Explore the vast range of titles available.

Post-translational protein modifications such as phosphorylation and ubiquitinylation are common molecular targets of conflict between viruses and their hosts. However, the role of other post-translational modifications, such as ADP-ribosylation, in host-virus interactions is less well characterized. ADP-ribosylation is carried out by proteins encoded by the PARP (also called ARTD) gene family. The majority of the 17 human PARP genes are poorly characterized. However, one PARP protein, PARP13/ZAP, has broad antiviral activity and has evolved under positive (diversifying) selection in primates. Such evolution is typical of domains that are locked in antagonistic 'arms races' with viral factors. To identify additional PARP genes that may be involved in host-virus interactions, we performed evolutionary analyses on all primate PARP genes to search for signatures of rapid evolution. Contrary to expectations that most PARP genes are involved in 'housekeeping' functions, we found that nearly one-third of PARP genes are evolving under strong recurrent positive selection. We identified a >300 amino acid disordered region of PARP4, a component of cytoplasmic vault structures, to be rapidly evolving in several mammalian lineages, suggesting this region serves as an important host-pathogen specificity interface. We also found positive selection of PARP9, 14 and 15, the only three human genes that contain both PARP domains and macrodomains. Macrodomains uniquely recognize, and in some cases can reverse, protein mono-ADP-ribosylation, and we observed strong signatures of recurrent positive selection throughout the macro-PARP macrodomains. Furthermore, PARP14 and PARP15 have undergone repeated rounds of gene birth and loss during vertebrate evolution, consistent with recurrent gene innovation. Together with previous studies that implicated several PARPs in immunity, as well as those that demonstrated a role for virally encoded macrodomains in host immune evasion, our evolutionary analyses suggest that addition, recognition and removal of ADP-ribosylation is a critical, underappreciated currency in host-virus conflicts.

PARP15 is a mono-ADP-ribosyltransferase that targets an unknown set of proteins as well as RNA. Its evolutionary relationship with PARP14 suggests roles in antiviral defence; its localization to stress granules points to functions in the regulation of translation. Here we show that the transferase domain of PARP15 dimerizes in solution; the formation of dimers is a prerequisite for catalytic activity and monomeric mutant variants of the domain are inactive. In cells, dimer-disrupting mutations abrogate catalytic activity and alter the subcellular localization of the full-length protein. Using biophysical methods, including X-ray crystallography and HDX-MS, we provide evidence for a regulatory mechanism by which dimerization enables correct target engagement rather than NAD + co-substrate binding, and by which the two protomers of the dimer operate independently of one another. Together, our results uncover a regulatory mechanism in a PARP family enzyme. PARP enzymes play key roles in human biology, but their regulation remains poorly understood. This study shows that PARP15 is activated through dimerization of its catalytic domain and reveals how this event primes the domain for ADP-ribosyl transfer.

Key Points The emergence of bioinformatics has changed how the study of microbial pathogenesis is carried out, which has facilitated the development of 'reverse proteomics' strategies. Researchers no longer need to clinically identify a pathogen before identifying putative virulence factors. These analyses can be used to identify new bacterial ADP-ribosyltransferase toxins (bARTTs) on the basis of conserved function, even when the proteins have novel structural organizations. This strategy has been proven by the identification of putative bARTTs, followed by experimental confirmation of their ADP-ribosyltransferase activity. Several novel protein toxins have been identified and characterized in this manner. New toxins, such as cholix toxin (ChxA), SpyA, HopU1, SpvB and the TcC proteins have been identified using this strategy of reverse proteomics and have been experimentally shown to have the predicted ADP-ribosyltransferase enzymatic activity. However, this strategy still has limitations. Many of the novel toxins have unique substrates or structural or delivery properties that were not predicted by bioinformatic methods. Experimental confirmation of the importance of a putative virulence factor according to Falkow's molecular postulates also needs to be considered. The continuing growth of the family of bacterial ADP-ribosylating toxins should enable greater ability to create 'rules' to properly identify substrates and the structural organization of novel toxins. Extrapolation of this technique to other toxins or families of proteins may greatly improve the field of microbiology. Bacterial ADP-ribosyltransferase toxins (bARTTs) transfer ADP-ribose to a range of eukaryotic proteins to promote bacterial pathogenesis. In this Review, the authors discuss the structural and functional properties of the most recently identified novel bARTTs, which are produced by various human, insect and plant pathogens and were identified using bioinformatic analyses. Bacterial ADP-ribosyltransferase toxins (bARTTs) transfer ADP-ribose to eukaryotic proteins to promote bacterial pathogenesis. In this Review, we use prototype bARTTs, such as diphtheria toxin and pertussis toxin, as references for the characterization of several new bARTTs from human, insect and plant pathogens, which were recently identified by bioinformatic analyses. Several of these toxins, including cholix toxin (ChxA) from Vibrio cholerae , SpyA from Streptococcus pyogenes , HopU1 from Pseudomonas syringae and the Tcc toxins from Photorhabdus luminescens , ADP-ribosylate novel substrates and have unique organizations, which distinguish them from the reference toxins. The characterization of these toxins increases our appreciation of the range of structural and functional properties that are possessed by bARTTs and their roles in bacterial pathogenesis.

