Component of a binary toxin active against Culex and some Aedes mosquito larvae (PubMed:1512580, PubMed:8419297). This subunit is responsible for localized binding to specific regions of the host larval gut. The individual subunits are not toxic. BinAB and this subunit alone bind to the gastric caecum and posterior midgut of C.quinquefasciatus larvae. Binary toxin internalization into host gut cells requires both proteins. Does not bind to the midgut of Aedes aegypti (PubMed:1512580). Toxic to Aedes atropalpus mosquito larvae; mortality towards both C.quinquefasciatus and A.atropalpus is maximal by 48 hours. A.aegypti is not very susceptible to this toxin (PubMed:8419297). Binding component of binary toxin. The 51 kDa polypeptide acts synergetically with the 42 kDa polypeptide for expression of a larvicidal toxin. Forms a heterodimer with BinA. The parasporal crystal protein is produced during sporulation and accumulates as a spore inclusion; crystals are separated from the forespores by a branch of the exosporium across the cell. The matrix of the paraspore is dissolved within 15 minutes following C.quinquefasciatus larvae feeding. Has an N-terminal beta-trefoil domain and a C-terminal pore-forming domain. The trefoil domain has barrel and cap subdomains; the cap has 3 possible carbohydrate-binding modules while the barrel is involved in host cell receptor binding. Processed by proteases extracted from mosquito larval gut. Belongs to the toxin_10 family.