Bovine brain GO isoforms have distinct gamma subunit compositions

G protein gamma subunit, a hidden master regulator of GPCR signaling

Heterotrimeric G proteins (αβγ subunits) that are activated by G protein-coupled receptors (GPCRs) mediate the biological responses of eukaryotic cells to extracellular signals. The α subunits and the tightly bound βγ subunit complex of G proteins have been extensively studied and shown to control the activity of effector molecules. In contrast, the potential roles of the large family of γ subunits have been less studied. In this review, we focus on present knowledge about these proteins. Induced loss of individual γ subunit types in animal and plant models result in strikingly distinct phenotypes indicating that γ subtypes play important and specific roles. Consistent with these findings, downregulation or upregulation of particular γ subunit types result in various types of cancers. Clues about the mechanistic basis of γ subunit function have emerged from imaging the dynamic behavior of G protein subunits in living cells. This shows that in the basal state, G proteins are not constrained to the plasma membrane but shuttle between membranes and on receptor activation βγ complexes translocate reversibly to internal membranes. The translocation kinetics of βγ complexes varies widely and is determined by the membrane affinity of the associated γ subtype. On translocating, some βγ complexes act on effectors in internal membranes. The variation in translocation kinetics determines differential sensitivity and adaptation of cells to external signals. Membrane affinity of γ subunits is thus a parsimonious and elegant mechanism that controls information flow to internal cell membranes while modulating signaling responses.

Preferential assembly of G-αβγ complexes directed by the γ subunits

Assembly of the G-αβγ heterotrimer is required for receptor signaling. Although much has been learned about the assembly process itself, the identities of the G-αβγ combinations that actually exist in physiological setting are largely unknown. Moreover, there is uncertainty regarding whether the individual subunits associate by a random process, or combine by a regulated process to form quasi-stable G-αβγ complexes. In this chapter, we will focus on emerging genetic -evidence that supports the latter model. Specifically, we will discuss how use of gene targeted mice has revealed preferential assembly of the striatal-specific Gα(olf)β(2)γ(7) complex occurs by a sequential process that is directed by the γ(7) subunit. The existence of specific G-αβγ complexes responsible for transducing the signals from different receptors may have profound implications by providing a possible explanation for biased agonism.

Conventional and novel Gγ protein families constitute the heterotrimeric G-protein signaling network in soybean

Heterotrimeric G-proteins comprised of Gα, Gβ and Gγ proteins are important signal transducers in all eukaryotes. The Gγ protein of the G-protein heterotrimer is crucial for its proper targeting at the plasma membrane and correct functioning. Gγ proteins are significantly smaller and more diverse than the Gα and Gβ proteins. In model plants Arabidopsis and rice that have a single Gα and Gβ protein, the presence of two canonical Gγ proteins provide some diversity to the possible heterotrimeric combinations. Our recent analysis of the latest version of the soybean genome has identified ten Gγ proteins which belong to three distinct families based on their C-termini. We amplified the full length cDNAs, analyzed their detailed expression profile by quantitative PCR, assessed their localization and performed yeast-based interaction analysis to evaluate interaction specificity with different Gβ proteins. Our results show that ten Gγ genes are retained in the soybean genome and have interesting expression profiles across different developmental stages. Six of the newly identified proteins belong to two plant-specific Gγ protein families. Yeast-based interaction analyses predict some degree of interaction specificity between different Gβ and Gγ proteins. This research thus identifies a highly diverse G-protein network from a plant species. Homologs of these novel proteins have been previously identified as QTLs for grain size and yield in rice.

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

Proteomic analysis of bovine brain G protein gamma subunit processing heterogeneity

We characterized the variable processing of the G protein gamma subunit isoforms associated with bovine brain G proteins, a primary mediator of cellular communication. Ggamma subunits were isolated from purified brain G proteins and characterized by Edman sequencing, by MALDI MS, by chemical and/or enzymatic fragmentation assayed by MALDI MS, and by MS/MS fragmentation and sequencing. Multiple forms of six different Ggamma isoforms were detected. Significant variation in processing was found at both the amino termini and particularly the carboxyl termini of the proteins. All Ggamma isoforms contain a carboxyl-terminal CAAX motif for prenylation, carboxyl-terminal proteolysis, and carboxymethylation. Characterization of these proteins indicates significant variability in the normal processing of all of these steps in the prenylation reaction, including a new variation of prenyl processing resulting from cysteinylation of the carboxyl terminus. These results have multiple implications for intracellular signaling mechanisms by G proteins, for the role of prenyl processing variation in cell signaling, and for the site of action and consequences of drugs that target the prenylation modification.

