NAD(+)-dependent ADP-ribosylation of T lymphocyte alloantigen RT6.1 reversibly proceeding in intact rat lymphocytes

Identification of a novel pathway of transforming growth factor-β1 regulation by extracellular NAD+ in mouse macrophages: in vitro and in silico studies

Extracellular β-nicotinamide adenine dinucleotide (NAD(+)) is anti-inflammatory. We hypothesized that NAD(+) would modulate the anti-inflammatory cytokine Transforming Growth Factor (TGF)-β1. Indeed, NAD(+) led to increases in both active and latent cell-associated TGF-β1 in RAW 264.7 mouse macrophages as well as in primary peritoneal macrophages isolated from both C3H/HeJ (TLR4-mutant) and C3H/HeOuJ (wild-type controls for C3H/HeJ) mice. NAD(+) acts partially via cyclic ADP-ribose (cADPR) and subsequent release of Ca(2+). Treatment of macrophages with the cADPR analog 3-deaza-cADPR or Ca(2+) ionophores recapitulated the effects of NAD(+) on TGF-β1, whereas the cADPR antagonist 8-Br-cADPR, Ca(2+) chelation, and antagonism of L-type Ca(2+) channels suppressed these effects. The time and dose effects of NAD(+) on TGF-β1 were complex and could be modeled both statistically and mathematically. Model-predicted levels of TGF-β1 protein and mRNA were largely confirmed experimentally but also suggested the presence of other mechanisms of regulation of TGF-β1 by NAD(+). Thus, in vitro and in silico evidence points to NAD(+) as a novel modulator of TGF-β1.

Salmonella enterica serotype Typhimurium DT104 ArtA-dependent modification of pertussis toxin-sensitive G proteins in the presence of [32P]NAD

Salmonella enterica serotype Typhimurium (S. Typhimurium) definitive phage type (DT) 104 has become a widespread cause of human and other animal infections worldwide. The severity of clinical illness in S. Typhimurium DT104 outbreaks suggests that this strain possesses enhanced virulence. ArtA and ArtB - encoded by a prophage in S. Typhimurium DT104 - are homologues of components of pertussis toxin (PTX), including its ADP-ribosyltransferase subunit. Here, we show that exposing DT104 to mitomycin C, a DNA-damaging agent, induced production of prophage-encoded ArtA/ArtB. Pertussis-sensitive G proteins were labelled in the presence of [(32)P]NAD and ArtA, and the label was released by HgCl(2), which is known to cleave cysteine-ADP-ribose bonds. ADP-dependent modification of G proteins was markedly reduced in in vitro-synthesized ArtA(6Arg-Ala) and ArtA(115Glu-Ala), in which alanine was substituted for the conserved arginine at position 6 (necessary for NAD binding) and the predicted catalytic glutamate at position 115, respectively. A cellular ADP-ribosylation assay and two-dimensional electrophoresis showed that ArtA- and PTX-induced ADP-ribosylation in Chinese hamster ovary (CHO) cells occur with the same type of G proteins. Furthermore, exposing CHO cells to the ArtA/ArtB-containing culture supernatant of DT104 resulted in a clustered growth pattern, as is observed in PTX-exposed CHO cells. Hydrogen peroxide, an oxidative stressor, also induced ArtA/ArtB production, suggesting that these agents induce in vivo synthesis of ArtA/ArtB. These results, taken together, suggest that ArtA/ArtB is an active toxin similar to PTX.

Structure of the ecto-ADP-ribosyl transferase ART2.2 from rat

The mammalian extracellular ADP-ribosyl transferases ART1 through ART5 are sequence-related to each other. Among them ART2 is involved in immuno regulation. The variant ART2.2 was expressed in the periplasm of Escherichia coli and crystallized. Its structure was determined by X-ray diffraction at 1.7A resolution in one crystal form and at slightly lower resolutions in two others. The active center was indicated by a ligated nicotinamide analogue, which also revealed a small induced-fit. The centerpiece of the chainfold of ART2.2 agrees with those of all bacterial ADP-ribosyl transferases. This correspondence and the nicotinamide position were used to model the binding structure of the whole substrate NAD(+) at ART2.2. Two of the bacterial enzymes are structurally more closely related to ART2.2 while the others are more closely related to the eukaryotic poly(ADP-ribosyl)polymerase. This splits the ADP-ribosyl transferases into two distinct subfamilies. A special feature of ART2.2 is its long N-terminal extension and two disulfide bridges that are far away from the active center. They stabilize the protein against denaturation and presumably also against shearing forces parallel with the membrane where ART2.2 is anchored.

