Characterization of a new cry2Ab gene of Bacillus thuringiensis with high insecticidal activity against Plutella xylostella L

Engineering of Bacillus thuringiensis Cry2Ab toxin for improved insecticidal activity

Bacillus thuringiensis Cry2Ab toxin was a widely used bioinsecticide to control lepidopteran pests all over the world. In the present study, engineering of Bacillus thuringiensis Cry2Ab toxin was performed for improved insecticidal activity using site-specific saturation mutation. Variants L183I were screened with lower LC50 (0.129 µg/cm2) against P. xylostella when compared to wild-type Cry2Ab (0.267 µg/cm2). To investigate the molecular mechanism behind the enhanced activity of variant L183I, the activation, oligomerization and pore-formation activities of L183I were evaluated, using wild-type Cry2Ab as a control. The results demonstrated that the proteolytic activation of L183I was the same as that of wild-type Cry2Ab. However, variant L183I displayed higher oligomerization and pore-formation activities, which was consistence with its increased insecticidal activity. The current study demonstrated that the insecticidal activity of Cry2Ab toxin could be assessed using oligomerization and pore-formation activities, and the screened variant L183I with improved activity might contribute to Cry2Ab toxin's future application.

Key residues of Bacillus thuringiensis Cry2Ab for oligomerization and pore-formation activity

As a pore-forming toxin, activation, oligomerization and pore-formation were both required for the mode of action of Cry toxins. Previous results revealed that the helices α4-α5 of Domain I were involved in the oligomerization of Cry2Ab, however, the key residues for Cry2Ab aggregation remained ambiguous. In present studies, we built 20 Cry2Ab alanine mutants site-directed in the helices α4-α5 of Domain I and demonstrated that mutants N151A, T152A, F157A, L183A, L185A and I188A could reduce the assembly of the 250 kDa oligomers, suggesting that these mutation residues might be essential for Cry2Ab oligomerization. As expected, all of these variants showed lower insecticidal activity against P. xylostella. Furthermore, we found that the pore-forming activities of these mutants also decreased when compared to wild-type Cry2Ab. Taken together, our data identified key residues for Cry2Ab oligomerization and emphasized that oligomerization was closely related to the insecticidal activity and pore-forming activity of Cry2Ab.

Characterization of the Expression and Functions of Two Odorant-Binding Proteins of Sitophilus zeamais Motschulsky (Coleoptera: Curculionoidea)

Odorant-binding proteins (OBPs) are important in insect chemical communication. The objective of this research was to identify the functions of two OBPs in Sitophilus zeamais. qRT-PCR and western blot (WB) were performed to investigate the expression profiles at the transcript and protein levels, respectively. Fluorescence competitive binding assays were used to measure the ability of the OBPs to bind to host volatiles, and a Y-tube olfactometer was used to verify the results (attraction/no response) via behavioral experiments. The RNAi was used to verify the function by knocking down the ability of proteins to bind odorants. qRT-PCR showed the highest expression SzeaOBP1 and SzeaOBP28 at the low-instar larva (LL) and eclosion adult (EA) stages, respectively. WB showed that both SzeaOBP1 and SzeaOBP28 were highly expressed in the EA stage. Fluorescence competitive binding assays indicated that SzeaOBP1 exhibited extremely high binding affinity with cetanol. SzeaOBP28 exhibited a pronounced binding affinity for 4-hydroxy-3-methoxybenzaldehyde. The behavioral experiment showed that the adult S. zeamais responded strongly to 4-hydroxy-3-methoxybenzaldehyde and valeraldehyde from Sorghum bicolor. The RNAi knockdown individuals displayed behavioral differences between normal insects and dsRNA (SzeaOBP1)-treated insects. We infer that they both have functions in perception and recognition of host volatiles, whereas SzeaOBP28 may also have other functions.

