Developmental expression of G protein alpha subunits in mouse spermatogenic cells: evidence that G alpha i is associated with the developing acrosome.

Guanine nucleotide-binding proteins (G proteins) are important signal transducing molecules found in all cells. G proteins are associated with the plasma membrane/outer acrosomal membrane region of acrosome-intact sperm and at least one G protein is involved in the zona pellucida-induced acrosome reaction. With the goal of elucidating the functions of these proteins during spermatogenesis, we investigated the types of G proteins present in spermatogenic cells and when they first become associated with the developing acrosome. Using bacterial toxin-catalyzed [32P]ADP-ribosylation in conjunction with immunoprecipitation and immunofluorescence utilizing antibodies directed against specific regions of various G protein isotypes, the alpha subunits of Gi1, Gi2, Gi3, and G(o) were detected in mouse spermatocytes and spermatids. An antiserum recognizing a conserved sequence of G alpha i subtypes localized to the proacrosomal granules of spermatocytes and the developing acrosome of spermatids. Levels of G alpha o diminished as spermatocytes developed into spermatids such that G alpha o was not detected in cauda epididymal sperm. Immunoreactivity using G alpha o-specific antisera did not display a distinct regionalization within any of the spermatogenic cell types. G alpha s was not detected in the developing spermatogenic cells or sperm. The association of G alpha i with the developing acrosome suggests a role for G proteins may have a role in acrosome biogenesis as well as being part of a complex required later for signal transduction leading to acrosomal exocytosis.     

Evolving insights regarding mechanisms for the inhibition of insulin release by norepinephrine and heterotrimeric G proteins

Norepinephrine has for many years been known to have three major effects on the pancreatic β-cell which lead to the inhibition of insulin release. These are activation of K(+) channels to hyperpolarize the cell and prevent the gating of voltage-dependent Ca(2+) channels that increase intracellular Ca(2+) concentration ([Ca(2+)](i)) and trigger release; inhibition of adenylyl cyclases, thus preventing the augmentation of stimulated insulin release by cyclic AMP; and a "distal" effect that occurs downstream of increased [Ca(2+)](i) to inhibit exocytosis. All three are mediated by the pertussis toxin (PTX)-sensitive heterotrimeric Gi and Go proteins. The distal inhibitory effect on exocytosis is now known to be due to the binding of G protein βγ subunits to the synaptosomal-associated protein of 25 kDa (SNAP-25) on the soluble NSF attachment protein receptor (SNARE) complex. Recent studies have uncovered two more actions of norepinephrine on the β-cell: 1) retardation of the refilling of the readily releasable granule pool after it has been discharged, an action that is mediated by Gαi(1) and/or Gαi(2); and 2) inhibition of endocytosis that is mediated by Gz. Of importance also are new findings that Gαo regulates the number of docked granules in the β-cell, and that Gαo(2) maintains a tonic inhibitory influence on secretion. The latter provides another explanation as to why PTX, which blocks the effect of Gαo(2), was initially called "islet activating protein." Finally, there is clear evidence that overexpression of α(2A)-adrenergic receptors in β-cells can cause type 2 diabetes.     

Comparative analysis of the efficacy of A1 adenosine receptor activation of Gi/o alpha G proteins following coexpression of receptor and G protein and expression of A1 adenosine receptor-Gi/o alpha fusion proteins

