Immunological and structural conservation of mammalian skeletal muscle glycosylphosphatidylinositol-linked ADP-ribosyltransferases

Expression of the mono-ADP-ribosyltransferase ART1 by tumor cells mediates immune resistance in non-small cell lung cancer

Most patients with non-small cell lung cancer (NSCLC) do not achieve durable clinical responses from immune checkpoint inhibitors, suggesting the existence of additional resistance mechanisms. Nicotinamide adenine dinucleotide (NAD)-induced cell death (NICD) of P2X7 receptor (P2X7R)-expressing T cells regulates immune homeostasis in inflamed tissues. This process is mediated by mono-adenosine 5'-diphosphate (ADP)-ribosyltransferases (ARTs). We found an association between membranous expression of ART1 on tumor cells and reduced CD8 T cell infiltration. Specifically, we observed a reduction in the P2X7R⁺ CD8 T cell subset in human lung adenocarcinomas. In vitro, P2X7R⁺ CD8 T cells were susceptible to ART1-mediated ADP-ribosylation and NICD, which was exacerbated upon blockade of the NAD⁺-degrading ADP-ribosyl cyclase CD38. Last, in murine NSCLC and melanoma models, we demonstrate that genetic and antibody-mediated ART1 inhibition slowed tumor growth in a CD8 T cell-dependent manner. This was associated with increased infiltration of activated P2X7R⁺CD8 T cells into tumors. In conclusion, we describe ART1-mediated NICD as a mechanism of immune resistance in NSCLC and provide preclinical evidence that antibody-mediated targeting of ART1 can improve tumor control, supporting pursuit of this approach in clinical studies.

The ARH and Macrodomain Families of α-ADP-ribose-acceptor Hydrolases Catalyze α-NAD+ Hydrolysis

ADP-ribosyltransferases transfer ADP-ribose from β-NAD⁺ to acceptors; ADP-ribosylated acceptors are cleaved by ADP-ribosyl-acceptor hydrolases (ARHs) and proteins containing ADP-ribose-binding modules termed macrodomains. On the basis of the ADP-ribosyl-arginine hydrolase 1 (ARH1) stereospecific hydrolysis of α-ADP-ribosyl-arginine and the hypothesis that α-NAD⁺ is generated as a side product of β-NAD⁺/ NADH metabolism, we proposed that α-NAD⁺ was a substrate of ARHs and macrodomain proteins. Here, we report that ARH1, ARH3, and macrodomain proteins (i.e., MacroD1, MacroD2, C6orf130 (TARG1), Af1521, hydrolyzed α-NAD⁺ but not β-NAD⁺. ARH3 had the highest α-NADase specific activity. The ARH and macrodomain protein families, in stereospecific reactions, cleave ADP-ribose linkages to N- or O- containing functional groups; anomerization of α- to β-forms (e.g., α-ADP-ribosyl-arginine to β-ADP-ribose- (arginine) protein) may explain partial hydrolysis of ADP-ribosylated acceptors with an increase in content of ADP-ribosylated substrates. Af1521 and ARH3 crystal structures with bound ADP-ribose revealed similar ADP-ribose-binding pockets with the catalytic residues of the ARH and macrodomain protein families in the N-terminal helix and loop. Although the biological roles of the ARHs and macrodomain proteins differ, they share enzymatic and structural properties that may regulate metabolites such as α-NAD⁺.

Mono-ADP-Ribosylation Catalyzed by Arginine-Specific ADP-Ribosyltransferases

Methods are described for determination of arginine-specific mono-ADP-ribosyltransferase activity of purified proteins and intact cells by monitoring the transfer of ADP-ribose from NAD⁺ to a model substrate, e.g., arginine, agmatine, and peptide (human neutrophil peptide-1 [HNP1]), and for the nonenzymatic hydrolysis of ADP-ribose-arginine to ornithine, a noncoded amino acid. In addition, preparation of purified ADP-ribosylarginine is included as a control substrate for ADP-ribosylation reactions.

