Cloning and expression of cDNA for arginine-specific ADP-ribosyltransferase from chicken bone marrow cells

Overview of the mammalian ADP-ribosyl-transferases clostridia toxin-like (ARTCs) family

Mono-ADP-ribosylation is a reversible post-translational protein modification that modulates the function of proteins involved in different cellular processes, including signal transduction, protein transport, transcription, cell cycle regulation, DNA repair and apoptosis. In mammals, mono-ADP-ribosylation is mainly catalyzed by members of two different classes of enzymes: ARTCs and ARTDs. The human ARTC family is composed of four structurally related ecto-mono-ARTs, expressed at the cell surface or secreted into the extracellular compartment that are either active mono-ARTs (hARTC1, hARTC5) or inactive proteins (hARTC3, hARTC4). The human ARTD enzyme family consists of 17 multidomain proteins that can be divided on the basis of their catalytic activity into polymerases (ARTD1-6), mono-ART (ARTD7-17), and the inactive ARTD13. In recent years, ADP-ribosylation was intensively studied, and research was dominated by studies focusing on the role of this modification and its implication on various cellular processes. The aim of this review is to provide a general overview of the ARTC enzymes. In the following sections, we will report the mono-ADP-ribosylation reactions that are catalysed by the active ARTC enzymes, with a particular focus on hARTC1 that recently has been intensively studied with the discovery of new targets and functions.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

A new detection method for arginine-specific ADP-ribosylation of protein -- a combinational use of anti-ADP-ribosylarginine antibody and ADP-ribosylarginine hydrolase

Arginine-specific ADP-ribosylation is one of the posttranslational modifications of proteins by transferring one ADP-ribose moiety of NAD to arginine residues of target proteins. This modification, catalyzed by ADP-ribosyltransferase (Art), is reversed by ADP-ribosylarginine hydrolase (AAH). In this study, we describe a new method combining an anti-ADP-ribosylarginine antibody (alphaADP-R-Arg Ab) and AAH for detection of the target protein of ADP-ribosylation. We have raised alphaADP-R-Arg Ab with ADP-ribosylated histone and examined the reactivity of the antibody with proteins treated by Art and/or AAH, as well as in situ ADP-ribosylation system with mouse T cells. Our results indicate that the detection of ADP-ribosylated protein with alphaADP-R-Arg Ab and AAH is a useful tool to explore the target proteins of ADP-ribosylation. We applied the method to search endogenously ADP-ribosylated protein in the rat, and detected possible target proteins in the skeletal muscle, which has high Art activity.

Purification, characterization and molecular cloning of glycosylphosphatidylinositol-anchored arginine-specific ADP-ribosyltransferases from chicken

Mono-ADP-ribosylation is a post-translational modification that regulates the functions of target proteins or peptides by attaching an ADP-ribose moiety. Here we report the purification, molecular cloning, characterization and tissue-specific distribution of novel arginine-specific Arts (ADP-ribosyltransferases) from chicken. Arts were detected in various chicken tissues as GPI (glycosylphosphatidylinositol)-anchored forms, and purified from the lung membrane fraction. By molecular cloning based on the partial amino acid sequence using 5'- and 3'-RACE (rapid amplification of cDNA ends), two full-length cDNAs of chicken GPI-anchored Arts, cgArt1 (chicken GPI-anchored Art1) and cgArt2, were obtained. The cDNA of cgArt1 encoded a novel polypeptide of 298 amino acids which shows a high degree of identity with cgArt2 (82.9%), Art6.1 (50.2%) and rabbit Art1 (42.1%). In contrast, the nucleotide sequence of cgArt2 was identical with that of Art7 cloned previously from chicken erythroblasts. cgArt1 and cgArt2 proteins expressed in DT40 cells were shown to be GPI-anchored Arts with a molecular mass of 45 kDa, and these Arts showed different enzymatic properties from the soluble chicken Art, Art6.1. RNase protection assays and real-time quantitative PCR revealed distinct expression patterns of the two Arts; cgArt1 was expressed predominantly in the lung, spleen and bone marrow, followed by the heart, kidney and muscle, while cgArt2 was expressed only in the heart and skeletal muscle. Thus GPI-anchored Arts encoded by the genes cgArt1 and cgArt2 are expressed extensively in chicken tissues. It may be worthwhile determining the functional roles of ADP-ribosylation in each tissue.

Mono-ADP-ribosylation: a tool for modulating immune response and cell signaling

Mono-ADP-ribosylation is a posttranslational modification of cellular proteins that has the potential to regulate various cell functions. This reaction consists of the enzymatic transfer of ADP-ribose to specific acceptor amino acid residues (predominantly arginine and cysteine). The best-known cellular ADP-ribosyltransferases (the enzymes that catalyze this reaction) are the seven ectoenzymes, members of the ART family. Recently, ADP-ribosylated human neutrophil-derived peptide (HNP-1, an antimicrobial peptide secreted by immune cells) has been identified in the bronchoalveolar lavage fluid from individuals who smoke cigarettes. This demonstrates that ADP-ribosylation of HNP-1 occurs in vivo. In vitro experiments have indicated that ART-1, an enzyme also present in the airway epithelium, specifically modifies Arg(14) of the HNP-1, causing the loss of the peptide's antimicrobial and cytotoxic activity, while preserving its chemotactic activity. From a functional point of view, these data support a role of ADP-ribosylation in the innate immune response. Additional functions proposed for the ADP-ribosylation reaction involve the intracellular ADP-ribosyltransferases, which are molecularly unrelated to the ARTs and intervene in cell signaling and metabolism cascades. The growing understanding of the biological roles of protein and peptide ADP-ribosylation represents a powerful tool for novel pharmacological interventions.

Genomewide identification of proteins secreted by the Hrp type III protein secretion system of Pseudomonas syringae pv. tomato DC3000

The ability of Pseudomonas syringae pv. tomato DC3000 to be pathogenic on plants depends on the Hrp (hypersensitive response and pathogenicity) type III protein secretion system and the effector proteins it translocates into plant cells. Through iterative application of experimental and computational techniques, the DC3000 effector inventory has been substantially enlarged. Five homologs of known avirulence (Avr) proteins and five effector candidates, encoded by genes with putative Hrp promoters and signatures of horizontal acquisition, were demonstrated to be secreted in culture and/or translocated into Arabidopsis in a Hrp-dependent manner. These 10 Hrp-dependent outer proteins (Hops) were designated HopPtoC (AvrPpiC2 homolog), HopPtoD1 and HopPtoD2 (AvrPphD homologs), HopPtoK (AvrRps4 homolog), HopPtoJ (AvrXv3 homolog), HopPtoE, HopPtoG, HopPtoH, HopPtoI, and HopPtoS1 (an ADP-ribosyltransferase homolog). Analysis of the enlarged collection of proteins traveling the Hrp pathway in P. syringae revealed an export-associated pattern of equivalent solvent-exposed amino acids in the N-terminal five positions, a lack of Asp or Glu residues in the first 12 positions, and amphipathicity in the first 50 positions. These characteristics were used to search the unfinished DC3000 genome, yielding 32 additional candidate effector genes that predicted proteins with Hrp export signals and that also possessed signatures of horizontal acquisition. Among these were genes encoding additional ADP-ribosyltransferases, a homolog of SrfC (a candidate effector in Salmonella enterica), a catalase, and a glucokinase. One ADP-ribosyltransferase and the SrfC homolog were tested and shown to be secreted in a Hrp-dependent manner. These proteins, designated HopPtoS2 and HopPtoL, respectively, bring the DC3000 Hrp-secreted protein inventory to 22.

