Pierisins and CARP-1: ADP-ribosylation of DNA by ARTCs in butterflies and shellfish

Mammalian N1-adenosine PARylation is a reversible DNA modification

Poly-ADP-ribosylation (PARylation) is regarded as a protein-specific modification. However, some PARPs were recently shown to modify DNA termini in vitro. Here, we use ultrasensitive mass spectrometry (LC-MS/MS), anti-PAR antibodies, and anti-PAR reagents to show that mammalian DNA is physiologically PARylated and to different levels in primary tissues. Inhibition of PAR glycohydrolase (PARG) increases DNA PARylation, supporting that the modification is reversible. DNA PARylation requires PARP1 and in vitro PARP1 PARylates single-stranded DNA, while PARG reverts the modification. DNA PARylation occurs at the N1-position of adenosine residues to form N1-Poly(ADP-ribosyl)-deoxyadenosine. Through partial hydrolysis of mammalian gDNA we identify PAR-DNA via the diagnostic deamination product N1-ribosyl-deoxyinosine to occur in vivo. The discovery of N1-adenosine PARylation as a DNA modification establishes the conceptual and methodological framework to elucidate its biological relevance and extends the role of PARP enzymes.

Intracellular mono-ADP-ribosyltransferases at the host-virus interphase

The innate immune system, the primary defense mechanism of higher organisms against pathogens including viruses, senses pathogen-associated molecular patterns (PAMPs). In response to PAMPs, interferons (IFNs) are produced, allowing the host to react swiftly to viral infection. In turn the expression of IFN-stimulated genes (ISGs) is induced. Their products disseminate the antiviral response. Among the ISGs conserved in many species are those encoding mono-ADP-ribosyltransferases (mono-ARTs). This prompts the question whether, and if so how, mono-ADP-ribosylation affects viral propagation. Emerging evidence demonstrates that some mono-ADP-ribosyltransferases function as PAMP receptors and modify both host and viral proteins relevant for viral replication. Support for mono-ADP-ribosylation in virus–host interaction stems from the findings that some viruses encode mono-ADP-ribosylhydrolases, which antagonize cellular mono-ARTs. We summarize and discuss the evidence linking mono-ADP-ribosylation and the enzymes relevant to catalyze this reversible modification with the innate immune response as part of the arms race between host and viruses.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

ADP-ribosylation of RNA and DNA: from in vitro characterization to in vivo function

The functionality of DNA, RNA and proteins is altered dynamically in response to physiological and pathological cues, partly achieved by their modification. While the modification of proteins with ADP-ribose has been well studied, nucleic acids were only recently identified as substrates for ADP-ribosylation by mammalian enzymes. RNA and DNA can be ADP-ribosylated by specific ADP-ribosyltransferases such as PARP1-3, PARP10 and tRNA 2'-phosphotransferase (TRPT1). Evidence suggests that these enzymes display different preferences towards different oligonucleotides. These reactions are reversed by ADP-ribosylhydrolases of the macrodomain and ARH families, such as MACROD1, TARG1, PARG, ARH1 and ARH3. Most findings derive from in vitro experiments using recombinant components, leaving the relevance of this modification in cells unclear. In this Survey and Summary, we provide an overview of the enzymes that ADP-ribosylate nucleic acids, the reversing hydrolases, and the substrates' requirements. Drawing on data available for other organisms, such as pierisin1 from cabbage butterflies and the bacterial toxin-antitoxin system DarT-DarG, we discuss possible functions for nucleic acid ADP-ribosylation in mammals. Hypothesized roles for nucleic acid ADP-ribosylation include functions in DNA damage repair, in antiviral immunity or as non-conventional RNA cap. Lastly, we assess various methods potentially suitable for future studies of nucleic acid ADP-ribosylation.

ADP-ribosylation systems in bacteria and viruses

ADP-ribosylation is an ancient posttranslational modification present in all kingdoms of life. The system likely originated in bacteria where it functions in inter- and intra-species conflict, stress response and pathogenicity. It was repeatedly adopted via lateral transfer by eukaryotes, including humans, where it has a pivotal role in epigenetics, DNA-damage repair, apoptosis, and other crucial pathways including the immune response to pathogenic bacteria and viruses. In other words, the same ammunition used by pathogens is adapted by eukaryotes to fight back. While we know quite a lot about the eukaryotic system, expanding rather patchy knowledge on bacterial and viral ADP-ribosylation would give us not only a better understanding of the system as a whole but a fighting advantage in this constant arms race. By writing this review we hope to put into focus the available information and give a perspective on how this system works and can be exploited in the search for therapeutic targets in the future. The relevance of the subject is especially highlighted by the current situation of being amid the world pandemic caused by a virus harbouring and dependent on a representative of such a system.

