Dilated cardiomyopathy in two transgenic mouse lines expressing activated G protein alpha(q): lack of correlation between phospholipase C activation and the phenotype

We previously described a transgenic mouse line (alpha(q)*52) in which cardiac-specific expression of activated G alpha(q)protein (HA alpha(q)*) leads to activation of phospholipase C beta (PLC beta), the immediate downstream target of HA alpha(q)*, with subsequent development of cardiac hypertrophy and dilation. We now describe a second, independent line in the same genetic background (alpha(q)*44h) with lower expression of HA alpha(q)* protein that ultimately results in the same phenotype: dilated cardiomyopathy (DCM) with severely impaired left ventricular systolic function (assessed by M-mode and 2D echocardiography), but with a much delayed disease onset. We asked if PLC activation correlates with the development of the phenotype. At 12-14 months, 65% of alpha(q)*44h mice still had normal cardiac function and ventricular weight/body weight ratios (VW/BW). However, their basal PLC activity, which began to increase in ventricles at 6 months, was threefold higher than in wild-type by 12 months. This increase was even more pronounced than in 2.5-month-old alpha(q)*52 mice, in which a twofold increase was accompanied by a 25% increase in VW/BW. Furthermore, at 12-14 months the increase in PLC activity in alpha(q)*44h mice with and without DCM was comparable. Thus, the delayed time course in alpha(q)*44h mice unmasked a lack of correlation between PLC activation and development of DCM in response to HA alpha(q)* expression, suggesting a role for additional pathways and/or mechanisms. It also revealed a differential temporal regulation of protein kinase C isoform expression. The markedly different ages of disease onset in these two mouse lines provide a model for studying both genetic modifying factors and potential environmental influences in DCM.

Reassembly of phospholipase C-beta2 from separated domains: analysis of basal and G protein-stimulated activities

Phosphatidylinositol-specific phospholipase C-betas (PLC-betas) are the only PLC isoforms that are regulated by G protein subunits. To further understand the regulation of PLC-beta(2) by G proteins and the functional roles of PLC-beta(2) structural domains, we tested whether the separately expressed amino and carboxyl halves of PLC-beta(2) could associate to form catalytically active enzymes as two polypeptides, and we explored how the complexes thus formed would be regulated by G protein betagamma subunits (Gbetagamma). We expressed cDNA constructs encoding PLC-beta(2) fragments of different lengths in COS-7 cells and demonstrated by coimmunoprecipitation that the coexpressed fragments could assemble and functionally reconstitute an active PLC-beta(2). The pleckstrin homology domain of PLC-beta(2) was required for its targeting to the membrane and for substrate hydrolysis. Reconstituted enzymes that contained the linker region that joins the two catalytic domains were as active or more active than the wild-type PLC-beta(2). When the linker region was removed, basal PLC-beta(2) enzymatic activity was increased further, suggesting that the linker region exerts an inhibitory effect on basal PLC-beta(2) activity. The reconstituted enzymes, like wild-type PLC-beta(2), were activated by Gbetagamma; when the C-terminal region was present in these constructs, they were also activated by Galpha(q). Gbetagamma and Galpha(q) activated these PLC-beta(2) constructs equally in the presence or absence of the linker region. We conclude that the linker region is an inhibitory element in PLC-beta(2) and that Gbetagamma and Galpha(q) do not stimulate PLC-beta(2) through easing the inhibition of enzymatic activity by the linker region.

Altered regulation of potassium and calcium channels by GABA(B) and adenosine receptors in hippocampal neurons from mice lacking Galpha(o)

To examine the role of G(o) in modulation of ion channels by neurotransmitter receptors, we characterized modulation of ionic currents in hippocampal CA3 neurons from mice lacking both isoforms of Galpha(o). In CA3 neurons from Galpha(o)(-/-) mice, 2-chloro-adenosine and the GABA(B)-receptor agonist baclofen activated inwardly rectifying K(+) currents and inhibited voltage-dependent Ca(2+) currents just as effectively as in Galpha(o)(+/+) littermates. However, the kinetics of transmitter action were dramatically altered in Galpha(o)(-/-) mice in that recovery on washout of agonist was much slower. For example, recovery from 2-chloro-adenosine inhibition of calcium current was more than fourfold slower in neurons from Galpha(o)(-/-) mice [time constant of 12.0 +/- 0.8 (SE) s] than in neurons from Galpha(o)(+/+) mice (time constant of 2.6 +/- 0.2 s). Recovery from baclofen effects was affected similarly. In neurons from control mice, effects of both baclofen and 2-chloro-adenosine on Ca(2+) currents and K(+) currents were abolished by brief exposure to external N-ethyl-maleimide (NEM). In neurons lacking Galpha(o), some inhibition of Ca(2+) currents by baclofen remained after NEM treatment, whereas baclofen activation of K(+) currents and both effects of 2-chloro-adenosine were abolished. These results show that modulation of Ca(2+) and K(+) currents by G protein-coupled receptors in hippocampal neurons does not have an absolute requirement for Galpha(o). However, modulation is changed in the absence of Galpha(o) in having much slower recovery kinetics. A likely possibility is that the very abundant Galpha(o) is normally used but, when absent, can readily be replaced by G proteins with different properties.

Signal transduction in atria and ventricles of mice with transient cardiac expression of activated G protein alpha(q)

We recently showed that the transient expression of a hemagglutinin (HA) epitope-tagged, constitutively active mutant of the G protein alpha(q) subunit (HAalpha(q)*) in the hearts of transgenic mice is sufficient to induce cardiac hypertrophy and dilatation that continue to progress after HAalpha(q)* protein becomes undetectable. We demonstrated that the activity of phospholipase Cbeta, the immediate downstream target of activated Galpha(q), is increased at 2 weeks, when HAalpha(q)* is expressed, but also at 10 weeks, when HAalpha(q)* is no longer detectable. This observation suggested that the transient HAalpha(q)* expression causes multiple, persistent changes in cellular signaling pathways. We now demonstrate changes in the level, activity, or both of several signaling components, including changes in the amount and hormone responsiveness of phospholipase Cbeta enzymes, in the basal level of diacylglycerol (which predominantly reflects activation of phospholipase D), in the amount or distribution of protein kinase C (PKC) isoforms (PKCalpha, PKCdelta, and PKCepsilon), and in the amount of several endogenous G proteins. These changes vary depending on the isoform of the signaling molecule, the chamber in which it is expressed, and the presence or absence of HAalpha(q)*. Our results suggest that a network of linked signaling functions determines the development of hypertrophy. They also suggest that atria and ventricles represent different signaling domains. It is likely that such changes occur in other model systems in which the activity of a single signaling component is increased, either due to an activating mutation or due to overexpression of the wild type.

The WD repeat: a common architecture for diverse functions

Our knowledge of the large family of proteins that contain the WD repeat continues to accumulate. The WD-repeat proteins are found in all eukaryotes and are implicated in a wide variety of crucial functions. The solution of the three-dimensional structure of one WD-repeat protein and the assumption that the structure will be common to all members of this family has allowed subfamilies of WD-repeat proteins to be defined on the basis of probable surface similarity. Proteins that have very similar surfaces are likely to have common binding partners and similar functions.