Functional genomic dissection of multimeric protein families in zebrafish

The study of multimeric protein function in the postgenomicera has become complicated by the discovery of multiple isoforms for each subunit of those proteins. A correspondingly large number of potential isoform combinations offer the multicellular organism a constellation of protein assemblies from which to generate a variety of functions across different cells, tissues, and organs. At the same time, the multiplicity of potential subunit isoform combinations presents a significant challenge when attempting to dissect the functions of particular isoform combinations. Biochemical and cell culture methods have brought us to a significant state of understanding of multimeric proteins but are unable to answer questions of function within the context of the many tissues and developmental stages of the multicellular organism. Answering those questions can be greatly facilitated in model systems in which expression can be determined over time, in the context of the whole organism, and in which hypomorphic function of each subunit can be studied individually and in combination. Fortunately, the potential for high-throughput in situ hybridization studies and antisense-based reverse genetic knockdowns in zebrafish offers exciting opportunities to meet this challenge. Some of these opportunities, along with cautions of interpretation and gaps in the existing technologies, are discussed in the context of ongoing investigations of the dimeric Na,K-ATPases and heterotrimeric G proteins.

Translating G protein subunit diversity into functional specificity

Historically, it has been assumed that the functional roles of G proteins in receptor recognition and effector regulation are specified by their diverse alpha subunits. However, the discovery of similarly diverse betagamma subunits that participate in both of these functional processes has called this assumption into question; recent work suggests that G proteins function as heterotrimers whose roles in particular receptor signaling pathways are determined by their specific alphabetagamma subunit combinations. Although much remains to be learned, the assembly of specific alphabetagamma subunit combinations seems to involve both structural and spatial factors.

Expression of novel splice variants of the G protein subunit, Go alpha, is tissue-specific and age-dependent in the rat

Heterotrimeric G proteins play an essential role in coupling numerous surface membrane receptors to intracellular signal transduction pathways. Relatively little is known about the splice variants of G proteins, including whether they undergo differential expression as a function of aging. We screened for splice variants of the alpha subunit of the dominant inhibitory G protein family member G(o) in a variety of tissues from rat and examined the expression of these splice variants during development. The splice variants were characterized using specific primers for G(o)alpha1 and G(o)alpha2 in conjunction with reverse transcription-polymerase chain reaction, and subsequently sequenced. G(o)alpha1 expression dominated over G(o)alpha2 in all neuronal tissues screened, including cerebral cortex, pituitary, spinal cord, colon myenteric plexus, dorsal root ganglion, and prenatal cortex. The sequence data of G(o)alpha1 supports the presence of three splice variants: G(o)alpha1a, G(o)alpha1b, G(o)alpha1c. The G(o)alpha1a variant was reported previously [J. Biol. Chem. 262 (1987) 14241], whereas G(o)alpha1b and G(o)alpha1c represent novel variants. The G(o)alpha1b splice variant demonstrates a 94 bp deletion using a cryptic donor site in exon 10. The G(o)alpha1c variant demonstrates a complete deletion of exon 10. A protein product with a molecular weight of approximately 34 kDa consistent with that expected for G(o)alpha1c was identified using Western blot analysis and two-dimensional gel electrophoresis. The expression of G(o)alpha1a decreased postnatally, supporting a potential physiological role during fetal development, whereas G(o)alpha1c expression increased postnatally. The age-dependent and tissue-specific expression of the G(o)alpha1 splice variants presage a broader functional role than has been observed historically with G(o).

Lymphocyte trafficking through the blood-brain barrier is dependent on endothelial cell heterotrimeric G-protein signaling

We have previously shown that the engagement of ICAM-1 on brain endothelial cells (EC) results in the propagation of EC signaling pathways that are necessary for efficient lymphocyte migration across the tight vascular barriers of the brain. Signaling via this receptor alone, however, is unlikely to explain the differential recruitment of leukocytes at different vascular beds. In this study, we investigated the role of EC heterotrimeric G-protein-mediated signaling in supporting transendothelial migration of T lymphocytes. Treatment of brain EC monolayers with pertussis toxin (PTX) resulted in ADP-ribosylation of G-protein alpha subunits and inhibition (>80%) of lymphocyte migration without affecting lymphocyte adhesion. Aortic and high endothelial venule EC treated identically resulted in only partial inhibition of lymphocyte migration (<40%). Expression of ribosylation-resistant (PTX-insensitive) G-protein alpha subunits in brain EC restored their ability to support lymphocyte migration after pretreatment with PTX. Treatment of brain EC with PTX did not inhibit ICAM-1-stimulated tyrosine phosphorylation of focal adhesion kinase, suggesting the effects of PTX in inhibiting EC facilitation of lymphocyte migration are distinct from activation of EC through ICAM-1. We conclude that a heterotrimeric G-protein-mediated signaling pathway in brain EC is essential for efficient transendothelial migration of T lymphocytes into the brain.

