pH-induced conformational transitions of Cry IA(a), CryIA(c), and Cry IIIA delta-endotoxins in Bacillus thuringiensis

Three protoxins and corresponding delta-endotoxins from Bacillus thuringiensis (BT) were studied by means of circular dichroism spectroscopy and size-exclusion HPLC. At neutral pH, the Cry IIIA toxin exists only as a 65-kDa monomer. The toxins of Cry IA(a) and Cry IA(c) exist both as 66-kDa monomers and as oligomers with apparent molecular masses greater than 220 kDa. At neutral pH, interconversion between monomer and oligomer is slow, and the two, separate forms exist for several days. Equilibration between the monomer and oligomer in both Cry IA(a) and Cry IA(c) toxins is facilitated by increasing the pH of the solutions to above 10. The relative amounts of monomer and oligomer depend upon temperature, pH, and buffer composition. CD spectra of the protoxins and toxins indicate a large helix content. The CD spectra of HPLC-isolated, monomeric Cry IA(a) and Cry IA(c) are quite similar, but are different from the spectrum of Cry IIIA. The Cry IA(a) and Cry IA(c) protoxins exhibit more helical CD spectra than the corresponding toxins. The CD spectra, with the pH titration of Cry IA(a), reveal that there is a significant increase in helical content as the pH is changed from neutral to alkaline values, but no decrease at low pH. A similar titration of Cry IIIA revealed no significant change in structure from pH 6 to pH 11. The CD spectrum of Cry IIIA at pH 2 indicates that the helical content of the toxin has significantly decreased. The magnitude of the 222-nm signal decreases from pH 7 to 2, with a midpoint of approximately pH 4.5. From neutral pH to weakly alkaline pH, where a great many experimental studies have been carried out, structural changes occur for all three delta-endotoxins studied. Specifically, the conformational changes take place at a pH where each toxin exhibits maximal biological activity.