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KINETICS OF THE PRION PROTEIN: STRUCTURE,
MISFOLDING, DISEASE, AND STABILITY
Rhiannon Aguilar
HONR299J
Final Presentation Spring 2014
BACKGROUND ON THE PRION
“Proteinaceous Infectious Particle”
Amyloid disease: visible protein deposits that can be
stained
Stanley Prusiner, 1982
Plaques found in 10% of CJD, higher percentage in other TSEs
Two distinct forms, PrP-C and PrR-res
Membrane-bound protein, 254 amino acids, 2
glycosylation sites
Conserved between species, but with slight changes
resulting in a disease species barrier
PrP 27-30, fragment created by digestion, can form
amyloid
Highly expressed in CNS, lymphatic tissue
HELICES AND PLEATED SHEETS
α-Helix: 3.6 amino
acids/turn, righthanded spiral
β-pleated sheet: parallel
or anti-parallel sheets
with a kinked shape,
connected by a loop
http://www.mun.ca/biology/scarr/MGA2_0318b.html
PRION STRUCTURE
Cellular PrP
Lots of alpha helices
Point mutations can
cause slight changes
in structure that make
misfolding favorable
Biologically
interesting fragment:
108-218
Huang, Prusiner, and Cohen 1996
MISFOLDED STRUCTURE
Predominantly beta-pleated
sheets
Presumably, this structure is
more likely to form
aggregates
Same biologically
interesting fragment (108218)
Huang, Prusiner, and Cohen 1996
DISEASE MECHANISM: REFOLDING VS SEEDING
Refolding:
Conversion is very
slow normally
Misfolded protein
acts as enzyme to
re-fold normal
Seeding: Conversion
is in constant
equilibrium
Seeds form when
Sc form
accumulates,
prevents return to
normal state
http://www.nature.com/nri/journal/v4/n9/images/nri14
37-f1.jpg
ANIMATION OF “REFOLDING” MODEL
http://learn.genetics.utah.edu/content/molecu
les/prions/
(Slide
7)
PRP DIMERIZATION
PrP can form a
covalent dimer
Third helix swaps
position to form a
covalent bond with a
second molecule
Forms a β-sheet at
the interface
Possibly a precursor
to aggregation in
disease
http://www.nature.com/nsmb/journal/v8/n9/full/nsb0901770.html
MORE PICTURES OF DIMERIZATION
Top: Green/Pink are
the two halves of the
dimer, Blue is the
monomer
superimposed
Bottom: Left is the
two halves of the
dimer, pulled apart,
and right is two
monomers
http://www.nature.com/nsmb/journal/v8/n9/full/nsb0901770.html
SIGNIFICANCE OF THIS DIMERIZATION?
17 amino acids present
in familiar SE’s are
located on the flipped
helix
Mutations may make this
flipping easier, facilitate
protein conformational
change
Covalent dimers present
in hamster scrapie
brains
Formation of new
covalent linkages =
protein
unfolding/refolding
Red: Amino acids mutated in familial SE’s
Bottom left: Met129, site of the Val mutation
that is a marker for CJD
http://www.nature.com/nsmb/journal/v8/n9/full/nsb090
1-770.html
FOLDING KINETICS
Fast-folding
Easily folds incorrectly
Has
mutations which perturb folding but do not
change stability
Important “nucleus” located between helices 2
and 3
(3rd
helix is moved in dimer formation)
KINETICS: EFFECT OF TEMPERATURE ON
STABILITY
Folded + GnHCL
Unfolded
Plot relates to reverse
reaction (Unfolded
Folded)
Native protein is most
stable at ~285K
(11.85°C)
http://www.pnas.org/content/106/14/5651.full
KINETICS: FOLDING OF NATIVE PRP
J. Biol. Chem. (2002)
Mutate Trp to Phe gives fluorescence to folded
protein
Experiments done at 5°C b/c too fast at 25°C
Results: Prion folds/unfolds with a kinetic
intermediate
First conclusive evidence for a folding intermediate
Prev. results say that mouse PrP does not have an
intermediate
Possible reason for species barrier?
KINETICS: EFFECT OF MUTATIONS
Folding/unfolding goes through a
partially-folded intermediate state
In most familial mutant versions,
the intermediate is extra stable
Intermediate is highly stabilized by
7/9 mutations
The intermediate is likely to
aggregate
Native protein needs PrP-res
seed, but maybe mutant
intermediate states can aggregate
on their own?
http://www.jbc.org/content/279/17/18008.long
STABILITY OF AGGREGATES: AMYLOID STABILITY
Amyloid analogue synthesized: KFFEAAAKKFFE
Aromatic pi-pi stacking (phenylalanine)
Charge attraction
Β-sheet interactions similar to silk
http://www.pnas.org/content/102/2/315.full
http://www.pearsonhighered.co
m/mathews/ch06/fi6p12.htm
STRUCTURAL STABILITY OF PRION AGGREGATES
Differences in structure at aggregate core
results in differential stability
Less
stable = shorter incubation time (Prusiner)
Synthesize PrP aggregates in two conditions:
2M GnHCl and 4M GnHCl
Produces
2 different stabilities when denaturation
is attempted
Open circles: 2M, Closed
circles: 4M
4M shows significantly
higher stability
http://www.jbc.org/content/289/
5/2643.long
STRUCTURAL STABILITY OF PRION AGGREGATES
Differential stability seems to only relate to
packing arrangement, not the protein secondary
structure
Tighter packing =
protease resistance?
Stability of disease
amyloid may relate to
conformation of amyloid
innoculated
http://www.jbc.org/content/289/5/264
3.long
THERMODYNAMIC STABILITY: STUDIES OF
INSULIN AMYLOID
Thermal
decomposition
results in loss of
mass and release of
gas
Occurs
at lower
temperature for
native molecule
than for amyloid
http://www.plosone.org/article/info%3Adoi%
2F10.1371%2Fjournal.pone.0086320
THERMODYNAMIC STABILITY: STUDIES OF
INSULIN AMYLOID
http://www.plosone.org/article/
info%3Adoi%2F10.1371%2Fjou
rnal.pone.0086320
Incubation at increasing temperatures decomposes
fibril structure
Incubation at 100°C shows little effect on structure (if
anything, may be more stable?)
Might be unfolding/refolding to a more stable structure?
Autoclaves at 120-130°C not sufficient!
Higher temperatures seem very effective