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 th is strain (Bin4) with those of the three other known toxin types (Bin1, Bin 2 and Bin3) revealed mutations at six positions, including a serine at posi tion 93 of BinA4, whereas all other types of BinA toxin from B. sphaericus had a leucine at this position. Reciprocal site-directed mutagenesis was pe rformed to replace this serine in BinA4 from LP1-G with a leucine and the l eucine in the BinA2 protein from strain 1593 with a serine. Native and muta ted genes were cloned and overexpressed. Inclusion bodies were tested on C. pipiens larvae. Unlike the native Bin4 toxin, the mutated protein was toxi c, and the reciprocal mutation in Bin2 led to a significant loss of toxicit y. In vitro receptor-binding studies showed similar binding behaviour for n ative and mutated toxins. In the absence of any experimental data on the 3D structure of these proteins, sequence analysis and secondary-structure pre dictions were performed. Amino acid 93 of the BinA polypeptide probably bel ongs to an alpha helix that is sensitive to amino-acid modifications. Posit ion 93 may be a key element in the formation of the BinA-BinB complex respo nsible for the toxicity and stability of B. sphaericus Bin toxins.