HEK293T cells were transiently transfected to express either the human A1 adenosine receptor together with pertussis toxin-resistant cysteine-to-glycine forms of the alpha subunits of Gi1 (C351G), Gi2 (C352G), and Gi3 (C351G) and wild-type Go1alpha or fusion proteins comprising the A1 adenosine receptor and these Gi/o G proteins to compare A1 adenosine receptor agonist-mediated activation of these Gi family G proteins upon coexpression of individual Gi/o G proteins and receptor versus expression as receptor-G protein fusion proteins. Addition of the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) to membranes of pertussis toxin-treated cells resulted in a concentration-dependent stimulation of [35S]GTPgammaS binding with comparable amounts of NECA required to produce half-maximal stimulation following transfection of A1 adenosine receptor and Gi/o G proteins either as fusion proteins or as separate polypeptides. However, the magnitude of agonist-mediated activation of GTPgammaS binding was greatly enhanced by expressing the A1 adenosine receptor and Gi family G proteins from chimaeric open reading frames. This observation was consistent following the study of more than 40 agonists. No preferential activation of any G protein was observed with more than 40 A1 receptor agonists following cotransfection of receptor with G protein or transfection of receptor-G protein fusion proteins. These studies demonstrate the utility of using fusion proteins to study receptor-G protein interaction, show that the A1 adenosine receptor couples equally well to the Gi/o G proteins Gi1alpha, G i2alpha, Gi3alpha, and Go1alpha, and demonstrate that for a range of agonists there is no selectivity for activation of any particular A1 adenosine receptor-Gi/o G protein combination.     

Regulatory Role of the GTP-Binding Protein, Go, in the Mechanism of Exocytosis in Adrenal Chromaffin Cells

To elucidate the possible involvement of GTP-binding proteins (G proteins) in the mechanism of exocytosis, we studied effects of pertussis toxin (PTX), guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S), and antibodies against the G proteins (Gi and G(o)) on the secretory function of bovine adrenal chromaffin cells. Pretreatment of chromaffin cells with PTX resulted in an increase in acetylcholine-evoked catecholamine release. High K(+)-, histamine-, or gamma-aminobutyric acid-evoked catecholamine release was also potentiated by PTX pretreatment. The concentration of extracellular Ca2+ required for maximal release by 10(-4) M acetylcholine was decreased significantly in PTX-treated cells. In digitonin-permeabilized cells, PTX pretreatment resulted in a decrease of the half-maximal concentration (Km) of Ca2+ required for exocytosis with no significant change in the maximal stimulation (Vmax). Exposure of permeabilized cells to GTP-gamma-S (a nonhydrolyzable GTP analogue) inhibited Ca(2+)-dependent exocytosis by reducing the affinity for Ca2+. The effects of PTX pretreatment were mimicked by treatment of permeabilized cells with polyclonal antibodies selective for the alpha subunit of the PTX-sensitive G protein, G(o). Treatment with similar antibodies against the alpha subunit of Gi had no effect. These findings suggest that G(o) directly controls the Ca(2+)-triggered process in the machinery of exocytosis by lowering the affinity of the unknown target for Ca2+.     

Norepinephrine exocytosis stimulated by alpha-latrotoxin requires both external and stored Ca2+ and is mediated by latrophilin, G proteins and phospholipase C

alpha-latrotoxin (LTX) stimulates massive release of neurotransmitters by binding to a heptahelical transmembrane protein, latrophilin. Our experiments demonstrate that latrophilin is a G-protein-coupled receptor that specifically associates with heterotrimeric G proteins. The latrophilin-G protein complex is very stable in the presence of GDP but dissociates when incubated with GTP, suggesting a functional interaction. As revealed by immunostaining, latrophilin interacts with G alpha q/11 and G alpha o but not with G alpha s, G alpha i or G alpha z, indicating that this receptor may couple to several G proteins but it is not promiscuous. The mechanisms underlying LTX-evoked norepinephrine secretion from rat brain nerve terminals were also studied. In the presence of extracellular Ca2+, LTX triggers vesicular exocytosis because botulinum neurotoxins E, Cl or tetanus toxin inhibit the Ca(2+)-dependent component of the toxin-evoked release. Based on (i) the known involvement of G alpha q in the regulation of inositol-1,4,5-triphosphate generation and (ii) the requirement for Ca2+ in LTX action, we tested the effect of inhibitors of Ca2+ mobilization on the toxin-evoked norepinephrine release. It was found that aminosteroid U73122, which inhibits the coupling of G proteins to phospholipase C, blocks the Ca(2+)-dependent toxin's action. Thapsigargin, which depletes intracellular Ca2+ stores, also potently decreases the effect of LTX in the presence of extracellular Ca2+. On the other hand, clostridial neurotoxins or drugs interfering with Ca2+ metabolism do not inhibit the Ca2(+)-independent component of LTX-stimulated release. In the absence of Ca2+, the toxin induces in the presynaptic membrane non-selective pores permeable to small fluorescent dyes; these pores may allow efflux of neurotransmitters from the cytoplasm. Our results suggest that LTX stimulates norepinephrine exocytosis only in the presence of external Ca2+ provided intracellular Ca2+ stores are unperturbed and that latrophilin, G proteins and phospholipase C may mediate the mobilization of stored Ca2+, which then triggers secretion.     