Glypican-3 is a cell surface glycoprotein that associates with Wnt in liver cancer. We develop two antibodies targeting glypican-3, HN3 and YP7. The first antibody recognizes a functional epitope and inhibits Wnt signalling, whereas the second antibody recognizes a C-terminal epitope but does not inhibit Wnt signalling. Both are fused to a fragment of Pseudomonas exotoxin A (PE38) to create immunotoxins. Interestingly, the immunotoxin based on HN3 (HN3-PE38) has superior antitumor activity as compared with YP7 (YP7-PE38) both in vitro and in vivo . Intravenous administration of HN3-PE38 alone, or in combination with chemotherapy, induces regression of Hep3B and HepG2 liver tumour xenografts in mice. This study establishes glypican-3 as a promising candidate for immunotoxin-based liver cancer therapy. Our results demonstrate immunotoxin-induced tumour regression via dual mechanisms: inactivation of cancer signalling via the antibody and inhibition of protein synthesis via the toxin. Tumour-targeted antibodies can kill cancer cells by blocking pro-survival signalling or by delivering a toxin. Here the authors show that glypican-3 antibody fused to a bacterial toxin suppresses tumour growth more efficiently if designed to block Wnt signalling downstream of glypican-3.

The astounding number of anti-phage defenses encoded by bacteria is countered by an elaborate set of phage counter-defenses, though their evolutionary origins are often unknown. Here, we report the discovery of an orphan antitoxin counter-defense element in T4-like phages that can overcome the bacterial toxin-antitoxin phage defense system, DarTG1. The DarT1 toxin, an ADP-ribosyltransferase, modifies phage DNA to prevent replication while its cognate antitoxin, DarG1, is a NADAR superfamily ADP-ribosylglycohydrolase that reverses these modifications in uninfected bacteria. We show that some phages carry an orphan DarG1-like NADAR domain protein, which we term anti-DarT factor NADAR (AdfN), that removes ADP-ribose modifications from phage DNA during infection thereby enabling replication in DarTG1-containing bacteria. We find divergent NADAR proteins in unrelated phages that likewise exhibit anti-DarTG1 activity, underscoring the importance of ADP-ribosylation in bacterial-phage interactions, and revealing the function of a substantial subset of the NADAR superfamily. Toxin-antitoxin systems make up a branch of the bacterial anti-phage immune system. Here, Johannesman et al demonstrate that some phages have co-opted an orphan antitoxin to counter a DNA ADP-ribosyltransferase defense system.

Clostridium difficile infection is the main cause of healthcareacquired diarrhea in the developed world. In addition to the main virulence factors toxin A and B, epidemic, PCR Ribotype 027 strains, such as R20291, produce a third toxin, CDT. To develop effective medical countermeasures, it is important to understand the importance of each toxin. Accordingly, we created all possible combinations of isogenic toxin mutants of R20291 and assessed their virulence. We demonstrated that either toxin A or toxin B alone can cause fulminant disease in the hamster infection model and present tantalizing data that C. difficile toxin may also contribute to virulence.

