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Biology of infectious proteins: lessons from yeast prions
Comment
Biology of infectious proteins: lessons from yeast prions
www.thelancet.com Vol 364 October 23, 2004
genetic changes needed to conserve the beneficial phenotypes are acquired.1 One is almost tempted to wonder whether yeast prions can act as subtle mutagens, perhaps via functional impairment of enzymes involved in DNA replication and repair. A slightly lowered fidelity of DNA replication combined with selective pressures in new environments could explain the rapid genetic fixation of phenotypes in infected cells and later in their prionnegative progeny. Yeast research has provided extremely valuable insights into the biological concept of infectious proteins. Indeed, formal proof of the prion hypothesis was first generated in this system.8–10 Prusiner’s group confirmed these data by showing that recombinant murine PrP can be converted in vitro into prions that are replication-competent and pathogenic.11 Moreover, a last resort for prion sceptics, the existence of prion strains, was rendered untenable by studies in the yeast system. As the prion hypothesis demands, prion strains indeed store and stably inherit the necessary information for diverse biological properties in different conformations of the same aminoacid sequence without the participation of nucleic acids.9,10 Prions cause devastating neurodegenerative diseases such as Creutzfeldt-Jakob disease in human beings and bovine spongiform encephalopathy in cattle. Because of discoveries about beneficial functions of yeast prions, we may have to ask ourselves whether prions are more than just dangerous pathogens. Recent evidence suggests that in the sea slug, Aplysia californica, aspects of long-term memory involve the conformational switch of the cytoplasmic polyadenylation element binding protein into
Rights were not granted to include this image in electronic media. Please refer to the printed journal.
Science Photo Library
Recently, Heather True and colleagues1 provided further insight into the astonishing biology of yeast prions. Rather than acting in the role of the nasty pathogen, in which prions are usually cast, it seems that prions can actually help the infected host. The existence of yeast prions is sometimes surprising to those not working in the area because the term prion is often confused with prion-protein (PrP), a protein probably encoded by all vertebrate genomes, but not yeast genomes.2 Generally, the term prion describes the folding transformation of harmless proteins into infectious agents. Prions replicate by forcing proteins with identical aminoacid sequences to assume prion conformations. PrP is just one of several proteins susceptible to being converted to prion folds. The idea of transmitting biological information without involvement of nucleic acids, as originally formulated in 1998 by Prusiner,3 has been intensely controversial. However, many essential insights into the prion concept arose not from the analysis of prion diseases but from research into non-chromosomally inherited genetic elements of yeast, the existence of which was first explained with the aid of the prion concept by Wickner in 1994.4 Sup35p is a yeast protein that normally terminates translation at stop codons. It can also adopt a prion conformation. Because the available Sup35p protein is no longer functional in prion-infected yeast, the ribosomal machinery reads beyond stop codons. Although one might expect this to be bad news for the infected yeast, they are actually quite happy to live with the prions. By translating genetic information normally hidden behind stop codons, infected cells are equipped with new and, occasionally, advantageous phenotypes. Proteins with altered properties therefore appear and contribute to the diversity of the population.5–7 Diversity leads to enhanced phenotypic plasticity in prion-infected yeast and facilitates the yeast’s adaption to environmental changes. When exposed to toxic environments that would normally prevent growth of uninfected yeast, prion-infected cells survive.6,7 Thus the yeast benefits from the increased biological diversity in the population caused by the sloppy termination of translation. If the prion infection favours a cell in particular circumstances, the cell passes on this trait to its progeny. However, because the prion infection can be lost spontaneously, the advantage provided by the acquired phenotypes would be even more profound if it could be maintained in the absence of the prion state of Sup35p. Indeed, True and colleagues showed that the traits can be genetically fixed by multiple mechanisms, including reassortment of genetic polymorphisms, or new mutations, or both. In other words, the beneficial trait can become prion independent. The Sup35p prions therefore promote evolutionary adaptation by allowing cells to thrive in hostile environments until the permanent
Scanning electron micrograph of Saccharomyces cerevisiae
1471
Comment
a prion fold.12 Hence, the beneficial effect of prions does not seem limited to yeasts; it may also be operating in multicellular organisms. For now, it is safe to say that the unexpected twists and turns in prion biology will keep researchers busy for years.
