in characterizing G protein-effector interactions. The identification of the contact sites between proteins involved in G protein signal transduction should aid in the development of non-peptide mimetic therapeutics which could specifically modify G protein-mediated cellular responses.

Inhibition of adenylate cyclase activity by galanin in rat insulinoma cells is mediated by the G-protein Gi3

Galanin inhibits adenylate cyclase activity and insulin secretion and modulates ion channels in pancreatic beta-cells through pertussis-toxin-sensitive G-protein(s). Antibodies directed against the C-terminal region of specific G-protein alpha-subunits were used to determine which G-protein(s) couple galanin receptors to inhibition of adenylate cyclase in the rat insulinoma cell line RINm5F. Preincubation of membranes with EC antibody (anti-alpha i3) decreased the inhibition of forskolin-stimulated adenylate cyclase activity by galanin (100 nM) by 45% compared with control IgG (P < 0.05) whereas preincubation with AS (anti-alpha i1, alpha i2) or GO (anti-alpha o) antibodies had no significant effect. To confirm these results, RINm5F cells were exposed intermittently over a 4-day period to phosphorothioate oligodeoxynucleotides that were either sense or antisense to alpha i1, alpha i2, alpha i3 or alpha o. Oligodeoxynucleotides antisense to alpha i2, alpha i3 and alpha o specifically decreased the levels of the targeted alpha-subunit in membranes. alpha i1 was undetectable in these cells. Inhibition of adenylate cyclase activity by galanin was largely abolished in membranes from cells exposed to the oligodeoxynucleotide antisense to alpha i3, whereas all other oligodeoxynucleotides had no significant effect on this pathway. Indirect immunofluorescence and immunoblotting of specific membrane fractions with EC antibody show significant localization of alpha i3 to intracellular membrane compartments. These results suggest that Gi3 is the G protein that couples galanin receptors to inhibition of adenylate cyclase activity in RINm5F cells.

Complex information processing by the transmembrane signaling system involving G proteins

Much of the information cells receive is transduced by a membranous signaling system that uses heterotrimeric guanine nucleotide binding proteins (G proteins) to functionally couple cell surface receptors to a variety of effectors. During recent years it has been shown that receptors, G protein alpha, beta and gamma subunits as well as effectors involved in this signaling system exhibit a remarkable structural diversity and that the interactions of these components display a bewildering complexity. Even though many questions remain to be answered, it is becoming obvious that G proteins form the basis of a complex membranous signaling network which allows the cell to coordinate and to process incoming signals already on the level of the plasma membrane.

Isolation of cDNAs encoding guanine nucleotide-binding protein beta-subunit homologues from maize (ZGB1) and Arabidopsis (AGB1)

We have isolated cDNAs from maize (ZGB1) and Arabidopsis (AGB1) encoding proteins homologous to beta subunits of guanine nucleotide-binding protein (G protein). The predicted ZGB1 and AGB1 gene products are 76% identical to each other and 41% or more identical to animal G protein beta subunits. Both predicted proteins contain seven repeats of the so-called "WD-40" motif, where WD is Trp-Asp. RNA blot analysis indicates that ZGB1 mRNA is present in the root, leaf, and tassel and that AGB1 mRNA is expressed in the root, leaf, and flower. DNA blot hybridizations indicate that maize and Arabidopsis genomes contain no other genes that are highly similar to ZGB1 and AGB1, respectively, suggesting that the newly isolated G protein beta-subunit homologues are likely to have unique functions. Furthermore, these G protein beta-subunit homologues are conserved among other plant species and may play important role(s) in plant signaling.

Electrical activity and calcium channels in neuroendocrine cells

Similar to neuronal cells, neuroendocrine cells express voltage-dependent ion channels and fire action potentials. Ca2+ influx through voltage-dependent Ca2+ channels couples changes in membrane potential to Ca(2+)-dependent cellular processes, such as hormone release. Using the patch-clamp technique, we studied the spontaneous electrical activity as well as voltage-dependent Ca2+ channels in cholecystokinin-producing pancreatic cells (RIN 1056E cell line), in prolactin-secreting pituitary cells (GH3 cell line), and in calcitonin-secreting cells of the thyroid (rMTC 44-2 cell line). All three cell types displayed spontaneous electrical activity, that is, they spontaneously produced action potentials. RIN 1056E cells, GH3 cells, and rMTC cells exhibited (various types of) voltage-dependent Ca2+ channels that were regulated by various neurotransmitters and hormones, such as somatostatin.