The Buzz about ADP-Ribosylation Toxins from Paenibacillus larvae, the Causative Agent of American Foulbrood in Honey Bees

The Gram-positive, spore-forming bacterium Paenibacillus larvae is the etiological agent of American Foulbrood, a highly contagious and often fatal honey bee brood disease. The species P. larvae comprises five so-called ERIC-genotypes which differ in virulence and pathogenesis strategies. In the past two decades, the identification and characterization of several P. larvae virulence factors have led to considerable progress in understanding the molecular basis of pathogen-host-interactions during P. larvae infections. Among these virulence factors are three ADP-ribosylating AB-toxins, Plx1, Plx2, and C3larvin. Plx1 is a phage-born toxin highly homologous to the pierisin-like AB-toxins expressed by the whites-and-yellows family Pieridae (Lepidoptera, Insecta) and to scabin expressed by the plant pathogen Streptomyces scabiei. These toxins ADP-ribosylate DNA and thus induce apoptosis. While the presumed cellular target of Plx1 still awaits final experimental proof, the classification of the A subunits of the binary AB-toxins Plx2 and C3larvin as typical C3-like toxins, which ADP-ribosylate Rho-proteins, has been confirmed experimentally. Normally, C3-exoenzymes do not occur together with a B subunit partner, but as single domain toxins. Interestingly, the B subunits of the two P. larvae C3-like toxins are homologous to the B-subunits of C2-like toxins with striking structural similarity to the PA-63 protomer of Bacillus anthracis.

Mapping the DNA-Binding Motif of Scabin Toxin, a Guanine Modifying Enzyme from Streptomyces scabies

Scabin is a mono-ADP-ribosyltransferase toxin/enzyme and possible virulence factor produced by the agriculture pathogen, Streptomyces scabies. Recently, molecular dynamic approaches and MD simulations revealed its interaction with both NAD⁺ and DNA substrates. An Essential Dynamics Analysis identified a crab-claw-like mechanism, including coupled changes in the exposed motifs, and the R_β1-R_La-N_Lc-STT_β2-W_PN-W_ARTT-(QxE)_ARTT sequence motif was proposed as a catalytic signature of the Pierisin family of DNA-acting toxins. A new fluorescence assay was devised to measure the kinetics for both RNA and DNA substrates. Several protein variants were prepared to probe the Scabin-NAD-DNA molecular model and to reveal the reaction mechanism for the transfer of ADP-ribose to the guanine base in the DNA substrate. The results revealed that there are several lysine and arginine residues in Scabin that are important for binding the DNA substrate; also, key residues such as Asn110 in the mechanism of ADP-ribose transfer to the guanine base were identified. The DNA-binding residues are shared with ScARP from Streptomyces coelicolor but are not conserved with Pierisin-1, suggesting that the modification of guanine bases by ADP-ribosyltransferases is divergent even in the Pierisin family.

Several New Putative Bacterial ADP-Ribosyltransferase Toxins Are Revealed from In Silico Data Mining, Including the Novel Toxin Vorin, Encoded by the Fire Blight Pathogen Erwinia amylovora

Mono-ADP-ribosyltransferase (mART) toxins are secreted by several pathogenic bacteria that disrupt vital host cell processes in deadly diseases like cholera and whooping cough. In the last two decades, the discovery of mART toxins has helped uncover the mechanisms of disease employed by pathogens impacting agriculture, aquaculture, and human health. Due to the current abundance of mARTs in bacterial genomes, and an unprecedented availability of genomic sequence data, mART toxins are amenable to discovery using an in silico strategy involving a series of sequence pattern filters and structural predictions. In this work, a bioinformatics approach was used to discover six bacterial mART sequences, one of which was a functional mART toxin encoded by the plant pathogen, Erwinia amylovora, called Vorin. Using a yeast growth-deficiency assay, we show that wild-type Vorin inhibited yeast cell growth, while catalytic variants reversed the growth-defective phenotype. Quantitative mass spectrometry analysis revealed that Vorin may cause eukaryotic host cell death by suppressing the initiation of autophagic processes. The genomic neighbourhood of Vorin indicated that it is a Type-VI-secreted effector, and co-expression experiments showed that Vorin is neutralized by binding of a cognate immunity protein, VorinI. We demonstrate that Vorin may also act as an antibacterial effector, since bacterial expression of Vorin was not achieved in the absence of VorinI. Vorin is the newest member of the mART family; further characterization of the Vorin/VorinI complex may help refine inhibitor design for mART toxins from other deadly pathogens.

