Prion????

[Ruth Goldstone]

In article <4jvvgu$65u at kellia.nioz.nl>, Gert Jan Gast <gjgast at nioz.nl> says:
>
>
>Hi all,
>
>As an ecologist I'm very much puzzled by this prion thing. 
>To be honest, I had never even heard the word before. So if I'm  
>being naive here, please forgive me. Could anyone explain or give 
>references to what a prion is and how it is supposed to work? I 
>understand that a protein can have harmful effects. However, as 
>it does not have DNA or RNA how does (or is) it multiply(ied) so 
>that it can spread through a population? Does a cell make copies 
>simply because the prion is there? Or forces the prion the cell 
>to do that, like a virus does? If so, how could a simple protein 
>do that? Also: as the original prion is the information for the 
>copy, it would have to be read first. That would give the cell 
>opportunities to destroy it, wouldn't it? Or is a prion some kind 
>of weird "side product" from something else? Like nonsense DNA, 
>mutations or transcription mistakes? 
>
>Thanks and good eastern weekend, GJ.
>
>
>-- 
>
>
>Gert Jan Gast <gjgast at NIOZ.NL>
>Netherlands Institute for Sea Research
>Department of Marine Ecology
>P.O. Box 59, 1790 AB Den Burg, The Netherlands.
>Phone: 31 (0)222 369573. Fax: 31 (0)222 319674.
>
>
 It is generally thought that prions lack nucleic acid completely. 
Stanley B. Prusiner hypothesized that prions replicate by converting 
(usually harmless ) proteins into more prions which, in turn convert 
other proteins to prions, and so on, until prions reach dangerous levels.
Prions are unusual in that they appear to underlie both infectious and 
inherited diseases. Creutzfeldt Jacob Disease (the old strain-not the new,
possibly BSE related strain) is inherited in 10 to 15% of cases but has
also been transmitted by corneal transplantation and injection of human 
growth hormone extracted from cadavers.
 The prion-type protein seems to exist in two forms. PrPc is the normal
form of the protein and is found in all cells and can be broken down 
by enzymes. PrPsc, the disease-causing form of the protein is resistant 
to degradation by enzymes and is found  in extracts from diseased brains.
PrPc and PrPsc molecules seem to be identical in terms of their amino 
acid sequences, yet they act differently in  cells. Researchers suspect 
that this is because they adopt different molecular shapes. It is thought
that when prion proteins come into contact with the near-identical normal
proteins, they induce these normal proteins to flip from the `healthy` 
molecular shape to a `disease causing` shape.
 People with inherited forms of prion diseases may carry mutated versions
of the gene that codes for the prion-like protein found normally in cells.
The mutated genes may code for a less stable form of the protein which
is much more likely to flip into the abnormal form.