Impact of Q139R substitution of MEB4-Cry2Aa toxin on its stability, accessibility and toxicity against Ephestia kuehniella

The Bacillus thuringiensis subsp. kurstaki strain MEB4 was previously found to be highly toxic to Ephestia kuehniella. SDS-PAGE analysis of the recombinant strain DH5α (pBS-cry2Aa-MEB4) showed that Cry2Aa-MEB4 delta-endotoxins were forming inclusion bodies, and were 2.75 fold more toxic towards E. kuehniella than those of Cry2Aa-BNS3. Besides to the 65 kDa active toxin, proteolysis activation of Cry2Aa-BNS3 protein with E. kuehniella midgut juice generated an extra proteolysis form of 49 kDa, which was the result of another chymotrypsin cleavage located in Leu144. The amino acid sequences alignment of Cry2Aa-MEB4 and Cry2Aa-BNS3 showed that among the different 15 amino acids, the Q139R substitution was found to be interesting. In fact, due to its presence within the loop α3-α4, the chymotrypsin-like protease was unable to access to its site in Cry2Aa-MEB4, resulting to the production of only the 65 kDa form. The accessible surface and the stability studies of the structure model of the Cry2Aa-BNS3-49 form showed a lower hydrophobicity surface due to the omission of 144 amino acids from the N-terminal comparing with the active Cry2Aa-MEB4 protein. All these features caused the diminishing of Cry2Aa-BNS3 toxicity towards E. kuehniella.