Does distant homology with Evf reveal a lipid binding site in Bacillus thuringiensis cytolytic toxins?

Serial femtosecond crystallography on in vivo-grown crystals drives elucidation of mosquitocidal Cyt1Aa bioactivation cascade

Cyt1Aa is the one of four crystalline protoxins produced by mosquitocidal bacterium Bacillus thuringiensis israelensis (Bti) that has been shown to delay the evolution of insect resistance in the field. Limiting our understanding of Bti efficacy and the path to improved toxicity and spectrum has been ignorance of how Cyt1Aa crystallizes in vivo and of its mechanism of toxicity. Here, we use serial femtosecond crystallography to determine the Cyt1Aa protoxin structure from sub-micron-sized crystals produced in Bti. Structures determined under various pH/redox conditions illuminate the role played by previously uncharacterized disulfide-bridge and domain-swapped interfaces from crystal formation in Bti to dissolution in the larval mosquito midgut. Biochemical, toxicological and biophysical methods enable the deconvolution of key steps in the Cyt1Aa bioactivation cascade. We additionally show that the size, shape, production yield, pH sensitivity and toxicity of Cyt1Aa crystals grown in Bti can be controlled by single atom substitution.

Bacillus thuringiensis israelensis (Bti) produces the naturally-crystalline proteinaceous toxin Cyt1Aa that is toxic to mosquito larvae. Here the authors grow recombinant nanocrystals of the Cyt1Aa protoxin in vivo and use serial femtosecond crystallography to determine its structure at different redox and pH conditions and by combining their structural data with further biochemical, toxicological and biophysical analyses provide mechanistic insights into the Cyt1Aa bioactivation cascade.

Structural characterization and heterologous expression of a new cyt gene cloned from Bacillus thuringiensis

Bacillus thuringiensis (Bt) strains produce Cry (crystal) and Cyt (cytolytic) proteins belonging to the group of bacterial toxins known as pore-forming toxins (PFTs), which interact with midgut cells of target insects to create pores, disruption of ion homeostasis and eventual death. PFTs have synergistic insecticidal activities and have been used as biopesticides against agriculturally important insects. Identification of new Cyt proteins is important because of their specific toxicity towards hemipteran pests, against which the Cry proteins are not effective. We have structurally characterized a cyt (cyt1007) gene from an Indian Bt isolate SK-1007. The presence of a "Bacillus thuringiensis toxin" domain and maximum identity of 36% with Cyt2Ca in the deduced amino acid sequence indicated Cyt1007 protein to be a new member of Cyt family. Three dimensional (3D) modeling (PMDB ID: PM0081490) revealed that it adopts a typical ferredoxin-like fold, and is composed of a single domain of α/β architecture, in which a single β sheet is surrounded by two α helical layers. The putative lipid binding site and probable mode of action of Cyt1007 protein were predicted through comparative analysis with other Cyt toxins and their distant homologs Evf (Erwinia virulence factor) and VVA2 (Volvatoxin A2). Heterologous expression of cyt1007 gene as a 25 kDa protein in Escherichia coli was achieved at high levels in both soluble and insoluble fractions. Affinity chromatography-based purification yielded 83.6% purified Cyt1007 protein, which can be used for downstream applications for the investigation of its toxicity. Graphical abstract Steps in the structural characterization and heterologous expression of a new cyt gene cloned from Bacillus thuringiensis.

Time influence on the interaction between Cyt2Aa2 and lipid/cholesterol bilayers

Protein-membrane interactions are still an important topic of investigation. One of the suitable experimental techniques used by the scientific community to address such question is atomic force microscopy. In a previous work, we have reported that the binding mechanism between the cytolytic and antimicrobial protein (Cyt2Aa2) and lipid/cholesterol bilayers was concentration-dependent, leading to either the formation of holes in the bilayer or aggregates. Here we study such binding mechanism as a function of time at low protein concentrations (10 µg/mL). We demonstrate that although holes are formed during the first stages of the protein-lipid interaction, a reparation process due to molecular mobility in the bilayer leads to a homogenous and isotropic protein-lipid/cholesterol layer within 3 hr. The combination of imaging, force spectroscopy, and phase contrast delivered information about topography dynamics (molecular mobility), layer thickness, and mechanical properties of the protein-lipid/cholesterol system. These results highlight the importance of the observation time in (such type of) protein-lipid interactions (at low protein concentrations).

