BioEd Online

Prions, the twisted proteins usually linked to disease, could help organisms adapt to tough situations by subtly altering the proteins manufactured by a cell¹. The discovery backs the idea that proteins as well as DNA are vital in driving evolution.

Prions are proteins that twist into one of two shapes. In mammals, one type of prion seems to be harmless in one form but is infectious in the other. It is thought to underlie mad cow disease and its human equivalent, variant Creutzfeldt-Jakob disease (vCJD).

But scientists studying yeast (Saccharomyces cerevisiae) have found that, in some cases, infectious prions may have an important role. In a colony of yeast cells, some cells carry the 'normal' type of the protein, whereas others harbour the infectious form, which accumulates into clumps and is passed from one cell to another.

Four years ago, Susan Lindquist and Heather True of the Whitehead Institute in Cambridge, Massachusetts, showed that this yeast prion can change the way that cells behave. In their infectious form, the prions sometimes helped the yeast to adapt, changing their rates of survival when they were grown in various nutrients or temperatures.

Now Lindquist and her colleagues have worked out how the prions do this. In its non-infectious form, the protein normally helps to read and convert the DNA code into other proteins. But in its infectious form, the prion stops working. This means that many proteins are manufactured slightly sloppily.

Evolutionary short-cut

The team believes that prions may therefore offer a speedy way for yeast to evolve, because those cells with the infectious prion churn out a whole range of slightly altered proteins. Normally this is bad news for the yeast, but when the cells find themselves in a tough spot, one or two of them may grow better in the new conditions as a result, and so help the colony to survive.

This mechanism may be important for helping yeast stay alive over the short term, says True. It gives the cells time to pick up the permanent genetic changes they need to survive, which are then passed on to subsequent generations.

The finding runs against the general assumption in evolution that when organisms adapt to a change in their environment, they do so by acquiring random mutations in their DNA.

"I think the whole concept is very important," says molecular biologist Michael Snyder at Yale University in New Haven, Connecticut. Other proteins in different organisms could do a similar thing, Snyder suggests, by subtly altering the shape or amount of proteins made. "We don't know how extensive this is," he says.