Characterization of GTP-binding proteins in Golgi-associated membrane vesicles from rat adipocytes

Gαi protein subunit: A step toward understanding its non-canonical mechanisms

The heterotrimeric G protein family plays essential roles during a varied array of cellular events; thus, its deregulation can seriously alter signaling events and the overall state of the cell. Heterotrimeric G-proteins have three subunits (α, β, γ) and are subdivided into four families, Gαi, Gα12/13, Gαq, and Gαs. These proteins cycle between an inactive Gα-GDP state and active Gα-GTP state, triggered canonically by the G-protein coupled receptor (GPCR) and by other accessory proteins receptors independent also known as AGS (Activators of G-protein Signaling). In this review, we summarize research data specific for the Gαi family. This family has the largest number of individual members, including Gαi1, Gαi2, Gαi3, Gαo, Gαt, Gαg, and Gαz, and constitutes the majority of G proteins α subunits expressed in a tissue or cell. Gαi was initially described by its inhibitory function on adenylyl cyclase activity, decreasing cAMP levels. Interestingly, today Gi family G-protein have been reported to be importantly involved in the immune system function. Here, we discuss the impact of Gαi on non-canonical effector proteins, such as c-Src, ERK1/2, phospholipase-C (PLC), and proteins from the Rho GTPase family members, all of them essential signaling pathways regulating a wide range of physiological processes.

New paradigms in cAMP signalling

Signalling through adenosine 3'5' monophosphate (cAMP) is known to be important in virtually every cell. The mapping of the human genome over the past two decades has revealed an unexpected complexity of cAMP signalling, which is shared from insects to mammals. A more recent technical advance is the ability to monitor intracellular cAMP levels at subcellular spatial resolution within the time-domains of fast biochemical reactions. Thus, new light has been shed on old paradigms, some of which turn out to be multiple new ones. The novel aspects of cAMP signalling are highlighted here: (1) agonist induced plasticity - showing how the repertory of cAMP signalling genes supports homeostatic adaptation; (2) sustained cAMP signalling after endocytosis; (3) pre-assembled receptor-Gs-adenylyl cyclase complexes. Finally, a hypothetical model of propagating neuronal cAMP signals travelling form dendrites to the cell body is presented.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Heterotrimeric G-protein beta-subunit is localized in the plasma membrane and nuclei of tobacco leaves

Heterotrimeric G-proteins are involved in a variety of cellular responses, but relatively little is known about their function and biochemistry in plants. Antibodies raised against the tobacco heterotrimeric G-protein beta-subunit (Gbeta) were used to analyse its distribution in tobacco leaves. In young tissue the protein level was relatively high, while it declined substantially during later stages of leaf development. Cell fractionation revealed that Gbeta is tightly associated with plasma membrane, but can also be detected in purified nuclei.

Rab 3D in rat adipose cells and its overexpression in genetic obesity (Zucker fatty rat)

Members of the Rab 3 subfamily of low-molecular-mass GTP-binding proteins have been functionally implicated in regulated exocytosis. The aim of the present study was to examine the subcellular distribution of a member of this family, Rab 3D, in rat adipose cells, given the hypothesis that this protein might be involved in insulin-stimulated GLUT4 exocytosis. We show that Rab 3D immunoreactivity is associated predominantly with the high-density microsomal fraction, where the signal intensity is 3- and 7-fold greater than that in plasma membranes and low-density microsomes respectively. Rab 3D does not co-localize with GLUT4 on immuno-isolated intracellular vesicles and, unlike GLUT4, it is not redistributed in response to insulin. Thus, if Rab 3D plays a role in GLUT4 trafficking, it relies on mechanisms independent of relocation. We observed that Rab 3D is overexpressed in adipose cells of obese (fa/fa) Zucker rats, in a tissue- and isoform-specific manner. The pathophysiological significance of this defect remains elusive. This could form the molecular basis for altered adipose secretory function in obesity.

