The natural history of ADP-ribosyltransferases and the ADP-ribosylation system

Legionella pneumophila-mediated host posttranslational modifications

Legionella pneumophila is a Gram-negative bacterium ubiquitously present in freshwater environments and causes a serious type of pneumonia called Legionnaires' disease. During infections, L. pneumophila releases over 300 effector proteins into host cells through an Icm/Dot type IV secretion system to manipulate the host defense system for survival within the host. Notably, certain effector proteins mediate posttranslational modifications (PTMs), serving as useful approaches exploited by L. pneumophila to modify host proteins. Some effectors catalyze the addition of host protein PTMs, while others mediate the removal of PTMs from host proteins. In this review, we summarize L. pneumophila effector-mediated PTMs of host proteins, including phosphorylation, ubiquitination, glycosylation, AMPylation, phosphocholination, methylation, and ADP-ribosylation, as well as dephosphorylation, deubiquitination, deAMPylation, deADP-ribosylation, dephosphocholination, and delipidation. We describe their molecular mechanisms and biological functions in the regulation of bacterial growth and Legionella-containing vacuole biosynthesis and in the disruption of host immune and defense machinery.

Structure-Based High-Throughput Virtual Screening and Molecular Dynamics Simulation for the Discovery of Novel SARS-CoV-2 NSP3 Mac1 Domain Inhibitors

Since the emergence of SARS-CoV-2, many genetic variations within its genome have been identified, but only a few mutations have been found in nonstructural proteins (NSPs). Among this class of viral proteins, NSP3 is a multidomain protein with 16 different domains, and its largest domain is known as the macrodomain or Mac1 domain. In this study, we present a virtual screening campaign in which we computationally evaluated the NCI anticancer library against the NSP3 Mac1 domain, using Molegro Virtual Docker. The top hits with the best MolDock and Re-Rank scores were selected. The physicochemical analysis and drug-like potential of the top hits were analyzed using the SwissADME data server. The binding stability and affinity of the top NSC compounds against the NSP3 Mac1 domain were analyzed using molecular dynamics (MD) simulation, using Desmond software, and their interaction energies were analyzed using the MM/GBSA method. In particular, by applying subsequent computational filters, we identified 10 compounds as possible NSP3 Mac1 domain inhibitors. Among them, after the assessment of binding energies (ΔGbind) on the whole MD trajectories, we identified the four most interesting compounds that acted as strong binders of the NSP3 Mac1 domain (NSC-358078, NSC-287067, NSC-123472, and NSC-142843), and, remarkably, it could be further characterized for developing innovative antivirals against SARS-CoV-2.

Metabolically distinct roles of NAD synthetase and NAD kinase define the essentiality of NAD and NADP in Mycobacterium tuberculosis

Nicotinamide adenine dinucleotide (NAD) and its phosphorylated derivative (NADP) are essential cofactors that participate in hundreds of biochemical reactions and have emerged as therapeutic targets in cancer, metabolic disorders, neurodegenerative diseases, and infections, including tuberculosis. The biological basis for the essentiality of NAD(P) in most settings, however, remains experimentally unexplained. Here, we report that inactivation of the terminal enzyme of NAD synthesis, NAD synthetase (NadE), elicits markedly different metabolic and microbiologic effects than those of the terminal enzyme of NADP biosynthesis, NAD kinase (PpnK), in Mycobacterium tuberculosis (Mtb). Inactivation of NadE led to parallel reductions of both NAD and NADP pools and Mtb viability, while inactivation of PpnK selectively depleted NADP pools but only arrested growth. Inactivation of each enzyme was accompanied by metabolic changes that were specific for the affected enzyme and associated microbiological phenotype. Bacteriostatic levels of NAD depletion caused a compensatory remodeling of NAD-dependent metabolic pathways in the absence of an impact on NADH/NAD ratios, while bactericidal levels of NAD depletion resulted in a disruption of NADH/NAD ratios and inhibition of oxygen respiration. These findings reveal a previously unrecognized physiologic specificity associated with the essentiality of two evolutionarily ubiquitous cofactors. IMPORTANCE The current course for cure of Mycobacterium tuberculosis (Mtb)-the etiologic agent of tuberculosis (TB)-infections is lengthy and requires multiple antibiotics. The development of shorter, simpler treatment regimens is, therefore, critical to the goal of eradicating TB. NadE, an enzyme required for the synthesis of the ubiquitous cofactor NAD, is essential for survival of Mtb and regarded as a promising drug target. However, the basis of this essentiality was not clear due to its role in the synthesis of both NAD and NADP. Here, we resolve this ambiguity through a combination of gene silencing and metabolomics. We specifically show that NADP deficiency is bacteriostatic, while NAD deficiency is bactericidal due to its role in Mtb's respiratory capacity. These results argue for a prioritization of NAD biosynthesis inhibitors in anti-TB drug development.

3D-QSAR, molecular docking, and molecular dynamics analysis of dihydrodiazaindolone derivatives as PARP-1 inhibitors

CONTEXT

PARP-1 plays an important role in DNA repair and apoptosis, and PARP-1 inhibitors have shown to be effective in the treatment of several malignancies. To evaluate the function of new PARP-1 inhibitors as anticancer adjuvant medicines, 3D-QSAR, molecular docking, and molecular dynamics (MD) simulations of a sequence of dihydrodiazepinoindolone derivatives PARP-1 inhibitors were undertaken in this study.

METHODS

In this paper, 43 PARP-1 inhibitors were studied in a three-dimensional quantitative structure-activity relationship (3D-QSAR) using comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA). CoMFA with q2 of 0.675 and r2 of 0.981 was achieved, as was CoMSIA with q2 of 0.755 and r2 of 0.992. The changed areas of these compounds are shown by steric, electrostatic, hydrophobic, and hydrogen-bonded acceptor field contour maps. Subsequently, molecular docking, and molecular dynamics simulations further confirmed that key residues Gly863 and Ser904 of PARP-1 are vital residues for protein interactions and their binding affinity. The effects of 3D-QSAR, molecular docking and molecular dynamics simulations supply a new route for the search of new PARP-1 inhibitors. Finally, we designed eight new compounds with exact activity and ADME/T properties.

New biochemistry in the Rhodanese-phosphatase superfamily: emerging roles in diverse metabolic processes, nucleic acid modifications, and biological conflicts

The protein-tyrosine/dual-specificity phosphatases and rhodanese domains constitute a sprawling superfamily of Rossmannoid domains that use a conserved active site with a cysteine to catalyze a range of phosphate-transfer, thiotransfer, selenotransfer and redox activities. While these enzymes have been extensively studied in the context of protein/lipid head group dephosphorylation and various thiotransfer reactions, their overall diversity and catalytic potential remain poorly understood. Using comparative genomics and sequence/structure analysis, we comprehensively investigate and develop a natural classification for this superfamily. As a result, we identified several novel clades, both those which retain the catalytic cysteine and those where a distinct active site has emerged in the same location (e.g. diphthine synthase-like methylases and RNA 2' OH ribosyl phosphate transferases). We also present evidence that the superfamily has a wider range of catalytic capabilities than previously known, including a set of parallel activities operating on various sugar/sugar alcohol groups in the context of NAD+-derivatives and RNA termini, and potential phosphate transfer activities involving sugars and nucleotides. We show that such activities are particularly expanded in the RapZ-C-DUF488-DUF4326 clade, defined here for the first time. Some enzymes from this clade are predicted to catalyze novel DNA-end processing activities as part of nucleic-acid-modifying systems that are likely to function in biological conflicts between viruses and their hosts.

