Role of α-Helix Seven ofBacillus thuringiensisCry1Ab δ-Endotoxin in Membrane Insertion, Structural Stability, and Ion Channel Activity

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Abstract
Domain I of the Cry1Ab insecticidal toxic protein has seven α-helices and is considered to be involved in the ion channel activity. While other α-helices, particularly α-4 and α-5, have been extensively explored, the remaining α-helices have been slightly studied. Site-directed mutagenesis was used to generate mutations throughout sequences encoding the α-helix 7 to test its role in ion channel function. Every amino acid residue in α-helix 7 was mutated to alanine. Most resultant proteins, e.g., D225A, W226A, Y229A, N230A, R233A, R234A, D242A, and F247A yielded no protoxin or were sensitive to degradation by trypsin or Manduca sexta midgut juice. Other mutant proteins, R224A, R228A, and E235A, were resistant to degradation to the above proteases but were 8, 30, and 12 times less toxic to M. sexta, respectively, than the wild-type Cry1Ab. Circular dichroism spectroscopy indicated a very small change in the R228A spectrum, while R224A and E235A display the same spectrum as the wild-type protein. These three mutant proteins showed little differences from Cry1Ab when analyzed by saturation binding and competition binding kinetics with 125I-labeled toxin or by surface plasmon resonance to M. sexta brush border membrane vesicles. More conservative amino acid substitutions were introduced into α-helix 7 residues: R228K, F232Y, E235Q, and F247Y. In comparison with wild-type Cry1Ab, mutant proteins R228K, F232Y, E235A, and E235Q selectively discriminate between K+ and Rb+, while R224A and R228A had reduced inhibition of short-circuit current for both ions, when analyzed by voltage clamping of M. sexta midguts.