Botulinum neurotoxins (BoNTs) cause flaccid paralysis by inhibiting
neurotransmission at cholinergic nerve terminals. BoNTs consist of three essential domains
for toxicity: the cell binding domain (Hc), the translocation domain (Hn) and the catalytic
domain (LC). The binding function of the Hc domain is essential for BoNTs to bind the
neuronal cell membrane, therefore removal of the Hc domain results in a product that retains
the endopeptidase activity of the LC but is non-toxic. Functional derivatives (LHn) of the
parent neurotoxin composed of Hn and LC domains have been recombinantly produced and
characterised. The crystallographic structures of LHn from serotypes A and B are reported
here and demonstrate the stability of the LHn fragment in comparison to the full length
toxins. The activity of LHn has been assessed on recombinant substrates and on cultured
neuronal cells. LHn retains the capacity to internalise and cleave its intracellular SNARE
substrate when applied to the cells at high concentration. These activities demonstrate the
utility of engineered botulinum neurotoxin fragments as analytical tools to study the
mechanisms of action of BoNT neurotoxins and of SNARE proteins. Targeted secretion
inhibitors (TSI) are a new class of engineered biopharmaceutical molecules derived from the
botulinum neurotoxins. These functional derivatives are expressed as single-chain proteins
and require post-translational activation into di-chain molecules for function. A range of
BoNT derivatives are presented and demonstrate the successful use of engineered SNARE
substrate peptides at the LC-Hn interface to give these molecules self-activating capabilities
while retaining the functions of LHn. Several novel molecules with therapeutic potential
have been produced and their crystallisation for structural investigation is reported. These
results provide an understanding of the structural implications and challenges of engineering
therapeutic molecules that combine functional properties of the LHn fragment from BoNTs
with specific ligand partners to target different cell types.