Gbeta gamma isoforms selectively rescue plasma membrane localization and palmitoylation of mutant Galphas and Galphaq

Mutation of Galpha(q) or Galpha(s) N-terminal contact sites for Gbetagamma resulted in alpha subunits that failed to localize at the plasma membrane or undergo palmitoylation when expressed in HEK293 cells. We now show that overexpression of specific betagamma subunits can recover plasma membrane localization and palmitoylation of the betagamma-binding-deficient mutants of alpha(s) or alpha(q). Thus, the betagamma-binding-defective alpha is completely dependent on co-expression of exogenous betagamma for proper membrane localization. In this report, we examined the ability of beta(1-5) in combination with gamma(2) or gamma(3) to promote proper localization and palmitoylation of mutant alpha(s) or alpha(q). Immunofluorescence localization, cellular fractionation, and palmitate labeling revealed distinct subtype-specific differences in betagamma interactions with alpha subunits. These studies demonstrate that 1) alpha and betagamma reciprocally promote the plasma membrane targeting of the other subunit; 2) beta(5), when co-expressed with gamma(2) or gamma(3), fails to localize to the plasma membrane or promote plasma membrane localization of mutant alpha(s) or alpha(q); 3) beta(3) is deficient in promoting plasma membrane localization of mutant alpha(s) and alpha(q), whereas beta(4) is deficient in promoting plasma membrane localization of mutant alpha(q); 4) both palmitoylation and interactions with betagamma are required for plasma membrane localization of alpha.

The G protein beta subunit is a determinant in the coupling of Gs to the beta 1-adrenergic and A2a adenosine receptors

The signaling specificity of five purified G protein betagamma dimers, beta(1)gamma(2), beta(2)gamma(2), beta(3)gamma(2), beta(4)gamma(2), and beta(5)gamma(2), was explored by reconstituting them with G(s) alpha and receptors or effectors in the adenylyl cyclase cascade. The ability of the five betagamma dimers to support receptor-alpha-betagamma interactions was examined using membranes expressing the beta(1)-adrenergic or A2a adenosine receptors. These receptors discriminated among the defined heterotrimers based solely on the beta isoform. The beta(4)gamma(2) dimer demonstrated the highest coupling efficiency to either receptor. The beta(5)gamma(2) dimer coupled poorly to each receptor, with EC(50) values 40-200-fold higher than those observed with beta(4)gamma(2). Strikingly, whereas the EC(50) of the beta(1)gamma(2) dimer at the beta(1)-adrenergic receptor was similar to beta(4)gamma(2), its EC(50) was 20-fold higher at the A2a adenosine receptor. Inhibition of adenylyl cyclase type I (AC1) and stimulation of type II (AC2) by the betagamma dimers were measured. betagamma dimers containing Gbeta(1-4) were able to stimulate AC2 similarly, and beta(5)gamma(2) was much less potent. beta(1)gamma(2), beta(2)gamma(2), and beta(4)gamma(2) inhibited AC1 equally; beta(3)gamma(2) was 10-fold less effective, and beta(5)gamma(2) had no effect. These data argue that the beta isoform in the betagamma dimer can determine the specificity of signaling at both receptors and effectors.

Selective coupling of G protein beta gamma complexes to inhibition of Ca2+ channels

Several mechanisms couple heterotrimeric guanine nucleotide-binding proteins (G proteins) to cellular effectors. Although alpha subunits of G proteins (Galpha) were the first recognized mediators of receptor-effector coupling, Gbetagamma regulation of effectors is now well known. Five Gbeta and 12 Ggamma subunit genes have been identified, suggesting through their diversity that specific subunits couple selectively to effectors. The molecular determinants of Gbetagamma-effector coupling, however, are not well understood, and most studies of G protein-effector coupling do not support selectivity of Gbetagamma action. To explore this issue further, we have introduced recombinant Gbetagamma complexes into avian sensory neurons and measured the inhibition of Ca(2+) currents mediated by an endogenous phospholipase Cbeta- (PLCbeta) and protein kinase C-dependent pathway. Activities of Gbetagamma in the native cells were compared with enzyme assays performed in vitro. We report a surprising selective activation of the PLCbeta pathway by Gbetagamma complexes containing beta(1) subunits, whereas beta(2)-containing complexes produced no activation. In contrast, when assayed in vitro, PLCbeta and type II adenylyl cyclase did not discriminate among these same Gbetagamma complexes, suggesting the possibility that additional cellular determinants confer specificity in vivo.

