Cellular ADP-ribosyltransferases Research Articles

State of the art of protein mono-ADP-ribosylation: biological role and therapeutic potential.

Mono-ADP-ribosylation is a post-translational modification that was discovered more than five decades ago, and it consists of the enzymatic transfer of ADP-ribose from NAD⁺ to acceptor proteins. In viruses and prokaryotes, mono-ADP-ribosylation is mainly, but not exclusively, a mechanism used to take control of the host cell. In mammals, mono-ADP-ribosylation serves to regulate protein functions, and it is catalysed by two families of toxin-related cellular ADP-ribosyltransferases: ecto-enzymes that modify various cell-surface proteins, like integrins and receptors, and intracellular enzymes that act on a variety of nuclear and cytosolic proteins. These two families have been recently renamed the ARTCs (clostridia toxin like) and ARTDs (diphtheria toxin like), depending on their conserved structural features, and in terms of their relationships to the bacterial toxins. In addition, two members of the structurally non-related sirtuin family can also modify cellular proteins by mono-ADP-ribosylation. Recently, new examples of ADP-ribosylation of proteins involved in signal transduction and intracellular trafficking have been discovered, thus opening the route to the better molecular understanding of this reaction and of its role in human cell physiology and pathology.

Crosstalk between SET7/9-dependent methylation and ARTD1-mediated ADP-ribosylation of histone H1.4

BackgroundDifferent histone post-translational modifications (PTMs) fine-tune and integrate different cellular signaling pathways at the chromatin level. ADP-ribose modification of histones by cellular ADP-ribosyltransferases such as ARTD1 (PARP1) is one of the many elements of the histone code. All 5 histone proteins were described to be ADP-ribosylated in vitro and in vivo. However, the crosstalk between ADP-ribosylation and other modifications is little understood.ResultsIn experiments with isolated histones, it was found that ADP-ribosylation of H3 by ARTD1 prevents H3 methylation by SET7/9. However, poly(ADP-ribosyl)ation (PARylation) of histone H3 surprisingly allowed subsequent methylation of H1 by SET7/9. Histone H1 was thus identified as a new target for SET7/9. The SET7/9 methylation sites in H1.4 were pinpointed to the last lysine residues of the six KAK motifs in the C-terminal domain (K121, K129, K159, K171, K177 and K192). Interestingly, H1 and the known SET7/9 target protein H3 competed with each other for SET7/9-dependent methylation.ConclusionsThe results presented here identify H1.4 as a novel SET7/9 target protein, and document an intricate crosstalk between H3 and H1 methylation and PARylation, thus implying substrate competition as a regulatory mechanism. Thereby, these results underline the role of ADP-ribosylation as an element of the histone code.

PARP16/ARTD15 Is a Novel Endoplasmic-Reticulum-Associated Mono-ADP-Ribosyltransferase That Interacts with, and Modifies Karyopherin-ß1

BackgroundProtein mono-ADP-ribosylation is a reversible post-translational modification that modulates the function of target proteins. The enzymes that catalyze this reaction in mammalian cells are either bacterial pathogenic toxins or endogenous cellular ADP-ribosyltransferases. The latter include members of three different families of proteins: the well characterized arginine-specific ecto-enzymes ARTCs, two sirtuins and, more recently, novel members of the poly(ADP-ribose) polymerase (PARP/ARTD) family that have been suggested to act as cellular mono-ADP-ribosyltransferases. Here, we report on the characterisation of human ARTD15, the only known ARTD family member with a putative C-terminal transmembrane domain.Methodology/Principal FindingsImmunofluorescence and electron microscopy were performed to characterise the sub-cellular localisation of ARTD15, which was found to be associated with membranes of the nuclear envelope and endoplasmic reticulum. The orientation of ARTD15 was determined using protease protection assay, and is shown to be a tail-anchored protein with a cytosolic catalytic domain. Importantly, by combining immunoprecipitation with mass spectrometry and using cell lysates from cells over-expressing FLAG-ARTD15, we have identified karyopherin-ß1, a component of the nuclear trafficking machinery, as a molecular partner of ARTD15. Finally, we demonstrate that ARTD15 is a mono-ADP-ribosyltransferase able to induce the ADP-ribosylation of karyopherin-ß1, thus defining the first substrate for this enzyme.Conclusions/SignificanceOur data reveal that ARTD15 is a novel ADP-ribosyltransferase enzyme with a new intracellular location. Finally, the identification of karyopherin-ß1 as a target of ARTD15-mediated ADP-ribosylation, hints at a novel regulatory mechanism of karyopherin-ß1 functions.

Side chain specificity of ADP-ribosylation by a sirtuin.

