AB5 ADP-ribosylating toxins: comparative anatomy and physiology

Characterization of pertussis-like toxin from Salmonella spp. that catalyzes ADP-ribosylation of G proteins

Salmonella Typhimurium definitive phage type (DT) 104 produces a pertussis-like toxin (ArtAB-DT104), which catalyzes ADP-ribosylation of pertussis toxin sensitive G proteins. However, the prevalence of ArtAB and its toxicity have not been established. We report here that, in addition to DT104, S. Worthington, and S. bongori, produce ArtAB homologs, designated ArtAB-SW and ArtAB-Sb, respectively. We purified and characterized these ArtAB toxins, which comprise a 27-kDa A subunit (ArtA) and 13.8-kDa pentameric B subunits (ArtB). While the sequence of the A subunit, which is ADP-ribosyltransferase, is similar to the A subunit sequences of other ArtABs, the B subunit of ArtAB-Sb is divergent compared to the B subunit sequences of other ArtABs. Intraperitoneal injection of purified ArtABs was fatal in mice; the 50% lethal doses of ArtAB-DT104 and ArtAB-SW were lower than that of ArtAB-Sb, suggesting that ArtB plays an influential role in the toxicity of ArtABs. ArtABs catalyzed ADP-ribosylation of G proteins in RAW 264.7 murine macrophage-like cells, and increased intracellular cyclic AMP levels. ArtAB-DT104 and ArtAB-SW, but not ArtAB-Sb, stimulated insulin secretion in mice; however, unlike Ptx, ArtABs did not induce leukocytosis. This disparity in biological activity may be explained by differences in ADP-ribosylation of target G proteins.

[Molecular mechanism of AB5 toxin A-subunit translocation into the target cells]

AB5 toxins are pore-forming protein complexes, which destroy eukaryotic target cells inactivating essential enzyme complexes through protein ADP-ribosylation or glycosylation by enzymatically active A1 subunits. The B-subunit pentamer interacts with the target cell receptor, induces membrane pore formation, and initiates receptor-mediated endocytosis. In the present article, we propose a model of A1-subunit translocation in the form of a globular structure, as opposed to the generally accepted hypothesis of A-subunit unfolding in the acidic milieu of the endosome followed by its transport in the form of unfolded polypeptide and refolding in the cytoplasm. This model is based on physical-chemical processes and explains why an endosome, but not an exosome, is formed. A-subunit translocation into the cytosol is driven by the proton potential difference generated by K/Na- and H(+)-ATPases. After reduction of the disulphide bond between A1 and A2 fragments by intracellular enzymes, B-subunit returns back into the endosome, where they are destroyed by endosomal proteases, and the pore is closed. Endosome integrates into the cellular membrane, and membrane-bound enzymatic complexes (ATPases and others) return back to their initial position. The proposed model of receptor-mediated endocytosis is a universal molecular mechanism of translocation of effector toxin molecule subunits or any other proteins into the target cell, as well as of cell membrane reparation after any cell membrane injury by pore-forming complexes.

Salmonella enterica serotype Typhimurium DT104 ArtA-dependent modification of pertussis toxin-sensitive G proteins in the presence of [32P]NAD

Salmonella enterica serotype Typhimurium (S. Typhimurium) definitive phage type (DT) 104 has become a widespread cause of human and other animal infections worldwide. The severity of clinical illness in S. Typhimurium DT104 outbreaks suggests that this strain possesses enhanced virulence. ArtA and ArtB - encoded by a prophage in S. Typhimurium DT104 - are homologues of components of pertussis toxin (PTX), including its ADP-ribosyltransferase subunit. Here, we show that exposing DT104 to mitomycin C, a DNA-damaging agent, induced production of prophage-encoded ArtA/ArtB. Pertussis-sensitive G proteins were labelled in the presence of [(32)P]NAD and ArtA, and the label was released by HgCl(2), which is known to cleave cysteine-ADP-ribose bonds. ADP-dependent modification of G proteins was markedly reduced in in vitro-synthesized ArtA(6Arg-Ala) and ArtA(115Glu-Ala), in which alanine was substituted for the conserved arginine at position 6 (necessary for NAD binding) and the predicted catalytic glutamate at position 115, respectively. A cellular ADP-ribosylation assay and two-dimensional electrophoresis showed that ArtA- and PTX-induced ADP-ribosylation in Chinese hamster ovary (CHO) cells occur with the same type of G proteins. Furthermore, exposing CHO cells to the ArtA/ArtB-containing culture supernatant of DT104 resulted in a clustered growth pattern, as is observed in PTX-exposed CHO cells. Hydrogen peroxide, an oxidative stressor, also induced ArtA/ArtB production, suggesting that these agents induce in vivo synthesis of ArtA/ArtB. These results, taken together, suggest that ArtA/ArtB is an active toxin similar to PTX.

