Pertussis toxin-insensitive activation of the heterotrimeric G-proteins Gi/Go by the NG108-15 G-protein activator

A ligand-independent activator of heterotrimeric brain G-protein was partially purified from detergent-solubilized extracts of the neuroblastoma-glioma cell hybrid NG108-15. The G-protein activator (NG108-15 G-protein activator (NG-GPA)) increased [(35)S]guanosine 5'-O-(thiotriphosphate) ([(35)S]GTPgammaS) to purified brain G-protein in a magnesium-dependent manner and promoted GDP dissociation from Galpha(o). The NG-GPA also increased GTPgammaS binding to purified, recombinant Galpha(i2), Galpha(i3), and Galpha(o), but minimally altered nucleotide binding to purified transducin. The NG-GPA increased GTPgammaS binding to membrane-bound G-proteins and inhibited basal, forskolin- and hormone-stimulated adenylyl cyclase activity in DDT(1)-MF-2 cell membranes. In contrast to G-protein coupled receptor-mediated activation of heterotrimeric G-proteins in DDT(1)-MF-2 cell membrane preparations, the action of the NG-GPA was not altered by treatment of the cells with pertussis toxin. ADP-ribosylation of purified brain G-protein also failed to alter the increase in GTPgammaS binding elicited by the NG-GPA. Thus, the NG-GPA acts in a manner distinct from that of a G-protein coupled receptor and other recently described receptor-independent activators of G-protein signaling. These data indicate the presence of unexpected regulatory domains on G(i)/G(o) proteins and suggest the existence of pertussis toxin-insensitive modes of signal input to G(i)/G(o) signaling systems.     

G-proteins in skeletal muscle. Evidence for a 40 kDa pertussis-toxin substrate in purified transverse tubules.

In muscle, it has been established that guanosine 5'-[gamma-thio]triphosphate (GTP[S]), a non-hydrolysable GTP analogue, elicits a rise in tension in chemically skinned fibres, and that pretreatment with Bordetella pertussis toxin (PTX) decreases GTP[S]-induced tension development [Di Virgilio, Salviati, Pozzan & Volpe (1986) EMBO J. 5, 259-262]. In the present study, G-proteins were analysed by PTX-catalysed ADP-ribosylation and by immunoblotting experiments at cellular and subcellular levels. First, the nature of the G-proteins present in neural and aneural zones of rat diaphragm muscle was investigated. PTX, known to catalyse the ADP-ribosylation of the alpha subunit of several G-proteins, was used to detect G-proteins. Three sequential extractions (low-salt-soluble, detergent-soluble and high-salt-soluble) were performed, and PTX was found to label two substrates of 41 and 40 kDa only in the detergent-soluble fraction. The addition of pure beta gamma subunits of G-proteins to the low-salt-soluble extract did not provide a way to detect PTX-catalysed ADP-ribosylation of G-protein alpha subunits in this hydrophilic fraction. In neural as well as in aneural zones, the 39 kDa PTX substrate, very abundant in the nervous system (Go alpha), was not observed. We then studied the nature of the G alpha subunits present in membranes from transverse tubules (T-tubules) purified from rabbit skeletal muscle. Only one 40 kDa PTX substrate was found in T-tubules, known to be the key element of excitation-contraction coupling. The presence of a G-protein in T-tubule membranes was further confirmed by the immunoreactivity detected with an anti-beta-subunit antiserum. A 40 kDa protein was also detected in T-tubule membranes with an antiserum raised against a purified bovine brain Go alpha. The presence of two PTX substrates (41 and 40 kDa) in equal amounts in total muscle extracts, compared with only one (40 kDa) found in purified T-tubule membranes, suggests that this 40 kDa PTX substrate might be involved in excitation-contraction coupling.     

