Directed evolution of the Bacillus thuringiensis insecticidal toxin for the creation of proteinaceous insecticide.

点击翻译按钮获取中文摘要

In AB-type toxins, which consist of active and binding domains, activity should theoretically be improved if binding affinity to the receptor is improved. The insecticidal toxins (Cry toxins) from B.thuringiensis are AB-type toxins that have been used as organic insecticides and as sources for the development of transgenic plants. In the present report, we describe a method for directed evolution of Cry toxin. This method can be used to increase the binding affinity of Cry toxins for their receptors in target insect pests. Using a commercial T7 phage-display system, we expressed Cry1Aa toxin on the phage surface as fusions with the capsid protein 10B. These recombinant phages bound to a cadherin-like protein that is one of the Cry1Aa toxin receptors in the model target insect Bombyx mori. The affinity of Cry1Aa-expressing phage for the receptor was higher than that of Cry1Ab-expressing phage. Toxin-expressing phages were effectively isolated from T7 wild-type phages using magnetic beads coated with cadherin-like protein. Phages expressing Cry1Aa were isolated from a mixed suspension of phages expressing Cry1Ab and concentrated by up to 1.3 x 10^<5->fold. Finally, random mutations were made in amino acid residues 369-375 in domain 2 of Cry1Aa toxin, the mutant toxins were expressed on phages, and the resulting phage library was screened five times with cadherin-like protein-coated beads. As a result, phages expressing abnormal or low-affinity mutant toxins were excluded, and phages with high-affinity mutant toxins were selected. The insecticidal activity of one of these toxins against B.mori was increased four-fold over that of wild-type Cry1Aa toxin. These results indicate that a method combining T7 phage display with selection using cadherin-like protein-coated magnetic beads can be used to increase the activity of easily obtained, low-activity Cry toxins from bacteria.