The artAB genes encode a putative ADP-ribosyltransferase toxin homologue associated with Salmonella enterica serovar Typhimurium DT104

Many bacterial pathogens encode ADP-ribosyltransferase toxins. The authors identified an ADP-ribosyltransferase toxin homologue (ArtA, ArtB) in Salmonella enterica serovar Typhimurium (S. typhimurium) DT104. ArtA is most homologous to a putative pertussis-like toxin subunit present in Salmonella typhi (STY1890) and Salmonella paratyphi A (SPA1609), while ArtB shows homology to a hypothetical periplasmic protein of S. typhi (STY1364) and S. paratyphi A (SPA1188), and a putative pertussis-like toxin subunit in S. typhi (STY1891) and S. paratyphi A (SPA1610). The artA gene was detected from the phage particle fraction upon mitomycin C induction, and the flanking region of artAB contains a prophage-like sequence, suggesting that these putative toxin genes reside within a prophage. Southern blotting analysis revealed that artA is conserved in 12 confirmed DT104 strains and in four related strains which are not phage-typed but are classified into the same group as DT104 by both amplified-fragment length polymorphism and pulsed-field gel electrophoresis. Except for one strain, NCTC 73, all 13 S. typhimurium strains which were classified into different groups from that of DT104 lacked the artA locus. The results suggest that phage-mediated recombination has resulted in the acquisition of art genes in S. typhimurium DT104 strains.

Identification of SpyA, a novel ADP-ribosyltransferase of Streptococcus pyogenes

Streptococcus pyogenes, the aetiological agent of both respiratory and skin infections, produces numerous exotoxins to establish infection. This report identifies a new exotoxin produced by this organism, termed SpyA, for S. pyogenesADP-ribosylating toxin. SpyA, MW 24.9, has amino acid identity with the ADP-riboslytransferases (ADPRTs) Staphylococcus aureus EDIN and Clostridium botulinum C3. Recombinant SpyA was able to hydrolyse beta-NAD(+), and this activity was dependent on a glutamate at position 187. SpyA has a putative biglutamate active site, and similar to most biglutamate ADPRTs, was able to ADP-ribosylate poly-l-arginine. SpyA modified numerous proteins in both CHO and HeLa cell lysates. Two-dimesional gel analysis and MALDI-TOF MS analysis of modified proteins indicated that vimentin, tropomyosin and actin, all cytoskeletal proteins, are targets. Expression of spyA in HeLa cells resulted in loss of actin microfilaments. We hypothesize that SpyA is produced by S. pyogenes to disrupt cytoskeletal structures and promote colonization of the host.

Involvement of G proteins in the mycelial photoresponses of Phycomyces

Many responses of the zygomycete fungus Phycomyces blakesleeanus are mediated by blue light, e.g. the stimulation of beta-carotene synthesis (photocarotenogenesis) and the formation of fruiting bodies (photomorphogenesis). Even though both responses have been described in detail genetically and biophysically, the underlying molecular events remain unknown. Applying a pharmacological approach in developing mycelia, we investigated the possible involvement of heterotrimeric G proteins in the blue-light transduction chains of both responses. G protein agonists (guanosine triphosphate analogues, cholera toxin, pertussis toxin) mimicked in darkness the effect of blue light for both responses, except for cholera toxin, which was ineffective in increasing the beta-carotene content of dark-grown mycelia. Experiments combining the two toxins indicated that photocarotenogenesis could involve an inhibitory G protein (Gi) type, whereas photomorphogenesis may depend on a transducin (Gt type)-like heterotrimer. The determination of the carB (phytoene dehydrogenase) and chs1 (chitin synthase 1) gene expression under various conditions of exogenous challenge supports the G protein participation. The fluctuations of the time course measurements of the carB and chs1 transcripts are discussed.

