Toxic Trypsin Digest Fragment from the Bacillus thuringiensis Parasporal Protein

Abstract

Enzymatic digestion in vitro of the Bacillus thuringiensis protoxin presumably releases and activates the toxin in a manner analogous to that which occurs when a B. thuringiensis sporulated fermentation preparation passes through the midgut of a lepidopteran larva. Therefore, a sporulated culture of B. thuringiensis subsp. kurstaki (serotype 3a3b) HD-263 was treated with trypsin to release an activated toxin soluble in bicarbonate buffer. A 63-kilodalton protein, toxic to cabbage looper larvae ( Trichoplusia ni ) and to lepidopteran cells in culture, was purified to homogeneity from this trypsin digest. The larvicide, a glycoprotein containing 5% carbohydrate (wt/wt), was purified from the soluble B. thuringiensis trypsin digest by using ammonium sulfate precipitation, anion-exchange chromatography, and hydrophobic-interaction chromatography. Its amino acid composition was high in nonpolar residues and unusually low in lysine and histidine. The isoelectric point was 6.5, and the amino acid on the N terminus was identified as isoleucine. The toxin was only slightly soluble in aqueous buffers unless the chaotropic agent potassium thiocyanate was added. Partial characterization of the toxin indicated that it corresponds well with reported sequences deduced from cloned genes.