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Prion proteins
THE 'protein only' hypothesis' states that a modified form of
normal prion protein triggers infectious neurodegenerative
diseases, such as bovine spongiform encephalopathy (BSE), or
Creutzfeldt-Jakob disease (CJD) in humans.
Prion proteins are thought to exist in two different conformations:
the 'benign' PrPc form, and the infectious 'scrapie form', PrPSc.
Knowledge of the three-dimensional structure of prpc is essential
for understanding the transition to PrPSc.
Schematic representation of the post-translational modifications
(a) and secondary structure (b) of murine PrP
Structure of the PrPc
• 210 aminoacids
• 33 - 35 kDa
• 2 N-linked oligosaccharydes
• GPI anchor (membrane-bound)
• C-terminal 3 a-helices and 2
antiparalell b-sheets
Solution structure of PrPc obtained last year by Wüthrich,
Glockshuber, and coworkers at the Swiss Federal Institute
of Technology shows three - helices and an antiparallel ßsheet at the protein's carboxy terminus and a "flexibly
disordered" segment at its amino terminus.
PrP gene
MANLGCWMLVLFVATWSDLGLCKKRPKPGGWNTGGSRYPGQGSPGGNRYP
PQGGGGWGQPHGGGWGQPHGGGWGQPHGGGWGQPHGGGWGQ
GGGTHSQWNKPSKPKTNMKHMAGAAAAGAVVGGLGGYMLGSAMSRPIIHFGS
DYEDRYYRENMHRYPNQ
VYYRPMDEYSNQNNFVHDCVNITIKQHTVTTTTKGENFTETDVKMMERVVEQ
MCITQYERESQAYYQRGS SMVLFSSPPVILLISFLIFLIVG
Ribbon diagram of the structure of the
mouse prion protein domain PrP(121-231)
Globular fold and surface properties of PrP(121-231).
The propagation of infectious prion protein
occurs via conversion of normal prion protein
(PrPc, left) to a disease-causing form (PrPSc,
right). In the refolding process, some of the helical regions (purple coils) in PrPc unfold,
forming an extended ß-sheet region (flat blue
arrows).
Transmission electron micrographs obtained
by Lindquist and coworkers of the University
of Chicago show fibers formed by yeast Sup35
protein. At right is a single fiber at higher
magnification.
Mutations in the PrP gene
Spongiform Encephalopathies
This is your Brain . . . . . . . this is your Brain on Beef
CJD - human
BSE - (cow)
Kuru - human
Scrapie - (sheep)
Vacuolization of neuronal cytoplasm results in the typical sponge-like appearance of
brain parenchyma.
Virus hypothesis
•
The genetic information defining the properties of the various scrapie strains is
encoded by a nucleic acid.
•
The important role of PrP in disease might be explained in that PrP plays an
essential role in the infection and spreading of the virus in the host.
•
Mutations in PrPC would alter susceptibility to the disease.
•
The species barrier for infection could be caused by a reduced affinity of a given
virus to the PrPC molecule of a different host species.
•
Conversion of PrPC into PrPSc would be brought about by the interaction of the
virus with PrPC.
•
The virus hypothesis readily explains the existence of distinct scrapie strains;
however, it is challenged by the claim that in preparations of highly enriched
infectivity, the average size of a nucleic acid per infectious unit is not larger than
80 nucleotides
Virino hypothesis
•
Postulates that the infectious particle consists of a small nucleic acid coated by a
host protein.
•
Variations in the nucleic acid sequence could account for the existence of distinct
strains, as for example, in viroids.
•
The size of the nucleic acid could be very small since it does not have to encode
for any proteins.
•
The unusual resistance to treatments destroying or modifying nucleic acids could
result from a protective effect of the coat.
•
The most likely candidate for the host protein forming the coat of the virino is
PrPSc. Standard preparations of purified infectivity usually contain approximately
105 PrPSc molecules per infectious
•
Infectivity might be associated to a subpopulation of PrPSc molecules,
denominated PrP*, which may have escaped detection because it is extremely rare
or difficult to distinguish from the rest of PrPSc.
‘Protein only’ hypothesis
•
The information for strain specificity is carried by an infectious protein.
•
To explain the mechanism of propagation of such an unusual infectious agent,
it was proposed that the agent consisted of a modified host protein which was
able to convert the host protein into a likeness of itself.
•
In order to account for the strain specificities, the modified host protein would
have to exist in various different isoforms. The most likely candidate for an
infectious protein causing TSEs is PrPSc (or PrP*) because it represents a
disease-associated posttranslational modification of a host-encoded protein
and because of its alleged association with infectivity.
Two ‘protein only’ models for
the molecular mechanism
‘refolding’ model - the rate-determining step is the
conformational conversion of PrPC into PrPSc, a process which is
catalyzed by the formation of a PrPC - PrPSc heterodimer
‘nucleation-dependent polymerization’ model the ratedetermining step is the formation of a nucleus of polymerized
PrPSc, which, once formed, promotes further polymerization of
PrPSc. Such nuclei would act as polymerization seeds in an
infected host thereby rapidly driving host PrP into the aggregated
state.
Microscopy of the normal brain (top left) shows dark-staining
neurons in the cerebral cortex with no spongy change in
surrounding tissue. In classical CJD (top right), brain cortex
contains numerous small vacuoles that give a spongelike
appearance. In new-variant CJD (bottom left), the cortex
shows less severe sponginess but contains "florid" plaques-aggregates of amloid material surrounded by spongy change.
Florid plaques in new-variant CJD (bottom right) stain
intensely for prion protein (brown).
According to the template
assistance (or heterodimer)
model, PrPc monomer adds to
PrPSc monomer to form PrPSc PrPC heterdimer, which
rapidly converts to PrPSc
homodimer. The homodimer
may then dissociate, releasing
new PrPSc monomer. The
nucleated polymerization
mechanism proposed that a
PrPSc oligomer or polymer
recruits PrPc, which then
converts to PrPSc.
the conversion of PrPC to PrPSc as a consequence of
a disease process in which PrPC might act as a
receptor for the infectious agent
Scrapie. Naturally occurring scrapie in sheep and goats has a duration of 2-6 months and typically appears
between the age of 2.5-4.5.
Bovine Spongiform Encephalopathy Clinical signs of bovine spongiform encephalopathy
(BSE) typically appear at the age of 4-5 as progressive apprehension, hyperaesthesia, and incoordination of
gait with a duration of 1-6 months.
Human TSEs:
Creutzfeldt-Jakob disease. The age of onset of the disorder typically lies
between the age of 50 and 75. CJD occurs worldwide with an incidence of approximately
0.5 cases per million per annum. Iatrogenic cases are extremely rare. Ca. 15% of all
CJD cases represent familial CJD with a dominant pattern of inheritance.
GSS (Gertsmann-Straussler syndrome) in patients typically
between 35-55 years of age as a slowly progressive ataxia of 2-10 years duration. GSS is
nearly always described in a familial context; it occurs in approximately 1 per 107 people.
Kuru. It was observed predominantly among children and adult females and had a
duration of 3-9 months. It is confined to the Fore tribe of Papua New Guinea (PNG), and
is caused by cannibalistic rituals, specifically the preparation and eating of human brains
Fatal familial insomnia (FFI) Most patients were between the ages of 40
and 60; disease duration was 7-18 months. The occurrence of fatal familial insomnia (FFI)
is associated with the same codon 178 aspartic acid to asparagine mutation.
Experimental TSE. Neuropathology of experimental TSE is mostly studied in hamsters and mice
because of the relatively short incubation times and the convenience of animal keeping. Animals are usually
inoculated intracerebrally.