Crystal structure of BinB: a receptor binding component of the binary toxin from Lysinibacillus sphaericus.

The binary toxin (Bin), produced by Lysinibacillus sphaericus, is composed of BinA (42 kDa) and BinB (51 kDa) proteins, which are both required for full toxicity against Culex and Anopheles mosquito larvae. Specificity of Bin toxin is determined by the binding of BinB component to a receptor present on the midgut epithelial membranes, while BinA is proposed to be a toxic component. Here, we determined the first crystal structure of the active form of BinB at a resolution of 1.75 Å. BinB possesses two distinct structural domains in its N- and C-termini. The globular N-terminal domain has a β-trefoil scaffold which is a highly conserved architecture of some sugar binding proteins or lectins, suggesting a role of this domain in receptor-binding. The BinB β-rich C-terminal domain shares similar three-dimensional folding with aerolysin type β-pore forming toxins, despite a low sequence identity. The BinB structure, therefore, is a new member of the aerolysin-like toxin family, with probably similarities in the cytolytic mechanism that takes place via pore formation.