A Simple Method to Study ADP-Ribosylation Reversal: From Function to Drug Discovery

ADP-ribosylation is an ancient modification of proteins, nucleic acids, and other biomolecules found in all kingdoms of life as well as in certain viruses. The regulation of fundamental (patho)physiological processes by ADP-ribosylation, including the cellular stress response, inflammation, and immune response to bacterial and viral pathogens, has created a strong interest into the study of modification establishment and removal to explore novel therapeutic approaches. Beyond ADP-ribosylation in humans, direct targeting of factors that alter host ADP-ribosylation signaling (e.g., viral macrodomains) or utilize ADP-ribosylation to manipulate host cell behavior (e.g., bacterial toxins) were shown to reduce virulence and disease severity. However, the realization of these therapeutic potentials is thus far hampered by the unavailability of simple, high-throughput methods to study the modification "writers" and "erasers" and screen for novel inhibitors.Here, we describe a scalable method for the measurement of (ADP-ribosyl)hydrolase activity. The assay relies on the conversion of ADP-ribose released from a modified substrate by the (ADP-ribosyl)hydrolase under investigation into AMP by the phosphodiesterase NudT5 into bioluminescence via a commercially available detection assay. Moreover, this method can be utilized to study the role of nudix- or ENPP-type phosphodiesterases in ADP-ribosylation processing and may also be adapted to investigate the activity of (ADP-ribosyl)transferases. Overall, this method is applicable for both basic biochemical characterization and screening of large drug libraries; hence, it is highly adaptable to diverse project needs.

PARP10 Mediates Mono-ADP-Ribosylation of Aurora-A Regulating G2/M Transition of the Cell Cycle

Intracellular mono-ADP-ribosyltransferases (mono-ARTs) catalyze the covalent attachment of a single ADP-ribose molecule to protein substrates, thus regulating their functions. PARP10 is a soluble mono-ART involved in the modulation of intracellular signaling, metabolism and apoptosis. PARP10 also participates in the regulation of the G1- and S-phase of the cell cycle. However, the role of this enzyme in G2/M progression is not defined. In this study, we found that genetic ablation, protein depletion and pharmacological inhibition of PARP10 cause a delay in the G2/M transition of the cell cycle. Moreover, we found that the mitotic kinase Aurora-A, a previously identified PARP10 substrate, is actively mono-ADP-ribosylated (MARylated) during G2/M transition in a PARP10-dependent manner. Notably, we showed that PARP10-mediated MARylation of Aurora-A enhances the activity of the kinase in vitro. Consistent with an impairment in the endogenous activity of Aurora-A, cells lacking PARP10 show a decreased localization of the kinase on the centrosomes and mitotic spindle during G2/M progression. Taken together, our data provide the first evidence of a direct role played by PARP10 in the progression of G2 and mitosis, an event that is strictly correlated to the endogenous MARylation of Aurora-A, thus proposing a novel mechanism for the modulation of Aurora-A kinase activity.

Cyclic ADP ribose isomers: Production, chemical structures, and immune signaling

Cyclic adenosine diphosphate (ADP)-ribose (cADPR) isomers are signaling molecules produced by bacterial and plant Toll/interleukin-1 receptor (TIR) domains via nicotinamide adenine dinucleotide (oxidized form) (NAD+) hydrolysis. We show that v-cADPR (2'cADPR) and v2-cADPR (3'cADPR) isomers are cyclized by O-glycosidic bond formation between the ribose moieties in ADPR. Structures of 2'cADPR-producing TIR domains reveal conformational changes that lead to an active assembly that resembles those of Toll-like receptor adaptor TIR domains. Mutagenesis reveals a conserved tryptophan that is essential for cyclization. We show that 3'cADPR is an activator of ThsA effector proteins from the bacterial antiphage defense system termed Thoeris and a suppressor of plant immunity when produced by the effector HopAM1. Collectively, our results reveal the molecular basis of cADPR isomer production and establish 3'cADPR in bacteria as an antiviral and plant immunity-suppressing signaling molecule.

Mechanism of threonine ADP-ribosylation of F-actin by a Tc toxin

Tc toxins deliver toxic enzymes into host cells by a unique injection mechanism. One of these enzymes is the actin ADP-ribosyltransferase TccC3, whose activity leads to the clustering of the cellular cytoskeleton and ultimately cell death. Here, we show in atomic detail how TccC3 modifies actin. We find that the ADP-ribosyltransferase does not bind to G-actin but interacts with two consecutive actin subunits of F-actin. The binding of TccC3 to F-actin occurs via an induced-fit mechanism that facilitates access of NAD⁺ to the nucleotide binding pocket. The following nucleophilic substitution reaction results in the transfer of ADP-ribose to threonine-148 of F-actin. We demonstrate that this site-specific modification of F-actin prevents its interaction with depolymerization factors, such as cofilin, which impairs actin network turnover and leads to steady actin polymerization. Our findings reveal in atomic detail a mechanism of action of a bacterial toxin through specific targeting and modification of F-actin.

Entomopathogenic bacteria used for pest control secrete potent Tc toxins. Here, the authors combine biochemistry, solution and solid-state NMR spectroscopy and cryo-EM to show in atomic detail how the toxin disrupts the host cell cytoskeleton and kills the target cell.

Research Progress on Mono-ADP-Ribosyltransferases in Human Cell Biology

ADP-ribosylation is a well-established post-translational modification that is inherently connected to diverse processes, including DNA repair, transcription, and cell signaling. The crucial roles of mono-ADP-ribosyltransferases (mono-ARTs) in biological processes have been identified in recent years by the comprehensive use of genetic engineering, chemical genetics, and proteomics. This review provides an update on current methodological advances in the study of these modifiers. Furthermore, the review provides details on the function of mono ADP-ribosylation. Several mono-ARTs have been implicated in the development of cancer, and this review discusses the role and therapeutic potential of some mono-ARTs in cancer.

Genomic diversity across the Rickettsia and 'Candidatus Megaira' genera and proposal of genus status for the Torix group

Members of the bacterial genus Rickettsia were originally identified as causative agents of vector-borne diseases in mammals. However, many Rickettsia species are arthropod symbionts and close relatives of ‘Candidatus Megaira’, which are symbiotic associates of microeukaryotes. Here, we clarify the evolutionary relationships between these organisms by assembling 26 genomes of Rickettsia species from understudied groups, including the Torix group, and two genomes of ‘Ca. Megaira’ from various insects and microeukaryotes. Our analyses of the new genomes, in comparison with previously described ones, indicate that the accessory genome diversity and broad host range of Torix Rickettsia are comparable to those of all other Rickettsia combined. Therefore, the Torix clade may play unrecognized roles in invertebrate biology and physiology. We argue this clade should be given its own genus status, for which we propose the name ‘Candidatus Tisiphia’.

The bacterial genus Rickettsia includes vector-borne pathogens and arthropod symbionts that are close relatives of symbionts of microeukaryotes classified under the genus ‘Candidatus Megaira’. Here, Davison et al. clarify the evolutionary relationships between these organisms by assembling 28 genomes of understudied species, and propose that a distinct clade known as Torix Rickettsia should be considered a separate genus.

ADP-Ribosylation Post-Translational Modification: An Overview with a Focus on RNA Biology and New Pharmacological Perspectives

Cellular functions are regulated through the gene expression program by the transcription of new messenger RNAs (mRNAs), alternative RNA splicing, and protein synthesis. To this end, the post-translational modifications (PTMs) of proteins add another layer of complexity, creating a continuously fine-tuned regulatory network. ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules, regulating a multitude of key functional processes as diverse as DNA damage repair (DDR), transcriptional regulation, intracellular transport, immune and stress responses, and cell survival. Additionally, due to the emerging role of ADP-ribosylation in pathological processes, ADP-ribosyltransferases (ARTs), the enzymes involved in ADPr, are attracting growing interest as new drug targets. In this review, an overview of human ARTs and their related biological functions is provided, mainly focusing on the regulation of ADP-ribosyltransferase Diphtheria toxin-like enzymes (ARTD)-dependent RNA functions. Finally, in order to unravel novel gene functional relationships, we propose the analysis of an inventory of human gene clusters, including ARTDs, which share conserved sequences at 3′ untranslated regions (UTRs).

