Dominant negative mutants of Bacillus thuringiensis Cry1Ab toxin function as anti-toxins: demonstration of the role of oligomerization in toxicity.
BACKGROUND
Bacillus thuringiensis Cry toxins, that are used worldwide in insect control, kill insects by a mechanism that depends on their ability to form oligomeric pores that insert into the insect-midgut cells. These toxins are being used worldwide in transgenic plants or spray to control insect pests in agriculture. However, a major concern has been the possible effects of these insecticidal proteins on non-target organisms mainly in ecosystems adjacent to agricultural fields.


RESULTS
We isolated and characterized 11 non-toxic mutants of Cry1Ab toxin affected in different steps of the mechanism of action namely binding to receptors, oligomerization and pore-formation. These mutant toxins were analyzed for their capacity to block wild type toxin activity, presenting a dominant negative phenotype. The dominant negative phenotype was analyzed at two levels, in vivo by toxicity bioassays against susceptible Manduca sexta larvae and in vitro by pore formation activity in black lipid bilayers. We demonstrate that some mutations located in helix alpha-4 completely block the wild type toxin activity at sub-stoichiometric level confirming a dominant negative phenotype, thereby functioning as potent antitoxins.


CONCLUSIONS
This is the first reported case of a Cry toxin dominant inhibitor. These data demonstrate that oligomerization is a fundamental step in Cry toxin action and represent a potential mechanism to protect special ecosystems from the possible effect of Cry toxins on non-target organisms.
Instituto de Biotecnología, Universidad Nacional Autónoma de México, Morelos, Mexico.
Claudia
C
Rodríguez-Almazán
Luis Enrique
LE
Zavala
Carlos
C
Muñoz-Garay
Nuria
N
Jiménez-Juárez
Sabino
S
Pacheco
Luke
L
Masson
Mario
M
Soberón
Alejandra
A
Bravo
D000818:Animals; D001413:Bacillus thuringiensis / Q000378:metabolism*; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000494:pharmacology / Q000633:toxicity*; D015153:Blotting, Western; D004731:Endotoxins / Q000737:chemistry* / Q000235:genetics / Q000494:pharmacology / Q000633:toxicity*; D006460:Hemolysin Proteins / Q000737:chemistry* / Q000235:genetics / Q000494:pharmacology / Q000633:toxicity*; D007306:Insecticides / Q000737:chemistry / Q000494:pharmacology / Q000633:toxicity; D007814:Larva / Q000187:drug effects; D008051:Lipid Bilayers / Q000737:chemistry; D018402:Manduca / Q000187:drug effects; D016297:Mutagenesis, Site-Directed; D055503:Protein Multimerization
D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins; D004731:Endotoxins; D006460:Hemolysin Proteins; D007306:Insecticides; D008051:Lipid Bilayers; C030956:insecticidal crystal protein, Bacillus Thuringiensis