Unusual enzymatic hydrolysis of NAD by solubilized form of NAD(+) glycohydrolase

Using solubilized form (sNADase) of membrane-bound porcine brain NAD(+) glycohydrolase (pNADase), the NADase-catalyzed hydrolysis and transglycosidation reactions of NAD (1) were examined. Unexpectedly, products in the reactions were found to be nicotinamide (5'-O-diphosphono)-beta-D-ribofuranoside (4) and adenosine (5). Adenosine 5'-diphosphate (ADP)-ribose (2) and nicotinamide (3) as well as a transglycosylated product, which are formed in a usual NAD/pNADase reaction system, were scarcely produced in the NAD/sNADase system. Setting aside the mechanical aspects of this unusual cleaving, it is quite interesting that the sNADase-catalyzed hydrolytic reaction of NAD resulted in the selective cleavage of the P-O bond of the adenosine side without the appreciable hydrolysis of the labile quaternary nicotinamide-ribose pyridinium linkage.

Extracellular nicotinamide adenine dinucleotide induces t cell apoptosis in vivo and in vitro

Incubation of mouse T cells expressing the cell surface enzyme ADP ribosyltransferase with nicotinamide adenine dinucleotide (NAD) had been reported to cause ADP ribosylation of cell surface molecules, inhibition of transmembrane signaling, and suppression of immune responses. In this study, we analyze the reasons for these effects and report that contact of T cells with NAD causes cell death. Naive T cells when incubated with NAD and adoptively transferred into semiallogeneic mice fail to cause graft-vs-host disease, and when injected into syngeneic, T cell-deficient recipients do not reconstitute these mice. Rather, they accumulate in the liver, leading to an increase of apoptotic lymphocytes in this organ. Similar effects are induced by injection of NAD, shown to cause a dramatic increase of apoptotic CD3(+), CD4(+), and CD8(+) cells in the liver. Consistent with this, in vitro incubation of naive T cells with NAD is shown to induce apoptosis. In contrast, no cell death is demonstrable when T cells are activated before incubation with NAD. It is concluded that ecto-NAD, as substrate of ADP ribosyltransferase, acts on naive, but not on activated CD69(+) T cells.

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.

O-ADP-Ribosylation in the NAD/NADase system: 2-alkanols as efficient substrates

Several 2-alkanols (2-propanol, 2-butanol, 2-pentanol, etc.) were examined as substrates for ADP-ribosylation in the NAD/NADase enzymatic system. Even though these secondary alcohols have hydroxy groups that are subject to the steric influence of a methyl group, they were shown to be efficiently ADP-ribosylated. However, in the case of 3-alkanol (3-butanol), only slight ADP-ribosylation was observed. In this enzymatic reaction, 1,2-propanediol provided both 1-O- and 2-O-ADP-ribosylation products in the ratio 1:1 as determined by 1H-NMR spectrometry. On the other hand, an equimolar mixture system of 1- and 2-propanols provided major 1-O- and minor 2-O-ribosylation products in the ratio 4:1. This is the first report of O-ADP-ribosylation of terminal secondary alcohols with the NAD/NADase enzymatic system.

A New Monoclonal Antibody Detects a Developmentally Regulated Mouse Ecto-ADP-Ribosyltransferase on T Cells: Subset Distribution, Inbred Strain Variation, and Modulation Upon T Cell Activation

ADP-ribosylation of membrane proteins on mouse T cells by ecto-ADP-ribosyltransferase(s) (ARTs) can down-regulate proliferation and function. The lack of mAbs against mouse ARTs has heretofore prevented analysis of ART expression on T cell subsets. Using gene gun technology, we immunized a Wistar rat with an Art2b expression vector and produced a novel mAb, Nika102, specific for ART2.2, the Art2b gene product. We show that ART2.2 is expressed as a GPI-anchored protein on the surface of mature T cells. Inbred strain-dependent differences in ART2.2 expression levels were observed. C57BL/6J and C57BLKS/J express the Ag at high level, with up to 70% of CD4+ and up to 95% of CD8+ peripheral T cells expressing ART2.2. CBA/J and DBA/2J represent strains with lowest expression levels. T cell-deficient mice and NZW/LacJ mice with a defective structural gene for this enzyme were ART2.2 negative. In the thymus, ART2.2 expression is restricted to subpopulations of mature cells. During postnatal ontogeny, increasing percentages of T cells express ART2.2, reaching a peak at 6–8 wk of age. Interestingly, ART2.2 and CD25 are reciprocally expressed: activation-induced up-regulation of CD25 is accompanied by loss of ART2.2 from the cell surface. Nika102 thus defines a new differentiation/activation marker of thymic and postthymic T cells in the mouse and should be useful for further elucidating the function of the ART2.2 cell surface enzyme.

