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I've been reading my textbook and it refers to prions as a normal protein with a helpful function but it can turn into a disease causing form. However, I look in my other textbook and it refers to the word prion as solely being a disease causing protein.

I'd like to know which is the correct definition. Ie. Would I be correct in saying "The prion protein is normally involved in synaptic transmission but can turn into a disease causing form"?

Thanks in advance!

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The normal isoform of the protein is called PrPC, which stands for cellular prion protein, while the infectious isoform is called PrPSC, which stands for scrapie prion protein.

According to Riesner (2003):

The biochemical properties of the prion protein which is the major, if not only, component of the prion are outlined in detail. PrP is a host-encoded protein which exists as PrPC (cellular) in the non-infected host, and as PrPSc (scrapie) as the major component of the scrapie infectious agent. (emphasis mine)

If you search for "cellular prion protein" you're gonna find several papers that use the name prion protein to the normal isoform. Some examples:

And many others.

Therefore, following this nomenclature, the answer to your question ("Would I be correct in saying 'The prion protein is normally involved in synaptic transmission but can turn into a disease causing form'?") is yes. The difference is the adjective: cellular or scrapie.

Finally, pay attention to this: you have two different questions here. In the title you say "Is prion a term used...", but in the last paragraph you say ""Is the prion protein normally involved in...". As extensively discussed in the other answer, the term prion alone (instead of prion protein) is normally used only when referring to the abnormal isoform. More on that here: https://www.cdc.gov/prions/pdfs/public-health-impact.pdf

Source: Detlev Riesner; Biochemistry and structure of PrPC and PrPSc. Br Med Bull 2003; 66 (1): 21-33.

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    $\begingroup$ This is also a fairly good review that outlines some of the history of the research and how the naming came to be. THE PUBLIC HEALTH IMPACT OF PRION DISEASES $\endgroup$ – AMR May 25 '17 at 3:37
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    $\begingroup$ @AMR Thanks, I just linked that in the question. $\endgroup$ – user24284 May 25 '17 at 5:15
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If we are considering prions in general, I disagree with the answer supplied by Gerardo Furtado.

Here is a definition taken from an article by Susan Lindquist:

[Prions are]...self-perpetuating and heritable protein conformations that cause multiple phenotypes, represent an unusual mechanism of information transfer that occurs via protein instead of nucleic acid.

Compare Wikipedia:

Prions are infectious agents...

If a prion is self-perpetuating or infectious then the normal form of such a protein is not a prion. In yeast the term prion is used to refer to the infectious form of a protein in a formal genetic sense. For example the [PSI+] prion was discovered as a genetic element and then subsequently found to be associated with the translation termination factor Sup35, a protein with a known cellular role. All yeast strains have Sup35 protein but only certain strains contain and transmit the prion form of this protein. So [PSI+] is referred to as the prion form of the protein.

The problem with interpreting the language used to describe the TSE prions is that they were discovered because of their prion activity so the normal form is referred to as prion protein (as far as I am aware the function of the normal form isn't known). This doesn't mean that this form of the protein IS a prion, merely that this protein is associated with the original prion phenomenon so is called 'prion protein'.

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    $\begingroup$ And if you didn't truncate your Lindquist quote: "Prion proteins can assume at least two conformations" $\endgroup$ – OrangeDog May 24 '17 at 16:44
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    $\begingroup$ I don't understand why you think that makes a difference. If a prion is the infectious form ('infectious agent') of a protein that can fold in different ways, that doesn't mean that the non-infectious form is a prion. A cell which contains only the normal form of a prion-forming protein doesn't contain any prions since it contains no infectious agents. $\endgroup$ – Alan Boyd May 24 '17 at 16:49
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    $\begingroup$ Yes, Lindquist refers to proteins that can form prions as 'prion proteins' but not as prions. Just like the cellular form of the TSE agent is called 'prion protein'. But please, write your own answer. $\endgroup$ – Alan Boyd May 24 '17 at 16:53
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    $\begingroup$ I don't need to. I just need to upvote the correct answer and downvote the wrong one. $\endgroup$ – OrangeDog May 24 '17 at 16:53
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    $\begingroup$ @GerardoFurtado There are two explicit questions and it seems that you answered one and this post the other. Anyways, you've got my upvote after your edit. $\endgroup$ – canadianer May 25 '17 at 2:08
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The Wikipedia definition of prions is quite clear I think:

Prions are infectious agents composed entirely of a protein material that can fold in multiple, structurally abstract ways, at least one of which is transmissible to other prion proteins, leading to disease in a manner that is epidemiologically comparable to the spread of viral infection.

Your definition: "The prion protein is normally involved in synaptic transmission but can turn into a disease causing form" it is misleading. First, it sounds like there is a protein called prion that is involved in synaptic transmission, which is not true. Prions are a subset of the group proteins. Moreover, not all the prions are derived from proteins involved in the synaptic transmission. Prions have been found in other organisms like bacteria and fungi, so there is little to do with synaptic transmission there. The wiki page I have linked is full of links to the related literature, start there your journey!

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