Tetanus and botulinum neurotoxins are the most potent toxins known. They bind to nerve cells, penetrate the cytosol and block neurotransmitter release. Comparison of their predicted amino acid sequences reveals a highly conserved segment that contains the HexxH zinc binding motif of metalloendopeptidases. The metal content of tetanus toxin was then measured and it was found that one atom of zinc is bound to the light chain of tetanus toxin. Zinc could be reversibly removed by incubation with heavy metal chelators. Zn2+ is coordinated by two histidines with no involvement in cysteines, suggesting that it plays a catalytic rather than a structural role. Bound Zn2+ was found to be essential for the tetanus toxin inhibition of neurotransmitter release in Aplysia neurons injected with the light chain. The intracellular activity of the toxin was blocked by phosphoramidon, a very specific inhibitor of zinc endopeptidases. Purified preparations of light chain showed a highly specific proteolytic activity against synaptobrevin, an integral membrane protein of small synaptic vesicles. The present findings indicate that tetanus toxin, and possibly also the botulinum neurotoxins, are metalloproteases and that they block neurotransmitter release via this protease activity.

No abstract

A B S T R A C T The form and time sequence of spikes generated by orthodromic, antidromic, and direct stimulation and during spontaneous activity have been studied with intracellular electrodes simultaneously introduced in the soma and in different parts of the axon of the giant nerve cell of AiOlysla. Evidence was obtained that :under normal conditions of excitability, the spike originates at some distance from the soma in an axonal region with a higher excitability surpassing that of the surrounding membranes. Between the trigger zone and the soma is situated a region of transitional excitability where the conduction of the spike towards the soma may be blocked at a functionally determined and variable locus. The cell body is electrically excitable, but has the highest threshold of all parts of the neuron. The inactivation or even the removal of the cell body does not suppress synaptic transmission.The use of modern electrophysiological methods allowing intracellular and extremely localized extracellular recordings has provided recent evidence of the heterogeneity of membrane properties in different regions of a single neuron. Thus, it was shown that the site of spike initiation is not to be found in the part of the neuron in which the synaptic contacts are most dense, but in a special region in which the intrinsic membrane excitability is particularly high. Suggestions by Gesell (12) and Bishop (3) that the spike takes origin in the axon hillock region having no or few synapses, received a splendid confirmation with the work of Araki and Otani (1), Fuortes, Frank, and Becker (11), Coombs, Curtis, and Eccles (4, 5), and others on the vertebrate spinal motoneuron. In the crustacean stretch receptor Edwards and Ottoson (8) have localized the site of spike initiation in the axon relatively far from the cell body, and Diamond, Gray, and Sato (6) have concluded that the discharge of the Pacinian corpuscle originates in the first axonal node.The Aplysia giant neuron (GN) with its large cell body (300 to 800/~ in

No abstract

No abstract