Arginine ADP-ribosyltransferase 1 promotes angiogenesis in colorectal cancer via the PI3K/Akt pathway

Arginine adenosine diphosphate (ADP)-ribosyl-transferase 1 (ART1) is known to play an important role in many physiological and pathological processes. Previous studies have demonstrated that ART1 promotes proliferation, invasion and metastasis in colon carcinoma. However, it was unclear whether ART1 is involved in angiogenesis in cases of colorectal cancer (CRC). In the present study, lentiviral vector‑mediated ART1‑cDNA or ART1-shRNA were transfected into LoVo cells, and the LoVo cells transfected with ART1-cDNA or ART1-shRNA were co-cultured with human umbilical vein endothelial cells (HUVECs) to determine the influence of ART1 on HUVECs. The proliferation, migration and angiogenesis of HUVECs were monitored using a cell counting kit-8 assay, a Transwell migration assay and immunohistochemical analysis in intrasplenic allograft tumors, respectively. Hypoxia‑inducible factor 1-α (HIF-1α), total (t-)Akt, phosphorylated (p-)Akt, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) expression levels were detected via western blot analysis. Our results revealed that HUVECs which were co-cultured with ART1-cDNA LoVo cells showed higher proliferation, migration and angiogenic abilities, but a reduction was noted in those cultured with ART1-shRNA LoVo cells; p-Akt, HIF-1α, VEGF and bFGF expression was increased in HUVECs cultured with ART1‑cDNA-transfected LoVo cells, but reduced in ART1-shRNA-transfected LoVo cells. In a mouse xenograft model, we noted that the tumor microvessel density (MVD) was significantly increased in intrasplenic transplanted ART1‑cDNA CT26 tumors but decreased in intrasplenic transplanted ART1‑shRNA tumors. These data suggest that ART1 promoted the expression of HIF-1α via the Akt pathway in tumor cells. It also upregulated VEGF and bFGF and enhanced angiogenesis in HUVECs. Thus, we suggest that ART1 plays an important role in the invasion of CRC cells and the metastasis of CRC.

Critical role for NAD glycohydrolase in regulation of erythropoiesis by hematopoietic stem cells through control of intracellular NAD content

NAD glycohydrolases (NADases) catalyze the hydrolysis of NAD to ADP-ribose and nicotinamide. Although many members of the NADase family, including ADP-ribosyltransferases, have been cloned and characterized, the structure and function of NADases with pure hydrolytic activity remain to be elucidated. Here, we report the structural and functional characterization of a novel NADase from rabbit reticulocytes. The novel NADase is a glycosylated, glycosylphosphatidylinositol-anchored cell surface protein exclusively expressed in reticulocytes. shRNA-mediated knockdown of the NADase in bone marrow cells resulted in a reduction of erythroid colony formation and an increase in NAD level. Furthermore, treatment of bone marrow cells with NAD, nicotinamide, or nicotinamide riboside, which induce an increase in NAD content, resulted in a significant decrease in erythroid progenitors. These results indicate that the novel NADase may play a critical role in regulating erythropoiesis of hematopoietic stem cells by modulating intracellular NAD.

Arginine-specific mono ADP-ribosylation in vitro of antimicrobial peptides by ADP-ribosylating toxins