Heterotrimeric G proteins involved in the modulation of voltage-dependent calcium channels of neuroendocrine cells

Various mechanisms have been identified by which hormones and neurotransmitters, interacting with heptahelical receptors, modulate the intracellular Ca2+ concentration in neuronal, endocrine, and neuroendocrine cells. All of them involve heterotrimeric G proteins. Best documented are hormonal stimulations and inhibitions of voltage-dependent Ca2+ channels. Stimulation is caused by agonists interacting with receptors known to induce phosphatidylinositol 4,5-bisphosphate hydrolysis, that is, a PI response. Although the PI response triggers a transient secretion by fast Ca2+ release, the stimulation of Ca2+ channels is assumed to be responsible for prolonged cell responses and for refilling of IP3-sensitive Ca2+ pools after repeated stimulations. Using antisense oligonucleotide microinjection in rat pituitary GH3 cells, Gi2 has been identified as the pertussis toxin-sensitive G protein stimulating Ca2+ channels, whereas Gq/G11 are involved in the concurrent PI response with subsequent protein kinase C activation, which is required for Ca2+ channel stimulation. Inhibitory modulations of Ca2+ channels are assumed to be the basis of inhibitions of transmitter or hormone secretion. Experiments in GH3 cells have revealed that Go subforms composed of alpha o1 x beta 3 x gamma 4 and alpha o2 x beta 1 x gamma 3 are the active G-protein heterotrimers transferring inhibitory signals from muscarinic M4 and somatostatin receptors to the Ca2+ channel, respectively.

G-protein gamma 7 subunit is selectively expressed in medium-sized neurons and dendrites of the rat neostriatum

We used subtractive hybridization to isolate clones of gamma 7, a 68 residue G-protein gamma subunit. Northern blotting and in situ hybridization reveal that the gamma 7 subunit mRNA is expressed primarily in medium-sized neurons of the neostriatum and nucleus accumbens and neurons of the olfactory tubercle, and at low levels in the dentate gyrus of the hippocampal formation and laminae II-III, and V of the neocortex. The gamma 7 mRNA is translocated into dendrites of neurons in the neostriatum and the dentate gyrus of the hippocampus. gamma 7 is expressed at relatively very low concentrations in peripheral tissues. The selective pattern of gamma 7 expression within the brain is highly reminiscent of those of the striatum-enriched adenylyl cyclase ACST, dopamine receptors, and the alpha subunit of G(olf), suggesting that, in striatum, gamma 7 may be a subunit of a G(olf) alpha-containing G protein that couples dopamine receptors selectively to ACST.

Engrailed controls glial/neuronal cell fate decisions at the midline of the central nervous system

The molecular mechanisms involved in glial/neuronal fate decisions during embryonic development are largely unknown. Here we show that the segment-polarity gene engrailed, which encodes a homeodomain protein, controls these decisions within an insect CNS lineage. The grasshopper median neuroblast (MNB) generates both neurons and midline glia in distinct temporal phases. engrailed expression in MNB progeny can be inhibited by injection of antisense oligodeoxynucleotides into the MNB nucleus. This produces a phenotype in which the midline glia do not develop and extra midline neurons are generated. In the absence of engrailed function, midline glial precursors are apparently converted into neuronal precursors. Thus, engrailed is required for execution of the decision between the glial and neuronal fates.

Lateral mobility of tetramethylrhodamine (TMR) labelled G protein alpha and beta gamma subunits in NG 108-15 cells

