title: Helix alpha 4 of the Bacillus thuringiensis Cry1Aa toxin plays a critical role in the postbinding steps of pore formation.
abstract: Helix alpha 4 of Bacillus thuringiensis Cry toxins is thought to play a critical role in the toxins' mode of action. Accordingly, single-site substitutions of many Cry1Aa helix alpha 4 amino acid residues have previously been shown to cause substantial reductions in the protein's pore-forming activity. Changes in protein structure and formation of intermolecular disulfide bonds were investigated as possible factors responsible for the inactivity of these mutants. Incubation of each mutant with trypsin and chymotrypsin for 12 h did not reveal overt structural differences with Cry1Aa, although circular dichroism was slightly decreased in the 190- to 210-nm region for the I132C, S139C, and V150C mutants. The addition of dithiothreitol stimulated pore formation by the E128C, I132C, S139C, T142C, I145C, P146C, and V150C mutants. However, in the presence of these mutants, the membrane permeability never reached that measured for Cry1Aa, indicating that the formation of disulfide bridges could only partially explain their loss of activity. The ability of a number of inactive mutants to compete with wild-type Cry1Aa for pore formation in brush border membrane vesicles isolated from Manduca sexta was also investigated with an osmotic swelling assay. With the exception of the L147C mutant, all mutants tested could inhibit the formation of pores by Cry1Aa, indicating that they retained receptor binding ability. These results strongly suggest that helix alpha 4 is involved mainly in the postbinding steps of pore formation.
authors.affiliation: Groupe d'Etude des Protéines Membranaires, Université de Montréal, Montreal, Quebec, Canada.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Girard
authors.forename: Vincent
authors.initials: V
authors.lastname: Vachon
authors.forename: Gabrielle
authors.initials: G
authors.lastname: Préfontaine
authors.forename: Lucie
authors.initials: L
authors.lastname: Marceau
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Schwartz
authors.forename: Luke
authors.initials: L
authors.lastname: Masson
authors.forename: Raynald
authors.initials: R
authors.lastname: Laprade
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism* / Q000633:toxicity*; D001665:Binding Sites; D002463:Cell Membrane Permeability; D002942:Circular Dichroism; D004229:Dithiothreitol / Q000494:pharmacology; D004731:Endotoxins / Q000737:chemistry / Q000378:metabolism* / Q000633:toxicity*; D006460:Hemolysin Proteins / Q000737:chemistry / Q000378:metabolism* / Q000633:toxicity*; D018402:Manduca; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D019163:Reducing Agents / Q000494:pharmacology; D022142:Secretory Vesicles / Q000187:drug effects
chemical_list: D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins; D004731:Endotoxins; D006460:Hemolysin Proteins; D019163:Reducing Agents; C030956:insecticidal crystal protein, Bacillus Thuringiensis; D004229:Dithiothreitol