Botulinum toxin type A

Botulinum toxin is a 150-kDa molecular weight protein consisting of a light chain (50 kDa) and heavy chain (100 kDa) linked by a single disulfide bond. The crystal structure reveals 3 lobes - the light chain, the amino-terminal portion of the heavy chain, and the carboxyl-terminal portion of the heavy chain.

Botulinum toxin type A blocks neuromuscular transmission on motor or sympathetic nerve terminals, inhibiting the release of acetylcholine. Botulinum toxins have actions on various regions: the neuromuscular junction, autonomic ganglia, and both postganglionic sympathetic and parasympathetic nerve endings. The heavy chain of the toxin binds selectively at the presynaptic surface of cholinergic neurons in an irreversible fashion. After binding, the toxin-receptor complex is transported into the cell by endocytosis. The disulfide bond between the two chains is cleaved and the botulism toxin enters the cytoplasm. The light chain specifically interacts with SNAP-25 in the nerve terminals to block binding of acetylcholine vesicles with the cell membrane. SNAP-25 is required for successful binding and release of acetylcholine from vesicles in nerve endings.