The hemagglutinin (HA) component of the progenitor toxin protects the structural integrity of botulinum neurotoxin; may increase internalization of the neurotoxin into the bloodstream of the host (PubMed:9421908). The HA component is involved in binding to the upper small intestine through interactions with glycolipids and glycoproteins containing sialic acid moieties (Probable). Whole protein and the HA-53 chain (but not the HA-22-23 chain) bind to bovine mucin; if the mucin is pretreated with neuraminidase (removes the terminal sialic acid of glycoconjugates) mucin binding is decreased (PubMed:19071137). Has higher affinity for alpha-2,3-sialylated oligosaccharides than alpha-2-6 sialylated oligosaccharides (PubMed:22684008). Botulinum toxins are produced as progenitor toxins of large molecular sizes of 12S (M toxin) and 16S (L toxin). M toxin consists of a non-toxic, non-hemagglutinin component (NTNHA) and the neurotoxin (Probable). L toxin consists of the M toxin and the 3 subcomponents of hemagglutinin (HA) (PubMed:7802661). HA is composed of subcomponents of 70, 33, and 17 kDa (PubMed:7802661). The 70 kDa subcomponent undergoes proteolytic processing and is split into HA-53 (also called HA3b) and HA-22-23 (collectively called HA3a) (PubMed:7802661). Upon crystallization this cleaved component forms a three-lobed trimer of dimers with a hole in the center (PubMed:19071137, PubMed:22684008). Preliminary crystallization studies show a tri-lobed HA-70 where each HA-70 contacts one HA-17 molecule (Probable). The stoichiometry of the whole complex has been modeled as one BoNT/C, one NTNHA, three HA-70, six HA-33 and three HA-17 (By similarity). The HA-22-23 chain and the N-terminus of the HA-55 chain are structurally similar, as are the domain 2 and domain 3 of HA-55. This protein can also be encoded on a prophage.