Rogue proteins give yeast an edge

Yeast uses the prions as evolutionary stepping stones

Rogue proteins that behave like those linked to the human diseases vCJD and Alzheimer's can give yeast a survival advantage, scientists have found.

Prions, abnormal proteins which change normal ones into copies of themselves, are thought to cause some neurodegenerative diseases.

But US scientists found that yeast uses them to develop beneficial traits that give them an evolutionary advantage.

The study from the Whitehead Institute for Biomedical Research, US, appears in the journal Nature.

This is the first time we've seen a prion affect a cell in a beneficial way that can determine the evolution of an organism Dr Heather True

The research has revealed a new twist in the way genetic traits can be passed down from parent to offspring.

In diseases such as variant Creutzfeldt-Jakob Disease (vCJD), prions enter the brain and reproduce, converting other proteins into copies of themselves.

Lead author Heather True and colleagues found the yeast protein Sup35 could alter the properties - or phenotype - of the cell when it misfolds into the prion state.

"This is the first time we've seen a prion affect a cell in a beneficial way that can determine the evolution of an organism," said Dr True.

Normally, Sup35 helps guide the process by which cells manufacture protein molecules.

But in its prion state it forms amyloid fibres similar to those found in Alzheimer's patients.

Stepping stone

This is often harmful to the cell, but in about 20% of the cases tested, the new phenotype gave the yeast a survival advantage.

In these rare cases, the yeast cell will pass on the trait to its progeny.

But the surprise came when the researchers studied what happened when daughter cells were mated, allowing genetic shuffling to took place.

They found that these cells could pass on the beneficial trait to their progeny without the rogue protein that gave them the advantage in the first place.

"We don't know yet how the daughter cells do this, but they do it quickly, often after a single mating," said Susan Lindquist, director of the Whitehead Institute, which is based in Cambridge, Massachusetts.

The prion seems to act as an evolutionary stepping stone, giving cells the chance to survive in a new environment until they can acquire the genetic changes that produce the same effect.

The ability to acquire the Sup35 prion state has been conserved in yeast for at least 100 million years.

The complexity of these new genetic traits leads the authors to suggest that the prion state of Sup35 affects several genes at the same time.

"This prion has a capacity to hide and release genetic information throughout the entire genome that can contribute to new traits in a complex way," Professor Lindquist said.