Synthesis and Characterization of Cry2Ab-AVM Bioconjugate: Enhanced Affinity to Binding Proteins and Insecticidal Activity

Bacillus thuringiensis insecticidal proteins (Bt toxins) have been widely used in crops for agricultural pest management and to reduce the use of chemical insecticides. Here, we have engineered Bt toxin Cry2Ab30 and bioconjugated it with 4"-O-succinyl avermectin (AVM) to synthesize Cry2Ab-AVM bioconjugate. It was found that Cry2Ab-AVM showed higher insecticidal activity against Plutella xylostella, up to 154.4 times compared to Cry2Ab30. The binding results showed that Cry2Ab-AVM binds to the cadherin-like binding protein fragments, the 10th and 11th cadherin repeat domains in the P. xylostella cadherin (PxCR_10-11), with a much higher affinity (dissociation equilibrium constant K_D = 3.44 nM) than Cry2Ab30 (K_D = 28.7 nM). Molecular docking suggested that the macrolide lactone group of Cry2Ab-AVM ligand docking into the PxCR_10-11 is a potential mechanism to enhance the binding affinity of Cry2Ab-AVM to PxCR_10-11. These findings offer scope for the engineering of Bt toxins by bioconjugation for improved pest management.

Molecular and structural characterization of a novel Cry1D toxin from Bacillus thuringiensis with high toxicity to Spodoptera littoralis (Lepidoptera: Noctuidae)

The investigation of new Bacillus thuringiensis (Bt) insecticidal proteins (Cry) with specific toxicity is one of the alternative measures used for Lepidopteran pest control. In the present study, a new Cry toxin was identified from a promising Bt strain BLB250 which was previously selected for its high toxicity against Spodoptera littoralis. The corresponding gene, designated cry1D-250, was cloned. It showed an ORF of 3498bp, encoding a protein of 1165 amino acid residues with a putative molecular mass of 132kDa which was confirmed by SDS-PAGE and Western blot analyses. The corresponding toxin named Cry1D-250 showed a higher insecticidal activity towards S. littoralis than Cry1D-133 (LC₅₀ of 224.4ngcm^-2) with an LC₅₀ of only 166ngcm^-2. Besides to the 65kDa active toxin, proteolysis activation of Cry1D-133 protein with S. littoralis midgut juice generated an extra form of 56kDa, which was the result of a second cleavage. Via activation study and 3D structure analysis, novel substitutions found in the Cry1D-250 protein compared to Cry1D-133 toxin were shown to be involved in the protein stability and toxicity. Therefore, the Cry1D-250 toxin can be considered to be an effective alternative for the control of S. littoralis.

Exposure of helices α4 and α5 is required for insecticidal activity of Cry2Ab by promoting assembly of a prepore oligomeric structure

Cry2Ab, a pore-forming toxin derived from Bacillus thuringiensis, is widely used as a bio-insecticide to control lepidopteran pests around the world. A previous study revealed that proteolytic activation of Cry2Ab by Plutella xylostella midgut juice was essential for its insecticidal activity against P. xylostella, although the exact molecular mechanism remained unknown. Here, we demonstrated for the first time that proteolysis of Cry2Ab uncovered an active region (the helices α4 and α5 in Domain I), which was required for the mode of action of Cry2Ab. Either the masking or the removal of helices α4 and α5 mediated the pesticidal activity of Cry2Ab. The exposure of helices α4 and α5 did not facilitate the binding of Cry2Ab to P. xylostella midgut receptors but did induce Cry2Ab monomer to aggregate and assemble a 250-kDa prepore oligomer. Site-directed mutagenesis assay was performed to generate Cry2Ab mutants site directed on the helices α4 and α5, and bioassays suggested that some Cry2Ab variants that could not form oligomers had significantly lowered their toxicities against P. xylostella. Taken together, our data highlight the importance of helices α4 and α5 in the mode of action of Cry2Ab and could lead to more detailed studies on the insecticidal activity of Cry2Ab.

Molecular characterization of Cry1D-133 toxin from Bacillus thuringiensis strain HD133 and its toxicity against Spodoptera littoralis

Bacillus thuringiensis subsp. aizawai strain HD133, known by its effectiveness against Spodoptera species, produces bipyramidal crystals encompassing the insecticidal proteins Cry1Ab, Cry1Ca and Cry1D-133 in the proportions 60:37:3, respectively. In this study, we dealt with the relevance of the low rate of Cry1D-133. The cry1D-133 gene from HD133 was cloned and sequenced. Both nucleotide and amino acid sequence similarity analyses with the cry1D genes available in the GenBank database revealed that cry1D-133 is a new variant of cry1Da-type genes with 99% identity with cry1Da1. Molecular modeling of the Cry1D-133 toxin showed that its higher toxicity is correlated to a higher number of toxin-receptor interactions. Optimal culture conditions of 4 h post-induction time, 160 rpm agitation and 37 °C post-induction temperature were determined and adopted to overproduce Cry1D-133 toxin at adequate amounts to carryout bioassays. A gradual increase of the proportion of Cry1D-133 to the HD133 insecticidal proteins forming the crystal (Cry1D-133, Cry1Ca and Cry1Ab) showed an improvement of the toxicity against Spodoptera littoralis larvae. Therefore, the potential of Cry1D-133 to enhance HD133 toxicity could promote its combination with other B. thuringiensis insecticidal proteins toxins in order to increase target range or to delay the emergence of resistance.

