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Fungal prions have been investigated, leading to a deeper understanding of disease-forming mammalian prions. A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ A prion (ˈpriːɒn is thought to be an infectious agent that according to current scientific consensus is comprised entirely of a propagated, mis-folded Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands

Prion-like proteins are found naturally in some plants and non-mammalian animals. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Some of these are not associated with any disease state and may possibly even have a useful role[1]. Because of this, scientists reasoned that such proteins could give some sort of evolutionary advantage to their host. eVolution is the third Album by eLDee, it was due to be released in 2008 This was suggested to be the case in a species of fungus, Podospora anserina. Genetically compatible colonies of this fungus can merge together and share cellular contents such as nutrients and cytoplasm. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance A nutrient is food or chemicals that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. A natural system of protective "incompatibility" proteins exists to prevent promiscuous sharing between unrelated colonies. One such protein, called HET-S, adopts a prion-like form in order to function properly [2]. The prion form of HET-S spreads rapidly throughout the cellular network of a colony and can convert the non-prion form of the protein to a prion state after compatible colonies have merged [3]. However, when an incompatible colony tries to merge with a prion-containing colony, the prion causes the "invader" cells to die, ensuring that only related colonies obtain the benefit of sharing resources.

Contents

Sup35p & Ure2p

In 1965, Brian Cox, a geneticist working with the yeast Saccharomyces cerevisiae, described a genetic trait (termed PSI+) with an unusual pattern of inheritance. Yeasts are a growth form of eukaryotic Microorganisms classified in the kingdom Fungi, with about 1500 Species currently described Saccharomyces cerevisiae is a Species of Budding Yeast. It is perhaps the most useful Yeast owing to its use since ancient times Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is A trait is a distinct phenotypic character of an organism that may be inherited environmentally determined or somewhere in between The initial discovery of PSI+ was made in a strain auxotrophic for adenine due to a nonsense mutation [1] Despite many years of effort, Cox could not identify a conventional mutation that was responsible for the PSI+ trait. Adenine is a Purine with a variety of roles in Biochemistry including Cellular respiration, in the form of both the energy-rich Adenosine In biology mutations are changes to the Nucleotide sequence of the Genetic material of an organism

In 1994, yeast geneticist Reed Wickner correctly hypothesized that PSI+ as well as another mysterious heritable trait, URE3, resulted from prion forms of certain normal cellular proteins [4]. Reed B Wickner (c 1942-) is an American Yeast Geneticist. Wickner proposed that the and Phenotypes in Saccharomyces cerevisiae It was soon noticed that heat shock proteins (which help other proteins fold properly) were intimately tied to the inheritance and transmission of PSI+ and many other yeast prions. Heat shock proteins ( HSP) are a group of Proteins whose expression is increased when the cells are exposed to elevated temperatures or other stress Fungal Prions have been investigated leading to a deeper understanding of disease-forming Mammalian prions Since then, researchers have unravelled how the proteins that code for PSI+ and URE3 can convert between prion and non-prion forms, as well as the consequences of having intracellular prions. When exposed to certain adverse conditions, PSI+ cells actually fare better than their prion-free siblings [5]; this finding suggests that, in some proteins, the ability to adopt a prion form may result from positive evolutionary selection [6]. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of It has been speculated that the ability to convert between prion infected and prion-free forms enables yeast to quickly and reversibly adapt in variable environments. Nevertheless, Wickner maintains that URE3 and PSI+ are diseases [7].

Further investigation found that PSI+ is the misfolded form of Sup35, which is an important factor for translation termination during protein synthesis [2]. Protein synthesis is the creation of proteins using DNA and' RNA'. It is believed that [PSI+] causes suppression of nonsense mutations by sequestering functional Sup35 in non-functional aggregates, thereby allowing stop codon readthrough. [PIN+], in turn, is the misfolded form of the protein Rnq1. However, the normal function of this protein is unknown to date. It is of note that for the induction of most variants of [PSI+], the presence of [PIN+] is required. Though reasons for this are poorly understood, it is suggested that [PIN+] aggregates may act as “seeds” for the polymerization of [PSI+] [3].

