Photoaffinity guanosine 5'-triphosphate analogs as a tool for the study of GTP-binding proteins

Differential expression profile of membrane proteins in Aplysia pleural–pedal ganglia under the stress of methyl parathion

This study was aimed to analyze the alteration of membrane protein profiles in Aplysia juliana Quoy & Gaimard (A. juliana) pleural–pedal ganglia under MP exposure. Both the results of GC–MS analysis and the activity assay of acetylcholinesterase (AChE), superoxide dismutase (SOD), catalase (CAT) reveal that MP toxicological effects on Aplysia left and right pleural–pedal ganglia are different under 7 and 14 days of exposure. Therefore, Aplysia were subjected for exposure at two concentrations (1 and 2 mg/l) of MP for 7 and 14 days for membrane proteomic study. As a result, 19 and 14 protein spots were differentially expressed in A. juliana left pleural–pedal ganglia under 7 and 14 days treatment, and 20 and 14 protein spots found with differential expressions in their right ganglia under the same treatment, respectively. Several proteins with expression variations were detected from both the left and right pleural–pedal ganglia; however, most proteins have distinctive expressions, indicating different mechanisms might be involved in initiating MP toxicology in left and right ganglia. Among the total differential protein spots obtained, 29 proteins were classed as membrane proteins. These proteins are mainly involved in the metabolism process, cell redox homeostasis, signal transduction, immunology, intracellular transport and catalysis, indicating MP toxicity in mollusks seems to be complex and diverse. Some differentially expressed proteins were further confirmed by Western blotting and quantitative real-time PCR. These results might provide renovated insights to reveal the mechanism of MP-induced neurotoxicity, and the novel candidate biomarkers might have potential application for environmental evaluation of MP pollution level.

Bacteriorhodopsin chimeras containing the third cytoplasmic loop of bovine rhodopsin activate transducin for GTP/GDP exchange

The mechanisms by which G-protein-coupled receptors (GPCRs) activate G-proteins are not well understood due to the lack of atomic structures of GPCRs in an active form or in GPCR/G-protein complexes. For study of GPCR/G-protein interactions, we have generated a series of chimeras by replacing the third cytoplasmic loop of a scaffold protein bacteriorhodopsin (bR) with various lengths of cytoplasmic loop 3 of bovine rhodopsin (Rh), and one such chimera containing loop 3 of the human beta2-adrenergic receptor. The chimeras expressed in the archaeon Halobacterium salinarum formed purple membrane lattices thus facilitating robust protein purification. Retinal was correctly incorporated into the chimeras, as determined by spectrophotometry. A 2D crystal (lattice) was evidenced by circular dichroism analysis, and proper organization of homotrimers formed by the bR/Rh loop 3 chimera Rh3C was clearly illustrated by atomic force microscopy. Most interestingly, Rh3C (and Rh3G to a lesser extent) was functional in activation of GTPgamma35S/GDP exchange of the transducin alpha subunit (Galphat) at a level 3.5-fold higher than the basal exchange. This activation was inhibited by GDP and by a high-affinity peptide analog of the Galphat C terminus, indicating specificity in the exchange reaction. Furthermore, a specific physical interaction between the chimera Rh3C loop 3 and the Galphat C terminus was demonstrated by cocentrifugation of transducin with Rh3C. This Galphat-activating bR/Rh chimera is highly likely to be a useful tool for studying GPCR/G-protein interactions.

Effect of tumor necrosis factor-alpha on the reciprocal G-protein-induced regulation of norepinephrine release by the alpha2-adrenergic receptor

Alpha2-adrenergic receptors control norepinephrine (NE) release and tumor necrosis factor-alpha (TNF) production from neurons. TNF regulates NE release, depending on alpha2-adrenergic receptor functioning. The relationship between TNF production in the brain and alpha2-adrenergic receptor activation could have profound control over NE release. TNF and alpha2-adrenergic regulation of NE release was investigated in rat hippocampal slices incubated with pertussis toxin (PTX). The alpha2-adrenergic receptor couples to Galpha(i/o)-proteins to inhibit NE release; however, in slices preexposed to PTX, alpha2-adrenergic receptor activation facilitates NE release. TNF exposure subsequent to PTX restores alpha2-adrenergic inhibition of NE release. PTX exposure of hippocampal slices prevents agonist-induced increases in Galpha(i/o) labeling with a GTP analog; after subsequent TNF exposure, agonist-induced increases in Galpha(i/o) labeling are restored. TNF regulation of NE release transforms from inhibition to facilitation depending on alpha2-adrenergic receptor activation following PTX exposure. Therefore, TNF directs the coupling of the alpha2-adrenergic receptor, ultimately affecting NE release.

