In article <4fupi4$lpf at melon.myna.com>, burris at myna.com wrote:
> Evidence suggests that prions do not contain nucleic acid since there is
a concomitant loss of infectiousness with nuclease
> treatment. If true, what evidence exists to confirm or refute the
protein, glycoprotein or lipoprotein based nature of the agent?
Not a lot of people would refute the role of PrP in the disease process
these days (although I can think of one person, off hand, who still
maintains it is a virus). The best evidence that there is a nucleic acid
involved somewhere is the extensive strain data in sheep, mice, and mink.
I think the most lucid papers (or a good starting point) are Bruce &
Dickinson (1987) J.Gen.Virol. 68, 79-89 and the papers by Bessen & Marsh
on TME (1992) J.Gen.Virol. 73, 329-334 and J.Virol. 66, 2096-2101. Even
the strain data may now fall under the auspices of the prion hypothesis
with a recent Nature paper (Race et al., 1995 I think) which continues the
work of Kocisko et al., (1994) Nature 370, 471-4 and reproduces the
phenotypic properties of TME strains in an in vitro system.
> If prions are solely based on protein, glycoprotein or lipoprotein, or
any combuination there of, shouldn't a lability
> or alteration in the 3-D conformation of the agent's protein based
structure be expected when the agent is exposed to heat?
> Would not protease treatment also be expected to affect the
infectiousness of the agent? Some reports suggest that heat and
> protease treatment does not reduce or eliminate the infective nature of
the agent. How can this be explained based on the
> above hypothesis?
PrP-res (the infectious 'organism') is a remarkably resiliant little
bugger and simple autoclaving is not enough to destroy infectivity
totally. it is also relatively resistant to treatment with proteinase K
and varies in its resistance to protein denaturants (chaotropic salts do
the job quite nicely). PrP is a very hydrophobic protein and in its
predominantly B-sheet infectious form I suppose it just curls up and
resists most things that are thrown at it (including host cell endosomal
and lysosomal enzymes). In contrast, the predominantly a-helix normal
host cell protein, PrP-sen, is not resistant to these treatments. The
conformational change associated with infectivity somehow allows for this
Olympian resistance to all things corrosive and nasty. Many proteins show
resiliance to abnormal conditions such as heat, UV, proteases, and
denaturants but we do not say they are not proteins. I think it is merely
the nature of the prion hypothesis that gets people worked up!
> If prion replication is accomplished by the reverse translation of
protein to mRNA shouldn't we be able to detect prion associated
> mRNA in infected neurological tissue? What evidence suggests that
reverse translation of protein is the actual mode of
> replication used by prions?
There is no reverse translation of protein to RNA in prion diseases. The
infectious organism is the abnormal form of a normal host cell protein
(Basler et al., (1986) Cell 46, 417-428. mRNA levels of the prp gene
remain the same in infected and uninfected animals. The infectious,
disease causing form (PrPsc/PrP-res: whatever nomenclature you are using)
causes a conformational change in the normal host cell protein
(PrPc/PrP-sen) by a mechanism that has not been fully elucidated. The
primary structures of PrPsc and PrPc are indistiguishable-to date.
I hope this helps. Feel free to contact me.
> I welcome your comments and ideas.
>> Burris Ormsby, Professor Microbiology, Sheridan College, Canada