3 4
Michael Burwinkel, Nikola Holtkamp, *Michael Baier
7
Project Neurodegenerative Diseases, Robert-Koch-Institut, 13353 Berlin, Germany (M Burwinkel, M Baier); and Institute of Neuropathology, Humboldt University, 13353 Berlin, Germany (NH) [email protected] We declare that we have no conflict of interest. 1
2
True HL, Berlin I, Lindquist SL. Epigenetic regulation of translation reveals hidden genetic variation to produce complex traits. Nature 2004; 431: 184–87. Oidtmann B, Simon D, Holtkamp N, Hoffmann R, Baier M. Identification of cDNAs from Japanese pufferfish (Fugu rubripes) and Atlantic salmon (Salmo salar) coding for homologues to tetrapod prion proteins. FEBS Lett 2003; 538: 96–100.
5 6
8
9 10
11 12
Prusiner SB. Prions. Proc Natl Acad Sci USA 1998; 95: 13363–83. Wickner RB. [URE3] as an altered URE2 protein: evidence for a prion analog in Saccharomyces cerevisiae. Science 1994; 264: 566–69. Cox BS. , a cytoplasmatic suppressor of super-suppressor in yeast. Heredity 1965; 20: 505–21. Eaglestone SS, Cox BS, Tuite MF. Translation termination efficiency can be regulated in Saccharomyces cerevisiae by environmental stress through a prion-mediated mechanism. EMBO J 1999; 18: 1974–81. True HL, Lindquist SL. A yeast prion provides a mechanism for genetic variation and phenotypic diversity. Nature 2000; 407: 477–83. Sparrer HE, Santoso A, Szoka FC Jr, Weissman JS. Evidence for the prion hypothesis: induction of the yeast [PSI+] factor by in vitro-converted Sup35 protein. Science 2000; 289: 595–99. King CY, Diaz-Avalos R. Protein-only transmission of three yeast prion strains. Nature 2004; 428: 319–23. Tanaka M, Chien P, Naber N, Cooke R, Weissman JS. Conformational variations in an infectious protein determine prion strain differences. Nature 2004; 428: 323–28. Legname G, Baskakov IV, Nguyen HO, et al. Synthetic mammalian prions. Science 2004; 305: 673–76. Bailey CH, Kandel ER, Si K. The persistence of long-term memory; a molecular approach to self-sustaining changes in learning-induced synaptic growth. Neuron 2004; 44: 49–57.
Not-for-profit drugs—no longer an oxymoron? Chinese cultivation of sweet wormwood (Artemisia annua) is set to increase ten-fold next year in an attempt to meet the rising demand for artemisinin, but this drug will still be
Science Photo Library
Rights were not granted to include this image in electronic media. Please refer to the printed journal.
Sweet wormwood, Artemisia annua Cultivation will have to increase to meet demand for artemisinin.
1472
expensive. The semi-synthetic agents, artemether and artesunate, are not perfect either. However, medicinal chemists and pharmacologists have continued to play around with these peroxidic antimalarials. Jonathan Vennerstrom and colleagues1 recently reported that a wholly synthetic trioxolane now offers “economically feasible and scalable synthesis, superior antimalarial activity and an improved biopharmaceutical profile”. Human studies with OZ277 (or RBx-11160) are only just beginning and the interest in this antimalarial candidate owes as much to the manner of its development as to its clinical potential. In 2002, a Médecins Sans Frontières working group recorded a grave imbalance between pharmaceutical innovation and global burdens of disease.2 The traditional pharmaceutical industry’s research base had lost interest in tropical illnesses. These are not the rare “orphan diseases” as recognised by the US Orphan Drug Act—for example, visceral leishmaniasis kills an estimated 200 000 people every year. Nonetheless to attract big investment in research and development for profit, a disease has to afflict not just large numbers of patients but also those who can pay. Only 10% of the world’s disease burden is targeted by 90% of annual global spending on health research and development. There has been no shortage of imaginative social-market solutions to this problem—including straight gifts such as the Mectizan Donation Program and discounted pricing— but most of the recent ideas take the form of publicprivate partnerships. These partnerships have flowered since the arrival of the International AIDS Vaccine Initiative in 1996. The Geneva-based Initiative on Public-Private www.thelancet.com Vol 364 October 23, 2004
Biology of infectious proteins: lessons from yeast prions
www.thelancet.com Vol 364 October 23, 2004