The relationship of G(o)alpha subunit deamidation to the tissue distribution, nucleotide binding properties, and betagamma dimer interactions of G(o)alpha subunit isoforms

The distribution and properties in brain of the alpha subunits of the major bovine brain Go isoforms, GoA, GoB and GoC, were characterized. The alpha(o)A and alpha(o)B isoforms arise from alternative splicing of RNAs from a single alpha(o) gene, whereas alpha(o)C is a deamidated form of alpha(o)A. All three Go isoforms purify from brain with different populations of betagamma dimers. This variable subunit composition of Go heterotrimers is likely a consequence of their functional differences. This study examined the biochemical properties of the alpha(o) isoforms to see if these properties explain the variable betagamma composition of their heterotrimers. The brain distribution of alpha(o)B differed substantially from that of alpha(o)A and alpha(o)C, as did its guanine nucleotide binding properties. The unique subunit composition of GoB can be explained by its expression in different brain regions. The alpha(o)A and alpha(o)C showed slight differences in guanine nucleotide binding properties but no preference for particular betagamma dimers when reassociated with a heterogeneous betagamma pool. The alpha(o)C protein occurred in a constant ratio to alpha(o)A throughout the brain, but was a much larger percent of total brain alpha(o) than previously thought, approximately 35%. These results suggest that alpha(o)A is a precursor of alpha(o)C and that the association of G(o)alpha subunits with different betagamma dimers reflects the function of an adaptive, G-protein signaling mechanism in brain.

Stable association of G proteins with beta 2AR is independent of the state of receptor activation

beta 2-Adrenergic receptors expressed in Sf9 cells activate endogenous Gs and adenylyl cyclase [Mouillac B., Caron M., Bonin H., Dennis M. and Bouvier M. (1992) J. Biol. Chem. 267, 21733-21737]. However, high affinity agonist binding is not detectable under these conditions suggesting an improper stoichiometry between the receptor and the G protein and possibly the effector molecule as well. In this study we demonstrate that when beta 2-adrenergic receptors were co-expressed with various mammalian G protein subunits in Sf9 cells using recombinant baculoviruses signalling properties found in native receptor systems were reconstituted. For example, when beta 2AR was co-expressed with the Gs alpha subunit, maximal receptor-mediated adenylyl cyclase stimulation was greatly enhanced (60 +/- 9.0 versus 150 +/- 52 pmol cAMP/min/mg protein) and high affinity, GppNHp-sensitive, agonist binding was detected. When G beta gamma subunits were co-expressed with Gs alpha and the beta 2AR, receptor-stimulated GTPase activity was also demonstrated, in contrast to when the receptor was expressed alone, and this activity was higher than when beta 2AR was co-expressed with Gs alpha alone. Other properties of the receptor, including receptor desensitization and response to inverse agonists were unaltered. Using antisera against an epitope-tagged beta 2AR, both Gs alpha and beta gamma subunits could be co-immunoprecipitated with the beta 2AR under conditions where subunit dissociation would be expected given current models of G protein function. A desensitization-defective beta 2AR (S261, 262, 345, 346A) and a mutant which is constitutively desensitized (C341G) could also co-immunoprecipitate G protein subunits. These results will be discussed in terms of a revised view of G protein-mediated signalling which may help address issues of specificity in receptor/G protein coupling.

Identification and expression of Go1 and Go2 alpha-subunit transcripts in human myometrium in relation to pregnancy

The 39-kDa Goalpha protein, the alpha subunit of a major heterotrimeric G protein of brain and neuroendocrine cells, was found to be present in human myometrium. Using three different antisera, we showed its strong expression in myometrium from pregnant patients as compared to nonpregnant ones. This is in agreement with the high expression level of its two isoforms (alphao1 and alphao2), previously described in late pregnancy. To better ascertain the nature of these immunoreactive isoforms, we investigated transcripts of the Goalpha gene in myometrium from pregnant and nonpregnant patients by reverse transcription-polymerase chain reaction (RT-PCR). In this tissue, the amplified cDNA product of a region common to both Go1alpha and Go2alpha mRNA variants was recognized as the Goalpha nucleotide sequence. Transcripts of Go1alpha and Go2alpha were identified by sequencing. A partial cDNA Go2alpha sequence was described, which differed from the Goalpha gene by two nucleotides in exon 8B. Levels of Go1alpha and Go2alpha transcripts analyzed by semi-quantitative RT-PCR were significantly higher in myometrium from pregnant than from nonpregnant patients. It is suggested that Goalpha gene expression in this tissue may contribute to modifications seen in the signaling pathways observed at the end of pregnancy.