Endogenous mono-ADP-ribosylation in eukaryotes is involved in regulating protein synthesis, signal transduction, cytoskeletal integrity, and cell proliferation, although few cellular ADP-ribosyltransferases have been identified. The sirtuins constitute a highly conserved family of protein deacetylases, and several family members have also been reported to perform protein ADP-ribosylation. We characterized the ADP-ribosylation reaction of the nuclear sirtuin homolog Trypanosoma brucei SIR2-related protein 1 (TbSIR2RP1) on both acetylated and unacetylated substrates. We demonstrated that an acetylated substrate is not required for ADP-ribosylation to occur, indicating that the reaction performed by TbSIR2RP1 is a genuine enzymatic reaction and not a side reaction of deacetylation. Biochemical and MS data showed that arginine is the major ADP-ribose acceptor for unacetylated substrates, whereas arginine does not appear to be the major ADP-ribose acceptor in reactions with acetylated histone H1.1. We performed combined ab initio quantum mechanical/molecular mechanical molecular dynamics simulations, which indicated that sirtuin ADP-ribosylation at arginine is energetically feasible, and involves a concerted mechanism with a highly dissociative transition state. In comparison with the corresponding nicotinamide cleavage in the deacetylation reaction, the simulations suggest that sirtuin ADP-ribosylation would be several orders slower but less sensitive to nicotinamide inhibition, which is consistent with experimental results. These results suggest that TbSIR2RP1 can perform ADP-ribosylation using two distinct mechanisms, depending on whether or not the substrate is acetylated.

Combining affinity purification by ADP-ribose-binding macro domains with mass spectrometry to define the mammalian ADP-ribosyl proteome

Mono-ADP-ribosylation is a reversible posttranslational modification that modulates the function of target proteins. The enzymes that catalyze this reaction in mammalian cells are either bacterial pathogenic toxins or endogenous cellular ADP-ribosyltransferases. For the latter, both the enzymes and their targets have largely remained elusive, mainly due to the lack of specific techniques to study this reaction. The recent discovery of the macro domain, a protein module that interacts selectively with ADP-ribose, prompted us to investigate whether this interaction can be extended to the identification of ADP-ribosylated proteins. Here, we report that macro domains can indeed be used as selective baits for high-affinity purification of mono-ADP-ribosylated proteins, which can then be identified by mass spectrometry. Using this approach, we have identified a series of cellular targets of ADP-ribosylation reactions catalyzed by cellular ADP-ribosyltransferases and toxins. These proteins include most of the known targets of ADP-ribosylation, indicating the validity of this method, and a large number of other proteins, which now need to be individually validated. This represents an important step toward the discovery of new ADP-ribosyltransferase targets and an understanding of the physiological role and the pharmacological potential of this protein modification.

NEW EMBO MEMBER'S REVIEW: Functional aspects of protein mono-ADP-ribosylation

Mono-ADP-ribosylation is the enzymatic transfer of ADP-ribose from NAD(+) to acceptor proteins. It is catalysed by cellular ADP-ribosyltransferases and certain bacterial toxins. There are two subclasses of cellular enzymes: the ectoenzymes that modify targets such as integrins, defensin and other cell surface molecules; and the intracellular enzymes that act on proteins involved in cell signalling and metabolism, such as the beta-subunit of heterotrimeric G proteins, GRP78/BiP and elongation factor 2. The genes that encode the ectoenzymes have been cloned and their protein products are well characterized, yet little is known about the intracellular ADP-ribosyltransferases, which may be part of a novel protein family with an important role in regulating cell function. ADP-ribosylation usually leads to protein inactivation, providing a mechanism to inhibit protein functions in both physiological and pathological conditions.

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

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

Mono-ADP-Ribosylation: A Tool for Modulating Immune Response and Cell Signaling

Mono-ADP-ribosylation is a posttranslational modification of cellular proteins that has the potential to regulate various cell functions. This reaction consists of the enzymatic transfer of ADP-ribose to specific acceptor amino acid residues (predominantly arginine and cysteine). The best-known cellular ADP-ribosyltransferases (the enzymes that catalyze this reaction) are the seven ectoenzymes, members of the ART family.

Cellular ADP-ribosylation of elongation factor 2.

A cellular ADP-ribosyltransferase activity has been found in a variety of animals and tissues. The enzyme transfers ADP-ribose from NAD to elongation factor 2, inactivating the factor and thus inhibiting in vitro protein synthesis. Although, the mechanism of action of the cellular enzyme appears similar to diphtheria toxin and Pseudomonas exotoxin A, it differs from the toxins in that only a fraction of the EF-2 pool is modified. The endogenously ADP-ribosylated EF-2 has been detected by a variety of methods including two-dimensional electrophoresis and immunoprecipitation with elongation factor 2 antibody. The nature of the cellular ADP-ribosyltransferase and its physiological significance are unknown.