genetic changes needed to conserve the beneficial phenotypes are acquired.1 One is almost tempted to wonder whether yeast prions can act as subtle mutagens, perhaps via functional impairment of enzymes involved in DNA replication and repair. A slightly lowered fidelity of DNA replication combined with selective pressures in new environments could explain the rapid genetic fixation of phenotypes in infected cells and later in their prionnegative progeny. Yeast research has provided extremely valuable insights into the biological concept of infectious proteins. Indeed, formal proof of the prion hypothesis was first generated in this system.8–10 Prusiner’s group confirmed these data by showing that recombinant murine PrP can be converted in vitro into prions that are replication-competent and pathogenic.11 Moreover, a last resort for prion sceptics, the existence of prion strains, was rendered untenable by studies in the yeast system. As the prion hypothesis demands, prion strains indeed store and stably inherit the necessary information for diverse biological properties in different conformations of the same aminoacid sequence without the participation of nucleic acids.9,10 Prions cause devastating neurodegenerative diseases such as Creutzfeldt-Jakob disease in human beings and bovine spongiform encephalopathy in cattle. Because of discoveries about beneficial functions of yeast prions, we may have to ask ourselves whether prions are more than just dangerous pathogens. Recent evidence suggests that in the sea slug, Aplysia californica, aspects of long-term memory involve the conformational switch of the cytoplasmic polyadenylation element binding protein into
Rights were not granted to include this image in electronic media. Please refer to the printed journal.
Science Photo Library
Recently, Heather True and colleagues1 provided further insight into the astonishing biology of yeast prions. Rather than acting in the role of the nasty pathogen, in which prions are usually cast, it seems that prions can actually help the infected host. The existence of yeast prions is sometimes surprising to those not working in the area because the term prion is often confused with prion-protein (PrP), a protein probably encoded by all vertebrate genomes, but not yeast genomes.2 Generally, the term prion describes the folding transformation of harmless proteins into infectious agents. Prions replicate by forcing proteins with identical aminoacid sequences to assume prion conformations. PrP is just one of several proteins susceptible to being converted to prion folds. The idea of transmitting biological information without involvement of nucleic acids, as originally formulated in 1998 by Prusiner,3 has been intensely controversial. However, many essential insights into the prion concept arose not from the analysis of prion diseases but from research into non-chromosomally inherited genetic elements of yeast, the existence of which was first explained with the aid of the prion concept by Wickner in 1994.4 Sup35p is a yeast protein that normally terminates translation at stop codons. It can also adopt a prion conformation. Because the available Sup35p protein is no longer functional in prion-infected yeast, the ribosomal machinery reads beyond stop codons. Although one might expect this to be bad news for the infected yeast, they are actually quite happy to live with the prions. By translating genetic information normally hidden behind stop codons, infected cells are equipped with new and, occasionally, advantageous phenotypes. Proteins with altered properties therefore appear and contribute to the diversity of the population.5–7 Diversity leads to enhanced phenotypic plasticity in prion-infected yeast and facilitates the yeast’s adaption to environmental changes. When exposed to toxic environments that would normally prevent growth of uninfected yeast, prion-infected cells survive.6,7 Thus the yeast benefits from the increased biological diversity in the population caused by the sloppy termination of translation. If the prion infection favours a cell in particular circumstances, the cell passes on this trait to its progeny. However, because the prion infection can be lost spontaneously, the advantage provided by the acquired phenotypes would be even more profound if it could be maintained in the absence of the prion state of Sup35p. Indeed, True and colleagues showed that the traits can be genetically fixed by multiple mechanisms, including reassortment of genetic polymorphisms, or new mutations, or both. In other words, the beneficial trait can become prion independent. The Sup35p prions therefore promote evolutionary adaptation by allowing cells to thrive in hostile environments until the permanent
Scanning electron micrograph of Saccharomyces cerevisiae
1471
Comment
a prion fold.12 Hence, the beneficial effect of prions does not seem limited to yeasts; it may also be operating in multicellular organisms. For now, it is safe to say that the unexpected twists and turns in prion biology will keep researchers busy for years.
