Protective antigen-like proteins (exotoxin B proteins) found in the bacterial taxonomic class Bacilli, are well-known as translocase proteins that mediate the membrane transit of enzymatic toxins (proteinases, adenylate cyclases and/or ADP-ribosyl transferases) to their target sites. These exotoxin B proteins assemble into heptameric transmembrane structures that facilitate the unfolding and transit of their cargo proteins as they interact with a series of characterised structural features.
Genomic analysis reveals increasing numbers of exotoxin B family genes that do not appear to have associated genes encoding cargo proteins. This group includes Vpb4 proteins from Bacillus thuringiensis, examples of which are able to kill a narrow range of target insects. We have studied the pore form of Vpb4Aa2 and demonstrate a heptameric pore that, overall, is very similar to the structures of translocases such as PA. However, Vpb4Aa2 shows distinct electrophysiological characteristics, indicating that it acts as a simple transmembrane pore. Detailed structural analysis shows that key features of the related translocases, the Phi clamp and charged rings in the lumen of the pore, are altered in Vpb4Aa2, explaining its ability to act as a toxin independently of any associated enzymatic toxins.