Variants of Hemagglutinin-33 in the Serotype C Botulinum Progenitor Toxin Complex Boost its Oral Toxicity

Abstract

　Botulinum neurotoxin (BoNT), which causes food-borne botulism, is produced by Clostridium botulinum. BoNT associates with nontoxic non-hemagglutinin and a complex of hemagglutinin (HA) composed of three HA-70, six HA-33 and three HA-17, forming a large progenitor toxin complex (PTC). Since the PTC displays higher oral toxicity than pure BoNT, it was presumed that nontoxic proteins are critical for food poisoning. A unique serotype C variant strain (C-Yoichi) produced a PTC with many amino acid substitutions in the C-terminal region of HA-33. Here, we demonstrated that the oral toxicity of PTC from variant strain (_33vPTC) in mice was approximately 90-times higher than that of PTC from wild-type strain (_wtPTC), although both PTCs showed similar intraperitoneal toxicities in mice. Co-administration of sugar containing galactose moieties with _33vPTC reduced oral toxicity of _33vPTC in mice. Thus, the _33vPTC dominantly binds to the cell surface galactose moieties when the _33vPTC is internalized into the body via the small intestine. Furthermore, _wtPTC can bind to mucins; however, the _33vPTC displayed a significantly lower mucin-binding capacity than that of _wtPTC. Consequently, the intensive amino acid substitutions in HA-33 in the _33vPTC may causeavoidance of the trapping by mucins before toxin enters the body via intestinal wall.