The CTA1-DD vaccine adjuvant binds to human B cells and potentiates their T cell stimulating ability

The present study demonstrates that the novel CTA1-DD-adjuvant, which combines the full enzymatic activity of the A1 subunit of cholera toxin (CT) with an immunoglobulin-binding domain of Staphylococcus aureus protein A (SpA), binds directly to human peripheral blood B lymphocytes of all classes and greatly augments B cell functions in vitro. These effects were reflected in strongly enhanced co-stimulation, resulting in augmented T cell responses to polyclonal-specific as well as Ag-specific activation in vitro. The CTA1-DD-adjuvant had pronounced effects on B cell functions with up-regulated expression of several important activation and co-stimulatory molecules, in particular CD86. Moreover, the adjuvant alone promoted cytokine and chemokine secretion by targeted B cells and in the presence of additional stimuli proliferative responses were augmented. These effects were dependent on the whole enzymatically active CTA1-DD molecule, since DD alone had no effects on the B cells. Collectively our data suggest that CTA1-DD acted via enhanced co-stimulation, which holds promise as to the use of CTA1-DD as a non-toxic adjuvant in future vaccines for human use.

Enhanced delivery of exogenous peptides into the class I antigen processing and presentation pathway

Current immunization strategies, using peptide or protein antigens, generally fail to elicit cytotoxic-T-lymphocyte responses, since these antigens are unable to access intracellular compartments where loading of major histocompatibility complex class I (MHC-I) molecules occurs. In an attempt to circumvent this, we investigated whether the GM1 receptor-binding B subunit of Escherichia coli heat-labile toxin (EtxB) could be used to deliver class I epitopes. When a class I epitope was conjugated to EtxB, it was delivered into the MHC-I presentation pathway in a GM1-binding-dependent fashion and resulted in the appearance of MHC-I-epitope complexes at the cell surface. Importantly, we show that the efficiency of EtxB-mediated epitope delivery could be strikingly enhanced by incorporating, adjacent to the class I epitope, a 10-amino-acid segment from the C terminus of the DNA polymerase (Pol) of herpes simplex virus. The replacement of this 10-amino-acid segment by a heterologous sequence or the introduction of specific amino acid substitutions within this segment either abolished or markedly reduced the efficiency of class I epitope delivery. If the epitope was extended at its C terminus, EtxB-mediated delivery into the class I presentation pathway was found to be completely dependent on proteasome activity. Thus, by combining the GM1-targeting function of EtxB with the 10-amino-acid Pol segment, highly efficient delivery of exogenous epitopes into the endogenous pathway of class I antigen processing and presentation can be achieved.

Chemoattractant-stimulated NF-kappaB activation is dependent on the low molecular weight GTPase RhoA

Chemoattractants bind to seven transmembrane-spanning, G-protein-coupled receptors on monocytes and neutrophils and induce a variety of functional responses, including activation of the transcription factor NF-kappaB. The signaling mechanisms utilized by chemoattractants to activate NF-kappaB in human peripheral blood monocytes are poorly defined. We previously demonstrated that fMet-Leu-Phe (fMLP) stimulates NF-kappaB activation, and this function of fMLP requires phosphatidylinositol 3-kinase (PI3K). Here we present evidence that fMLP activates RhoA and that fMLP-induced NF-kappaB activation requires this small GTPase. Stimulation of monocytes with fMLP rapidly activated RhoA as well as NF-kappaB, and their activation was markedly reduced by pertussis toxin treatment. Pretreatment of monocyte with a RhoA inhibitor, C3 transferase from Clostridium botulinum, effectively blocked fMLP-induced NF-kappaB activation as well as interleukin-1beta gene expression. A dominant negative form of RhoA (T19N) also inhibited fMLP-stimulated reporter gene expression in a kappaB-dependent manner. Cotransfection of the monocytic THP1 cells with a constitutively active form of RhoA (Q63L) with the promoter reporter plasmid results in a marked increase in NF-kappaB-mediated reporter gene expression. Furthermore, the PI3K inhibitors wortmannin and LY294002 block RhoA activation induced by fMLP. These results demonstrate that low molecular weight GTPase RhoA is a novel signal transducer for fMLP-induced NF-kappaB activation and Galpha(i) or Galpha(o) class of heterotrimeric G proteins likely mediate RhoA activation via PI3K in human peripheral blood monocytes.