Legionella pneumophila modulates host energy metabolism by ADP-ribosylation of ADP/ATP translocases

The intracellular pathogen Legionella pneumophila delivers more than 330 effectors into host cells by its Dot/Icm secretion system. Those effectors direct the biogenesis of the Legionella-containing vacuole (LCV) that permits its intracellular survival and replication. It has long been documented that the LCV is associated with mitochondria and a number of Dot/Icm effectors have been shown to target to this organelle. Yet, the biochemical function and host cell target of most of these effectors remain unknown. Here, we found that the Dot/Icm substrate Ceg3 (Lpg0080) is a mono-ADP-ribosyltransferase that localizes to the mitochondria in host cells where it attacks ADP/ATP translocases by ADP-ribosylation, and blunts their ADP/ATP exchange activity. The modification occurs on the second arginine residue in the -RRRMMM- element, which is conserved among all known ADP/ATP carriers from different organisms. Our results reveal modulation of host energy metabolism as a virulence mechanism for L. pneumophila.

Biological Properties of Vitamins of the B-Complex, Part 1: Vitamins B1, B2, B3, and B5

This review summarizes the current knowledge on essential vitamins B₁, B₂, B₃, and B₅. These B-complex vitamins must be taken from diet, with the exception of vitamin B₃, that can also be synthetized from amino acid tryptophan. All of these vitamins are water soluble, which determines their main properties, namely: they are partly lost when food is washed or boiled since they migrate to the water; the requirement of membrane transporters for their permeation into the cells; and their safety since any excess is rapidly eliminated via the kidney. The therapeutic use of B-complex vitamins is mostly limited to hypovitaminoses or similar conditions, but, as they are generally very safe, they have also been examined in other pathological conditions. Nicotinic acid, a form of vitamin B₃, is the only exception because it is a known hypolipidemic agent in gram doses. The article also sums up: (i) the current methods for detection of the vitamins of the B-complex in biological fluids; (ii) the food and other sources of these vitamins including the effect of common processing and storage methods on their content; and (iii) their physiological function.

PKD-dependent PARP12-catalyzed mono-ADP-ribosylation of Golgin-97 is required for E-cadherin transport from Golgi to plasma membrane

Adenosine diphosphate (ADP)-ribosylation is a posttranslational modification involved in key regulatory events catalyzed by ADP-ribosyltransferases (ARTs). Substrate identification and localization of the mono-ADP-ribosyltransferase PARP12 at the trans-Golgi network (TGN) hinted at the involvement of ARTs in intracellular traffic. We find that Golgin-97, a TGN protein required for the formation and transport of a specific class of basolateral cargoes (e.g., E-cadherin and vesicular stomatitis virus G protein [VSVG]), is a PARP12 substrate. PARP12 targets an acidic cluster in the Golgin-97 coiled-coil domain essential for function. Its mutation or PARP12 depletion, delays E-cadherin and VSVG export and leads to a defect in carrier fission, hence in transport, with consequent accumulation of cargoes in a trans-Golgi/Rab11-positive intermediate compartment. In contrast, PARP12 does not control the Golgin-245-dependent traffic of cargoes such as tumor necrosis factor alpha (TNFα). Thus, the transport of different basolateral proteins to the plasma membrane is differentially regulated by Golgin-97 mono-ADP-ribosylation by PARP12. This identifies a selective regulatory mechanism acting on the transport of Golgin-97- vs. Golgin-245-dependent cargoes. Of note, PARP12 enzymatic activity, and consequently Golgin-97 mono-ADP-ribosylation, depends on the activation of protein kinase D (PKD) at the TGN during traffic. PARP12 is directly phosphorylated by PKD, and this is essential to stimulate PARP12 catalytic activity. PARP12 is therefore a component of the PKD-driven regulatory cascade that selectively controls a major branch of the basolateral transport pathway. We propose that through this mechanism, PARP12 contributes to the maintenance of E-cadherin-mediated cell polarity and cell-cell junctions.

Phenotypic and Genomic Comparison of Klebsiella pneumoniae Lytic Phages: vB_KpnM-VAC66 and vB_KpnM-VAC13

Klebsiella pneumoniae is a human pathogen that worsens the prognosis of many immunocompromised patients. Here, we annotated and compared the genomes of two lytic phages that infect clinical strains of K. pneumoniae (vB_KpnM-VAC13 and vB_KpnM-VAC66) and phenotypically characterized vB_KpnM-VAC66 (time of adsorption of 12 min, burst size of 31.49 ± 0.61 PFU/infected cell, and a host range of 20.8% of the tested strains). Transmission electronic microscopy showed that vB_KpnM-VAC66 belongs to the Myoviridae family. The genomic analysis of the phage vB_KpnM-VAC66 revealed that its genome encoded 289 proteins. When compared to the genome of vB_KpnM-VAC13, they showed a nucleotide similarity of 97.56%, with a 93% of query cover, and the phylogenetic study performed with other Tevenvirinae phages showed a close common ancestor. However, there were 21 coding sequences which differed. Interestingly, the main differences were that vB_KpnM-VAC66 encoded 10 more homing endonucleases than vB_KpnM-VAC13, and that the nucleotidic and amino-acid sequences of the L-shaped tail fiber protein were highly dissimilar, leading to different three-dimensional protein predictions. Both phages differed significantly in their host range. These viruses may be useful in the development of alternative therapies to antibiotics or as a co-therapy increasing its antimicrobial potential, especially when addressing multidrug resistant (MDR) pathogens.

Community-Acquired Respiratory Distress Syndrome Toxin: Unique Exotoxin for M. pneumoniae

Mycoplasma pneumoniae infection often causes respiratory diseases in humans, particularly in children and adults with atypical pneumonia and community-acquired pneumonia (CAP), and is often exacerbated by co-infection with other lung diseases, such as asthma, bronchitis, and chronic obstructive pulmonary disorder. Community-acquired respiratory distress syndrome toxin (CARDS TX) is the only exotoxin produced by M. pneumoniae and has been extensively studied for its ADP-ribosyltransferase (ADPRT) activity and cellular vacuolization properties. Additionally, CARDS TX induces inflammatory responses, resulting in cell swelling, nuclear lysis, mucus proliferation, and cell vacuolization. CARDS TX enters host cells by binding to the host receptor and is then reverse transported to the endoplasmic reticulum to exert its pathogenic effects. In this review, we focus on the structural characteristics, functional activity, distribution and receptors, mechanism of cell entry, and inflammatory response of CARDS TX was examined. Overall, the findings of this review provide a theoretical basis for further investigation of the mechanism of M. pneumoniae infection and the development of clinical diagnosis and vaccines.