Activation of Go-proteins by membrane depolarization traced by in situ photoaffinity labeling of galphao-proteins with [alpha32P]GTP-azidoanilide

Evidence for depolarization-induced activation of G-proteins in membranes of rat brain synaptoneurosomes has been previously reported (Cohen-Armon, M., and Sokolovsky, M. (1991) J. Biol. Chem. 266, 2595-2605; Cohen-Armon, M., and Sokolovsky, M. (1993) J. Biol. Chem. 268, 9824-9838). In the present work we identify the activated G-proteins as Go-proteins by tracing their depolarization-induced in situ photoaffinity labeling with [alpha32P]GTP-azidoanilide (GTPAA). Labeled GTPAA was introduced into transiently permeabilized rat brain-stem synaptoneurosomes. The resealed synaptoneurosomes, while being UV-irradiated, were depolarized. Relative to synaptoneurosomes at resting potential, the covalent binding of [alpha32P]GTPAA to Galphao1- and Galphao3-proteins, but not to Galphao2- isoforms, was enhanced by 5- to 7-fold in depolarized synaptoneurosomes, thereby implying an accelerated exchange of GDP for [alpha32P]GTPAA. Their depolarization-induced photoaffinity labeling was independent of stimulation of Go-protein-coupled receptors and could be reversed by membrane repolarization, thus excluding induction by transmitters release. It was, however, dependent on depolarization-induced activation of the voltage-gated sodium channels (VGSC), regardless of Na+ current. The alpha subunit of VGSC was cross-linked and co-immunoprecipitated with Galphao-proteins in depolarized brain-stem and cortical synaptoneurosomes. VGSC alpha subunit most efficiently cross-linked with guanosine 5'-O-2-thiodiphosphate-bound rather than to guanosine 5'-O-(3-thiotriphosphate)-bound Galphao-proteins in isolated synaptoneurosomal membranes. These findings support a possible involvement of VGSC in depolarization-induced activation of Go-proteins.

Posttranslational modification of Galphao1 generates Galphao3, an abundant G protein in brain

Galphao, the most abundant G protein in mammalian brain, occurs at least in two subforms, i.e., Galphao1 and Galphao2, derived by alternative splicing of the mRNA. A third Galphao1-related isoform, Galphao3, has been purified, representing about 30% of total Go in brain. Initial studies revealed distinct biochemical properties of Galphao3 as compared with other Galphao isoforms. In matrix-assisted laser desorption/ionization peptide mass mapping of gel-isolated Galphao1 and Galphao3, C-terminal peptides showed a difference of +1 Da for Galphao3. Nanoelectrospray tandem mass spectrometry sequencing revealed an Asp instead of an Asn at position 346 of Galphao3. Gel electrophoretic analysis of recombinant Galphao3 showed the same mobility as native Galphao3 but distinct to Galphao1. The conversion of 346Asn-->Asp changed the signaling properties, including the velocity of the basal guanine nucleotide-exchange reaction, which points to the involvement of the C terminus in basal guanosine 5'-[gamma-thio]triphosphate binding. No cDNA coding for Galphao3 was detected, suggesting an enzymatic deamidation of Galphao1 by a yet-unidentified activity. Therefore, Galpha heterogeneity is generated not only at the DNA or RNA levels, but also at the protein level. The relative amount of Galphao1 and Galphao3 differed from cell type to cell type, indicating an additional principle of G protein regulation.