Among the several toxins used by pathogenic bacteria to target eukaryotic host cells, proteins that exert ADP-ribosylation activity represent a large and studied family of dangerous and potentially lethal toxins. These proteins alter cell physiology catalyzing the transfer of the ADP-ribose unit from NAD to cellular proteins involved in key metabolic pathways. In the present study, we tested the capability of four of these toxins, to ADP-ribosylate α- and β- defensins. Cholera toxin (CT) from Vibrio cholerae and heat labile enterotoxin (LT) from Escherichia coli both modified the human α-defensin (HNP-1) and β- defensin-1 (HBD1), as efficiently as the mammalian mono-ADP-ribosyltransferase-1. Pseudomonas aeruginosa exoenzyme S was inactive on both HNP-1 and HBD1. Neisseria meningitidis NarE poorly recognized HNP-1 as a substrate but it was completely inactive on HBD1. On the other hand, HNP-1 strongly influenced NarE inhibiting its transferase activity while enhancing auto-ADP-ribosylation. We conclude that only some arginine-specific ADP-ribosylating toxins recognize defensins as substrates in vitro. Modifications that alter the biological activities of antimicrobial peptides may be relevant for the innate immune response. In particular, ADP-ribosylation of antimicrobial peptides may represent a novel escape mechanism adopted by pathogens to facilitate colonization of host tissues.

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.

Toward a unified nomenclature for mammalian ADP-ribosyltransferases

ADP-ribosylation is a post-translational modification of proteins catalyzed by ADP-ribosyltransferases. It comprises the transfer of the ADP-ribose moiety from NAD+ to specific amino acid residues on substrate proteins or to ADP-ribose itself. Currently, 22 human genes encoding proteins that possess an ADP-ribosyltransferase catalytic domain are known. Recent structural and enzymological evidence of poly(ADP-ribose)polymerase (PARP) family members demonstrate that earlier proposed names and classifications of these proteins are no longer accurate. Here we summarize these new findings and propose a new consensus nomenclature for all ADP-ribosyltransferases (ARTs) based on the catalyzed reaction and on structural features. A unified nomenclature would facilitate communication between researchers both inside and outside the ADP-ribosylation field.

Identification of two regulatory binding sites which confer myotube specific expression of the mono-ADP-ribosyltransferase ART1 gene

BACKGROUND

Mono-ADP-ribosyltransferase (ART) 1 belongs to a family of mammalian ectoenzymes that catalyze the transfer of ADP-ribose from NAD+ to a target protein. ART1 is predominantly expressed in skeletal and cardiac muscle. It ADP-ribosylates alpha7-integrin which together with beta1-integrin forms a dimer and binds to laminin, a protein of the extracellular matrix involved in cell adhesion. This posttranslational modification leads to an increased laminin binding affinity.

RESULTS

Using C2C12 and C3H-10T 1/2 cells as models of myogenesis, we found that ART1 expression was restricted to myotube formation. We identified a fragment spanning the gene 1.3 kb upstream of the transcriptional start site as the functional promoter of the ART1 gene. This region contains an E box and an A/T-rich element, two conserved binding sites for transcription factors found in the promoters of most skeletal muscle specific genes. Mutating the DNA consensus sequence of either the E box or the A/T-rich element resulted in a nearly complete loss of ART1 promoter inducibility, indicating a cooperative role of the transcription factors binding to those sites. Gel mobility shift analyses carried out with nuclear extracts from C2C12 and C3H-10T 1/2 cells revealed binding of myogenin to the E box and MEF-2 to the A/T-rich element, the binding being restricted to C2C12 and C3H-10T 1/2 myotubes.

CONCLUSION

Here we describe the molecular mechanism underlying the regulation of the ART1 gene expression in skeletal muscle cells. The differentiation-dependent upregulation of ART1 mRNA is induced by the binding of myogenin to an E box and of MEF-2 to an A/T-rich element in the proximal promoter region of the ART1 gene. Thus the transcriptional regulation involves molecular mechanisms similar to those used to activate muscle-specific genes.

Ecto-ADP-ribose transferases: cell-surface response to local tissue injury

Ecto-ADP-ribose transferases (ecto-ARTs) catalyze the transfer of ADP-ribose from NAD(+) to arginine residues in cell-surface proteins. Since the concentration of extracellular NAD(+) is very low under normal physiological conditions but rises significantly upon tissue injury or membrane stress, it is postulated that the main role of ecto-ARTs is to ADP-ribosylate and regulate the function of certain membrane receptors in response to elevated levels of NAD(+).