Identification of G Protein Subtypes in Peripheral Nerve and Cultured Schwann Cells

In this study, we investigated the expression of various G proteins in whole sciatic nerves, in myelin and nonmyelin fractions from these nerves, and in membranes of immortalized Schwann cells. In myelin, nonmyelin, and Schwann cell membranes we detected two 39-40-kDa pertussis toxin substrates that were resolved on separation on urea-gradient gels. Two cholera toxin substrates with apparent molecular masses of 42 and 47 kDa were present in nerve and brain myelin and in Schwann cell membranes. In these membranes, a third 45-kDa cholera toxin substrate, which displayed the highest labeling, was also present. Immunoblotting with specific antisera allowed the identification of G(o) alpha, Gi1 alpha, Gi2 alpha, Gi3 alpha, Gq/G11 alpha, and the two isoforms of Gs alpha in nerve homogenates, nerve, and brain myelin fractions. In Schwann cell membranes we identified G(o) alpha, Gi2 alpha, Gi3 alpha, and proteins from the Gq family, but no immunoreactivity toward anti-Gi1 alpha antiserum was detected. In these membranes, anti-Gs alpha antibody recognized the three cholera toxin substrates mentioned above, with the 45-kDa band displaying the highest immunoreactivity. Relative to sciatic nerve myelin, the Schwann cell membranes revealed a significantly higher expression of Gi3 alpha and the absence of Gi1 alpha. The different distribution of G proteins among the different nerve compartments might reflect the very specialized function of Schwann cells and myelin within the nerve.     

Influence of thyroid hormone status on expression of genes encoding G protein subunits in the rat heart.

We have studied the influence of thyroid hormone status in vivo on expression of the genes encoding guanine nucleotide-binding regulatory protein (G protein) alpha-subunits Gs alpha, Gi alpha(2), Gi alpha(3), and both the 36-kDa form (beta 1) and the 35-kDa form (beta 2) of the beta-subunit in rat ventricle. The relative amounts of immunoactive Gi alpha(2) and Gi alpha(3) were greater in ventricular membranes from hypothyroid animals than from euthyroid animals (1.9- and 2.6-fold, respectively). A corresponding 2.3-fold increase in Gi alpha(2) mRNA was observed as well as a 1.5-fold increase in Gi alpha(3) mRNA. The relative amounts of immunoactive beta 1 and beta 2 polypeptides were also increased (2.8- and 1.8-fold, respectively) in the hypothyroid state and corresponded with comparable increases in the relative levels of beta 1 and beta 2 mRNAs. No difference was seen between the amounts of Gi alpha(2), Gi alpha(3), beta 1, and beta 2 in the euthyroid state and the hyperthyroid state. In contrast to these effects of thyroid hormone status on Gi alpha and beta, the steady-state amounts of Gs alpha protein and mRNA were not altered by thyroid hormone status. Thyroid hormone status did not alter sensitivity of adenylyl cyclase to stimulation by sodium fluoride or guanyl-5'-yl imidodiphosphate (GppNHp), nor did it influence GppNHp-induced inhibition of forskolin-stimulated enzyme activity. These results demonstrate that thyroid hormone status in vivo can regulate expression of specific G protein subunits in rat myocardium. However, the physiological consequences of these changes remain unclear.     

The NH2-terminal alpha subunit attenuator domain confers regulation of G protein activation by beta gamma complexes.

