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Toxin binding proteins and the mode of action of Bacillus thuringiensis Cry toxins
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Toxin binding proteins and the mode of action of Bacillus thuringiensis Cry toxins
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Dissertation
Toxin binding proteins and the mode of action of Bacillus thuringiensis Cry toxins
2002
Overview
Aminopeptidases and cadherin-like proteins have been identified as Bacillus thuringiensis Cry toxin-binding proteins from several insect species. In the Cry1A-susceptible lepidopteran insects, Heliothis virescens and Manduca sexta, aminopeptidases are preferentially associated with Triton X-100 insoluble lipid rafts membrane microdomains. Unlike aminopeptidases, cadherin-like proteins are not raft-associated. However, binding of Cry1A toxins to cadherin-like proteins caused relocation of cadherin-like protein to the lipid rafts microdomain and further activation of Cry1A toxins by removal of toxin domain I α-helix 1. Association of these toxin-binding proteins as well as Cry1A toxins with lipid rafts indicates an important role of lipid rafts in Cry1A toxicity. Moreover, it is showed that lipid rafts integrity is essential for Cry1A toxin pore formation, possibly by providing a platform for toxin aggregation and pore formation. The importance of cadherin-like protein in Cry toxicity has been implied by its correlation with a Cry1A resistant H. virescens strain. H. virescens and M. sexta cadherin-like proteins are present in insect midgut columnar cell apical membranes, which are the Cry toxin target sites in the susceptible insects. Cry1A treatment of the H. virescens cadherin-like protein expressing stable mammalian cells HEK293 affects cell-cell contact and cell membrane integrity, indicating direct involvement of cadherin-like protein in Cry1A toxin action. Moreover, the Cry1A-binding region in H. virescens cadherin-like protein is mapped to amino acids 1422–1461, and this region is not only involved in toxin binding, but is also important in toxicity. Another intracellular protein, Hvp85, also binds Cry1Ac specifically. Binding of Hvp85 to Dor suggests that Hvp85 may be involved in lysosomal delivery and its potential involvement in Cry1A action. In addition to the known receptors aminopeptidases and cadherin-like proteins, several smaller proteins with sizes of 45–90 kDa bound Cry1Ac. Our study suggests that Cry protein action is complex and the role of p85 in Cry1Ac toxicity remains unclear but warrants further investigation.