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.

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.

Characterization of High Density Lipoprotein-Bound and Soluble RT6 Released Following Administration of Anti-RT6.1 Monoclonal Antibody

RT6 is a rat lymphocyte glycosylphosphatidylinositol (GPI)-anchored alloantigen with nicotinamide adenine dinucleotide (NAD) glycohydrolase (NADase) and auto-ADP-ribosyltransferase activities. RT6 may have immunoregulatory properties based in part on the observation that injection of diabetes-resistant (DR)-BB rats with depleting doses of anti-RT6.1 mAb induced autoimmune diabetes and thyroiditis. We now report that injection of DR-BB rats with anti-RT6.1 mAb increased plasma NADase activity, which localized, by fluid phase liquid chromatography fractionation, to the high density lipoprotein (HDL) fraction. Following ultracentrifugation in high salt, however, RT6 was found in the nonlipoprotein fraction, where it existed, under nondenaturing conditions, as a 200-kDa complex and, by SDS-PAGE, as a 30- to 36-kDa species. Thy-1, another GPI-linked protein, and proteins that reacted with anti-GPI-oligosaccharide Abs also translocated from HDL to the nonlipoprotein fraction under similar conditions. Injection of anti-RT6.1 mAb into thymectomized DR and diabetes-prone-BB rats increased soluble RT6 to levels comparable to those observed in euthymic DR-BB rats, suggesting that HDL-bound RT6 is not derived from peripheral lymphocytes. In agreement, NADase activity in the plasma of eviscerated DR-BB rats did not increase following injection of anti-RT6 mAb. These data suggest that HDL is a carrier of plasma RT6 and other GPI-linked proteins, with equilibrium between the lipoprotein and nonlipoprotein fractions being salt dependent. Since GPI-linked proteins in HDL can transfer to cells in a functionally active form, the presence of RT6 in HDL is consistent with it having a role in signaling in nonlymphoid cells.

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.

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.

Mono(ADP-ribosyl)transferase genes and diabetes in NOD mice. Is there a relationship?

The answer to the question posed by the title (is there a relationship between aberrant Art gene expression and IDDM pathogenesis in NOD mice?) remains elusive. Conclusions are currently based almost entirely upon analysis of mRNA transcript levels rather than on T-cell-specific mono-ADP ribosylation activities. Our unpublished data, as well as data published in abstract form by Dr. L. Chatenoud and colleagues (48) indicate that gene transcription is not impaired in splenic leukocytes of older NOD mice, including those with spontaneous IDDM development. Based upon the limited data showing that there may be reduced expression of Art gene products in the earliest T cell immigrants from the NOD thymus, one would have to surmise that If there is a regulatory defect, it may be in allowing single positive thymic T cells to emigrate before they are fully mature. Therefore, development of anti-Art monoclonal antibody together with further studies regarding functions of mono(ADP-ribosyl)transferase in immunoregulation of different subpopulation of T-cells, may finally resolve the role that altered mono(ADP-ribosyl)transferase activities play in the pathogenesis of IDDM in NOD mice.

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.

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.

Expression of the ectoenzyme RT6 is not restricted to resting peripheral T cells and is differently regulated in normal peripheral T cells, intestinal IEL, and NK cells

The RT6 alloantigenic system of the rat has originally been defined on T lymphocytes of the peripheral lymphatic organs and has been considered to be selectively expressed on mature peripheral T cells. Studying NK cells and intestinal intraepithelial lymphocytes (IEL), we have now found that both cell types also express RT6 and that the expression patterns found for IEL and NK cells were markedly different from each other and also from the expression pattern previously described for T cells of the peripheral lymphatic organs. In lymph nodes, spleen, and blood both RT6- and RT6+ T cells have been found and the density of RT6 expression on the positive cells has been shown to vary over a broad range. In contrast more than 98% of intestinal IEL stained for RT6 and the RT6 density was about tenfold higher than on strongly positive T cells of the peripheral lymphatic organs. Furthermore, the same high RT6 density was also found on IEL of athymic nude rats althogh these cells, to a large extent, lacked other T cell markers. This probably indicates that RT6 expression is an early event in the maturation of intestinal IEL which can occur already before the expression of T cell-specific membrane molecules. The conclusion that the expression of RT6 may be differently regulated in IEL and other T cell populations was further substantiated by the observation that RT6 was also present on IEL of diabetes-prone BB rats which are known to lack RT6 positive T cells in peripheral lymphatic organs. For NK cells still another pattern of RT6 expression was found. Unlike peripheral T cells and IEL, only a small subset of NK cells in blood and spleen expressed RT6. The percentage of RT6 positive cells was increased by in vitro stimulation of isolated NK cells with high concentrations of recombinant rat IL-2 indicating that RT6 expression may be associated with an activated state in NK cells. Taken together, these findings demonstrate that the expression of RT6 is not restricted to T cells and is differently regulated in normal peripheral T cells, intestinal IEL, and NK cells. Since it has recently been demonstrated that the RT6 gene contains two functional promoter regions with major structural disparity it is very likely that the distinct patterns of RT6 expression in different cell types reflect the differential use of the two promoters. The development of this complex control of RT6 expression in evolution may have been driven by a beneficial effect resulting from the use of the RT6 molecular function by several different lymphocyte populations.