Proteolytic activation of Bacillus thuringiensis Vip3Aa protein by Spodoptera exigua midgut protease

Proteolysis of Vip3Aa by insect midgut proteases is essential for their toxicity against target insects. In the present study, proteolysis of Vip3Aa was evaluated by Spodoptera exigua midgut proteases (MJ). Trypsin was verified involved in the activation of Vip3Aa and three potential cleavage sites (Lys¹⁹⁵, Lys¹⁹⁷ and Lys¹⁹⁸) were identified. Four Vip3Aa mutants (KKK195197198AAA, KK197198AA, KK195198AA and KK195197AA) were designed and constructed by replacing residues Lys^195,197,198, Lys^197,198, Lys^195,198 and Lys^195,197 with Ala, respectively. Proteolytic processing assays revealed that mutants KK197198AA, KK195198AA and KK195197AA could be processed into 66kDa activated toxins by trypsin or MJ while mutant KKK195197198AAA was not cleaved by trypsin and less susceptible to MJ. Bioassays demonstrated that mutants KK197198AA, KK195198AA and KK195197AA were toxic against S. exigua resembled that of wild-type Vip3Aa, however, the LC₅₀ of mutant KKK195197198AAA against S. exigua was higher than wild-type. Those results suggested that proteolysis by MJ was associated with the insecticidal toxicity of Vip3Aa against S. exigua. It also revealed that trypsin played an important role in the formation of Vip3Aa activated toxin. Our studies characterized the proteolytic processing of Vip3Aa and provided new insight into the activation of this novel Bt toxin.

Microbial Biofertilizer Decreases Nicotine Content by Improving Soil Nitrogen Supply

Biofertilizers have been widely used in many countries for their benefit to soil biological and physicochemical properties. A new microbial biofertilizer containing Phanerochaete chrysosporium and Bacillus thuringiensis was prepared to decrease nicotine content in tobacco leaves by regulating soil nitrogen supply. Soil NO3--N, NH4+-N, nitrogen supply-related enzyme activities, and nitrogen accumulation in plant leaves throughout the growing period were investigated to explore the mechanism of nicotine reduction. The experimental results indicated that biofertilizer can reduce the nicotine content in tobacco leaves, with a maximum decrement of 16-18 % in mature upper leaves. In the meantime, the total nitrogen in mature lower and middle leaves increased with the application of biofertilizer, while an opposite result was observed in upper leaves. Protein concentration in leaves had similar fluctuation to that of total nitrogen in response to biofertilizer. NO3--N content and nitrate reductase activity in biofertilizer-amended soil increased by 92.3 and 42.2 %, respectively, compared to those in the control, whereas the NH4+-N and urease activity decreased by 37.8 and 29.3 %, respectively. Nitrogen uptake was improved in the early growing stage, but this phenomenon was not observed during the late growth period. Nicotine decrease is attributing to the adjustment of biofertilizer in soil nitrogen supply and its uptake in tobacco, which result in changes of nitrogen content as well as its distribution in tobacco leaves. The application of biofertilizer containing P. chrysosporium and B. thuringiensis can reduce the nicotine content and improve tobacco quality, which may provide some useful information for tobacco cultivation.