Transient cardiac expression of constitutively active Galphaq leads to hypertrophy and dilated cardiomyopathy by calcineurin-dependent and independent pathways

Cardiac hypertrophy and dilatation can result from stimulation of signal transduction pathways mediated by heterotrimeric G proteins, especially Gq, whose alpha subunit activates phospholipase Cbeta (PLCbeta). We now report that transient, modest expression of a hemagglutinin (HA) epitope-tagged, constitutively active mutant of the Gq alpha subunit (HAalpha*q) in hearts of transgenic mice is sufficient to induce cardiac hypertrophy and dilatation that continue to progress after the initiating stimulus becomes undetectable. At 2 weeks, HAalpha*q protein is expressed at less than 50% of endogenous alphaq/11, and the transgenic hearts are essentially normal morphologically. Although HAalpha*q protein declines at 4 weeks and is undetectable by 10 weeks, the animals develop cardiac hypertrophy and dilatation and die between 8 and 30 weeks in heart failure. As the pathology develops, endogenous alphaq/11 rises (2.9-fold in atria; 1.8-fold in ventricles). At 2 weeks, basal PLC activity is increased 9- to 10-fold in atria but not ventricles. By 10 weeks, it is elevated in both, presumably because of the rise in endogenous alphaq/11. We conclude that the pathological changes initiated by early, transient HAalpha*q expression are maintained in part by compensatory changes in signal transduction and other pathways. Cyclosporin A (CsA) prevents hypertrophy caused by activation of calcineurin [Molkentin, J. D., Lu, J.-R., Antos, C. L., Markham, B., Richardson, J., Robbins, J., Grant, S. R. & Olson, E. N. (1998) Cell 93, 215-228]. Because HAalpha*q acts upstream of calcineurin, we hypothesized that HAalpha*q might initiate additional pathways leading to hypertrophy and dilatation. Treating HAalpha*q mice with CsA diminished some, but not all, aspects of the hypertrophic phenotype, suggesting that multiple pathways are involved.

Cardiac myocytes express mRNA for ten RGS proteins: changes in RGS mRNA expression in ventricular myocytes and cultured atria

Regulators of G-protein signalling (RGS) are recently identified proteins that shorten the lifetime of the activated G protein. We now show that rat cardiac myocytes express mRNA for at least 10 RGS. The mRNA for RGS-r is barely detectable in rat ventricles, but increases more than 20-fold during the 60- to 90-min process of isolating ventricular myocytes, and after 90 min of culture of atrial pieces in medium with Ca2+. Both in myocytes and in atria, the rise in RGS-r is transient. The mRNA for cardiac RGS5, but not RGS-r, is developmentally regulated. These studies suggest that rapid regulation of RGS levels may be a new mechanism that governs how signals are transmitted across the cardiac cell membrane.

Sites important for PLCbeta2 activation by the G protein betagamma subunit map to the sides of the beta propeller structure

The betagamma subunits of the heterotrimeric GTP-binding proteins (G proteins) that couple heptahelical, plasma membrane-bound receptors to intracellular effector enzymes or ion channels directly regulate several types of effectors, including phospholipase Cbeta and adenylyl cyclase. The beta subunit is made up of two structurally different regions: an N-terminal alpha helix followed by a toroidal structure made up of 7 blades, each of which is a twisted beta sheet composed of four anti-parallel beta strands (Wall, M. A., Coleman, D. E., Lee, E., Iñiguez-Lluhi, J. A., Posner, B. A., Gilman, A. G., and Sprang, S. R. (1995) Cell 83, 1047-1058; Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319). We have previously shown that sites for activation of PLCbeta2, PLCbeta3, and adenylyl cyclase II overlap on the "top" surface of the propeller, where Galpha also binds (Li, Y., Sternweis, P. M., Charnecki, S., Smith, T. F., Gilman, A. G., Neer, E. J., and Kozasa, T. (1998) J. Biol. Chem. 273, 16265-16272). The present study was undertaken to identify the regions on the side of the torus that might be important for effector interactions. We made mutations in each of the outer beta strands of the G protein beta1 propeller, as well as mutations in the loops that connect the outer strands to the adjacent beta strands. Our results suggest that activation of PLCbeta2 involves residues in the outer strands of blades 2, 6, and 7 of the propeller. We tested three of the mutations that most severely affected PLCbeta2 activity against two forms of adenylyl cyclase (ACI and ACII). Both inhibition of ACI and activation of ACII were unaffected by these mutations, suggesting that if ACI and ACII contact the outer strands, the sites of contact are different from those for PLCbeta2. We propose that distinct sets of contacts along the sides of the propeller will define the specificity of the interaction of betagamma with effectors.

Effect of deletion of the major brain G-protein alpha subunit (alpha(o)) on coordination of G-protein subunits and on adenylyl cyclase activity

Heterotrimeric G-proteins, composed of alpha and betagamma subunits, transmit signals from cell-surface receptors to cellular effectors and ion channels. Cellular responses to receptor agonists depend on not only the type and amount of G-protein subunits expressed but also the ratio of alpha and betagamma subunits. Thus far, little is known about how the amounts of alpha and betagamma subunits are coordinated. Targeted disruption of the alpha(o) gene leads to loss of both isoforms of alpha(o), the most abundant alpha subunit in the brain. We demonstrate that loss of alpha(o) protein in the brain is accompanied by a reduction of beta protein to 32+/-2% (n = 4) of wild type. Sucrose density gradient experiments show that all of the betagamma remaining in the brains of alpha(o)-/- mice sediments as a heterotrimer (s20,w = 4.4 S, n = 2), with no detectable free alpha or betagamma subunits. Thus, the level of the remaining betagamma subunits matches that of the remaining alpha subunits. Protein levels of alpha subunits other than alpha(o) are unchanged, suggesting that they are controlled independently. Coordination of betagamma to alpha occurs posttranscriptionally because the mRNA level of the predominant beta1 subtype in the brains of alpha(o)-/- mice was unchanged. Adenylyl cyclase can be positively or negatively regulated by betagamma. Because the level of other alpha subunits is unchanged and alpha(o) itself has little or no effect on adenylyl cyclase, we could examine how a large change in the level of betagamma affects this enzyme. Surprisingly, we could not detect any difference in the adenylyl cyclase activity between brain membranes from wild-type and alpha(o)-/- mice. We propose that alpha(o) and its associated betagamma are sequestered in a distinct pool of membranes that does not contribute to the regulation of adenylyl cyclase.

Sites for Galpha binding on the G protein beta subunit overlap with sites for regulation of phospholipase Cbeta and adenylyl cyclase

Heterotrimeric G proteins, composed of alpha and betagamma subunits, forward signals from transmembrane receptors to intracellular effector enzymes and ion channels. Free betagamma activates downstream targets, but its action is terminated by association with GDP-liganded alpha subunits. Because alpha can inhibit activation of many effectors by betagamma, it is likely that the alpha subunit binding surfaces on betagamma overlap the surfaces necessary for effector activation. To test this hypothesis, we mutated residues on beta shown to contact alpha in the recently published crystal structures of the alphabetagamma heterotrimer (Wall, M. A., Coleman, D. E., Lee, E., Iniguez-Lluhi, J. A., Posner, B. A., Gilman, A. G., and Sprang, S. R. (1995) Cell 83, 1047-1058; Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319.). The alpha subunit binds to the flat, top surface of the toroidal beta subunit and also extends a helix along the side of the beta subunit at blade 1. We mutated four residues on the top surface of beta (Hbeta1[L117A], Hbeta1[D228R], Hbeta1[D246S], and Hbeta1[W332A]) and two residues on the side of beta that contacts alpha (Hbeta1[N88A/K89A]). Each of the mutant proteins was able to form beta gamma dimers, but they differed in their ability to bind alpha and to activate phospholipase C beta2 (PLCbeta2), PLCbeta3, and adenylyl cyclase II. Mutation of residues along the side of the torus at blade 1 diminish affinity for alpha but do not prevent activation of any of the effectors. Mutations on the alpha binding surface differentially affected PLCbeta2, PLCbeta3, and adenylyl cyclase II. Residues that affect PLCbeta and adenylyl cyclase II activity are found on opposite sides of the central tunnel, suggesting that PLC and adenylyl cyclase, like the alpha subunit, make many contacts on the top surface. None of the mutations affected the ability of betagamma to inhibit adenylyl cyclase I. We conclude that alpha, PLCbeta2, PLCbeta3, and adenylyl cyclase II share an interaction on the top surface of beta. The importance of individual residues is different for alpha binding and for effector activation and differs even between closely related isoforms of the same effector.