Tubulin as a regulator of G-protein signaling

Tubulin is known to form high-affinity complexes with certain G proteins. The formation of such complexes allows tubulin to activate Galpha and fosters a system whereby elements of the cytoskeleton can influence G-protein signaling. This article describes the interaction between tubulin and G proteins and discusses methods for examining this interaction.

G beta gamma mediates the interplay between tubulin dimers and microtubules in the modulation of Gq signaling

Agonist stimulation causes tubulin association with the plasma membrane and activation of PLC beta 1 through direct interaction with, and transactivation of, G alpha q. Here we demonstrate that G beta gamma interaction with tubulin down-regulates this signaling pathway. Purified G beta gamma, alone or with phosphatidylinositol 4,5-bisphosphate (PIP2), inhibited carbachol-evoked membrane recruitment of tubulin and G alpha q transactivation by tubulin. Polymerization of microtubules elicited by G beta gamma overrode tubulin translocation to the membrane in response to carbachol stimulation. G beta gamma sequestration of tubulin reduced the inhibition of PLC beta 1 observed at high tubulin concentration. G beta 1 gamma 2 interacted preferentially with tubulin-GDP, whereas G alpha q was transactivated by tubulin-GTP. Prenylation of the gamma 2 polypeptide was required for G beta gamma/tubulin interaction. Both confocal microscopy and coimmunoprecipitation studies revealed the spatiotemporal pattern of G beta gamma/tubulin interaction during carbachol stimulation of neuroblastoma SK-N-SH cells. In resting cells G beta gamma localized predominantly at the cell membrane, whereas tubulin was found in well defined microtubules in the cytosol. Within 2 min of agonist exposure, a subset of tubulin translocated to the plasma membrane and colocalized with G beta. Fifteen min post-carbachol addition, tubulin and G beta colocalized in vesicle-like structures in the cytosol. G beta/tubulin colocalization increased after pretreatment of cells with the microtubule-depolymerizing agent, colchicine, and was inhibited by taxol. Taxol also inhibited carbachol-induced PIP2 hydrolysis. It is suggested that G beta gamma/tubulin interaction mediates internalization of membrane-associated tubulin at the offset of PLC beta 1 signaling. Newly cytosolic G beta gamma/tubulin complexes might promote microtubule polymerization attenuating further tubulin association with the plasma membrane. Thus G protein-coupled receptors might evoke G alpha and G beta gamma to orchestrate regulation of phospholipase signaling by tubulin dimers and control of cell shape by microtubules.

G protein signaling and the molecular basis of antidepressant action

Over the past four decades, a variety of interventions have been used for the treatment of clinical depression and other affective disorders. Several distinct pharmacological compounds show therapeutic efficacy. There are three major classes of antidepressant drugs: monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic compounds. There are also a variety of atypical antidepressant drugs, which defy ready classification. Finally, there is electroconvulsive therapy, ECT. All require chronic (2-3 weeks) treatment to achieve a clinical response. To date, no truly inclusive hypothesis concerning a mechanism of action for these diverse therapies has been formed. This review is intended to give an overview of research concerning G protein signaling and the molecular basis of antidepressant action. In it, the authors attempt to discuss progress that has been made in this arena as well as the possibility that some point (or points) along a G protein signaling cascade represent a molecular target for antidepressant therapy that might lead toward a unifying hypothesis for depression. This review is not designed to address the clinical studies. Furthermore, as it is a relatively short paper, citations to the literature are necessarily selective. The authors apologize in advance to authors whose work we have failed to cite.