Gs and Gi, respectively, activate and inhibit the enzyme adenylyl cyclase. Regulation of adenylyl cyclase by the heterotrimeric Gs and Gi proteins requires the dissociation of GDP and binding of GTP to the alpha s or alpha i subunit. The beta gamma subunit complex of Gs and Gi functions, in part, to inhibit GDP dissociation and alpha subunit activation by GTP. Multiple beta and gamma polypeptides are expressed in different cell types, but the functional significance for this heterogeneity is unclear. The beta gamma complex from retinal rod outer segments (beta gamma t) has been shown to discriminate between alpha i and alpha s subunits (Helman et al: Eur J Biochem 169:431-439, 1987). beta gamma t efficiently interacts with alpha i-like G protein subunits, but poorly recognizes the alpha s subunit. beta gamma t was, therefore, used to define regions of the alpha i subunit polypeptide that conferred selective regulation compared to the alpha s polypeptide. A series of alpha subunit chimeras having NH2-terminal alpha i and COOH-terminal alpha s sequences were characterized for their regulation by beta gamma t, measured by the kinetics of GTP gamma S activation of adenylyl cyclase. A 122 amino acid NH2-terminal region of the alpha i polypeptide encoded within an alpha i/alpha s chimera was sufficient for beta gamma t to discriminate the chimera from alpha s. A shorter 54 amino acid alpha i sequence substituted for the corresponding NH2-terminal region of alpha s was insufficient to support the alpha i-like interaction with beta gamma t. The findings are consistent with our previous observation (Osawa et al: Cell 63:697-706, 1990) that a region in the NH2-terminal moiety functions as an attenuator domain controlling GDP dissociation and GTP activation of the alpha subunit polypeptide and that the attenuator domain is involved in functional recognition and regulation by beta gamma complexes.     

An Asp79Asn mutation of the alpha2A-adrenoceptor interferes equally with agonist activation of individual Gialpha-family G protein subtypes

The quantitative effects of an Asp79Asn mutation in the porcine alpha2A-adrenoceptor on adrenaline-mediated stimulation of the alpha subunit of individual members of the Gi family of G proteins were assessed by measuring GTP turnover number for fusion proteins between the wild type or mutated receptor and pertussis toxin-resistant forms of each of Gi1, Gi2 and Gi3. In each case the receptor mutation limited activation of the G protein to 8-14% of that produced by the wild type receptor. Previous demonstration that in a single cell this mutation selectively interferes with alpha2A-adrenoceptor regulation of distinct effector end points transduced by Gi family members must therefore reflect differential requirements for amplification or the cellular location of individual, co-expressed, G proteins.     

The G alpha z gene product in human erythrocytes. Identification as a 41-kilodalton protein

A cDNA encoding a previously unknown G protein alpha-subunit lacking the site for pertussis toxin-catalyzed ADP-ribosylation was recently cloned and its putative protein product named Gz (Fong, H. K. W., Yoshimoto, K. K., Eversole-Cire, P., and Simon, M. I. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 3066-3070) or Gx (Matsuoka, M., Itoh, H. Kozasa, T., and Kaziro, Y. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 5384-5388). A synthetic peptide corresponding to the deduced carboxyl-terminal decapeptide of this putative protein (alpha z) has been synthesized and used to prepare a polyclonal rabbit antiserum directed against the protein. The specificity and cross-reactivity of this antiserum was assessed using bacterially expressed recombinant G protein alpha-subunit fusion proteins (r alpha). The crude antiserum strongly recognizes r alpha z in immunoblots. Pretreatment of antiserum with antigen peptide greatly reduces the interaction of the antiserum with r alpha z. Affinity purified antiserum strongly recognizes expressed r alpha z, does not recognize r alpha s1, r alpha s1, r alpha o, or r alpha i3, and very weakly interacts with r alpha i1 and r alpha i2. In contrast, the alpha-subunits of purified bovine brain Gi1 and human erythrocyte Gi2 and Gi3 did not react with the alpha z-antiserum. Partially purified mixtures of human erythrocyte G proteins contain a 41-kDa protein that reacts specifically in immunoblots with both crude and affinity purified alpha z-specific antiserum. Quantitative immunoblotting using r alpha z as a standard indicates that there is 60-100 ng of alpha z/micrograms of 40/41-kDa alpha-subunit protein in partially purified human erythrocyte G protein preparations. We conclude that we have identified the alpha z gene product as a 41-kDa trace protein in human erythrocytes.     