G-protein dependent potentiation of calcium release from sarcoplasmic reticulum of skeletal muscle

Skinned fibre experiments were conducted to determine if guanine nucleotide-binding proteins play a role in excitation-contraction coupling of skeletal muscle. By itself, the GTP-gamma S, a non hydrolysable GTP analogue was unable to induce calcium release from the sarcoplasmic reticulum, even at concentrations as high as 500 microM. However, calcium- or caffeine-induced calcium releases were enhanced by GTP-gamma S in micromolar concentrations. This response was blocked by GDP-beta S or Pertussis toxin. 32P-ADP-ribosylation catalysed by Pertussis toxin, radiolabelled G-protein alpha subunits in the range of 40 kDa on membrane subcellular fractions of rat skeletal muscle. Using Western blot analysis with antibodies raised against the bovine transducin, G-proteins were identified in frog and rat skeletal muscle subcellular fractions. In most of the muscle fractions (plasma membrane, T-tubules, triads, sarcoplasmic reticulum), the anti-beta subunit antibodies recognized a 36 kDa protein which comigrated with transducin beta subunit. It appears therefore that some of the G-proteins identified by ADP-ribosylation or immunostaining in several subcellular fractions from skeletal muscle, are implicated in the modulation of calcium release from sarcoplasmic reticulum. These results suggest that a Pertussis toxin sensitive G-protein is present at the loci of E-C coupling, and that it serves to regulate the calcium release.     

Activation of heterotrimeric G proteins by a high energy phosphate transfer via nucleoside diphosphate kinase (NDPK) B and Gbeta subunits. Complex formation of NDPK B with Gbeta gamma dimers and phosphorylation of His-266 IN Gbeta

G protein betagamma dimers can be phosphorylated in membranes from various tissues by GTP at a histidine residue in the beta subunit. The phosphate is high energetic and can be transferred onto GDP leading to formation of GTP. Purified Gbetagamma dimers do not display autophosphorylation, indicating the involvement of a separate protein kinase. We therefore enriched the Gbeta-phosphorylating activity present in preparations of the retinal G protein transducin and in partially purified G(i/o) proteins from bovine brain. Immunoblots, autophosphorylation, and enzymatic activity measurements demonstrated enriched nucleoside diphosphate kinase (NDPK) B in both preparations, together with residual Gbetagamma dimers. In the retinal NDPK B-enriched fractions, a Gbeta-specific antiserum co-precipitated phosphorylated NDPK B, and an antiserum against the human NDPK co-precipitated phosphorylated Gbetagamma. In addition, the NDPK-containing fractions from bovine brain reconstituted the phosphorylation of purified Gbetagamma. For identification of the phosphorylated histidine residue, bovine brain Gbetagamma and G(t)betagamma were thiophosphorylated with guanosine 5'-O-(3-[(35)S]thio)triphosphate, followed by digestion with endoproteinase Glu-C and trypsin, separation of the resulting peptides by gel electrophoresis and high pressure liquid chromatography, respectively, and sequencing of the radioactive peptides. The sequence information produced by both methods identified specific labeled fragments of bovine Gbeta(1) that overlapped in the heptapeptide, Leu-Met-Thr-Tyr-Ser-His-Asp (amino acids 261-267). We conclude that NDPK B forms complexes with Gbetagamma dimers and contributes to G protein activation by increasing the high energetic phosphate transfer onto GDP via intermediately phosphorylated His-266 in Gbeta(1) subunits.     

Immunochemical evidence for a novel pertussis toxin substrate in human neutrophils

Pertussis toxin catalyzes incorporation of 20.2 pmol of ADP-ribose/mg of protein into approximately 40-kDa protein(s) in human neutrophil membranes compared with 14.1 pmol/mg in bovine brain membranes. Based on these measurements we estimate that pertussis toxin substrate(s) should represent at least 0.085% of total membrane protein in neutrophils. Both brain and neutrophil membranes show high concentrations (0.34 versus 0.16% of total membrane protein, respectively) of the common beta subunit of guanine nucleotide binding proteins. Affinity purified antibodies specific for Go-alpha fail to detect any protein in immunoblots of neutrophil membranes (150 micrograms) under conditions where as little as 10 ng of purified Go-alpha is detectable, and Go-alpha is readily detected in brain membranes (100 micrograms). An antiserum against transducin that cross-reacts strongly with Gi-alpha, detects as little as 5 ng of purified Gi-alpha and readily detects Gi-alpha in brain membranes, but in neutrophil membranes, the antiserum detects an approximately 40-kDa band that corresponds to less than 10% of the expected amount of pertussis toxin substrate(s). The results show that human neutrophil membranes contain relatively large amounts of pertussis toxin substrate(s), but that the predominant pertussis toxin substrate is immunochemically distinct from previously identified substrates, transducin, Gi, and Go.     

A third form of the G protein beta subunit. 1. Immunochemical identification and localization to cone photoreceptors.