Folding a WD repeat propeller. Role of highly conserved aspartic acid residues in the G protein beta subunit and Sec13

The beta subunit of the heterotrimeric G proteins that transduce signals across the plasma membrane is made up of an amino-terminal alpha-helical segment followed by seven repeating units called WD (Trp-Asp) repeats that occur in about 140 different proteins. The seven WD repeats in Gbeta, the only WD repeat protein whose crystal structure is known, form seven antiparallel beta sheets making up the blades of a toroidal propeller structure (Wall, M. A., Coleman, D. E., Lee, E., Iniguez-Lluhi, J. A., Posner, B. A., Gilman, A. G., and Sprang, S. R. (1995) Cell 83, 1047-1058; Sondek, J., Bohm, A., Lambright, D. G., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 369-374). It is likely that all proteins with WD repeats form a propeller structure. Alignment of the sequence of 918 unique WD repeats reveals that 85% of the repeats have an aspartic acid (D) residue (not the D of WD) in the turn connecting beta strands b and c of each putative propeller blade. We mutated each of these conserved Asp residues to Gly individually and in pairs in Gbeta and in Sec13, a yeast WD repeat protein involved in vesicular traffic, and then analyzed the ability of the mutant proteins to fold in vitro and in COS-7 cells. In vitro, most single mutant Gbeta subunits fold into Gbetagamma dimers more slowly than wild type to a degree that varies with the blade. In contrast, all single mutants form normal amounts of Gbetagamma in COS-7 cells, although some dimers show subtle local distortions of structure. Most double mutants assemble poorly in both systems. We conclude that the conserved Asp residues are not equivalent and not all are essential for the folding of the propeller structure. Some may affect the folding pathway or the affinity for chaperonins. Mutations of the conserved Asp in Sec13 affect folding equally in vitro and in COS-7 cells. The repeats that most affected folding were not at the same position in Sec13 and Gbeta. Our finding, both in Gbeta and in Sec13, that no mutation of the conserved Asp entirely prevents folding suggests that there is no obligatory folding order for each repeat and that the folding order is probably not the same for different WD repeat proteins, or even necessarily constant for the same protein.

G alpha(o) is necessary for muscarinic regulation of Ca2+ channels in mouse heart

Heterotrimeric G proteins, composed of G alpha and G betagamma subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The G alpha(o) protein is the most abundant G alpha subtype in the nervous system, but it is also found in the heart. Its function is not completely known, although it is required for regulation of N-type Ca2+ channels in GH3 cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of G alpha(o), we have generated mice lacking both isoforms of G alpha(o) by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that G alpha(o) makes up 0.2-0.5% of brain particulate protein and 10% of the growth cone membrane. The G alpha(o)-/- mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, G alpha(o)-/- mice have a clear and specific defect in ion channel regulation in the heart. Normal muscarinic regulation of L-type calcium channels in ventricular myocytes is absent in the mutant mice. The L-type calcium channel responds normally to isoproterenol, but there is no evident muscarinic inhibition. Muscarinic regulation of atrial K+ channels is normal, as is the electrocardiogram. The levels of other G alpha subunits (G alpha(s), G alpha(q), and G alpha(i)) are unchanged in the hearts of G alpha(o)-/- mice, but the amount of G betagamma is decreased. Whichever subunit, G alpha(o) or G betagamma, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca2+ channels requires coupling of the muscarinic receptor to G alpha(o). Other cardiac G alpha subunits cannot substitute.

Intracellular signalling: turning down G-protein signals

The recently discovered family of proteins known as 'regulators of G-protein signalling' offers a solution to an important puzzle about the termination of signalling by G proteins and may also be important in more long-term modulation of signalling via G proteins.

G protein beta gamma subunits

Guanine nucleotide binding (G) proteins relay extracellular signals encoded in light, small molecules, peptides, and proteins to activate or inhibit intracellular enzymes and ion channels. The larger G proteins, made up of G alpha beta gamma heterotrimers, dissociate into G alpha and G beta gamma subunits that separately activate intracellular effector molecules. Only recently has the G beta gamma subunit been recognized as a signal transduction molecule in its own right; G beta gamma is now known to directly regulate as many different protein targets as the G alpha subunit. Recent X-ray crystallography of G alpha, G beta gamma, and G alpha beta gamma subunits will guide the investigation of structure-function relationships.

Analysis of the physical properties and molecular modeling of Sec13: A WD repeat protein involved in vesicular traffic

WD repeat proteins are a family of proteins that contain a series of highly conserved internal repeat motifs, usually ending with WD (Trp-Asp). The G beta subunit of heterotrimeric guanine nucleotide binding protein is a member of this family, and its crystal structure has been recently solved at high resolution (Wall et al. (1995) Cell 83, 1047-1058; Sondek et al. (1996) Nature 379, 369-374). Based on the coordinates of G beta, we have constructed a model for the structure of Sec13, a 33 kDa WD repeat protein from Saccharomyces cerevesiae essential for vesicular traffic. The model has been tested using a combination of biophysical and biochemical methods. Sec13 was expressed in Escherichia coli as a hexa-His-tagged protein (H6Sec13) and purified to homogeneity. In contrast to some other WD repeat proteins that are unable to fold into monomeric structures when expressed in E. coli, H6Sec13 was soluble and monomeric in the absence of detergent. The far-UV circular dichroism (CD) spectra of H6Sec13 indicated less than 10% alpha-helix consistent with the model which predicts primarily beta-sheets. H6Sec13 shows a cooperative and irreversible thermal denaturation curve consistent with a tightly packed structure. The CD spectrum shows an unusual positive ellipticity at 229 nm that was attributed to interactions of surface tryptophans since the 229 nm maximum could be abolished by modification of 6.3 +/- 0.3 (n = 3) tryptophans (out of 15 total in the molecule) with N-bromosuccinimide. Our model predicts that three sets of tryptophans are clustered near the surface. As predicted by the model, purified H6Sec13 was completely resistant to trypsin digestion. The concordance of the model of Sec13 presented in this paper with the biochemical and biophysical studies suggests that this model can be useful as a guide to further experiments designed to elucidate the function of Sec13 in vesicular traffic.

