Commitment in an uncertain world - lessons from yeast

Even yeast cells have their share of romantic drama: not only must they choose who to mate with and when, but also they can opt for asexual reproduction. Researchers studying how yeast make these decisions have discovered a new chemical ‘switch’ involving a protein that also exists in people.

 

SULSA Professor Peter Swain from the University of Edinburgh was one of the four authors behind this work, which was recently published in Nature. The other authors hailed from Imperial College, London, and McGill University and the University of Montreal in Canada.

 

At the heart of the study was the puzzle that although yeast encounter many chemical signals, they are surprisingly sensitive to tiny changes in levels of pheromone released from potential mates. As soon as the pheromone level rises above a certain threshold, a cell will act fast to extend a protrusion towards its potential mate – a process known as “shmooing” - before another cell can beat them to it.

 

Although yeast’s strategic mating habits are well known to scientists, no one knew how the proteins within the cells could reliably switch on shmooing. To uncover the switch mechanism, the authors monitored which proteins came together while yeast cells were deciding whether or not to shmoo. They then used mathematical models to test how the proteins coordinate to bring about the switch.

 

The researchers’ mathematical approach led them to discover an unanticipated new switch mechanism centred on a ‘scaffold’ protein that is also found in human cells. As Swain explains, “Scaffold proteins are common, but even the name implies it’s something static; we’ve shown that this protein is active, not passive.”

 

The researchers discovered that two enzymes compete to modify the scaffold in opposite ways. When the pheromone level rises above the threshold, one enzyme always wins, triggering a chemical cascade that results in the cell shmooing.

 

Understanding the decision-making process in the single-celled yeast can help us understand similar processes in human cells. “Although yeast are dramatically different from people, at a molecular and cellular level, we have a lot in common,” explains co-author Stephen Michnick of the University of Montreal. “Similar switching decisions to those made by yeast are made by stem cells during embryonic development and become dysfunctional in cancers.”

 

Email: leitac@gmail.com 

 

The scaffold protein Ste5 directly controls a switch-like mating decision in yeast. Malleshaiah MK, Shahrezaei V, Swain PS, Michnick SW. Nature. 2010 May 6;465(7294):101-5.