New & Noteworthy

Yeast Gets By With a Little Help From its Friends

October 09, 2014

If you spend any time looking at social media, you’ve seen the viral videos about interspecies “friendships” – heartwarming scenes of elephants playing with dogs, or lions cuddling with antelopes. These animal relationships strike a chord with most people. Maybe they make us feel there’s hope for harmony within the human species, if such different creatures can get along with each other.

It may not give you quite as warm and fuzzy a feeling, but in a recent Cell paper, Jarosz and colleagues have shown that yeast and bacteria enjoy a friendship too.  However, these microbes have taken it a step further than the larger animals.

Not only do the yeast and bacteria get something good out of the relationship, but the yeast also get a permanent change that they can pass down to their daughters. It is as if being friends with an elephant could give a dog (and her puppies!) the ability to survive on grasses and fruit.

Like koalas with their eucalyptus leaves and pandas with their bamboo, yeast is a nutritional specialist. It is very good at consuming glucose, and will eat nothing else if glucose is available. All the genes necessary to metabolize other carbon sources are tightly turned off in the presence of glucose, a phenomenon termed glucose repression.

As Jarosz and coworkers studied this glucose repression, they stumbled upon the finding that contaminating bacteria could short circuit this process in yeast.  In other words, when yeast and these bacteria grew together, the yeast gained the ability to metabolize other carbon sources in the presence of glucose!  And even more surprisingly, that trait was passed on to the yeast’s future generations.

Here’s how this discovery unfolded.  The authors had plated yeast on medium containing glycerol as a carbon source, plus a small amount of glucosamine, which is a nonmetabolizable glucose analog. Wild-type cells cannot grow on this medium because the presence of the glucose analog makes it seem like glucose is present, causing glucose repression and preventing utilization of the glycerol.

However, there happened to be a contaminating bacterial colony on one plate, and the yeast cells immediately around this colony were able to grow on the glycerol + glucosamine medium.  When those yeast cells were re-streaked onto a fresh glycerol + glucosamine plate, with no bacteria present, they were still able to grow: they had undergone a heritable change. The ability to utilize glycerol in the presence of glucosamine was stably inherited for many generations, even without any selective pressure.

Although the first observation was serendipitous, this proved not to be an isolated phenomenon. The researchers were able not only to reconfirm it, but also to show that it could happen in 15 diverse S. cerevisiae strains. They identified the original bacterial contaminant as Staphylococcus hominis, but showed that some other bacterial species could also give yeast the ability to bypass glucose repression.

This group had previously found a way that yeast could become a nutritional generalist: by acquiring the [GAR+] prion. Prions are proteins that take on an altered conformation and can be inherited from generation to generation.  They usually confer certain phenotypes; one of the best known is bovine spongiform encephalopathy, or mad cow disease.

Luckily for the yeast, the [GAR+] prion is not nearly so devastating. Instead of a deteriorating brain, S. cerevisiae cells carrying the [GAR+] prion can grow on multiple carbon sources even in the presence of glucose.

Since this phenotype was suspiciously similar to that of the yeast that had been exposed to bacteria, Jarosz and colleagues tested them for the presence of the [GAR+] prion, and found by several different criteria that the cells had indeed acquired it. They looked to see if the yeast got other prions as well, but found that bacterial contact specifically induced only the [GAR+] prion.

The next step was to find out how the bacteria were communicating with the yeast.  Since active extracts could be boiled, frozen and thawed, digested with RNAse, DNAse, or proteases, or filtered through a 3 kDa filter without losing activity, the signaling molecule(s) was probably small.  But the researchers ended up with a complex mixture of small molecules, and more work will be needed to find which compound(s) are responsible for this effect.

In the case of animal friendships, it’s believable that intelligent animals are getting some emotional reward from their relationships (If you don’t believe it, the story of Tarra the elephant and Bella the dog in the video below may convince you!). We can’t exactly invoke this for microbes, so why would these organisms have evolved to affect each other in this way?  It seems there must be a “reward” of some kind.

The benefit to yeast cells from their bacterial friendship is that when they carry the [GAR+] prion, they can grow much better in mixed carbon sources and have better viability during aging.

Conversely, the bacteria benefit because [GAR+] yeast cells produce less ethanol than do cells without the prion. This makes a better environment for bacteria to grow, since too much ethanol is toxic. Interestingly, although the bacterial species that were the best inducers of [GAR+] are not phylogenetically closely related to each other, several of them share an ecological niche. They are often found in arrested wine fermentations, which are unsuccessful fermentations in which the yeast stop growing and bacteria take over.

So interspecies “friendships” can have more profound effects than just tugging at the heartstrings of viewers.  One example is the cat that acts as the eyes for that blind dog.  Another is this case, where bacteria can do yeast some permanent good and make a more hospitable environment for themselves in the process.

And this study reminds scientists of two important things. First, that the laboratory environment cannot tell us everything about biology. How often do yeast cells in nature grow as a monoculture on pure glucose, anyway?  And second, that sometimes accidental occurrences in the laboratory, in this case “contamination,” can broaden our findings…if we pay attention to them.  Just ask Alexander Fleming!

by Maria Costanzo, Ph.D., Senior Biocurator, SGD

Categories: Research Spotlight

Tags: bacteria, glucose repression, prions, Saccharomyces cerevisiae

Like People, Prions Need Intimate Contact to Spread

June 19, 2014

In the Matrix Trilogy, the delicate balance of a virtual world is upset by a rogue computer program that goes by the name of Agent Smith.  This program finds and touches other agent programs, converting them into copies of itself.  Eventually, all the agent programs are copies of Agent Smith and only the hero Neo can save humanity in an epic battle within the virtual world of the Matrix.