Expression of Non-ADP-ribosylatable, Diphtheria Toxin-Resistant Elongation Factor 2 in Saccharomyces cerevisiae

Eucaryotic elongation factor 2 (EF-2) contains a post-translationally modified histidine residue termed diphthamide that is specifically ADP-ribosylated by diphtheria toxin (DT) or Pseudomonas exotoxin A. To analyze the potential physiological role of ADP-ribosylation of EF-2 by cellular ADP-ribosyl transferase, we constructed DT-resistant, non-ADP-ribosylatable Saccharomyces cerevisiae EF-2 by site-directed mutagenesis and expressed the mutant EF-2 in yeast. Substitution of Arg for Gly(701) in yeast EF-2 conferred complete resistance to DT in vivo and in vitro. However, when only non-ribosylatable EF-2 was expressed in cells using genetic manipulation, the mutated EF-2 did not affect vegetative cell growth, mating, sporulation and germination of ascospores.

Cellular mono(ADP-ribosyl) transferase inhibits protein synthesis

A reticulocyte translation system was depleted of functional EF-2 by treatment with diphtheria toxin (DT) fragment A and NAD. After dialysis to remove NAD, the system was reconstituted using preparation of EF-2 derived from pyBHK cells. Untreated and reconstituted lysates permitted similar rates of translation. As expected, when DT-treated EF-2 was used to reconstitute the system, no translation occurred. Furthermore EF-2, reacting with the endogenous ADP-ribosyl transferase from pyBHK cells, was also unable to restore protein synthesis in the reconstituted system. These studies suggest that eukaryotic cellular ADP-ribosyl transferases may play a role in regulating protein synthesis.

On the nature of cellular ADP-ribosyltransferase from rat liver specific for elongation factor 2

A cellular ADP-ribosyltransferase, specific for elongation factor 2 (EF-2), is found in extracts from rat liver. Co-migrating with EF-2 throughout purification, this activity is, moreover, located in the protein bands corresponding to EF-2 after native or sodium dodecyl sulfate polyacrylamide gel electrophoresis. The observed activity is thus implicated to be an inherent property of EF-2. Preincubation of EF-2 with GuoPPCH2Pox inhibits endogenous, but not diphtheria toxin catalyzed ADP-ribosylation.

Cellular ADP-ribosyltransferase with the same mechanism of action as diphtheria toxin and Pseudomonas toxin A.

An ADP-ribosyltransferase was found in elongation factor 2 (EF-2) preparations from polyoma virus-transformed baby hamster kidney (pyBHK) cells. Like fragment A of diphtheria toxin and Pseudomonas toxin A, this eukaryotic cellular enzyme transfers [14C]adenosine from NAD+ to EF-2. However, the cellular transferase is immunologically distinct from fragment A. The transferase also can be distinguished from fragment A and Pseudomonas toxin A by the inhibition of the activity of the former by cytoplasmic extracts and by histamine. Snake venom phosphodiesterase digestion of the [14C]adenosine-labeled EF-2 product of the cellular transferase reaction yielded [14C]AMP, indicating that the cellular enzyme is a mono(ADP-ribosyl)transferase. The forward ADP-ribosylation reaction catalyzed by the cellular enzyme is reversed by fragment A, yielding [14C]NAD+. The results strongly suggest that the cellular transferase is a mono(ADP-ribosyl)transferase, which ADP-ribosylates the same diphthamide residue of EF-2 as does fragment A and Pseudomonas toxin A.

Adenosine diphosphate ribosyltransferase and protein acceptors associated with cytoplasmic free messenger ribonucleoprotein particles

ADP-ribosyltransferase activity has been characterized in free messenger ribonucleoprotein particles (mRNP) from mouse plasmacytoma cells. This enzymatic activity appears to be associated with the free mRNP and not due to nuclear contamination. The enzyme activity is not stimulated by added DNA or histone H1 and represents 34 per cent of the total cellular ADP-ribosyltransferase activity while the DNA contamination in free mRNP is less than 4 per cent of the total cellular DNA. Moreover, the ADP-ribosyltransferase specific activity per mg of DNA is about 75-fold higher in free mRNP than in the nuclei. During CsCl gradient centrifugation of the cytoplasmic fraction, the ADP-ribosylated material separates out at a buoyant density similar to that of free mRNP. This ADP-ribosyltransferase activity is inhibited by thymidine, nicotinamide and 3-aminobenzamide, while it is highly stimulated by exogenous pancreatic RNase. The in vitro synthesized acid insoluble material is rendered partly soluble by treatment by a proteolytic enzyme or by snake venom phosphodiesterase resulting in phosphoribosyl-AMP formation: the pancreatic RNase does not solubilize this material. Several ADP-ribosylated proteins are detected by lithium dodecylsulfate gel electrophoresis. Such an ADP-ribosyltransferase activity has also been detected in free mRNP from rat liver. It is suggested that this ADP-ribosylation of specific free mRNP proteins may be associated with free mRNP structure and/or with some chemical covalent type of modification rendering mRNA available for translation.