Oligomerization is a key step for Bacillus thuringiensis Cyt1Aa insecticidal activity but not for toxicity against red blood cells

Bacillus thuringiensis (Bt) Cyt1Aa toxin shows toxicity to mosquitoes, to certain coleopteran pests and also to red blood cells (RBC). However, its mode of action in the different target cells is not well defined. This protein is a single alpha-beta domain pore-forming toxin, where a beta sheet is wrapped by two alpha-helices layers. The Cyt1Aa alpha-helix hairpin in the N-terminal has been proposed to be involved in initial membrane binding and oligomerization, while the beta sheet inserts into the membrane to form a pore that lyze the cells. To determine the role of the N-terminal alpha-helix hairpin region of Cyt1Aa in its mode of action, we characterized different single point mutations located in helices alpha-1 and alpha-2. Eight cysteine substitutions in different residues were produced in Bt, and we found that three of them: Cyt1AaA65C, Cyt1AaL85C and Cyt1AaN89C, lost insecticidal toxicity against Aedes aegypti larvae but retained similar or increased hemolytic activity towards rabbit RBC. Analysis of toxin binding and oligomerization using Ae. aegypti midgut brush border membrane vesicles showed that the three Cyt1Aa mutants non-toxic to Ae. aegypti were affected in oligomerization. However, these mutants were still hemolytic. Our data shows that oligomerization of Cyt1Aa toxin is essential for its toxicity to Ae. aegypti but not for its toxicity against RBC indicating that the mode of action of Cyt1Aa is different in these distinct target membranes.