Structure, chromosomal localization, and expression of the gene for mouse ecto-mono(ADP-ribosyl)transferase ART5

Mono(ADP-ribosyl)transferases regulate the function of target proteins by attaching ADP-ribose to specific amino acid residues in their target proteins. The purpose of this study was to determine the structure, chromosomal localization, and expression profile of the gene for mouse ecto-ADP-ribosyltransferase ART5. Southern blot analyses indicate that Art5 is a single copy gene which maps to mouse chromosome 7 at offset 49.6 cM in close proximity to the Art1, Art2a and Art2b genes. Northern blot and RT-PCR analyses demonstrate prominent expression of Art5 in testis, and lower levels in cardiac and skeletal muscle. Sequence analyses reveal that the Art5 gene encompasses six exons spanning 8 kb of genomic DNA. The 5' end of the Art5 gene overlaps with that of the Art1 gene. A single long exon encodes the predicted ART5 catalytic domain. Separate exons encode the N-terminal leader peptide and a hydrophilic C-terminal extension. Sequencing of RT-PCR products and ESTs identified six splice variants. The deduced amino acid sequence of ART5 shows 87% sequence identity to its orthologue from the human, and 37 and 32% identity to its murine paralogues ART1 and ART2. Unlike ART1 and ART2, ART5 lacks a glycosylphosphatidylinositol-anchor signal sequence and is predicted to be a secretory enzyme. This prediction was confirmed by transfecting an Art5 cDNA expression construct into Sf9 insect cells. The secreted epitope-tagged ART5 protein resembled rat ART2 in exhibiting potent NAD-glycohydrolase activity. This study provides important experimental tools to further elucidate the function of ART5.

Nicotinamide adenine dinucleotide (NAD) and its metabolites inhibit T lymphocyte proliferation: role of cell surface NAD glycohydrolase and pyrophosphatase activities

The presence of NAD-metabolizing enzymes (e.g., ADP-ribosyltransferase (ART)2) on the surface of immune cells suggests a potential immunomodulatory activity for ecto-NAD or its metabolites at sites of inflammation and cell lysis where extracellular levels of NAD may be high. In vitro, NAD inhibits mitogen-stimulated rat T cell proliferation. To investigate the mechanism of inhibition, the effects of NAD and its metabolites on T cell proliferation were studied using ART2a+ and ART2b+ rat T cells. NAD and ADP-ribose, but not nicotinamide, inhibited proliferation of mitogen-activated T cells independent of ART2 allele-specific expression. Inhibition by P2 purinergic receptor agonists was comparable to that induced by NAD and ADP-ribose; these compounds were more potent than P1 agonists. Analysis of the NAD-metabolizing activity of intact rat T cells demonstrated that ADP-ribose was the predominant metabolite, consistent with the presence of cell surface NAD glycohydrolase (NADase) activities. Treatment of T cells with phosphatidylinositol-specific phospholipase C removed much of the NADase activity, consistent with at least one NADase having a GPI anchor; ART2- T cell subsets contained NADase activity that was not releasable by phosphatidylinositol-specific phospholipase C treatment. Formation of AMP from NAD and ADP-ribose also occurred, a result of cell surface pyrophosphatase activity. Because AMP and its metabolite, adenosine, were less inhibitory to rat T cell proliferation than was NAD or ADP-ribose, pyrophosphatases may serve a regulatory role in modifying the inhibitory effect of ecto-NAD on T cell activation. These data suggest that T cells express multiple NAD and adenine nucleotide-metabolizing activities that together modulate immune function.

New functions of a long-known molecule. Emerging roles of NAD in cellular signaling

Over the past decades, the pyridine nucleotides have been established as important molecules in signaling pathways, besides their well known function in energy transduction. Similarly to another molecule carrying such dual functions, ATP, NAD(P)+ may serve as substrate for covalent protein modification or as precursor of biologically active compounds. Protein modification is catalyzed by ADP-ribosyl transferases that attach the ADP-ribose moiety of NAD+ to specific amino-acid residues of the acceptor proteins. For a number of ADP ribosylation reactions the specific transferases and their target proteins have been identified. As a result of the modification, the biological activity of the acceptor proteins may be severely changed. The cell nucleus contains enzymes catalyzing the transfer of ADP-ribose polymers (polyADP-ribose) onto the acceptor proteins. The best known enzyme of this type is poly(ADP-ribose) polymerase 1 (PARP1), which has been implicated in the regulation of several important processes including DNA repair, transcription, apoptosis, neoplastic transformation and others. The second group of reactions leads to the synthesis of an unusual cyclic nucleotide, cyclic ADP-ribose (cADPR). Moreover, the enzymes catalyzing this reaction may also replace the nicotinamide of NADP+ by nicotinic acid resulting in the synthesis of nicotinic acid adenine dinucleotide phosphate (NAADP+). Both cADPR and NAADP+ have been reported to be potent intracellular calcium-mobilizing agents. In concert with inositol 1,4,5-trisphosphate, they participate in cytosolic calcium regulation by releasing calcium from intracellular stores.

Modification of the ADP-ribosyltransferase and NAD glycohydrolase activities of a mammalian transferase (ADP-ribosyltransferase 5) by auto-ADP-ribosylation

Mono-ADP-ribosylation, a post-translational modification in which the ADP-ribose moiety of NAD is transferred to an acceptor protein, is catalyzed by a family of amino acid-specific ADP-ribosyltransferases. ADP-ribosyltransferase 5 (ART5), a murine transferase originally isolated from Yac-1 lymphoma cells, differed in properties from previously identified eukaryotic transferases in that it exhibited significant NAD glycohydrolase (NADase) activity. To investigate the mechanism of regulation of transferase and NADase activities, ART5 was synthesized as a FLAG fusion protein in Escherichia coli. Agmatine was used as the ADP-ribose acceptor to quantify transferase activity. ART5 was found to be primarily an NADase at 10 microM NAD, whereas at higher NAD concentrations (1 mM), after some delay, transferase activity increased, whereas NADase activity fell. This change in catalytic activity was correlated with auto-ADP-ribosylation and occurred in a time- and NAD concentration-dependent manner. Based on the change in mobility of auto-ADP-ribosylated ART5 by SDS-polyacrylamide gel electrophoresis, the modification appeared to be stoichiometric and resulted in the addition of at least two ADP-ribose moieties. Auto-ADP-ribosylated ART5 isolated after incubation with NAD was primarily a transferase. These findings suggest that auto-ADP-ribosylation of ART5 was stoichiometric, resulted in at least two modifications and converted ART5 from an NADase to a transferase, and could be one mechanism for regulating enzyme activity.

11,12-Epoxyeicosatrienoic acid stimulates endogenous mono-ADP-ribosylation in bovine coronary arterial smooth muscle

The role of endogenous ADP-ribosylation in mediating the activation of the Ca(2+)-activated K(+) channels was determined in bovine coronary arteries. Endogenous ADP-ribosylation was examined by incubating coronary arterial homogenates or lysates of cultured coronary arterial smooth muscle cells with [adenylate-(32)P]NAD. Four (32)P-labeled proteins were observed at 51, 52, 80, and 124 kDa in the homogenates and lysates. This reaction was enhanced by the addition of 11,12-epoxyeicosatrienoic acid (11,12-EET), a cytochrome P450-derived eicosanoid, and GTP to the incubation. By Western blot analysis, 42- and 70-kDa proteins were recognized by specific antibodies against ADP-ribosyltransferase in the coronary arterial homogenates and smooth muscle cell lysate but not in the lysate of endothelial cells. The 52-kDa acceptor protein of endogenous ADP-ribosylation comigrated with a protein ADP-ribosylated by cholera toxin and was recognized and immunoprecipitated by an anti-G(S)alpha antibody. These results suggest that G(S)alpha is one of several acceptors of the ADP-ribose moiety. As shown by the patch-clamp technique, 11,12-EET stimulated the activation of the K(+) channels in the smooth muscle cells, and this activation was completely blocked by novobiocin, vitamin K(1), 3-aminobenzamide, and m-iodobenzylguanidine, inhibitors of endogenous mono-ADP-ribosyltransferases. We conclude that endogenous mono-ADP-ribosyltransferases are present in smooth muscle from bovine coronary arteries. These enzymes transfer ADP-ribose to the cellular proteins such as G(S)alpha and may mediate intracellular signal transduction in coronary vascular smooth muscle. In the coronary circulation, the ADP-ribosylation signaling pathway may play an important role in mediating the activation of the K(+) channels induced by 11,12-EET.

