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Yeast prions show unusual inheritance
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- A prion is a proteinaceous
infectious agent that behaves as an inheritable trait, although it
contains no nucleic acid. Examples are PrPSc, the agent of scrapie in sheep and bovine spongiform encephalopathy, and Psi, which confers an inherited state in yeast.
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The Sup35 protein in its wild-type soluble form is a termination factor for translation.
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It can also exist in an alternative form of oligomeric aggregates, in which it is not active in protein synthesis.
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The presence of the oligomeric form causes newly synthesized protein to acquire the inactive structure.
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Conversion between the two forms is influenced by chaperones.
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The wild-type form has the recessive genetic state psi–
and the mutant form has the dominant genetic state PSI+.
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One of the clearest cases of the dependence of epigenetic inheritance on the condition of a protein is
provided by the behavior of prions — proteinaceous infectious agents. They have been
characterized in two circumstances: by genetic effects in yeast; and as the causative agents of neurological
diseases in mammals, including man. A striking epigenetic effect is found in yeast, where two different states
can be inherited that map to a single genetic locus, although the sequence of the gene is the same in both
states. The two different states are [psi– ] and [PSI+].
A switch in condition occurs at a low frequency as the result of a spontaneous transition between the states
(for review see 211; 212; 4864 ).
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Figure 23.44
The state of the Sup35 protein determines whether termination of translation occurs.
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The psi genotype maps to the locus sup35, which codes for a translation termination factor. Figure 23.44 summarizes the effects of the Sup35 protein in yeast. In wild-type cells, which are characterized as [psi–
], the gene is active, and Sup35 protein terminates protein synthesis. In cells of the mutant [PSI+]
type, the factor does not function, causing a failure to terminate protein synthesis properly. (This was originally detected by the lethal
effects of the enhanced efficiency of suppressors of ochre codons in [PSI+] strains.)
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[PSI+] strains have unusual genetic properties. When a [psi–] strain
is crossed with a [PSI+] strain, all of the progeny are [PSI+].
This is a pattern of inheritance that would be expected of an extrachromosomal agent, but the [PSI+]
trait cannot be mapped to any such nucleic acid. The [PSI+]
trait is metastable, which means that, although it is inherited by most
progeny, it is lost at a higher rate than is consistent with mutation.
Similar behavior is shown also by the locus URE2, which codes
for a protein required for nitrogen-mediated repression of certain
catabolic enzymes. When a yeast strain is converted into an alternative
state, called [URE3], the Ure2 protein is no longer functional (637).
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Figure 23.45
Newly synthesized Sup35 protein is converted into the [PSI+] state by the presence of pre-existing [PSI+] protein.
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The [PSI+] state is determined by the conformation of the Sup35 protein.
In a wild-type [psi–] cell, the protein displays its normal function.
But in a [PSI+] cell, the protein is present in an alternative conformation in which
its normal function has been lost. To explain the unilateral dominance
of [PSI+] over [psi–]
in genetic crosses, we must suppose that the presence of protein in the [PSI+]
state causes all the protein in the cell to enter this state. This requires an interaction between
the [PSI+] protein and newly synthesized protein, probably reflecting the generation of
an oligomeric state in which the [PSI+] protein has a nucleating role, as illustrated in
Figure 23.45 (for review see 997).
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A feature common to both the Sup35 and Ure2 proteins is that each consists of two domains that function
independently. The C-terminal domain is sufficient for the activity of the protein. The N-terminal domain is
sufficient for formation of the structures that make the protein inactive. So yeast in which the N-terminal
domain of Sup35 has been deleted cannot acquire the [PSI+] state; and the presence of an
[PSI+] N-terminal domain is sufficient to maintain Sup35 protein in the
[PSI+] condition (639). The critical feature of the N-terminal domain is that it
is rich in glutamine and asparagine residues.
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Loss of function in the [PSI+] state is due to the sequestration of the protein in an oligomeric complex.
Sup35 protein in [PSI+] cells is clustered in discrete foci, whereas the protein in [psi
–] cells is diffused in the cytosol. Sup35 protein from [PSI+] cells forms amyloid fibers
in vitro— these have a characteristic high content of
ß sheet structures (640).
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The involvement of protein conformation (rather than covalent modification) is suggested by the effects of
conditions that affect protein structure. Denaturing treatments cause loss of the [PSI+]
state. And in particular, the chaperone Hsp104 is involved in inheritance of [PSI+].
Its effects are paradoxical. Deletion of HSP104 prevents maintenance of the [PSI+]
state. And overexpression of Hsp104 also causes loss of the [PSI+]
state. This suggests that Hsp104 is required for some change in the structure of Sup35 that is necessary for
acquisition of the [PSI+] state, but that must be transitory (638;
for review see 213).
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Figure 23.46
Purified protein can convert the[[psi-] state of yeast to [PSI+].