Vertebrate retinal cones play a major role in both photopic vision and color perception. Although the molecular mechanism of visual excitation in the cone is not as well understood as in the rod, it is generally thought to involve a cone-specific G protein (cone transducin) that couples the cone visual pigment to a cGMP phosphodiesterase. Like all other G proteins, cone transducin is most likely a heterotrimer consisting of G alpha, G beta, and G gamma subunits. A G alpha subunit of cone transducin has been localized to the outer segment of bovine cones, but its associated G beta and G gamma subunits are unknown. To identify the G beta subunit involved in the phototransduction process of cones, we have developed a panel of antipeptide antisera against the most diverse region of the amino acid sequences encoded by G beta 1, G beta 2, and G beta 3 cDNAs and used them to determine the distribution of the G beta isoforms in different retinal preparations. We found that the G beta 3 subunit is present in bovine retinal transducin and phosducin-T beta gamma complex preparations which were previously thought to contain only G beta 1. Analysis of its subcellular distribution indicated that G beta 3 is predominantly cytoplasmic. Immunocytochemical staining of bovine retinal sections with the anti-G beta 3 antiserum further revealed a specific localization of G beta 3 in cones but not in rods. In contrast, anti-G beta 1 antiserum stained only the rods. These results suggest that G beta 3 is the G beta subunit of cone transducin and confirms the proposition that rods and cones utilize distinct signaling proteins for phototransduction.     

Purification and immunochemical characterization of the major pertussis-toxin-sensitive guanine-nucleotide-binding protein of bovine-neutrophil membranes

Bovine peripheral neutrophils contain high levels of a 40-kDa pertussis toxin substrate, which was found highly enriched in a light membrane fraction upon subcellular fractionation of neutrophil homogenates. The 40-kDa pertussis toxin substrate, referred to as alpha n, was purified to near homogeneity from this fraction by sequential ion-exchange, gel-filtration and hydrophobic chromatography. Purified alpha n was shown to interact with beta gamma subunits, undergo ADP-ribosylation by pertussis toxin, and bind guanine nucleotides with high affinity. The mobility of purified alpha n on SDS/polyacrylamide gels was intermediate between those of the alpha subunits of Gi and Go, purified from bovine brain, and slightly lower than the mobility of the alpha subunit of transducin (Gt). Several polyclonal antisera against the alpha subunits of bovine Gt and Go did not react with alpha n on immunoblots. CW 6, a polyclonal antiserum reactive against the bovine alpha i, reacted only minimally with alpha n. These results suggest that the major pertussis toxin substrate of bovine neutrophils, designated Gn, is structurally different from previously identified pertussis toxin substrates and may represent a novel guanine-nucleotide-binding protein.     

Purification of an AlF4- and G-protein beta gamma-subunit-regulated phospholipase C-activating protein.

A 150-kDa phospholipase C has previously been purified from turkey erythrocytes and has been shown by reconstitution with turkey erythrocyte membranes to be a receptor- and G-protein-regulated enzyme (Morris, A. J., Waldo, G. L., Downes, C.P., and Harden, T. K. (1990) J. Biol. Chem. 265, 13501-13507; Morris, A.J., Waldo, G.L., Downes, C.P., and Harden, T.K. (1990) J. Biol. Chem. 265, 13508-13514). Combination of this 150-kDa protein with phosphoinositide substrate-containing phospholipid vesicles prepared with a cholate extract from purified turkey erythrocyte plasma membranes resulted in conferrence of AlF4- sensitivity to the purified phospholipase C. Guanosine 5'-3-O-(thio)triphosphate also activated the reconstituted phospholipase C in a manner that was inhibited by guanosine 5'-2-O-(thio)-diphosphate. The magnitude of the AlF4- stimulation was increased with increasing amounts of plasma membrane extract, and was also dependent on the concentration of purified phospholipase C. Using reconstitution of AlF4- sensitivity as an assay, the putative G-protein conferring regulation to the 150-kDa phospholipase C was purified to near homogeneity by sequential chromatography over Q-Sepharose, Sephacryl S-300, octyl-Sepharose, hydroxylapatite, and Mono-Q. Reconstituting activity co-purified with an approximately 43-kDa protein identified by silver staining; lesser amounts of a 35-kDa protein was present in the final purified fractions, as was a minor 40-kDa protein. The 43-kDa protein strongly reacted with antiserum against a 12-amino acid sequence found at the carboxyl terminus of Gq and G11, the 35-kDa protein strongly reacted with G-protein beta-subunit antiserum, and the 40-kDa protein reacted with antiserum that recognizes Gi3. Immunoprecipitation of the 43-kDa protein resulted in loss of phospholipase C-stimulating activity of the purified fraction. The idea that this is a phospholipase C-regulating G-protein is further supported by the observation that co-reconstitution of G-protein beta gamma-subunit with the purified phospholipase C-activating fraction resulted in a beta gamma-subunit-dependent inhibition of AlF(4-)-stimulated phospholipase C activity in the reconstituted preparation.     

