Prions, Darwin's Friend

blamanj writes "Prions, the recently discovered bits of protein thought to be responsible for mad cow disease are turning up all over. It has been shown that prions change the behavior of yeasts, and may therefore offer a speedy way for yeast to evolve."

Comments

Re:Mad Yeast Disease?

[Anonymous Coward]

The same prions that are suspected to be the cause of Mad Cow Disease. See the connection? Yeah, the parent is definitely on-topic.

Except that these prions are not at all related to the Mad cow disease ones. "Prion" is a class of infectious substance, akin to "bacterium" or "virus". It's a relatively new class, so the mad cow disease ones are best known, but saying "Yeast prions will give you vCJD" is like saying "plant bacteria will give you tuberculosis".

mechanism for variable expression in vertabrates?

[oliphaunt]

from the article:

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.

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 is interesting for a number of reasons, but the one that sticks out in my mind is the transfer of plasmids between prokaryotes. Eukaryotes, like eyu and me, don't pass our DNA around as casually as, say, Staphlococcus Aureus does. This has the benefit that our DNA is relatively stable, meaning that we are almost guaranteed to be able to breed with someone two or three generations younger than we are ourselves (hellOOOO, Hef!) but the trade-off is that we're not very responsive to environmental stressors that require quick adaptation to survive.

In simpler terms, through conjugation live bacteria can pick up spare DNA from other living bacteria and immediately "evolve" to meet a new environmental challenge without needing to wait for another generation to be born before the mutation can take effect. The plasmid DNA is then passed along to the daughter cells of successuful bacteria. This obviously confers huge selective advantages on bacteria which pick up plasmids that code for antibiotic resistance genes.

I interpret this prion research in yeast to suggest that yeast can experience the same kind of variable protein expression as bacteria, throught the mechanism of prion inactivation. This is significant because eukaryotic cells do not experience conjugation, and generally do not have plasmids, but DO transport various small proteins across the cell wall and into the cytoplasm.

And if it works for yeast, perhaps the prion mechanism is intended to perform the same function in humans, or in the cows that end up with BSE. The test would be to find some gene in a higher mammal that expresses one way when a prion is in conformation A, but expresses another way in conformation B...

Re:Now if they could only get humans to evolve aga

[gl4ss]

well, human gene base is quite small compared to animals i recall, something like two chimps from the same tribe can have more variation in the genes than two humans chosen randomly from anywhere from the planet.

(though, there's differences in average height and whatever, i guess that would count as evolution that those change between distant generations)

Re:Mad Yeast Disease?

[AndrewRUK]

And, in addition to what my sibling posts have already pointed out, you can make bread without yeast. You can use another raising agent, as in soda bread (which uses baking soda), or consider the many unleavened breads, which (by definition) don't have anything in them to make them rise.

And as for beer... well, some things that are sold a beer bear a remarkable similarity to a liquid that we all produce without using any yeast ;-)

Re:mechanism for variable expression in vertabrate

[Dr. GeneMachine]

I wouldn't compare this effect to plasmid based inheritance. As far as I understand the article, the yeast prion PSI is a suppressor of nonsense mutation. This means that PSI works at the level of translation, the production of proteins from genetic information. Translation is normally a well regulated process which terminates at so called stop codons. PSI now enables the yeast translation machinery to read over this stop codons, thereby producing longer proteins with somewhat altered functions, or even new proteins which where silenced by stop codons. At this point this mechanism does not created inheritable phenotypes. Nevertheless, the group of True et al. found inheritably changed phenotypes in some PSI+ stems. This might be - and I'm speculating here - due to the production of altered replication proteins by this mechanism, which then introduce more mutations into the yeast genome.

In this light, the mechanism of yeast prion action is more akin to some error prone repair and replication mechanisms in bacteria, which are triggered by stress and result in an increase in spontaneous mutations. This is different from plasmid transfer, which is a directed transfer of well-defined genetic information

In case of mammalian prions, I see no indication of a similar mechanism. Human prion protein huPrP is a cell surface protein which does not interfere with the translation apparatus or with genetic processes in the nucleus. It rather appears to be involved in regulatory processes which transduce signals from outside into the cell.
Yeast and mammalian prions share no common characteristics except for the ability to exist in to different conformations and to autocatalytically propagate one of these conformations. In general, prions are no functionally homogenous group. Another example would be the prions of slime molds, which play a role in the exchange of genetic information by cell fusion. Here the prion-conversion mechanism creates some kind of incompatibility groups, shutting down the fusion zone if two cells belonging to different compatibility groups try to fuse.

The prion phenomenon is a basic biophysical characteristic, it has not to be bound to a certain function. The fact that yeasts and fungi have found a way to use this physical property of certain proteins does not implicate that other prions have to have useful functions also. It is absolutely possible that mammalian prions are a purely pathological phenomenon.