Tetanus neurotoxins (TeNT) and botulinum neurotoxins (BoNTs) are closely related ~150 kDa protein toxins that together comprise the group of clostridial neurotoxins (CNTs) expressed by various species of Clostridia. While TeNT is expressed as a single polypeptide, BoNTs are always produced alongside multiple non-toxic proteins that form a stabilizing complex with BoNT and are encoded in a conserved toxin gene cluster. It is unknown how tent evolved without a similar gene cluster and why complex-free TeNT is secreted as a stable and soluble protein by C. tetani, whereas complexing proteins appear to be essential for BoNT stability in culture supernatants of C. botulinum. To assess whether the stability of TeNT is due to an innate property of the toxin or is a result of C. tetani's intra- and extra-cellular environment, both TeNT and complex-free BoNT/A1ERY were expressed recombinantly in atoxic C. tetani and analyzed for expression and stability. The strong clostridial ferredoxin (fdx) promotor resulted in the expression of recombinant TeNT at greater levels and earlier time points than endogenously produced TeNT. Recombinant BoNT/A1ERY was similarly expressed by atoxic C. tetani, although partial degradation was observed. The rBoNT/A1ERY produced in C. tetani was also partially proteolytically processed to the dichain form. Investigations of bacterial growth media and pH conditions found that the stability of rTeNT and rBoNT/A1ERY in spent media of C. tetani or C. botulinum was affected by growth media but not by pH. These data indicate that the distinct metabolism of C. tetani or C. botulinum under various growth conditions is a primary factor in creating a more or less favorable environment for complex-free CNT stability.
Keywords: Clostridium tetani; botulinum neurotoxin; expression system; protein stability; recombinant; tetanus toxin.