Antisera against the 3-17 sequence of rat G alpha i recognize only a 40 kDa G-protein in brain

To obtain antisera specific for the GTP-binding protein Gi alpha we immunized rabbits against a synthetic peptide derived from the N-terminal (3-17) sequence predicted from the rat Gi alpha cDNA clone published by Itoh et al. (1986) (Proc. Natl. Acad. Sci. USA 83, 3776-3780). Western-blot analysis of bovine brain G-proteins purified and resolved by hydrophobic chromatography and of rat striatal membranes, indicate that this antiserum does not recognize 41 kDa alpha i or 39 kDa alpha o. However, it reacts with a 40 kDa alpha-subunit. The data suggest that the sequence deduced from the rat G alpha i cDNA corresponds to a G40 alpha protein and that N-terminus directed antisera are useful tools to discriminate between two different G alpha i-like types of G-proteins present in mammalian brain.     

Localization of a visual Gq protein in the photoreceptors of a polychaete, Perinereis brevicirris (Annelida)

Heterotrimeric GTP-binding proteins (G proteins) play an important role in phototransduction. The presence of G-protein subclasses has been reported in photoreceptive membranes, e.g., the Gi subgroup (transducin) in vertebrate rods, and the Gq subgroup in the eyes of the Arthropoda and the Mollusca. We examined the immunoreactivity and distribution of a Gq homologue in the cerebral ocelli of Perinereis brevicirris (Polychaeta, Annelida) using an anti-GqC antibody raised against a conserved sequence at the C-terminal of the alpha-subunit of Gq (Gq-alpha). The anti-GqC antibody labeled a 48-kDa band on the Western blot of proteins from the Perinereis ocelli. The anti-GtC antibody, which is raised against the C-terminal sequence of bovine transducin alpha-subunit (Gt-alpha), did not cross-react to the ocellar proteins of Perinereis. The rhabdomeric layers of the anterior and posterior ocelli were strongly labeled by anti-GqC on light-microscopic immunohistology. Immunoelectron microscopy showed that the Gq molecules were specifically localized in the photoreceptive membrane of the rhabdomeric microvilli. These results suggest that the Gq protein plays a role in the phototransduction of the Perinereis ocelli.     

Identification of a retina-specific MEKA protein as a 33 K protein

A photoreceptor-specific MEKA protein was purified from bovine retinal soluble fraction. The purified sample was eluted as a single peak of 74 kDa protein from a Superose column, which was dissolved into three components, MEKA protein (32 kDa), beta-(36 kDa) and gamma-(10 kDa) subunits of transducin on a SDS-PAGE. From several lines of evidence, we concluded that MEKA protein is identical with a 33k phosphoprotein reported by Lee et al (1).     

Immunological identification of G protein alpha- and beta-subunits in tail membranes of bovine spermatozoa.

Heterotrimeric G proteins are believed to play important roles as signal transducing components in various mammalian sperm functions. To assess the distribution of G proteins in bovine sperm tails, we purified membranes by hypoosmotic swelling of bovine spermatozoa followed by disruption of plasma membranes in a homogenizer and various centrifugation steps. Electron microscopy revealed highly purified membranes of bovine sperm tails. Subsequently, antisera against synthetic peptides were used to identify G proteins in immunoblots. An antiserum directed against the C-terminal decapeptide of Gi3 and detecting all known pertussis toxin-sensitive alpha-subunits, reacted specifically with a 40-kDa protein. In contrast, various other specific peptide antisera against alpha-subunits did not detect any G protein in enriched tail membranes. An antiserum recognizing the beta 2-subunit of G proteins and an antiserum reacting with both beta 1- and beta 2-subunits identified a 35-kDa protein in sperm tail membranes. In contrast, antisera against the 36-kDa beta 1-subunit did not detect any relevant proteins in the membrane fraction. Neither G protein alpha-subunits nor G protein beta-subunits were found in the cytosol. Our results suggest that G proteins in membranes of tails of bovine spermatozoa most likely belong to a novel subtype of G protein alpha-subunits, whereas the putative beta-subunit could be identified as a beta 2-subunit.     

