Botulinum neurotoxin types A, B, and E: fragmentations by autoproteolysis and other mechanisms including by O-phenanthroline-dithiothreitol, and association of the dinucleotides NAD(+)/NADH with the heavy chain of the three neurotoxins.

The first evidence of autoproteolytic activity of the approximately 50-kDa light chain of the clostridial neurotoxins (NT) is traceable to the observations that the light chains of botulinum NT serotypes A and E, separated from their approximately 100-kDa heavy chain conjugate, were found cleaved at the amino side of Tyr250 and Arg244, respectively [DasGupta and Foley (1989). Biochimie 71: 1183-1200]. Specific cleavages of the recombinant light chain of NT type A, including at Tyr249-Tyr250, firmly established that the cleavages reported earlier were due to autoproteolysis [Ahmed et al. (2001). J. Protein Chem. 20: 221-231; Ahmed et al. (2003). Biochemistry 42:12539-12549] and not by contaminating proteases or non-enzymatic. We now report many cleavages in the NT types A, B and E and also in their separated light and heavy chains, and identification of several of the peptide bonds cleaved. None of the identified cleaved bonds (-P1-P1' -) in one serotype (except Asp-Pro) was found common in other serotypes or cleaved within itself at a second site. After separation from the heavy chain self-cleavages of the light chains of type A, B and E at Tyr249-Tyr250, Gln258-Ser259 and Ile243-Arg244, respectively indicate an intriguing feature (in the aligned sequences these bonds of type A and B are 2 and type A and E are 4 peptide bonds apart) that may have some role in the NT's structure-function relationship yet to be understood. We point out that autoproteolysis of a single peptide bond (Phe418-Thr419 or Phe422-Glu423) in NT type A reported by Ahmed et al. (2001) can potentially generate proteolytically active light chain freed of the heavy chain; this is an efficient pathway, that by-passes nicking by a trypsin-like protease(s) inside the intrachain disulfide bridge and its reductive cleavage. We offer probable explanations for the observed cleavages such as acid- and metal-mediated (non-catalytic and non-stoichiometric) reactions in addition to autoproteolysis but cannot predict which mechanism(s) of cleavage occur or prevail following NT's entry in the body as poison or therapeutic agent. The metal chelator O-phenanthroline (above critical miceller concentration) in the presence of dithiothreitol cleaved type E NT at limited sites generating discrete 114-, 87-, 49-, 42-, and 31-kDa fragments but degraded NTs type A and B extensively. The limited cleavage of type E NT was dependent on the presence of metal ion(s) bound to the protein and its native (urea sensitive) conformation. The self-cleavage of the NTs at specific sites prompted us to search for specific binding sites on the NTs analogous to SNARE-motifs-the 9-residuelong motifs present on the NT's natural substrates (SNAP-25, syntaxin, VAMP/synaptobrevin); such putative binding motifs (sites) noted on all clostridial NTs are reported here. Their relationship to the observed autoproteolysis remains to be determined experimentally. The dinucleotide NAD(+)/NADH associated with the NTs type A, B and E (2-3 NADH per protein molecule) via their H-chains, and a portion of the H-chain (toward the C-terminus) appears to exhibit limited amino acid sequence homology with lactate dehydrogenase-a representative NAD(+)/NADH binding protein.