Characterization of the major bovine brain Go alpha isoforms. Mapping the structural differences between the alpha subunit isoforms identifies a variable region of the protein involved in receptor interactions

Go is the major G protein in bovine brain, with at least three isoforms, GoA, GoB, and GoC. Whereas alphaoA and alphaoB arise from a single Goalpha gene as alternatively spliced mRNAs, alphaoA and alphaoC are thought to differ by covalent modification. To test the hypothesis that alphaoA and alphaoC have different N-terminal lipid modifications, proteolytic fragments of alphao isoforms were immunoprecipitated with an N terminus-specific antibody and analyzed by matrix-assisted laser desorption ionization mass spectrometry. The major masses observed in immunoprecipitates were the same for all three alphao isoforms and corresponded to the predicted mass of a myristoylated N-terminal fragment. Structural differences between alphaoA and alphaoC were also compared before and after limited tryptic proteolysis using SDS-polyacrylamide gel electrophoresis containing 6 M urea. Based upon the alphao subunit fragments produced under activating and nonactivating conditions, differences between alphaoA and alphaoC were localized to a C-terminal fragment of the protein. This region, involved in receptor and effector interactions, implies divergent signaling roles for these two alphao proteins. Finally, the structural difference between alphaoA and alphaoC is associated with a difference of at most 2 daltons based upon measurements by electrospay ionization mass spectrometry.

A major G protein alpha O isoform in bovine brain is deamidated at Asn346 and Asn347, residues involved in receptor coupling

The structural differences between two major forms of the alpha subunit of the heterotrimeric G protein GO were found to be due to deamidation of either of two Asn residues near the C-terminus of the proteins, in a region involved in receptor recognition. GO is the most abundant heterotrimeric G protein in mammalian brain. Two forms of the protein, GOA and GOB, are known to be generated by alternative splicing of a single GOalpha gene. A third isoform, alphaOC, represents about 1/3 of the alphaO protein in brain and is related to alphaOA, from which it is thought to be generated by protein modification. Mass spectrometry and chemical derivatization of tryptic fragments of the proteins were used to localize the structural difference between alphaOA and alphaOC to a C-terminal peptide. Sequence analysis of a C-terminal chymotryptic fragment both by ion trap mass spectrometry and by Edman degradation identified Asn346 and Asn347 of alphaOA as alternative deamidation sites in alphaOC. These structural differences have immediate implications for G protein function, as they occur in a conformationally sensitive part of the protein involved in receptor recognition and activation. Since Asn347 is a conserved residue present in most G protein alpha subunits outside the alphas family, these observations may have general significance for many G proteins. Deamidation may be a component of a novel process for modifying or adapting cellular responses mediated by G proteins.

Heterogeneous processing of a G protein gamma subunit at a site critical for protein and membrane interactions

The G protein gamma5 subunit is selectively associated with specific G protein alpha subunits [Wilcox, M. D., et al. (1995) J. Biol. Chem. 270, 4189] and is localized preferentially in focal adhesion plaques [Hansen, C. A., et al. (1996) J. Cell Biol. 126, 811]. What determines the differential association of G proteins and their subunits with specific cellular structures or compartments is not clear, but one factor could be variation in the pattern of processing of the proteins. To study gamma5 subunit diversity and modifications, G protein subunits were fractionated on an HPLC phenyl column and analyzed with a gamma5-specific antiserum. The gamma5 eluted from the column as two peaks of immunoreactivity. Analysis by matrix-assisted laser desorption ionization (MALDI) mass spectrometry and electrospray ionization tandem mass spectrometry revealed that the first immunoreactive peak corresponded to the predicted gamma5 isoform (N-terminally acetylated after removal of methionine, C-terminally geranylgeranylated and carboxymethylated with removal of the last three amino acids), while the second peak of immunoreactivity contained a gamma5 isoform isoprenylated at the C-terminus but retaining its three terminal amino acids. This alternatively processed protein is the predominant gamma5 subunit isoform associated with Go and Gi proteins purified from bovine brain. These results describe a new C-terminal processing pattern for G protein gamma subunits and establish the principle that G protein gamma subunits can be heterogeneously modified at their C-termini. This is a site on the gamma subunit critical for membrane and protein-protein interactions of G proteins. These results open the possibility that one determinant of the localization of G proteins in cells could be the pattern of processing of their gamma subunit constituents.

The G-protein betagamma complex

The vast majority of signalling pathways in mammalian cells are mediated by heterotrimeric (alpha betagamma) G proteins. Reviewed here is regulation of signal transduction by the betagamma complex at different protein interfaces: subunit-subunit, receptor-G protein and G protein-effector. The role of diverse beta and gamma subunit types in achieving specificity in signalling and potentially unidentified functions for these subunits also are discussed.

