Prions

Sometimes a scientific discovery shakes the confidence of scientists, making
them question whether they truly understand nature's ground rules.   That's exactly
what prions have done to scientists' understanding of the ground rules for
infectiousdiseases.   Prions cause diseases,but they aren't viruses or bacteria or
fungi or parasites. They are simply proteins, and proteins were never thought to
be infectious on their own. Organisms are infectious, proteins are not. Or, at least,
they never used to be.   Prions entered the public's consciousness during the mad
cow epidemic that hitEngland in 1986.   For decades, however, scientists had
searched for unusual, atypical infectious agents that they suspected caused some
puzzling diseases that could not be linked to any of the "regular" infectious
organisms. One possibility was that slow viruses-viruses that spent decades
wreaking havoc in their hosts-might be the culprits, and these putative viruses
were not only leisurely about multiplying but also hard to isolate. Now researchers
are coming around, although reluctantly, to accepting the shocking fact that
naked proteins can be infectious.  
    Prions enter cells and apparently convert normal proteins found within the
cells into prions just like themselves. The normal cell proteins have all the same
"parts" as the prions- specifically the same amino acid building blocks -but they
fold differently.   They are much like the toy "Transformers" that were around in
the 1980s. They could change themselves in to be different shapes with nothing
added and nothing subtracted.
    Prions enter brain cells and there convert the normal cell protein PrPC to the
prion form of the protein, called PrPSC. When normal cell proteins transform into
prions, amino acids that are folded tightly into alpha helical structures relax into
looser beta sheets. More and more PrPC molecules transform into PrPSC
molecules, until eventually prions... [continues]

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