Characterization of glycosylphosphatidylinositiol-anchored, secreted, and intracellular vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation is a posttranslational modification of proteins in which the ADP-ribose moiety of nicotinamide adenine dinucleotide is transferred to an acceptor amino acid. Five mammalian ADP-ribosyltransferases (ART1--ART5) have been cloned and expression is restricted to tissues such as cardiac and skeletal muscle, leukocytes, brain, and testis. ART1 and ART2 are glycosylphosphatidylinositol (GPI)-anchored ectoenzymes. ART5 appears not to be GPI-linked and may be secreted. In skeletal muscle and lymphocytes, ART1 modifies specific members of the integrin family of adhesion molecules, suggesting that ADP-ribosylation affects cell-matrix or cell-cell interactions. In lymphocytes, ADP-ribosylation of surface proteins is associated with changes in p56lck tyrosine kinase-mediated signaling. The catalytic sites of bacterial toxins and vertebrate transferases have conserved structural features, consistent with a common reaction mechanism. ADP-ribosylation can be reversed by ADP-ribosylarginine hydrolases, resulting in the regeneration of free arginine. Thus, an ADP-ribosylation cycle may play a regulatory role in vertebrate tissues.

Pseudomonas aeruginosa exoenzyme S, a double ADP-ribosyltransferase, resembles vertebrate mono-ADP-ribosyltransferases

Previous data indicated that Pseudomonas aeruginosa exoenzyme S (ExoS) ADP-ribosylated Ras at multiple sites. One site appeared to be Arg41, but the second site could not be localized. In this study, the sites of ADP-ribosylation of c-Ha-Ras by ExoS were directly determined. Under saturating conditions, ExoS ADP-ribosylated Ras to a stoichiometry of 2 mol of ADP-ribose incorporated per mol of Ras. Nucleotide occupancy did not influence the stoichiometry or velocity of ADP-ribosylation of Ras by ExoS. Edman degradation and mass spectrometry of V8 protease generated peptides of ADP-ribosylated Ras identified the sites of ADP-ribosylation to be Arg41 and Arg128. ExoS ADP-ribosylated the double mutant, RasR41K,R128K, to a stoichiometry of 1 mol of ADP-ribose incorporated per mol of Ras, which indicated that Ras possessed an alternative site of ADP-ribosylation. The alternative site of ADP-ribosylation on Ras was identified as Arg135, which was on the same alpha-helix as Arg128. Arg41 and Arg128 are located within two different secondary structure motifs, beta-sheet and alpha-helix, respectively, and are spatially separated within the three-dimensional structure of Ras. The fact that ExoS could ADP-ribosylate a target protein at multiple sites, along with earlier observations that ExoS could ADP-ribosylate numerous target proteins, were properties that have been attributed to several vertebrate ADP-ribosyltransferases. This prompted a detailed alignment study which showed that the catalytic domain of ExoS possessed considerably more primary amino acid homology with the vertebrate mono-ADP-ribosyltransferases than the bacterial ADP-ribosyltransferases. These data are consistent with the hypothesis that ExoS may represent an evolutionary link between bacterial and vertebrate mono-ADP-ribosyltransferases.

The effects of mono-ADP-ribosylation on desmin assembly-disassembly

Previous studies have shown that desmin, the muscle-specific intermediate filament protein, is a substrate for the endogenous muscle arginine-specific mono-ADP-ribosyltransferase and that ADP-ribosylation inhibits assembly of desmin into intermediate filaments (Huang et al., Exp. Cell Res. 226, 147-153, 1996). In this paper, the effects of mono-ADP-ribosylation on assembly and disassembly of desmin intermediate filaments were further characterized. First, it was found that ADP-ribosylated desmin does not coassemble with unmodified desmin and has no effect on assembly of unmodified desmin. Second, incubation of assembled desmin filaments with mono-ADP-ribosyltransferase and NAD+ results in disassembly of the filaments. Finally, the structural components of the attached ADP-ribose moiety responsible for altering the assembly of desmin into filaments were investigated by a stepwise cleavage of ADP-ribose with snake venom phosphodiesterase and alkaline phophatase, followed by analysis of assembly. The reactions catalyzed by these two enzymes were established using a desmin peptide as a substrate. Our results show that ribosylated desmin, but not phosphoribosylated desmin, was able to self-assemble into intermediate filaments, suggesting that the presence of a phosphate group is needed to alter desmin's assembly ability.

Mouse Rt6.1 is a thiol-dependent arginine-specific ADP-ribosyltransferase

Mouse T-cell antigens Rt6.1 and Rt6.2 are glycosylphosphatidylinositol-anchored arginine-specific adenosine diphosphate (ADP)-ribosyltransferases. In the present study, we obtained evidence that an arginine-specific ADP-ribosyltransferase activity liberated from BALB/c mouse splenocytes by phosphatidylinositol-specific phospholipase C increased fivefold in the presence of dithiothreitol and that the activity was immunoprecipitated by polyclonal antibodies generated against recombinant rat RT6.1. When mouse Rt6.1 was expressed as a recombinant protein, the transferase activity of Rt6.1 was stimulated by dithiothreitol, and inhibited by N-ethylmaleimide, while activities of recombinant mouse Rt6.2 and the Glu-207 mutant of rat RT6.1 [Hara, N., Tsuchiya, M., and Shimoyama, M. (1996) J. Biol. Chem. 271, 29552-29555] were unaffected by either agent. In addition to four cysteine residues conserved among mouse Rt6 and rat RT6 antigens, Rt6.1 has two extra cysteine residues at positions 80 and 201. To investigate a contribution of these extra cysteines in mouse Rt6.1 to thiol dependency of Rt6.1 transferase activity, Cys-80 and Cys-201 of Rt6.1 were replaced with serine and phenylalanine, respectively, the corresponding residues of mouse Rt6. 2 and rat RT6.1. Transferase activity of the Phe-201 mutant of Rt6.1 lost thiol dependency while that of the Ser-80 mutant remained thiol-dependent. Thus, we conclude that mouse Rt6.1 is a thiol-dependent arginine-specific ADP-ribosyltransferase, and that Cys-201 confers thiol dependency on Rt6.1 transferase. Our study indicates that arginine-specific ADP-ribosyltransferase activity detected on BALB/c mouse splenocytes is attributed to Rt6.1 and that Rt6.1 differs from Rt6.2 in enzymatic property of the transferase and perhaps in immunoregulatory functions.

Involvement of cytosolic NAD+ glycohydrolase in cyclic ADP-ribose metabolism

The NAD+ glycohydrolase homogeneously purified from bovine brain cytosol was found to catalyze the synthesis and hydrolysis of cyclic ADP-ribose (cADPR). Although the formation of cADPR from NAD+ does not exceed about 2% of the reaction products, the cyclase activity is clearly evidenced by its conversion of NGD+ to cyclic GDP-ribose (cGDPR), which cannot be hydrolyzed to GDPR. Importantly, a steep increase in cADPR hydrolytic activity was observed at cADPR concentrations above 60 microM, which could be reproduced on a Hill curve with a Hill coefficient of 2. Thus, the allosteric binding of cADPR to the NAD+ glycohydrolase (E) molecule promotes the hydrolysis of cADPR. These results suggest that NAD+ hydrolysis to ADPR and nicotinamide catalyzed by the NAD+ glycohydrolase occurs through the formation of a cADPR. E. cADP-ribosyl complex. The low production of cADPR by NAD+ glycohydrolase compared with invertebrate ADP-ribosyl cyclase is believed to be attributable to the fast hydrolysis of cADPR by the allosteric effect of cADPR bound to the same enzyme that produces it.

