Component of a binary toxin active against Culex and some Aedes mosquito larvae (Ref.6, PubMed:8419297). This subunit alone has no toxic larvicidal activity (PubMed:3926751). This subunit is responsible for localized binding to specific regions of the host larval gut. Binary toxin internalization into host gut cells requires both proteins (By similarity). Forms a heterodimer with BinA (PubMed:27680699). Upon toxin crystal solubilization with NaOH at pH 12, only the 63-kDa (binB) and 43-kDa (binA) proteins were detected (PubMed:3926751). Interacts with mosquito protein Cpm1 which acts as its host receptor (PubMed:11483434). Total crystal protein is produced during sporulation, it appears after 6 hours of growth, and represents about 4.8% of cellular dry weight in stationary phase. It probably accumulates next to spores within the exosporeum. 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. At neutral pH the carbohydrate-binding modules are accessible on the toxin surface but the barrel subdomain is not (PubMed:27680699). The crystal is very stable at neutral pH, upon ingestion by larvae the crystals dissolve in the alkaline midgut. As the pH rises the 2 subunits compact, while deprotonation at up to 4 sites (including the N- and C-termini) increases the accessibility of the propeptides and moves subdomains. The combined pH-induced changes are thought to expose the previously hidden receptor-binding motif and lead to crystal dissolution (Probable). Processed by proteases extracted from C.pipiens larval gut; unlike its partner BinA, it does not form a stable digestion product. In protein gels runs as a 63 kDa protein. Belongs to the toxin_10 family.