Component of a binary toxin active against Culex and some Aedes mosquito larvae; about 1000-fold more toxic against C.quinquefasciatus than A.aegypti (Probable) (Ref.10, PubMed:8419297). This subunit alone is active against C.quinquefasciatus, Anopheles gambiae, A.stephensi and Aedes aegypti mosquito cell lines; non Culex mosquitoes are less sensitive to the toxin (PubMed:2886104) (Probable). Binary toxin internalization into host gut cells requires both proteins (By similarity). Toxic activity of the cleaved 40 kDa form of this subunit alone on cells of C.quinquefasciatus is decreased about 50% by chitobiose, chitotriose, N-acetylmuramic acid and N-acetylneuraminic acid. Forms a heterodimer with BinB (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). Total crystal protein is produced during sporulation, 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 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; 6 amino acids are removed from the N-terminus while it is estimated about 20 residues are removed from the C-terminus to yield the 40 kDa toxin form that is seen in insects. The 40 kDa form is 50-fold more lethal against tissue culture cells than the precursor form. Larval gut extracts of Aedes aegypti and Anopheles gambiae also generate the same 40 kDa form all of which are toxic in C.pipiens. Belongs to the toxin_10 family.