Multi-step signal transducing events, such as those mediated by G proteins, have been difficult to study in intact cells. We prepared fluorescently labelled G protein subunits, tetramethylrhodamine-alpha o (TMR-alpha o) and TMR-beta gamma, in order to study their subcellular distribution and lateral mobility. Heterotrimeric G proteins labelled in the alpha (TMR-alpha o/beta gamma) or beta (TMR-beta gamma/alpha o) subunit were reconstituted into lipid vesicles and fused to NG-108-15 cells using polyethylene glycol (PEG). Vesicles fused completely to the cells as determined by dequenching of a fluorescent lipid probe, octadecyl rhodamine B. The orientation of G protein beta gamma subunits after fusion followed the expected random distribution; the quenching of surface fluorescence with anti-fluorescein antibodies showed that about 50% of the label was accessible extracellularly. G proteins incorporated by the fusion method were able to couple to endogenous alpha 2 adrenergic receptors based on the restoration of high affinity agonist binding to pertussis toxin-treated cells. The subcellular localization of TMR-alpha o and TMR-beta gamma determined by differential centrifugation and confocal microscopy indicated that TMR-alpha o was present in the plasma membrane and in intracellular membranes, whereas TMR-beta gamma was mainly localized in the plasma membrane. The lateral mobility of TMR-alpha o and TMR-beta gamma measured using fluorescence recovery after photobleaching (FRAP) demonstrated low mobile fractions of 0.34 +/- 0.03 and 0.16 +/- 0.03, respectively. The translational diffusion coefficients of the mobile components were similar, 4.0 x 10(-9) and 2.0 x 10(-9) cm2/s, for alpha and beta gamma respectively. Neither activation of Gi-linked receptors nor cytoskeletal disruption with nocodozole or cytochalasin D changed the mobile fraction or diffusion coefficient of the alpha or beta gamma subunits. The FRAP data combined with the localization of fluorescent subunits by confocal microscopy suggest that the beta gamma subunits are highly constrained to localized regions of the plasma membrane while the alpha subunit may diffuse in intracellular regions to transmit signals from receptors to effector proteins.

Ion channels and the signal transduction pathways in the regulation of growth hormone secretion

The secretion of GH from pituitary somatotrophs is mainly regulated by alterations in the levels of intracellular free Ca(2+) concentrations ([Ca(2+)](i)) that depend on the influx of Ca(2+) through voltage-gated Ca(2+) channels in the cell membrane. Hypothalamic stimulatory and inhibitory factors bind to specific receptors on the cell membrane to regulate membrane potential and activate second-messenger systems. The receptors are G-protein coupled, and activated G proteins directly influence membrane ion channels to regulate Ca(2+) influx. The function of cAMP-dependent protein kinase A is also modulated by receptor-coupled G proteins leading to the phosphorylation of Ca(2+) channel proteins and further alteration of Ca(2+) influx.

Localization of a heterotrimeric G protein gamma subunit to focal adhesions and associated stress fibers

Signal transducing heterotrimeric G proteins are responsible for coupling a large number of cell surface receptors to the appropriate effector(s). Of the three subunits, 16 alpha, 4 beta, and 5 gamma subunits have been characterized, indicating a potential for over 300 unique combinations of heterotrimeric G proteins. To begin deciphering the unique G protein combinations that couple specific receptors with effectors, we examined the subcellular localization of the gamma subunits. Using anti-peptide antibodies specific for each of the known gamma subunits, neonatal cardiac fibroblasts were screened by standard immunocytochemistry. The anti-gamma 5 subunit antibody yielded a highly distinctive pattern of intensely fluorescent regions near the periphery of the cell that tended to protrude into the cell in a fibrous pattern. Dual staining with anti-vinculin antibody showed co-localization of the gamma 5 subunit with vinculin. In addition, the gamma 5 subunit staining extended a short distance out from the vinculin pattern along the protruding stress fiber, as revealed by double staining with phalloidin. These data indicated that the gamma 5 subunit was localized to areas of focal adhesion. Dual staining of rat aortic smooth muscle cells and Schwann cells also indicated co-localization of the gamma 5 subunit and vinculin, suggesting that the association of the gamma 5 subunit with areas of focal adhesion was wide-spread.

Antisense knockouts: molecular scalpels for the dissection of signal transduction

The complexity of signal transduction is becoming increasingly apparent following the cloning of multiple families of receptors, G proteins, and effectors. Therefore, new tools are needed to assess the importance of particular subtypes in receptor-mediated signal transduction. One such tool is the use of antisense approaches to specifically 'knockout' particular G protein subtypes and then assess the functional consequences for receptor-signalling pathways. In this article by Paul Albert and Stephen Morris, various antisense approaches (including transfection of full-length cDNA) are discussed and compared for their specificity and efficiency. The antisense approach is argued to be applicable to a wide variety of signal-transduction systems, including G-protein-coupled receptor signalling, for analysis of the downstream events that dictate biological responsiveness.

