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J. Biol. Chem., Vol. 277, Issue 27, 23985-23987, July 5, 2002

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ACCELERATED PUBLICATION
N-terminal Activation Is an Essential Early Step in the Mechanism of Action of the Bacillus thuringiensis Cry1Ac Insecticidal Toxin^*

Alejandra Bravo, Jorge Sánchez, Thaleia Kouskoura^§, and Neil Crickmore^§¶

From the  Insituto de Biotechnología, Universidad Nacional Autónoma de México, Morelos 62210, México and the ^§ School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom

A variant form of the Bacillus thuringiensis Cry1Ac toxin that is not cleaved at the N terminus during proteolytic activation with trypsin was found to be incapable of forming pores in Manduca sexta brush border membrane vesicles in vitro and had reduced insecticidal activity in vivo. Binding studies indicated an altered binding pattern of the mutant toxin in that bound toxin could not be fully displaced by a high molar excess of fully trypsin-activated toxin. These results suggest that proteolytic removal of the N-terminal peptide of Cry1Ac is an important step in toxin activation.

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