Two-Site Autoinhibition of the ADP-Ribosylating Mosquitocidal Toxin (MTX) from Bacillus sphaericus by Its 70-kDa Ricin-like Binding Domain

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

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

Identification and characterization of two novel toxins expressed by the lethal honey bee pathogen Paenibacillus larvae, the causative agent of American foulbrood

Paenibacillus larvae is a Gram-positive bacterial pathogen causing the epizootic American foulbrood in honey bee larvae. Four so-called enterobacterial repetitive intergenic consensus (ERIC) genotypes of P. larvae exist with P. larvae genotypes ERIC I and ERIC II being responsible for disease outbreaks all over the world. Very few molecular data on the pathogen, on pathogenesis or on virulence factors exist. We now identified two genomic loci in P. larvae ERIC I coding for two binary AB toxins, Plx1 and Plx2. In silico analyses revealed that Plx1 is the third member of an enigmatic family of AB toxins so far only comprising MTX1 of Lysinibacillus sphaericus and pierisin-like toxins expressed by several butterflies. Plx2 is also remarkable because the A-domain is highly similar to C3 exoenzymes, which normally are single domain proteins, while the B-domain is homologous to B-domains of C2-toxins. We constructed P. larvae mutants lacking expression of Plx1, Plx2 or both toxins and demonstrated that these toxins are important virulence factors for P. larvae ERIC I.

From cosubstrate similarity to inhibitor diversity--inhibitors of ADP-ribosyltransferases from kinase inhibitor screening

ADP-ribosyltransferases (ADP-RTs) use NAD(+) to transfer an ADP-ribosyl group to target proteins. Although some ADP-RTs are bacterial toxins only few inhibitors are known. Here we present the development of fluorescence-based assays and a focussed library screening using kinase inhibitors as a new approach towards inhibitors of ADP-RTs. Different screening setups were established using surrogate small molecule substrates or the quantitation of the cofactor NAD(+). Proof-of-principle screening experiments were performed using a kinase inhibitor library in order to target the NAD(+) binding pockets. This led to the discovery of structurally different lead inhibitors for the mono-ADP-ribosyltransferases Mosquitocidal toxin (MTX) from Bacillus sphaericus SSII-1, C3bot toxin from Clostridium botulinum and CDTa from Clostridium difficile. The interaction of the inhibitors with the toxin proteins was analyzed by means of docking and binding free energy calculations. Binding at the nicotinamide subpocket, which shows a significant difference in the three enzymes, is used to explain the selectivity of the identified inhibitors and offers an opportunity for further development of potent and selective inhibitors.

Cell-free synthesis and characterization of a novel cytotoxic pierisin-like protein from the cabbage butterfly Pieris rapae

Pierisin-like proteins comprise a growing family of ADP-ribosyltransferases expressed in various species of white butterflies. The prototype pierisin-1 from the cabbage butterfly, Pieris rapae, was identified as a potent apoptosis-inducing agent, acting on various types of carcinoma cell lines by mono-ADP-ribosylation of DNA. The characterization of pierisin-like proteins is hampered by its potent toxicity, which prevents its expression as a recombinant protein in Escherichia coli. Here we characterized a new member of the pierisin protein family named pierisin-1b, which was cloned from P. rapae. Pierisin-1b consists of 849 amino acids residues and shares 63%-91% identity with already described pierisins. For expression of pierisin-1b a novel in vitro translation system was utilized. Obtained protein exhibits specific ADP-ribosyltransferase activity on deoxyguanosine residues of DNA leading to induction of apoptosis and cell death.

Structure and mode of action of a mosquitocidal holotoxin

The crystal structure of the full mosquitocidal toxin from Bacillus sphaericus (MTX(holo)) has been determined at 2.5 A resolution by the molecular replacement method. The resulting structure revealed essentially the complete chain consisting of four ricin B-type domains curling around the catalytic domain in a hedgehog-like assembly. As the structure was virtually identical in three different crystal packings, it is probably not affected by packing contacts. The structure of MTX(holo) explains earlier autoinhibition data. An analysis of published complexes comprising ricin B-type lectin domains and sugar molecules shows that the general construction principle applies to all four lectin domains of MTX(holo), indicating 12 putative sugar-binding sites. These sites are sequence-related to those of the cytotoxin pierisin from cabbage butterfly, which are known to bind glycolipids. It seems therefore likely that MTX(holo) also binds glycolipids. The seven contact interfaces between the five domains are predominantly polar and not stronger than common crystal contacts so that in an appropriate environment, the multidomain structure would likely uncurl into a string of single domains. The structure of the isolated catalytic domain plus an extended linker was established earlier in three crystal packings, two of which showed a peculiar association around a 7-fold axis. The catalytic domain of the reported MTX(holo) closely resembles all three published structures, except one with an appreciable deviation of the 40 N-terminal residues. A comparison of all structures suggests a possible scenario for the translocation of the toxin into the cytosol.

