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J. Biol. Chem., Vol. 282, Issue 29, 21222-21229, July 20, 2007

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Bacillus thuringiensis Cry1Ab Mutants Affecting Oligomer Formation Are Non-toxic to Manduca sexta Larvae^*

Nuria Jiménez-Juárez, Carlos Muñoz-Garay, Isabel Gómez, Gloria Saab-Rincon, Juanita Y. Damian-Almazo, Sarjeet S. Gill, Mario Soberón, and Alejandra Bravo¹

From the Instituto de Biotecnología, Universidad Nacional Autónoma de México, Apdo. 510-3, Av. Universidad 2002, Col. Chamilpa CP 62250, Cuernavaca, Morelos 62250, Mexico and the Department of Cell Biology and Neuroscience, University of California, Riverside, California 92506

Pore-forming toxins are biological weapons produced by a variety of living organisms, particularly bacteria but also by insects, reptiles, and invertebrates. These proteins affect the cell membrane of their target, disrupting permeability and leading eventually to cell death. The pore-forming toxins typically transform from soluble, monomeric proteins to oligomers that form transmembrane channels. The Cry toxins produced by Bacillus thuringiensis are widely used as insecticides. These proteins have been recognized as pore-forming toxins, and their primary action is to lyse midgut epithelial cells in their target insect. To exert their toxic effect, a prepore oligomeric intermediate is formed leading finally to membrane-inserted oligomeric pores. To understand the role of Cry oligomeric pre-pore formation in the insecticidal activity we isolated point mutations that affected toxin oligomerization but not their binding with the cadherin-like, Bt-R₁ receptor. We show the helix -3 in domain I contains sequences that could form coiled-coil structures important for oligomerization. Some single point mutants in this helix bound Bt-R₁ receptors with similar affinity as the wild-type toxin, but were affected in oligomerization and were severally impaired in pore formation and toxicity against Manduca sexta larvae. These data indicate the pre-pore oligomer and the toxin pore formation play a major role in the intoxication process of Cry1Ab toxin in insect larvae.

Received for publication, February 15, 2007 , and in revised form, May 23, 2007.

^* This work was supported by the Consejo Nacional de Ciencia y Tecnologia (Grant 46176-Q), by the Dirección General de Asuntos del Personal Académico-Universidad Nacional Autónoma de México (Grants IN206200 and IN216300), by National Institutes of Health Grant IH 1R01 AI066014, and by U. S. Department of Agriculture Grant 2207-35607-17780. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Tel.: 52-777-329-1635; Fax: 52-777-317-2388; E-mail: bravo{at}ibt.unam.mx.

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