Interactions in platelets between G proteins and the agonists that stimulate phospholipase C and inhibit adenylyl cyclase

Platelet responses to agonists are believed to be mediated by at least two pertussis toxin-sensitive guanine nucleotide-binding (G) proteins: Gi which inhibits adenylyl cyclase and Gp, which stimulates phospholipase C. The present studies compare the properties of Gi and Gp and examine their interactions with the receptors for various platelet agonists. In permeabilized platelets and platelet membranes, pertussis toxin [32P]ADP-ribosylated a protein(s) (alpha 41) which migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis fractionally below rabbit and bovine alpha i (Mr = 41,000). Prior exposure of the platelets to an agonist inhibited the [32P]ADP-ribosylation of alpha 41 to an extent which correlated with the pattern of responses to that agonist. Thrombin, which elicited responses that were mediated by both Gi and Gp, decreased radiolabeling by greater than 90%. Epinephrine, which was functionally coupled only to Gi, decreased radiolabeling by 50%, as did vasopressin and platelet-activating factor (PAF), which were coupled only to Gp. U46619, a thromboxane analog which neither inhibited cAMP formation nor caused pertussis toxin-sensitive phosphoinositide hydrolysis, had no effect on 32P-ADP-ribosylation. These results suggest that either G alpha 41 regulates more than one enzyme or that alpha subunits from more than one G protein comigrate within alpha 41. Two-dimensional electrophoresis was used to test the latter possibility. Upon isoelectric focusing, alpha 41 resolved into two distinct subspecies. However, these appear to be minor variants rather than functionally distinct alpha subunits since: 1) both proteins produced the same proteolytic fragments after digestion with chymotrypsin or Staphylococcus aureus V8 protease and 2) preincubation of the platelets with agonists, including those which appear to interact in intact platelets solely with Gp (PAF and vasopressin) or solely with Gi (epinephrine), inhibited the [32P]ADP-ribosylation of both proteins to the same extent. The pattern of functional responses produced by some of the agonists was found to depend upon the conditions used for the assay. Although unable to inhibit cAMP formation in intact platelets, both PAF and vasopressin caused pertussis toxin-sensitive inhibition of adenylyl cyclase in isolated membranes. Collectively, these observations suggest that 1) in platelets a single pertussis toxin-sensitive, alpha 41-containing G protein may be involved in the regulation of both adenylyl cyclase and phospholipase C and 2) additional constraints which are altered during membrane isolation may help to determine which enzyme is coupled to which agonist.     

CB1 receptor-G protein association. Subtype selectivity is determined by distinct intracellular domains.

The CB1 cannabinoid receptor in N18TG2 neuroblastoma cells inhibits adenylate cyclase, and this response can be mimicked by a peptide corresponding to the juxtamembrane C-terminal domain (CB(1)401-417). Guanosine 5'-O-(3-thio)triphosphate binding to G proteins can be stimulated by both peptide CB(1)401-417 and peptides corresponding to the third intracellular loop [Howlett, A.C., Song, C., Berglund, B.A., Wilken, G.H. & Pigg, J.J. (1998) Mol. Pharmacol. 53, 504-510; Mukhopadhyay, S., Cowsik, S.M., Welsh, W.J. & Howlett, A.C. (1999) Biochemistry 38, 3447-3455]. In Chaps-solubilized N18TG2 membranes, the CB1 receptor coimmunoprecipitated with all three Gi subtypes. Pertussis toxin significantly reduced the CB(1) receptor-G alpha(i) association and attenuated the CB(1)401-417-induced inhibition of adenylate cyclase. CB(1)401-417 significantly reduced the CB(1) receptor association with G alpha(i3), but not with G alpha(i1) or G alpha(i2). In contrast, third intracellular loop peptides significantly reduced the CB(1) receptor association with G alpha(i1) and G alpha(i2), but not G alpha(i3). These interactions are specific for the CB(1) receptor because a peptide corresponding to the juxtamembrane C-terminal domain of the CB(2) receptor failed to compete for the association of the CB1 receptor with any of the Gi alpha subtypes, and was not able to activate Gi proteins to inhibit adenylate cyclase. These studies indicate that different domains of the CB(1) receptor direct the interaction with specific G protein subtypes.     

