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Cry Toxins Use Multiple ATP-Binding Cassette Transporter Subfamily C Members as Low-Efficiency Receptors in Bombyx
Satomi Adegawa1,2, Yonghao Wang1, Ryusei Waizumi3
1Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Koganei, Koganei 184-8588, Tokyo, Japan.
Abstract:
Recent studies have suggested that ABC transporters are the main receptors of Cry toxins. However, the receptors of many Cry toxins have not been identified. In this study, we used a heterologous cell expression system to identify Bombyx mori ABC transporter subfamily C members (BmABCCs) that function as receptors for five Cry toxins active in Lepidopteran insects: Cry1Aa, Cry1Ca, Cry1Da, Cry8Ca, and Cry9Aa. All five Cry toxins can use multiple ABCCs as low-efficiency receptors, which induce cytotoxicity only at high concentrations. Surface plasmon resonance analysis revealed that the KD values between the toxins and BmABCC1 and BmABCC4 were 10-5 to 10-9 M, suggesting binding affinities 8- to 10,000-fold lower than those between Cry1Aa and BmABCC2, which are susceptibility-determining receptors for Cry1Aa. Bioassays in BmABCC-knockout silkworm strains showed that these low-efficiency receptors are not involved in sensitivity to Cry toxins. The findings suggest that each family of Cry toxins uses multiple BmABCCs as low-efficiency receptors in the insect midgut based on the promiscuous binding of their receptor-binding regions. Each Cry toxin seems to have evolved to utilize one or several ABC transporters as susceptibility-determining receptors.
Insights
Many ABC transporters act as low-efficiency receptors for Cry toxins in insects. However, specific ABC transporters determine insect susceptibility to these toxins, guiding future pest control strategies.
Area of Science:
- Molecular biology
- Insect toxicology
- Biochemistry
Background:
- ABC transporters are implicated as Cry toxin receptors.
- Receptors for numerous Cry toxins remain unidentified.
- Understanding Cry toxin-insect interactions is crucial for pest management.
Purpose of the Study:
- Identify Bombyx mori ABC transporter subfamily C members (BmABCCs) acting as receptors for five Lepidopteran-active Cry toxins.
- Characterize the binding affinities and functional roles of BmABCCs in Cry toxin sensitivity.
Main Methods:
- Heterologous cell expression system to test BmABCCs as Cry toxin receptors.
- Surface plasmon resonance to quantify toxin-receptor binding kinetics (K values).
- Bioassays using BmABCC-knockdown silkworm strains to assess toxin sensitivity.
Main Results:
- All five tested Cry toxins (Cry1Aa, Cry1Ca, Cry1Da, Cry8Ca, Cry9Aa) bind promiscuously to multiple BmABCCs with low affinity.
- BmABCC1 and BmABCC4 exhibit low binding affinities (10^-5 to 10^-9 M) for tested toxins.
- Low-affinity BmABCCs are not essential for Cry toxin sensitivity in silkworms, unlike high-affinity receptors like BmABCC2 for Cry1Aa.
Conclusions:
- Cry toxins utilize multiple BmABCCs as low-efficiency receptors in the insect midgut due to promiscuous binding.
- Specific ABC transporters function as high-affinity, susceptibility-determining receptors for individual Cry toxins.
- This promiscuity and specificity in receptor binding highlight evolutionary adaptations of Cry toxins for insect interaction.
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