Computational Structural Genomics Unravels Common Folds and Novel Families in the Secretome of Fungal Phytopathogen Magnaporthe oryzae

Structural biology has the potential to illuminate the evolution of pathogen effectors and their commonalities that cannot be readily detected at the primary sequence level. Recent breakthroughs in protein structure modeling have demonstrated the feasibility to predict the protein folds without depending on homologous templates. These advances enabled a genome-wide computational structural biology approach to help understand proteins based on their predicted folds. In this study, we employed structure prediction methods on the secretome of the destructive fungal pathogen Magnaporthe oryzae. Out of 1,854 secreted proteins, we predicted the folds of 1,295 proteins (70%). We showed that template-free modeling by TrRosetta captured 514 folds missed by homology modeling, including many known effectors and virulence factors, and that TrRosetta generally produced higher quality models for secreted proteins. Along with sensitive homology search, we employed structure-based clustering, defining not only homologous groups with divergent members but also sequence-unrelated structurally analogous groups. We demonstrate that this approach can reveal new putative members of structurally similar MAX effectors and novel analogous effector families present in M. oryzae and possibly in other phytopathogens. We also investigated the evolution of expanded putative ADP-ribose transferases with predicted structures. We suggest that the loss of catalytic activities of the enzymes might have led them to new evolutionary trajectories to be specialized as protein binders. Collectively, we propose that computational structural genomics approaches can be an integral part of studying effector biology and provide valuable resources that were inaccessible before the advent of machine learning-based structure prediction.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Comparative Phylogenomic Analysis Reveals Evolutionary Genomic Changes and Novel Toxin Families in Endophytic Liberibacter Pathogens

Liberibacter pathogens are the causative agents of several severe crop diseases worldwide, including citrus Huanglongbing and potato zebra chip. These bacteria are endophytic and nonculturable, which makes experimental approaches challenging and highlights the need for bioinformatic analysis in advancing our understanding about Liberibacter pathogenesis. Here, we performed an in-depth comparative phylogenomic analysis of the Liberibacter pathogens and their free-living, nonpathogenic, ancestral species, aiming to identify major genomic changes and determinants associated with their evolutionary transitions in living habitats and pathogenicity. Using gene neighborhood analysis and phylogenetic classification, we systematically uncovered, annotated, and classified all prophage loci into four types, including one previously unrecognized group. We showed that these prophages originated through independent gene transfers at different evolutionary stages of Liberibacter and only the SC-type prophage was associated with the emergence of the pathogens. Using ortholog clustering, we vigorously identified two additional sets of genomic genes, which were either lost or gained in the ancestor of the pathogens. Consistent with the habitat change, the lost genes were enriched for biosynthesis of cellular building blocks. Importantly, among the gained genes, we uncovered several previously unrecognized toxins, including new toxins homologous to the EspG/VirA effectors, a YdjM phospholipase toxin, and a secreted endonuclease/exonuclease/phosphatase (EEP) protein. Our results substantially extend the knowledge of the evolutionary events and potential determinants leading to the emergence of endophytic, pathogenic Liberibacter species, which will facilitate the design of functional experiments and the development of new methods for detection and blockage of these pathogens. IMPORTANCELiberibacter pathogens are associated with several severe crop diseases, including citrus Huanglongbing, the most destructive disease to the citrus industry. Currently, no effective cure or treatments are available, and no resistant citrus variety has been found. The fact that these obligate endophytic pathogens are not culturable has made it extremely challenging to experimentally uncover the genes/proteins important to Liberibacter pathogenesis. Further, earlier bioinformatics studies failed to identify key genomic determinants, such as toxins and effector proteins, that underlie the pathogenicity of the bacteria. In this study, an in-depth comparative genomic analysis of Liberibacter pathogens along with their ancestral nonpathogenic species identified the prophage loci and several novel toxins that are evolutionarily associated with the emergence of the pathogens. These results shed new light on the disease mechanism of Liberibacter pathogens and will facilitate the development of new detection and blockage methods targeting the toxins.

TARG1 protects against toxic DNA ADP-ribosylation

ADP-ribosylation is a modification that targets a variety of macromolecules and regulates a diverse array of important cellular processes. ADP-ribosylation is catalysed by ADP-ribosyltransferases and reversed by ADP-ribosylhydrolases. Recently, an ADP-ribosyltransferase toxin termed 'DarT' from bacteria, which is distantly related to human PARPs, was shown to modify thymidine in single-stranded DNA in a sequence specific manner. The antitoxin of DarT is the macrodomain containing ADP-ribosylhydrolase DarG, which shares striking structural homology with the human ADP-ribosylhydrolase TARG1. Here, we show that TARG1, like DarG, can reverse thymidine-linked DNA ADP-ribosylation. We find that TARG1-deficient human cells are extremely sensitive to DNA ADP-ribosylation. Furthermore, we also demonstrate the first detection of reversible ADP-ribosylation on genomic DNA in vivo from human cells. Collectively, our results elucidate the impact of DNA ADP-ribosylation in human cells and provides a molecular toolkit for future studies into this largely unknown facet of ADP-ribosylation.

Genomic epidemiology of rifampicin ADP-ribosyltransferase (Arr) in the Bacteria domain

Arr is an ADP-ribosyltransferase enzyme primarily reported in association with rifamycin resistance, which has been used to treat tuberculosis in addition to Gram-positive infections and, recently, pan-resistant Gram-negative bacteria. The arr gene was initially identified on the Mycolicibacterium smegmatis chromosome and later on Proteobacteria plasmids. This scenario raised concerns on the distribution and spread of arr, considering the Bacteria domain. Based on 198,082 bacterial genomes/metagenomes, we performed in silico analysis, including phylogenetic reconstruction of Arr in different genomic contexts. Besides, new arr alleles were evaluated by in vitro analysis to assess their association with rifampin resistance phenotype. The arr gene was prevalent in thousands of chromosomes and in hundreds of plasmids from environmental and clinical bacteria, mainly from the phyla Actinobacteria, Proteobacteria, Firmicutes, and Bacteroidetes. Furthermore, this gene was identified in other and new genomic contexts. Interestingly, Arr sequences associated with rifampin resistance were distributed across all phylogeny, indicating that, despite the diversity, their association with rifampin resistance phenotype were maintained. In fact, we found that the key residues were highly conserved. In addition, other analyzes have raised evidence of another Arr function, which is related to guanidine metabolism. Finally, this scenario as a whole also suggested the Actinobacteria phylum as a potential ancestral source of arr within the Bacteria domain.

Gut microbiome ADP-ribosyltransferases are widespread phage-encoded fitness factors

Bacterial ADP-ribosyltransferases (ADPRTs) have been described as toxins involved in pathogenesis through the modification of host proteins. Here, we report that ADPRTs are not pathogen restricted but widely prevalent in the human gut microbiome and often associated with phage elements. We validated their biochemical activity in a large clinical isolate collection and further examined Bxa, a highly abundant ADPRT in Bacteroides. Bxa is expressed, secreted, and enzymatically active in Bacteroides and can ADP-ribosylate non-muscle myosin II proteins. Addition of Bxa to epithelial cells remodeled the actin cytoskeleton and induced secretion of inosine. Bxa-encoding B. stercoris can use inosine as a carbon source and colonizes the gut to significantly greater numbers than a bxa-deleted strain in germ-free and altered Schaedler flora (ASF) mice. Colonization correlated with increased inosine concentrations in the feces and tissues. Altogether, our results show that ADPRTs are abundant in the microbiome and act as bacterial fitness factors.

Molecular basis for DarT ADP-ribosylation of a DNA base

ADP-ribosyltransferases use NAD+ to catalyse substrate ADP-ribosylation1, and thereby regulate cellular pathways or contribute to toxin-mediated pathogenicity of bacteria2-4. Reversible ADP-ribosylation has traditionally been considered a protein-specific modification5, but recent in vitro studies have suggested nucleic acids as targets6-9. Here we present evidence that specific, reversible ADP-ribosylation of DNA on thymidine bases occurs in cellulo through the DarT-DarG toxin-antitoxin system, which is found in a variety of bacteria (including global pathogens such as Mycobacterium tuberculosis, enteropathogenic Escherichia coli and Pseudomonas aeruginosa)10. We report the structure of DarT, which identifies this protein as a diverged member of the PARP family. We provide a set of high-resolution structures of this enzyme in ligand-free and pre- and post-reaction states, which reveals a specialized mechanism of catalysis that includes a key active-site arginine that extends the canonical ADP-ribosyltransferase toolkit. Comparison with PARP-HPF1, a well-established DNA repair protein ADP-ribosylation complex, offers insights into how the DarT class of ADP-ribosyltransferases evolved into specific DNA-modifying enzymes. Together, our structural and mechanistic data provide details of this PARP family member and contribute to a fundamental understanding of the ADP-ribosylation of nucleic acids. We also show that thymine-linked ADP-ribose DNA adducts reversed by DarG antitoxin (functioning as a noncanonical DNA repair factor) are used not only for targeted DNA damage to induce toxicity, but also as a signalling strategy for cellular processes. Using M. tuberculosis as an exemplar, we show that DarT-DarG regulates growth by ADP-ribosylation of DNA at the origin of chromosome replication.