Folding of proteins with WD-repeats: comparison of six members of the WD-repeat superfamily to the G protein beta subunit

The family of WD-repeat proteins comprises over 30 different proteins that share a highly conserved repeating motif [Neer, E. J., Schmidt, C. J., Nambudripad, R., & Smith, T. F. (1994) Nature 371, 297-300]. Members of this family include the signal-transducing G protein beta subunit, as well as other proteins that regulate signal transduction, transcription, pre-mRNA splicing, cytoskeletal organization, and vesicular fusion. The crystal structure of one WD-repeat protein (G beta) has now been solved (Wall et al., 1995; Sondek et al, 1996) and reveals that the seven repeating units form a circular, propeller-like structure with seven blades each made up of four beta strands. It is very likely that all WD-repeat proteins form a similar structure. If so, it will be possible to use information about important surface regions of one family member to predict properties of another. If WD proteins form structures similar to G beta, their hydrodynamic properties should be those of compact, globular proteins, and they should be resistant to cleavage by trypsin. However, the only studied example of a WD-repeat protein, G beta, synthesized in vitro in a rabbit reticulocyte lysate, is unable to fold into a native structure without its partner protein G gamma. The non-WD-repeat amino terminal alpha helix of G beta does not inhibit folding because G beta does not fold even when this region is removed. It is not known whether all WD-repeat proteins are unable to fold when synthesized in an in vitro system. We synthesized seven members of the family in a rabbit reticulocyte lysate, determined their Stokes radius, sedimentation coefficient, and frictional ratio, and assayed their stability to trypsin. Our working definition of folding was that the proteins from globular, trypsin-resistant structures because, except for G beta gamma, their functions are not known or cannot be assayed in reticulocyte lysates. We chose proteins that include amino and carboxyl extensions as well as proteins that are made up entirely of WD-repeats. We show that unlike G beta, several proteins with WD-repeats are able to fold into globular proteins in a rabbit reticulocyte lysate. One protein, beta Trcp, formed large aggregates like G beta, suggesting that it may also require a partner protein. Despite the presence of many potential tryptic cleavage sites, all of the proteins that did fold gave stable large products on tryptic proteolysis, as predicted on the basis of the structure of G beta. These studies suggest that other WD-repeat proteins are likely to form propeller structures similar to G beta.

Maintenance of cellular levels of G-proteins: different efficiencies of alpha s and alpha o synthesis in GH3 cells

G-proteins couple membrane-bound receptors to intracellular effectors. Each cell has a characteristic complement of G-protein alpha, beta and gamma subunits that partly determines the cell's response to external signals. Very little is known about the mechanisms that set and maintain cellular levels of G-proteins or about potential points of regulation. We have assayed the steady-state levels of mRNA and protein for two types of G-protein subunits, alpha s and alpha o, in rat brain, heart and GH3 cells, and found that in all these cases, it takes 9- to 20-fold more mRNA to produce a given amount of alpha s protein than to produce the same amount of alpha o protein. Such a situation could arise from a relatively rapid rate of alpha s protein degradation, requiring rapid protein synthesis to compensate, or from relatively inefficient translation of alpha s mRNA compared with alpha o mRNA. The latter appears to be the case in GH3 cells. These cells contain 94 times more mRNA for alpha s than for alpha o, yet the rate of alpha s protein synthesis is only 9 times greater than alpha o protein synthesis. The degradation rates of the two proteins are similar (13 h for alpha s and 18 h for alpha o). To begin to define the mechanism that accounts for the fact that it takes more mRNA to synthesize a given amount of alpha s than alpha o, we asked whether there is a pool of alpha s mRNA that does not participate in protein synthesis. We found that virtually all alpha s and alpha o mRNA is associated with ribosomes. Therefore, all the mRNA is likely to be capable of directing protein synthesis. Since the rate-limiting step in protein synthesis is usually binding of the ribosome to mRNA at initiation, our results suggest that the relatively slow rate of alpha s protein synthesis is regulated by a mechanism that acts beyond initiation at peptide elongation and/or termination.

Intersubunit surfaces in G protein alpha beta gamma heterotrimers. Analysis by cross-linking and mutagenesis of beta gamma

Heterotrimeric guanine nucleotide binding proteins (G proteins) are made up of alpha, beta, and gamma subunits, the last two forming a very tight complex. Stimulation of cell surface receptors promotes dissociation of alpha from the beta gamma dimer, which, in turn, allows both components to interact with intracellular enzymes or ion channels and modulate their activity. At present, little is known about the conformation of the beta gamma dimer or about the areas of beta gamma that interact with alpha. Direct information on the orientation of protein surfaces can be obtained from the analysis of chemically cross-linked products. Previous work in this laboratory showed that 1,6-bismaleimidohexane, which reacts with cysteine residues, specifically cross-links alpha to beta and beta to gamma (Yi, F., Denker, B. M., and Neer, E. J. (1991) J. Biol. Chem. 266, 3900-3906). To identify the residues in beta and gamma involved in cross-linking to each other or to alpha, we have mutated the cysteines in beta 1, gamma 2, and gamma 3 and analyzed the mutated proteins by in vitro translation in a rabbit reticulocyte lysate. All the mutants were able to form beta gamma dimers that could interact with the alpha subunit. We found that 1,6-bismaleimidohexane can cross-link beta 1 to gamma 3 but not to gamma 2. The cross-link goes from Cys25 in beta 1 to Cys30 in gamma 3. This cysteine is absent from any of the other known gamma isoforms and therefore confers a distinctive property to gamma 3. The beta subunit in the beta 1 gamma 2 dimer can be cross-linked to an unidentified protein in the rabbit reticulocyte lysate, generating a product slightly larger than cross-linked beta 1 gamma 3. The beta subunit can also be cross-linked to alpha, giving rise to two products on SDS-polyacrylamide gel electrophoresis, both of which were previously shown to be formed by cross-linking beta to Cys215 in alpha o (Thomas, T. C., Schmidt, C. J., and Neer, E. J. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 10295-10299). Mutation of Cys204 in beta 1 abolished one of these two products, whereas mutation of Cys271 abolished the other. Because both alpha-beta cross-linked products are formed in approximately equal amounts, Cys204 and Cys271 in beta are equally accessible from Cys215 in alpha o. Our findings begin to define intersubunit surfaces, and they pose structural constraints upon any model of the beta gamma dimer.

Localization of G alpha 0 to growth cones in PC12 cells: role of G alpha 0 association with receptors and G beta gamma

The heterotrimeric G protein G0 is highly enriched in the growth cones of neuronal cells and makes up 10% of the membrane protein of growth cones from neonatal rat brain. We have used PC12 cells, a cell line that differentiates to a neuron-like phenotype, as a model with which to study the mechanism of G protein localization. First, the role of the beta gamma-subunit was investigated. The attachment of the beta gamma-subunit to the membrane depends on the isoprenylation of the gamma-subunit. The drug lovastatin blocks isoprenylation by inhibiting a key enzyme in the biosynthetic pathway. After treatment of PC12 cells with 10 microM lovastatin for 48 hours 50% of the beta gamma-subunits were cytosolic compared with 100% membrane bound beta gamma in control cells, as determined by cell fractionation, gel electrophoresis and western blot. Addition of 200 microM mevalonic acid reverses this effect. However, lovastatin affects neither the membrane attachment of alpha 0 nor its localization to the growth cones as determined by immunohistochemistry. This suggests that the localization and retention of alpha 0 are independent of the membrane attachment of the full complement of beta gamma-subunits. Second, pertussis toxin was used to block the interaction between alpha 0 and receptors. PC12 cells were treated with 0.1 microgram/ml pertussis toxin prior to and during nerve growth factor-induced differentiation. In vitro [32P]ADP-ribosylation confirmed that alpha 0 and alpha i were completely ADP-ribosylated by this treatment. The ADP-ribosylation by pertussis toxin did not interfere with neurite outgrowth. The localization of alpha 0 to the growth cones was indistinguishable from that in untreated cells. We conclude that G protein-receptor interaction is not necessary for the distribution of alpha 0 to growth cones.