An abundance of bacterial ADP-ribosyltransferases--implications for the origin of exotoxins and their human homologues

ADP-ribosylation is a post-translational modification that can be seen in many contexts, including as the primary mechanism of action of many important bacterial exotoxins. By data-mining complete and incomplete bacterial genome sequences, we have discovered >20 novel putative ADP-ribosyltransferases, including several new potential toxins.

Requirement of phosphatidylinositol 3-kinase activity for bradykinin stimulation of NF-kappaB activation in cultured human epithelial cells

The signaling mechanisms utilized by bradykinin (BK) to activate the transcription factor nuclear factor kappaB (NF-kappaB) are poorly defined. We previously demonstrated that BK-stimulated NF-kappaB activation requires the small GTPase RhoA. We present evidence that BK-induced NF-kappaB activation both activates and requires phosphatidylinositol 3-kinase (PI 3-kinase) in A549 human epithelial cells. Pre-treatment with the PI 3-kinase-specific inhibitors, wortmannin, and LY294002 effectively blocked BK-induced PI 3-kinase activity. Wortmannin and LY294002 also abolished BK-induced NF-kappaB activation, as did transient transfection with a dominant negative mutant of the p85 subunit. BK-stimulated PI 3-kinase activity and NF-kappaB activation were sensitive to pertussis but not cholera toxin, suggesting that the B2 BK receptors transducing the response were coupled to Galphai or Galphao heterotrimeric G proteins. Tumor necrosis factor alpha (TNFalpha) also stimulated increased PI 3-kinase activity, however TNFalpha-stimulated NF-kappaB activation was not affected by the PI 3-kinase inhibitors or the p85 dominant negative mutant. These findings provide evidence that BK-induced NF-kappaB activation utilizes a signaling pathway that requires activity of both RhoA and PI 3-kinase and is distinct from the signaling pathway utilized by TNFalpha. Furthermore, we show that the p85 regulatory subunit is required for activation of PI 3-kinase activity by this G protein-coupled receptor.

Effect of in vivo injection of cholera and pertussis toxin on glucose transport in rat skeletal muscle

Cholera toxin (CTX) and pertussis toxin (PTX) were examined for their ability to inhibit glucose transport in perfused skeletal muscle. Twenty-five hours after an intravenous injection of CTX, basal transport was decreased approximately 30%, and insulin- and contraction-stimulated transport was reduced at least 86 and 49%, respectively, in both the soleus and red and white gastrocnemius muscles. In contrast, PTX treatment was much less efficient. Impairment of glucose transport appeared to develop 10-15 h after CTX administration, which coincided with development of hyperglycemia despite hyperinsulinimia, increased plasma free fatty acid levels, increased adenosine 3',5'-cyclic monophosphate (cAMP) concentrations in muscle, but no difference in plasma catecholamines. Twenty-five hours after CTX treatment, GLUT-4 protein in both soleus and red gastrocnemius muscles was decreased, whereas no change in GLUT-1 protein content was found. In contrast, GLUT-4 mRNA was unchanged, but transcripts for GLUT-1 were increased > or = 150% in all three muscles from CTX-treated rats. The findings suggest that CTX via increased cAMP impairs basal as well as insulin- and contraction-stimulated muscle glucose transport, at least in part from a decrease in intramuscular GLUT-4 protein.

