Process of Action of Dipteran-Specific Insecticidal Crystal Proteins from Bacillus thuringiensis subsp. israelensis

摘要

Dipteran-specific insecticidal protein Cry4A is produced as a protoxin of 130 kDa in Bacillus thuringiensis subsp. israelensis (Bti). The 130-kDa protoxin was processed in vitro into a 60-kDa intermediate, which is subsequently converted into two protease-resistant fragments of 20 and 45 kDa through intramolecular cleavage. The 20-kDa and 45-kDa fragments, each of which alone was inactive, were associated to form a complex actively toxic against the larvae of mosquito Culex pipiens. Thus, it was strongly suggested that the complex composed of the two fragments is an active form of Cry4A. Immunohistochemical analyses revealed that Cry4A bound to the epithelial cells of mosquito larval midgut. Several Cry4A-specific binding proteins were detected by the ligand blotting experiments using digoxigenin-labeled Cry4A as a probe. Moreover, it was suggested that the mode of Cry4A binding to the target cell membrane was distinct from that of the Cry1 toxin family. Bti produces another dipteran-specific insecticidal protein Cry 11A. Upon in vitro processing of the 70-kDa Cry11A protoxin, the 36-kDa and 32-kDa fragments were produced. The two fragments, each of which alone was inactive, were associated toform a complex actively toxic against the larvae of mosquito.