New details about how “cloaking” proteins protect the toxin that causes botulism may prove crucial to developing vaccines or therapeutics that could prevent or treat the disease.
The key discovery was made by deciphering the atomic-scale structure of one form of the botulinum neurotoxin bound to an associated protein. To determine these structures, the scientists studied crystallized samples of the bound toxin using electron microscopy and x-rays at Brookhaven’s National Synchrotron Light Source (NSLS).
The research was a collaborative effort by scientists at Brookhaven, Stony Brook University and the Institute of Advanced Sciences.
Though there are seven different types of botulinum toxin, all are released from Clostridium botulinum bacteria as a complex of the toxin plus at least one such associated protein. When ingested, the toxin/associated-protein complex stays together in the digestive system, but splits apart to free the toxin once it moves into the bloodstream, where it goes on to disable nerve cells, causing paralysis and death.
“We wanted to know how the toxin is protected, how the complex stays together in the harsh acidic environment of the human gastrointestinal (GI) tract, and how the toxin comes out once it leaves the GI tract and enters the bloodstream,” said Subramanyam Swaminathan, a biologist at Brookhaven.
The atomic-scale structures reveal the detailed mechanism of how the toxin/associated-protein complex is assembled and stays intact under acidic conditions, and also how it disassociates when it enters the neutral pH conditions of the blood.
“As the pH changes to neutral, some of the acidic amino acids at the interface between the two proteins—the toxin and the associated protein—become negatively charged. The repulsion between the negative charges causes the toxin and the associated protein to separate, leading to the dissolution of the complex and the emergence of the active toxin,” Swaminathan said.
The structures also revealed a strong similarity with one of the complexes formed by another botulinum neurotoxin, suggesting that there may be common elements across all seven varieties in the complex stage.
“The active versions of the toxins have quite different structures,” Swaminathan said. “So finding any similarities among the seven varieties could help simplify the search for strategies to combat the toxin.”
One promising approach, the scientists say, would be to use the toxin’s own cloaking system to develop vaccines or drugs. For example, the cloaking proteins might be used to deliver inactive forms of the toxin that could trigger a protective immune response in the bloodstream—in other words an easily administered oral vaccine.
The cloaking proteins might even be able to deliver a drug that could disable the toxin before it has a chance to exert its deadly effects. This would provide an antidote to a deadly toxin that currently has limited treatments.
The research was supported in part by grants from the Defense Threat Reduction Agency and the National Institutes of Health.
Read more at Scientific Reports: Molecular Assembly of Clostridium botulinum progenitor M complex of type E.