C3larvin toxin, an ADP-ribosyltransferase from Paenibacillus larvae.

C3larvin toxin was identified by a bioinformatic strategy as a putative mono-ADP-ribosyltransferase and a possible virulence factor from Paenibacillus larvae, which is the causative agent of American Foulbrood in honey bees. C3larvin targets RhoA as a substrate for its transferase reaction, and kinetics for both the NAD(+) (Km = 34 ± 12 µm) and RhoA (Km = 17 ± 3 µm) substrates were characterized for this enzyme from the mono-ADP-ribosyltransferase C3 toxin subgroup. C3larvin is toxic to yeast when expressed in the cytoplasm, and catalytic variants of the enzyme lost the ability to kill the yeast host, indicating that the toxin exerts its lethality through its enzyme activity. A small molecule inhibitor of C3larvin enzymatic activity was discovered called M3 (Ki = 11 ± 2 µm), and to our knowledge, is the first inhibitor of transferase activity of the C3 toxin family. C3larvin was crystallized, and its crystal structure (apoenzyme) was solved to 2.3 Å resolution. C3larvin was also shown to have a different mechanism of cell entry from other C3 toxins.

Characterization of Vis Toxin, a Novel ADP-Ribosyltransferase from Vibrio splendidus.

Vis toxin was identified by a bioinformatics strategy as a putative virulence factor produced by Vibrio splendidus with mono-ADP-ribosyltransferase activity. Vis was purified to homogeneity as a 28 kDa single-domain enzyme and was shown to possess NAD(+)-glycohydrolase [KM(NAD(+)) = 276 ± 12 µM] activity and with an R-S-E-X-E motif; it targets arginine-related compounds [KM(agmatine) = 272 ± 18 mM]. Mass spectrometry analysis revealed that Vis labels l-arginine with ADP-ribose from the NAD(+) substrate at the amino nitrogen of the guanidinium side chain. Vis is toxic to yeast when expressed in the cytoplasm under control of the CUP1 promotor, and catalytic variants lost the ability to kill the yeast host, indicating that the toxin exerts its lethality through its enzyme activity. Several small molecule inhibitors were identified from a virtual screen, and the most potent compounds were found to inhibit the transferase activity of the enzyme with Ki values ranging from 25 to 134 µM. Inhibitor compound M6 bears the necessary attributes of a solid candidate as a lead compound for therapeutic development. Vis toxin was crystallized, and the structures of the apoenzyme (1.4 Å) and the enzyme bound with NAD(+) (1.8 Å) and with the M6 inhibitor (1.5 Å) were determined. The structures revealed that Vis represents a new subgroup within the mono-ADP-ribosyltransferase toxin family.

The 1.8 Å cholix toxin crystal structure in complex with NAD+ and evidence for a new kinetic model.

Certain Vibrio cholerae strains produce cholix, a potent protein toxin that has diphthamide-specific ADP-ribosyltransferase activity against eukaryotic elongation factor 2. Here we present a 1.8 Å crystal structure of cholix in complex with its natural substrate, nicotinamide adenine dinucleotide (NAD(+)). We also substituted hallmark catalytic residues by site-directed mutagenesis and analyzed both NAD(+) binding and ADP-ribosyltransferase activity using a fluorescence-based assay. These data are the basis for a new kinetic model of cholix toxin activity. Further, the new structural data serve as a reference for continuing inhibitor development for this toxin class.

A cationic lumen in the Wzx flippase mediates anionic O-antigen subunit translocation in Pseudomonas aeruginosa PAO1.

Heteropolymeric B-band O-antigen (O-Ag) biosynthesis in Pseudomonas aeruginosa PAO1 follows the Wzy-dependent pathway, beginning with translocation of undecaprenyl pyrophosphate-linked anionic O-Ag subunits (O units) from the inner to the outer leaflets of the inner membrane (IM). This translocation is mediated by the integral IM flippase Wzx. Through experimentally based and unbiased topological mapping, our group previously observed that Wzx possesses many charged and aromatic amino acid residues within its 12 transmembrane segments (TMS). Herein, site-directed mutagenesis targeting 102 residues was carried out on the TMS and loops of Wzx, followed by assessment of each construct's ability to restore B-band O-Ag production, identifying eight residues important for flippase function. The importance of various charged and aromatic residues was highlighted, predominantly within the TMS of the protein, revealing functional 'hotspots' within the flippase, particularly within TMS2 and TMS8. Construction of a tertiary structure homology model for Wzx indicated that TMS2 and TMS8 line a central cationic lumen. This is the first report to describe a charged flippase lumen for mediating anionic O-unit translocation across the hydrophobic IM.

Needle in the haystack: structure-based toxin discovery.

In the current data-rich era, making the leap from sequence data to knowledge is a task that requires an elegant bioinformatics toolset to pinpoint pressing research questions. Therefore, a strategy to expand important protein-family knowledge is required, particularly in cases in which primary sequence identity is low but structural conservation is high. For example, the mono-ADP-ribosylating toxins fit these criteria and several approaches have been used to accelerate the discovery of new family members. The strategy evolved from conduction of PSI-BLAST searches through to the combination of secondary-structure prediction with pattern-based searches. However, a newly developed tactic, in which fold recognition dominates, reduces reliance on sequence similarity and advances scientists toward a true structure-based protein-family expansion methodology.

Photox, a novel actin-targeting mono-ADP-ribosyltransferase from Photorhabdus luminescens.

