Three spin-labeled mutant proteins, mutated at the beginning, middle, and end of alpha-helix 5 of the Bacillus thuringiensis Cry1Ab delta-endotoxin, were used to study the involvement of these specific amino acid residues in ion transport and to determine conformational changes in the vicinity of these residues when the protein was translocated into a biological membrane.Amino acid residue leucine 157, located in the N-terminal portion of alpha-helix 5, showed no involvement in ion transport, and the environment that surrounds the residue did not show any change when transferred into the biological membrane. Serine 170, located in the middle of the alpha-helix, showed no involvement in ion transport, but our findings indicate that in the membrane-bound state this residue faces an environment that makes the spin less mobile, as opposed to the mobility observed in an aqueous environment. Serine 176, located in the C-terminal end of the alpha-helix 5 is shown to be involved in ion transport activity.Ion transport data for L157, S170, and S176, along with the mobility of the spin-labels, structural characterization of the resulting proteins, and toxicity assays against a target insect, suggest that the toxin undergoes conformational changes upon protein translocation into the midgut membrane. These conformational changes result in the midregion of the alpha-helix 5 being exposed to a hydrophobic-like environment. The location of these three residues in the toxin suggests that the entire alpha-helix becomes inserted in the insect midgut membrane.