Differential localization of G proteins, Gi and Go, in the olfactory epithelium and the main olfactory bulb of the rat.

The immunohistochemical localization of proteins Gi1 (plus Gi3). Gi2 and Go was studied in the olfactory epithelium and the main olfactory bulb of rats, using purified antibodies to the respective alpha subunits and beta gamma subunits of these G proteins. In the olfactory epithelium, only a restricted population of olfactory cells was immunopositive for Gi2 alpha, but others were not. The immunoreactivity for Gi1 alpha/Gi3 alpha was not observed. The olfactory epithelium was immunopositive for both Go alpha and beta gamma, but its apical surface was immunopositive only for beta gamma. In the main olfactory bulb, all layers were intensely immunopositive for Go alpha and beta gamma but weakly for Gi2 alpha. In contrast to the negative or weak immunostainings in the olfactory nerve fiber layer and glomeruli, the molecular and the internal granular layers were intensely immunopositive for Gi1 alpha/Gi3 alpha. These findings suggest the functional difference among Gi1/Gi3, Gi2 and Go in the signal transduction in the olfactory system.     

Regulation of NHE3 activity by G protein subunits in renal brush-border membranes

NHE3 activity is regulated by phosphorylation/dephosphorylation processes and membrane recycling in intact cells. However, the Na(+)/H(+) exchanger (NHE) can also be regulated by G proteins independent of cytoplasmic second messengers, but the G protein subunits involved in this regulation are not known. Therefore, we studied G protein subunit regulation of NHE3 activity in renal brush-border membrane vesicles (BBMV) in a system devoid of cytoplasmic components and second messengers. Basal NHE3 activity was not regulated by G(s)alpha or G(i)alpha, because antibodies to these G proteins by themselves were without effect. The inhibitory effect of D(1)-like agonists on NHE3 activity was mediated, in part, by G(s)alpha, because it was partially reversed by anti-G(s)alpha antibodies. Moreover, the amount of G(s)alpha that coimmunoprecipitated with NHE3 was increased by fenoldopam in both brush-border membranes and renal proximal tubule cells. Furthermore, guanosine 5'-O-(3-thiotriphosphate) but not guanosine 5'-O-(2-thiodiphosphate), the inactive analog of GDP, increased the amount of G(s)alpha that coimmunoprecipitated with NHE3. The alpha(2)-adrenergic agonist, UK-14304 or pertussis toxin (PTX) alone had no effect on NHE3 activity, but UK-14304 and PTX treatment attenuated the D(1)-like receptor-mediated NHE3 inhibition. The ability of UK-14304 to attenuate the D(1)-like agonist effect was not due to G(i)alpha, because the attenuation was not blocked by anti-G(i)alpha antibodies or by PTX. Anti-Gbeta(common) antibodies, by themselves, slightly inhibited NHE3 activity but had little effect on D(1)-like receptor-mediated NHE3 inhibition. However, anti-Gbeta(common) antibodies reversed the effects of UK-14304 and PTX on D(1)-like agonist-mediated NHE3 inhibition. These studies provide concrete evidence of a direct regulatory role for G(s)alpha, independent of second messengers, in the D(1)-like-mediated inhibition of NHE3 activity in rat renal BBMV. In addition, beta/gamma dimers of heterotrimeric G proteins appear to have a stimulatory effect on NHE3 activity in BBMV.     

