I understand that prions are misfolded in a way that’s smaller and more spatially efficient than the normal proteins that brain tissue is made out of, and the spread of these smaller proteins causes the tissue to shrink and literally grow holes. What I don’t understand is why the proteins in an infected brain “want” to copy the improper fold pattern – they aren’t sentient, they can’t notice the pattern and think “Why didn’t I try folding like this before, it’s so much more efficient”. If proteins “want” to fold a certain way because of a physical property, what was stopping them from folding that way in the first place? What is it about the introduction of a prion that makes healthy nerve tissue “get the idea” to fold that way?
In: Biology
They don’t. The prions get them to fold that way.
Proteins are the cell’s factories, they do nearly everything in the cell from moving around molecules to folding proteins to making energy.
A protein’s shape determines its function, and its construction from amino acids determine how the protein will get folded, among other things. Many environmental factors determine how a protein folds and this is an area of big research, as this isnt the only misfolded protein disease out there.
Most misfolded proteins do nothing. Some misfolded proteins do bad but its only one protein which can only do so much damage. The bigger misfolding diseases are caused by either mutation or a bad environment that causes mass misfolding.
Prions are unique, because **they misfold other proteins to take their form.** These misfolded proteins become the prions themselves, that then go along and misfold other proteins into their prion form, in other words they begin to spread and infect and interrupt regular operation of that protein.
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Proteins get folded in a way that is locally stable, but there could be a more stable conformation that the protein could fold into if the conditions are right. Prions nudge healthy proteins into this other state.
Imagine you make a cool sculpture out of magnets stuck together. But then you stick a new magnet in the wrong place and suddenly the sculpture collapses into a new shape.
Now imagine instead of magnets those are atoms. That’s a bit like what’s happening with prions.
So proteins fold in a way determined by their amino acid structure, but there are multiple possible ways this can happen. The classic way to imagine this is a series of valleys of different depths. The depth of the valley corresponds to how energetically favourable that conformation is, and the mountains between represent the energy cost it takes to change conformation.
Prion proteins are one specific protein, and they only change their own kind, just to clarify. The misfolded conformation is a deeper valley so to speak, and they can bump other proteins of their kind over the mountain to get there. How they do that mechanically I don’t exactly know.
There are proteins called chaperones which assist in protein folding. They are the reason some proteins are in a shallow valley instead of a deep one, because the chaperone helps them get there, and once they are in the valley they don’t climb the mountain on their own. Prions can kick them over and this is the cause of the problem.
Imagine you had a paper folding machine, made out of paper, which folded paper into copies of itself.
If you dropped one onto a stack of papers, not only would it begin folding the papers, but the folded papers would theirselves begin folding papers.
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