During sporulation, Bacillus thuringiensis produces parasporal inclusions with insecticidal activity. The parasporal inclusions produced by most subspecies of B. thuringiensis are active only against the larvae of a few lepidopteran insects. Lepidopteran-active parasporal inclusions are usually bipyramidal crystals composed of one or more 130 × 103 −140 × 103Mr polypeptides. These polypeptides are designated as protoxins. The complete insecticidal activity of each protoxin resides in a 55 × 103−70 × 103Mr protease-resistant toxin which results from solubilization and partial digestion of the crystals in the larval midgut (Aronson et al. 1987).
The target of lepidopteran-active B. thuringiensis toxins is the brush-border membrane of larval lepidopteran midgut (Lüthy et al. 1986). This insect cell membrane contains specific high-affinity receptors, of unknown normal physiological function, for B. thuringiensis toxins (Hofmann et al. 1988a;...
