Domain Organization in Clostridium botulinum Neurotoxin Type E Is Unique: Its Implication in Faster Translocation
ABSTRACT Clostridium botulinum produces seven antigenically distinct neurotoxins [C. botulinum neurotoxins (BoNTs) A-G] sharing a significant sequence homology. Based on sequence and functional similarity, it was believed that their three-dimensional structures will also be similar. Indeed, the crystal structures of BoNTs A and B exhibit similar fold and domain association where the translocation domain is flanked on either side by binding and catalytic domains. Here, we report the crystal structure of BoNT E holotoxin and show that the domain association is different and unique, although the individual domains are similar to those of BoNTs A and B. In BoNT E, both the binding domain and the catalytic domain are on the same side of the translocation domain, and all three have mutual interfaces. This unique association may have an effect on the rate of translocation, with the molecule strategically positioned in the vesicle for quick entry into cytosol. Botulism, the disease caused by BoNT E, sets in faster than any other serotype because of its speedy internalization and translocation, and the present structure offers a credible explanation. We propose that the translocation domain in other BoNTs follows a two-step process to attain translocation-competent conformation as in BoNT E. We also suggest that this translocation-competent conformation in BoNT E is a probable reason for its faster toxic rate compared to BoNT A. However, this needs further experimental elucidation.
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ABSTRACT: Botulinum neurotoxins (BoNTs) are among the most deadly toxins known. They act rapidly in a highly specific manner to block neurotransmitter release by cleaving the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) complex at neuromuscular junctions. The extreme toxicity of BoNTs relies predominantly on their neurotropism that is accomplished by recognition of two host receptors, a polysialo-ganglioside and in the majority of cases a synaptic vesicle protein, through their receptor-binding domains. Two proteins, synaptotagmin and synaptic vesicles glycoprotein 2, have been identified as the receptors for various serotypes of BoNTs. Here, we review recent breakthroughs on the structural studies of BoNT-protein receptor recognitions that highlight a range of diverse mechanisms by which BoNTs manipulate host neuronal proteins for highly specific uptake at neuromuscular junctions. Copyright © 2015. Published by Elsevier Ltd.Progress in Biophysics and Molecular Biology 02/2015; 117(2-3). DOI:10.1016/j.pbiomolbio.2015.02.004
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ABSTRACT: OBJECTIVE: Botulinum neurotoxins act on nerve endings and block neurotransmitter release. Their potency is due to their enzymatic activity and high affinity binding to neurons. Botulinum toxin type A is used in the treatment of human diseases characterized by hyperactivity of peripheral cholinergic nerve terminals, but some patients are or become resistant to it. This can be overcome by using other botulinum toxins, and studies have been performed with different toxin serotypes. Botulinum neurotoxin type D has never been tested in humans in vivo, and, therefore, we investigated the action of this toxin in mouse and human muscles. METHODS: Botulinum toxin type D potency was determined on mouse hemidiaphragm and on rat neuronal cultures. From these experiments, doses to be injected in human volunteers were decided. The compound muscle action potential of toxin-injected Extensor Digitorum Brevis muscle was measured at different times points after injection in human volunteers. RESULTS: Botulinum toxin type D is poorly effective in inducing human skeletal muscle paralysis. CONCLUSIONS: Botulinum toxin type D is very potent in mice and almost ineffective in humans in vivo. SIGNIFICANCE: The results shed new light on the mechanism of toxin type D binding to the neuronal surface receptors.Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 12/2012; 124(5). DOI:10.1016/j.clinph.2012.11.004
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ABSTRACT: Clostridium botulinum produces seven antigenically distinct serotypes of botulinum neurotoxin (BoNTs, A-G) the most potent toxins to humans. They cause paralysis at less than picomolar concentration by blocking neurotransmitter release. BoNT consists of a heavy chain and a light chain. The C-terminal half of the heavy chain allows the toxin to bind to the presynaptic membrane while the N-terminal half helps in forming a channel in the endosomal membrane to allow the light chain to escape into the cytosol. While the binding and catalytic mechanisms are well understood now, the details of translocation still remain a mystery. A full length BoNT/A heavy chain (BAHC) has been cloned, over expressed and purified from inclusion bodies by solubilising with a detergent. Preliminary crystallographic results show that BAHC forms a tetramer in the crystal lending experimental support for tetrameric pore formation for the light chain to pass through the endosomal membrane.The Botulinum J 10/2012; 2(2). DOI:10.1504/TBJ.2012.050196