ADP-ribosyltransferase PARP11 suppresses Zika virus in synergy with PARP12

BACKGROUND

Zika virus (ZIKV) infection and ZIKV epidemic have been continuously spreading silently throughout the world and its associated microcephaly and other serious congenital neurological complications poses a significant global threat to public health. Type I interferon response to ZIKV infection in host cells suppresses viral replication by inducing the expression of interferon-stimulated genes (ISGs).

METHODS

The study aims to demonstrate the anti-ZIKV mechanism of PARP11. PARP11 knock out and overexpressing A549 cell lines were constructed to evaluate the anti-ZIKV function of PARP11. PARP11-/-, PARP12-/- and PARP11-/-PARP12-/- HEK293T cell lines were constructed to explain the synergistic effect of PARP11 and PARP12 on NS1 and NS3 protein degradation. Western blotting, immunofluorescence and immunoprecipitation assay were performed to illustrate the interaction between PARP11 and PARP12.

RESULTS

Both mRNA and protein levels of PARP11 were induced in WT but not IFNAR1-/- cells in response to IFNα or IFNβ stimulation and ZIKV infection. ZIKV replication was suppressed in cells expressed PARP11 but was enhanced in PARP11-/- cells. PARP11 suppressed ZIKV independently on itself PARP enzyme activity. PARP11 interacted with PARP12 and promoted PARP12-mediated ZIKV NS1 and NS3 protein degradation.

CONCLUSION

We identified ADP-ribosyltransferase PARP11 as an anti-ZIKV ISG and found that it cooperated with PARP12 to enhance ZIKV NS1 and NS3 protein degradation. Our findings have broadened the understanding of the anti-viral function of ADP-ribosyltransferase family members, and provided potential therapeutic targets against viral ZIKV infection.

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 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.

ADP-ribosylation in evasion, promotion and exacerbation of immune responses

ADP-ribosylation is the addition of one or more (up to some hundreds) ADP-ribose moieties to acceptor proteins. This evolutionary ancient post-translational modification (PTM) is involved in fundamental processes including DNA repair, inflammation, cell death, differentiation and proliferation, among others. ADP-ribosylation is catalysed by two major families of enzymes: the cholera toxin-like ADP-ribosyltransferases (ARTCs) and the diphtheria toxin-like ADP-ribosyltransferases (ARTDs, also known as PARPs). ARTCs sense and use extracellular NAD, which may represent a danger signal, whereas ARTDs are present in the cell nucleus and/or cytoplasm. ARTCs mono-ADP-ribosylate their substrates, whereas ARTDs, according to the specific family member, are able to mono- or poly-ADP-ribosylate target proteins or are devoid of enzymatic activity. Both mono- and poly-ADP-ribosylation are dynamic processes, as specific hydrolases are able to remove single or polymeric ADP moieties. This dynamic equilibrium between addition and degradation provides plasticity for fast adaptation, a feature being particularly relevant to immune cell functions. ADP-ribosylation regulates differentiation and functions of myeloid, T and B cells. It also regulates the expression of cytokines and chemokines, production of antibodies, isotype switch and the expression of several immune mediators. Alterations in these processes involve ADP-ribosylation in virtually any acute and chronic inflammatory/immune-mediated disease. Besides, pathogens developed mechanisms to contrast the action of ADP-ribosylating enzymes by using their own hydrolases and/or to exploit this PTM to sustain their virulence. In the present review, we summarize and discuss recent findings on the role of ADP-ribosylation in immunobiology, immune evasion/subversion by pathogens and immune-mediated diseases.

Enzymology of extracellular NAD metabolism

Extracellular NAD represents a key signaling molecule in different physiological and pathological conditions. It exerts such function both directly, through the activation of specific purinergic receptors, or indirectly, serving as substrate of ectoenzymes, such as CD73, nucleotide pyrophosphatase/phosphodiesterase 1, CD38 and its paralog CD157, and ecto ADP ribosyltransferases. By hydrolyzing NAD, these enzymes dictate extracellular NAD availability, thus regulating its direct signaling role. In addition, they can generate from NAD smaller signaling molecules, like the immunomodulator adenosine, or they can use NAD to ADP-ribosylate various extracellular proteins and membrane receptors, with significant impact on the control of immunity, inflammatory response, tumorigenesis, and other diseases. Besides, they release from NAD several pyridine metabolites that can be taken up by the cell for the intracellular regeneration of NAD itself. The extracellular environment also hosts nicotinamide phosphoribosyltransferase and nicotinic acid phosphoribosyltransferase, which inside the cell catalyze key reactions in NAD salvaging pathways. The extracellular forms of these enzymes behave as cytokines, with pro-inflammatory functions. This review summarizes the current knowledge on the extracellular NAD metabolome and describes the major biochemical properties of the enzymes involved in extracellular NAD metabolism, focusing on the contribution of their catalytic activities to the biological function. By uncovering the controversies and gaps in their characterization, further research directions are suggested, also to better exploit the great potential of these enzymes as therapeutic targets in various human diseases.

A novel predicted ADP-ribosyltransferase-like family conserved in eukaryotic evolution

The presence of many completely uncharacterized proteins, even in well-studied organisms such as humans, seriously hampers full understanding of the functioning of the living cells. ADP-ribosylation is a common post-translational modification of proteins; also nucleic acids and small molecules can be modified by the covalent attachment of ADP-ribose. This modification, important in cellular signalling and infection processes, is usually executed by enzymes from the large superfamily of ADP-ribosyltransferases (ARTs). Here, using bioinformatics approaches, we identify a novel putative ADP-ribosyltransferase family, conserved in eukaryotic evolution, with a divergent active site. The hallmark of these proteins is the ART domain nestled between flanking leucine-rich repeat (LRR) domains. LRRs are typically involved in innate immune surveillance. The novel family appears as putative novel ADP-ribosylation-related actors, most likely pseudoenzymes. Sequence divergence and lack of clearly detectable “classical” ART active site suggests the novel domains are pseudoARTs, yet atypical ART activity, or alternative enzymatic activity cannot be excluded. We propose that this family, including its human member LRRC9, may be involved in an ancient defense mechanism, with analogies to the innate immune system, and coupling pathogen detection to ADP-ribosyltransfer or other signalling mechanisms.

The complete genome and comparative analysis of the phage phiC120 infecting multidrug-resistant Escherichia coli and Salmonella strains

Phages infecting Salmonella and Escherichia coli are promising agents for therapeutics and biological control of these foodborne pathogens, in particular those strains with resistance to several antibiotics. In an effort to assess the potential of the phage phiC120, a virulent phage isolated from horse feces in Mexico, we characterized its morphology, host range and complete genome. Herein, we showed that phiC120 possesses strong lytic activity against several multidrug-resistant E. coli O157: H7 and Salmonella strains, and its morphology indicated that is a member of Myoviridae family. The phiC120 genome is double-stranded DNA and consists of 186,570 bp in length with a 37.6% G + C content. A total of 281 putative open reading frames (ORFs) and two tRNAs were found, where 150 ORFs encoded hypothetical proteins with unknown function. Comparative analysis showed that phiC120 shared high similarity at nucleotide and protein levels with coliphages RB69 and phiE142. Detailed phiC120 analysis revealed that ORF 94 encodes a putative depolymerase, meanwhile genes encoding factors associated with lysogeny, toxins, and antibiotic resistance were absent; however, ORF 95 encodes a putative protein with potential allergenic and pro-inflammatory properties, making needed further studies to guarantee the safety of phiC120 for human use. The characterization of phiC120 expands our knowledge about the biology of coliphages and provides novel insights supporting its potential for the development of phage-based applications to control unwanted bacteria.

