The bacteria responsible for the plague and some forms of food poisoning "paralyze" the immune system of their hosts in an unexpected way, according to a new study in the September 8, 2006 issue of the journal Cell.
The researchers found that these bacteria, which belong to the genus Yersinia, harbor a protein that mimics an apparently unrelated mammalian enzyme. That copycat protein blocks host cells' capacity to change shape and move, abilities important for cells of the immune system to track down and "eat" foreign invaders, the researchers explained.
The discovery marks the second way in which this protein, called YpkA, compromises the immune system. Earlier studies suggested that another portion of YpkA--which may have been derived from a mammalian enzyme and later co-opted by Yersinia--has activity that also influences cell shape by a separate, though incompletely understood mechanism.
The findings offer important new insight into the factors that lend Yersinia their ability to spawn disease, the researchers said. The results might also contribute to new strategies for fighting the bug.
"Yersinia injects several virulence factors into its host," said C. Erec Stebbins of Rockefeller University. "If we can discover which ones are critical, we might identify the pathogen's Achilles heel--an attractive target for antibacterial or anti-virulence compounds."
"We were quite excited to see such a critical and unexpected factor in the virulence of Yersinia--a bacteria historically responsible for some of the worst diseases," he added. Although improvements in sanitation have eliminated acute problems from diseases caused by Yersinia, concerns remain about the possibility that an untreatable strain might arise or that the bacteria might come into use as a biological weapon, he said.
Nearly 200 million people are estimated to have died in the plague epidemics that devastated the ancient world, the researchers said. The successful weaponization of plague in the former Soviet Union bioweapons program also made the pathogen a primary biodefense concern. Additional medical concerns have arisen from the evolution of multidrug-resistant strains of the plague bacterium found in patients from several locations.
The plague bacterium Yersinia pestis is closely related to Y. enterocolitica and Y. pseudotuberculosis, which are food-borne agents that cause inflammation of the stomach and intestines. All Yersinia bacteria have a virulence plasmid, which is necessary to cause disease. Plasmids are extra DNA molecules frequently found in bacteria containing genes that can be passed from one bacterial strain to another and that may confer an evolutionary advantage, such as antibiotic resistance.
In the case of Yersinia, the plasmid harbors numerous genes, including a large number that contribute to the ability of diverse pathogens to deliver virulence factors into host cells. One of these genes is YpkA, a protein with multiple domains, including one closely related to an enzyme, a type of kinase, not typically found in bacteria. Earlier studies found that mutations that eliminate this activity reduce but do not eliminate YpkA's ability to disrupt cell shape by modifying their cytoskeletal support system.
In the current study, the researchers solved the high-resolution crystal structure of a second YpkA domain, the "Rho-GTPase binding domain" along with the host protein, "Rac1," with which it interacts.
"The Yersinia structure was doing things to Rac1 that the host proteins normally do," Stebbins said, suggesting that the domain acted as a mimic.
Further examination confirmed the domain to be a mimic of mammalian "guanidine nucleotide dissociation inhibitor" (GDI) proteins with a critical role in the bacteria's ability to disrupt cell structure. The domain paralyzes cells by acting as an "off-switch" for host proteins involved in modifying cell shape, Stebbins said.
Mutations that prevented the bacterial proteins' interaction with the host protein significantly impaired YpkA's ability to disrupt the cytoskeleton. Moreover, a mutant strain of Y. pseudotuberculosis that lacked the GDI activity caused significantly fewer problems for infected mice compared to normal bacteria.
"Earlier studies that focused only on the protein's kinase activity had missed half the picture," Stebbins said. "The GDI domain seems to have an even bigger effect on host cells in culture, and a significant impact on virulence."
The results also add to broader themes in the evolution of bacterial diseases, the researchers added.
"It is becoming increasingly clear that a common strategy used by bacterial pathogens to manipulate host cell biology is the mimicry of their own biochemical processes," Stebbins said.