G proteins in adipocytes and preadipocytes: characterization, subcellular distribution, and potential roles for Gi2 and/or Gi3 in the control of cell proliferation

Guanosine triphosphate (GTP)-binding protein subunits were studied by immunoblot analysis in particulate fractions from mature adipocytes, confluent preadipocytes, and in vitro-differentiated preadipocytes. Mature adipocytes express Gi alpha 1, Gi alpha 2, Gi alpha 3, Go alpha, Gq/11 alpha, G13 alpha and the long and short isoforms of Gs alpha, but no Gz alpha or G12 alpha. Confluent and differentiated preadipocytes differ in having a higher content of Gi alpha 3 and G13 alpha and expressing G12 alpha. In contrast, they lack Gi alpha 1, Go alpha, and the short from of Gs alpha. The G-protein alpha subunits Gi alpha 2, Gs alpha (long isoform), and Gq/11 alpha, and G-protein beta subunits were unchanged throughout the differentiation process. By immunoblot and indirect immunofluorescence studies on confluent preadipocytes, we showed that Gi alpha 2 is present in the endoplasmic reticulum and marginally in plasma membranes and nuclei. In contrast, antibodies to Gi alpha 3 stained the Golgi apparatus. The role of G proteins on preadipocyte proliferation was studied using Bordetella pertussis toxin. Exposure of growing cells to this toxin in the presence of fetal calf serum (FCS) decreased [3H]thymidine incorporation by 40% and induced a 40% increase in doubling time. This resulted in a 30% decrease in cell number per well after 48 h. These effects of B. pertussis toxin did not appear to be related to an increase in cyclic adenosine monophosphate (cAMP) concentration, because forskolin had the opposite effect on cell proliferation. Finally, B. pertussis toxin prevented serum-induced Raf1 association to the plasma membrane, possibly by disrupting FCS-induced G beta gamma effects on the Ras/Raf1 pathway. Since Go alpha and Gi alpha 1 subunits were absent in preadipocytes, we conclude that Gi2 and/or Gi3 proteins transduce some mitogenic signals of FCS through release of G beta gamma subunits. The subcellular distribution of Gi alpha 2 and Gi alpha 3 suggests that part of their functions result from interactions with components other than the plasma membrane.

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

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

Insulin-induced translocation of the glucose transporter GLUT4 in cardiac muscle: studies on the role of small-molecular-mass GTP-binding proteins

Subcellular fractions obtained from rat cardiac ventricular tissue were used to elucidate a possible functional relationship between small-molecular-mass G-proteins and the insulin-responsive glucose transporter GLUT4. Proteins were separated by SDS/PAGE and transferred to nitrocellulose membranes. Incubation with [alpha-32P]GTP revealed the presence of two major distinct GTP-binding protein bands of 24 and 26 kDa in both plasma and microsomal membranes. Immunoadsorption of microsomal membranes to anti-GLUT4 antibodies was used to isolate GLUT4-enriched membrane vesicles. This material was found to contain a much decreased amount of small G-proteins, with the exclusive presence of the 24 kDa species. Insulin treatment in vivo had no effect on the microsomal membrane content of small GTP-binding proteins, but significantly decreased the 24 kDa species in GLUT4-enriched vesicles by 36 +/- 5% (n = 3). This correlated with a decreased (30-40%) recovery of GLUT4-enriched vesicles from insulin-treated animals. Western-blot analysis of microsomal membranes with a panel of antisera against rab GTP-binding proteins indicated the presence of rab4A, with a molecular mass of 24 kDa, whereas rab1A, rab2 and rab6 were not observed. rab4A was barely detectable in GLUT4-enriched vesicles; however, insulin produced an extensive shift of rab4A from the cytosol and the microsomal fraction to the plasma membrane with a parallel increase in GLUT4. These data show that a small GTP-binding protein is co-localized with GLUT4 in an insulin-responsive intracellular compartment, and strongly suggest that this protein is involved in the exocytosis of GLUT4 in cardiac muscle. Furthermore, the observed translocation of rab4A is compatible with insulin-induced endosome recycling processes, possibly including the glucose transporters.