The G protein gamma subunit. Requirements for dimerization with beta subunits

Guanine nucleotide-binding protein beta and gamma subunits form a tightly bound complex that can only be separated by denaturation. Assembly of beta and gamma subunits is a complicated process. The beta 1 and gamma 2 subunits can be synthesized in vitro in rabbit reticulocyte lysate and then assembled into dimers, but beta 1 cannot form beta gamma dimers when synthesized in a wheat germ extract. In contrast, gamma 2 translated in either system can dimerize with beta 1, suggesting that dimerization-competent gamma 2 can be synthesized without the aid of specific chaperonins or other cofactors. Dimerization-competent gamma 2 in solution forms an asymmetric particle with a Stokes radius of about 21 +/- 0.4 A (n = 4), s20,w of 0.9 S (range 0.8-1.0 S, n = 2), and frictional ratio of 1.57 (assuming no hydration). To define the part of gamma 2 that is needed for native beta gamma dimer formation, a series of N- and C-terminal truncations were generated, synthesized in vitro, and incubated with beta 1. Dimerization was assessed by stabilization of beta 1 to tryptic proteolysis. Truncation of up to 13 amino acids at the C terminus did not affect dimerization with beta 1, whereas removal of 27 amino acids prevented it. Therefore, a region between residues 45 and 59 of gamma 2 is important for dimerization. Truncation of 15 amino acids from the N terminus greatly diminished the formation of beta gamma dimers, while removal of 25 amino acids entirely blocked it. Thus, another region important for forming native beta gamma is near the N terminus. Extension of the N terminus by 12 amino acids that include the influenza virus hemagglutinin epitope did not prevent beta gamma dimerization. Furthermore, in intact 35S-labeled COS cells, epitope-tagged gamma 2 coimmunoprecipitates with beta and alpha subunits. The N-terminal epitope tag must lie at the surface of the heterotrimer since it prevents neither heterotrimer formation nor access of the antibody.

Interactions between the amino- and carboxyl-terminal regions of G alpha subunits: analysis of mutated G alpha o/G alpha i2 chimeras

Receptors activate the G alpha subunits of heterotrimeric G proteins by binding to the C-terminus and reducing their affinity for bound GDP, therefore promoting exchange of GDP for GTP. Although this general mechanism is the same for all G alpha subunits, different G alpha subunits vary in nucleotide binding and hydrolysis even though the residues that make up the guanine nucleotide binding site are virtually identical. We have shown previously that truncation of 14 amino acids from the C-terminus of G alpha o decreased the apparent affinity for GDP and permitted us to see an activated conformation with GTP [Denker, B. M., et al. (1992) J. Biol. Chem. 267, 9998-10002]. To test whether mutations in the receptor binding region lead to different phenotypes in closely related G alpha subunits, we made the equivalent deletions in G alpha i2, synthesized the proteins in vitro in a rabbit reticulocyte lysate and used the pattern of native tryptic proteolysis as an index of conformation. The phenotype of truncated G alpha i2 was different from that of truncated G alpha o: GDP affinity was reduced, but we could not detect an activated conformation with GTP (although GTP gamma S activated normally). Analysis of shorter deletions showed that loss of three hydrophobic residues (between 11 and 13 residues from the C-terminus) was responsible for the phenotypes. To define the regions of G alpha o and G alpha i2 that were responsible for their different phenotypes, we used a conserved BamHI site (codon 212) to make chimeras. Each chimera truncated at the C-terminus had the phenotype of the donor of the amino-terminal portion. Both truncated chimeras were activated by GTP gamma S-like wild-type proteins, and both had decreased apparent affinity for GDP. Full-length chimeric subunits behaved like wild-type proteins. The crystal structure of G alpha t and G alpha i1 shows that the three hydrophobic amino acids we have identified make contact with residues in the N- and C-terminal portions of the protein. Our studies point to the importance of the contacts in the N-terminal region (start of beta strands 1 and 3) that may stabilize the C-terminal alpha helix, affect nucleotide binding, and determine the characteristic features of different G alpha subunits.

Regulation of alpha o expression by the 5'-flanking region of the alpha o gene

Many responses of cells to external signals require activation of the heterotrimeric G proteins. These responses depend on the type and amount of G proteins that are expressed. Each cell has a characteristic complement of G protein subunits. For example, the alpha o subunit is very abundant in neural tissues. Very little is known about the mechanisms that determine cellular levels of G proteins. In the present study, we have isolated a genomic clone for mouse alpha o gene and identified the promoter region. There are multiple transcription initiation sites located about 750 base pairs upstream of the translational start site. The promoter region is GC-rich and contains neither a TATA-box nor a CAAT box. Transient expression assays using a series of constructs containing various lengths of the 5'-flanking region of the alpha o promoter demonstrated that the region 300-700 base pairs upstream of the transcription initiation sites is responsible for the basic promoter activity. The relative activity of alpha o promoter is 8-12-fold higher in cells expressing alpha o than in cells lacking alpha o. The level of alpha o in cells may also be regulated at the level of protein translation because deletions in the 5'-noncoding region of alpha o gene increase reporter enzyme expression without a corresponding increase in reporter enzyme mRNA level. Our results suggest that both transcriptional and post-transcriptional mechanisms are involved in regulating the expression of alpha o in vivo. Transcriptional regulation probably is important for control of tissue-specific expression, while posttranscriptional mechanisms may be used to regulate the alpha o level in cells.

The ancient regulatory-protein family of WD-repeat proteins

WD proteins are made up of highly conserved repeating units usually ending with Trp-Asp (WD). They are found in all eukaryotes but not in prokaryotes. They regulate cellular functions, such as cell division, cell-fate determination, gene transcription, transmembrane signalling, mRNA modification and vesicle fusion. Here we define the common features of the repeating units, and criteria for grouping such proteins into functional subfamilies.

A 14-amino acid region of the G protein gamma subunit is sufficient to confer selectivity of gamma binding to the beta subunit

Heterotrimeric guanine nucleotide-binding proteins are important signaling molecules composed of an alpha, beta, and gamma subunit. The beta subunits must form dimers with gamma subunits to function. Several subtypes of beta and gamma have been identified, but not all combinations of beta and gamma subtypes can form dimers. For example, the gamma 2 subunit can form dimers with beta 1 and beta 2, but gamma 1 forms dimers only with beta 1, not with beta 2. Selective dimerization may play a role in the regulation of beta gamma dimer-mediated signal transduction. In order to identify the region of gamma responsible for selective dimer formation, a series of gamma 1/gamma 2 chimeras was constructed, transcribed, and translated in vitro. The ability of these gamma chimeras to form dimers with beta 1 and beta 2 was assayed by trypsin protection and chemical cross-linking. When amino acids 36-49 of gamma 1 were substituted for 33-46 of gamma 2, the chimera behaved like gamma 1 and dimerized only with beta 1; the reciprocal chimera, in which 14 residues from gamma 2 were substituted for the corresponding amino acids of gamma 1, behaved like gamma 2 and interacted with both beta 1 and beta 2. This 14-amino acid region was sufficient for gamma 1 to discriminate between the beta subunits. All gamma chimeras were functional because they were able to interact with beta 1, which is capable of forming dimers with both gamma 1 and gamma 2. All dimers of chimeric gamma subunits plus beta 1 were able to interact with purified alpha o subunit, indicating that beta gamma dimers containing chimeric gamma molecules were capable of interacting with an appropriate third molecule. This lays the foundation for using these gamma chimeras to study selective dimer interactions with various effectors and receptors.

