title: A high-throughput, in-vitro assay for Bacillus thuringiensis insecticidal proteins.
abstract: A high-throughput, in-vitro assay for Bacillus thuringiensis (Bt) insecticidal proteins designated as Cry was developed and evaluated for screening a large number of Cry protein variants produced by DNA shuffling. This automation-amenable assay exploits an insect cell line expressing a single receptor of Bt Cry proteins. The Cry toxin used to develop this assay is a variant of the Cry1Ab protein called IP1-88, which was produced previously by DNA shuffling. Cell mortality caused by the activated Bt Cry toxin was determined by chemical cell viability assay in 96/384-well microtiter plates utilizing CellTiter 96(®) obtained from Promega. A widely-accepted mode-of-action theory of certain Bt Cry proteins suggests that the activated toxin binds to one or more receptors and forms a pore through the insect gut epithelial cell apical membrane. A number of insect proteins such as cadherin-like protein (Cad), aminopeptidase-N (APN), alkaline phosphatase (ALP) and ABC transporter (ABCC) have been identified as the receptors of Bt Cry toxins. In this study, Bt Cry toxin receptors Ostrinia nubilalis (European corn borer) cadherin-like protein (On-Cad) and aminopeptidase-N 1 and 3 (On-APN1, On-APN3) and Spodoptera frugiperda (fall armyworm) cadherin-like protein (Sf-Cad) were cloned in an insect cell line, Sf21, and a mammalian cell line, Expi293F. It was observed by ligand blotting and immunofluorescence microscopy that trypsin-activated IP1-88 bound to On-Cad and On-APN1, but not Sf-Cad or On-APN3. In contrast, IP1-88 bound only to APN1 in BBMV (Brush Border Membrane Vesicles) prepared from the third and fourth-instar O. nubilalis larval midgut. The sensitivity of the recombinant cells to the toxin was then tested. IP1-88 showed no toxicity to non-recombinant Sf21 and Expi293F. Toxicity was observed only when the On-Cad gene was cloned and expressed. Sf-Cad and On-APN1 were not able to make those cells sensitive to the toxin. Since the expression of On-Cad alone was sufficient to make both insect and mammalian cells sensitive to the IP1-88 toxin, it is not likely that a secondary receptor, which may exist specifically in the Sf21 insect cell but not in the Expi293F cell, is involved in the cytotoxicity of IP1-88.
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA. Electronic address: michi.izumi@pioneer.com.
authors.forename: Michi
authors.initials: M
authors.lastname: Izumi Willcoxon
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: Jaclyn R
authors.initials: JR
authors.lastname: Dennis
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: Sabina I
authors.initials: SI
authors.lastname: Lau
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: Weiping
authors.initials: W
authors.lastname: Xie
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: You
authors.initials: Y
authors.lastname: You
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: Song
authors.initials: S
authors.lastname: Leng
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: Ryan C
authors.initials: RC
authors.lastname: Fong
authors.affiliation: Plant Protection, Ag Biotechnology, DuPont Pioneer, 4010 Point Eden Way, Hayward, CA 94545, USA.
authors.forename: Takashi
authors.initials: T
authors.lastname: Yamamoto
mesh_terms: D000818:Animals; D001413:Bacillus thuringiensis / Q000737:chemistry* / Q000378:metabolism; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000032:analysis*; D001427:Bacterial Toxins / Q000032:analysis*; D018826:CD13 Antigens / Q000378:metabolism; D015820:Cadherins / Q000096:biosynthesis / Q000235:genetics; D002460:Cell Line; D004731:Endotoxins / Q000032:analysis*; D006460:Hemolysin Proteins / Q000032:analysis*; D057166:High-Throughput Screening Assays / Q000379:methods*; D007915:Lepidoptera; D061987:Sf9 Cells
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; D018826:CD13 Antigens
keywords: Bacillus thuringiensis; Cadherin-like protein; Cry protein; High throughput screening; Sf21