Bacterial Exotoxin B is a family of proteins functioning as the binding (B) component within the AB Toxin System. The family comprises pore-forming proteins that bind to the cell surface and perforate the cells, allowing the transport of the enzymatically active (A) component into the cell to confer its toxicity. While most family members are toxic to humans and livestock (such as Anthrax PA), the Vpb4 subclass was discovered to exhibit larvicidal activity against the economically important maize pests Western Corn Rootworm (labelled the billion-dollar-bug) and Two-spotted Leaf Beetle. Importantly, the Vpb4 proteins can function in the absence of any A component, making it a unique member of the class and family. The precise molecular mechanism regarding how Vpb4 targets and kills, however, remains limited.
Here, through single-particle cryo-electron microscopy, we elucidate the first inserted pore structure of the Vpb4 proteins at 3.12 Å global resolution. The structure revealed several distinguishing features compared to the typical Bacterial Exotoxin B pores, including the absence of the highly conserved molecular bottleneck and the overall neutral charge of the pore lumen. Accordingly, electrophysiology studies revealed that the Vpb4 pore conducts ions at a greater flux than the archetypal family member, Anthrax (PA), across lipid bilayers. Collectively, these results suggest that the Vpb4 subclass functions as bona fide pores to disrupt the insect guts through leakage of the cellular content. Through phylogenetic analysis, we were also able to predict several putative Vpb4-like proteins that could share this unique molecular mechanism. Our findings have significant implications for the design of Vpb4 and its implementation in pest control, as well as the identification of similar independent pores as potential candidates for novel biopesticides.