Physiological relevance of the endogenous mono(ADP-ribosyl)ation of cellular proteins

The mono(ADP-ribosyl)ation reaction is a post-translational modification that is catalysed by both bacterial toxins and eukaryotic enzymes, and that results in the transfer of ADP-ribose from betaNAD+ to various acceptor proteins. In mammals, both intracellular and extracellular reactions have been described; the latter are due to glycosylphosphatidylinositol-anchored or secreted enzymes that are able to modify their targets, which include the purinergic receptor P2X7, the defensins and the integrins. Intracellular mono(ADP-ribosyl)ation modifies proteins that have roles in cell signalling and metabolism, such as the chaperone GRP78/BiP, the beta-subunit of heterotrimeric G-proteins and glutamate dehydrogenase. The molecular identification of the intracellular enzymes, however, is still missing. A better molecular understanding of this reaction will help in the full definition of its role in cell physiology and pathology.

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.

ADP-ribosylation of integrin alpha7 modulates the binding of integrin alpha7beta1 to laminin

The extracellular domain of integrin alpha7 is ADP-ribosylated by an arginine-specific ecto-ADP-ribosyltransferase after adding exogenous NAD+ to intact C2C12 skeletal muscle cells. The effect of ADP-ribosylation on the structure or function of integrin alpha7beta1 has not been explored. In the present study, we show that ADP-ribosylation of integrin alpha7 takes place exclusively in differentiated myotubes and that this post-translational modification modulates the affinity of alpha7beta1 dimer for its ligand, laminin. ADP-ribosylation in the 37-kDa 'stalk' region of alpha7 that takes place at micromolar NAD+ concentrations increases the binding of the alpha7beta1 dimer to laminin. Increased in vitro binding of integrin alpha7beta1 to laminin after ADP-ribosylation of the 37-kDa fragment of alpha7 requires the presence of Mn2+ and it is not observed in the presence of Mg2+. In contrast, ADP-ribosylation of the 63-kDa N-terminal region comprising the ligand-binding site of alpha7 that occurs at approx. 100 microM NAD+ inhibits the binding of integrin alpha7beta1 to laminin. Furthermore, incubation of C2C12 myotubes with NAD+ increases the expression of an epitope on integrin beta1 subunit recognized by monoclonal antibody 9EG7. We discuss our results based on the current models of integrin activation. We also hypothesize that ADP-ribosylation may represent a mechanism of regulation of integrin alpha7beta1 function in myofibres in vivo when the continuity of the membrane is compromised and NAD+ is available as a substrate for ecto-ADP-ribosylation.

Reannotation of the CELO genome characterizes a set of previously unassigned open reading frames and points to novel modes of host interaction in avian adenoviruses

BACKGROUND

The genome of the avian adenovirus Chicken Embryo Lethal Orphan (CELO) has two terminal regions without detectable homology in mammalian adenoviruses that are left without annotation in the initial analysis. Since adenoviruses have been a rich source of new insights into molecular cell biology and practical applications of CELO as gene a delivery vector are being considered, this genome appeared worth revisiting. We conducted a systematic reannotation and in-depth sequence analysis of the CELO genome.

RESULTS

We describe a strongly diverged paralogous cluster including ORF-2, ORF-12, ORF-13, and ORF-14 with an ATPase/helicase domain most likely acquired from adeno-associated parvoviruses. None of these ORFs appear to have retained ATPase/helicase function and alternative functions (e.g. modulation of gene expression during the early life-cycle) must be considered in an adenoviral context. Further, we identified a cluster of three putative type-1-transmembrane glycoproteins with IG-like domains (ORF-9, ORF-10, ORF-11) which are good candidates to substitute for the missing immunomodulatory functions of mammalian adenoviruses. ORF-16 (located directly adjacent) displays distant homology to vertebrate mono-ADP-ribosyltransferases. Members of this family are known to be involved in immuno-regulation and similiar functions during CELO life cycle can be considered for this ORF. Finally, we describe a putative triglyceride lipase (merged ORF-18/19) with additional domains, which can be expected to have specific roles during the infection of birds, since they are unique to avian adenoviruses and Marek's disease-like viruses, a group of pathogenic avian herpesviruses.