Photorhabdus luminescens is a pathogenic bacterium that produces many toxic proteins. The mono-ADP-ribosyltransferases (mARTs) are an enzyme class produced by numerous pathogenic bacteria and participate in disease in plants and animals, including humans. Herein we report a novel mART from P. luminescens called Photox. This 46-kDa toxin shows high homology to other actin-targeting mARTs in hallmark catalytic regions and a similar core catalytic fold. Furthermore, Photox shows in vivo cytotoxic activity against yeast, with protection occurring when catalytic residues are substituted with alanine. In vitro, enzymatic activity (k(cat), 1680 +/- 75 min(-1)) is higher than that of the related iota toxin, and diminishes by nearly 14,000-fold following substitution of the catalytic Glu (E355A). This toxin specifically ADP-ribosylates monomeric alpha-skeletal actin and nonmuscle beta- and gamma-actin at Arg(177), inhibiting regular polymerization of actin filaments. These results indicate that Photox is indeed an ADP-ribosyltransferase, making it the newest member of the actin-targeting mART family.

Newly discovered and characterized antivirulence compounds inhibit bacterial mono-ADP-ribosyltransferase toxins.

The mono-ADP-ribosyltransferase toxins are bacterial virulence factors that contribute to many disease states in plants, animals, and humans. These toxins function as enzymes that target various host proteins and covalently attach an ADP-ribose moiety that alters target protein function. We tested compounds from a virtual screen of commercially available compounds combined with a directed poly(ADP-ribose) polymerase (PARP) inhibitor library and found several compounds that bind tightly and inhibit toxins from Pseudomonas aeruginosa and Vibrio cholerae. The most efficacious compounds completely protected human lung epithelial cells against the cytotoxicity of these bacterial virulence factors. Moreover, we determined high-resolution crystal structures of the best inhibitors in complex with cholix toxin to reveal important criteria for inhibitor binding and mechanism of action. These results provide new insight into development of antivirulence compounds for treating many bacterial diseases.

Cholera- and anthrax-like toxins are among several new ADP-ribosyltransferases.

Chelt, a cholera-like toxin from Vibrio cholerae, and Certhrax, an anthrax-like toxin from Bacillus cereus, are among six new bacterial protein toxins we identified and characterized using in silico and cell-based techniques. We also uncovered medically relevant toxins from Mycobacterium avium and Enterococcus faecalis. We found agriculturally relevant toxins in Photorhabdus luminescens and Vibrio splendidus. These toxins belong to the ADP-ribosyltransferase family that has conserved structure despite low sequence identity. Therefore, our search for new toxins combined fold recognition with rules for filtering sequences--including a primary sequence pattern--to reduce reliance on sequence identity and identify toxins using structure. We used computers to build models and analyzed each new toxin to understand features including: structure, secretion, cell entry, activation, NAD+ substrate binding, intracellular target binding and the reaction mechanism. We confirmed activity using a yeast growth test. In this era where an expanding protein structure library complements abundant protein sequence data--and we need high-throughput validation--our approach provides insight into the newest toxin ADP-ribosyltransferases.

Yeast as a tool for characterizing mono-ADP-ribosyltransferase toxins.

The emergence of bacterial antibiotic resistance poses a significant challenge in the pursuit of novel therapeutics, making new strategies for drug discovery imperative. We have developed a yeast growth-defect phenotypic screen to help solve this current dilemma. This approach facilitates the identification and characterization of a new diphtheria toxin (DT) group, ADP-ribosyltransferase toxins from pathogenic bacteria. In addition, this assay utilizes Saccharomyces cerevisiae, a reliable model for bacterial toxin expression, to streamline the identification and characterization of new inhibitors against this group of bacterial toxins that may be useful for antimicrobial therapies. We show that a mutant of the elongation factor 2 target protein in yeast, G701R, confers resistance to all DT group toxins and recovers the growth-defect phenotype in yeast. We also demonstrate the ability of a potent small-molecule toxin inhibitor, 1,8-naphthalimide (NAP), to alleviate the growth defect caused by toxin expression in yeast. Moreover, we determined the crystal structure of the NAP inhibitor-toxin complex at near-atomic resolution to provide insight into the inhibitory mechanism. Finally, the NAP inhibitor shows therapeutic protective effects against toxin invasion of mammalian cells, including human lung cells.

Cholix toxin, a novel ADP-ribosylating factor from Vibrio cholerae.

The ADP-ribosyltransferases are a class of enzymes that display activity in a variety of bacterial pathogens responsible for causing diseases in plants and animals, including those affecting mankind, such as diphtheria, cholera, and whooping cough. We report the characterization of a novel toxin from Vibrio cholerae, which we call cholix toxin. The toxin is active against mammalian cells (IC(50) = 4.6 +/- 0.4 ng/ml) and crustaceans (Artemia nauplii LD(50) = 10 +/- 2 mug/ml). Here we show that this toxin is the third member of the diphthamide-specific class of ADP-ribose transferases and that it possesses specific ADP-ribose transferase activity against ribosomal eukaryotic elongation factor 2. We also describe the high resolution crystal structures of the multidomain toxin and its catalytic domain at 2.1- and 1.25-A resolution, respectively. The new structural data show that cholix toxin possesses the necessary molecular features required for infection of eukaryotes by receptor-mediated endocytosis, translocation to the host cytoplasm, and inhibition of protein synthesis by specific modification of elongation factor 2. The crystal structures also provide important insight into the structural basis for activation of toxin ADP-ribosyltransferase activity. These results indicate that cholix toxin may be an important virulence factor of Vibrio cholerae that likely plays a significant role in the survival of the organism in an aquatic environment.