Research article

Ser170 of Bacillus thuringiensis Cry1Ab δ-endotoxin becomes anchored in a hydrophobic moiety upon insertion of this protein into Manduca sexta brush border membranes

Oscar Alzate^1,2,3,5,6 , Craig F Hemann³ , Cristina Osorio¹ , Russ Hille^1,2,4 and Donald H Dean^1,2

¹Biochemistry Department, The Ohio State University, Columbus, 43210; OH, USA

²Biophysics Program, The Ohio State University, Columbus, 43210; OH, USA

³The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, 43210; OH, USA

⁴Department of Biochemistry, University of California, Riverside, 92526; CA, USA

⁵School of Medicine, Cl 78B, # 72A-109, Universidad Pontificia Bolivariana, Medellín, Colombia

⁶Department of Cell and Developmental Biology, School of Medicine, CB 7090, University of North Carolina, Chapel Hill, 27599; NC, USA

author email corresponding author email

BMC Biochemistry 2009, 10:25doi:10.1186/1471-2091-10-25

Published:	19 October 2009

Abstract

Background

Three spin-labeled mutant proteins, mutated at the beginning, middle, and end of α-helix 5 of the Bacillus thuringiensis Cry1Ab δ-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.

Results

Amino acid residue leucine 157, located in the N-terminal portion of α-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 α-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 α-helix 5 is shown to be involved in ion transport activity.

Conclusion

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 α-helix 5 being exposed to a hydrophobic-like environment. The location of these three residues in the toxin suggests that the entire α-helix becomes inserted in the insect midgut membrane.