Phosphatidylinositol 4,5-bisphosphate modifies tubulin participation in phospholipase Cbeta1 signaling

Tubulin forms the microtubule and regulates certain G-protein-mediated signaling pathways. Both functions rely on the GTP-binding properties of tubulin. Signal transduction through Galpha(q)-regulated phospholipase Cbeta1 (PLCbeta1) is activated by tubulin through a direct transfer of GTP from tubulin to Galpha(q). However, at high tubulin concentrations, inhibition of PLCbeta1 is observed. This report demonstrates that tubulin inhibits PLCbeta1 by binding the PLCbeta1 substrate phosphatidylinositol 4,5-bisphosphate (PIP2). Tubulin binding of PIP2 was specific, because PIP2 but not phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-phosphate, phosphatidylinositol, phosphatidylcholine, phosphatidylethanolamine, or inositol 1,4,5-trisphosphate inhibited microtubule assembly. PIP2 did not affect GTP binding or GTP hydrolysis by tubulin. Muscarinic agonists promoted microtubule depolymerization and translocation of tubulin to the plasma membrane. PIP2 augmented this process in both Sf9 cells, containing a recombinant PLCbeta1 pathway, and SK-N-SH neuroblastoma cells. Colocalization of tubulin and PIP2 at the plasma membrane was demonstrated with confocal laser immunofluorescence microscopy. Although tubulin bound to both Galpha(q) and PLCbeta1, PIP2 facilitated the interaction between tubulin and PLCbeta1 but not that between tubulin and Galpha(q). However, PIP2 did augment formation of tubulin--Galpha(q)-PLCbeta1 complexes. Subsequent to potentiating PLCbeta1 activation, sustained agonist-independent membrane binding of tubulin at PIP2- and PLCbeta1-rich sites appeared to inhibit Galpha(q) coupling to PLCbeta1. Furthermore, colchicine increased membrane-associated tubulin and also inhibited PLCbeta1 activity in SK-N-SH cells. Thus, tubulin, depending on local membrane concentration, may serve as a positive or negative regulator of phosphoinositide hydrolysis. Rapid changes in membrane lipid composition or in the cytoskeleton might modify neuronal signaling through such a mechanism.

Activation of the luteinizing hormone/choriogonadotropin hormone receptor promotes ADP ribosylation factor 6 activation in porcine ovarian follicular membranes

Previously we demonstrated in a cell-free ovarian follicular plasma membrane model that agonist-dependent desensitization of the luteinizing hormone/choriogonadotropin receptor (LH/CG R) is GTP-dependent, mimicked by the addition of ADP-ribosylation factor (ARF) nucleotide binding site opener, which acts as a guanine nucleotide exchange factor for ARFs 1 and 6, and selectively inhibited by synthetic N-terminal ARF6 peptides. We therefore sought direct evidence that activation of the LH/CG R promotes activation of ARF1 and/or ARF6. Using a classic ARF activation assay, the cholera toxin-catalyzed ADP-ribosylation of G alpha(s), results show that LH/CG R activation stimulates an ARF protein by a brefeldin A-independent mechanism. Synthetic N-terminal inhibitory ARF6 but not ARF1 peptide blocks LH/CG R-stimulated ARF activity. LH/CG R activation also promotes the binding of a photoaffinity GTP analog to a protein that migrates on one- and two-dimensional polyacrylamide gel electrophoresis with ARF6. These results suggest that ARF6 is the predominant ARF activated by the LH/CG R. To activate ARF6, the LH/CG R does not appear to signal through the C-terminal regions of G alpha(i) or G alpha(q) or through the second or third intracellular loops or the N terminus of the cytoplasmic tail of the LH/CG R. Although exogenous recombinant ARNO promotes only a small increase in ARF6 activation in the presence of activated LH/CG R, hCG-stimulated ARF6 activation is reduced to basal levels by catalytically inactive ARF nucleotide binding-site opener. These results provide direct evidence that LH/CG R activation leads to the activation of membrane-delimited ARF6.

Differences in Ca(2+) signaling underlie age-specific effects of secretagogues on colonic Cl(-) transport

Taurodeoxycholic acid (TDC) stimulates Cl(-) transport in adult (AD), but not weanling (WN) and newborn (NB), rabbit colonic epithelial cells (colonocytes). The present study demonstrates that stimuli like neurotensin (NT) are also age specific and identifies the age-dependent signaling step. Bile acid actions are segment and bile acid specific. Thus although TDC and taurochenodeoxycholate stimulate Cl(-) transport in AD distal but not proximal colon, taurocholate has no effect in either segment. TDC increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in AD, but not in WN and NB, colonocytes. In AD cells, TDC (5 min) action on Cl(-) transport needs intra- but not extracellular Ca(2+). NT, histamine, and bethanechol increase Cl(-) transport and [Ca(2+)](i) in AD, but not WN, distal colonocytes. However, A-23187 increased [Ca(2+)](i) and Cl(-) transport in all age groups, suggesting that Ca(2+)-sensitive Cl(-) transport is present from birth. Study of the proximal steps in Ca(2+) signaling revealed that NT, but not TDC, activates a GTP-binding protein, Galpha(q), in AD and WN cells. In addition, although WN and AD colonocytes had similar levels of phosphatidylinositol 4,5-bisphosphate, NT and TDC increased 1,4,5-inositol trisphosphate content only in AD cells. Nonresponsiveness of WN cells to Ca(2+)-dependent stimuli, therefore, is due to the absence of measurable phospholipase C activity. Thus delays in Ca(2+) signaling afford a crucial protective mechanism to meet the changing demands of the developing colon.

