Taxonomic Structure of the Fungal Prions
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Prions |
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Prions are infectous proteins: altered forms of a normal cellular protein that may have lost their normal function, but have acquired the ability to change the normal form of the protein into the same abnormal form as themselves. In some cases they form filaments or filamentous aggregates, but this is not part of the definition of a prion, which in some cases could be based on covalent modification without substantial morphological effects.
Physicochemical and Physical Properties
The Ure2 protein (Ure2p) is relatively proteinase K resistant in [URE3] strains compared to wild-type strains. The Sup35 protein (Sup35p) is relatively proteinase K resistant in [PSI] strains compared to wild-type strains and forms large aggregates in these cells. The het-s protein (Hetsp) is relatively proteinase K - resistant in strains with the [Het-s] prion compared to wild-type cells.
None. The definition of a prion is that it is an infectious protein without the need for any accompanying nucleic acid.
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[URE3] |
Ure2p |
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[PSI] |
Sup35p |
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[Het-s] |
Hetsp |
None reported.
None reported.
Genome Organization and Replication
[URE3], [PSI] and [Het-s] are all inherited as non-Mendelian genetic elements (also called cytoplasmic genetic elements or non-chromosomal genetic elements). The ‘genetic material’ in this case is the altered protein which is self-replicating by changing the normal form of the protein (which nonetheless has the same primary sequence) into the altered form. Thus altered Ure2p, Sup35p and Hetsp are the ‘genomes’ of [URE3], [PSI] and [Het-s].
Not reported.
The yeast and fungal prions are passed from cell to cell during mating and hyphal anastomosis. There are no known natural vectors or extracellular transmission. Thus, fungal prions appear first as non-Mendelian genetic elements. Three biologic (genetic) criteria have been proposed to determine in a preliminary way whether a given non-Mendelian genetic element is a prion.
1. |
Prions are “reversibly curable”, meaning that even if a prion can be cured from a strain, it can arise again spontaneously at some low frequency in the cured strain. |
2. |
Overexpression of the normal protein increases the frequency with which the prion form arises. |
3. |
The phenotype of the presence of the prion is the same as the phenotype of a recessive mutant in the gene for the normal form, which gene first appears as a chromosomal gene necessary for the propagation of the prion. In some cases, the prion produces a phenotype by having an action beyond inactivation of the normal form. In this case, the phenotypic relation does not provide evidence that the non-Mendelian genetic element is a prion, but, of course, it is also not evidence against it being a prion. |
Further, physical or chemical evidence for an alteration of the protein in cells carrying the prion should be demonstrated.
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Fungal Prion |
[URE3] |
Saccharomyces cerevisiae
The synthetic prion domain of Ure2p, Ure2p1-65, forms amyloid in vitro, and induces a self-propagating amyloid formation by full-length native Ure2p purified from yeast.
Physicochemical and Physical Properties
Ure2p is more proteinase K-resistant in extracts of [URE3] strains than in wild-type strains. Ure2p is aggregated in vivo in [URE3] strains.
None.
Ure2p, in its altered form, is the primary component of [URE3]. Other proteins may also be involved.
Genome Organization and Replication
The URE2 gene has an ORF coding for a 354 amino acid polypeptide, of which the C-terminal part, including residues 81 to 354, is capable of carrying out the nitrogen regulation function of Ure2p. The N-terminal 80 residues is necessary for the initial formation of [URE3] or for its propagation or for a molecule to be affected by the presence of [URE3].
[URE3] makes cells able to take up ureidosuccinate from the media containing a good nitrogen source such as ammonia or glutamine. The ure2 mutants have the same phenotype, and the propagation of [URE3] requires the URE2 gene. Yeast cells turn off transcription of genes for enzymes involved in utilization of poor nitrogen sources when a good nitrogen source is available (nitrogen catabolite regulation). DAL5 encodes the permease for allantoate, a poor, but usable, nitrogen source for yeast. DAL5 is thus subject to nitrogen catabolite regulation. Allantoate is structurally similar to ureidosuccinate, and so Dal5p can take up ureidosuccinate. Ureidosuccinate is an intermediate in uracil biosynthesis, the product of the first step in the pathway, aspartate transcarbamylase (URA2). Thus, ura2 mutants can grow on ureidosuccinate in place of uracil if the cell has either the [URE3] prion or a mutation in ure2. [URE3] can be cured by growth in the presence of 5 mM guanidine HCl, but from the cured strain can again be isolated subclones which have again acquired [URE3]. Overproduction of Ure2p induces the de novo formation of [URE3] at 100 fold the normal frequency.
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Fungal Prion |
[PSI] |
Saccharomyces cerevisiae
Sup35p is aggregated in [PSI] strains. The Sup35p, synthesized in E. coli tends to aggregate to form filamentous structures.
Physicochemical and Physical Properties
The Sup35 protein is present as large aggregates in [PSI] strains.
None.
Sup35p (Mr 79 103).
[PSI] increases the efficiency with which ribosomes readthrough termination codons. The Sup35 protein is a subunit of the translation release factor that recognizes terination codons, and releases the peptidyl tRNA and cleaves the nascent peptide from the tRNA. Thus, sup35 mutants have the same phenotype as do [PSI] strains. Furthermore, the SUP35 gene is necessary for the propagation of [PSI]. [PSI] can be cured by growth in the presence of 5 mM guanidine HCl, but from the cured strains, can again be isolated subclones carrying [PSI]. Overproduction of Sup35p results in a 100-fold increase in the frequency with which [PSI] arises de novo.
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Fungal Prion |
[Het-s] |
Podospora anserina, a filamentous fungus
Not reported.
Physicochemical and Physical Properties
Not reported.
None.
The protein product of the het-s gene, a.
None reported.
None reported.
Not reported.
[Het-s] makes Podospora anserina strains differing at the het-s locus able to carry out heterokaryon incompatibility. It is unique among prions described to date in that it is responsible for a normal cellular function, rather than a disease of the organism. When two fungal colonies grow together, the hyphae of the two colonies fuse with each other. This process facilitates cooperation between the colonies in acquisition of nutrients, and perhaps other functions. One risk of this process is that viruses present in one colony will spread into the other colony. Apparently to prevent this, most fungi will only form heterokaryons with closely related strains, identical for alleles at each of 6 or more loci. In Podospora anserina, there are 8 such loci called Het loci. The Het-s locus has alleles het-s and het-S. The het-s strains show incompatibillity for heterokaryon formation with het-S strains only if the protein encoded by het-s is in a prion form. The presence of this prion is found as a non-Mendelian genetic element, called [Het-s]. [Het-s] is reversibly curable, the frequency of its appearance de novo is enhanced by overproduction of the protein encoded by the het-s gene, and the het-s gene is necessary for the propagation of [Het-s]. The protein encoded by the het-s gene is more protease-resistant in strains carrying [Het-s].
None reported.
Prion: sigla from infectious protein.
[URE3] and URE2 from ureidosuccinate.
[Het-s] and het-s from heterokaryon incompatibility.
[PSI] from the greek letter Psi
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