Structural and Functional Characterization of the α5 Segment of Bacillus thuringiensis δ-Endotoxin

One of the most conserved sequences in various δ-endotoxins is the 30 amino acid long block I. Block I of cryIIIA δ-endotoxin contains a 23 amino acid amphiphilic α-helix termed α5. The potential involvement of this α5 helix in the toxic mechanism of δ-endotoxin was examined. For this purpose, a peptide corresponding to the α5 segment and its proline incorporated analogue (P-α5) were synthesized and characterized. The α-helical content of the peptides, assessed in methanol by circular dichroism (CD), was 58% and 24% for α5 and P-α5, respectively. To monitor the interaction of α5 peptides with phospholipid membranes, they were selectively labeled at their N-terminal amino acids with the fluorescent probes 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or carboxyfluorescein. Fluorometric studies allowed the calculation of membrane surface partition constants, which were about 10⁴ M^-1 for both α5 and P-α5, and revealed that their N-terminals are located within the lipid bilayers. The shape of the binding isotherms indicated that α5 aggregated in both zwitterionic and acidic vesicles. Functional characterization of the α5 peptides was determined by assessing their ability to dissipate a diffusion potential from sonicated small unilamellar vesicles (SUV) composed of zwitterionic or acidic phospholipids and to lyse human erythrocytes. α5 was much more active than P-α5 in both assays. Moreover, membrane-bound α5 was more protected from enzymatic proteolysis than P-α5. The results reveal a good correlation between the structure and function of α5 and experimentally support the hypothesis that α5 is a structural component of the pores formed by δ-endotoxins via aggregation of amphiphilic α-helices.