Tubulin stimulates adenylyl cyclase activity in C6 glioma cells by bypassing the beta-adrenergic receptor: a potential mechanism of G protein activation

While the cytoskeleton is known to play several roles in the biology of the cell, one role, which has been revealed only recently, is that of a participant in the signal transduction process. Tubulin binds specifically to the alpha subunits of Gs (stimulatory GTP-binding regulatory protein of adenylyl cyclase), Gi1 (inhibitory protein of adenylyl cyclase), and Gq and transactivates those molecules through direct transfer of GTP. The relevance of this transactivation process to G proteins which are normally activated by a neurotransmitter-occupied receptor is the subject of this study. C6 glioma cells, made permeable with saponin, retained tight coupling between Gs and the beta-adrenergic receptor. Although 5-guanylylimidodiphosphate (GppNHp) was incapable of activating Gs (and subsequently, adenylyl cyclase) in the absence of agonist, tubulin with GppNHp bound (tubulin-GppNHp) activated adenylyl cyclase with an EC(50) of 30 nM. Desensitization of beta-adrenergic receptors by isoproterenol exposure had no effect on the ability of tubulin-GppNHp to activate Gs and adenylyl cyclase. When the photoaffinity GTP analog, azidoanilido GTP (AAGTP; P3(4-azidoanilido)-P1-5'-GTP), was added to C6 membranes or permeable C6 cells, it was only weakly incorporated by G alpha s in the absence of isoproterenol. When the same concentration of dimeric tubulin with AAGTP bound was introduced, AAGTP was transferred from tubulin to G alpha s, activating the latter species. Similar 'preferential' activation of G alpha s by tubulin-AAGTP versus the free nucleotide was seen using purified components. Thus, membrane-associated tubulin may serve to activate G alpha s, independent of signals not normally coupled to that protein. Tubulin may act as an agent to link a variety of membrane-associated signalling systems.

Optimized binding of [35S]GTPgammaS to Gq-like proteins stimulated with dopamine D1-like receptor agonists

Subtypes of dopamine D1-like receptors are coupled through the G proteins Gs or Gq to stimulate either adenylate cyclase or phospholipase C signaling cascades. In the present study, we have uncovered the marked enhancement by sodium deoxycholate of D1-like agonist-stimulated [35S]GTPgammaS binding to Gq-like G proteins in brain membranes, and determined the optimal experimental conditions for assessing agonist effects on [35S]GTPgammaS binding in the presence of the detergent. Factors and their optimal levels that were found to significantly enhance the sensitivity and robustness of the agonist-stimulated [35S]GTPyS binding reaction include protein concentration at 40 microg/ml, cationic concentrations of 120 mM Na+, 1.8 mM K+, and 20 mM Mg(2+), a molar guanine nucleotide ratio of 100,000 GDP to [35S]GTPgammaS, the presence of 1 mM deoxycholate, and an overall incubation duration of 30-120 min. Under the optimized conditions, the D1-like agonist SKF38393 induced potent and highly efficacious (up to 1000%) stimulation of [35S]GTPgammaS binding in membrane preparations from the striatum and other rat brain regions. In striatal membranes incubated with drug for 2 h, immunoprecipitation of the [35S]GTPgammaS-bound proteins with specific Galpha antibodies showed that at least 70% of SKF38393-stimulated [35S]GTPgammaS binding was to Galphaq. The present reaction parameters are consistent with conditions previously found to support dopaminergic stimulation of phospholipase C-mediated signaling in brain slice preparations. These results imply that different but equally physiologically relevant conditions can be obtained under which subtypes of dopaminergic receptors may couple preferentially to Galphas and the adenylate cyclase pathway or to Galphaq and the phospholipase C pathway.