3 4
Michael Burwinkel, Nikola Holtkamp, *Michael Baier
7
Project Neurodegenerative Diseases, Robert-Koch-Institut, 13353 Berlin, Germany (M Burwinkel, M Baier); and Institute of Neuropathology, Humboldt University, 13353 Berlin, Germany (NH) [email protected] We declare that we have no conflict of interest. 1
2
True HL, Berlin I, Lindquist SL. Epigenetic regulation of translation reveals hidden genetic variation to produce complex traits. Nature 2004; 431: 184–87. Oidtmann B, Simon D, Holtkamp N, Hoffmann R, Baier M. Identification of cDNAs from Japanese pufferfish (Fugu rubripes) and Atlantic salmon (Salmo salar) coding for homologues to tetrapod prion proteins. FEBS Lett 2003; 538: 96–100.
5 6
8
9 10
11 12
Prusiner SB. Prions. Proc Natl Acad Sci USA 1998; 95: 13363–83. Wickner RB. [URE3] as an altered URE2 protein: evidence for a prion analog in Saccharomyces cerevisiae. Science 1994; 264: 566–69. Cox BS. , a cytoplasmatic suppressor of super-suppressor in yeast. Heredity 1965; 20: 505–21. Eaglestone SS, Cox BS, Tuite MF. Translation termination efficiency can be regulated in Saccharomyces cerevisiae by environmental stress through a prion-mediated mechanism. EMBO J 1999; 18: 1974–81. True HL, Lindquist SL. A yeast prion provides a mechanism for genetic variation and phenotypic diversity. Nature 2000; 407: 477–83. Sparrer HE, Santoso A, Szoka FC Jr, Weissman JS. Evidence for the prion hypothesis: induction of the yeast [PSI+] factor by in vitro-converted Sup35 protein. Science 2000; 289: 595–99. King CY, Diaz-Avalos R. Protein-only transmission of three yeast prion strains. Nature 2004; 428: 319–23. Tanaka M, Chien P, Naber N, Cooke R, Weissman JS. Conformational variations in an infectious protein determine prion strain differences. Nature 2004; 428: 323–28. Legname G, Baskakov IV, Nguyen HO, et al. Synthetic mammalian prions. Science 2004; 305: 673–76. Bailey CH, Kandel ER, Si K. The persistence of long-term memory; a molecular approach to self-sustaining changes in learning-induced synaptic growth. Neuron 2004; 44: 49–57.
Not-for-profit drugs—no longer an oxymoron? Chinese cultivation of sweet wormwood (Artemisia annua) is set to increase ten-fold next year in an attempt to meet the rising demand for artemisinin, but this drug will still be
Science Photo Library
Rights were not granted to include this image in electronic media. Please refer to the printed journal.
Sweet wormwood, Artemisia annua Cultivation will have to increase to meet demand for artemisinin.
1472
expensive. The semi-synthetic agents, artemether and artesunate, are not perfect either. However, medicinal chemists and pharmacologists have continued to play around with these peroxidic antimalarials. Jonathan Vennerstrom and colleagues1 recently reported that a wholly synthetic trioxolane now offers “economically feasible and scalable synthesis, superior antimalarial activity and an improved biopharmaceutical profile”. Human studies with OZ277 (or RBx-11160) are only just beginning and the interest in this antimalarial candidate owes as much to the manner of its development as to its clinical potential. In 2002, a Médecins Sans Frontières working group recorded a grave imbalance between pharmaceutical innovation and global burdens of disease.2 The traditional pharmaceutical industry’s research base had lost interest in tropical illnesses. These are not the rare “orphan diseases” as recognised by the US Orphan Drug Act—for example, visceral leishmaniasis kills an estimated 200 000 people every year. Nonetheless to attract big investment in research and development for profit, a disease has to afflict not just large numbers of patients but also those who can pay. Only 10% of the world’s disease burden is targeted by 90% of annual global spending on health research and development. There has been no shortage of imaginative social-market solutions to this problem—including straight gifts such as the Mectizan Donation Program and discounted pricing— but most of the recent ideas take the form of publicprivate partnerships. These partnerships have flowered since the arrival of the International AIDS Vaccine Initiative in 1996. The Geneva-based Initiative on Public-Private www.thelancet.com Vol 364 October 23, 2004