(ADP-ribosyl)hydrolases: structure, function, and biology

ADP-ribosylation is an intricate and versatile posttranslational modification involved in the regulation of a vast variety of cellular processes in all kingdoms of life. Its complexity derives from the varied range of different chemical linkages, including to several amino acid side chains as well as nucleic acids termini and bases, it can adopt. In this review, we provide an overview of the different families of (ADP-ribosyl)hydrolases. We discuss their molecular functions, physiological roles, and influence on human health and disease. Together, the accumulated data support the increasingly compelling view that (ADP-ribosyl)hydrolases are a vital element within ADP-ribosyl signaling pathways and they hold the potential for novel therapeutic approaches as well as a deeper understanding of ADP-ribosylation as a whole.

Characterization of DNA ADP-ribosyltransferase activities of PARP2 and PARP3: new insights into DNA ADP-ribosylation

Poly(ADP-ribose) polymerases (PARPs) act as DNA break sensors and catalyze the synthesis of polymers of ADP-ribose (PAR) covalently attached to acceptor proteins at DNA damage sites. It has been demonstrated that both mammalian PARP1 and PARP2 PARylate double-strand break termini in DNA oligonucleotide duplexes in vitro. Here, we show that mammalian PARP2 and PARP3 can PARylate and mono(ADP-ribosyl)ate (MARylate), respectively, 5′- and 3′-terminal phosphate residues at double- and single-strand break termini of a DNA molecule containing multiple strand breaks. PARP3-catalyzed DNA MARylation can be considered a new type of reversible post-replicative DNA modification. According to DNA substrate specificity of PARP3 and PARP2, we propose a putative mechanistic model of PARP-catalyzed strand break–oriented ADP-ribosylation of DNA termini. Notably, PARP-mediated DNA ADP-ribosylation can be more effective than PARPs’ auto-ADP-ribosylation depending on the DNA substrates and reaction conditions used. Finally, we show an effective PARP3- or PARP2-catalyzed ADP-ribosylation of high-molecular-weight (∼3-kb) DNA molecules, PARP-mediated DNA PARylation in cell-free extracts and a persisting signal of anti-PAR antibodies in a serially purified genomic DNA from bleomycin-treated poly(ADP-ribose) glycohydrolase-depleted HeLa cells. These results suggest that certain types of complex DNA breaks can be effectively ADP-ribosylated by PARPs in cellular response to DNA damage.

Substrate N2 atom recognition mechanism in pierisin family DNA-targeting, guanine-specific ADP-ribosyltransferase ScARP

ScARP from the bacterium Streptomyces coelicolor belongs to the pierisin family of DNA-targeting ADP-ribosyltransferases (ARTs). These enzymes ADP-ribosylate the N² amino groups of guanine residues in DNA to yield N²-(ADP-ribos-1-yl)-2'-deoxyguanosine. Although the structures of pierisin-1 and Scabin were revealed recently, the substrate recognition mechanisms remain poorly understood because of the lack of a substrate-binding structure. Here, we report the apo structure of ScARP and of ScARP bound to NADH and its GDP substrate at 1.50 and 1.57 Å resolutions, respectively. The bound structure revealed that the guanine of GDP is trapped between N-ribose of NADH and Trp-159. Interestingly, N² and N³ of guanine formed hydrogen bonds with the OE1 and NE2 atoms of Gln-162, respectively. We directly observed that the ADP-ribosylating toxin turn-turn (ARTT)-loop, including Trp-159 and Gln-162, plays a key role in the specificity of DNA-targeting, guanine-specific ARTs as well as protein-targeting ARTs such as the C3 exoenzyme. We propose that the ARTT-loop recognition is a common substrate-recognition mechanism in the pierisin family. Furthermore, this complex structure sheds light on similarities and differences among two subclasses that are distinguished by conserved structural motifs: H-Y-E in the ARTD subfamily and R-S-E in the ARTC subfamily. The spatial arrangements of the electrophile and nucleophile were the same, providing the first evidence for a common reaction mechanism in these ARTs. ARTC (including ScARP) uses the ARTT-loop for substrate recognition, whereas ARTD (represented by Arr) uses the C-terminal helix instead of the ARTT-loop. These observations could help inform efforts to improve ART inhibitors.

Dna is a New Target of Parp3

Most members of the poly(ADP-ribose)polymerase family, PARP family, have a catalytic activity that involves the transfer of ADP-ribose from a beta-NAD+-molecule to protein acceptors. It was recently discovered by Talhaoui et al. that DNA-dependent PARP1 and PARP2 can also modify DNA. Here, we demonstrate that DNA-dependent PARP3 can modify DNA and form a specific primed structure for further use by the repair proteins. We demonstrated that gapped DNA that was ADP-ribosylated by PARP3 could be ligated to double-stranded DNA by DNA ligases. Moreover, this ADP-ribosylated DNA could serve as a primed DNA substrate for PAR chain elongation by the purified proteins PARP1 and PARP2 as well as by cell-free extracts. We suggest that this ADP-ribose modification can be involved in cellular pathways that are important for cell survival in the process of double-strand break formation.

