Prions in yeast
Protein particles similar to those suspected in "mad cow" disease found in yeast cells
August 2, 1996
In a finding that supports a new mechanism of inheritance and the leading theory as to the mysterious causative agent of "mad cow" disease and human Creutzfeldt-Jakob disease, researchers at the University of Chicago's Howard Hughes Medical Institute have shown that a defective cell trait can be propagated by a faulty protein, without any DNA or RNA serving as the genetic blueprint. The finding is reported in the August 2, 1996 issue of Science.
Susan Lindquist, PhD, professor of molecular genetics and cell biology, and colleagues showed that an improperly folded protein in yeast cells clumps together and then corrupts other, healthy molecules of the same protein to do likewise, in a process much like the "seeding" of a crystal. When the cell divides, the corrupt protein is transmitted to both daughter cells, where the process is repeated.
"We think this is a mechanism of inheritance that people haven't realized existed," Lindquist said. "We've shown that a change in the shape of a protein can perpetuate itself and be passed to other cells, without a change in DNA or RNA. It goes against our notions of inheritance, that DNA or RNA is the genetic material."
The new finding offers direct, physical evidence supporting protein-based inheritance, thus strengthening the "prion hypothesis" of the cause of neurodegenerative diseases in mammals, such as sheep scrapie, mad cow disease (or bovine spongiform encephalopathy) and the kuru disease of the Papua New Guinea tribes. These diseases are thought to be caused by prions (pronounced pree-ons), infectious protein particles--normal cellular proteins that supposedly change their shape and then mold other protein molecules into the same sickly altered form.
The hypothesis has been hotly debated for thirty years, since researchers showed that diseased brain tissue remained infectious even after treatment with radiation that would have destroyed any DNA or RNA. Prions have been the cold fusion of infectious disease research, but the Chicago researchers have proved that prion-like elements exist--at least in yeast.
Lindquist's group focused on a yeast protein called sup35, part of the normal yeast machinery for making all the other proteins in the cell. In certain strains--which appear to have identical DNA to normal strains--the sup35 protein doesn't work. By breaking open the cells and spinning the contents in a centrifuge, the researchers showed that in yeast strains with functional sup35, the protein was evenly dispersed in the cell. In affected strains, the protein was clumped together--which is also observed in the case of the mammalian prion diseases.
The researchers also attached a fluorescent tag to the sup35 molecule, allowing them to show that in affected cells the newly-synthesized protein was being converted to the defective, clumping form. The fluorescent tag also allowed them to visualize, for the first time, the spontaneous appearance of new genetic elements--the protein particles--in cells that were overproducing sup35.
Although the yeast sup35 protein and the mammalian prion protein are not at all related to each other--the yeast pose no risk to consumers of bread or beer--the researchers think that in-depth analysis of the yeast prion-like elements and other proteins that help them fold up may lead to new approaches to therapies for neurodegenerative diseases.
Lindquist said perhaps the most important implication of this work is that it shows that the ability of certain proteins to confer heritable properties by changing their shape is found widely in nature and may underlie other diseases or unexplained genetic phenomena.
The researchers note that in the mammalian brain, whose cells do not divide, prions pass between cells and function as infectious agents; in yeast, they produce heritable changes from one generation to the next.
"That a genetic property carried by protein shape can be responsible for inheritance from generation to generation or for an infection is a very powerful concept," Lindquist said.
Other authors on the Science paper are research associates Maria Patino and John Glover and graduate student Jia-Jia Liu. The research was funded by the Howard Hughes Medical Institute and the National Institutes of Health.
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