Assembles with botulinum neurotoxin type A (BoNT/A) and protects it against pH-mediated inactivation or protease activity at pH 2.6 (the pH of the animal gastrointestinal tract) but not at pH 6.0 (PubMed:22363010). Necessary for neurotoxicity. Forms a highly interlocked heterodimer with botulinum neurotoxin type A at pH 6.0 but not at pH 7.5, called the minimally functional progenitor toxin complex (M-PTC) (BoNT/A, botA) (PubMed:22363010). Has 3 domains that are structurally very similar to those in BoNT/A; light chain (nLC, equivalent to the light chain, residues 1-408), N-heavy chain (nHN, residues 409-829) and C-heavy chain (nHC, residues 830-1193) (PubMed:22363010). The 133-Lys-|-Lys-134 bond is cleaved during long-term storage; the cleavage site is masked in the M-PTC complex (PubMed:22363010). Belongs to the botulism non-toxic nonhemagglutinin family.