Functional complementation of nontoxic mutant binary toxins of Bacillus sphaericus 1593M generated by site-directed mutagenesis

Amino acid residues in the N-terminal region of the BinB subunit of Lysinibacillus sphaericus binary toxin play a critical role during receptor binding and membrane insertion

The binary toxin produced by Lysinibacillus sphaericus is composed of BinA and BinB subunits that work together in governing toxicity against mosquito larvae. BinA is proposed to be important for toxicity, whereas BinB has been shown to act as a specific receptor-binding component. The precise function of both subunits, however, is not well established. Here, we investigated the function of the N-terminal region of BinB subunit initially by introducing triple alanine substitutions at positions 35PEI37 and 41FYN43. Both block mutations abolished the larvicidal activity. Single point mutations (P35A, E36A, I37A, F41A, Y42A, N43A) were generated in order to identify amino acids that are critical for the toxin activity. Mosquito-larvicidal activity was significantly reduced in P35A, E36A, F41A and Y42A mutants. However, these mutants retained ability to form in vitro interaction with the BinA counterpart. Immunohistochemistry analysis revealed that P35A, F41A and N43A bind to the larval midgut membrane at comparable levels to that of the wild type BinB. In contrast, greatly reduced binding activity was observed in the Y42A, suggesting an important role of this residue in receptor binding. Alanine substitution at P35 resulted in a marked decrease in membrane penetration, indicating its functional importance for the membrane insertion. These results suggest the important roles of the N-terminal region of BinB in both the receptor recognition and the membrane interaction.

Identification of mosquito larvicidal bacterial strains isolated from north Sinai in Egypt

In the present study, two of the most toxic bacterial strains of Bacillus sphaericus against mosquito were identified with the most recent genetic techniques. The PCR product profiles indicated the presence of genes encoding Bin A, Bin B and Mtx1 in all analyzed strains; they are consistent with protein profiles. The preliminary bioinformatics analysis of the binary toxin genes sequence revealed that the open reading frames had high similarities when matched with nucleotides sequence in the database of other B. sphaericus strains. The biological activity of B. sphaericus strains varied according to growing medium, and cultivation time. The highest yield of viable counts, spores and larvicidal protein were attained after 5 days. Poly (P) medium achieved the highest yield of growth, sporulation, protein and larvicidal activity for all tested strains compared to the other tested media. The larvicidal protein produced by local strains (B. sphaericus EMCC 1931 and EMCC 1932) in P medium was more lethal against the 3rd instar larvae of Culex pipiens than that of reference strains (B. sphaericus 1593 and B. sphaericus 2297). The obtained results revealed that P medium was the most effective medium and will be used in future work in order to optimize large scale production of biocide by the locally isolated Bacillus sphaericus strains.

The bacterium, Lysinibacillus sphaericus, as an insect pathogen

Since the first bacteria with insecticidal activity against mosquito larvae were reported in the 1960s, many have been described, with the most potent being isolates of Bacillus thuringiensis or Lysinibacillus sphaericus (formerly and best known as Bacillus sphaericus). Given environmental concerns over the use of broad spectrum synthetic chemical insecticides and the evolution of resistance to these, industry placed emphasis on the development of bacteria as alternative control agents. To date, numerous commercial formulations of B. thuringiensis subsp. israelensis (Bti) are available in many countries for control of nuisance and vector mosquitoes. Within the past few years, commercial formulations of L. sphaericus (Ls) have become available. Because Bti has been in use for more than 30 years, its properties are well know, more so than those of Ls. Thus, the purpose of this review is to summarise the most critical aspects of Ls and the various proteins that account for its insecticidal properties, especially the mosquitocidal activity of the most common isolates studied. Data are reviewed for the binary toxin, which accounts for the activity of sporulated cells, as well as for other toxins produced during vegetative growth, including sphaericolysin (active against cockroaches and caterpillars) and the different mosquitocidal Mtx and Cry toxins. Future studies of these could well lead to novel potent and environmentally compatible insecticidal products for controlling a range of insect pests and vectors of disease.