ADP-ribosyl transferases (ARTs) are a family of enzymes which catalyze the addition of a chain (PARylation) or a single moiety (MARylation) of ADP-ribose to their substrates. PARP7 is a mono-ADP-ribosyl transferase (mono-ART) which has recently gained attention due to its emerging role as a negative regulator of the type I interferon (IFN-I) and nuclear receptor signaling, and due to its aberrant expression in cancer, contributing to disease progression and immune evasion. PARP7-mediated ADP-ribosylation can differentially affect protein stability. On the one hand, PARP7-mediated ADP-ribosylation of the transcription factor FRA1 protects it from proteosomal degradation and thereby supports its function in negatively regulating IRF1 and the expression of apoptosis and immune signaling genes. On the other hand, PARP7-mediated ADP-ribosylation of aryl hydrocarbon receptor (AHR) and estrogen receptor (ER) marks them for proteosomal degradation. PARP7 also ADP-ribosylates the ligand-bound androgen receptor (AR), which is recognized by DTX3L-PARP9 that modulate the AR transcriptional activity. In this review, we discuss PARP7 enzymatic properties, biological functions and known substrates, its role in various cancers, and its targeting by specific inhibitors. In this review, D. Slade and colleagues discuss PARP7 enzymatic properties, biological functions and known substrates, its role in various cancers, and its targeting by specific inhibitors.

Bacteria have evolved diverse immune systems to prevent phage infection, and, consequently, phages have evolved diverse methods of evading bacterial immune systems. To evade the bacterial CRISPR-Cas immune system, phages encode anti-CRISPR proteins (Acrs). Acrs disable CRISPR-Cas by either stably binding to the CRISPR-Cas complex or by enzymatic modification. However, Acr enzymes have not been characterized in vivo during lytic infection or lysogenic maintenance. Here, we report the benefits and drawbacks of enzymatic inhibition with AcrIF11, an ADP-ribosyltransferase. Under “high pressure” scenarios such as high CRISPR targeting or CRISPR-Cas overexpression, AcrIF11 outperforms a strong, stable binding Acr by very specifically modifying the Cas8 protein, but nothing else in the cell. AcrIF11 additionally stabilizes lysogeny effectively, but the ADP-ribose modification can potentially be removed by macrodomains, which are ADP-ribose eraser enzymes. AcrIF11 is therefore a potent and widespread plasmid/phage-encoded inhibitor of Type I-F CRISPR-Cas systems with catalytic activity.

The iota toxin produced by Clostridium perfringens type E is a binary toxin comprising two independent polypeptides: Ia, an ADP-ribosyltransferase, and Ib, which is involved in cell binding and translocation of Ia across the cell membrane. Here we report cryo-EM structures of the translocation channel Ib-pore and its complex with Ia. The high-resolution Ib-pore structure demonstrates a similar structural framework to that of the catalytic ϕ-clamp of the anthrax protective antigen pore. However, the Ia-bound Ib-pore structure shows a unique binding mode of Ia: one Ia binds to the Ib-pore, and the Ia amino-terminal domain forms multiple weak interactions with two additional Ib-pore constriction sites. Furthermore, Ib-binding induces tilting and partial unfolding of the Ia N-terminal α-helix, permitting its extension to the ϕ-clamp gate. This new mechanism of N-terminal unfolding is crucial for protein translocation.Structural elucidation of the pore form of binary Iota toxin Ib with its toxic subunit, Ia, visualizes interactions mediating Ia translocation through the pore and extension of the Ia N-terminus consistent with a Brownian ratchet translocation mechanism.

Immune responses can make protein therapeutics ineffective or even dangerous. We describe a general computational protein design method for reducing immunogenicity by eliminating known and predicted T-cell epitopes and maximizing the content of human peptide sequences without disrupting protein structure and function. We show that the method recapitulates previous experimental results on immunogenicity reduction, and we use it to disrupt T-cell epitopes in GFP and Pseudomonas exotoxin A without disrupting function.