The discovery in common brewer's yeast of a new, infectious, misfolded protein -- or prion -- by University of Illinois at Chicago molecular biologists raises new questions about the roles played by these curious molecules, often associated with degenerative brain diseases like "mad cow" and its human counterpart, Creutzfeldt-Jakob.
Susan Liebman, distinguished university professor of biological sciences, and postdoctoral research associate Basant Patel propagated the new prion from a normal yeast protein called Cyc8. They note that like the Cyc8 protein, the prion of Cyc8 can affect the expression of a large number of yeast genes.
"We know this prion turns on the expression of genes but we don't know if the prion forms naturally," said Liebman. "If it were to form, it would have this effect. But whether it happens out in the wild all the time, we don't know."
Liebman and her coworkers discovered that Cyc8 was a prion candidate using a genetic screen that looks for proteins that when overproduced can spur formation of new prions. To date, scientists have discovered only seven prions, six of which are only in fungi, including yeast. The latest two discovered, Cyc8 and another, identified as Swi1, came from genes screened in Liebman's lab. The Cyc8 prion was characterized by the UIC scientists, while the Swi1 prion was found by Northwestern University researchers.
The normal Cyc8 protein shuts down expression of more than 300 genes in yeast, says Patel, including some genes that are involved in stress tolerance.