Suppression of cell adhesion and spreading activities of fibronectin by arginine-specific ADP-ribosyltransferase from chicken polymorphonuclear leukocytes

Arginine-specific ADP-ribosyltransferase present in secretory granules of chicken polymorphonuclear leukocytes (so-called heterophils) was shown to be released into the extracellular space by secretagogues (Terashima et al., J. Biochem. 120 (1996) 1209-1215). In the present work, we examined fibronectin as an extracellular target protein of the released transferase. Fibronectin was ADP-ribosylated by purified transferase and stoichiometry of ADP-ribose incorporation into fibronectin was 1.0 mol/mol of fibronectin. Cell adhesion and spreading assays revealed that ADP-ribosylation of fibronectin markedly inhibited the adhesion activity of fibronectin. A proteolytic peptide map of ADP-ribosylated fibronectin demonstrated that the modification occurs in the cell binding domain of fibronectin. ADP-ribosylation of the RGD peptide suggests that the RGD sequence is the modification site in the domain. ADP-ribosylation of fibronectin in plasma means that fibronectin can probably serve as the substrate for extracellularly released ADP-ribosyltransferase in vivo. Thus, in the extracellular space, ADP-ribosyltransferase released from polymorphonuclear leukocytes may perhaps be involved in regulation of cell adhesion process by interfering with the activity of fibronectin.

ADP-ribosylation of tuftsin suppresses its receptor-binding capacity and phagocytosis-stimulating activity to murine peritoneal macrophages

Arginine-specific ADP-ribosyltransferase present in granules of chicken polymorphonuclear leukocytes (so-called heterophils) is released into the extracellular space by stimulus of calcium ionophore A23187 or opsonized zymosan [Terashima et al. (1996) J. Biochem. 120, 1209-1215]. In the present work, we examined extracellular targets of the released transferase and identified tuftsin, a phagocytosis-stimulating tetrapeptide derived from leukokinin, as a preferential substrate of the enzyme in chicken plasma. Specific binding of FITC-tuftsin to murine peritoneal macrophages, observed under a fluorescent microscope, was impaired by ADP-ribosylation of the labelled peptide. Phagocytic assay analyzed by flow cytometry revealed that ADP-ribosylation of tuftsin decreased its phagocytosis-stimulating activity towards the macrophages. Thus, the ADP-ribosylation of tuftsin apparently decreases its biological activity and ADP-ribosylation may possibly be involved in inflammatory processes through alterations in tuftsin activity.

Regulation of nitrogen fixation in Azospirillum brasilense

The regulation of nitrogen fixation in Azospirillum brasilense is very complicated, and it responds to exogenous fixed nitrogen or a change of oxygen concentration. This regulation occurs at both transcriptional and posttranslational levels. Unlike regulation seen in Klebsiella pneumoniae, transcription of nifA does not require NTRB/NTRC in A. brasilense and the expression of nifHDK is controlled by posttranslational regulation of NIFA activity. Addition of NH4+ or a shift from microaerobic to anaerobic conditions also causes a rapid loss of nitrogenase activity in A. brasilense. This posttranslational regulation of nitrogenase activity involves the DRAT/DRAG regulatory system, which is similar to that of Rhodospirillum rubrum. Both DRAT and DRAG activities are regulated in vivo, but the mechanisms for their regulation are unknown.

Characterization of chemical inhibitors of brefeldin A-activated mono-ADP-ribosylation

Brefeldin A, a toxin inhibitor of vesicular traffic, induces the selective mono-ADP-ribosylation of two cytosolic proteins, glyceraldehyde-3-phosphate dehydrogenase and the novel GTP-binding protein BARS-50. Here, we have used a new quantitative assay for the characterization of this reaction and the development of specific pharmacological inhibitors. Mono-ADP-ribosylation is activated by brefeldin A with an EC50 of 17.0 +/- 3.1 microg/ml, but not by biologically inactive analogs including a brefeldin A stereoisomer. Brefeldin A acts by increasing the Vmax of the reaction, whereas it does not influence the Km of the enzyme for NAD+ (154 +/- 13 microM). The enzyme is an integral membrane protein present in most tissues and is modulated by Zn2+, Cu2+, ATP (but not by other nucleotides), pH, temperature, and ionic strength. To identify inhibitors of the reaction, a large number of drugs previously tested as blockers of bacterial ADP-ribosyltransferases were screened. Two classes of molecules, one belonging to the coumarin group (dicumarol, coumermycin A1, and novobiocin) and the other to the quinone group (ilimaquinone, benzoquinone, and naphthoquinone), rather potently and specifically inhibited brefeldin A-dependent mono-ADP-ribosylation. When tested in living cells, these molecules antagonized the tubular reticular redistribution of the Golgi complex caused by brefeldin A at concentrations similar to those active in the mono-ADP-ribosylation assay in vitro, suggesting a role for mono-ADP-ribosylation in the cellular actions of brefeldin A.

An 18-kDa domain of a glycosylphosphatidylinositol-linked NAD:arginine ADP-ribosyltransferase possesses NAD glycohydrolase activity

Transfection of NMU (rat mammary adenocarcinoma) cells with NAD:arginine ADP-ribosyltransferase cDNAs from Yac-1 murine lymphoma cells or rabbit muscle increased NAD glycohydrolase and ADP-ribosyltransferase activities. The ADP-ribosyltransferase activity was released from transformed NMU cells by phosphatidylinositol-specific phospholipase C (PI-PLC) and hence glycosylphosphatidylinositol (GPI)-anchored, whereas the NAD glycohydrolase (NADase) activity remained cell-associated. By gel permeation chromatography, the size of the PI-PLC-released transferase was approximately 40 kDa and that of the detergent-solubilized NADase was approximately 100 kDa. Using polyclonal antibodies against rabbit muscle transferase on Western blots, approximately 18- and approximately 30-kDa band were visualized among proteins from the NADase fractions and 38-40-kDa bands with protein from the transferase fractions. Incubation of blots with [32P]NAD led to the incorporation of radioactivity into the immunoreactive transferase bands of 38 kDa and the immunoreactive NADase band of approximately 18 kDa. These data suggest that proteolysis of ADP-ribosyltransferase synthesized in transformed NMU cells might result in the formation of aggregates of an 18-kDa NAD glycohydrolase. A fusion protein with glutathione S-transferase linked to the amino terminus of Yac-1 transferase, from which the amino-terminal 121 amino acids had been deleted (GST-Yac-1-delta121), exhibited NADase, but not transferase, activity. The size of the recombinant fusion protein was similar to that of the proteolytic fragment seen in NMU cells transformed with transferase cDNA. These results are compatible with the conclusion that the NAD glycohydrolase activity was generated in NMU cells by proteolysis of ADP-ribosyltransferase, with release of a carboxyl-terminal fragment that possesses glycohydrolase but not transferase activity, i.e. the carboxyl-terminal portion of the transferase can exist as a catalytically active NADase.

Characterization of mouse Rt6.1 NAD:arginine ADP-ribosyltransferase

Rat RT6 proteins, and perhaps mouse Rt6, identify a set of immunoregulatory T lymphocytes. Rat RT6.1 (RT6.1) and rat RT6.2 (RT6. 2) are NAD glycohydrolases, which catalyze auto-ADP-ribosylation, but not ADP-ribosylation of exogenous proteins. Mouse Rt6.1 (mRt6.1) also catalyzes auto-ADP-ribosylation. The activity of mouse cytotoxic T lymphocytes is reportedly inhibited by ADP-ribosylation of surface proteins, raising the possibility that mRt6 may participate in this process. The reactions catalyzed by mRt6, would, however, need to be more diverse than those of the rat homologues and include the ADP-ribosylation of acceptors other than itself. To test this hypothesis, mRt6.1 and rat RT6.2 were synthesized in Sf9 insect cells and rat mammary adenocarcinoma (NMU) cells. mRt6.1, but not rat RT6.2, catalyzed the ADP-ribosylation of guanidino-containing compounds (e.g. agmatine). Unlike RT6.2, mRt6.1 was a weak NAD glycohydrolase. In the presence of agmatine, however, the ratio of [adenine-14C]ADP-ribosylagmatine formation from [adenine-14C]NAD to [carbonyl-14C]nicotinamide formation from [carbonyl-14C]NAD was approximately 1.0, demonstrating that mRt6.1 is primarily a transferase. ADP-ribosylarginine hydrolase, which preferentially hydrolyzes the alpha-anomer of ADP-ribosylarginine, released [U-14C]arginine from ADP-ribosyl[U-14C]arginine synthesized by mRT6.1, consistent with the conclusion that mRt6.1 catalyzes a stereospecific Sn2-like reaction. Thus, mRt6.1 is an NAD:arginine ADP-ribosyltransferase capable of catalyzing a multiple turnover, stereospecific Sn2-like reaction.

