title: Protein engineering of Bacillus thuringiensis delta-endotoxin: mutations at domain II of CryIAb enhance receptor affinity and toxicity toward gypsy moth larvae.
abstract: Substitutions or deletions of domain II loop residues of Bacillus thuringiensis delta-endotoxin CryIAb were constructed using site-directed mutagenesis techniques to investigate their functional roles in receptor binding and toxicity toward gypsy moth (Lymantria dispar). Substitution of loop 2 residue N372 with Ala or Gly (N372A, N372G) increased the toxicity against gypsy moth larvae 8-fold and enhanced binding affinity to gypsy moth midgut brush border membrane vesicles (BBMV) approximately 4-fold. Deletion of N372 (D3), however, substantially reduced toxicity (> 21 times) as well as binding affinity, suggesting that residue N372 is involved in receptor binding. Interestingly, a triple mutant, DF-1 (N372A, A282G and L283S), has a 36-fold increase in toxicity to gypsy moth neonates compared with wild-type toxin. The enhanced activity of DF-1 was correlated with higher binding affinity (18-fold) and binding site concentrations. Dissociation binding assays suggested that the off-rate of the BBMV-bound mutant toxins was similar to that of the wild type. However, DF-1 toxin bound 4 times more than the wild-type and N372A toxins, and it was directly correlated with binding affinity and potency. Protein blots of gypsy moth BBMV probed with labeled N372A, DF-1, and CryIAb toxins recognized a common 210-kDa protein, indicating that the increased activity of the mutants was not caused by binding to additional receptor(s). The improved binding affinity of N372A and DF-1 suggest that a shorter side chain at these loops may fit the toxin more efficiently to the binding pockets. These results offer an excellent model system for engineering delta-endotoxins with higher potency and wider spectra of target pests by improving receptor binding interactions.
authors.affiliation: Department of Biochemistry, Ohio State University, Columbus 43210-1292, USA.
authors.forename: F
authors.initials: F
authors.lastname: Rajamohan
authors.forename: O
authors.initials: O
authors.lastname: Alzate
authors.forename: J A
authors.initials: JA
authors.lastname: Cotrill
authors.forename: A
authors.initials: A
authors.lastname: Curtiss
authors.forename: D H
authors.initials: DH
authors.lastname: Dean
mesh_terms: D000818:Animals; D001413:Bacillus thuringiensis / Q000378:metabolism*; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism / Q000633:toxicity; D001427:Bacterial Toxins / Q000235:genetics* / Q000378:metabolism / Q000633:toxicity; D001665:Binding Sites / Q000235:genetics; D004731:Endotoxins / Q000235:genetics* / Q000378:metabolism / Q000633:toxicity; D006460:Hemolysin Proteins; D007814:Larva / Q000382:microbiology; D009036:Moths / Q000382:microbiology*; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D015202:Protein Engineering; D011971:Receptors, Immunologic / Q000378:metabolism*
chemical_list: D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D004731:Endotoxins; D006460:Hemolysin Proteins; D011971:Receptors, Immunologic; C048071:endotoxin receptor; C030956:insecticidal crystal protein, Bacillus Thuringiensis