快猫短视频

THE bizarre shape-changing proteins known as prions, infamous for their role
in diseases such as BSE, may be common and often beneficial, say biologists in
the US.

鈥淲hen people hear `prion鈥� they automatically think of diseases,鈥� says Susan
Lindquist of the University of Chicago. 鈥淏ut these results argue that prions are
a novel way to control proteins.鈥�

Prions have had an image problem ever since Stanley Prusiner at the
University of California, San Francisco, first proposed their existence.
Prusiner suggested that a brain protein called PrP causes diseases like BSE when
it changes shape.

PrP is found on the surface of nerve cells in most brains. Prusiner suggested
that in afflicted animals a twisted form of PrP, which he called a prion, makes
healthy PrP adopt the same twisted shape. In this way more and more PrP is
converted into the abnormal form, which forms clumps that kill nerve cells.

The prion story took its next twist when geneticists discovered that two
proteins in the yeast Saccharomyces cerevisiae, Sup35 and Ure2, also
appear to change shape and replicate as prions. When the prions form, the
proteins are inactivated in that yeast cell and all its descendants. Sup35, for
example, regulates the length of proteins, and cells carrying the prions make
proteins that are longer than normal.

This doesn鈥檛 seem to damage the cells, however, which made Lindquist and
others speculate that the yeast uses prions to inactivate proteins that aren鈥檛
needed, and that these changes can be inherited without DNA being involved
(快猫短视频,24 January 1998, p 24).

If the ability to form prions is a useful adaptation, it is likely to have
been conserved by evolution and shared by other species of yeast. To test this
idea, Jonathan Weissman of the University of California, San Francisco, has now
taken the genetic sequences that code for the shape-changing region of the Sup35
protein in three other yeasts and spliced them into the corresponding S.
cerevisiae gene. The resulting hybrid proteins could all form prions.

Meanwhile, Lindquist has shown that it isn鈥檛 difficult to create prions.
Grafting a fragment of Sup35 onto a rat protein called GR was enough to give
that protein the capacity to replicate as a prion in yeast cells. Her team and
Weissman鈥檚 have both identified more yeast proteins capable of forming prions.
Weissman has also identified a possible prion in a nematode worm.

If it turns out that prions can be beneficial as well as harmful, it might
open up a whole new way to treat prion diseases. If cells have to deal with
prions all the time, they might have evolved ways to control them, or flip them
back into their previous shape. 鈥淭hey might have mechanisms we can exploit to
prevent prion formation,鈥� says Weissman.