Immunocytochemical and biochemical characterization of guanine nucleotide-binding regulatory proteins in mammalian spermatozoa

Polyclonal antisera directed against conserved and subtype-specific peptide sequences of the alpha-subunits of guanine nucleotide-binding regulatory proteins (G proteins) were used to characterize the nature of mammalian sperm G proteins and to determine whether their localization was consistent with their proposed roles in mediating ZP3-induced acrosomal exocytosis. Mouse and guinea pig sperm exhibit positive immunofluorescence in the acrosomal region using an antiserum directed against a peptide region common to all alpha-subunits of G proteins (G alpha). The immunofluorescence disappears after sperm have undergone the acrosome reaction, suggesting that the immunoreactive material is associated with the plasma membrane/outer acrosomal membrane region overlying the acrosome. The presence of G proteins in this region is confirmed by the presence of a Mr 41,000 substrate for pertussis toxin (PT)-catalyzed [32P]ADP-ribosylation in purified plasma membrane/outer acrosomal membrane hybrid vesicles obtained from acrosome-reacted guinea pig sperm. Immunoprecipitation and polyacrylamide gel electrophoresis of PT-catalyzed [32P]ADP-ribosylated protein(s) using anti-peptide antisera generated against sequences unique to Gi alpha 1, Gi alpha 2, and Gi alpha 3 confirm the existence of all three Gi subtypes in mouse sperm extracts. Indirect immunofluorescence using an antiserum directed against a peptide region present in Gz alpha, a PT-insensitive G protein, demonstrates positive immunoreactivity in the postacrosomal/lateral face region of the mouse sperm head. This immunoreactivity is retained during acrosomal exocytosis in response to solubilized ZP and then disappears subsequent to this exocytotic event. These data demonstrate that Gi protein alpha-subunits are present in the acrosomal region of mammalian sperm, consistent with their postulated role in regulating ZP3-mediated acrosomal exocytosis, and that PT-insensitive Gz alpha is found in a region of the sperm head distinct from that of the Gi alpha subunits.     

Inhibition of G alpha i2 activation by G alpha i3 in CXCR3-mediated signaling

G protein-coupled receptors (GPCRs) convey extracellular stimulation into dynamic intracellular action, leading to the regulation of cell migration and differentiation. T lymphocytes express G alpha(i2) and G alpha(i3), two members of the G alpha(i/o) protein family, but whether these two G alpha(i) proteins have distinguishable roles guiding T cell migration remains largely unknown because of a lack of member-specific inhibitors. This study details distinct G alpha(i2) and G alpha(i3) effects on chemokine receptor CXCR3-mediated signaling. Our data showed that G alpha(i2) was indispensable for T cell responses to three CXCR3 ligands, CXCL9, CXCL10, and CXCL11, as the lack of G alpha(i2) abolished CXCR3-stimulated migration and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) incorporation. In sharp contrast, T cells isolated from G alpha(i3) knock-out mice displayed a significant increase in both GTPgammaS incorporation and migration as compared with wild type T cells when stimulated with CXCR3 agonists. The increased GTPgammaS incorporation was blocked by G alpha(i3) protein in a dose-dependent manner. G alpha(i3)-mediated blockade of G alpha(i2) activation did not result from G alpha(i3) activation, but instead resulted from competition or steric hindrance of G alpha(i2) interaction with the CXCR3 receptor via the N terminus of the second intracellular loop. A mutation in this domain abrogated not only G alpha(i2) activation induced by a CXCR3 agonist but also the interaction of G alpha(i3) to the CXCR3 receptor. These findings reveal for the first time an interplay of G alpha(i) proteins in transmitting G protein-coupled receptor signals. This interplay has heretofore been masked by the use of pertussis toxin, a broad inhibitor of the G alpha(i/o) protein family.     

