Structure Botulinum neurotoxin

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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. ... 

Among the toxins with intracellular targets, we will focus mostly on the botulinum neurotoxins although some references will be made to tetanus neurotoxin because of the structural and functional relationships between tetanus and botulinum neurotoxins. While... 

Figure 5. Primary amino acid sequence of type A botulinum neurotoxin and tetanus neurotoxin showing possible leucine zipper-like structures. Residues with asterisk( ) mark offset the zipper scheme by one residue.

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.

If botulinum and tetanus neurotoxins exist as dimer or trimer/tetramer in aqueous solutions (Fig. 7), what could be the physiological role of such structures The observation could be relevant to explain the behavior of botulinum neurotoxin with mouse phrenic hemidiaphragm at different concentrations (Bandyopadhyay, 1987 Maisey et al., 1988), Because both the neurotoxins exist in more than one oligomeric form, it is possible that these oligomeric forms are in equilibrium with each other (Fig. 7), and this equilibrium could be altered in different conditions such as in low pH and upon interaction with membranes. A model of oligomeric channel is shown in Figure 8 assuming a trimeric form of botulinum neurotoxin. 

Figure 7. Schematic representation of oligomeric structure of type Abotulinum and tetanus neurotoxins. Based on results in Ledoux et al. (1994) and modified after Singh (1993), it is assumed that botulinum neurotoxin exists as trimer and tetramer whereas tetanus neurotoxin exists as dimer and trimer. The arrows indicate possible interconversion between two oligomeric form. The shaded areas indicate the location of amphiphilic/trans-membrane region of the monomeric units.

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