Effect of the Concentration of Cytolytic Protein Cyt2Aa2 on the Binding Mechanism on Lipid Bilayers Studied by QCM-D and AFM

Bacillus thuringiensis is known by its insecticidal property. The insecticidal proteins are produced at different growth stages, including the cytolytic protein (Cyt2Aa2), which is a bioinsecticide and an antimicrobial protein. However, the binding mechanism (and the interaction) of Cyt2Aa2 on lipid bilayers is still unclear. In this work, we have used quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) to investigate the interaction between Cyt2Aa2 protein and (cholesterol-)lipid bilayers. We have found that the binding mechanism is concentration dependent. While at 10 μg/mL, Cyt2Aa2 binds slowly on the lipid bilayer forming a compliance protein/lipid layer with aggregates, at higher protein concentrations (100 μg/mL), the binding is fast, and the protein/lipid layer is more rigid including holes (of about a lipid bilayer thickness) in its structure. Our study suggests that the protein/lipid bilayer binding mechanism seems to be carpet-like at low protein concentrations and pore forming-like at high protein concentrations.

New Cyt-like δ-endotoxins from Dickeya dadantii: structure and aphicidal activity

In the track of new biopesticides, four genes namely cytA, cytB, cytC and cytD encoding proteins homologous to Bacillus thuringiensis (Bt) Cyt toxins have been identified in the plant pathogenic bacteria Dickeya dadantii genome. Here we show that three Cyt-like δ-endotoxins from D. dadantii (CytA, CytB and CytC) are toxic to the pathogen of the pea aphid Acyrthosiphon pisum in terms of both mortality and growth rate. The phylogenetic analysis of the comprehensive set of Cyt toxins available in genomic databases shows that the whole family is of limited taxonomic occurrence, though in quite diverse microbial taxa. From a structure-function perspective the 3D structure of CytC and its backbone dynamics in solution have been determined by NMR. CytC adopts a cytolysin fold, structurally classified as a Cyt2-like protein. Moreover, the identification of a putative lipid binding pocket in CytC structure, which has been probably maintained in most members of the Cyt-toxin family, could support the importance of this lipid binding cavity for the mechanism of action of the whole family. This integrative approach provided significant insights into the evolutionary and functional history of D. dadantii Cyt toxins, which appears to be interesting leads for biopesticides.

Bacillus thuringiensis toxins: an overview of their biocidal activity

Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

Cyt1Aa toxin: crystal structure reveals implications for its membrane-perforating function

During sporulation, Bacillus thuringiensis subsp. israelensis produces a mosquito larvicidal protein complex containing several crystalline and cytolytic (Cyt) toxins. Here, the activated monomeric form of Cyt1Aa, the most toxic Cyt family member, was isolated and crystallized, and its structure was determined for the first time at 2.2 Å resolution. Cyt1Aa adopts a typical cytolysin fold containing a β-sheet held by two surrounding α-helical layers. The absence of a β-strand (between residues V26 and I37) in the dimeric structure of Cyt2Aa led us to deduce that this is the only essential segment for dimer formation and that activation of the toxin occurs by proteolytic processing of its N-terminus. Based on the Cyt1Aa structure, we suggest that the toxicity of Cyt1Aa and other nonrelated proteins, all sharing a cytolysin fold, is correlated with their ability to undergo conformational changes that are necessary prior to their membrane insertion and perforation. This fold allows the α-helical layers to swing away, exposing the β-sheet to insert into the membrane. The identification of a putative lipid binding pocket between the β-sheet and the helical layer of Cyt1Aa supports this mechanism. Sequence-based structural analysis of Cyt1Aa revealed that the lack of activity of Cyt1Ca may be related to the latter's inability to undergo this conformational change due to its lack of flexibility. The pattern of the hemolytic activity of Cyt1Aa presented here (resembling that of pore-forming agents), while differing from that imposed by ionic and nonionic detergents, further supports the pore-forming model by which conformational changes occur prior to membrane insertion and perforation.