Multiple neurological abnormalities in mice deficient in the G protein Go

The G protein Go is highly expressed in neurons and mediates effects of a group of rhodopsin-like receptors that includes the opioid, alpha2-adrenergic, M2 muscarinic, and somatostatin receptors. In vitro, Go is also activated by growth cone-associated protein of Mr 43,000 (GAP43) and the Alzheimer amyloid precursor protein, but it is not known whether this occurs in intact cells. To learn about the roles that Go may play in intact cells and whole body homeostasis, we disrupted the gene encoding the alpha subunits of Go in embryonic stem cells and derived Go-deficient mice. Mice with a disrupted alphao gene (alphao-/- mice) lived but had an average half-life of only about 7 weeks. No Goalpha was detectable in homogenates of alphao-/- mice by ADP-ribosylation with pertussis toxin. At the cellular level, inhibition of cardiac adenylyl cyclase by carbachol (50-55% at saturation) was unaffected, but inhibition of Ca2+ channel currents by opioid receptor agonist in dorsal root ganglion cells was decreased by 30%, and in 25% of the alphao-/- cells examined, the Ca2+ channel was activated at voltages that were 13.3 +/- 1.7 mV lower than in their counterparts. Loss of alphao was not accompanied by appearance of significant amounts of active free betagamma dimers (prepulse test). At the level of the living animal, Go-deficient mice are hyperalgesic (hot-plate test) and display a severe motor control impairment (falling from rotarods and 1-inch wide beams). In spite of this deficiency, alphao-/- mice are hyperactive and exhibit a turning behavior that has them running in circles for hours on end, both in cages and in open-field tests. Except for one, all alphao-/- mice turned only counterclockwise. These findings indicate that Go plays a major role in motor control, in motor behavior, and in pain perception and also predict involvement of Go in Ca2+ channel regulation by an unknown mechanism.

Ribozyme-mediated suppression of the G protein gamma7 subunit suggests a role in hormone regulation of adenylylcyclase activity

Human HEK 293 cells present a simple and tractable system to directly test the hypothesis that the G protein gamma subunits contribute to the specificity of receptor signaling pathways in vivo. To begin to elucidate the functions of the individual gamma subunits in these cells, a ribozyme strategy was used to specifically inactivate the mRNA encoding the gamma7 subunit. A phosphorothioated DNA-RNA chimeric hammerhead ribozyme was constructed and analyzed for specificity toward the targeted gamma7 subunit. In vitro cleavage analysis of this ribozyme revealed a highly efficient cleavage activity directed exclusively toward the gamma7 RNA transcript. In particular, this ribozyme did not result in cleavage of the gamma12 RNA transcript, which is 75% identical to the gamma7 RNA transcript. Using a transient transfection assay, in vivo analysis of this ribozyme showed a specific reduction in both the mRNA and protein expression of the gamma7 subunit in HEK 293 cells. Coincident with this loss in gamma7 subunit, there was a specific reduction in the protein expression of the beta1 subunit, suggesting that the beta1 and gamma7 subunits may functionally interact to form a betagamma dimer in vivo. Functional analysis of the consequences of ribozyme-mediated suppression of the gamma7 subunit expression indicated that it was associated with significant attenuation of isoproterenol-, but not prostaglandin E1-, stimulated adenylylcyclase activity. Suppression of the gamma7 subunit expression had no effect on carbachol- and ATP-mediated stimulation of phosphatidylinositol turnover. Taken together, these results not only indicate the feasibility of using the ribozyme technology to determine the roles of individual gamma subunits in receptor-G protein-effector pathways in vivo, but they point to a specific role of the gamma7 subunit in the regulation of adenylylcyclase activity in response to isoproterenol.

Role of subunit diversity in signaling by heterotrimeric G proteins

The heterotrimeric G proteins are extensively involved in the regulation of cells by extracellular signals. The receptors that control them are often the targets of drugs. There are many isoforms of each of the three subunits that make up these proteins. Thus far, genes for at least sixteen alpha subunits, five beta subunits, and eleven gamma subunits have been identified. In addition, some of these proteins have splice variants or are differentially modified. Based upon what is already known, there are well over a thousand possible G protein heterotrimer combinations. The role of subunit diversity in heterotrimer formation and its effect on signaling by G proteins are still not well understood. However, many current lines of research are leading toward an understanding of these roles. The functional significance of subunit heterogeneity is related to the mechanisms used by G proteins to transmit and integrate the many signals coming into cells through this system. Described here are the basic mechanisms by which G proteins integrate cellular responses, the possible role of subunit heterogeneity in these mechanisms, and the evidence for and against their physiological significance. Recent studies suggest the likely possibility that subunit heterogeneity plays an important role in signaling by G proteins. This role has the potential to extend substantially the flexibility of G proteins in mediating cellular responses to extracellular signals. However, the details of this are yet to be worked out, and they are the subject of many different avenues of research.