Two novel human members of an emerging mammalian gene family related to mono-ADP-ribosylating bacterial toxins

Mono-ADP-ribosylation is one of the posttranslational protein modifications regulating cellular metabolism, e.g., nitrogen fixation, in prokaryotes. Several bacterial toxins mono-ADP-ribosylate and inactivate specific proteins in their animal hosts. Recently, two mammalian GPI-anchored cell surface enzymes with similar activities were cloned (designated ART1 and ART2). We have now identified six related expressed sequence tags (ESTs) in the public database and cloned the two novel human genes from which these are derived (designated ART3 and ART4). The deduced amino acid sequences of the predicted gene products show 28% sequence identity to one another and 32-41% identity vs the muscle and T cell enzymes. They contain signal peptide sequences characteristic of GPI anchorage. Southern Zoo blot analyses suggest the presence of related genes in other mammalian species. By PCR screening of somatic cell hybrids and by in situ hybridization, we have mapped the two genes to human chromosomes 4p14-p15.1 and 12q13.2-q13.3. Northern blot analyses show that these genes are specifically expressed in testis and spleen, respectively. Comparison of genomic and cDNA sequences reveals a conserved exon/intron structure, with an unusually large exon encoding the predicted mature membrane proteins. Secondary structure prediction analyses indicate conserved motifs and amino acid residues consistent with a common ancestry of this emerging mammalian enzyme family and bacterial mono(ADP-ribosyl)transferases. It is possible that the four human gene family members identified so far represent the "tip of an iceberg," i.e., a larger family of enzymes that influences the function of target proteins via mono-ADP-ribosylation.

Molecular cloning of a functional murine arginine-specific mono-ADP-ribosyltransferase and its expression in lymphoid cells

A protein mono-ADP-ribosyltransferase (ADPRT), anchored in the cell membrane as a glycosylphosphatidylinositol (GPI)-anchored cell-surface enzyme, was recently described on murine cytotoxic T cells (CTL). Expression of this enzyme was shown to exert regulatory functions on CTL proliferation and cytotoxic activity, presumably by modulating activity of the protein tyrosine kinase p56(lck), which is associated with the CTL co-receptor CD8. Here we report on the molecular cloning and expression of this important regulatory enzyme. The ADPRT coding sequence was derived by making use of ADPRT sequence homologies from different vertebrate species. A cDNA fragment of the enzyme coding sequence was generated by reverse transcription polymerase chain reaction (RT-PCR) from murine T-cell lymphoma SL12, which expresses the cell-surface ADPRT. The cDNA fragment was found to share extensive homology with the corresponding sequences of human and rabbit muscle ADPRT. In Northern blot hybridization, this cDNA fragment generates a strong hybridization signal with RNA from murine heart and skeletal muscle. Weak signals are seen with SL12, thymus, and spleen. Therefore, a murine skeletal muscle cDNA library was used to identify and obtain the coding sequence of the ADPRT gene. It is shown that the nucleic acid open reading frame sequence of the murine skeletal muscle gene shares 80.3% and 76.3% homology with the sequences of the human and rabbit muscle genes, respectively. Semiquantitative RT-PCR with intron-spanning primers shows that the ADPRT mRNA is present in lymphoid organs, cytotoxic T cells, and T-cell lines. Transfection of the ADPRT coding sequence into EL4 cells results in expression of the enzyme as a functional GPI-anchored cell-surface protein, able to ADP-ribosylate the arginine analog agmatine as well as cell-surface molecules.

Purification, characterisation, and molecular cloning of a chicken erythroblast mono(ADP-ribosyl)transferase

We have purified an arginine-specific mono(ADP-ribosyl)transferase from chicken erythrocytes. The purified transferase was free from poly (ADP-ribose) polymerase activity. The molecular weight of the purified enzyme was estimated to be 27.5 kDa by gel filtration through Sephadex G-75 in a non-denaturing solvent. Activity gel experiments indicate that the active enzyme has an apparent molecular weight in SDS gels of about 28 kDa. The optimum pH of the reaction is about 8.0. The K(m) value for NAD+ of the purified enzyme is about 130 microM. Small molecular weight inhibitors of poly (ADP-ribose) polymerase have no significant effect on the mono ADP-ribosyl transferase enzyme activity. A number of inhibitors of the arginine-specific mono(ADP-ribosyl)transferase activity have been identified. Among the more effective inhibitors are 1,4 naphthoquinone, 5,8-dihydroxy-1,4-naphthoquinone, 4-amino-1-naphthol and 1,2-naphthoquinone. We have also cloned a mono(ADP-ribosyl)transferase from chicken erythroblasts. This gene has been expressed in E. coli and ADP-ribosylation activity has been demonstrated using histones as substrate. The activity is shown to be arginine-specific by the use of poly-L-arginine as substrate. Use of a specific inhibitor has shown that this enzyme is indeed a mono(ADP-ribosyl)transferase and not a NAD glycohydrolase activity. The sequence of this gene is very similar to several other mono(ADP-ribosyl)transferase genes. There are thus at least three different chicken mono(ADP-ribosyl)transferase genes in the blood system alone; this suggests that there is a quite large family of mono(ADP-ribosyl)transferase genes in animals. We have also isolated the promoter region of this chicken gene and are able to identify several standard motifs in this promoter.

ADP-ribosylarginine hydrolases and ADP-ribosyltransferases. Partners in ADP-ribosylation cycles

Mono-ADP-ribosylation is a reversible modification of arginine residues in proteins, with NAD:arginine ADP-ribosyltransferases and ADP-ribosylarginine hydrolases constituting opposing arms of a putative ADP-ribosylation cycle. The enzymatic components of an ADP-ribosylation cycle have been identified in both prokaryotic and eukaryotic systems. The regulatory significance of the cycle has been best documented in prokaryotes. As shown by Ludden and coworkers, ADP-ribosylation controls the activity of dinitrogenase reductase in the phototropic bacterium Rhodospirillum rubrum. ADP-ribosylation of other amino acids, such as cysteine, has also been demonstrated, lending credence to the hypothesis that this modification is heterogeneous. In eukaryotes, the functional relationship between ADP-ribosyltransferases and ADP-ribosylarginine hydrolases is less well documented. The transferase-catalyzed reaction results in sterospecific formation of alpha-ADP-ribosylarginine from beta-NAD; ADP-ribosylarginine hydrolases specifically cleave the alpha-anomer, leading to release of ADP-ribose and regeneration of the free guanidino group of arginine. The two reactions can thus be coupled in vitro. Coupling in vivo is dependent on cellular localization. The deduced amino acid sequences of ADP-ribosyltransferases from avian and mammalian tissues have common consensus sequences involved in catalytic activity but, in some instances, enzyme-specific cellular localization signals. The presence of amino- and carboxy-terminal signal sequences is consistent with the glycosylphosphatidylinositol(GPI)-anchoring to the cell surface. The muscle and lymphocyte transferases ADP-ribosylate integrins. Some transferases lack the carboxy- terminal signal sequence needed for GPI-anchoring. Most ADP-ribosylarginine hydrolase activity is cytosolic, although perhaps some is located at the cell surface. Deduced amino acid sequences of hydrolases from a number of mammalian species are consistent with their cytoplasmic localization. Katada and coworkers have determined, however, that auto-ADP-ribosylated RT6, a GPI-linked protein, is metabolized by a hydrolase-like activity, consistent with the existence of an ADP-ribosylation cycle. ADP-ribosyl RT6 may be internalized, thereby coming in contact with the cytosolic hydrolase; alternatively, a novel form of the hydrolase may be located at the surface. The mechanism of coupling of ADP-ribosyltransferases and hydrolases in eukaryotic ADP-ribosylation cycles has yet to be clarified.