Antisense oligonucleotide strategies in physiology

Antisense oligonucleotides can inhibit gene expression in living cells by binding to complementary sequences of DNA, RNA or mRNA. The mechanisms include inhibition of RNA synthesis, RNA splicing, mRNA export, binding of initiation factors, assembly of ribosome subunits and of sliding of the ribosome along the mRNA coding sequence. The most efficient antisense oligonucleotides also activate RNAse H, an ubiquitous enzyme that cleaves the mRNA at sites of mRNA/oligonucleotide duplex formation. A staggering number of oligonucleotide modifications have been proposed to retard degradation by nucleases, enhance cellular uptake, increase binding to the target sequence, and minimize non-specific binding to related nucleic acid sequences. Phosphorothioates are the most popular oligonucleotides used in cell culture and in vivo, although sequence non-specificity remains an underreported problem. Recently developed chimeras between methylphosphonates and phosphodiester oligonucleotides appear to combine the advantages of water solubility, nuclease resistance, enhanced cellular uptake, activation of RNAse H, and high sequence selectivity. Antigene oligonucleotides are also promising, because they can inhibit gene expression by triple helix formation with DNA or by binding to one of the DNA strands. They have so far been little used in physiological studies. Cost is still a prohibitive factor, especially for suppressing the expression of a hormone or hormone receptor gene in rats, for example. However, patch-clamp dialysis of single cells or nuclear microinjections in culture, exposure of cultures to extracellular oligonucleotides, and intra-cerebral microinjections of oligonucleotides are feasible and highly rewarding approaches in physiology.

A novel phosphoinositide 3 kinase activity in myeloid-derived cells is activated by G protein beta gamma subunits

Phosphoinositide 3 kinase (PI3K) is a key signaling enzyme implicated in receptor-stimulated mitogenesis, oxidative bursting in neutrophils, membrane ruffling, and glucose uptake. A PI3K has already been purified, cloned, and shown to be regulated by receptors that act via tyrosine kinase-dependent regulatory mechanisms. We report that an immunologically, pharmacologically, and chromatographically distinct form of PI3K activity present in neutrophils and U937 cells is specifically activated by G protein beta gamma subunits. This data suggests PI3Ks conform to the paradigm set by receptor regulation of phosphoinositidase Cs: different receptor transduction systems specifically regulate dedicated isoforms of effector protein.

Developmental expression of G proteins in a migratory population of embryonic neurons

Directed neuronal migration contributes to the formation of many developing systems, but the molecular mechanisms that control the migratory process are still poorly understood. We have examined the role of heterotrimeric G proteins (guanyl nucleotide binding proteins) in regulating the migratory behavior of embryonic neurons in the enteric nervous system of the moth, Manduca sexta. During the formation of the enteric nervous system, a group of approx. 300 enteric neurons (the EP cells) participate in a precise migratory sequence, during which the undifferentiated cells populate a branching nerve plexus that lies superficially on the visceral musculature. Once migration is complete, the cells then acquire a variety of position-specific neuronal phenotypes. Using affinity-purified antisera against different G protein subtypes, we found no apparent staining for any G protein in the EP cells prior to their migration. Coincident with the onset of migration, however, the EP cells commenced the expression of one particular G protein, Go alpha. The intensity of immunostaining continued to increase as migration progressed, with Go alpha immunoreactivity being detectable in the leading processes of the neurons as well as their somata. The identity of the Go alpha-related proteins was confirmed by protein immunoblot analysis and by comparison with previously described forms of Go alpha from Drosophila. When cultured embryos were treated briefly with aluminium fluoride, a compound known to stimulate the activity of heterotrimeric G proteins, both EP cell migration and process outgrowth were inhibited. The effects of aluminium fluoride were potentiated by alpha toxin, a pore-forming compound that by itself caused no significant perturbations of migration. In preliminary experiments, intracellular injections of the non-hydrolyzable nucleotide GTP gamma-S also inhibited the migration of individual EP cells, supporting the hypothesis that G proteins play a key role in the control of neuronal motility in this system. In addition, once migration was complete, the expression of Go alpha-related proteins in the EP cells underwent a subsequent phase of regulation, so that only certain phenotypic classes among the differentiated EP cells retained detectable levels of Go alpha immunoreactivity. Thus Go may perform multiple functions within the same population of migratory neurons in the course of embryonic development.

The active role of beta gamma in signal transduction

Many receptors that sense the environment effect intracellular regulation through stimulation of heterotrimeric G proteins and the consequences thereof. While prominence was originally given to the alpha-subunits of G proteins as the pathway for downstream regulation, very active roles for the beta gamma-subunits have emerged in the past year. Recent experiments highlight the versatility of beta gamma-subunits in these regulatory pathways, but also emphasize some fundamental questions that remain.