Bacteria survive by building strong walls of protection around themselves, but these walls are more than just a passive defence measure. For many disease-causing bacteria, the cell wall also contains an arsenal of hidden weapons.
These weapons are called “secretion systems” — elaborate mechanisms through which bacteria release potent toxins into the world around them.
Depending on the type of bacteria, these systems can work like needles to inject toxins directly into nearby cells, or like release valves that let toxins slip out into the environment. In either case, these systems are how bacteria attack other microbes and cause disease in susceptible hosts.
“Evolution has arrived at the same solution for many different kinds of bacteria,” says John Whitney, an associate professor in the Department of Biochemistry and Biomedical Sciences at McMaster University. “The mechanisms may differ from type to type, but the intent is always to deliver toxins that tip the competitive balance in their favour.”
Whitney’s lab, part of the Michael G. DeGroote Institute for Infectious Disease Research, has long studied these bacterial secretion systems. Recently, his group made two breakthrough discoveries that shed new light on how toxins work and how they’re released.
Solving the structure
The first breakthrough came from studying a toxin family known as VasX, first discovered in the early-2000s in Vibrio cholerae, the bacterium that causes cholera. These toxins are considered “pore-formers” — proteins that punch tiny holes into the membranes of other cells, causing essential molecules to leak out.
“Cells are essentially contained bags of molecules and complex chemical processes,” explains Whitney. “Many toxins act by getting inside the bag and interfering with its contents, but these toxins go after the bag itself.”
Whitney’s team, working with collaborators in the Netherlands, used a mix of AI-derived predictions and cryo-electron microscopy to solve and describe the molecular structure of a VasX toxin produced by the deadly human pathogen Pseudomonas aeruginosa, which can cause infections in the lungs, skin, and urinary tract.
The study, led by MD/PhD student Jake Colautti and published in the journal Cell Reports, revealed that the while the toxin’s architecture was unlike that of any other known protein, understanding its structure helped researchers quickly identify the key features that make it so toxic.
“The structure gave us the clues that we needed to understand how it punches holes in cell membranes,” says Colautti.
According to researchers, the findings could have downstream implications on drug development.
“Because bacteria use this toxin to kill each other, understanding its structure and function may lead to new ideas about how we might turn the tables on harmful microbes,” Whitney says.
How toxins squeeze through
Understanding what bacterial toxins look like and how they function is only part of the equation, says Whitney. Equally important is discerning how bacteria physically secrete toxins in the first place.
In many bacteria, this happens via the “type VII secretion system” — a narrow, tunnel-like structure tailormade for exporting toxins. But the system itself is so narrow that even microscopic toxins shouldn’t fit through it — and yet they do, a phenomenon that’s puzzled scientists for years.
But the Whitney Lab’s second recent breakthrough — the identification of previously unknown “molecular chaperones” — has helped clarify this complex process.
Chaperones are helper proteins that fold other proteins into new shapes so that they can perform specific functions. In this case, the newly identified chaperones were found to fold toxins into a narrow shape that’s perfectly compatible with the bacteria’s secretion system.
After passing through, the toxins spontaneously snap back into their original, lethal shape.
Polina Gkragkopoulou.
“We were surprised to find these helper proteins hiding in plain sight,” says Stephen Garrett, a postdoctoral fellow in Whitney’s lab and co-first author of the study, with McMaster graduate student Polina Gkragkopoulou. “They’re absolutely essential for getting toxins through the secretion system.”
The discovery, published in the Proceedings of the National Academy of Sciences (PNAS), provides researchers with a new, more comprehensive understanding of how the type VII secretion system works.
“These chaperones were a major missing piece of the type VII secretion puzzle,” says Whitney. “Now, we have a much clearer picture of how these systems move toxins from inside a bacterium to the outside world.”
Tying it all together
In past work, Whitney’s lab identified how bacterial secretion systems distinguish between beneficial and toxic molecules. Now, with the detailed structure of an important toxin and the identification of these molecular chaperones, his group has provided critical new insights into how bacteria produce and release their toxins.
Collectively, this research not only highlights what Whitney describes as “the remarkable ingenuity of bacteria as they compete with one another and infect a host,” but also exposes critical vulnerabilities that could someday be exploited by new medicine.
“Understanding how bacterial toxin systems work could point to new ways to treat infections,” he says. “For instance, it’s possible that the chaperones that are essential for secretion in pathogens like Mycobacterium tuberculosis could be targeted — in this case, no chaperones means no toxins, and no toxins means no disease.”