Prion protein self-replication

Studies with mammalian prion PrP have failed to identify any cellular factor, be it a protein, a co-factor or a nucleic acid species that is essential for the generation or propagation of PrPSc. This has led to the assertion that mammalian prions are self-replicating . Fungal prions are however different they require one or more cellular proteins in order to propagate successfully the prion form in vivo. This difference reflects in part the requirement to propagate yeast prions in a matter of... 

In the cases above regarding urushiol and botulinum neurotoxin, the toxin is well defined and its release has a well-defined target and goal. However, in other cases the toxin is less well defined and its occurrence may be more accidental. As an example, consider prions. Prions are proteins that can be folded in different ways, including a normally configured prion protein that is harmless and an infectious form that causes disease. The proteins have the same amino acid sequence however, their three-dimensional shape (the way that the protein folds together) is different. The infectious prion has the ability to convert normal prion proteins into infectious ones, and as such the prions are self-replicating. 

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