GTPase-deficient G alpha i2 oncogene gip2 inhibits adenylylcyclase and attenuates receptor-stimulated phospholipase A2 activity

The GTPase activity of a G protein alpha subunit functions as a timer to control the lifetime of the activated conformation of the protein. Expression of the GTPase-deficient Gi2 alpha subunit oncogene, gip2 (alpha i2Q205L), in Chinese hamster ovary cells inhibited the stimulation of adenylylcyclase and altered the calcium regulation of the Gi2-phospholipase A2 (PLA2) effector complex. The phenotypic consequence of the activated alpha i2 mutant on hormonal stimulation of PLA2 varied depending on the cytoplasmic calcium transient elicited by different Gi2-linked receptors. The stimulation of PLA2 by thrombin, which mobilized calcium only from internal stores, was markedly attenuated in gip2-expressing cells. In contrast, the attenuation of the PLA2 response to ATP, a purinergic agonist which mobilizes calcium from both extracellular space and internal stores, was significantly less than that observed for thrombin. Ionomycin, a calcium ionophore, stimulated PLA2 activity in clones which expressed gip2 to a level similar to that observed in wild-type Chinese hamster ovary cells. Thus, the dominant GTPase-deficient gip2 polypeptide will constitutively inhibit adenylylcyclase but differentially modulate enzymes regulated by calcium and coupled to Gi2.     

Differential capacities of the RGS1, RGS16 and RGS-GAIP regulators of G protein signaling to enhance alpha2A-adrenoreceptor agonist-stimulated GTPase activity of G(o1)alpha

Recombinant RGS1, RGS16 and RGS-GAIP, but not RGS2, were able to substantially further stimulate the maximal GTPase activity of G(o1)alpha promoted by agonists at the alpha2A-adrenoreceptor in a concentration-dependent manner. Kinetic analysis of the regulation of an alpha2A-adrenoreceptor-G(o1)alpha fusion protein by all three RGS proteins revealed that they had similar affinities for the receptor-G protein fusion. However, their maximal effects on GTP hydrolysis varied over threefold with RGS16 > RGS1 > RGS-GAIP. Both RGS1 and RGS16 reduced the potency of the alpha2A-adrenoreceptor agonist adrenaline by some 10-fold. A lower potency shift was observed for the partial agonist UK14304 and the effect was absent for the weak partial agonist oxymetazoline. Each of these RGS proteins altered the intrinsic activity of both UK14304 and oxymetazoline relative to adrenaline. Such results require the RGS interaction with G(o1)alpha to alter the conformation of the alpha2A-adrenoreceptor and are thus consistent with models invoking direct interactions between RGS proteins and receptors. These studies demonstrate that RGS1, RGS16 and RGS-GAIP show a high degree of selectivity to regulate alpha2A-adrenoreceptor-activated G(o1)alpha rather than G(i1)alpha, G(i2)alpha or G(i3)alpha and different capacities to inactivate this G protein.     

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.     

Analysis of the fibroblast transformation potential of GTPase-deficient gip2 oncogenes.

Expression of GTPase-deficient Gi2 alpha subunit (alpha i2) mutant polypeptides and overexpression of the wild-type alpha i2 polypeptide in Rat 1a, Swiss 3T3, and NIH 3T3 fibroblasts altered normal growth regulation and induced a loss of contact inhibition. In Rat 1a cells (but not in NIH 3T3 or Swiss 3T3 cells), expression of the GTPase-deficient alpha i2 mutant polypeptides allowed colony formation in soft agar, which correlated with a loss in anchorage dependence and a decreased serum requirement. The altered growth regulatory properties of Rat 1a cells induced by expression of alpha i2 mutant polypeptides was not significantly inhibited by cotransfection with a dominant negative Ha-ras mutant polypeptide (Asn-17rasH), indicating that the activated Gi2 membrane signal transduction protein is uniquely capable of altering the regulation of Rat 1a cell growth by a predominantly c-ras-independent mechanism. The results show that GTPase-deficient alpha i2 mutant polypeptides have the properties of an oncogene that can induce the phenotypic characteristics of transformation in Rat 1a cells but that only a subset of these changes is observed with NIH 3T3 and Swiss 3T3 cells.