Characterization of a G-protein activator in the neuroblastoma-glioma cell hybrid NG108-15

Purified bovine brain G-protein was used in a solution phase assay to identify membrane-associated proteins that influenced the activation of heterotrimeric G-proteins. Detergent-solubilized membrane extracts from the neuroblastoma-glioma cell hybrid NG108-15, but not the parent C6B4 glioma cell line, increased [35S]GTPgammaS binding to purified G-protein by approximately 460%. The G-protein activator was heat-sensitive, and the magnitude of its action was related to the amount of extract protein. The biophysical and biochemical properties of the G-protein activator were determined using DEAE ion exchange chromatography, gel filtration, and a lectin affinity matrix. In the presence of added GDP (1 microM), the enriched G-protein activator increased the initial rate of [35S]GTPgammaS binding to brain G-protein by up to 4-fold. In the absence of added GDP, the G-protein activator elicited an initial burst in [35S]GTPgammaS binding to brain G-protein within the first 30 s, after which the rate of nucleotide binding to G-protein was similar in the absence or presence of the G-protein activator. The stimulation of nucleotide binding to brain G-protein by the activator was also observed after resolution of Galpha from Gbetagamma. The G-protein activator was distinct from other proteins (neuromodulin, tubulin, and beta-amyloid precursor protein) that influence nucleotide binding to G-protein, indicating the existence of a novel signal accelerator.

Effect of thyroid deficiency on Go alpha-subunit isoforms in developing rat cerebral cortex

Postnatal development of G alpha o isoforms in rat cerebral cortex was studied by SDS-PAGE and immunoblotting. When rat cerebral cortical membranes were resolved on separating gels containing 9% acrylamide and 8 M urea, three electrophoretically distinct G alpha o-immunoreactive proteins were evident. Comparison of their electrophoretic mobilities and partial tryptic digest pattern with recombinant G alpha o1 or G alpha o1-specific antibody revealed that the slowest and intermediate-migrating bands represent unmodified and fatty acylated forms of G alpha o1 protein, respectively. The fastest-migrating band corresponds to G alpha o2. While the fatty acylated form of G alpha o1 is the predominant species, its appearance paralleled that observed for G alpha o2 in developing rat cortex. Perinatal hypothyroidism induced by methimazole treatment did not significantly alter the appearance of cerebral cortical G alpha o1 and G alpha o2 between days 1 and 22 postpartum. Our findings support the earlier idea that heterogeneity of G alpha o proteins in mammalian brain is likely the result of different co- or post-translational processings of each splice variant of G alpha o. While the appearance of G alpha o isoforms is developmentally regulated, they likely do not play an obligatory role in neonatal brain development. Alternatively, the expression of G alpha o isoforms in developing rat cortex may be controlled by an intrinsic signal(s) that is independent of the thyroid status.

Isolation of cDNA clones encoding eight different human G protein gamma subunits, including three novel forms designated the gamma 4, gamma 10, and gamma 11 subunits

With the growing awareness that the G protein beta and gamma subunits directly regulate the activities of various enzymes and ion channels, the importance of identifying and characterizing these subunits is underscored. In this paper, we report the isolation of cDNA clones encoding eight different human gamma subunits, including three novel forms designated gamma 4, gamma 10, and gamma 11. The predicted protein sequence of gamma 4 shares the most identity (60-77%) with gamma 2, gamma 3, and gamma 7 and the least identity (38%) with gamma 1. The gamma 4 is modified by a geranylgeranyl group and is capable of interacting with both beta 1 and beta 2 but not with beta 3. The predicted protein sequence of gamma 10 shows only modest to low identity (35-53%) with the other known gamma subunits, with most of the differences concentrated in the N-terminal region, suggesting gamma 10 may interact with a unique subclass of alpha. The gamma 10 is modified by a geranylgeranyl group and is capable of interacting with beta 1 and beta 2 but not with beta 3. Finally, the predicted protein sequence of gamma 11 shows the most identity to gamma 1 (76% identity) and the least identity to the other known gamma (33-44%). Unlike most of the other known gamma subunits, gamma 11 is modified by a farnesyl group and is not capable of interacting with beta 2. The close resemblance of gamma 11 to gamma 1 raises intriguing questions regarding its function since the mRNA for gamma 11 is abundantly expressed in all tissues tested except for brain, whereas the mRNA for gamma 1 is expressed only in the retina where the protein functions in phototransduction.