Monday, 21 May 2012

Prions: The cause of a newly found style of evolution


By Alex Moore (42658867) Practical Group: Tuesday AM Tutor: Prajwal Bhattaram

Bovine spongiform encephalopathy, more commonly known as Mad Cow Disease, is an infectious disease that causes the degeneration and cell death of brain cells resulting in the formation of small holes in the brain. This fatal disease although most prevalent in cows can be transmitted across species by the ingestion of any contaminated meat, resulting in the death of many humans (NoVA, 2006).
Figure 1: Protein in the prion configuration (Source from: Coghlan A, 2012)
 
Yeast got the edge with prions <i>(Image: Gene Cox/SPL/Getty)</i>It wasn’t until 1982 that Stanley Prusiner discovered the cause of the fatal disease. By extracting and examining the infectious agent Prusiner found no traces of DNA or a nucleus, thus ruling out bacterial or viral infection. What he did find was protein, and thus he discovered the prion. Prions are self replicating; infectious proteins whose irregular folded

protein structure make them very resilient to modern medication (Nova, 2006). Prions are formed through the conversion of a normal protein by the interaction with its prion counterpart made of the same amino acid sequence (Lindquist et al, 2004).
Although prions seem to only cause disease and appear to pose no real benefit for living organisms it has recently been confirmed that this is not the case. Prions are in fact responsible for a newly discovered form of evolution that allows the survival and further propagation of low order organisms in harsh and normally unliveable environments by altering the phenotype of the organism without changing the genotype (Lindquist et al., 2010).

Text Box: Figure 1: The effect of Sup532 and [PSI+] on the translation process (Source from: Coghlan A, 2012)http://www.newscientist.com/data/images/ns/cms/mg21328524.800/mg21328524.800-2_253.jpg One particular prion is [PSI+], the prion derivative of Sup53, a protein found in a species of yeast, Saccharomyces cerevisiae (Coghlan A, 2012). Sup53 plays a crucial role in the translation of mRNA into proteins to be expressed by an organism. Specifically Sup53 acts as a guideline for where ribosomes, an rRNA and protein complex, ‘start and stop’ protein synthesis(Coghlan A, 2012). When Sup53 is converted into [PSI+] it loses the ability to regulate protein synthesis. This means that previously untranslated segments of mRNA undergo protein synthesis, resulting in the creation of approximately 100 new proteins that were previously unexpressed(Coghlan A, 2012). But                                                                                      what does this mean for the host organism?

In a study conducted by Susan Lindquist it was found that when the yeast S. cerevisiae was grown in extreme conditions, such as oxygen deprived and acidic environments,  the yeast with the [PSI+] prion survived due to the extra protein. This was confirmed by treating the remaining yeast with prion eliminating agents causing the surviving yeast to die. It was also found that these prions were inherited between parent and daughter cells (Coghlan A, 2012).
It was theorized that these alterations to the cells phenotype may in the future become a permanent addition to the cells genotype; however it is too early to tell (Coghlan A, 2012). This newfound form of evolution has not only shed a light on the beneficial functions of prions but may also lead to the further understanding of their role in the natural evolution of lower order organisms. It is not yet known if there is any similar evolution function in higher orders of life but it is a further stepping stone in our understanding of evolution.

Bibliography

NOVA – 2006, Prions – morphing agents of disease, Australian Academy of Science, 17 March 2012, <http://www.science.org.au/nova/003/003key.htm>.
Coghlan, A 2012, ‘Prions point to a new style of evolution’, New Scientist, Issue Number 2852.
Lindquist, S et al. 2004, ‘Effects of Q/N-rich, polyQ, and non-polyQ amyloids on the de novo formation of the [PSI+] prion in yeast and aggregation of Sup53 in vitro’, Proceedings of the National Academy of Sciences of the United States of America, vol. 101, no. 39.
Lindquist, S et al. 2010, ‘Prions, protein homeostasis and phenotypic diversity’, Trends in Cell Biology, vol. 20, no. 3.
Campbell, Reece, Meyers, Urry, Cain, Wasserman, Minorsky, Jackson, 2009, Biology, 8th edition, Pearson Education, Australia.

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