|Photo courtesy Mary Beth Mudgett, University of California, Berkeley|
U-M scientists now have discovered the molecular mechanism Yersenia uses to sever these vital cell signaling pathways. It turns out to be an ancient agent of deathso effective that both plant and animal bacteria have been using it throughout long periods of evolutionary history.
Results from the U-M studycompleted in collaboration with scientists at the University of California-Berkeley, the State University of New York at Stony Brook and Brookhaven National Laboratoryare published in the Nov. 24 issue of Science.
YopJ, the protein Yersenia uses to cut cell signaling pathways, is one of six proteins the bacterium injects into immune cells called macrophages, says Jack E. Dixon, the Minor J. Coon Professor of Biological Chemistry and co-director of the Life Sciences Institute. Every Yop has a specific function, and the proteins work together to get inside cells and destroy the bodys defense systems.
In research published last year in Science, Dixons team reported that YopJ attacks two vital cellular signaling pathways called MAPK and NFkB, which regulate immune response and help prevent cell death.
Now we have found closely related variants of YopJ in several species of pathogenic plant and animal bacteria, as well as in Rhizobiumsymbiotic bacteria that live on plant roots, says Dixon, who directed the research project.
When Mary Beth Mudgett, a postdoctoral fellow at Berkeley, infected leaves with the plant equivalent of YopJa protein called AvrBsTblack patches of dead cells appeared around the infection site. The leaf induces cell death in areas exposed to the bacteria to prevent it from spreading through the entire plant, explains Kim Orth, a research investigator in the Medical School and first author on the Science paper.
Zhaohui Xu, assistant professor of biological chemistry, found that all these plant and animal YopJ-related proteins look like cysteine proteasesspecialized enzymes that cut up proteins. Xu is a Biological Sciences Scholara program started by Gilbert S. Omenn, executive vice president for medical affairs, to recruit promising faculty candidates from the countrys top research institutions.
Detailed comparisons of the molecular structure of YopJ-related proteins in the study found that they all shared a key catalytic sitefour amino acids nestled in a pocket, which must be present for YopJ to do its protein-cutting work. YopJ mutants that lacked even one of these amino acids could not block MAPK pathways and had no effect on macrophage immune response.
Although future research is needed to confirm their hypothesis, Dixon and Orth believe that YopJs disrupt a vital, but previously unappreciated, step in cell signaling pathways called ubiquitination.
Until recently, scientists believed that ubiquitin proteins simply mark other proteins for destruction, Orth says. This study shows that ubiquitin-like proteins 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 dont know YopJs target molecule, but at least now we know it must be a member of the ubiquitin protein family, Orth says. Identification of the molecule could have important implications in medicine, Orth adds, because these pathways are critical in development of cancer and immune-related diseases.
The study was funded by the National Institutes of Health, the U.S. Department of Energy and the Walther Cancer Institute. Additional collaborators on the study included Zhao Qin Bao, research associate; Brian Staskawicz, professor of plant and microbial biology at Berkeley; Lance E. Palmer and James B. Bliska, from Stony Brook; and Walter F. Mangel, professor of biology at the Brookhaven National Laboratory.