By David N. Leff
In medieval Europe, the bubonic plague pandemics of the 1300s and 1600s killed 40 percent to 60 percent of the population in England and Italy alone. The dark splotches that marked its victims' skin gave their name to the scourge, known ever since as the Black Death.
Now switch to the tomatoes growing in your back yard. The "black spot" infection that blemishes their leaves and flesh is directly related (at the molecular level) to the lethal plague pathogen, Yersinia pestis.
When this blood-borne bacterium encounters a macrophage - the immune system's front-line defense - it doesn't invade that sentinel cell. Rather, the pathogen stands off, and fires six rounds of its infective agent, Yop (Yersinia outer surface protein), into the macrophage. This salvo triggers a domino-like cascade of molecular signals that commands the immune system to shut up shop - thus leaving the pathogen free to get on with its serial killing.
Here's how postgraduate biochemist Kim Orth, at the University of Michigan, Ann Arbor, recounts the process: "The way Yersinia works is it has this so-called type III secretion system, which is really cool. It looks like a syringe perched on its cell surface. When the bacterium comes in contact with a macrophage, it shoots these effector molecules into that cell. And YopJ knocks out the immune response, while other molecules debilitate the macrophage from engulfing the Yersinia."
Orth is lead author of a paper in the current issue of Science, dated Nov. 24, 2000, and titled: "Disruption of signaling by Yersinia effector, YopJ, a ubiquitin-like protein protease."
"What we found," she told BioWorld Today, "was a novel mechanism used to regulate the pathways that elicit an immune response at a molecular level. The other really exciting thing," Orth added, "is that this molecule is used not only by animal pathogens but by plant pathogens. That suggests that they alter the same host defense response in plants as in mammals. So the same thing that causes the Black Death is what causes the black spot on your tomato plants."
Black Death For Plant Kingdom, Too?
"It was discovered this year," Orth continued, "because we collaborated with a group of plant and molecular biologists at the University of California, Berkeley. When they infected leaves with the plant equivalent of YopJ, black patches appeared around the infection site. The leaves induced cell death in areas exposed to the bacteria to prevent it from spreading through the entire plant.
"A year ago," she recalled, "we had found that YopJ was blocking a number of different signaling pathways." (See BioWorld Today, Sept. 27, 1999, p. 1.)
This year we figured out what YopJ is - not exactly what it's targeting in those signaling pathways, but the mechanism that it might be using to disrupt those communications. Now we think that there's another post-translational modification that's necessary for signaling in addition. And this is shared both by the plant world and the animal world.
"Our next paper, hopefully, will describe what exactly the targets of YopJ are. But that might take another year."
Presumably, pathogenic bacteria don't want to kill off their human host because that would be the death of them, too. Why, Orth was asked, is Yersinia perfectly happy to kill off 90 percent of its human victims?
"That's because we're not its targeted host," she pointed out. "Yersinia works in another ecosystem - between a rodent and a flea. The flea carries the pathogen from one rodent host to another, by biting it and then regurgitating the Yersinia into the rodents' bloodstream. So the flea dies - 100 percent mortality rate in the flea, from ingesting the pathogen. But in the rodent it kills only about 60 percent. So the disease persists within this rodent/flea population.
"Thus in the Western U.S.," Orth said, "you've got these little colonies of rodents and fleas that co-exist, and the pathogen persists. So if a human hiker goes through there and he's bitten - and isn't diagnosed the right way - he's dead in four days. But Homo sapiens weren't the original targeted hosts for this pathogen. The only reason we became a host is because of what happened in the Middle Ages with some rodents and fleas being around. With that kind of dynamics we eventually became the host.
"We also know that other homologues of YobJ are found in other types of pathogens, including the Salmonella bacterium, which isn't lethal. Also in Rhizobia, which are nitrogen-fixing soil bacteria - symbiotic with the roots of plants such as clover and beans. So YopJ probably goes in and modulates the signaling pathways for a host defense response. It says to its target, 'I'm here and you're here. We can co-exist, without inducing a host immune response.'
"So now we think," Orth summed up, "that we know what mechanisms they are using. It's basically a ubiquitin-like protein mechanism. These molecules are put on and taken off, and tell the cells different things - just like phosphorylation."
"When it comes to killing cells," observed the Science paper's senior author, biological chemist Jack Dixon, "Yersinia pestis is the stealth assassin of the pathogen world. It kills quietly and efficiently by first slipping inside immune system sentinel cells, and cutting off the communication lines they need to call for help."
Clinical Payoffs Await YopJ's Hidden Molecule
Although future research is needed to confirm their hypotheses, Dixon and Orth believe that YopJs disrupt a vital but previously unappreciated step in cell signaling pathways - namely ubiquitination.
"During the last few years," Orth pointed out, "a lot of scientific papers that are making sense now have been discovering that ubiquitin is used for more than just marking other proteins for degradation. Our study shows that they are required to activate these critical cellular signaling pathways. When YopJ breaks the bond between ubiquitin and its target molecule, the pathway is blocked and cell communication shuts down.
"We still don't know YopJ's target molecule," she went on, "but at least now we know it must be a member of the ubiquitin protein family. Identification of the molecule," she concluded, "could have important implications in medicine, because these pathways are critical in the development of cancer and autoimmune diseases."