title: Bacillus thuringiensis Cry1Ab mutants affecting oligomer formation are non-toxic to Manduca sexta larvae.
abstract: 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-R(1) receptor. We show the helix alpha-3 in domain I contains sequences that could form coiled-coil structures important for oligomerization. Some single point mutants in this helix bound Bt-R(1) 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.
authors.affiliation: Instituto de Biotecnología, Universidad Nacional Autónoma de México, Apdo. 510-3, Av. Universidad 2002, Col. Chamilpa CP 62250, Cuernavaca, Morelos 62250, Mexico.
authors.forename: Nuria
authors.initials: N
authors.lastname: Jiménez-Juárez
authors.forename: Carlos
authors.initials: C
authors.lastname: Muñoz-Garay
authors.forename: Isabel
authors.initials: I
authors.lastname: Gómez
authors.forename: Gloria
authors.initials: G
authors.lastname: Saab-Rincon
authors.forename: Juanita Y
authors.initials: JY
authors.lastname: Damian-Almazo
authors.forename: Sarjeet S
authors.initials: SS
authors.lastname: Gill
authors.forename: Mario
authors.initials: M
authors.lastname: Soberón
authors.forename: Alejandra
authors.initials: A
authors.lastname: Bravo
mesh_terms: D000818:Animals; D001413:Bacillus thuringiensis / Q000378:metabolism*; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000235:genetics*; D001427:Bacterial Toxins / Q000235:genetics*; D001681:Biological Assay; D015820:Cadherins / Q000378:metabolism; D003001:Cloning, Molecular; D004305:Dose-Response Relationship, Drug; D004731:Endotoxins / Q000235:genetics*; D006460:Hemolysin Proteins / Q000235:genetics*; D007700:Kinetics; D007814:Larva / Q000382:microbiology*; D018402:Manduca / Q000382:microbiology*; D009154:Mutation; D017354:Point Mutation; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D015820:Cadherins; D004731:Endotoxins; D006460:Hemolysin Proteins; C030956:insecticidal crystal protein, Bacillus Thuringiensis