Scientists identify prion's infectious
secret
CAMBRIDGE, Mass. (May 9, 2007) — Researchers
have known for decades that certain neurodegenerative
diseases, such as mad cow disease or its human equivalent,
Cruetzfeldt-Jakob disease, result from a kind of infectious
protein called a prion. Remarkably, in recent years
researchers also have discovered non-pathogenic prions
that play beneficial roles in biology, and prions even
may act as essential elements in learning and memory.
But although prions have received a great deal of
scrutiny, scientists still don't understand many of
the most fundamental mechanisms of how prions form,
replicate and cross from one species to another.
Now, through studying non-toxic yeast prions, scientists
at Whitehead Institute have discovered small but critical
regions within prions that determine much of their
behavior.
“We've seen just one small part of this
prion inducing proteins to fold,” says
Whitehead Member Susan Lindquist. “This
is an entirely new concept.” |
"These findings provide a new framework for us
to begin exploring properties of prion biology that,
up until now, have proven difficult to investigate," says
Whitehead Member and MIT biology professor Susan
Lindquist,
senior author on the paper, which will appear in the
May 9 online issue of Nature.
Proteins are the cell's workhorses, and they need
to fold into complex and precise shapes to do their
jobs. Prions are proteins that start out normally,
but then at some point misfold-rather like an origami
swan that comes out looking and acting instead like
a vulture.
But prions have another characteristic that enables
them to wreak havoc. They recruit other, properly folded
proteins into misforming along with them, a process
Lindquist calls a "conformational cascade." In
many organisms, this conformational cascade creates
long fibers called amyloids. (The brains of animals
that have died from prion infections are literally
packed with amyloid clumps.)
In order to glean insights into the mechanics that
enable amyloid formation, Peter Tessier, a postdoctoral
scientist in Lindquist's lab, used peptide arrays-glass
slides covered with thousands of tiny protein fragments.
Traditionally, these arrays are used for finding binding
sites within well-behaved proteins. Here, Tessier designed
the arrays so that he could observe protein folding
and amyloid formation in real time.
Tessier covered the array with peptides from baker's
yeast and then added prion protein to the array, also
from the same yeast species. He found that a small
cluster of peptides recruited the prion proteins to
misfold into an amyloid structure. This region of the
protein, which Tessier called a "recognition element," constitutes
about 10 percent of the prion. Tessier repeated this
experiment with peptides and a prion taken from pathogenic
fungi. The results were the same.
Both prions also maintained a rigid species barrier.
The baker's yeast prion could not recruit peptides
from the pathogenic fungi cells, and vice versa.
To further verify these results, Tessier accessed
a synthetic yeast prion, one that another research
group had assembled from pieces of both the baker's
yeast and the pathogenic fungi prion. Earlier studies
had shown that this synthetic prion could cross the
species barrier but did not identify the mechanism.
Tessier found that this synthetic prion contained two
recognition elements, one for baker's yeast and one
for pathogenic fungi. When the prion was placed with
peptide fragments from baker's yeast, the baker's yeast
recognition element was activated, and likewise for
the pathogenic fungi.
Even more striking, Tessier could activate different
recognition elements by manipulating environmental
conditions, such as temperature. For example, when
he conducted the experiment at 4 degrees Celsius, the
baker's yeast recognition element switched on. At 37
degrees Celsius, the pathogenic fungi element was activated.
In other words, temperature alone could dictate which
yeast species the prion could infect. Additionally,
the prion's behavior could be altered by subtle alterations
in the recognition element's amino acid sequence.
While this prion is a laboratory construct not found
in nature, these findings provide researchers with
a new way to approach old questions, such as why some
prion diseases can jump from one species to another
but others can't. Tessier and Lindquist say it is likely
that natural prions contain more than one recognition
element, and recognition elements can slide into a
neighboring region. Many external factors can determine
which recognition element is activated, in turn influencing
the downstream behavior of the prion.
"These findings are remarkable for two reasons," says
Lindquist, who is also an investigator for Howard Hughes
Medical Institute. "For one thing, this is the
first time that these peptide arrays have been used
to study protein folding. We've taken this platform
to a whole new level. Also, we've seen just one small
part of this prion inducing proteins to fold. This
is an entirely new concept."
Earlier research from the Lindquist lab, published
in Nature in 2005, identified the amino acid regions
where prions connect with one another to form amyloids.
Those interaction regions turn out to be the same regions
Tessier identified as recognition elements-
further confirmation that these regions are key to
prion activity.
Tessier and his colleagues plan to further investigate
this process in mammalian prions, such as those responsible
for mad cow and Cruetzfeldt-Jakob diseases, as well
as in other non-prion proteins that can also form amyloid
structures. |