Inhibition of the GTPase activity of transducin by an NAD+:arginine ADP-ribosyltransferase from turkey erythrocytes.

The bacterial toxins, choleragen and pertussis toxin, inhibit the light-stimulated GTPase activity of bovine retinal rod outer segments by catalysing the ADP-ribosylation of the alpha-subunit (T alpha) of transducin [Abood, Hurley, Pappone, Bourne & Stryer (1982) J. Biol. Chem. 257, 10540-10543; Van Dop, Yamanaka, Steinberg, Sekura, Manclark, Stryer & Bourne (1984) J. Biol. Chem. 259, 23-26]. Incubation of retinal rod outer segments with NAD+ and a purified NAD+:arginine ADP-ribosyltransferase from turkey erythrocytes resulted in approx. 60% inhibition of GTPase activity. Inhibition was dependent on both enzyme and NAD+, and was potentiated by the non-hydrolysable GTP analogues guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) and guanosine 5'-[beta gamma-methylene]triphosphate (p[CH2]ppG). The transferase ADP-ribosylated both the T alpha and T beta subunits of purified transducin. T alpha (39 kDa), after ADP-ribosylation, migrated as two distinct peptides with molecular masses of 42 kDa and 46 kDa on SDS/polyacrylamide-gel electrophoresis. T beta (36 kDa), after ADP-ribosylation, migrated as a 38 kDa peptide. With purified transducin subunits, it was observed that the GTPase activity of ADP-ribosylated T alpha, reconstituted with unmodified T beta gamma and photolysed rhodopsin, was decreased by 80%; conversely, reconstitution of T alpha with ADP-ribosyl-T beta gamma resulted in only a 19% inhibition of GTPase. Thus ADP-ribosylation of T alpha, the transducin subunit that contains the guanine nucleotide-binding site, has more dramatic effects on GTPase activity than does modification of the critical 'helper subunits' T beta gamma. To elucidate the mechanism of GTPase inhibition by transferase, we studied the effect of ADP-ribosylation on p[NH]pp[3H]G binding to transducin. It was shown previously that modification of transducin by choleragen, which like transferase ADP-ribosylates arginine residues, did not affect guanine nucleotide binding. ADP-ribosylation by the transferase, however, decreased p[NH]pp[3H]G binding, consistent with the hypothesis that choleragen and transferase inhibit GTPase by different mechanisms.     

Activated cGMP phosphodiesterase of retinal rods. A complex with transducin alpha subunit.

Purified G-protein (transducin) activated with the nonhydrolyzable analog guanosine 5'-O-(thiotriphosphate) (GTP gamma S) and cGMP phosphodiesterase (PDE) from retinal rods are added to protein-stripped disc membranes. Specific binding of the mainly soluble alpha subunit of G-protein with GTP gamma S bound (G alpha GTP gamma S, activator of the PDE) to the disc membrane in the presence of PDE is measured from gel scans or experiments with labeled G-protein alpha subunit (G alpha). Its variation as a function of G concentration matches the theoretical variation of G alpha involved in the activation of PDE calculated with previously estimated dissociation constants (Bennett, N., and Clerc, A. (1989) Biochemistry 28, 7418-7424), and the G alpha bound/PDE ratio at saturation is close to 2. No increase of G alpha binding to the membrane is observed when purified inhibitory subunit of PDE (PDE gamma) is added together with or instead of total PDE, and excess PDE gamma remains soluble. These results suggest that activated PDE is a complex with the activator G alpha GTP rather than PDE from which the inhibitory subunits have been removed. A method for purifying PDE gamma with a high yield of recovery and activity is described.     

