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Not sure if there's a better Stack Exchange for this, but How could Darwin be sure all life has a common origin triggered this question.

As I understand it, prions are pretty much protein-based viruses. I know that there's no perfect definition of "living," but if you put a virus in that set, ought you not also include prions?

Is there any evidence of prions evolving from a xNA-based organism?

-- Edit --

@OliverC spoke most directly to the thoughts I had in mind, which I suppose I can restate as a question about "natural origin of replicating, (perhaps) evolutionarily-capable entities." Admittedly, it requires a protein "environment" which originates from nucleic acids, but if prion replication began without evolving from, e.g., viral replication, then it seems to me an astonishing thing.

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Larry OBrien
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    I was asking myself this question yesterday :) – Chris Dennett May 14 '11 at 00:55
  • "qualify as living" implies that "living" is a yes/no attribute. For all we know, neither living or dead are on/off. – Sklivvz May 14 '11 at 09:15
  • Could you maybe clarify what you mean by "evolving from a xNA-based organism"? Like all proteins prions are translated from DNA via RNA into the final prion protein, I'm not sure what you're asking there. – Mad Scientist May 14 '11 at 09:32
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    By the way, viruses aren’t alive either, by any common definition of the term. – Konrad Rudolph May 14 '11 at 13:32
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    How is this question on-topic? 1. There is *no actual claim* that prions are alive; 2. The question in the body is too vague to be answered. – Sklivvz May 14 '11 at 14:22
  • @Sklivvz This will be the next big thing in the anti-evolution pseudosciences. I'm taking bets. – Rusty May 14 '11 at 18:54
  • @Rusty In what way do prions fit into an anti-evolution agenda? Just the opposite, I would think, as their replication mechanism is considerably less elaborate than that of organisms based on *NA. They don't argue for a watchmaker. – Larry OBrien May 14 '11 at 19:05
  • @Larry Because they already [have](http://www.evolutionnews.org/2010/01/does_lifeless_prion_evolution030701.html). – Rusty May 14 '11 at 20:15
  • @Larry Please don't take my comments as any sort of personal slight or attack. We can always discuss things in chat _(where I can also pester you with questions about the Gemini telescopes)_. – Rusty May 14 '11 at 20:27
  • @Rusty No problem. So the anti-evolutionists take evidence of evolution as supporting their contention that evolution doesn't explain origin? Huh. – Larry OBrien May 14 '11 at 22:05
  • @Larry Yup. More evidence of [irreducible complexity](http://en.wikipedia.org/wiki/Irreducible_complexity) will also be claimed. BTW: Questions with accepted answers do not receive many new answers and voting falls off for both. You might consider waiting a bit longer before accepting an answer (especially over a weekend). – Rusty May 14 '11 at 22:45
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    If prion is alive, then religion is alive too. Prion is a way protein fold. Religion is a way people behave. So in that sense yea, life do have different origins. Once computer virus can mutate, it'll be alive too. Life means: reproduce, inherit, mutate. –  Nov 14 '11 at 08:13

3 Answers3

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A common definition of life consists of the following characteristics:

  • Homeostasis
  • Metabolism
  • Organization
  • Adaptation
  • Growth
  • Response to stimuli
  • Reproduction

You could argue that they fulfill the reproduction critera, as the disease-causing, misfolded prion protein PrPSc can convert other prions into the misfolded state. To reproduce PrPSc needs the whole protein synthesis machinery of the cell to produce normal prions it can convert, it cannot reproduce independently.

This does not mean they are alive, they don't fulfill any of the other conditions usually associated with life. Don't take me wrong, the reproduction of prions is an amazing and previously unexpected phenomenon, but that doesn't mean they are alive in any way.

Viruses are also not generally considered alive, as they don't fulfill all of the common criteria for live. The exact definition of life is more of a debate, you could certainly argue that viruses are alive in some way, but that is not a universally accepted claim in any way.

Mad Scientist
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    Prions don’t even reproduce properly. They are able, by shoehorning the service of other proteins, to impart *certain aspects* of their information onto other proteins. The important difference here is that prions in themselves are not a complete reproductive machinery (much like viruses) and don’t replicate all their information (unlike viruses). – Konrad Rudolph May 14 '11 at 13:31
  • @Konrad I've clarified the point about reproduction – Mad Scientist May 14 '11 at 13:53
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Prions are proteins and we all have them in us, in abundance.

PrPC is the normal form and actually good for us.

Then there is

the abnormal form, the "evil twin" : PrPSc or PrPCJD

Evil Twins Image Source


When the evil twin encounters the good twin things go awry, because

good joins the dark side and turns into evil

Prions Image Source


So like zombies or Agent Smith in The Matrix,

Matrix Image Source

PrPSc / PrPCJD multiplies by "touch".

That's why it doesn't need DNA or RNA to reproduce.


Interestingly, prions might be able to evolve and adapt:

If prions can evolve, and if they can show the same sort of adaptive resistance as bacteria or fungi, does this mean that they are alive?

Charles Weissman doesn't think so on the grounds that prions are completely dependent on their hosts for reproduction. They need normal proteins that are encoded within the genome of their host to make more copies of themselves.

He says, "The remarkable finding that prions can mutate and adapt to their environment imbues them with a further attribute of living things, without however elevating them to the status of being 'alive'."

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Oliver_C
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Setting aside the issue of whether prions are considered "alive" (which is as much about English definitions as any facts), there is the question of whether they came from RNA/DNA.

In ScienceDaily, they reported about a study that looked at the evolutionary origins.

"The prion protein was discovered over twenty years ago and has been studied intensively. Nobody, however, knew its evolutionary origin and much confusion surrounds its physiological function," says Prof. Schmitt-Ulms. The team's analysis suggests that the prion gene is descended from the more ancient ZIP family of metal ion transporters. Members of the ZIP protein family are well known for their ability to transport zinc and other metals across cell membranes.
Although no single evidence firmly established the phylogenetic relationship between ZIP and prion genes, Schmitt-Ulms is confident that the many corroborating pieces of evidence collected and, equally important, the absence of any conflicting observations, allow no other conclusion to be drawn.

References:

  • University of Toronto. "Evolutionary Origins Of Prion Disease Gene Uncovered." ScienceDaily, 29 Sep. 2009. Web. 14 May 2011.
  • Gerold Schmitt-Ulms, Sepehr Ehsani, Joel C. Watts, David Westaway, Holger Wille. Evolutionary Descent of Prion Genes from the ZIP Family of Metal Ion Transporters. PLoS ONE, September 28, 2009 DOI: 10.1371/journal.pone.0007208
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