Muscarinic receptor activation promotes the membrane association of tubulin for the regulation of Gq-mediated phospholipase Cbeta(1) signaling

The microtubule protein tubulin regulates adenylyl cyclase and phospholipase Cbeta(1) (PLCbeta(1)) signaling via transactivation of the G-protein subunits Galphas, Galphai1, and Galphaq. Because most tubulin is not membrane associated, this study investigates whether tubulin translocates to the membrane in response to an agonist so that it might regulate G-protein signaling. This was studied in SK-N-SH neuroblastoma cells, which possess a muscarinic receptor-regulated PLCbeta(1)-signaling pathway. Tubulin, at nanomolar concentrations, transactivated Galphaq by the direct transfer of a GTP analog and potentiated carbachol-activated PLCbeta(1). A specific and time-dependent association of tubulin with plasma membranes was observed when SK-N-SH cells were treated with carbachol. The same phenomenon was observed with membranes from Sf9 cells, expressing a recombinant PLCbeta(1) cascade. The time course of this event was concordant both with transactivation of Galphaq by the direct transfer of [(32)P]P(3)(4-azidoanilido)-P(1)-5'-GTP from tubulin as well as with the activation of PLCbeta(1). In SK-N-SH cells, carbachol induced a rapid and transient translocation of tubulin to the plasma membrane, microtubule reorganization, and a change in cell shape as demonstrated by confocal immunofluorescence microscopy. These observations presented a spatial and temporal resolution of the sequence of events underlying receptor-evoked involvement of tubulin in G-protein-mediated signaling. It is suggested that G-protein-coupled receptors might modulate cytoskeletal dynamics, intracellular traffic, and cellular architecture.

Human substance P receptor expressed in Chinese hamster ovary cells directly activates G(alpha q/11), G(alpha s), G(alpha o)

Substance P receptor (SPR) stably expressed in Chinese hamster ovary (CHO) cells stimulates at least three second messenger systems including phosphoinositide hydrolysis, cyclic AMP (cAMP) formation, and arachidonic acid release. Whether these second messenger systems are activated via single or multiple G proteins is not known. Therefore, in the present study we examined whether human SPR (hSPR) stably expressed in CHO cells activates multiple G proteins. This was achieved by photoaffinity labeling of G(alpha)-subunits with [32P]azidoanilido-GTP ([32P]AA-GTP) upon hSPR stimulation in CHO-hSPR membranes followed by immunoprecipitation of the labeled G(alpha)-subunits with antibodies specific for various G(alpha)-subunits. These experiments reveal that hSPR directly activates G(alpha q/11), G(alpha s) and G(alpha o). While hSPR is known to couple G(alpha q/11), the present study provides the first evidence that hSPR can also activate G(alpha s) and G(alpha o) in a mammalian system.

Human substance P receptor expressed in Sf9 cells couples with multiple endogenous G proteins

To identify the G proteins involved in the function of human substance P receptor (hSPR), the receptor was expressed in Sf9 cells using the baculovirus expression system. Maximal hSPR expression was up to 65 pmol/mg membrane protein. The following data indicated that hSPR in Sf9 membranes is coupled to endogenous G proteins: 1) binding of agonist radioligand [125I]BHSP to the receptor was sensitive to guanine nucleotides; and 2) stimulation of the receptor increased [35S]GTPgammaS binding. The hSPR-associated G proteins were identified by photoaffinity labeling with [alpha-32P]-azidoanilido GTP ([alpha-32P]AAGTP), followed by immunoprecipitation of the labeled G proteins with antibodies specific for various Galpha-subunits. These experiments showed that stimulation of hSPR in Sf9 membranes activated multiple endogenous G proteins including Galpha(o), Galpha(q/11), and Galpha(s). While hSPR's ability to associate with Gq/11 is well-documented, the present study provides the first evidence of hSPR's potential to activate Galpha(o) and Galpha(s).