Photoaffinity labelling of cardiac membrane GTP-binding proteins in response to insulin

Plasma membranes from rat cardiac ventricular tissue and insulin receptors partially purified by wheat-germ-agglutinin chromatography were subjected to direct photoaffinity labelling with [alpha-32P]GTP in order to elucidate the presence of insulin-receptor-coupled GTP-binding proteins. In plasma membranes three proteins have been identified that exhibit an enhanced photolabelling with the nucleotide in response to insulin. The apparent molecular masses of these proteins were found to be 56, 60 and 74 kDa. Photolabelling of partially purified insulin receptors showed the copurification of the 60-kDa species, whereas the 56-kDa and 74-kDa proteins could not be detected. Furthermore, the 60-kDa G-protein was found to be specifically co-immunoprecipitated with the insulin receptor. Incubation of insulin receptors with insulin increased the labelling of the 60-kDa band to 205 +/- 27% (n = 5) of control. Immuno- and ligand-blotting experiments revealed the additional presence of a 39-kDa G(o)-like protein and two G-proteins with molecular masses of 24 and 26 kDa in the receptor preparation. Under basal conditions the insulin receptor and the 60-kDa G-protein exhibited an apparent inverse distribution between plasma and microsomal membranes with the G-protein being extensively labelled in the microsomal fraction. In conclusion, our data show that, in its native environment, the cardiac insulin receptor couples to at least three GTP-binding proteins. Out of these, a 60-kDa species of microsomal origin, copurifies with the insulin receptor. It is suggested that this G-protein is associated with the insulin receptor and may be involved in insulin receptor signalling in target cells.

Transgenic mice expressing the human GLUT4/muscle-fat facilitative glucose transporter protein exhibit efficient glycemic control

To examine the physiological role of the GLUT4/muscle-fat specific facilitative glucose transporter in regulating glucose homeostasis, we have generated transgenic mice expressing high levels of this protein in an appropriate tissue-specific manner. Examination of two independent founder lines demonstrated that high-level expression of GLUT4 protein resulted in a marked reduction of fasting glucose levels (approximately 70 mg/dl) compared to wild-type mice (approximately 130 mg/dl). Surprisingly, 30 min following an oral glucose challenge the GLUT4 transgenic mice had only a slight elevation in plasma glucose levels (approximately 90 mg/dl), whereas wild-type mice displayed a typical 2- to 3-fold increase (approximately 250-300 mg/dl). In parallel to the changes in plasma glucose, insulin levels were approximately 2-fold lower in the transgenic mice compared to the wild-type mice. Furthermore, isolated adipocytes from the GLUT4 transgenic mice had increased basal glucose uptake and subcellular fractionation indicated elevated levels of cell surface-associated GLUT4 protein. Consistent with these results, in situ immunocytochemical localization of GLUT4 protein in adipocytes and cardiac myocytes indicated a marked increase in plasma membrane-associated GLUT4 protein in the basal state. Taken together these data demonstrate that increased expression of the human GLUT4 gene in vivo results in a constitutively high level of cell surface GLUT4 protein expression and more efficient metabolic control over fluctuations in plasma glucose concentrations.

Intracellular translocation of a 28 kDa GTP-binding protein during osmotic shock-induced cell volume regulation in Dunaliella salina

The primary aim of this study was to determine if small GTP-binding proteins play a role in the conspicuous and much-examined volume control process in Dunaliella salina. We confirmed the previous identification by Rodriguez et al. (Rodriguez Rosales, M.P., Herrin, D.L. and Thompson, G.A., Jr. (1992) Plant Physiol. 98, 446-451) of small GTP-binding proteins in the green alga Dunaliella salina and revealed the presence of at least five such proteins, having molecular masses of approx. 21, 28, 28.5, 29 and 30 kDa. These proteins were concentrated largely in the endoplasmic reticulum (ER) and in an intermediate density organelle fraction (GA) containing mainly Golgi vesicles, mitochondria and flagella. The chloroplast fraction and plasma membrane contained the 28-kDa GTP-binding protein exclusively, while the cytosol contained both the 28-kDa component and small amounts of a 21-kDa GTP-binding protein. Immunodetection analysis showed that the D. salina 28-kDa protein cross-reacted strongly with a polyclonal antibody raised against a Volvox carteri yptV1 type GTP-binding protein. This antibody was utilized for quantitative GTP-binding protein measurements as described below. Certain anti-GTP-binding protein antibodies derived from non-plant sources, namely, monoclonal antibodies raised against yeast and mouse ypt1 GTP-binding proteins, cross-reacted not only with the D. salina 28-kDa protein but also the 29-kDa component. The 30-kDa GTP-binding protein of D. salina did not bind the antibodies mentioned above but did cross-react with an anti-yeast ypt1 polyclonal antibody. None of the D. salina GTP-binding proteins reacted positively with polyclonal antibodies raised against SEC4, rab1 or rab6 proteins. When D. salina cells were subjected to hypoosmotic swelling by abruptly reducing the NaCl concentration of their medium from 1.7 M to 0.85 M, the increase in cell surface area was accompanied by a substantial translocation of the 28-kDa GTP-binding protein from the ER and GA fractions to the plasma membrane, chloroplast and cytosolic fractions, as determined by quantitative [32P]GTP binding and [125I]antibody binding on nitrocellulose blots. This translocation increased the content of the 28-kDa component in the plasma membrane, chloroplast and cytosol by 3-4-fold. No net movement of the 30-kDa GTP-binding protein from either the ER or GA fractions was observed following hypoosmotic shock.(ABSTRACT TRUNCATED AT 400 WORDS)