CONCLUSIONS

We could characterize most of the previously unassigned ORFs pointing to functions in host-virus interaction. The results provide new directives for rationally designed experiments.

Identification of regulatory domains in ADP-ribosyltransferase-1 that determine transferase and NAD glycohydrolase activities

Mono-ADP-ribosyltransferases (ART1-7) transfer ADP-ribose from NAD+ to proteins (transferase activity) or water (NAD glycohydrolase activity). The mature proteins contain two domains, an alpha-helical amino terminus and a beta-sheet-rich carboxyl terminus. A basic region in the carboxyl termini is encoded in a separate exon in ART1 and ART5. Structural motifs are conserved among ART molecules. Successive amino- or carboxyl-terminal truncations of ART1, an arginine-specific transferase, identified regions that regulated transferase and NAD glycohydrolase activities. In mouse ART1, amino acids 24-38 (ART-specific extension) were needed to inhibit both activities; amino acids 39-45 (common ART coil) were required for both. Successive truncations of the alpha-helical region reduced transferase and NAD glycohydrolase activities; however, truncation to residue 106 enhanced both. Removal of the carboxyl-terminal basic domain decreased transferase, but enhanced NAD glycohydrolase, activity. Thus, amino- and carboxyl-terminal regions of ART1 are required for transferase activity. The enhanced glycohydrolase activity of the shorter mutants indicates that sequences, which are not part of the NAD binding, core catalytic site, exert structural constraints, modulating substrate specificity and catalytic activity. These functional domains, defined by discrete exons or structural motifs, are found in ART1 and other ARTs, consistent with conservation of structure and function across the ART family.

The family of toxin-related ecto-ADP-ribosyltransferases in humans and the mouse

ADP-ribosyltransferases including toxins secreted by Vibrio cholera, Pseudomonas aerurginosa, and other pathogenic bacteria inactivate the function of human target proteins by attaching ADP-ribose onto a critical amino acid residue. Cross-species polymerase chain reaction (PCR) and database mining identified the orthologs of these ADP-ribosylating toxins in humans and the mouse. The human genome contains four functional toxin-related ADP-ribosyltransferase genes (ARTs) and two related intron-containing pseudogenes; the mouse has six functional orthologs. The human and mouse ART genes map to chromosomal regions with conserved linkage synteny. The individual ART genes reveal highly restricted expression patterns, which are largely conserved in humans and the mouse. We confirmed the predicted extracellular location of the ART proteins by expressing recombinant ARTs in insect cells. Two human and four mouse ARTs contain the active site motif (R-S-EXE) typical of arginine-specific ADP-ribosyltransferases and exhibit the predicted enzyme activities. Two other human ARTs and their murine orthologues deviate in the active site motif and lack detectable enzyme activity. Conceivably, these ARTs may have acquired a new specificity or function. The position-sensitive iterative database search program PSI-BLAST connected the mammalian ARTs with most known bacterial ADP-ribosylating toxins. In contrast, no related open reading frames occur in the four completed genomes of lower eucaryotes (yeast, worm, fly, and mustard weed). Interestingly, these organisms also lack genes for ADP-ribosylhydrolases, the enzymes that reverse protein ADP-ribosylation. This suggests that the two enzyme families that catalyze reversible mono-ADP-ribosylation either were lost from the genomes of these nonchordata eucaryotes or were subject to horizontal gene transfer between kingdoms.

