Structure of New Botulism Nerve Toxin

Computer-generated "ribbon" representations of the molecular structure of botulinum neruotoxin subtypes E (left) and B (right). The accompanying schematics show that in subtype E, both the binding domain (yellow) and the catalytic domain (red - which cleaves cellular proteins to block the release of neurotransmitters) lie on one side of the translocation domain (green). On subtype B, the binding and catalytic domains flank the central translocation domain. This structural difference may explain why subtype E is a faster-acting toxin.

All seven neurotoxin subtypes cause their deadly effects using a common mechanism, with each step being activated by a different portion, or domain, of the toxin protein. First the neurotoxin binds to a nerve cell; then it moves into the cell; and then it cleaves specific proteins that block the release of neurotransmitters, the chemicals nerve cells use to communicate with one another and with muscles. Without that communication, muscles, including those used to breathe, become paralyzed.

"Blocking any of these steps could thwart the toxins' deadly action," Swaminathan said. "But to do that, we need to understand the details of the proteins' structures".

Swaminathan and his team had previously analyzed the molecular-level structures of various fragments of botulinum neurotoxin subtypes A to F, and that of the whole neurotoxin B, using x-ray crystallography at the National Synchrotron Light Source (NSLS) at Brookhaven Lab. In this technique, researchers beam high-intensity x-rays at a crystalline sample of the protein and measure how the x-rays scatter off the sample to locate the positions of individual atoms.

These studies revealed that in subtypes A and B, the three domains were arranged in the same way: with the binding and protein-cleaving domains "flanking" a longer central region known as the translocation domain, essential for moving the toxin into the cell.

"Because the genes that code for these proteins have a large degree of similarity and all the subtypes incapacitate nerve cells in a very similar way, a number of biologists had assumed that all seven botulinum neurotoxins would have a similar structural arrangement," Swaminathan said.

The current study of botulinum subtype E, also conducted at the NSLS, disproved that assumption, taking the researchers by surprise. Instead of the flanking arrangement, the binding and protein-cleaving domains of subtype E are both on the same side of the translocation domain. In addition, while all other subtypes are made of two protein chains, subtype E is a single-chain molecule.

Posted by: Emily Source