Botulinum neurotoxins: perspective on their existence and as polyproteins harboring viral proteases.

no cross-reactivity) proteins known as botulinum neurotoxin (serotypes A–G) and tetanus neurotoxin (NT) have apparent similar structures and similar structurefunction relationships (Humeau et al., 2000; Schiavo et al., 2000). These proteins do not appear to serve any physiological function for the producer anaerobic bacteria Clostridium botulinum, C. baratii, C. butyricum, C. argentinense and C. tetani, nor to be essential for their survival (Allen et al., 1999; Minton, 1995). Are the NTs vestiges or relics of evolution? Or is the ancestral NT gene (Collins and East, 1998; Henderson et al., 1997; Popoff and Marvaud, 1999) still evolving to acquire a “useful existence”? The NT genes, seemingly superfluous for the survival and reproduction of C. botulinum, express the most toxic poison in nature that does not kill any cell. This conundrum is considered with some conjectures, viewed from the published structural and functional properties of the 150-kDa proteins. Assessment of the literature provided rationale to propose a consideration of the NT as a polyprotein harboring viral protease (the 50-kDa light chain) and to provide a perspective on the evolution of the NT into the present state where 8 NTs (botulinum plus tetanus) cleave 3 neuronal and 1 non-neuronal proteins at 7 sites. Clostridial NTs are polyproteins harboring viral metalloproteinases—a proposal: Each NT synthesized as a 150-kDa single chain protein shapes into three clearly visible (X-ray diffraction of crystals) structural domains each of 50 kDa (Lacy et al., 1998; Swaminathan and Eswaramoorthy, 2000). Proteolytic cleavage(s) at the two junctions between the three structural-functional domains is a facile process as is their ensuing separation(s) (Prabakaran et al., 2001, and refs. therein). The selective and relative susceptibilities of these junctional segments indicate that these segments are mere hinges, as is evident from the 3-dimensional structures of type A and B botulinum NTs, acting as purely provisional links between the three structurally independent domains. Each of the three 50-kDa domains of the 150-kDa NTs performs its individual function without the presence of the other two. The 50-kDa C-terminal domain (the binding domain) mediates NT’s binding to the receptors on the presynaptic membrane of neuromuscular junctions. The adjacent 50-kDa domain forms channels in the endosomal membrane (channel former or transmembrane domain) and promotes insertion/passage of the Botulinum neurotoxins: Perspective on their existence and as polyproteins harboring viral proteases