Crystal structure of diphtheria toxin bound to nicotinamide adenine dinucleotide

The crystal structure of diphtheria toxin (DT) in complex with nicotinamide adenine dinucleotide (NAD) has been determined by x-ray crystallography to 2.3 A resolution. NAD binds to a cleft on the surface of the catalytic (C) domain of DT, interacting closely with the side chains of Tyr54, Tyr65, His21, Thr23, and Glu 48. The carboxylate group of Glu148 of Dt lies approximately 4 A from the scissile, N-glycosidic bound of NAD, suggesting a possible catalytic role for Glu148 in stabilizing a positively charged oxocarbonium intermediate. Residues 39-46 of the active-site loop of the C-domain become disordered upon NAD-binding, suggesting a potential role for these residues in binding to elongation facor-2 (EF-2). Structural alignments of the DT-NAD complex with the structures of other ADP-ribosylating toxins suggest how NAD may bind to these other enzymes.

Mutants of the Escherichia coli heat-labile enterotoxin with reduced ADP-ribosylation activity or no activity retain the immunogenic properties of the native holotoxin

The Escherichia coli heat-labile enterotoxin (LT) is a potent inducer of mucosal immune responses. In a previous study (L. DeHaan, W. R. Verweij, M. Holtrop, E. Agsteribbe, and J. Wilschut, Vaccine 14:620-626, 1996), we have shown that efficient induction of an LTB-specific mucosal immune response by LT requires the presence of the LTA chain, suggesting a possible role of the enzymatic activity of LTA in the induction of these responses. In the present study, we generated LT mutants with altered ADP-ribosylation activities and evaluated their immunogenicity upon intranasal administration to mice. The results demonstrate that the mucosal immunogenicity of LT is not dependent on its ADP-ribosylation activity.

Crystal structure of a new heat-labile enterotoxin, LT-IIb

BACKGROUND

Cholera toxin from Vibrio cholerae and the type I heat-labile enterotoxins (LT-Is) from Escherichia coli are oligomeric proteins with AB5 structures. The type II heat-labile enterotoxins (LT-IIs) from E. coli are structurally similar to, but antigenically distinct from, the type I enterotoxins. The A subunits of type I and type II enterotoxins are homologous and activate adenylate cyclase by ADP-ribosylation of a G protein subunit, G8 alpha. However, the B subunits of type I and type II enterotoxins differ dramatically in amino acid sequence and ganglioside-binding specificity. The structure of LT-IIb was determined both as a prototype for other LT-IIs and to provide additional insights into structure/function relationships among members of the heat-labile enterotoxin family and the superfamily of ADP-ribosylating protein toxins.

RESULTS

The 2.25 A crystal structure of the LT-IIb holotoxin has been determined. The structure reveals striking similarities with LT-I in both the catalytic A subunit and the ganglioside-binding B subunits. The latter form a pentamer which has a central pore with a diameter of 10-18 A. Despite their similarities, the relative orientation between the A polypeptide and the B pentamer differs by 24 degrees in LT-I and LT-IIb. A common hydrophobic ring was observed at the A-B5 interface which may be important in the cholera toxin family for assembly of the AB5 heterohexamer. A cluster of arginine residues at the surface of the A subunit of LT-I and cholera toxin, possibly involved in assembly, is also present in LT-IIb. The ganglioside receptor binding sites are localized, as suggested by mutagenesis, and are in a position roughly similar to the sites where LT-I binds its receptor.

CONCLUSIONS

The structure of LT-IIb provides insight into the sequence diversity and structural similarity of the AB5 toxin family. New knowledge has been gained regarding the assembly of AB5 toxins and their active-site architecture.

Protein structure. Why have six-fold symmetry?

Why do proteins proteins that encircle DNA have six-fold symmetry? One important factor may be the economy in protein mass with which DNA can be encircled by six globular subunits arranged in a ring.

AB5 toxins

Crystal structures of shiga and pertussis toxins have recently revealed a remarkable degree of structural homology among the members of the AB5 class of bacterial toxins. Other structures have provided a detailed view of the molecular basis of receptor binding specificity of cholera toxin, and of the heat-labile enterotoxin of Escherichia coli. These structures also provide tantalizing, but as yet incomplete, information on the site of ADP-ribosylation in the homologous A-subunits of the Escherichia coli heat-labile toxin, cholera toxin, and pertussis toxin.