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Using the ability of Sup35 to form the inactive structure in vitro, it is possible
to provide biochemical proof for the role of the protein. Figure 23.46 illustrates a striking experiment
in which the protein was converted to the inactive form in vitro,
put into liposomes (when in effect the protein is surrounded by an artificial membrane), and then introduced
directly into cells by fusing the liposomes with [psi–] yeast (1069).
The yeast cells were converted to [PSI+]!
This experiment refutes all of the objections that were raised to the
conclusion that the protein has the ability to confer the epigenetic
state. Experiments in which cells are mated, or in which extracts are
taken from one cell to treat another cell, always are susceptible to
the possibility that a nucleic acid has been transferred. But when the
protein by itself does not convert target cells, but protein converted
to the inactive state can do so, the only difference is the treatment
of the protein — which must therefore be responsible for the conversion.
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>The ability of yeast to form the [PSI+] prion state depends on the genetic background.
The yeast must be [PIN+] in order for the [PSI+] state to form.
The [PIN+] condition itself is an epigenetic state (1953). It can be created by
the formation of prions from any one of several different proteins (1954).
These proteins share the characteristic of Sup35 that they have
Gln/Asn-rich domains. Overexpression of these domains in yeast
stimulates formation of the [PSI+] state (1955).
This suggests that there is a common model for the formation of the
prion state that involves aggregation of the Gln/Asn domains into
self-propagating amyloid structure (for review see 5861).
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How does the presence of one Gln/Asn
protein influence the formation of prions by another? We know that the
formation of Sup35 prions is specific to Sup35 protein, that is, it
does not occur by cross-aggregation with other proteins. This suggests
that the yeast cell may contain soluble proteins that antagonize prion
formation. These proteins are not specific for any one prion. As a
result, the introduction of any Gln/Asn domain protein that interacts
with these proteins will reduce the concentration. This will allow
other Gln/Asn proteins to aggregate more easily.
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211 Wickner, R. B.
(1996).
Prions and RNA viruses of S. cerevisiae.
Annu. Rev. Genet. 30, 109-139.
PubMed Journal
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212 Lindquist, S.
(1997).
Mad cows meet psi-chotic yeast: the expansion of the prion hypothesis.
Cell 89, 495-498.
PubMed Journal
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213 Horwich, A. L. and Weissman, J. S.
(1997).
Deadly conformations: protein misfolding in prion disease.
Cell 89, 499-510.
PubMed Journal
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997 Serio, T. R. and Lindquist, S. L.
(1999).
[PSI+]: an epigenetic modulator of translation termination efficiency.
Annu. Rev. Cell Dev. Biol. 15, 661-703.
PubMed Journal
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4864 Wickner, R. B., Edskes, H. K., Roberts, B. T., Baxa, U., Pierce, M. M., Ross, E. D., and Brachmann, A.
(2004).
Prions: proteins as genes and infectious entities.
Genes Dev. 18, 470-485.
PubMed
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5861 Wickner, R. B., Edskes, H. K., Ross, E. D., Pierce, M. M., Baxa, U., Brachmann, A., and Shewmaker, F.
(2004).
Prion genetics: new rules for a new kind of gene.
Annu. Rev. Genet. 38, 681-707.
PubMed
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637 Wickner, R. B.
(1994).
[URE3] as an altered URE2 protein: evidence for a prion analog in S. cerevisiae.
Science 264, 566-569.
PubMed
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638 Chernoff, Y. O. et al.
(1995).
Role of the chaperone protein Hsp104 in propagation of the yeast prion-like factor [PSI+].
Science 268, 880-884.
PubMed
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639 Masison, D. C. and Wickner, R. B.
(1995).
Prion-inducing domain of yeast Ure2p and protease resistance of Ure2p in prion-containing cells.
Science 270, 93-95.
PubMed
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640 Glover, J. R. et al.
(1997).
Self-seeded fibers formed by Sup35, the protein determinant of [PSI+], a heritable prion-like factor of S. cerevisiae.
Cell 89, 811-819.
PubMed
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1069 Sparrer, H E, Santoso, A, Szoka, F C, and Weissman, J S
(2000).
Evidence for the prion hypothesis: induction of the yeast [PSI+] factor by in vitro-converted Sup35 protein.
Science 289, 595-599.
PubMed Journal
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1954 Derkatch, I. L., Bradley, M. E., Hong, J. Y., and Liebman, S. W.
(2001).
Prions affect the appearance of other prions: the story of [PIN(+)].
Cell 106, 171-182.
PubMed Journal
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1955 Osherovich, L. Z. and Weissman, J. S.
(2001).
Multiple gln/asn-rich prion domains confer susceptibility to induction of the yeast.
Cell 106, 183-194.
PubMed Journal
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© Jones and Bartlett Publishers (2007)
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