By Vivienne Baillie Gerritsen
When referring to prions, most of us think ‘Mad Cow disease’ (Bovine Spongiform Encephalopathy or BSE). Which is not incorrect but rather narrow-minded. Indeed, the term ‘prion’ - coined in the early 1980s by the American biochemist Stanley Prusiner - simply means ‘infectious protein’, from which were derived the letters which make up the word. One may wonder which ‘i’, ‘o’ and ‘n’ were promoted but that is not the issue here… Prion proteins are not only found in cattle, sheep and humans, but also other vertebrates as well as yeast and certain fungi. And will no doubt continue to be discovered in many other organisms, if not all. The URE2 protein, a candidate prion in Saccharomyces cerevisiae, was the first prion to be crystallized. Clearly, the understanding of a prion’s 3D structure and the conformational changes it undergoes - and passes onto its peers - is of great interest in the quest for therapeutic treatments of diseases caused by certain prions.
Scrapie, the fatal neurodegenerative disease which affects sheep and goats, and which could very well be named the ‘Mad Sheep disease’, was first described in the 1700s but its infectious nature was only recognized in the 1930s. It took a further 70 years before it was discovered that the infectious agent of such diseases was in fact a protein. Prusiner made the discovery and tentatively suggested it to the scientific community.
The concept that a protein could be infectious did not get a standing ovation and the media helped generously in kindling disagreement. It took some time before the idea seeped in and was finally accepted. Prusiner went through a rough time but was ultimately rewarded the Nobel Prize in Medecine in 1997 for his discovery, where he wisely asserted that '…while it is quite reasonable for scientists to be skeptical of new ideas that do not fit within the accepted realm of scientific knowledge, the best science often emerges from situations where results carefully obtained do not fit within the accepted paradigms.’
Contrary to popular belief, prion proteins do not necessarily bring on disease. They are not ‘infectious’ in the common sense. What a prion is capable of doing is undergoing a conformational change which, in turn, it can transmit - and in this sense it is being infectious - to a second identical protein, and so on. In the event of BSE, the prions form fibrillar structures in the cells, which are ultimately harmful to the organism. Such an affliction is also termed a conformational disease. On the other hand, a number of prions are absolutely harmless to the cell and the organism as a whole; despite the ‘infection’, there is no disease. It may be that some organisms actually use prion infection as regulator of a function. That is to say, the infection can actually switch on a function at a given time. In the event of S.cerevisiae and URE2, the protein’s function is disrupted but causes no subsequent harm to the yeast.
Fig. 1 3D structure of the functional region of URE2p in its monomeric form (Source Ref.3)*
What function does URE2 have in its native form? Well, no one knows for sure. It does have sequence similarity and even structural similarity to glutathione S-transferases but no one yet has been able to show that this is actually its function. Although it has been shown that URE2 is required for detoxification of glutathione S-transferase substrates and cellular oxidants. What is known, however, is that it has a role in nitrogen metabolism, where it regulates a number of transcription factors. When there is plenty nitrogen, yeast turns off enzymes and transporters needed in the event of a poor nitrogen source. URE2 helps out in this process by binding to transcription factors in the cytoplasm, thereby preventing their entry into the nucleus where they would promote the transcription of a certain number of genes.
URE2 is a rather globular protein with a flexible cap region and a poorly structured N-terminal region. The belly of the protein does sport a cleft - like the glutathione S-transferases - and could well be there for an unknown ligand. In its active form, URE2 acts as a dimer, where it seems likely that the N-terminal region of one monomer interacts with the belly (functional) region of the other to hold the dimer together in an appealing embrace.
The poorly-structured N-terminal is also known as the prion region. It may be that the interaction of the latter with the globular functional region - within the same monomer - prevents the conversion of the protein into its prion form. What molecular changes occur to ease URE2 into its prion form, URE2p? No one knows. What happens though is that the structural change causes the subsequent loss of URE2 function altogether and promotes the assembly of URE2p into fibrils through the interaction of URE2p monomers. It is not known to date whether it is the subtle change in conformation which causes the loss of URE2 function or whether the novel fibril formation simply hinders the URE2/GLN3 interaction. How is the ‘infection’ propagated from one yeast cell to another? By cytoplasmic mixing. When yeast mates, the cytoplasm of the parental strains mix even though the nucleii do not fuse. When the URE2p of one strain flows into the other, it infects the second strain by transmitting the URE2p conformational change.
Naturally, the whole point in understanding how a prion protein such as URE2p becomes infectious and assembles into fibrils, how it loses its function and how it ultimately affects an organism is to help in the future design of drugs which could counter neurodegenerative diseases such as BSE and the unfortunate human form: Creutzfeldt-Jakob disease.
Cross-references to Swiss-Prot
P23202: Saccharomyces cerevisiae (Baker’s yeast) URE2 protein,
References
1. Bousset L., Redeker V., Decottignies P., Dubois S., Le Maréchal P., Melki R.
Structural characterization of the fibrillar form of the yeast Saccharomyces cerevisiae prion Ure2p
PMID: 15109261.
2. Rai R., Tate J.J., Cooper T.G.
Ure2, a prion precursor with homology to glutathione S-transferase, protects Saccharomyces cerevisiae cells from heavy metal ion and oxidant toxicity
J. Biol. Chem. 278:12826-12833(2003)
PMID: 12562760.
3. Bousset L, Belrhali H., Janin J., Melki R., Morera S.
Structure of the globular region of the prion protein Ure2 from the yeast Saccharomyces cerevisiae
Structure 9:39-46(2001)
PMID: 11342133.
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