Abundance and subcellular distribution of GTP-binding proteins in 3T3-L1 cells before and after differentiation to the insulin-sensitive phenotype

The abundance and the subcellular distribution of GTP-binding proteins was studied in membrane fractions (plasma membranes and low-density microsomes) from 3T3-L1 cells before and after differentiation to the insulin-sensitive phenotype. After differentiation, the abundance of alpha i (alpha subunit of GTP-binding protein Gi), alpha o (alpha subunit of GTP-binding protein G(o)), and of a 47-kDa alpha s (alpha subunit of GTP-binding protein Gs) as detected by immunoblotting with specific antisera was reduced by 10-50% when normalized per membrane protein. In contrast, a 43-kDa alpha s was increased about threefold after differentiation. Furthermore, cholera-toxin-catalyzed ADP-ribosylation of both 43-kDa and 47-kDa alpha s was disproportionately increased ninefold and threefold, respectively, possibly reflecting the increased production of an ADP-ribosylation factor in the differentiated cells. The small GTP-binding protein Ha-ras was reduced by 50%, whereas rab1 and other small GTP-binding proteins tentatively identified as rab-isoforms (ras-homologous gene products from brain) were increased by 100% and 70%, respectively. Since the total protein content of 3T3-L1 cells was increased threefold after differentiation, the observed increase of the 43-kDa alpha s, rab1 and of the other rab isoforms was eightfold, sixfold and fivefold, respectively, when normalized/cell count. With the exception of the rab isoforms, all GTP-binding proteins were predominantly, if not exclusively, located in the plasma membrane; comparable amounts of the rab isoforms were found in plasma membranes and low-density microsomes. Insulin induced the characteristic redistribution of glucose transporters GLUT4 from low-density microsomes to the plasma membranes, but failed to alter the subcellular distribution of any of the GTP-binding proteins investigated. These data suggest that the increase in the abundance of the 43-kDa alpha s subunit and of several rab isoforms might be related to specific functions of the adipocyte-like phenotype, but that none of the investigated guanine-nucleotide-binding regulatory (G)-proteins appears to be tightly associated with the GLUT4.

Stimulation of glucose transport by guanine nucleotides in permeabilized rat adipocytes

Effects of guanine nucleotides on glucose transport were studied in permeabilized rat epididymal fat cells. GTP gamma S and Gpp(NH)p, but not App(NH)p, stimulated 3-O-methylglucose transport. Effect of GTP gamma S was dose-dependent, being detectable at 0.1 mM, and 1.0 mM GTP gamma S stimulated glucose transport to the same extent as insulin. GTP gamma S (0.3 mM) enhanced insulin-stimulated glucose transport while 1 mM GTP gamma S did not affect insulin-mediated transport. GDP beta S had no effect on glucose transport by itself but rather enhanced insulin action. NaF, which is known to activate trimeric G proteins, increased glucose transport to the same extent as insulin. Likewise, mastoparan augmented glucose transport. These results indicate that a certain type of trimeric G protein(s) is involved in the regulation of glucose transport.