Immunological properties of O2.- generating oxidase from bovine neutrophils

Two antisera have been prepared against the O2.- generating oxidase purified from bovine polymorphonuclear neutrophils (PMNs). The first antiserum was directed against the enzymatically active fraction obtained after isoelectric focusing (pI oxidase), which consisted of a major protein of Mr 65,000 [(1985) Biochemistry 24, 7231-7239]. The second antiserum was directed against the 65 kDa band excised from an SDS-polyacrylamide gel after electrophoresis of the pI oxidase preparation. The pI oxidase antiserum inhibited O2.- generation by PMN cells, PMN membranes and detergent-solubilized membranes. The 65 kDa band antiserum was virtually non-inhibitory against PMN cells; in contrast, it was nearly as potent as the pI oxidase antiserum on PMN membranes and detergent-solubilized membranes. Inhibition of O2.- generation by the pI oxidase antiserum was correlated with the immunoreactivity of four membrane-bound proteins of 65, 54, 18 and 16 kDa; the 65 kDa band antiserum reacted only with the two proteins of 65 and 54 kDa. It is concluded that the 18 and 16 kDa proteins, present in trace amounts in the pI oxidase preparation, are probably potent catalysts of the respiratory burst.     

The membrane-binding domain of a 23-kDa G-protein is carboxyl methylated

We have purified to homogeneity a 23-kDa protein from bovine brain membranes using [35S]guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding as an assay. GTP gamma S binding to the purified protein is inhibited by GDP, GTP, and GTP analogs but not by cGMP, GMP, or adenine nucleotides, consistent with the nucleotide-binding behavior of members of the family of GTP-binding regulatory proteins. On addition of the methyl donor S-adenosyl-L-methionine and a methyltransferase present in bovine brain membranes, the purified 23-kDa G-protein is carboxyl methylated. When subjected to limited tryptic proteolysis, the 23-kDa protein is converted to a 22-kDa major fragment with concomitant release of a carboxyl methylated protein fragment of 1 kDa. Furthermore, when the cleaved protein is reconstituted with stripped bovine brain membranes, the small carboxyl-methylated fragment but not the 22-kDa major fragment is found to reassociate with the membranes. These results indicate that the site of carboxyl methylation and the region responsible for membrane anchoring, most likely, are localized to a small region at the carboxyl terminus. It is attractive to speculate that carboxyl methylation and membrane anchoring are interrelated processes and play key roles in the function of this small G-protein.     

Receptor-dependent G-protein activation in lipidic cubic phase

The primary step in cellular signaling by G-protein-coupled receptors (GPCRs) is the interaction of the agonist-activated transmembrane receptor with an intracellular G-protein. Understanding the underlying molecular mechanisms requires the structural determination of receptor G-protein complexes that are not yet achieved. The crystal structure of the bovine photoreceptor rhodopsin, a prototypical GPCR, was solved recently and the structures of different states of engineered G-proteins were reported. Posttranslational hydrophobic modifications of G-proteins are in most cases removed for crystallization but play functional roles for interactions among G-protein subunits with receptors, as well as membranes. Bovine rhodopsin is reconstituted into lipidic cubic phases to assess their potential for crystallization of receptor G-protein complexes under conditions that may preserve the structural and functional roles of hydrophobic protein modifications. Three-dimensional bilayers of a bicontinuous lipidic cubic phase are successfully employed for crystallization of membrane and soluble proteins. UV-visible absorption and attenuated total reflection Fourier transform IR difference spectroscopy reveal that light activation of cubic phase reconstituted rhodopsin results in the generation of a metarhodopsin II-like state. Via diffusion along aqueous channels, transducin couples efficiently to this photoproduct as evidenced by the nucleotide-dependent increase of transducin fluorescence. Thus, rhodopsin transducin interactions do not crucially depend on the presence of sn1 and sn2 acyl chains, phospholipid head groups, or membrane planarity. Because lipidic cubic phases preserve the essential functional and structural properties of native rhodopsin and transducin, they appear suitable for the detergent-free crystallization of receptor G-protein complexes carrying a normal pattern of hydrophobic modifications.     

Chimeric microbial rhodopsins containing the third cytoplasmic loop of bovine rhodopsin