G(O), a guanine nucleotide binding protein, is expressed during neurite extension in the embryonic mouse

The developmental pattern of expression of the G protein alpha o subunit and GAP43 were compared by immunohistochemical staining of mouse embryos. Staining for alpha o and GAP43 was identical and detected throughout the developing nervous system, and the antigens first appeared in neurons at the beginning of neuronal differentiation. GAP43 and alpha o were not detected in regions containing only neuroblasts. These observations suggest that alpha o and GAP43 may not be required for the decision to pass from neuroblast to differentiated neuron, but may play a role in signal transduction during early neuronal development.

G proteins: critical control points for transmembrane signals

Heterotrimeric GTP-binding proteins (G proteins) that are made up of alpha and beta gamma subunits couple many kinds of cell-surface receptors to intracellular effector enzymes or ion channels. Every cell contains several types of receptors, G proteins, and effectors. The specificity with which G protein subunits interact with receptors and effectors defines the range of responses a cell is able to make to an external signal. Thus, the G proteins act as a critical control point that determines whether a signal spreads through several pathways or is focused to a single pathway. In this review, I will summarize some features of the structure and function of mammalian G protein subunits, discuss the role of both alpha and beta gamma subunits in regulation of effectors, the role of the beta gamma subunit in macromolecular assembly, and the mechanisms that might make some responses extremely specific and others rather diffuse.

G-protein alpha o subunit: mutation of conserved cysteines identifies a subunit contact surface and alters GDP affinity

The reversible association of alpha and beta gamma subunits of GTP-binding proteins is important for signal transmission from a variety of cell-surface receptors to intracellular effectors. Previous work showed that 1,6-bis(maleimido)hexane, which crosslinks cysteine residues, crosslinks alpha o and alpha i-1 to beta gamma. These crosslinks are likely to form through a conserved cysteine because 1,6-bis(maleimido)hexane can also crosslink alpha i-2, alpha 1, alpha s and Drosophila alpha 1 to give products of the same apparent molecular weight as crosslinked alpha o beta gamma and alpha i-1 beta gamma. These proteins have only two cysteines in common. Therefore, we mutated each of the two conserved cysteines of alpha o to alanines. Mutation of Cys215 prevents crosslinking to beta gamma, but does not affect binding of guanosine 5'-[gamma-thio]triphosphate or the ability of the mutated alpha subunit to bind beta gamma. In models of the alpha subunit based on the crystal structure of p21ras, Cys215 is located on the face opposite to the GTP-binding site and near an area that changes conformation depending on the nucleotide bound. This surface on the alpha subunit overlaps a putative effector binding region, raising important questions about the spatial organization of the proteins as they form ternary complexes. Mutation of Cys325 has no effect on crosslinking but, surprisingly, decreases by a factor of 10 the affinity of the mutated protein for GDP, relative to wild type, without changing the affinity for guanosine 5'-[gamma-thio]triphosphate. This mutation falls within a region thought to contact receptors and may represent a site through which receptors enhance the release of GDP.

New roles for G-protein beta gamma-dimers in transmembrane signalling

When a membrane-bound receptor acts on a G protein, the GTP-binding or G alpha subunit dissociates from the G beta gamma dimer. Until recently, the G alpha subunit alone was thought to act on the enzymes and ion channels controlled by these proteins. Newer evidence indicates that the G beta gamma dimer also plays a major part in signal transmission, enhancing the complexity of the possible interactions between the G proteins and their targets.

G protein beta gamma subunit: physical and chemical characterization

The beta gamma subunits of heterotrimeric G proteins play a central role in regulating the function of the G protein alpha subunits and in modulating the activity of several enzymes and ion channels. We have used the signature tryptic cleavage pattern of native beta gamma from bovine brain as a starting point for our analysis of its physical and chemical properties. Digestion of bovine brain beta gamma with trypsin yields only 2 beta-derived fragments, with relative mobilities on SDS-PAGE of 14 kDa (amino terminal) and 27 kDa (carboxyl terminal), despite the presence of 32 potential tryptic cleavage sites in the beta 1 subunit. Trypsin-cleaved beta gamma remains in a complex that has the same apparent sedimentation coefficient as intact beta gamma, and retains its ability to associate functionally with the alpha o subunit. Comparison of the incorporation of [14C]iodoacetamide into reduced denatured beta and unreduced denatured beta showed that there are no disulfide bonds in the molecule to hold the complex together. The brain beta and gamma subunits can be cross-linked by 1,6-bis(maleimido)hexane to form a 46-kDa product on SDS-PAGE, and trypsin cleavage of cross-linked beta gamma shows that gamma is cross-linked to the 14-kDa amino-terminal fragment of the beta subunit. On the basis of its primary sequence, the beta subunit is predicted to form a repetitive structure encompassing the 27-kDa fragment and part of the 14-kDa fragment. Analysis of the thermal denaturation of trypsin-cleaved beta gamma supports this prediction and confirms that both fragments retain stable tertiary structures following tryptic cleavage.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor changes G protein levels and localization in PC12 cells

Growth cones at the growing tips of developing neurites contain the machinery to transmit information from receptors to a variety of intracellular enzymes and ion channels. In order to understand how signals are transmitted across the membrane, we asked whether the multiplicity of signalling pathways in the growth cone is reflected by the diversity of G proteins found in this organelle. Our immunohistochemical analysis indicated that growth cones of differentiated PC12 cells contain at least 4 alpha G protein subunits, 3 that are pertussis toxin substrates (alpha o, alpha i-1, alpha i-2) and 1 that is not (alpha q). In addition to localization in the neurites and growth cones, alpha o, alpha i-1, alpha i-2, and alpha q were detected in intracellular perinuclear structures. We also analyzed the temporal change in G proteins in PC12 cells differentiated by treatment with nerve growth factor (NGF). Time course experiments have shown that alpha o and beta proteins coordinately increase after 2 days of treatment with NGF, reach a maximum at 4 days, and remain elevated. In contrast to alpha o, alpha i-2 reached a peak at 4 days, then declined to almost the basal level by day 7 of treatment with NGF. These data indicated that the levels of alpha o, alpha i-2, and beta are differentially regulated during NGF-induced neuronal differentiation in PC12 cells. The alpha o protein was highly concentrated at the tips of the growth cones before the cellular level of alpha o had increased appreciably, suggesting that the alpha subunits are translocated during the first stage of neurite development. In addition, not every neural process has the same high level of alpha o, suggesting that G proteins may help define the specialized functions of particular neurites within a single cell.

Specificity of G protein beta and gamma subunit interactions

Multiple heterotrimeric guanine nucleotide binding protein (G protein) subunits have evolved to couple a large variety of receptors to intracellular effectors. G protein beta gamma subunits are essential for efficient coupling of alpha subunits to receptors, and they are also important for modulation of effectors. Several different beta and gamma subunits exist, but it is not known whether all possible combinations of beta and gamma can form functional dimers. To answer this question, we have compared the ability of in vitro translated beta 1, beta 2, and beta 3 to form dimers with either gamma 1 or gamma 2. Dimerization was monitored by gel filtration, resistance to tryptic digestion, and chemical cross-linking. The results indicate that beta 1 binds both gamma subunits, beta 2 binds only gamma 2, and beta 3 will bind neither gamma 1 or gamma 2. Hence, the occurrence of beta gamma dimers may be partially regulated by the ability of the subunits to associate. Specificity of dimerization might allow cells to co-express multiple beta and gamma subunits while maintaining efficient and specific signal transduction.