NAD-dependent inhibition of the NAD-glycohydrolase activity in A549 cells

NAD glycohydrolases are enzymes that catalyze the hydrolysis of NAD to produce ADP-ribose and nicotinamide. Regulation of these enzymes has not been fully elucidated. We have identified a NAD-glycohydrolase activity associated with the outer surface of the plasma membrane in human lung epithelial cell line A549. This activity is negatively regulated by its substrate beta-NAD but not by alpha-NAD. Partial restoration of NADase activity after incubation of the cells with arginine or histidine, known ADP-ribose acceptors, suggests that inhibition be regulated by ADP-ribosylation. A549 do not undergo to apoptosis upon NAD treatment indicating that this effect be likely mediated by a cellular component(s) lacking in epithelial cells.

Inactivation of platelet-derived growth factor-BB following modification by ADP-ribosyltransferase

1. Arginine-specific ADP-ribosyltransferase (ART1) is expressed on the surface of a number of cell types, and catalyses the transfer of ADP-ribose from NAD(+) to target proteins. We investigated whether extracellular proteins such as growth factors may serve as substrates for this enzyme, with subsequent alteration in their biological activity. Experiments were performed with rat skeletal muscle membranes and V79 Chinese hamster lung fibroblasts with doxycycline-inducible expression of human ART. 2. From a panel of growth factors, platelet-derived growth factor-BB (PDGF-BB) was found to be the best substrate for ART1, whereas the structural homologue PDGF-AA was not a substrate. Under conditions of maximum labelling 5 mol ADP-ribose was incorporated per mol of PDGF-BB. 3. Purified (ADP-ribosyl)-PDGF-BB did not stimulate a mitogenic or chemotactic response in human pulmonary smooth muscle cells, and showed a reduced capacity to bind to PDGF receptors in competition binding experiments, when compared to unmodified PDGF-BB. 4. PDGF-dependent [(3)H-methyl]-thymidine incorporation was measured in the ART1-transfected fibroblast cell line at physiological concentrations of PDGF-BB, and without addition of extracellular NAD(+). Fibroblasts expressing human ART1 at the cell surface showed reduced mitogenic responses to PDGF-BB, but not to PDGF-AA. This loss of mitogenic response in cells expressing ART1 activity was reversed by the addition of agmatine (an ART1 substrate). 5. In conclusion, we propose that PDGF-BB-dependent signalling may be regulated by post-translational modification of the growth factor by ART1 at the cell surface. This has been demonstrated in membranes of rat skeletal muscle, and the reaction confirmed in ART1-transfected fibroblasts.

Endogenous ADP-ribosylation of the G protein beta subunit prevents the inhibition of type 1 adenylyl cyclase

Mono-ADP-ribosylation is a post-translational modification of cellular proteins that has been implicated in the regulation of signal transduction, muscle cell differentiation, protein trafficking, and secretion. In several cell systems we have observed that the major substrate of endogenous mono-ADP-ribosylation is a 36-kDa protein. This ADP-ribosylated protein was both recognized in Western blotting experiments and selectively immunoprecipitated by a G protein beta subunit-specific polyclonal antibody, indicating that this protein is the G protein beta subunit. The ADP-ribosylation of the beta subunit was due to a plasma membrane-associated enzyme, was sensitive to treatment with hydroxylamine, and was inhibited by meta-iodobenzylguanidine, indicating that the involved enzyme is an arginine-specific mono-ADP-ribosyltransferase. By mutational analysis, the target arginine was located in position 129. The ADP-ribosylated beta subunit was also deribosylated by a cytosolic hydrolase. This ADP-ribosylation/deribosylation cycle might be an in vivo modulator of the interaction of betagamma with specific effectors. Indeed, we found that the ADP-ribosylated betagamma subunit is unable to inhibit calmodulin-stimulated type 1 adenylyl cyclase in cell membranes and that the endogenous ADP-ribosylation of the beta subunit occurs in intact Chinese hamster ovary cells, where the NAD(+) pool was labeled with [(3)H]adenine. These results show that the ADP-ribosylation of the betagamma subunit could represent a novel cellular mechanism in the regulation of G protein-mediated signal transduction.