A new study out in GENETICS by Li and Du provides additional evidence that prions in the yeast Saccharomyces cerevisiae work similarly to Agent Smith, in that they spread through a direct contact model.  These prions are proteins that have entered a rogue conformation, and they end up converting all copies of the same protein into a similar rogue conformation.  The proteins change from a hardworking Agent Smith trying to do its job into something that mucks up the working of a cell.  And the results, at least in humans, can be as catastrophic for the cell as Agent Smith was for the Matrix. 

Mad cow disease, for example, is caused by prions converting the prion protein (PrP) in the brain cells of people from a useful conformation to a dangerous one that spreads.  As the conformation spreads throughout the cell, these prions form amyloid fibrils that eventually kill the cell.  When enough brain cells are killed, the person dies.

The authors chose to work in yeast because unlike in people, there are multiple examples of proteins in yeast that can go prion.  The list includes Sup35p, Ure2p, Rnq1p, Swi1p, Cyc8p, Mot3p, Sfp1p, Mod5p and Nup100p.  As you might guess from the sheer number of these prion-ready proteins, prions actually do more than kill a cell in yeast; they can serve useful functions. Scientists have yet to identify any useful functions for the prion form of PrP in people. 

Having multiple prions in a cell allowed Li and Du to perform some experiments to try to distinguish between two models of prion conformation spreading.  In the first, called the cross-seeding model, the prion acts very much like Agent Smith in that it needs to contact a “healthy” protein to convert it into a prion.  In the second model, the titration model, factors in the cell that prevent prion formation are titrated out when prions form.  As the factors are taken out of commission, prions are free to form.

The main evidence in this study that supports the cross-seeding model has to do with the localization of pre-existing prions during the de novo formation of a new prion.  Li and Du found that the prion [SWI+] localized to newly forming [PSI+] prions but not to already formed [PSI+] prions.  This is not the result we would expect if prion formation were due to titrating out of inhibitors of prion formation.  If that were the mechanism, then there would be no reason for [SWI+] to colocalize with newly forming [PSI+]. These experiments are like having a google map of the Matrix where we could see Smiths converting other agents by touch and then moving on and touching other agents.

Work like this is important for helping to find treatments for prion associated diseases and, perhaps, other amyloid fibril forming diseases like Huntington’s or Alzheimer’s.  Scientists need to focus on the amyloid fiber forming proteins themselves instead of trying, for example, to ramp up the activity of factors that inhibit formation.  Scientists probably need to eliminate Agent Smith to prevent the destruction of the Matrix and all of mankind.

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This is how prions turn other proteins into copies of themselves:

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight, Yeast and Human Disease

Tags: prions, Saccharomyces cerevisiae

Prions Let Yeast Take Traits for a Test Drive

August 27, 2012

For the most part, prions have a bad rep. They are the proverbial bad apple that spoils the whole bunch.

A prion is a protein that misfolds in a certain way that creates a chain reaction to misfold many additional copies of that particular protein in a cell. This misfolding en masse can cause severe problems like mad cow disease or Alzheimer’s.

As if that weren’t bad enough, this misfoldedness can spread from one organism to another. Once a prion gets into a cell and/or a part of the body, it will cause many of its properly folded brethren to misfold too. This is true even though the prion gene in the new host is happily churning out properly folded protein.

These things look like a nightmare. Why on Earth are prions still around? Because in addition to their bad side, they can sometimes be an advantage too (at least in yeast).

In a study published in Nature in February 2012, Halfmann and coworkers provide compelling evidence that prions can help both laboratory and wild yeast strains to adapt rapidly to a changing environment, by unlocking survival traits hidden in yeast DNA. In other words, prions are a way for a yeast population to hedge its bets against a world of changing environments.

The authors focused on the most famous prion in yeast, the translation termination protein Sup35p. When Sup35p switches to prion mode ([PSI+]), it becomes bound up in insoluble fibers, causing translation termination to become leaky. Now normally untranslated parts of mRNAs become part of their respective proteins. And this can change these proteins’ functions.

Sure, most of this newfound variation will have no effect or maybe even be harmful, but occasionally the prion will reveal a beneficial trait. This yeast can then go on to survive and even thrive in this new environment.

This mechanism may apply to other prions in addition to Sup35p. Prions tend to come from proteins that are global regulators of transcription or translation. In the non-prion form, these proteins do their usual job making sure transcription and translation are following the rules. But when these proteins become misfolded into a prion, they can no longer perform their usual function. This uncovers previously silent bits of DNA or RNA for transcription or translation.

These authors also convincingly showed that prions are not some weird phenomenon found only in laboratory strains of yeast. They found evidence for prions in 255 out of the 690 wild strains they surveyed (although only ten had Sup35p based prions). Not only that, but many of these prions also conferred new traits on the yeast that could be beneficial in certain circumstances. It looks like prions may serve an important function in yeast.

A more surprising result from the study is that these prion-derived traits carry on in later generations even after the prion has been removed. For example, the authors looked at the wine yeast UCD978. They found that when Sup35p was in its prion form in this strain, UCD978 could effectively penetrate agar surfaces and that this trait was lost when the prion was cured, reverting Sup35p to its functional form.

They then took the study further and showed that after meiosis and sporulation, 5/30 haploid progeny of UCD978 retained the trait even after the prion was removed. These five had fixed the new trait and no longer required the prion to maintain it. They got all the benefits with none of the costs.

It isn’t obvious how this trait became independent of the original inducing prion. But that is for another study (or two or ten).

If the results of this study pan out, they show that prions are not just part of a disease but are really just another way to adapt to environmental changes and to pass them down to future generations. Maybe these apples aren’t so bad after all!

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: evolution, prions, S. cerevisiae, SUP35, variation

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