Promotion of the GTP-liganded state of the Go alpha protein by deletion of the C terminus

G proteins are active as long as GTP is bound to the alpha subunit. Activation ends when GTP is cleaved to GDP that then stays bound to the active site. Agonist-liganded receptors allow formation of the active state by decreasing the affinity of alpha subunits for GDP allowing exchange of GDP for GTP. Since receptors interact with the C terminus of the alpha subunits, we tested whether deletion of the C terminus could mimic activation by receptors. Three deletions and one point mutation at the C terminus of alpha o were engineered in alpha o cDNA by the polymerase chain reaction, transcribed into RNA, and translated in a rabbit reticulocyte lysate. The ability of in vitro synthesized protein to bind guanine nucleotide was inferred from analysis of native tryptic cleavage patterns, while the ability of the proteins to associate with beta gamma was measured by sucrose density gradient centrifugation. Deletion of 14 amino acids, alpha oD[341], from the C terminus causes a large decrease in GDP affinity, with little or no change in guanosine 5'-3-O-(thio)triphosphate affinity. When GTP is present, alpha oD[341] remains in the activated conformation because exchange of GTP for GDP is rapid. Deletion of 10 amino acids, alpha oD[345], lowers GDP affinity, but less dramatically than in alpha oD[341]. Deletion of 5 amino acids, alpha oD[350], or mutation of Arg-349 to proline alpha oR[349P] has no detectable effects on GDP affinity. Deletion of up to 10 amino acids from the C terminus does not prevent formation of alpha beta gamma heterotrimers. We propose that the C terminus of the alpha subunit is a mobile region that blocks dissociation of GDP. Agonist-liganded receptors may move it aside to allow release of GDP, exchange for GTP, and activation of the alpha subunit.

Mutagenesis of the amino terminus of the alpha subunit of the G protein Go. In vitro characterization of alpha o beta gamma interactions

Heterotrimeric guanine nucleotide-binding proteins are composed of alpha and beta gamma subunits and couple a variety of cell-surface receptors to intracellular enzymes or ion channels. The heterotrimer dissociates into alpha and beta gamma subunits when the alpha subunit is activated by guanine nucleoside triphosphates. Several lines of evidence show that the amino terminus of the alpha subunit is important for the interaction with the beta gamma subunit (Neer, E. J., Pulsifer, L., and Wolf, L. G. (1988) J. Biol. Chem. 263, 8996-9000; Fung, B. K.-K., and Nash, C. R. (1983) J. Biol. Chem. 258, 10503-10510). We have mutagenized the amino terminus of alpha o to dissect the relative contributions of amino-terminal myristoylation and specific amino acid sequences to subunit interaction. Wild-type and mutant alpha o cDNAs were translated in vitro in a rabbit reticulocyte lysate. All proteins were able to bind guanosine 5'-(gamma-thio)triphosphate and to achieve the necessary conformation for protection from tryptic digestion. Two assays of alpha o beta gamma interactions were used: sucrose density gradients to look for stable heterotrimer formation and ADP-ribosylation by pertussis toxin to detect weak or transient alpha o beta gamma interactions. Our results indicate that myristoylation is essential for stable heterotrimer formation, but that nonmyristoylated proteins are also capable of interacting with the beta gamma subunit. Amino acids 7-10 have an important role in alpha o beta gamma interactions whether alpha o is myristoylated or not. Deletion of this region diminishes the ability of alpha o to interact with the beta gamma subunit, but substitutions at this position indicate that other amino acids can be tolerated without affecting subunit interaction.

Regional localization of the human G protein alpha i2 (GNAI2) gene: assignment to 3p21 and a related sequence (GNAI2L) to 12p12-p13

Gi alpha proteins, members of the G protein signal transduction family, include a small number of polypeptides: Gi alpha 1 (GNAI1), Gi alpha 2 (GNAI2), and Gi alpha 3 (GNAI3). A cDNA for the human GNAI2 gene has been isolated from a human T-cell library and is mapped by chromosomal in situ hybridization to the short arm of chromosome 3 at 3p21. A related sequence, GNAI2L, is mapped by in situ hybridization to the short arm of chromosome 12 at p12-p13. These mapping results are further supported by amplification of GNAI2-specific sequences in a monochromosomal human/rodent somatic cell hybrid containing only human chromosome 3. Of note, these assignments are to chromosome regions in which other G proteins reside. Localization of GNAI2 to 3p21 is of great interest as this region of the short arm of chromosome 3 is frequently involved in rearrangements in various human tumors.

Expression of a G protein subunit, alpha i-1, in Balb/c 3T3 cells leads to agonist-specific changes in growth regulation

Cellular receptors for many hormones, neurotransmitters, and growth factors are coupled to intracellular effector enzymes or ion channels through a set of heterotrimeric G proteins. In order to determine whether isoforms of G protein alpha subunits contribute differentially to mitogenic responses, we introduced an alpha subunit isoform, alpha i-1, into Balb/c 3T3 cells that normally lack this subtype. Balb/c 3T3 cells transfected with a plasmid containing cDNA encoding alpha i-1 expressed the alpha i-1 protein as judged both by the appearance of immunoreactive alpha i-1 protein on Western blots and by two-dimensional analysis of the proteins [32P]ADP-ribosylated by pertussis toxin. The amount of alpha i-1 expressed is less than the amount of alpha subunits endogenously present in these cells. Expression of alpha i-1 in the transfected cells slightly blunts stimulation of adenylylcyclase by GTP, guanosine 5'-3-O-(thio)triphosphate, or forskolin, but has no major effect on the ability of thrombin to inhibit the enzyme. In contrast, the expression of alpha i-1 has significant effects on cell growth and on the mitogenic response to thrombin. The alpha i-1-transfected cells have a doubling time that is twice as long as control cells transfected with the same plasmid without a cDNA insert. Despite their slower growth, thymidine incorporation in response to thrombin is greater in transfected than in control cells. Thrombin-stimulated DNA synthesis is sensitive to inhibition by pertussis toxin and is 5-fold more sensitive to inhibition by pertussis toxin in transfected cells than in control cells. The changes are receptor-specific since the mitogenic response to platelet-derived growth factor is indistinguishable between control and transfected cells. These studies suggest that the alpha i subunit composition of the cell may have profound effects on its growth and its response to stimulation through a specific cell surface receptor.

Characterization of a mastoparan-stimulated nucleotidase from bovine brain

Mastoparan is a 14-amino-acid peptide that stimulates secretion from several cell types. Secretion can be partially blocked by pertussis toxin and may be mediated by guanine-nucleotide-binding proteins (G-proteins). Mastoparan can act directly on G-proteins, probably at the hormone receptor-binding site, to stimulate guanosine 5'-[gamma-thio]triphosphate binding and GTPase activities of pertussis-toxin substrates Go and Gi [Higashijima, Uzu, Nakajima & Ross (1988) J. Biol. Chem. 263, 6491-6494]. We now describe a nucleotidase from bovine brain that is not a known G-protein whose GTPase and ATPase activities are stimulated by mastoparan. This nucleotidase hydrolyses ATP faster than GTP, but has similar affinities for both (0.4 microM). Mastoparan maximally stimulates both ATPase and GTPase activities by about 8-fold after insertion of the protein into phospholipid vesicles, but does not affect the EC50 (concentration at which half the maximal effect is observed) for ATP and GTP. The EC50 for mastoparan stimulation of GTPase and ATPase is 6 and 12 microM respectively. The native molecular mass of the partially purified mastoparan-stimulated nucleotidase is 87 kDa. This nucleotidase may be another receptor-activated enzyme, and its identification may be useful for understanding mastoparan-stimulated processes.