Molecular cloning and characterization of lymphocyte and muscle ADP-ribosyltransferases

Mono-ADP-ribosylation, catalyzed by ADP-ribosyltransferases, is a posttranslational modification of proteins in which the ADP-ribose moiety of NAD is transferred to an acceptor protein(arginine). Several of the bacterial toxin ADP-ribosyltransferases have been well characterized in their ability to alter cellular metabolism. It has been postulated that these bacterial toxins mimic the actions of transferases from mammalian cells. We have cloned and characterized ADP-ribosyltransferases from rabbit and human skeletal muscle, and mouse lymphocytes. The muscle transferases are glycosylphosphatidylinositol (GPI)-anchored proteins that are conserved among species. Two distinct transferases, termed Yac-1 and Yac-2 were cloned from mouse lymphoma (Yac-1) cells. The Yac-1 transferase, like the muscle enzymes, is a GPI-linked exoenzyme. The Yac-2 transferase, on the other hand, is membrane-associated but appears not to be GPI-linked. In contrast to Yac-1, the Yac-2 enzyme had significant NAD glycohydrolase activity and may preferentially hydrolyze NAD. The bacterial toxin ADP-ribosyltransferases contain three noncontiguous regions of sequence similarity, which are involved in formation of the catalytic site. Alignment of the deduced amino acid sequences of the mammalian transferases and the rodent RT6 enzymes, along with results from site-directed mutagenesis of the muscle enzyme, are consistent with the notion of a common mechanism of NAD binding and catalysis among ADP-ribosyltransferases.

Molecular cloning and characterization of arginine-specific ADP-ribosyltransferases from chicken bone marrow cells

Among a number of tissues and peripheral blood cells in chicken, leukocytes, bone marrow cells, liver and spleen showed high ADP-ribosyltransferase activity, with leukocytes having the highest. Density gradient centrifugation of the leukocytes revealed that the leukocyte ADP-ribosyltransferase originates in the polymorphonuclear cells, so called heterophils. Subcellular distribution of the cells showed the localization of the enzyme in the granule fraction. Based on the obtained amino acid sequences of arginine-specific ADP-ribosyltransferase purified from chicken peripheral heterophils, two arginine-specific ADP-ribosyltransferase cDNAs (designated AT1 and AT2) were obtained from chicken bone marrow cells. Each cDNA encodes a different peptide of 312 amino acid residues. Homology of the deduced amino acid sequences between AT1 and AT2 was 78.3%. Arginine-specific ADP-ribosyltransferase activity was detected in culture medium of COS 7 cells transiently transfected with AT1 cDNA, while activity from the cells transfected with AT2 cDNA was found in both culture medium and cell lysate. AT1 transferase required 2-mercaptoethanol (MSH) for the activity and in the presence of NaCl, the activity was inhibited while the AT2 enzyme was activated by either agent. Highly conserved regions were observed among the deduced amino acid sequences of AT1, AT2, chicken erythroblast and rabbit and human skeletal muscle ADP-ribosyltransferases, and rodent T-cell surface antigen RT6. Two forms of the transferase with much the same properties as AT1 and AT2 proteins, regarding the effect of NaCl and MSH, were detected in bone marrow cells. Based on these results it seems that AT1 and AT2 cDNAs encode the two forms of arginine-specific ADP-ribosyltransferase detected in chicken bone marrow cells.

Mono(ADP-ribosyl)transferases and related enzymes in animal tissues. Emerging gene families

Mono-ADP-ribosylation, like phosphorylation, is an enzyme-catalyzed, reversible post-translational modification that modulates protein function. It was originally discovered as the pathogenic principle of diphtheria-, cholera-, and other potent bacterial toxins. By analogy, corresponding enyzmes were postulated to exist in animal tissues, and mounting biochemical evidence indicates that such enzymes, indeed, play important regulatory roles in cellular functions. The molecular cloning of the first mammalian mono(ADP-ribosyl)transferase from rabbit skeletal muscle, the finding of its homology to a well-studied T-cell marker, RT6, and the molecular cloning of additional gene family members from mammals and birds is providing fresh impetus to research in this field. Intriguingly, these vertebrate enzymes are predicted to be secretory or membrane proteins. They are expressed in lymphatic tissues, muscle, testis, bone marrow, and erythroblasts. Here we review the relationship between this novel family of eucaryotic mono(ADP-ribosyl)transferases (mADPRTs), ADP-ribosylating bacterial toxins, the poly(ADP-ribose)polymerase (PARP), the ADP-ribosyl cyclases, and the ADP-ribosylprotein hydrolase (ARH) in terms of their structure, enzymatic properties and possible biological functions.

Arginine-specific mono(ADP-ribosyl)transferase activity in human neutrophil polymorphs. A possible link with the assembly of filamentous actin and chemotaxis

Mono(ADP-ribosyl)transferase activity has been detected on the external surface of human polymorphonuclear neutrophil leucocytes (PMNs). The corresponding cDNA has been cloned and shown to be identical to that derived from human skeletal muscle. Our results suggest that mono(ADP-ribosyl)transferase is involved in the transduction pathway mediating (i) receptor-dependent re-alignment of cytoskeletal actin and (ii) chemotaxis of PMNs.

Molecular characterization of rat T lymphocyte alloantigen RT6.1 as an ADP-ribosyltransferase

A family of glycosylphosphatidylinositol-linked ADP-ribosyltransferases, of which cDNAs were cloned from various mammalian cells, possess a common Glu-rich motif (EEEVLIP) near their carboxyl termini. Although the first Glu in the common motif is replaced by Gln (Q207EEVLIP) in rat T lymphocyte alloantigens RT6.1 and RT6.2, the two RT6s appear to have ADP-ribosyltransferase activity. To investigate the significance of the Glu-rich motif in the enzyme activity, we produced a mutant RT6.1, in which Gln207 was replaced by Glu (Q207E), together with wild-type RT6s, in Escherichia coli. The recombinant RT6.1 and RT6.2 displayed extremely low auto-ADP-ribosylation, though the latter modification was somewhat higher than the former one. In contrast, much higher the auto-modification was observed for Q207E mutant. Moreover, the mutant could effectively ADP-ribosylate agmatine as a substrate. Thus, the single amino acid mutation of RT6.1 caused remarkable increase in its ADP-ribosyltransferase activity, indicating that the Glu-rich motif near the carboxy terminus plays an important role in the enzyme activity.

The alpha 7 integrin as a target protein for cell surface mono-ADP-ribosylation in muscle cells

A membrane-associated arginine-specific mono-ADP-ribosyltransferase was purified 215,000-fold from rabbit skeletal muscle and its gene was isolated from a skeletal muscle cDNA library. The enzyme was a glycosylphosphatidyl-inositol-linked protein, present on the surface of differentiated skeletal muscle myoblasts (myotubes). Following incubation of cultured, intact myotubes with [adenylate-32P]NAD and analysis by SDS-PAGE, a major radiolabeled protein of 97/140 kDa (reduced/nonreduced conditions) was observed. It was identified as integrin alpha 7 based on its size, binding to a laminin affinity column, immunoprecipitation with a monoclonal antibody, and partial amino acid sequencing. Since ADP-ribosylarginine hydrolase, the enzyme responsible for cleavage of the ADP-ribosylarginine bond and a component with the transferase of a putative ADP-ribosylation cycle, is cytosolic, whereas the transferase is attached via a GPI-anchor to the cell surface, the processing of ADP-ribosylated integrin alpha 7 was investigated. 32P label was rapidly removed from [32P]ADP-ribosylated integrin alpha 7, a process inhibited by free ADP-ribose or p-nitrophenylthymidine-5'-monophosphate, alternative substrates for 5'-nucleotide phosphodiesterase. The processed integrin alpha 7 was not susceptible to subsequent ADP-ribosylation, although the amount of surface integrin alpha 7 remained constant. During the processing, no loss of label was observed from integrin alpha 7 radiolabeled with [14C]NAD, containing 14C in the nicotinamide-proximal ribose, consistent with a degradation of the ADP-ribose moiety by a cell surface 5'-nucleotide phosphodiesterase. Thus, cell surface ADP-ribosylation, in contrast to intracellular ADP-ribosylation, is not readily reversed by the presently known ADP-ribosylarginine hydrolase and seems to operate outside the postulated ADP-ribosylation cycle.

