Structure and function of Clostridium botulinum toxins

Clostridium botulinum strains produce seven immunologically distinct neurotoxins, types A to G. The neurotoxins inhibit the release of acetylcholine (Ach) at the neuromuscular junctions and synapses, and cause botulism in humans and animals. The molecular mass (Mr) of all types of neurotoxins are approximately 150 kDa. The neurotoxins associate with nontoxic components in culture, and become large complexes varying from 300 kDa (12S) to 900 kDa (19S), which are designated as progenitor toxins. Recently, the genes coding for type A to G neurotoxins have been cloned, and their whole nucleotide sequences have been determined. Furthermore, it has become clear that the neurotoxins are Zn2+ -binding proteins and possess protease activities. Structures of the nontoxic components of the progenitor toxins have also been investigated genetically in types C, E and F. In this paper, the gene organization of the neurotoxins and the nontoxic components is summarized, and their structure and function are discussed.