Constitutive activation of the gastrin-releasing peptide receptor expressed by the nonmalignant human colon epithelial cell line NCM460

Gastrin-releasing peptide (GRP) causes multiple effects in humans by activating a specific heptaspanning receptor. Within the gastrointestinal tract, GRP receptors (GRP-R) are not normally expressed by mucosal epithelial cells except for those lining the gastric antrum. In contrast, recent studies have shown that up to 40% of resected colon cancers aberrantly express this receptor. This is important because the GRP-R can cause the proliferation of many, but not all, tissues in which it is expressed. Since GRP and other agonists are not known to exist in the colonic lumen, it has not been clear how or even if GRP-R expression in colon cancer contributes to cell proliferation. To evaluate the functional consequence of GRP-R expression on colonic epithelium, we transfected the recently isolated nonmalignant human colon epithelial cell line NCM460 with the cDNA for this receptor. All NCM460 cell lines expressing varying numbers of GRP-R bound selected agonists and antagonists indistinguishably from receptors expressed by other human tissues. Furthermore GRP-R-expressing transfected cell lines, but not wild-type NCM460 cells, proliferated independently of serum or other growth factors. Further evaluation revealed that GRP-R in these cells tonically stimulated G alpha q/11, resulting in increased phospholipase C activation. Since transfected cells do not secrete GRP, nor is their growth influenced by exposure to receptor-specific antagonists, these data indicate that GRP-R ectopically expressed by NCM460 cells are constitutively active. This report provides the first evidence of mutation-independent heptaspanning receptor constitutive activation resulting in cell proliferation, and identifies a potential mechanism whereby the GRP-R may act as an oncogene in human colon cancer.

Tubulin, Gq, and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cbeta1 signaling

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein alpha subunits, Galphas or Galphai1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Galpha conformation. To study the possibility of tubulin involvement in Galphaq-mediated phospholipase Cbeta1 (PLCbeta1) signaling, the m1 muscarinic receptor, Galphaq, and PLCbeta1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCbeta1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCbeta1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Galphaq and PLCbeta1, accompanied by guanine nucleotide transfer from tubulin to Galphaq, is suggested as a mechanism for the enzyme activation. The PLCbeta1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCbeta1 by tubulin, since high tubulin concentrations might prevent the access of PLCbeta1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Galphaq for tubulin. Involvement of tubulin in the articulation between PLCbeta1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events.

Synthesis of azidoaniline derivatives of oligonucleotides and investigation of their photochemical behavior

A series of aryl azides, p-N3C6H4NH(CH2)nNH2 with n = 2-6, have been synthesized and used to prepare oligonucleotide derivatives carrying photoreactive the p-azidoaniline residue. Reactive moieties have been coupled to the 5'-terminal phosphate of d(pGATACCAA) [compounds IV(b), IV(c), and IV(e) with n = 3, 4, and 6, respectively] and of d(pGCC) [compound V(b) with n = 3] via a phosphoamide bond. Irradiation at wavelengths over > 300 nm of IV(b) and V(b) (n = 3) resulted in cleavage of the P-N bond. However, under the same reaction conditions, the P-N bond remained intact for compounds containing longer spacers [IV(c) and IV(e)]. Intraduplex reaction of the latter derivatives with d(GGTATCp)NH(CH2)6NH2 resulted in cross-linking dependent on the presence of an aliphatic amino group. The results obtained have demonstrated that the azidoaniline derivatives of oligonucleotides capable of the affinity modification of a specific target can be prepared. However, the sufficiently long aliphatic spacer group is necessary to prevent P-N bond cleavage within the photoreactive oligonucleotide.

Evidence that the 50-kDa substrate of brefeldin A-dependent ADP-ribosylation binds GTP and is modulated by the G-protein beta gamma subunit complex

Brefeldin A, a fungal metabolite that inhibits membrane transport, induces the mono(ADP-ribosyl)ation of two cytosolic proteins of 38 and 50 kDa as judged by SDS/PAGE. The 38-kDa substrate has been previously identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). We report that the 50-kDa BFA-induced ADP-ribosylated substrate (BARS-50) has native forms of 170 and 130 kDa, as determined by gel filtration of rat brain cytosol, indicating that BARS-50 might exist as a multimeric complex. BARS-50 can bind GTP, as indicated by blot-overlay studies with [alpha-32P]GTP and by photoaffinity labeling with guanosine 5'-[gamma-32P] [beta,gamma-(4-azidoanilido)]triphosphate. Moreover, ADP-ribosylation of BARS-50 was completely inhibited by the beta gamma subunit complex of G proteins, while the ADP-ribosylation of GAPDH was unmodified, indicating that this effect was due to an interaction of the beta gamma complex with BARS-50, rather than with the ADP-ribosylating enzyme. Two-dimensional gel electrophoresis and immunoblot analysis shows that BARS-50 is a group of closely related proteins that appear to be different from all the known GTP-binding proteins.