A newly identified glycosylphosphatidylinositol-anchored arginine-specific ADP-ribosyltransferase in chicken spleen

An arginine-specific ADP-ribosyltransferase activity was detected in chicken spleen membrane fraction using a capillary electrophoresis assay and the activity was extracted by phosphatidylinositol-specific phospholipase C but not by 1 M NaCl or 1% Triton X-100. The enzyme protein was purified from chicken spleen membrane fraction to apparent homogeneity with a six-step method containing phosphatidylinositol-specific phospholipase C treatment, ammonium sulfate precipitation and conventional column chromatographies. Apparent molecular mass of the purified enzyme estimated with SDS/PAGE was 44 kDa. N-glycanase treatment of the enzyme reduced the apparent molecular size on SDS/PAGE. The enzyme was recognized by anti-cross reacting determinant antibodies. Partial amino acid sequence of the purified enzyme protein showed high homologies with primary structures of previously reported chicken arginine-specific ADP-ribosyltransferases.

Glutamic acid 207 in rodent T-cell RT6 antigens is essential for arginine-specific ADP-ribosylation

A rat T-cell antigen RT6.1 catalyzes NAD glycohydrolysis but not ADP-ribose transfer, even though the antigen has significant amino acid identity with eucaryotic arginine-specific ADP-ribosyltransferases. Since a highly conserved Glu in the catalytic region of these transferases is substituted with Gln at position 207 in RT6.1, we replaced the Gln with Glu, Asp, or Ala, by site-directed mutagenesis. The Glu-207 mutant produced ADP-ribosylarginine during incubation with NAD and L-arginine. The Asp-207 mutant but not the Ala-207 mutant produced ADP-ribosylarginine, but at a lower rate. In contrast, these mutations affected NAD glycohydrolase activity of RT6.1 to a much lesser extent. Kinetic studies of transferase reaction revealed that kcat of the Glu-207 mutant increased compared to findings with the Asp-207 mutant. Moreover, the mouse homologue of rat RT6 lost arginine-specific ADP-ribosyltransferase activity when Glu-207 was replaced with Gln. Thus, Glu-207 in rodent T-cell RT6 antigens is essential for transfer reaction of ADP-ribose to arginine.

Cloning and characterization of a novel membrane-associated lymphocyte NAD:arginine ADP-ribosyltransferase

Mono-ADP-ribosylation is a post-translational modification of proteins in which the ADP-ribose moiety of NAD is transferred to proteins and is responsible for the toxicity of some bacterial toxins (e.g. cholera toxin and pertussis toxin). NAD:arginine ADP-ribosyltransferases cloned from human and rabbit skeletal muscle and from mouse lymphoma (Yac-1) cells are glycosylphosphatidylinositol-anchored and have similar enzymatic and physical properties; transferases cloned from chicken heterophils and red cells have signal peptides and may be secreted. We report here the cloning and characterization of an ADP-ribosyltransferase (Yac-2), also from Yac-1 lymphoma cells, that differs in properties from the previously identified eukaryotic transferases. The nucleotide and deduced amino acid sequences of the Yac-1 and Yac-2 transferases are 58 and 33% identical, respectively. The Yac-2 protein is membrane-bound but, unlike the Yac-1 enzyme, appears not to be glycosylphosphatidylinositol-anchored. The Yac-1 and Yac-2 enzymes, expressed as glutathione S-transferase fusion proteins in Escherichia coli, were used to compare their ADP-ribosyltransferase and NAD glycohydrolase activities. Using agmatine as the ADP-ribose acceptor, the Yac-1 enzyme was predominantly an ADP-ribosyltransferase, whereas the transferase and NAD glycohydrolase activities of the recombinant Yac-2 protein were equivalent. The deduced amino acid sequence of the Yac-2 transferase contained consensus regions common to several bacterial toxin and mammalian transferases and NAD glycohydrolases, consistent with the hypothesis that there is a common mechanism of NAD binding and catalysis among ADP-ribosyltransferases.

Reduction by inhibitors of mono(ADP-ribosyl)transferase of chemotaxis in human neutrophil leucocytes by inhibition of the assembly of filamentous actin

1. Chemotaxis of human neutrophils is mediated by numerous agents [e.g. N-formyl-methionyl-leucyl-phenylalanine (FMLP) and platelet activating factor (PAF)] whose receptors are coupled to phospholipase C. However, the subsequent transduction pathway mediating cell movement remains obscure. We now propose involvement of mono(ADP-ribosyl)transferase activity in receptor-dependent chemotaxis. 2. Human neutrophils were isolated from whole blood and measurements were made of FMLP or PAF-dependent actin polymerization and chemotaxis. The activity of cell surface Arg-specific mono(ADP-ribosyl)transferase was also measured. Each of these activities was inhibited by vitamin K3 and similar IC50 values obtained (4.67 +/- 1.46 microM, 2.0 +/- 0.1 microM and 4.7 +/- 0.1 microM respectively). 3. There were similar close correlations between inhibition of (a) enzyme activity and (b) actin polymerization or chemotaxis by other known inhibitors of mono(ADP-ribosyl)transferase, namely vitamin K1, novobiocin, nicotinamide and the efficient pseudosubstrate, diethylamino(benzylidineamino)guanidine (DEA-BAG). 4. Intracellular Ca2+ was measured by laser scanning confocal microscopy with two fluorescent dyes (Fluo-3 and Fura-Red). Exposure of human neutrophils to FMLP or PAF was followed by transient increases in intracellular Ca2+ concentration, but the inhibitors of mono(ADP-ribosyl)transferase listed above had no effect on the magnitude of the response. 5. A panel of selective inhibitors of protein kinase C, tyrosine kinase, protein kinases A and G or phosphatases 1 and 2A showed no consistent inhibition of FMLP-dependent polymerization of actin. 6. We conclude that eukaryotic Arg-specific mono(ADP-ribosyl)transferase activity may be implicated in the transduction pathway mediating chemotaxis of human neutrophils, with involvement in the assembly of actin-containing cytoskeletal microfilaments.

Increase in ADP-ribosyltransferase activity of rat T lymphocyte alloantigen RT6.1 by a single amino acid mutation

A family of glycosylphosphatidylinositol-linked ADP-ribosyltransferases, of which cDNAs were cloned from various mammalian cells, possess a common Glu-rich motif (EEEVLIP) near their carboxyl termini. Although the first Glu in the common motif is replaced by Gln (Q207EEVLIP) in rat T lymphocyte alloantigens RT6.1 and RT6.2, the two RT6s appear to have both activities of NAD+ glycohydrolase and ADP-ribosyltransferase to a lesser extent. To investigate the significance of the Glu-rich motif in the two enzyme activities, we produced a mutant RT6.1 (Q207E), in which Gln207 was replaced by Glu, together with wild-type RT6s, in Escherichia coli. Kinetic analysis revealed that there were no marked differences in the Vmax and Km values of NAD+ glycohydrolases among the three recombinant proteins. The recombinant RT6.1 and RT6.2 displayed extremely low auto-ADP-ribosylation, although the latter modification was somewhat higher than the former. In contrast, much greater auto-modification was observed for the Q207E mutant. Moreover, the mutant could effectively ADP-ribosylate agmatine as a substrate. Thus, the single amino acid mutation of RT6.1 caused a marked increase in its ADP-ribosyltransferase activity, indicating that the Glu-rich motif near the carboxy terminus plays an important role in the enzyme activity.