Two modified versions of Sup35 have been created that can induce PSI+ in the absence of [PIN+] when overexpressed. One version was created by digestion of the gene with BalI, which results in a protein consisting of only the M and N portions of Sup35 [4]. The other is a fusion of Sup35NM with HPR, a human membrane receptor protein.

Laboratories commonly identify [PSI+] by growth of a strain auxotrophic for adenine on media lacking adenine, similar to that used by Cox et al. These strains cannot synthesize adenine due to a nonsense mutation in one of the enzymes involved in biosynthetic pathway. When the strain is grown on yeast-extract/dextrose/peptone media (YPD), the blocked pathway results in buildup of a red-colored intermediate compound, which is exported from the cell due to its toxicity. Hence, color is an alternative method of identifying [PSI+] -- [PSI+] strains are white or pinkish in color, and [psi-] strains are red. A third method of identifying [PSI+] is by the presence of Sup35 in the pelleted fraction of cellular lysate.

Classification

Fungal Prions
Protein Natural Host Normal Function Prion State Prion Phenotype
Ure2p Saccharomyces cerevisiae Nitrogen catabolite repressor [URE3] Growth on poor nitrogen sources
Sup35p Saccharomyces cerevisiae Translation termination factor [PSI+] Increased levels of nonsense suppression
Rnq1p Saccharomyces cerevisiae Protein template factor [RNQ+] Promotes aggregation of other prions
HET-S Podospora anserina Regulates heterokaryon incompatibility [Het-s] Heterokaryon formation between incompatible strains

As of 2003, the following proteins in Saccharomyces cerevisiae had been identified or postulated as prions:

References

  1. ^ Cox, B. S. , M. F. Tuite and C. S. McLaughlin (1988). "The PSI+-Factor of Yeast - a Problem in Inheritance". Yeast 4, 159–178
  2. ^ Paushkin, S. V. , V. V. Kushnirov, V. N. Smirnov and M. D. Ter-Avanesyan (1996). "Propagation of the yeast prion-like PSI+ determinant is mediated by oligomerization of the SUP35-encoded polypeptide chain release factor". EMBO (European Molecular Biology Organization) Journal 15, 3127–3134
  3. ^ Chernoff, Y. O. (2001). "Mutation processes at the protein level: Is Lamarck back?". Mutation Research 488, 39–64
  4. ^ Derkatch, I. L. , M. E. Bradley, P. Zhou, Y. O. Chernoff and S. W. Liebman (1997). "Genetic and Environmental Factors Affecting the de novo Appearance of the [PSI+] Prion in Saccharomyces cerevisiae". Genetics 147 507–519
  1. ^ A census of glutamine/asparagine-rich regions: Implications for their conserved function and the prediction of novel prions. PNAS USA. 2000 Oct 24; 97(22): 11910-5 Free text
  2. ^ The protein product of the het-s heterokaryon incompatibility gene of the fungus Podospora anserina behaves as a prion analog. PNAS USA. 1997 Sep 2; 94(18): 9773-8 Free text
  3. ^ Amyloid aggregates of the HET-S prionprotein are infectious. PNAS USA. 2002 May 28; 99(11): 7402-7 Free text
  4. ^ [URE3] as an altered URE2 protein: evidence for a prion analog in Saccharomyces cerevisiae. Science. 1994 Apr 22; 264(5158): 566-9 Abstract
  5. ^ A yeast prion provides a mechanism for genetic variation and phenotypic diversity. Nature. 2000 Sep 28; 407(6803): 477-83 Abstract
  6. ^ A small reservoir of disabled ORFs in the yeast genome and its implications for the dynamics of proteome evolution. J Mol Biol. 2002 Feb 22; 316(3): 409-19 Abstract
  7. ^ Yeast prions [URE3] and [PSI+] are diseases. PNAS USA. 2005 July 26; 102(30): 10575-80 Free text

See also

A prion (ˈpriːɒn is thought to be an infectious agent that according to current scientific consensus is comprised entirely of a propagated, mis-folded Sup35p is the Saccharomyces cerevisiae (a Yeast) Eukaryotic translation Termination factor. In Biology, the term epigenetics refers to changes in Gene expression caused by mechanisms other than changes in the underlying DNA sequence
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