Common Mechanism for Target Specificity of Protein- and DNA-Targeting ADP-Ribosyltransferases

Many bacterial pathogens utilize ADP-ribosyltransferases (ARTs) as virulence factors. The critical aspect of ARTs is their target specificity. Each individual ART modifies a specific residue of its substrates, which could be proteins, DNA, or antibiotics. However, the mechanism underlying this specificity is poorly understood. Here, we review the substrate recognition mechanism and target residue specificity based on the available complex structures of ARTs and their substrates. We show that there are common mechanisms of target residue specificity among protein- and DNA-targeting ARTs.

Jumbo Phages: A Comparative Genomic Overview of Core Functions and Adaptions for Biological Conflicts

Jumbo phages have attracted much attention by virtue of their extraordinary genome size and unusual aspects of biology. By performing a comparative genomics analysis of 224 jumbo phages, we suggest an objective inclusion criterion based on genome size distributions and present a synthetic overview of their manifold adaptations across major biological systems. By means of clustering and principal component analysis of the phyletic patterns of conserved genes, all known jumbo phages can be classified into three higher-order groups, which include both myoviral and siphoviral morphologies indicating multiple independent origins from smaller predecessors. Our study uncovers several under-appreciated or unreported aspects of the DNA replication, recombination, transcription and virion maturation systems. Leveraging sensitive sequence analysis methods, we identify novel protein-modifying enzymes that might help hijack the host-machinery. Focusing on host–virus conflicts, we detect strategies used to counter different wings of the bacterial immune system, such as cyclic nucleotide- and NAD⁺-dependent effector-activation, and prevention of superinfection during pseudolysogeny. We reconstruct the RNA-repair systems of jumbo phages that counter the consequences of RNA-targeting host effectors. These findings also suggest that several jumbo phage proteins provide a snapshot of the systems found in ancient replicons preceding the last universal ancestor of cellular life.

A Legionella effector ADP-ribosyltransferase inactivates glutamate dehydrogenase

ADP-ribosyltransferases (ARTs) are a widespread superfamily of enzymes frequently employed in pathogenic strategies of bacteria. Legionella pneumophila, the causative agent of a severe form of pneumonia known as Legionnaire's disease, has acquired over 330 translocated effectors that showcase remarkable biochemical and structural diversity. However, the ART effectors that influence L. pneumophila have not been well defined. Here, we took a bioinformatic approach to search the Legionella effector repertoire for additional divergent members of the ART superfamily and identified an ART domain in Legionella pneumophila gene0181, which we hereafter refer to as Legionella ADP-Ribosyltransferase 1 (Lart1) (Legionella ART 1). We show that L. pneumophila Lart1 targets a specific class of 120-kDa NAD+-dependent glutamate dehydrogenase (GDH) enzymes found in fungi and protists, including many natural hosts of Legionella. Lart1 targets a conserved arginine residue in the NAD+-binding pocket of GDH, thereby blocking oxidative deamination of glutamate. Therefore, Lart1 could be the first example of a Legionella effector which directly targets a host metabolic enzyme during infection.

Identification of Uncharacterized Components of Prokaryotic Immune Systems and Their Diverse Eukaryotic Reformulations

Nucleotide-activated effector deployment, prototyped by interferon-dependent immunity, is a common mechanistic theme shared by immune systems of several animals and prokaryotes. Prokaryotic versions include CRISPR-Cas with the CRISPR polymerase domain, their minimal variants, and systems with second messenger oligonucleotide or dinucleotide synthetase (SMODS). Cyclic or linear oligonucleotide signals in these systems help set a threshold for the activation of potentially deleterious downstream effectors in response to invader detection. We establish such a regulatory mechanism to be a more general principle of immune systems, which can also operate independently of such messengers. Using sensitive sequence analysis and comparative genomics, we identify 12 new prokaryotic immune systems, which we unify by this principle of threshold-dependent effector activation. These display regulatory mechanisms paralleling physiological signaling based on 3'-5' cyclic mononucleotides, NAD⁺-derived messengers, two- and one-component signaling that includes histidine kinase-based signaling, and proteolytic activation. Furthermore, these systems allowed the identification of multiple new sensory signal sensory components, such as a tetratricopeptide repeat (TPR) scaffold predicted to recognize NAD⁺-derived signals, unreported versions of the STING domain, prokaryotic YEATS domains, and a predicted nucleotide sensor related to receiver domains. We also identify previously unrecognized invader detection components and effector components, such as prokaryotic versions of the Wnt domain. Finally, we show that there have been multiple acquisitions of unidentified STING domains in eukaryotes, while the TPR scaffold was incorporated into the animal immunity/apoptosis signal-regulating kinase (ASK) signalosome.IMPORTANCE Both prokaryotic and eukaryotic immune systems face the dangers of premature activation of effectors and degradation of self-molecules in the absence of an invader. To mitigate this, they have evolved threshold-setting regulatory mechanisms for the triggering of effectors only upon the detection of a sufficiently strong invader signal. This work defines general templates for such regulation in effector-based immune systems. Using this, we identify several previously uncharacterized prokaryotic immune mechanisms that accomplish the regulation of downstream effector deployment by using nucleotide, NAD⁺-derived, two-component, and one-component signals paralleling physiological homeostasis. This study has also helped identify several previously unknown sensor and effector modules in these systems. Our findings also augment the growing evidence for the emergence of key animal immunity and chromatin regulatory components from prokaryotic progenitors.

Structural, Biophysical, and Biochemical Elucidation of the SARS-CoV-2 Nonstructural Protein 3 Macro Domain

The pandemic outbreak of a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has threatened the global public health and economy since late December 2019. SARS-CoV-2 encodes the conserved macro domain within nonstructural protein 3, which may reverse cellular ADP-ribosylation and potentially cut the signal of a viral infection in the cell. Herein, we report that the SARS-CoV-2 macro domain was examined as a poly-ADP-ribose (ADPR) binding module and possessed mono-ADPR cleavage enzyme activity. After confirming the ADPR binding ability via a biophysical approach, the X-ray crystal structure of the SARS-CoV-2 macro domain was determined and structurally compared with those of other viruses. This study provides structural, biophysical, and biochemical bases to further evaluate the role of the SARS-CoV-2 macro domain in the host response via ADP-ribose binding but also as a potential target for drug design against COVID-19.

Novel Immunoglobulin Domain Proteins Provide Insights into Evolution and Pathogenesis of SARS-CoV-2-Related Viruses

A novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was recently identified as the causative agent for the coronavirus disease 2019 (COVID-19) outbreak that has generated a global health crisis. We use a combination of genomic analysis and sensitive profile-based sequence and structure analysis to understand the potential pathogenesis determinants of this virus. As a result, we identify several fast-evolving genomic regions that might be at the interface of virus-host interactions, corresponding to the receptor binding domain of the Spike protein, the three tandem Macro fold domains in ORF1a, and the uncharacterized protein ORF8. Further, we show that ORF8 and several other proteins from alpha- and beta-CoVs belong to novel families of immunoglobulin (Ig) proteins. Among them, ORF8 is distinguished by being rapidly evolving, possessing a unique insert, and having a hypervariable position among SARS-CoV-2 genomes in its predicted ligand-binding groove. We also uncover numerous Ig domain proteins from several unrelated metazoan viruses, which are distinct in sequence and structure but share comparable architectures to those of the CoV Ig domain proteins. Hence, we propose that SARS-CoV-2 ORF8 and other previously unidentified CoV Ig domain proteins fall under the umbrella of a widespread strategy of deployment of Ig domain proteins in animal viruses as pathogenicity factors that modulate host immunity. The rapid evolution of the ORF8 Ig domain proteins points to a potential evolutionary arms race between viruses and hosts, likely arising from immune pressure, and suggests a role in transmission between distinct host species.IMPORTANCE The ongoing COVID-19 pandemic strongly emphasizes the need for a more complete understanding of the biology and pathogenesis of its causative agent SARS-CoV-2. Despite intense scrutiny, several proteins encoded by the genomes of SARS-CoV-2 and other SARS-like coronaviruses remain enigmatic. Moreover, the high infectivity and severity of SARS-CoV-2 in certain individuals make wet-lab studies currently challenging. In this study, we used a series of computational strategies to identify several fast-evolving regions of SARS-CoV-2 proteins which are potentially under host immune pressure. Most notably, the hitherto-uncharacterized protein encoded by ORF8 is one of them. Using sensitive sequence and structural analysis methods, we show that ORF8 and several other proteins from alpha- and beta-coronavirus comprise novel families of immunoglobulin domain proteins, which might function as potential immune modulators to delay or attenuate the host immune response against the viruses.

