Crystallization and preliminary X-ray diffraction studies of a mosquito-larvicidal toxin from Bacillus thuringiensis subsp. israelensis

The Cry4B δ-endotoxin from Bacillus thuringiensis subsp. israelensis is specifically toxic to mosquito larvae. For a better understanding of the mechanism of toxicity, chymotrypsin-activated Cry4B toxin (68 kDa) has been purified and crystallized in sodium bromide at neutral pH. The well formed crystals belong to the rhombohedral space group R32, with unit-cell parameters a = b = 185.82, c = 187.93 Å, and diffracted X-rays to 1.75 Å resolution. The asymmetric unit contains one toxin molecule and 74% solvent content, as shown by molecular replacement from a composite model of the homologous Cry3A and Cry1Aa. The purified protein and crystals both possessed mosquitocidal activity.