Roles of ABC proteins in the mechanism and management of Bt resistance.

Abstract Genetic studies of strains of insects that have developed resistance to pore-forming Cry toxins from Bacillus thuringiensis (Bt) have provided useful and unexpected insights into the mode of action of the toxin. Independent approaches in five species of Lepidoptera have converged on the same result: that mutations in a member of the superfamily of ABC transporters confer resistance to Cry toxins. These mutations range from a single amino acid insertion to truncations that delete most of the protein. This result is surprising, because since the first detection of the specific binding of Cry toxins to sites in the lepidopteran midgut in 1988, no studies had documented any sort of interaction between Cry toxins and ABC proteins. It is hypothesized that ABC transporters mediate the critical step of Cry pore insertion into the membrane. Heterologous expression of ABC proteins has recently shown that they can facilitate cell swelling by Cry toxins, which is indirect evidence of pore formation. It is suggested that the 'ATP-switch' mechanism of ABC proteins that drives the transport of small molecules across cell membranes might be exploited to potentiate Bt action and to guide the design of improved Cry toxins.