G-protein-coupled receptors transmit stimuli (light, taste, hormone, neurotransmitter, etc.) to the intracellular signaling systems, and rhodopsin (Rh) is the most-studied G-protein-coupled receptor. Rh possesses an 11-cis retinal as the chromophore, and 11-cis to all-trans photoisomerization leads to the protein structural changes in the cytoplasmic loops to activate G-protein. Microbial rhodopsins are similar heptahelical membrane proteins that function as bacterial sensors, light-driven ion-pumps, or light-gated channels. Microbial rhodopsins possess an all-trans retinal, and all-trans to 13-cis photoisomerization triggers protein structural changes for each function. Despite these similarities, there is no sequence homology between visual and microbial rhodopsins, and microbial rhodopsins do not activate G-proteins. However, it was reported that bacteriorhodopsin (BR) chimeras containing the third cytoplasmic loop of bovine Rh are able to activate G-protein, suggesting a common mechanism of protein structural changes. Here we design chimeric proteins for Natronomonas pharaonis sensory rhodopsin II (SRII, also called pharaonis phoborhodopsin), which has a two-orders-of-magnitude slower photocycle than BR. Light-dependent transducin activation was observed for most of the nine SRII chimeras containing the third cytoplasmic loop of bovine Rh (from Y223, G224, Q225 to T251, R252, and M253), but the activation level was 30,000–140,000 times lower than that of bovine Rh. The BR chimera, BR/Rh223-253, activates a G-protein transducin, whereas the activation level was 37,000 times lower than that of bovine Rh. We interpret the low activation by the chimeric proteins as reasonable, because bovine Rh must have been optimized for activating a G-protein transducin during its evolution. On the other hand, similar activation level of the SRII and BR chimeras suggests that the lifetime of the M intermediates is not the simple determinant of activation, because SRII chimeras have two-orders-of-magnitude's slower photocycle than the BR chimera. Activation mechanism of visual and microbial rhodopsins is discussed on the basis of these results.     

Possible involvement of different GTP-binding proteins in noradrenaline- and thrombin-stimulated release of arachidonic acid in rabbit platelets.

Noradrenaline (NA) stimulated the release of arachidonic acid (AA) from the [3H]AA-labelled rabbit platelets via alpha 2-adrenergic receptors, since the effect of NA was inhibited by yohimbine. The stimulatory effect of NA in digitonin-permeabilized platelets was completely dependent on the simultaneous presence of GTP and Ca2+. The NA- and thrombin-stimulated releases of AA were markedly decreased by the prior ADP-ribosylation of the permeabilized platelets with pertussis toxin. Antiserum directed against the pig brain Go (a GTP-binding protein of unknown function), recognizing both alpha 39 and beta 35,36 subunits, but not alpha 41, of pig brain, reacted with 41 kDa and 40 kDa bands, with not one of 39 kDa, in rabbit platelet membranes. Anti-Go antiserum inhibited guanosine 5'-[gamma-thio]triphosphate-, A1F4(-)-, NA- and thrombin-stimulated AA releases in the membranes. Although the effect of thrombin was inhibited by low concentrations of anti-Go antiserum, high concentrations of the antiserum was needed for inhibition of the NA effect. Antiserum directed against the pig brain G1 (inhibitory G-protein), recognizing both alpha 41 and beta 35,36 subunits, but not alpha 39, of pig brain, reacted with the 41 kDa band in platelets. Anti-G1 antiserum inhibited only the effect of NA. Reconstitution of the platelet membranes ADP-ribosylated by pertussis toxin with Go, not Gi, purified from pig brain restored the thrombin-stimulated release of AA. In contrast, reconstitution of those membranes with Gi, not Go, restored the NA-stimulated release of AA. These results indicate that different GTP-binding proteins, Gi- and Go-like proteins, may be involved in the mechanism of signal transduction from alpha 2-adrenergic receptors and thrombin receptors to phospholipase A2 in rabbit platelets.     

High molecular weight forms of GTP-binding regulatory proteins from the bovine cerebellum

A new form of a low Km GTPase belonging to the family of regulatory GTP-binding G-proteins has been identified in bovine cerebellum. The molecular weight of this G-protein is several times as high as that of other G-proteins known to be alpha beta gamma heterotrimers: i. e., Gs, Gi, Go, transducin and a new G-protein which had recently been isolated in our laboratory from bovine cerebellum. The high molecular weight G-protein is stable against dissociation; its molecular mass does not change after treatment with DTT, colchicine and NaF. Using antibodies against the alpha-subunit of the formerly isolated cerebellar G-protein and the transducin beta-subunit, it was demonstrated that the both immunoreactive subunits are present in the high molecular weight G-protein. The two forms of the cerebellar G-proteins, i. e., "high" and "low molecular weight" ones, differ drastically in terms of the Mg2+ effect on their GTPase activity. Whereas at submicromolar concentrations of Mg2+ the GTPase activity of the former is virtually absent, the GTPase activity of the latter is more elevated in the presence of EDTA than in the presence of Mg2+.