Botulinum neurotoxins formation

Reuner KH, Schlegel K, Just I, et al. (1991) Autoregulatory control of actin synthesis in cultured rat hepatocytes. In FEBS Letters. 286 100-4 Schmid A, Benz R, Just I, et al. (1994) Interaction of Olostridium botulinum 02 toxin with lipid bilayer membranes. Formation of cation-selective channels and inhibition of channel function by chloroquine. In J Biol Chem. 269 16706-11 Simpson LL (1982) A comparison of the pharmacological properties of Clostridium botulinum type 01 and 02 toxins. In J Pharmacol Exp Then 223 695-701 Simpson LL (1989a) Botulinum Neurotoxin andTetanus Toxin, pp 1 -422, San Diego Academic Press... 

Otto H, Hanson PI, Chapman ER eta/. (1995) Poisoning by botulinum neurotoxin A does not inhibit formation or disassembly of the synaptosomal fusion complex. Biochem. Biophys. Res, Commun. 212 945-52. 

Vaidyanathan W, Yoshino K, Jahnz M, et al. 1999 Proteolysis of SNAP-25 isoforms by botulinum neurotoxin types A, C, and E Domains and amino acid residues controlling the formation of enzyme-substrate complexes and deav Age. Journal of Neurochemistry 72(1) 327-337. 

Hollow structures can also be prepared by the self-assembly of stave or rod-like subunits into barrel or bundle-shaped frameworks. This is one of the most common strategies in nature for channel formation, where the rod-like molecules of the barrel-stave type are /S-sheets or a-helices of amphipathic character. The central cavity has hydrophilic properties, while the lipophilic area is oriented outward, in contact with the membrane of the cell. A natural example of this type of protein is a-hemolysin, a bacterium toxin formed by seven identical subunits that self-assemble when in contact with the cell membrane. This assembly gives rise to a mushroom-shaped structure, whose trunk is formed by a -barrel that is inserted into the cell membrane. The resulting channel has a diameter of 13 A at its narrowest point and can transport ions and other molecules. Other natural examples based on this model —but using an a-helix instead of f-sheets—include cholera toxin, potassium channels, porins, aquaporins, and the most powerful toxin known to date, botulinum neurotoxin (BoNT, known as Botox), broadly studied by Mental s group. ... 

It is also possible that a combination of both modes suggested above is involved in the formation of membrane channel for the translocation botulinum neurotoxin. The main assumptions of the hypothesis are (i) The presence of amphiphilic and transmembrane segments in the light and heavy chains as predicted by the hydrophobic moment analysis, (ii) Existence of botulinum neurotoxin in oligomeric form. 

Figure 6. Schematic diagram depicting possible model for association between type A botulinum neurotoxin molecules involving leucine-zipper like structure. (A) Representation of an association between the light chains of the monomeric neurotoxin molecules for a dimer formation. (B) Representation of a helical structure which is assumed for the leucine-like structure. (C) Depiction of amino acid residues which may be in favorable contact. The amino acid sequence corresponds to the residues 270 to 291 on the light chain of type A botulinum neurotoxin (Fig. 5). The two sequences are represented antiparallel to depict favorable ionic contacts.

Botulinum and tetanus neurotoxins, which are extensively soluble in water, are known to form efficient membrane channels at a low pH in artificial membranes (Hoch et al. 1985 Boquet and Duflot, 1982). Membrane channel formation by a water soluble protein is an intriguing phenomenon, because for water solubility, hydrophobic domains are needed on the surface of a protein whereas for membrane channel formation, adequate hydrophobic segments will be required for the interaction with non-polar membrane bilayer. A major question to be answered is how are the polypeptides integrated in the lipid bilayer. Are the hydrophobic segments of these neurotoxins hidden in aqueous medium which get exposed... 

Low pH is apparently required for the strong channel formation activity of botulinum and tetanus neurotoxins and their heavy chains (Boquet and Duflot, 1982 Hoch et al., 1985). A pH of 5.0 or lower induces the channel formation. Two possible effects of a low pH are... 

Singh, B. R., Ledoux, D. N. and Fu, F.-N., 1994, An analysis of the protein structure of botulinum and tetanus neurotoxins to understand molecular basis of membrane channel formation. In Advances in Venom and Toxin Research (Tan, N. H., Oo, S. L., Thambyrajah, V. and Azila, N.. eds.), Malaysian Society on Toxinology, Kuala Lumpur, pp. 103-108. 

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