Reversible mono-ADP-ribosylation of DNA breaks

Adenosine diphosphate (ADP)-ribosylation is a chemical modification of macromolecules that plays an important role in regulation of quintessential biological processes such as DNA repair, transcription, chromatin remodelling, stress response, apoptosis, bacterial metabolism and many others. ADP-ribosylation is carried out by ADP-ribosyltransferase proteins, such as poly (ADP-ribose) polymerases (PARPs) that transfer either monomer or polymers of ADP-ribose onto the molecular targets by using nicotinamide adenine dinucleotide (NAD+ ) as a cofactor. Traditionally, proteins have been described as primary targets of ADP-ribosylation; however, there has been growing evidence that DNA may be a common target as well. Here, we show using biochemical studies that PARP3, a DNA damage-activated ADP-ribosyltransferase, can mono-ADP-ribosylate double-stranded DNA ends. ADP-ribosylation of DNA mediated by PARP3 attaches a single mono-ADP-ribose moiety to the phosphate group at the terminal ends of DNA. We further show that mono ADP-ribosylation at DNA ends can be efficiently reversed by several cellular hydrolases (PARG, MACROD2, TARG1 and ARH3). This suggests that mono ADP-ribosylated DNA adducts can be efficiently removed in cells by several mechanisms.

ADP-ribosylation: new facets of an ancient modification

Rapid response to environmental changes is achieved by uni- and multicellular organisms through a series of molecular events, often involving modification of macromolecules, including proteins, nucleic acids and lipids. Amongst these, ADP-ribosylation is of emerging interest because of its ability to modify different macromolecules in the cells, and its association with many key biological processes, such as DNA-damage repair, DNA replication, transcription, cell division, signal transduction, stress and infection responses, microbial pathogenicity and aging. In this review, we provide an update on novel pathways and mechanisms regulated by ADP-ribosylation in organisms coming from all kingdoms of life.

The Toxin-Antitoxin System DarTG Catalyzes Reversible ADP-Ribosylation of DNA

The discovery and study of toxin-antitoxin (TA) systems helps us advance our understanding of the strategies prokaryotes employ to regulate cellular processes related to the general stress response, such as defense against phages, growth control, biofilm formation, persistence, and programmed cell death. Here we identify and characterize a TA system found in various bacteria, including the global pathogen Mycobacterium tuberculosis. The toxin of the system (DarT) is a domain of unknown function (DUF) 4433, and the antitoxin (DarG) a macrodomain protein. We demonstrate that DarT is an enzyme that specifically modifies thymidines on single-stranded DNA in a sequence-specific manner by a nucleotide-type modification called ADP-ribosylation. We also show that this modification can be removed by DarG. Our results provide an example of reversible DNA ADP-ribosylation, and we anticipate potential therapeutic benefits by targeting this enzyme-enzyme TA system in bacterial pathogens such as M. tuberculosis.

Scabin, a Novel DNA-acting ADP-ribosyltransferase from Streptomyces scabies

A bioinformatics strategy was used to identify Scabin, a novel DNA-targeting enzyme from the plant pathogen 87.22 strain of Streptomyces scabies Scabin shares nearly 40% sequence identity with the Pierisin family of mono-ADP-ribosyltransferase toxins. Scabin was purified to homogeneity as a 22-kDa single-domain enzyme and was shown to possess high NAD(+)-glycohydrolase (Km (NAD) = 68 ± 3 μm; kcat = 94 ± 2 min(-1)) activity with an RSQXE motif; it was also shown to target deoxyguanosine and showed sigmoidal enzyme kinetics (K0.5(deoxyguanosine) = 302 ± 12 μm; kcat = 14 min(-1)). Mass spectrometry analysis revealed that Scabin labels the exocyclic amino group on guanine bases in either single-stranded or double-stranded DNA. Several small molecule inhibitors were identified, and the most potent compounds were found to inhibit the enzyme activity with Ki values ranging from 3 to 24 μm PJ34, a well known inhibitor of poly-ADP-ribosyltransferases, was shown to be the most potent inhibitor of Scabin. Scabin was crystallized, representing the first structure of a DNA-targeting mono-ADP-ribosyltransferase enzyme; the structures of the apo-form (1.45 Å) and with two inhibitors (P6-E, 1.4 Å; PJ34, 1.6 Å) were solved. These x-ray structures are also the first high resolution structures of the Pierisin subgroup of the mono-ADP-ribosyltransferase toxin family. A model of Scabin with its DNA substrate is also proposed.