Expression and comparative analysis of recombinant rat and mouse RT6 T cell mono(ADP-ribosyl)transferases in E. coli

Recombinant RT6 proteins of rat and mouse were analyzed for NAD-metabolizing, i.e. mono(ADP-ribosyl)transferase, NAD-glycohydrolase (NADase) and ADP-ribosyl cyclase activities. The results reveal surprising intra- as well as inter-species differences in enzyme activities. While mouse Rt6 proteins were found to be strong arginine-specific transferases, but comparatively weak NADases, the opposite held true for rat RT6, for which transferase activity could only be detected in the form of arginine-specific auto-ADP-ribosylation, displayed by RT6.2 but not by RT6.1. NADase activity of rat RT6 was not accompanied by production of cyclic ADPR (cADPR). Rat RT6 gained potent arginine-specific transferase activity by exchange of a single amino acid for the corresponding residue of the mouse proteins.

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.

Regulation of NAD+ glycohydrolase activity by NAD(+)-dependent auto-ADP-ribosylation

NAD+ glycohydrolase (NADase; EC 3.2.2.5) is an enzyme that catalyses hydrolysis of NAD+ to produce ADP-ribose and nicotinamide. Its physiological role and the regulation of its enzymic activity have not been fully elucidated. In the present study, the mechanism of self-inactivation of NADase by its substrate, NAD+, was investigated by using intact rabbit erythrocytes and purified NADase. Our results suggest that inactivation of NADase was due an auto-ADP-ribosylation reaction. ADP-ribosylated NADase of rabbit erythrocytes was deADP-ribosylated when incubated without NAD+, and thus enzyme activity was simultaneously restored. These findings suggest that reversible auto-ADP-ribosylation of NADase might regulate the enzyme's activity in vivo.

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.

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.

Inhibition of NAD+ glycohydrolase and ADP-ribosyl cyclase activities of leukocyte cell surface antigen CD38 by gangliosides

We have recently reported that gangliosides act as inhibitors of ADP-ribosyltransferases and NAD+ glycohydrolases (NADase) of pertussis toxin and the C3 exoenzyme from Clostridium botulinum (Hara-Yokoyama, M., Hirabayashi, Y., Irie, F., Syuto, B., Moriishi, K., Sugiya, H., and Furuyama, S. (1995) J. Biol. Chem. 270, 8115-8121). Here, we investigated the effect of gangliosides on the enzymatic activity of leukocyte cell surface antigen CD38, which is identified as an ecto-NADase (Kontani, K., Nishina, H., Ohoka, Y., Takahashi, K., and Katada, T. (1993) J. Biol. Chem. 268, 16895-16898). Gangliosides GM1a and GQ1balpha inhibited the NADase activity in the immunoprecipitate of anti-CD38 antibody from the membrane extract of retinoic acid-treated human leukemic HL-60 cells. Gangliosides also inhibited the NADase activity of the extracellular domain of CD38 antigen that was deprived of the transmembrane domain and was expressed in Escherichia coli as a fusion protein with maltose-binding protein (MBP-CD38). The order of the inhibitory effect of purified ganglioside species on the NADase activity on MBP-CD38 was as follows: GQ1balpha > GT1b, GQ1b > GD1a, GD1b, GM1a, GM1b, GD3, GM3. GQ1balpha inhibited the NADase of MBP-CD38 in a noncompetitive manner versus NAD+ with a Ki value of about 0.3 microM. Neither ceramide nor the oligosaccharide moiety of GQ1balpha had an effect on the NADase activity. GQ1balpha, GT1b, and GQ1b also efficiently inhibited the ADP-ribosyl cyclase activity of MBP-CD38. At present, gangliosides are the only endogenous species that can block the enzymatic activity of CD38 antigen. The present results suggest a potential role of gangliosides as inhibitors of the ecto-NADases.

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.

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.