Implications of high-molecular-weight oligomers of the binary toxin from Bacillus sphaericus

The mosquito-larvicidal binary toxin produced by Bacillus sphaericus is composed of BinB and BinA, which have calculated molecular weights of 51.4 and 41.9 kDa, respectively. NaOH extracts of B. sphaericus spores were analyzed using SDS-PAGE. Stained gels showed bands with molecular weights corresponding to those of BinB and BinA as well as two additional bands at 110 and 125 kDa. The matrix-assisted laser desorption/ionization mass spectrum of the purified 110 and 125 kDa bands showed two peaks at 104,160 and 87,358 Da that are assigned to dimers of BinB and BinA, respectively. Mass spectral analysis of trypsin-digested 110 and 125 kDa bands showed peaks at 51,328, 43,523, 43,130, and 40,832 Da that assigned to undigested BinB, two forms of digested BinB and digested BinA, respectively. Dynamic light scattering studies showed a solution of the purified 110 and 125 kDa bands was comprised almost entirely (99.6% of total mass) of a particle with a hydrodynamic radius of 5.6+/-1.2 nm and a calculated molecular weight of 186+/-38 kDa. These data demonstrate that the binary toxin extracted from B. sphaericus spores can exist in solution as an oligomer containing two copies each of BinB and BinA.

Identification and molecular structural prediction analysis of a toxicity determinant in the Bacillus sphaericus crystal larvicidal toxin

The operon containing the genes encoding the subunits of the binary crystal toxin of Bacillus sphaericus strain LP1-G, BinA and BinB (41.9 kDa and 51.4 kDa, respectively), was cloned and sequenced. Purified crystals were not toxic to Culex pipiens larvae. Comparison of the amino-acid sequences of this strain (Bin4) with those of the three other known toxin types (Bin1, Bin2 and Bin3) revealed mutations at six positions, including a serine at position 93 of BinA4, whereas all other types of BinA toxin from B. sphaericus had a leucine at this position. Reciprocal site-directed mutagenesis was performed to replace this serine in BinA4 from LP1-G with a leucine and the leucine in the BinA2 protein from strain 1593 with a serine. Native and mutated genes were cloned and overexpressed. Inclusion bodies were tested on C. pipiens larvae. Unlike the native Bin4 toxin, the mutated protein was toxic, and the reciprocal mutation in Bin2 led to a significant loss of toxicity. In vitro receptor-binding studies showed similar binding behaviour for native and mutated toxins. In the absence of any experimental data on the 3D structure of these proteins, sequence analysis and secondary-structure predictions were performed. Amino acid 93 of the BinA polypeptide probably belongs to an alpha helix that is sensitive to amino-acid modifications. Position 93 may be a key element in the formation of the BinA-BinB complex responsible for the toxicity and stability of B. sphaericus Bin toxins.

Evidence for intermolecular interaction as a necessary step for pore-formation activity and toxicity of Bacillus thuringiensis Cry1Ab toxin

Based on the observation of large conductance states formed by Bacillus thuringiensis Cry toxins in synthetic planar lipid bilayers and the estimation of a pore size of 10-20 A, it has been proposed that the pore could be formed by an oligomer containing four to six Cry toxin monomers. However, there is a lack of information regarding the insertion of Cry toxins into the membrane and oligomer formation. Here we provide direct evidence showing that the intermolecular interaction between Cry1Ab toxin monomers is a necessary step for pore formation and toxicity. Two Cry1Ab mutant proteins affected in different steps of their mode of action (F371A in receptor binding and H168F in pore formation) were affected in toxicity against Manduca sexta larvae. Binding analysis showed that F371A protein bound more efficiently to M. sexta brush border membrane vesicles when mixed with H168F in a one to one ratio. These mutant proteins also recovered pore-formation activity, measured with a fluorescent dye with isolated brush border membrane vesicles, and toxicity against M. sexta larvae when mixed, showing that monomers affected in different steps of their mode of action can form functional hetero-oligomers.

Identification of the functional site in the mosquito larvicidal binary toxin of Bacillus sphaericus 1593M by site-directed mutagenesis

To study the mode of action of the binary toxin (51- and 42-kDa) of Bacillus sphaericus, amino acid residues were substituted at selected sites of the N- and C-terminal regions of both peptides. Bioassay results of the mutant binary toxins tested against mosquito larvae, Culex quinquefasciatus, revealed that most of the substitutions made on both peptides led to either decrease or total loss of the activity. Furthermore, receptor binding studies carried out for some of the mutants of the 42-kDa peptide showed mutations in N- and C-terminal regions of the 42-kDa peptide did not affect the binding of the binary toxin to brush border membrane vesicles of mosquito larvae. One of the mutants having a single amino acid substitution at the C-terminal region ((312)R) of the 42-kDa peptide completely abolished the biological activity, implicating the role of this residue in membrane pore formation. These results indicate the importance of the C-terminal region of the 42-kDa of binary toxin, in general, and particularly the residue (312)R for biological activity against mosquito larvae.