Glycosylphosphatidylinositol-anchored and Secretory Isoforms of Mono-ADP-ribosyltransferases

Abstract

Mono-ADP-ribosylation, a post-translational modification of proteins in which the ADP-ribose moiety of NAD is transferred to an acceptor amino acid, occurs in viruses, bacteria, and eukaryotic cells (1). The reaction is distinct from that catalyzed by poly(ADPribose) polymerase, a nuclear protein involved in DNA repair, cell differentiation, and the maintenance of chromatin structure (2). Among mono-ADP-ribosyltransferases, the bacterial toxins, cholera toxin, pertussis toxin, diphtheria toxin, and Pseudomonas aeruginosa exotoxin A are the best characterized in molecular structure, function, and substrate specificity (reviewed in Ref. 1). Mono-ADP-ribosyltransferases from mammalian and avian cells have been cloned and characterized, and specific target proteins have been identified (3,4). In lymphocytes, a glycosylphosphatidylinositol (GPI) 1 -anchored transferase appears to be involved in immune modulation, whereas other isoforms in lymphocytes (5) and chicken heterophil granules (6) are membrane-associated but appear to be processed for secretion. Further, ADP-ribosyltransferases have been purified from brain, and data from several independent laboratories demonstrate that ADP-ribosylation is involved in neuronal function (7,8). Deduced amino acid sequences of the vertebrate ADP-ribosyltransferases have similarities to those of viral and bacterial toxin transferases (9, 10) in regions that form, in part, an active site cleft, consistent with a common mechanism of NAD binding and ADP-ribose transfer (9).The majority of the eukaryotic enzymes are arginine-specific transferases. ADP-ribosylation of arginine appears to be a reversible process; free arginine can be regenerated in ADP-ribosylated proteins by ADP-ribosylarginine hydrolases (1). ADP-ribosylarginine hydrolase activity was detected in the soluble fraction of turkey erythrocytes, cultured mouse cells, and rat skeletal muscle with deduced amino acid sequences known for rat, mouse, and human brain ADP-ribosylarginine hydrolases (11,12).ADP-ribosylation of cysteine was reported in bovine erythrocytes (13), and an NAD:cysteine ADP-ribosyltransferase that modified G␣ i was purified from human erythrocyte and platelet membranes (14). Consistent with this, ADP-ribosylcysteine linkages were detected in rat liver plasma membranes (15). ADP-ribosylation of cysteine can, however, occur nonenzymatically via the reaction of ADP-ribose, generated from NAD by NAD glycohydrolases, with cysteine to form an ADP-ribosylthiazolidine, a linkage distinct from the thioglycoside formed by pertussis toxin (PT)-catalyzed ADP-ribosylation of a cysteine in the heterotrimeric guanine nucleotide-binding (G) proteins (16). Nonenzymatic ADP-ribosylation of cysteine in proteins, however, yielded a product with the same chemical sensitivity as the linkage formed by PT (17). Based on these data, the ADP-ribose-cysteine produced by the human erythrocyte enzyme may have been generated nonenzymatically from free ADP-ribose. Because nitric oxide (NO) induced the noncovalent binding of the ent...

Hydrolysis of O-Acetyl-ADP-ribose Isomers by ADP-ribosylhydrolase 3

“…In addition, mammalian cells possess hydrolases that cleave the ADPr-protein linkage, releasing ADPr and regenerating the unmodified protein (5,6). An ADP-ribosyl(arginine) hydrolase, termed ARH1, catalyzes in a stereospecific manner, hydrolysis of the ␣-linkage of arginine-ribose found in ADP-ribosyl(arginine)-protein to ADPr and (arginine)-protein (7,8), consistent with the regulation of ADP-ribosyl(arginine)-protein levels by opposing activities of transferases and hydrolases, participating in an ADP-ribosylation cycle (4,9).…”

NAD+ surfaces again

“…On the other hand, although also present within cells, mono-ADP-ribosylation appears to be largely an extracellular mechanism of NAD + -mediated regulation. All identified ARTs (mono-ADP-ribosyl transferases) of higher eukaryotes are either GPI (glycosylphosphatidylinositol)-anchored or secreted [2,3]. These enzymes transfer the ADPR (ADP-ribose) moiety of NAD + on to acceptor proteins, thereby modulating their biological activities.…”

“…These enzymes transfer the ADPR (ADP-ribose) moiety of NAD + on to acceptor proteins, thereby modulating their biological activities. ARTs are apparently primarily expressed in blood cells and may contribute to immune functions by modifying proteins, for example, of the extracellular matrix [2,3]. Surprisingly, the known mammalian enzymes catalysing the synthesis of cADPR, designated ADP-ribosyl cyclases, are also located on the cell surface (again, primarily of blood cells).…”

Expression, purification, crystallization and preliminary X-ray analysis of rat ecto-ADP-ribosyltransferase 2 (ART2.2)

“…The suspicion that these toxins mimic the function of mammalian homologues was nurtured by the discovery of ART activities in uninfected tissues. This initiated a search that revealed a small family of extracellular mammalian enzymes, including the GPIanchored membrane proteins, ecto-ART1 through ecto-ART4 and the secretory ecto-ART5 (Koch-Nolte & Haag, 1997;Okazaki & Moss, 1998).…”