Proteolytic Activation of Bacillus thuringiensis Cry2Ab through a Belt-and-Braces Approach

Proteolytic processing of Bacillus thuringiensis (Bt) crystal toxins by insect midgut proteases plays an essential role in their insecticidal toxicities against target insects. In the present study, proteolysis of Bt crystal toxin Cry2Ab by Plutella xylostella L. midgut proteases (PxMJ) was evaluated. Both trypsin and chymotrypsin were identified involving the proteolytic activation of Cry2Ab and cleaving Cry2Ab at Arg(139) and Leu(144), respectively. Three Cry2Ab mutants (R139A, L144A, and R139A-L144A) were constructed by replacing residues Arg(139), Leu(144), and Arg(139)-Leu(144) with alanine. Proteolysis assays revealed that mutants R139A and L144A but not R139A-L144A could be cleaved into 50 kDa activated toxins by PxMJ. Bioassays showed that mutants R139A and L144A were highly toxic against P. xylostella larvae, while mutant R139A-L144A was almost non-insecticidal. Those results demonstrated that proteolysis by PxMJ was associated with the toxicity of Cry2Ab against P. xylostella. It also revealed that either trypsin or chymotrypsin was enough to activate Cry2Ab protoxin. This characteristic was regarded as a belt-and-braces approach and might contribute to the control of resistance development in target insects. Our studies characterized the proteolytic processing of Cry2Ab and provided new insight into the activation of this Bt toxin.

Transgenic Cotton Plants Expressing Cry1Ia12 Toxin Confer Resistance to Fall Armyworm (Spodoptera frugiperda) and Cotton Boll Weevil (Anthonomus grandis)

Gossypium hirsutum (commercial cooton) is one of the most economically important fibers sources and a commodity crop highly affected by insect pests and pathogens. Several transgenic approaches have been developed to improve cotton resistance to insect pests, through the transgenic expression of different factors, including Cry toxins, proteinase inhibitors, and toxic peptides, among others. In the present study, we developed transgenic cotton plants by fertilized floral buds injection (through the pollen-tube pathway technique) using an DNA expression cassette harboring the cry1Ia12 gene, driven by CaMV35S promoter. The T0 transgenic cotton plants were initially selected with kanamycin and posteriorly characterized by PCR and Southern blot experiments to confirm the genetic transformation. Western blot and ELISA assays indicated the transgenic cotton plants with higher Cry1Ia12 protein expression levels to be further tested in the control of two major G. hirsutum insect pests. Bioassays with T1 plants revealed the Cry1Ia12 protein toxicity on Spodoptera frugiperda larvae, as evidenced by mortality up to 40% and a significant delay in the development of the target insects compared to untransformed controls (up to 30-fold). Also, an important reduction of Anthonomus grandis emerging adults (up to 60%) was observed when the insect larvae were fed on T1 floral buds. All the larvae and adult insect survivors on the transgenic lines were weaker and significantly smaller compared to the non-transformed plants. Therefore, this study provides GM cotton plant with simultaneous resistance against the Lepidopteran (S. frugiperda), and the Coleopteran (A. grandis) insect orders, and all data suggested that the Cry1Ia12 toxin could effectively enhance the cotton transgenic plants resistance to both insect pests.

High-level expression, purification, and enzymatic characterization of truncated human plasminogen (Lys531-Asn791) in the methylotrophic yeast Pichia pastoris

Background

Plasmin is a serine protease that plays a critical role in fibrinolysis, which is a process that prevents blood clots from growing and becoming problematic. Recombinant human microplasminogen (rhμPlg) is a derivative of plasmin that solely consists of the catalytic domain of human plasmin and lacks the five kringle domains found in the native protein. Developing an industrial production method that provides high yields of this protein with high purity, quality, and potency is critical for preclinical research.

Results

The human microplasminogen gene was cloned into the pPIC9K vector, and the recombinant plasmid was transformed into Pichia pastoris strain GS115. The concentration of plasmin reached 510.1 mg/L of culture medium. Under fermentation conditions, the yield of rhμPlg was 1.0 g/L. We purified rhμPlg to 96 % purity by gel-filtration and cation-exchange chromatography. The specific activity of rhμPlg reached 23.6 U/mg. The K_m of substrate hydrolysis by recombinant human microplasmin was comparable to that of human plasmin, while rhμPlm had higher k_cat/Km values than plasmin. The high purity and activity of the rhμPlg obtained here will likely prove to be a valuable tool for studies of its application in thrombotic diseases and vitreoretinopathies.

Conclusions

Reliable rhμPlg production (for use in therapeutic applications) is feasible using genetically modified P. pastoris as a host strain. The successful expression of rhμPlg in P. pastoris lays a solid foundation for its downstream application.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0179-z) contains supplementary material, which is available to authorized users.