(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.

Highly regulated, diversifying NTP-dependent biological conflict systems with implications for the emergence of multicellularity

Social cellular aggregation or multicellular organization pose increased risk of transmission of infections through the system upon infection of a single cell. The generality of the evolutionary responses to this outside of Metazoa remains unclear. We report the discovery of several thematically unified, remarkable biological conflict systems preponderantly present in multicellular prokaryotes. These combine thresholding mechanisms utilizing NTPase chaperones (the MoxR-vWA couple), GTPases and proteolytic cascades with hypervariable effectors, which vary either by using a reverse transcriptase-dependent diversity-generating system or through a system of acquisition of diverse protein modules, typically in inactive form, from various cellular subsystems. Conciliant lines of evidence indicate their deployment against invasive entities, like viruses, to limit their spread in multicellular/social contexts via physical containment, dominant-negative interactions or apoptosis. These findings argue for both a similar operational 'grammar' and shared protein domains in the sensing and limiting of infections during the multiple emergences of multicellularity.

DNA ADP-Ribosylation Stalls Replication and Is Reversed by RecF-Mediated Homologous Recombination and Nucleotide Excision Repair

ADP-ribosylation of proteins is crucial for fundamental cellular processes. Despite increasing examples of DNA ADP-ribosylation, the impact of this modification on DNA metabolism and cell physiology is unknown. Here, we show that the DarTG toxin-antitoxin system from enteropathogenic Escherichia coli (EPEC) catalyzes reversible ADP-ribosylation of single-stranded DNA (ssDNA). The DarT toxin recognizes specific sequence motifs. EPEC DarG abrogates DarT toxicity by two distinct mechanisms: removal of DNA ADP-ribose (ADPr) groups and DarT sequestration. Furthermore, we investigate how cells recognize and deal with DNA ADP-ribosylation. We demonstrate that DNA ADPr stalls replication and is perceived as DNA damage. Removal of ADPr from DNA requires the sequential activity of two DNA repair pathways, with RecF-mediated homologous recombination likely to transfer ADP-ribosylation from single- to double-stranded DNA (dsDNA) and subsequent nucleotide excision repair eliminating the lesion. Our work demonstrates that these DNA repair pathways prevent the genotoxic effects of DNA ADP-ribosylation.

Different ways to transport ammonia in human and Mycobacterium tuberculosis NAD+ synthetases

NAD+ synthetase is an essential enzyme of de novo and recycling pathways of NAD+ biosynthesis in Mycobacterium tuberculosis but not in humans. This bifunctional enzyme couples the NAD+ synthetase and glutaminase activities through an ammonia tunnel but free ammonia is also a substrate. Here we show that the Homo sapiens NAD+ synthetase (hsNadE) lacks substrate specificity for glutamine over ammonia and displays a modest activation of the glutaminase domain compared to tbNadE. We report the crystal structures of hsNadE and NAD+ synthetase from M. tuberculosis (tbNadE) with synthetase intermediate analogues. Based on the observed exclusive arrangements of the domains and of the intra- or inter-subunit tunnels we propose a model for the inter-domain communication mechanism for the regulation of glutamine-dependent activity and NH3 transport. The structural and mechanistic comparison herein reported between hsNadE and tbNadE provides also a starting point for future efforts in the development of anti-TB drugs.

Poly(ADP-Ribosylation) in Age-Related Neurological Disease

A central and causative feature of age-related neurodegenerative disease is the deposition of misfolded proteins in the brain. To devise novel approaches to treatment, regulatory pathways that modulate these aggregation-prone proteins must be defined. One such pathway is post-translational modification by the addition of poly(ADP-ribose) (PAR), which promotes protein recruitment and localization in several cellular contexts. Mounting evidence implicates PAR in seeding the abnormal localization and accumulation of proteins that are causative of neurodegenerative disease. Inhibitors of PAR polymerase (PARP) activity have been developed as cancer therapeutics, raising the possibility that they could be used to treat neurodegenerative disease. We focus on pathways regulated by PAR in neurodegenerative disease, with emphasis on amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD).

Targeting ADP-ribosylation as an antimicrobial strategy

ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules controlling major biological processes as diverse as DNA damage repair, transcriptional regulation, intracellular transport, immune and stress responses, cell survival and proliferation. Furthermore, enzymatic reactions of ADPr are central in the pathogenesis of many human diseases, including infectious conditions. By providing a review of ADPr signalling in bacterial systems, we highlight the relevance of this chemical modification in the pathogenesis of human diseases depending on host-pathogen interactions. The post-antibiotic era has raised the need to find alternative approaches to antibiotic administration, as major pathogens becoming resistant to antibiotics. An in-depth understanding of ADPr reactions provides the rationale for designing novel antimicrobial strategies for treatment of infectious diseases. In addition, the understanding of mechanisms of ADPr by bacterial virulence factors offers important hints to improve our knowledge on cellular processes regulated by eukaryotic homologous enzymes, which are often involved in the pathogenesis of human diseases.

Antimicrobial Peptides, Polymorphic Toxins, and Self-Nonself Recognition Systems in Archaea: an Untapped Armory for Intermicrobial Conflicts

Diverse and highly variable systems involved in biological conflicts and self-versus-nonself discrimination are ubiquitous in bacteria but much less studied in archaea. We performed comprehensive comparative genomic analyses of the archaeal systems that share components with analogous bacterial systems and propose an approach to identify new systems that could be involved in these functions. We predict polymorphic toxin systems in 141 archaeal genomes and identify new, archaea-specific toxin and immunity protein families. These systems are widely represented in archaea and are predicted to play major roles in interactions between species and in intermicrobial conflicts. This work is expected to stimulate experimental research to advance the understanding of poorly characterized major aspects of archaeal biology.

Numerous, diverse, highly variable defense and offense genetic systems are encoded in most bacterial genomes and are involved in various forms of conflict among competing microbes or their eukaryotic hosts. Here we focus on the offense and self-versus-nonself discrimination systems encoded by archaeal genomes that so far have remained largely uncharacterized and unannotated. Specifically, we analyze archaeal genomic loci encoding polymorphic and related toxin systems and ribosomally synthesized antimicrobial peptides. Using sensitive methods for sequence comparison and the “guilt by association” approach, we identified such systems in 141 archaeal genomes. These toxins can be classified into four major groups based on the structure of the components involved in the toxin delivery. The toxin domains are often shared between and within each system. We revisit halocin families and substantially expand the halocin C8 family, which was identified in diverse archaeal genomes and also certain bacteria. Finally, we employ features of protein sequences and genomic locus organization characteristic of archaeocins and polymorphic toxins to identify candidates for analogous but not necessarily homologous systems among uncharacterized protein families. This work confidently predicts that more than 1,600 archaeal proteins, currently annotated as “hypothetical” in public databases, are components of conflict and self-versus-nonself discrimination systems.

