title: Improvement of crystal solubility and increasing toxicity against Caenorhabditis elegans by asparagine substitution in block 3 of Bacillus thuringiensis crystal protein Cry5Ba.
abstract: The crystal proteins from Bacillus thuringiensis are widely used for their specific toxicity against insects and nematodes. The highly conserved sequence blocks play an important role in Cry protein stability and flexibility, the basis of toxicity. The block 3 in Cry5Ba subfamily has a shorter sequence (only 12 residues) and more asparagine residues than that of others which harbor about 48 residues but only one asparagine. Based on the theoretical structure model of Cry5Ba, all three asparagines in block 3 are closely located in the interface of putative three domains, implying their probable importance in structure and function. In this study, all three asparagines in Cry5Ba2 block 3 were individually substituted with alanine by site-directed mutagenesis. The wild-type and mutant proteins were overexpressed and crystallized in acrystalliferous B. thuringiensis strain BMB171. However, the crystals formed in one of the mutants, designated N586A, abnormally disappeared and dissolved into the culture supernatant once the sporulation cells lysed, whereas the Cry5Ba crystal and the other mutant crystals were stable. The mutant N586A crystal, isolated from sporulation cells by the ultrasonic process, was found to be easily dissolved at wide range of pH value (5.0 to 10.0). Moreover, the toxicity assays showed that the mutant N586A exhibited nearly 9-fold-higher activity against nematodes and damaged the host's intestine more efficiently than the native Cry5Ba2. These data support the presumption that the amide residue Asn586 at the interface of domains might adversely affect the protein flexibility, solubility and resultant toxicity of Cry5Ba.
authors.affiliation: State Key Laboratory of Agricultural Microbiology, College of Life Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China.
authors.forename: Fenshan
authors.initials: F
authors.lastname: Wang
authors.forename: Yingying
authors.initials: Y
authors.lastname: Liu
authors.forename: Fengjuan
authors.initials: F
authors.lastname: Zhang
authors.forename: Lujun
authors.initials: L
authors.lastname: Chai
authors.forename: Lifang
authors.initials: L
authors.lastname: Ruan
authors.forename: Donghai
authors.initials: D
authors.lastname: Peng
authors.forename: Ming
authors.initials: M
authors.lastname: Sun
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D001216:Asparagine / Q000235:genetics; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000633:toxicity*; D017173:Caenorhabditis elegans / Q000187:drug effects* / Q000502:physiology*; D003460:Crystallization; D004731:Endotoxins / Q000737:chemistry* / Q000235:genetics / Q000633:toxicity*; D006460:Hemolysin Proteins / Q000737:chemistry* / Q000235:genetics / Q000633:toxicity*; D006863:Hydrogen-Ion Concentration; D016297:Mutagenesis, Site-Directed; D012995:Solubility; D016019:Survival Analysis
chemical_list: D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins; D004731:Endotoxins; D006460:Hemolysin Proteins; C030956:insecticidal crystal protein, Bacillus Thuringiensis; D001216:Asparagine