Embryonic stem cells lacking a functional inhibitory G-protein subunit (alpha i2) produced by gene targeting of both alleles

The alpha i2 subunit of the inhibitory heterotrimeric guanine nucleotide-binding proteins is highly conserved in mammals and is expressed in all cell types, but its exact function is not yet defined. We have investigated the role of this protein by producing embryonic stem (ES) cells lacking a functional alpha i2 gene. These alpha i2-null cell lines regulate adenylyl cyclase and grow and differentiate in vitro the same as wild-type ES cells. Homologous recombination was used to sequentially inactivate both copies of the alpha i2 gene. The first allele was inactivated by insertion of a neomycin-resistance gene. We modified the hygromycin B-resistance gene for improved expression in ES cells and used this gene to inactivate the remaining normal allele. The techniques used should be generally applicable to other genes whether or not they are expressed in ES cells.

Identification of a retinal protein in Drosophila with antibody to the alpha subunit of bovine brain G(o) protein

An antibody directed against the alpha(o) subunit of bovine brain G(o) (R4) was used to identify a Drosophila retinal protein which may be the analogue of vertebrate transducin. The immunoreactivity appears predominantly in the retinal and occellar rhabdomeres. On a Western blot, the antibody recognizes a 41 kDa protein that is present in the heads of yellow white flies, but not in the heads of eyeless mutant flies, eyes absent. This protein is not recognized by an antibody raised against Drosophila alpha(o). Antibody R4 intensely stains rhabdomeres and, to a lesser extent, the neuropil of the central nervous system in tissue sections of adult flies. Antibody to Drosophila alpha(o) stains the neuropil of the central nervous system, but does not stain rhabdomeres. In developing flies, faint immunoreactivity appears in the retinal rhabdomeres at about 70% of the time through pupal development and increases to its apparent adult maximal level about 1 day after eclosion. Tissue sections from a phototransduction mutant, norp A, have retinal immunoreactivity at normal levels up to about 1 week after eclosion, but by 2 weeks, immunoreactivity has largely disappeared. This disappearance parallels the degeneration of the retina in norp A mutants. In Drosophila and other invertebrates, light activates a phospholipase C in the retina. The identification of a protein in Drosophila rhabdomeres with an antibody raised against a mammalian G protein alpha subunit thought to be involved in phospholipase C activation suggests that there may be common structural features between the putative Drosophila transducin and alpha(o). The identification of regions common to mammalian alpha(o) and Drosophila transducin may then provide clues to the structural requirements for PLC activation.

In vitro synthesis of G protein beta gamma dimers

The guanine nucleotide-binding proteins (G proteins), which play a central role in coupling membrane-bound receptors to intracellular effectors, are heterotrimers composed of alpha, beta, and gamma subunits. The beta and gamma subunits form a functional monomer that does not appear to separate under physiological conditions. This has made it difficult to differentiate the individual roles of beta and gamma subunits in signal transduction. To characterize the individual subunits, the 36-kDa beta subunit (beta 1), brain gamma (gamma 2), and transducin gamma (gamma t) were translated in vitro in a rabbit reticulocyte lysate system. Hydrodynamic studies and tryptic proteolysis were used to compare the physical properties of the in vitro translation products with those of beta gamma dimers purified from bovine brain. The hydrodynamic studies indicate that, without gamma subunits, the beta subunits are not stable but tend to aggregate into high molecular weight complexes. When beta and gamma subunits were co-translated, stable beta gamma dimers formed that bound alpha 0 in a guanine nucleotide-dependent manner. The beta gamma dimers were less hydrophobic than those purified from bovine brain. This may reflect a lack of post-translational modification in the reticulocyte lysate or other differences between the in vitro translation products and the purified beta gamma. When beta and gamma were translated separately and then mixed, beta gamma dimers also formed. Analysis of in vitro translated beta gamma subunits will provide ways to assess the function of these subunits and to determine the structural requirements for beta gamma formation.

Structural and functional studies of cross-linked Go protein subunits

The guanine nucleotide binding proteins (G proteins) that couple hormone and other receptors to a variety of intracellular effector enzymes and ion channels are heterotrimers of alpha, beta, and gamma subunits. One way to study the interfaces between subunits is to analyze the consequences of chemically cross-linking them. We have used 1,6-bismaleimidohexane (BMH), a homobifunctional cross-linking reagent that reacts with sulfhydryl groups, to cross-link alpha to beta subunits of Go and Gi-1. Two cross-linked products are formed from each G protein with apparent molecular masses of 140 and 122 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both bands formed from Go reacted with anti-alpha o and anti-beta antibody. The mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis is anomalous since the undenatured, cross-linked proteins have the same Stokes radius as the native, uncross-linked alpha beta gamma heterotrimer. Therefore, each cross-linked product contains one alpha and one beta subunit. Activation of Go by guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) does not prevent cross-linking of alpha to beta gamma, consistent with an equilibrium between associated and dissociated subunits even in the presence of GTP gamma S. The same cross-linked products of Go are formed in brain membranes reacted with BMH as are formed in solution, indicating that the residues cross-linked by BMH in the pure protein are accessible when Go is membrane bound. Analysis of tryptic peptides formed from the cross-linked products indicates that the alpha subunit is cross-linked to the 26-kDa carboxyl-terminal portion of the beta subunit. The cross-linked G protein is functional, and its alpha subunit can change conformation upon binding GTP gamma S. GTP gamma S stabilizes alpha o to digestion by trypsin (Winslow, J.W., Van Amsterdam, J.R., and Neer, E.J. (1986) J. Biol. Chem. 261, 7571-7579) and also stabilizes the alpha subunit in the cross-linked product. Cross-linked G o can be ADP-ribosylated by pertussis toxin. This ADP-ribosylation is inhibited by GTP gamma S with a concentration dependence that is indistinguishable from that of the control, uncross-linked G o. These two kinds of experiments indicate that alpha o is able to change its conformation even though it cannot separate completely from beta gamma. Thus, although dissociation of the subunits accompanies activation of G o in solution, it is not obligatory for a conformational change to occur in the alpha subunit.

GO associates with another 40 kDa brain protein

Guanine nucleotide binding proteins (G proteins) mediate a variety of cellular responses to external stimuli. Pure G protein, receptor, and effector are sufficient to reconstitute hormonal activation of an effector in phospholipid vesicles, but other components may be important for specificity or localization in vivo. If another protein associates with GO, the molecular weight of GO solubilized from membranes would be larger than the molecular weight of GO after purification. We find that GO solubilized from bovine brain membranes by Triton X-100 behaves as a single population of molecules on sucrose density gradients and gel filtration columns. Its molecular mass is about 40 kDa larger than pure GO. Association of GO with the other protein is fragile as the proteins dissociate on further purification. There was no difference in ADP-ribosylation or tryptic cleavage of GO in larger and smaller form. These studies provide a basis for future experiments to stabilize the interaction and identify the protein.

G0 is a major growth cone protein subject to regulation by GAP-43

G0, a GTP-binding protein that transduces information from transmembrane receptors, has been found to be a major component of the neuronal growth cone membrane. GAP-43, an intracellular growth cone protein closely associated with neuronal growth, stimulates GTP-gamma-S binding to G0. It does so through an amino-terminal domain homologous to G-linked transmembrane receptors. Thus, G0 in the growth cone may be regulated by intracellular as well as extracellular signals.