Effects of mutations within surface-exposed loops in the pore-forming domain of the Cry9Ca insecticidal toxin of Bacillus thuringiensis. 2010

Jean-Frédéric Brunet, and Vincent Vachon, and Mireille Marsolais, and Greta Arnaut, and Jeroen Van Rie, and Lucie Marceau, and Geneviève Larouche, and Charles Vincent, and Jean-Louis Schwartz, and Raynald Laprade
Groupe d'étude des protéines membranaires, Université de Montréal, P.O. Box 6128, Centre Ville Station, Montreal, QC, H3C 3J7, Canada. jean-frederic.brunet@umontreal.ca

The pore-forming domain of Bacillus thuringiensis insecticidal Cry toxins is formed of seven amphipathic α-helices. Because pore formation is thought to involve conformational changes within this domain, the possible role of its interhelical loops in this crucial step was investigated with Cry9Ca double mutants, which all share the previously characterized R164A mutation, using a combination of homology modeling, bioassays and electrophysiological measurements. The mutations either introduced, neutralized or reversed an electrical charge carried by a single residue of one of the domain I loops. The ability of the 28 Cry9Ca double mutants to depolarize the apical membrane of freshly isolated Manduca sexta larval midguts was tested in the presence of either midgut juice or a cocktail of protease inhibitors because these conditions had been shown earlier to greatly enhance pore formation by Cry9Ca and its R164A single-site mutant. Most mutants retained toxicity toward neonate larvae and a pore-forming ability in the electrophysiological assay, which were comparable to those of their parental toxin. In contrast, mutants F130D, L186D and V189D were very poorly toxic and practically inactive in vitro. On the other hand, mutant E129A depolarized the midgut membrane efficiently despite a considerably reduced toxicity, and mutant Q192E displayed a reduced depolarizing ability while conserving a near wild-type toxicity. These results suggest that the conditions found in the insect midgut, including high ionic strength, contribute to minimizing the influence of surface charges on the ability of Cry9Ca and probably other B. thuringiensis toxins to form pores within their target membrane.

UI MeSH Term Description Entries
D007814 Larva Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Maggots,Tadpoles,Larvae,Maggot,Tadpole
D008564 Membrane Potentials The voltage differences across a membrane. For cellular membranes they are computed by subtracting the voltage measured outside the membrane from the voltage measured inside the membrane. They result from differences of inside versus outside concentration of potassium, sodium, chloride, and other ions across cells' or ORGANELLES membranes. For excitable cells, the resting membrane potentials range between -30 and -100 millivolts. Physical, chemical, or electrical stimuli can make a membrane potential more negative (hyperpolarization), or less negative (depolarization). Resting Potentials,Transmembrane Potentials,Delta Psi,Resting Membrane Potential,Transmembrane Electrical Potential Difference,Transmembrane Potential Difference,Difference, Transmembrane Potential,Differences, Transmembrane Potential,Membrane Potential,Membrane Potential, Resting,Membrane Potentials, Resting,Potential Difference, Transmembrane,Potential Differences, Transmembrane,Potential, Membrane,Potential, Resting,Potential, Transmembrane,Potentials, Membrane,Potentials, Resting,Potentials, Transmembrane,Resting Membrane Potentials,Resting Potential,Transmembrane Potential,Transmembrane Potential Differences
D009154 Mutation Any detectable and heritable change in the genetic material that causes a change in the GENOTYPE and which is transmitted to daughter cells and to succeeding generations. Mutations
D004594 Electrophysiology The study of the generation and behavior of electrical charges in living organisms particularly the nervous system and the effects of electricity on living organisms.
D004731 Endotoxins Toxins closely associated with the living cytoplasm or cell wall of certain microorganisms, which do not readily diffuse into the culture medium, but are released upon lysis of the cells. Endotoxin
D006460 Hemolysin Proteins Proteins from BACTERIA and FUNGI that are soluble enough to be secreted to target ERYTHROCYTES and insert into the membrane to form beta-barrel pores. Biosynthesis may be regulated by HEMOLYSIN FACTORS. Hemolysin,Hemolysins,Hemalysins,Proteins, Hemolysin
D000083722 Bacillus thuringiensis Toxins Endotoxins produced by BACILLUS THURINGIENSIS used in transgenic plants and insecticides. When eaten by a susceptible insect they are protease activated in the insect midgut resulting in death from bacterial septicemia. B thuringiensis Toxins,B. thuringiensis Toxins,Bt Toxin,Bt Toxins,Toxin, Bt,Toxins, B thuringiensis,Toxins, B. thuringiensis
D000818 Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, Animalia was one of the kingdoms. Under the modern three domain model, Animalia represents one of the many groups in the domain EUKARYOTA. Animal,Metazoa,Animalia
D001426 Bacterial Proteins Proteins found in any species of bacterium. Bacterial Gene Products,Bacterial Gene Proteins,Gene Products, Bacterial,Bacterial Gene Product,Bacterial Gene Protein,Bacterial Protein,Gene Product, Bacterial,Gene Protein, Bacterial,Gene Proteins, Bacterial,Protein, Bacterial,Proteins, Bacterial
D013329 Structure-Activity Relationship The relationship between the chemical structure of a compound and its biological or pharmacological activity. Compounds are often classed together because they have structural characteristics in common including shape, size, stereochemical arrangement, and distribution of functional groups. Relationship, Structure-Activity,Relationships, Structure-Activity,Structure Activity Relationship,Structure-Activity Relationships

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