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.

Selective expression of RT6 superfamily in human bronchial epithelial cells

RT6 proteins are glycosylphosphatidylinositol (GPI)-linked alloantigens that are localized to cytotoxic T lymphocytes and that have nicotinamide adenine dinucleotide glycohydrolase and adenosine diphosphate (ADP)-ribosyltransferase activities. In view of the importance of GPI-linked surface proteins in mediating interactions of cells with their milieu, and the varied functions of airway cells in inflammation, we undertook the present study to determine whether human homologues of the RT6 superfamily of ADP-ribosyltransferases (ART) are expressed in pulmonary epithelial cells. We hypothesized that these surface proteins or related family members may be present in cells that interact with inflammatory cells, and that they may thereby be involved in intercellular signaling. Using in situ analysis and Northern blot analysis, we identified ART1 messenger RNA (mRNA) in airway epithelial cells. As expected for GPI-anchored proteins, the localization of ART1 at the apical surface of ciliated epithelial cells was demonstrated by staining with polyclonal anti-ART1 antibody, and was confirmed by loss of this immunoreactivity after treatment with phosphatidylinositol-specific phospholipase C (PI-PLC), which selectively cleaves GPI anchors and releases proteins from the plasma membrane. Using in situ hybridization with specific ART3 and ART4 oligonucleotides, we also identified two additional members of the RT6 superfamily in epithelial cells. In accord with these findings, we identified ART3 and ART4 mRNAs through reverse transcription- polymerase chain reaction of polyadenine-positive RNA from human trachea. Interestingly, these proteins appeared to be preferentially localized to the airway epithelium. The localized expression of these members of the RT6 superfamily in human pulmonary epithelial cells may reflect a role for them in cell-cell signaling during immune responses within the airway.

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.

Polymorphic forms of human ADP-ribosyltransferase-1 differences in their catalytic activities revealed by labeling of membrane-associated substrates

Full length cDNA encoding ADP-ribosyltransferase-1 (ART1) was generated from human skeletal muscle. A single base variation from the published sequence was observed (C770-->T), and was established as a polymorphism by the screening of a population of 50 Caucasians. The base variation predicted a nonconservative substitution of Leu for Pro at codon 257. Cell lines with stable and doxycycline-inducible expression of the two polymorphic forms of ART1 were generated from Chinese hamster V79 cells, and exploited in studies to compare the activities of ART1-Pro257 and ART1-Leu257. The results revealed no differences in the capacity of phosphoinositide-specific phospholipase C to cleave the two ART1 isoforms from the plasma membrane. Furthermore, the capacities of ART1-Pro257 and ART1-Leu257 to ADP-ribosylate agmatine or fibroblast growth factor-2 were similar. Differences in the catalytic activities of ART1-Pro257 and ART1-Leu257 were however, identified when measurements were made of their capacities to ADP-ribosylate membrane-associated proteins on the surface of V79 cells. Protein(s) of molecular mass 80-110 kDa were more extensively ADP-ribosylated by ART1-Pro257 than ART1-Leu257, in accordance with the Vmax (59.5 +/- 5.5 and 37.0 +/- 3.0) and Km values (12.5 +/- 4.5 and 5.0 +/- 1. 9) for ART1-Pro257 and ART1-Leu257, respectively.

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.