Molecular characterization of mouse T-cell ecto-ADP-ribosyltransferase Rt6: cloning of a second functional gene and identification of the Rt6 gene products

RT6 is an enzymatically active GPI-anchored membrane protein that was originally discovered in the rat as a peripheral T cell alloantigen. It has attracted interest as an activation antigen and because defective RT6-expression coincides with increased susceptibility for autoimmune type I diabetes in the BB rat. Southern blot analyses indicate that the rat carries a single copy RT6 gene whereas the mouse carries a duplication of the homologous locus. We had previously cloned and sequenced a RT6-homologous cDNA from BALB/c mouse spleen. We now report the cloning and characterization of a second RT6-homologue from BALB/c and 129/Sv mice. The two mouse Rt6 genes (designated Rt6-1 and Rt6-2) encode similar open reading frames that are disrupted by conserved introns. The nucleotide sequences of the Rt6-1 and Rt6-2 coding regions show 87% sequence identity, the deduced amino acid sequences 79% identity. The amino acid sequences reveal significant similarity to recently cloned ADP-ribosylating ectoenzymes from rabbit and human skeletal muscle as well as chicken bone marrow cells. RT-PCR analyses reveal that the two Rt6 genes are differentially expressed in distinct inbred mouse strains and that their transcripts are properly processed. Western blot analyses demonstrate that the respective gene products are released from cells by treatment with PI-PLC. The results further show that both mouse Rt6 genes are translated into GPI-anchored cell surface molecules and that Rt6 gene expression is restricted to peripheral lymphoid tissues.

Mouse T cell membrane proteins Rt6-1 and Rt6-2 are arginine/protein mono(ADPribosyl)transferases and share secondary structure motifs with ADP-ribosylating bacterial toxins

Mono ADP-ribosylation is a posttranslational protein modification that has been implicated in the regulation of key biological functions in bacteria as well as in animals. Recently, the first cDNAs for eucaryotic mono(ADPribosyl)transferases were cloned and found to exhibit significant sequence similarity only to one other known protein, the T cell differentiation antigen Rt6. In this paper we describe secondary structure analyses of Rt6 and related proteins and show conserved structure motifs and amino acid residues consistent with a common ancestry of these eucaryotic proteins and bacterial ADP-ribosyltransferases. Moreover, we have expressed soluble mouse Rt6-1 and Rt6-2 gene products in which C-terminal tags (FLAG-His6) replace the native glycosylphosphatidylinositol anchor signal sequences. Purified recombinant Rt6-2, but not Rt6-1, shows NAD+ glycohydrolase activity, which is inhibited by the arginine analogue agmatine. Immunoprecipitation of recombinant Rt6-1 and Rt6-2 with anti-FLAG M2 antibody followed by incubation with [32P]NAD+ leads to rapid and covalent incorporation of radioactivity into the light chain of the M2 antibody. The bound label is resistant to treatment with HgCl2 but sensitive to NH2OH, characteristic of arginine-linked ADP-ribosylation. These results demonstrate that Rt6-1 and RT6-2 possess the enzymatic activities typical for NAD+-dependent arginine/protein mono(ADPribosyl)transferases (EC 2.4.2.31). They are the first such enzymes to be molecularly characterized in the immune system.

Mono-ADP-ribosylation: a reversible posttranslational modification of proteins

Mono-ADP-ribosyltransferase activity has been detected in numerous vertebrate tissues and transferase cDNAs from a few species have recently been cloned. In vitro ADP-ribosylation has been demonstrated with diverse substrates such as phosphorylase kinase, actin, and Gs alpha resulting in the alteration of substrate function. ADP-ribosylation of endogenous target proteins has been observed in chicken heterophils, rat brain, and human platelets, and integrin alpha 7 was found to be the endogenous substrate of the GPI-anchored rabbit skeletal muscle transferase. The reversibility of ADP-ribosylation is made possible by ADP-ribosylarginine hydrolases which have been isolated and cloned from rodent and human tissues. The transferases and hydrolases could in principle form an intracellular ADP-ribosylation regulatory cycle. In the case of the skeletal muscle transferases, however, processing of ADP-ribosylated integrin alpha 7 is carried out by phosphodiesterases and possibly phosphatases (Fig. 1). Most bacterial toxin and eukaryotic mono-ADP-ribosyltransferases, and perhaps other NAD-utilizing enzymes such as the RT6 family of proteins, share a common catalytic-site structure despite a lack of overall sequence identity. The transferases that have been studied thus far possess a critical glutamic acid and other amino acids at the catalytic cleft which function to position NAD for nucleophilic attack at the N-glycosidic linkage for either ADP-ribose transfer or NAD hydrolysis. The amino acid differences among transferases at the active site may reflect different catalytic mechanisms of ADP-ribosylation or may be required for accommodating the different ADP-ribose acceptor molecules.

Structure and function of eukaryotic mono-ADP-ribosyltransferases

ADP-ribosylation of proteins has been observed in numerous animal tissues including chicken heterophils, rat brain, human platelets, and mouse skeletal muscle. ADP-ribosylation in these tissues is thought to modulate critical cellular functions such as muscle cell development, actin polymerization, and cytotoxic T lymphocyte proliferation. Specific substrates of the ADP-ribosyltransferases have been identified; the skeletal muscle transferase ADP-ribosylates integrin alpha 7 whereas the chicken heterophil enzyme modifies the heterophil granule protein p33 and the CTL enzyme ADP-ribosylates the membrane-associated protein p40. Transferase sequence has been determined which should assist in elucidating the role of ADP-ribosylation in cells. There is sequence similarity among the vertebrate transferases and the rodent RT6 alloantigens. The RT6 family of proteins are NAD glycohydrolases that have been shown to possess auto-ADP-ribosyltransferase activity whereas the mouse Rt6-1 is also capable of ADP-ribosylating histone. Absence of RT6+ T cells has been associated with the development of an autoimmune-mediated diabetes in rodents. Humans have an RT6 pseudogene and do not express RT6 proteins. The reversal of ADP-ribosylation is catalyzed by ADP-ribosylarginine hydrolases, which have been purified and cloned from rodent and human tissues. In principle, the transferases and hydrolases could form an intracellular ADP-ribosylation regulatory cycle. In skeletal muscle and lymphocytes, however, the transferases and their substrates are extracellular membrane proteins whereas the hydrolases described thus far are cytoplasmic. In cultured mouse skeletal muscle cells, processing of the ADP-ribosylated integrin alpha 7 was carried out by phosphodiesterases and possibly phosphatases, leaving a residual ribose attached to the (arginine)protein. Several bacterial toxin and eukaryotic mono-ADP-ribosyltransferases, and perhaps other NAD-utilizing enzymes such as the RT6 alloantigens share regions of amino acid sequence similarity, which form, in part, the catalytic site. The catalytic cleft, found in the bacterial toxins that have been studied thus far, contains a critical glutamate and other amino acids that function to position NAD for nucleophilic attack at the N-glycosidic linkage, for either ADP-ribose transfer or NAD hydrolysis. Amino acid differences among the transferases at the active site may be required for accommodating the different ADP-ribose acceptor molecules.

Sequence of a chicken erythroblast mono(ADP-ribosyl)transferase-encoding gene and its upstream region

We have cloned the MADPRT gene encoding the 300-amino-acid mono(ADP-ribosyl)transferase (MADPRT) from chicken erythroblasts. The protein has homology to the rabbit and human skeletal muscle (50% identity) and two chicken heterophil (52% identity) NAD+:arginine MADPRT. The active site region is particularly conserved. The upstream region of the MADPRT gene from erythroblasts has several features characteristic of promoter sequences.

Both allelic forms of the rat T cell differentiation marker RT6 display nicotinamide adenine dinucleotide (NAD)-glycohydrolase activity, yet only RT6.2 is capable of automodification upon incubation with NAD

The finding that recently cloned mono-ADP-ribosyltransferases show sequence similarity to the rat T cell differentiation marker RT6 has led us to investigate the enzymatic activity of this alloantigenic system. To search for ADP-ribosylation of cell surface proteins, T cell populations from RT6.1- and RT6.2-expressing rat strains, as well as RT6.1+ and RT6.2+ T-T hybridoma cell lines, were incubated with [32P]nicotinamide adenine dinucleotide (NAD). All RT6.2+, but no RT6.1+ or RT6- cells, show incorporation of radioactivity into a single protein which could be identified as RT6.2 by immunoprecipitation with monoclonal antibodies. This automodification of RT6.2 is covalent, requires intact NAD as substrate, and displays characteristics typical for linkage of ADP-ribose to arginine. The alloantigens RT6.1 and RT6.2 differ in ten amino acids, RT6.2 having two arginine residues not present in RT6.1. Both alloantigens were found to display potent NAD-glycohydrolase activity.