Deciphering the Nucleotide and RNA Binding Selectivity of the Mayaro Virus Macro Domain

Mayaro virus (MAYV) is a member of Togaviridae family, which also includes Chikungunya virus as a notorious member. MAYV recently emerged in urban areas of the Americas, and this emergence emphasized the current paucity of knowledge about its replication cycle. The macro domain (MD) of MAYV belongs to the N-terminal region of its non-structural protein 3, part of the replication complex. Here, we report the first structural and dynamical characterization of a previously unexplored Alphavirus MD investigated through high-resolution NMR spectroscopy, along with data on its ligand selectivity and binding properties. The structural analysis of MAYV MD reveals a typical "macro" (ββαββαβαβα) fold for this polypeptide, while NMR-driven interaction studies provide in-depth insights into MAYV MD-ligand adducts. NMR data in concert with thermodynamics and biochemical studies provide convincing experimental evidence for preferential binding of adenosine diphosphate ribose (ADP-r) and adenine-rich RNAs to MAYV MD, thus shedding light on the structure-function relationship of a previously unexplored viral MD. The emerging differences with any other related MD are expected to enlighten distinct functions.

An NAD+ Phosphorylase Toxin Triggers Mycobacterium tuberculosis Cell Death

Toxin-antitoxin (TA) systems regulate fundamental cellular processes in bacteria and represent potential therapeutic targets. We report a new RES-Xre TA system in multiple human pathogens, including Mycobacterium tuberculosis. The toxin, MbcT, is bactericidal unless neutralized by its antitoxin MbcA. To investigate the mechanism, we solved the 1.8 Å-resolution crystal structure of the MbcTA complex. We found that MbcT resembles secreted NAD+-dependent bacterial exotoxins, such as diphtheria toxin. Indeed, MbcT catalyzes NAD+ degradation in vitro and in vivo. Unexpectedly, the reaction is stimulated by inorganic phosphate, and our data reveal that MbcT is a NAD+ phosphorylase. In the absence of MbcA, MbcT triggers rapid M. tuberculosis cell death, which reduces mycobacterial survival in macrophages and prolongs the survival of infected mice. Our study expands the molecular activities employed by bacterial TA modules and uncovers a new class of enzymes that could be exploited to treat tuberculosis and other infectious diseases.

Biochemical and Biophysical Assays of PAR-WWE Domain Interactions and Production of iso-ADPr for PAR-Binding Analysis

The poly(ADP-ribose) polymerase (PARP) family of proteins utilize NAD⁺ as the substrate to modify protein acceptors with either mono(ADP-ribose) (MAR) or poly(ADP-ribose) (PAR). MAR and PAR have been shown to regulate distinct cellular processes. Iso-ADP-ribose (iso-ADPr) is the smallest internal PAR structural unit containing the characteristic ribose-ribose glycosidic bond formed during poly(ADP-ribosyl)ation. The WWE domain of RNF146 specifically recognizes the iso-ADPr moiety in PAR but does not interact with MAR. This provides a way to distinguish PAR from MAR modification and to isolate PARylated proteins. Iso-ADPr can be used to detect the PAR-specific binding properties of interested proteins. Here we describe the detailed method to generate and purify iso-ADPr and its use in PAR-binding analysis through isothermal titration calorimetry (ITC) analysis.

Structure of tRNA splicing enzyme Tpt1 illuminates the mechanism of RNA 2'-PO4 recognition and ADP-ribosylation

Tpt1 is an essential agent of fungal tRNA splicing that removes the 2′-PO₄ at the splice junction generated by fungal tRNA ligase. Tpt1 catalyzes a unique two-step reaction whereby the 2′-PO₄ attacks NAD⁺ to form an RNA-2′-phospho-ADP-ribosyl intermediate that undergoes transesterification to yield 2′-OH RNA and ADP-ribose-1″,2″-cyclic phosphate products. Because Tpt1 is inessential in exemplary bacterial and mammalian taxa, Tpt1 is seen as an attractive antifungal target. Here we report a 1.4 Å crystal structure of Tpt1 in a product-mimetic complex with ADP-ribose-1″-phosphate in the NAD⁺ site and pAp in the RNA site. The structure reveals how Tpt1 recognizes a 2′-PO₄ RNA splice junction and the mechanism of RNA phospho-ADP-ribosylation. This study also provides evidence that a bacterium has an endogenous phosphorylated substrate with which Tpt1 reacts.

Tpt1 catalyzes the final essential step in yeast tRNA splicing and is a potential antifungal target. Here the authors provide structural insights into how Tpt1 recognizes a 2’-PO₄ RNA splice junction and the mechanism of RNA phospho-ADP-ribosylation.

Poly(ADP-Ribose) Polymerases in Host-Pathogen Interactions, Inflammation, and Immunity

The literature review presented here details recent research involving members of the poly(ADP-ribose) polymerase (PARP) family of proteins. Among the 17 recognized members of the family, the human enzyme PARP1 is the most extensively studied, resulting in a number of known biological and metabolic roles. This review is focused on the roles played by PARP enzymes in host-pathogen interactions and in diseases with an associated inflammatory response. In mammalian cells, several PARPs have specific roles in the antiviral response; this is perhaps best illustrated by PARP13, also termed the zinc finger antiviral protein (ZAP). Plant stress responses and immunity are also regulated by poly(ADP-ribosyl)ation. PARPs promote inflammatory responses by stimulating proinflammatory signal transduction pathways that lead to the expression of cytokines and cell adhesion molecules. Hence, PARP inhibitors show promise in the treatment of inflammatory disorders and conditions with an inflammatory component, such as diabetes, arthritis, and stroke. These functions are correlated with the biophysical characteristics of PARP family enzymes. This work is important in providing a comprehensive understanding of the molecular basis of pathogenesis and host responses, as well as in the identification of inhibitors. This is important because the identification of inhibitors has been shown to be effective in arresting the progression of disease.

The chemistry of the vitamin B3 metabolome

The functional cofactors derived from vitamin B3 are nicotinamide adenine dinucleotide (NAD⁺), its phosphorylated form, nicotinamide adenine dinucleotide phosphate (NADP⁺) and their reduced forms (NAD(P)H). These cofactors, together referred as the NAD(P)(H) pool, are intimately implicated in all essential bioenergetics, anabolic and catabolic pathways in all forms of life. This pool also contributes to post-translational protein modifications and second messenger generation. Since NAD⁺ seats at the cross-road between cell metabolism and cell signaling, manipulation of NAD⁺ bioavailability through vitamin B3 supplementation has become a valuable nutritional and therapeutic avenue. Yet, much remains unexplored regarding vitamin B3 metabolism. The present review highlights the chemical diversity of the vitamin B3-derived anabolites and catabolites of NAD⁺ and offers a chemical perspective on the approaches adopted to identify, modulate and measure the contribution of various precursors to the NAD(P)(H) pool.

Bifunctional Immunity Proteins Protect Bacteria against FtsZ-Targeting ADP-Ribosylating Toxins

ADP-ribosylation of proteins can profoundly impact their function and serves as an effective mechanism by which bacterial toxins impair eukaryotic cell processes. Here, we report the discovery that bacteria also employ ADP-ribosylating toxins against each other during interspecies competition. We demonstrate that one such toxin from Serratia proteamaculans interrupts the division of competing cells by modifying the essential bacterial tubulin-like protein, FtsZ, adjacent to its protomer interface, blocking its capacity to polymerize. The structure of the toxin in complex with its immunity determinant revealed two distinct modes of inhibition: active site occlusion and enzymatic removal of ADP-ribose modifications. We show that each is sufficient to support toxin immunity; however, the latter additionally provides unprecedented broad protection against non-cognate ADP-ribosylating effectors. Our findings reveal how an interbacterial arms race has produced a unique solution for safeguarding the integrity of bacterial cell division machinery against inactivating post-translational modifications.