Chemotaxin-dependent translocation of immunoreactive ADP-ribosyltransferase-1 to the surface of human neutrophil polymorphs

mRNA from human polymorphonuclear neutrophil leucocytes (PMNs) was probed with cDNA encoding human skeletal muscle arginine-specific ADP-ribosyltransferase (ART1). A single 2.6-kb transcript was identified, which was similar in size to that observed in human skeletal muscle RNA. An 872-bp cDNA fragment, corresponding to the amino acid sequence of the processed human skeletal muscle enzyme, was generated by reverse transcription-PCR amplification of RNA from human PMNs, and was found to be identical to the ART1 cDNA derived from human skeletal muscle. ART1 was expressed as a fusion protein with glutathione S-transferase (GST) in insect cells, and antibodies were raised against the fusion protein in a rabbit. Following removal of GST immunoreactivity by immunoprecipitation, these antibodies were used to measure the abundance of immunoreactive ART1 on the surface of PMNs. Exposure of PMNs to formyl-Met-Leu-Phe (FMLP) was followed by a rapid increase in the abundance of cell surface ART1 (T1/2 = 1.9 min), and the concentration of FMLP for half-maximum response was 28.6 nM. Similar responses were observed after exposure of the cells to platelet-activating factor or interleukin-8, and we conclude that some of the effects of these chemotaxins are mediated by translocation of an intracellular pool of ART1 to its site of catalytic activity on the outer aspect of the plasma membrane.

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.

Molecular characterization and expression of the gene for mouse NAD+:arginine ecto-mono(ADP-ribosyl)transferase, Art1

Mono(ADP-ribosyl)transferases regulate the function of target proteins by attaching ADP-ribose to specific amino acid residues in the proteins. We have characterized the gene for mouse arginine-specific ADP-ribosyltransferase, Art1. Southern blot analyses indicate that Art1 is a single-copy gene. Northern blot and reverse transcription-PCR analyses demonstrate prominent expression of Art1 in cardiac and skeletal muscle, and lower levels in spleen, lung, liver and fetal tissues. While human ART1 is not represented in the public expressed sequence tag (EST) database, the database contains 14 mouse Art1 ESTs. The Art1 gene encompasses four exons spanning 20 kb of genomic DNA. The deduced amino acid sequence of Art1 exhibits the characteristic features of a glycosylphosphatidylinositol-anchored membrane protein. It shows 75-77% sequence identity with its orthologues from the human and rabbit, and 33-34% identity with its paralogues from the mouse, Art2-1 and Art2-2. Separate exons encode the N- and C-terminal signal peptides, and a single long exon encodes the entire predicted native polypeptide chain. We expressed Art1 in 293T cells as a recombinant fusion protein with the Fc portion of human IgG1. This soluble protein exhibits enzyme activities characteristic of arginine-specific ADP-ribosyltransferases. The availability of the Art1 gene provides the basis for applying transgene and knockout technologies to further probe the function of this gene product.

Interaction of integrin alpha 7 beta 1 in C2C12 myotubes and in solution with laminin

The dimer of integrin alpha 7 and beta 1 is a major laminin-binding receptor in skeletal muscle. We studied interactions of integrin alpha 7 beta 1 with the extracellular matrix protein laminin in solution and in intact cells. Integrin alpha 7 beta 1 bound to EHS laminin (laminin-1, composed of alpha 1, beta 1, and gamma 1 chains), but not to endogenous laminin expressed in C2C12 myotubes. Northern blot analysis demonstrated that C2C12 myotubes synthesized laminin-1 alpha, beta, and gamma subunits mRNAs. C2C12 laminin was, however, immunologically distinct from EHS laminin; it was not recognized by 5D3 anti-laminin-1 monoclonal antibody, whereas 5A2 and LT3 antibodies reacted equally well with C2C12 and EHS laminins. Following deglycosylation of EHS laminin, separation of the subunits by SDS-PAGE, Western blotting, and partial amino acid sequencing of the protein bands, the epitope recognized by 5D3 antibody was localized to the gamma 1 laminin chain. Following binding in vitro, the complex of EHS laminin and integrin alpha 7 beta 1 was subject to chemical cross-linking. The two proteins did not undergo cross-linking at the cell surface, consistent with the fact that in intact, resting myotubes integrin alpha 7 beta 1 interacted poorly with EHS laminin, which may reflect a limited accessibility of integrin alpha 7 beta 1 in the membrane to laminin or an inactive state of the integrin.

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.

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.