Lack of cross-resistance to Mtx1 from Bacillus sphaericus in B. sphaericus-resistant Culex quinquefasciatus (Diptera: Culicidae)

The toxicities of Mtx1 toxin against dipteran and lepidopteran species have been evaluated in this study. It was shown that Mtx1 has little or no toxicity to the tested lepidopteran species, but has moderate-level toxicity to Aedes albopictus Skuse (Diptera: Culicidae) and high-level toxicity to both susceptible and binary toxin-resistant Culex quinquefasciatus Say (Diptera: Culicidae). The LC(50) values of Mtx1 against a susceptible C. quinquefasciatus colony SLCq and two resistant colonies RLCq1/C3-41 and RLCq2/IAB59 selected in the laboratory with Bacillus sphaericus (Mayer & Neide) strains C3-41 and IAB59 respectively were 0.508, 0.854 and 0.675 mg L(-1) respectively. The data indicate that Mtx1 has a different mode of action from the binary toxin, and that it could be an alternative toxin to delay or overcome resistance development to binary toxin in C. quinquefasciatus.

Bacillus sphaericus mosquitocidal toxin (MTX) and pierisin: the enigmatic offspring from the family of ADP-ribosyltransferases

The mosquitocidal toxin (MTX) from Bacillus sphaericus and the apoptosis-inducing pierisin-1 from the cabbage butterfly Pieris rapae are two of the most intriguing members of the family of ADP-ribosyltransferases. They are both approximately 100 kDa proteins, composed of an N-terminal ADP-ribosyltransferase (approximately 27 kDa) and a C-terminal putative binding and translocation domain (approximately 70 kDa) consisting of four ricin-B-like domains. While they both share structural homologies, with an overall amino acid sequence identity of approximately 30% that becomes approximately 50% at the level of the catalytic core, and functional similarities, notably in terms of enzyme regulation, they seem to largely differ with regard to their targets or cell internalization mechanisms. MTX ADP-ribosylates numerous proteins in lysates of target insect cells at arginine residues, whereas pierisin-1 modifies DNA of insect and mammalian cells by ADP-ribosylation at 2'-deoxyguanosine residues resulting in DNA adducts, mutations and eventually apoptosis. This target specificity differentiates pierisin-1 from all other ADP-ribosyltransferases described so far, and implies that the enzyme must reach the nucleus of target cells. The recently solved crystal structure of MTX catalytic domain is helpful to reveal new insights into structural organization, catalytic mechanisms, proteolytic activation and autoinhibition of both enzymes. The uptake and processing of the ADP-ribosyltransferases is discussed.

Mosquitocidal toxin from Bacillus sphaericus induces stronger delayed effects than binary toxin on Culex quinquefasciatus (Diptera: Culicidae)

We studied the toxicity and delayed effects of a mosquitocidal toxin (Mtx1) and a binary toxin (Bin) produced in Escherchia coli E-TH21 and Bacillus thuringiensis B-CW1, respectively, on Culex quinquefasciatus (Diptera: Culicidae). Bioassay results showed that both E-TH21 powder and B-CW1 sporulated culture were highly toxic against susceptible Cx. quinquefasciatus, with LC50 values of 0.65 and 1.70 mg/liter against third and fourth instars at 48 h, respectively. After initial 48-h exposure of larvae to different concentrations of Mtx1 and Bin, significant continued mortality could be observed in larval, pupal, and emergence stages of Cx. quinquefasciatus. Importantly, the Mtx1 could induce higher cumulative larval and preadult mortalities than Bin toxin on the target mosquito. This finding is important for understanding the mode of action of Mtx1 and Bin toxins and for developing a new bioassay procedure for evaluation of toxicity of Bacillus sphaericus Neide, some strains of which produce Mtx1 and Bin, in the laboratory and field.

Structure of the mosquitocidal toxin from Bacillus sphaericus

The catalytic domain of a mosquitocidal toxin prolonged by a C-terminal 44 residue linker connecting to four ricin B-like domains was crystallized. Three crystal structures were established at resolutions between 2.5A and 3.0A using multi-wavelength and single-wavelength anomalous X-ray diffraction as well as molecular replacement phasing techniques. The chainfold of the toxin fragment corresponds to those of ADP-ribosylating enzymes. At pH 4.3 the fragment is associated in a C(7)-symmetric heptamer in agreement with an aggregate of similar size observed by size-exclusion chromatography. In two distinct crystal forms, the heptamers formed nearly spherical, D(7)-symmetric tetradecamers. Another crystal form obtained at pH 6.3 contained a recurring C(2)-symmetric tetramer, which, however, was not stable in solution. On the basis of the common chainfold and NAD(+)-binding site of all ADP-ribosyl transferases, the NAD(+)-binding site of the toxin was assigned at a high confidence level. In all three crystal forms the NAD(+) site was occupied by part of the 44 residue linker, explaining the known inhibitory effect of this polypeptide region. The structure showed that the cleavage site for toxin activation is in a highly mobile loop that is exposed in the monomer. Since it contains the inhibitory linker as a crucial part of the association contact, the observed heptamer is inactive. Moreover, the heptamer cannot be activated by proteolysis because the activation loop is at the ring center and not accessible for proteases. Therefore the heptamer, or possibly the tetradecamer, seems to represent an inactive storage form of the toxin.