Amino acids at N- and C-termini are required for the efficient production and folding of a cytolytic δ-endotoxin from Bacillus thuringiensis

Abstract

Bacillus thuringiensis Cyt2Aa toxin is a mosquito-larvicidal and cytolytic δ-endotoxin, which is synthesized as a protoxin and forms crystalline inclusions within the cell. These inclusions are solubilized under alkaline conditions and are activated by proteases within the larval gut. In order to assess the functions of the N-and C-terminal regions of the protoxin, several N- and C-terminal truncated forms of Cyt2Aa were constructed. It was determined that amino acid removal at the N-terminal, which disrupts the β1 structure, might critically influence toxin production and inclusion formation. The deletion of 22 amino acids from the C-terminus reduced the production and solubility of the toxin. However, the removal of more than 22 amino acids from the C-terminus or the addition of a bulky group to this region could result in the inability of the protein to adopt the proper folding. These findings directly demonstrated the critical roles of N- and C- terminal amino acids on the production and folding of the B. thuringiensis cytolytic δ-endotoxin.