Rearrangement of N-Terminal α-Helices of Bacillus thuringiensis Cry1Ab Toxin Essential for Oligomer Assembly and Toxicity.

Cry proteins produced by Bacillus thuringiensis are pore-forming toxins that disrupt the membrane integrity of insect midgut cells. The structure of such pore is unknown, but it has been shown that domain I is responsible for oligomerization, membrane insertion and pore formation activity. Specifically, it was proposed that some N-terminal alpha-helices are lost, leading to conformational changes that trigger oligomerization. We designed a series of mutants to further analyze the molecular rearrangements at the N-terminal region of Cry1Ab toxin that lead to oligomer assembly. For this purpose, we introduced Cys residues at specific positions within alpha-helices of domain I for their specific labeling with extrinsic fluorophores to perform Foster resonance energy transfer analysis to fluorescent labeled Lys residues located in Domains II-III, or for disulfide bridges formation to restrict mobility of conformational changes. Our data support that helix alpha-1 of domain I is cleaved out and swings away from the toxin core upon binding with Manduca sexta brush border membrane vesicles. That movement of helix alpha-2b is also required for the conformational changes involved in oligomerization. These observations are consistent with a model proposing that helices alpha-2b and alpha-3 form an extended helix alpha-3 necessary for oligomer assembly of Cry toxins.