The University of Kent, Canterbury, Kent, CT2 7NZ, T +44 (0)1227 764000
Professor Mick Tuite
Professor of Molecular Biology
School of Biosciences
- 01227 (82)3699
Professor Mick Tuite joined the School of Biosciences in 1983 after conducting postdoctoral research at the University of California (1978-81) and the University of Oxford (1981-83). He began his research studies on yeast whilst a PhD student under the supervision of Dr Brian Cox at the 'Botany School' in Oxford. His research interests have largely focused on the mechanism and control of translation in yeast but more recently his interests have moved to yeast prion proteins and molecular chaperones. The research in his group has been extensively funded by grants from the BBSRC, Wellcome Trust and the Leverhulme Trust.back to top
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Protein misfolding, prions and molecular chaperones
Creutzfeldt-Jakob Disease (CJD) is an unusual infectious disease of the human brain that leads to dementia and death. The most baffling fact about this and related diseases such as sheep scrapie and bovine spongiform encephalopathy (BSE, Mad Cow Disease) is that they appear to be triggered by a unique class of infectious agent known as a prion (protein-only infectious agent). Although CJD is a comparatively rare disease in humans (approximately 1 in a million people will contract the disease in any one year), sufferers show many of the pathological features seen in individuals suffering from other more common, non-infectious diseases of the brain such as Alzheimer's Disease and Parkinson's Disease. In spite of its infectious nature, the majority of cases of human CJD (~80%) are known to occur 'sporadically' i.e. appear spontaneously, without any evidence of the diseased individual acquiring an infectious prion from a third party, or through a mutation in the individual's genes. Prions are highly unusual infectious agents because they consist only of a single protein that has become structurally altered. In humans the prion protein responsible for CJD is called PrP, a protein normally found in the brain. Yet we know very little about how the PrP-based prions take up their altered structure spontaneously to cause sporadic CJD, what will trigger their formation or how prions are propagated and transmitted once formed in the cell.
To help us address these important questions we study prions that are found in Baker's yeast (Saccharomyces cerevisiae). Prions were first described in this fungus some 15 years ago by Reed Wickner and the many subsequent studies on yeast prions have revealed fascinating often surprising new aspects of prion biology not least the fact that their presence can be of benefit to the host they infect rather than detrimental.
The yeast Saccharomyces cerevisiae in its many guises
Recent reviews published by the Tuite Laboratory
- Tuite, M.F. and Serio, T.R. (2010). The prion hypothesis: from biological anomaly to basic regulatory mechanism. Nature Reviews in Molecular Cell Biology 11, 823-833.
- Naeimi, W.R. and Tuite, M.F. (2010) Fungal prions as epigenetic determinants. The Biochemist 32: 30-33.
- Tuite, M.F., Marchante, R. and Kushnirov, V.V. (2011) Fungal prions: structure, function and propagation. In: Prion Proteins (Topics in Current Chemistry, volume 305), Ed. Jorg Tatzelt, Springer, pp 257-98.
- Staniforth, G.L and Tuite, M.F. (2012) Fungal prions. Progress in Molecular Biology and Translational Science 107: 417–456.
- Naeimi, W.R. and Tuite, M.F. (2012) Prions as epigenetic regulators of phenotype in fungi. In Encyclopaedia of Molecular Cell Biology and Molecular Medicine: Epigenetic Regulation and Epigenomics (Ed. R A Meyers), Wiley-VCH Verlag GmbH & Co. pp741-770.
....and on-line seminar
- Tuite, M.F.. (2008), "Mechanisms of yeast prion propagation", in Wickner, R. (ed.), Prions and Amyloids: Self-propagating protein structures in mammals, yeast and fungi, The Biomedical & Life Sciences Collection, Henry Stewart Talks Ltd, London (online at http://www.hstalks.com/?t=BL0521794-Tuite
Defining the sequence and structural features important for amyloidogenesis and prion propagation in yeast
The main objective of this project is to identify the role of a particular region of the yeast prion protein in question, namely the protein Sup35p, in the continued replication of the transmissible prion form. The region of the Sup35p protein molecule that will be investigated lies at the extreme amino terminus of the protein, a region known as the prion-forming domain (PrD). This region in turn has two functionally distinct elements: a 40 residue ‘aggregation domain’ and a 56 residue ‘propagation domain’. The important feature of the aggregation domain is its ability to drive the formation of amyloid-like fibres of the protein in the test tube. This property is due in part to its high content of amino acid residues glutamine (Q) and asparagine (N). This region, which is referred to as the QN-rich (QNR) region, will be the focus of the project. The major objectives of this project are:
- to define the minimal region of the QNR region with respect to prion propagation;
- to study the structure of the QNR region using NMR;
- to identify the important sequence and/or structural features of the oligopeptide repeat region (OPR) of the Sup35p-PrD necessary for stable propagation of the prion form of Sup35p.
Researchers: Dr Ricardo Marchante (postdoc), Faye Russell (MSc student)
Modelling prion dynamics in the living yeast cell
Two models have been used to explain the self-propagation of prions in yeast and mammals; the template-directed refolding model ('seeded polymerisation') and the inhibitor titration model. The former model has gained general acceptance and invokes the formation of oligomeric seeds that drive the structural alteration and polymerisation of soluble molecules of the prion protein thus forming the characteristic high molecular weight amyloid fibrils. Stable propagation of yeast prions thus requires continued generation of new prion seeding molecular entities (which we call propagons) and their efficient transmission to daughter cells at cell division. In this project we are exploiting a novel, stochastic modelling approach to establish the key molecular events in the propagation and transmission of the [PSI+] prion.
Our current models provide an important phenomenological description of how the [PSI+] prion is eliminated in the absence of a mechanism to generate new propagons. In this new project we are developing new mechanistic models that not only take into account the kinetics of prion protein polymerisation, but also polymer fragmentation in the growing yeast cell. To achieve this we are generating stochastic simulation models for yeast prion polymer kinetics and supporting this model building by experimentally establishing the values of the important model factors including the cellular levels of Sup35p and the key chaperone proteins Hsp104 and Sis1p, the number and sizes of Sup35p prion polymers and the rate of synthesis and turnover of Sup35p in its various cellular forms.
To evaluate the emerging simulation models we are also assessing the in vivo consequences of modulating levels of Sup35p, Hsp104 and Sis1p all of which are essential for [PSI+] propagation. One crucial parameter in our models is pi, the proportion of propagons transmitted to the daughter cell. We wish to identify factors that lead to a change in pi, particularly in relation to the number and length of Sup35p polymers in the cell. We also carry out model sensitivity and validation studies in the light of experimental data obtained in order to generate a definitive stochastic simulation model for yeast prion polymer kinetics in the dividing cell. This project involves a close interplay between bioscience-led research and integrated mathematical modelling with the aim of shedding new light on how infectious amyloids are generated and transmitted in vivo.
Researchers: Dr Wesley Naeimi (postdoc), Dr Vasileios Giagos (postdoc), Jintana Wongwigkarn (PhD student)
Collaborators: Professors Byron Morgan and Martin Ridout (School of Mathematics, Statistics and Actuarial Science, University of Kent)
Funded by: BBSRC
Induction of yeast prions by reactive oxygen species (ROS)
How yeast and mammalian prions form spontaneously into infectious amyloid-like structures is poorly understood at present yet the majority of cases of the human prion disease Creutzfeldt Jakob Disease (CJD) are sporadic i.e. without underlying infection or genetic change. The overall objective of this project is to establish the molecular mechanism by which 'sporadic' prion formation is triggered by oxidative stress and other environmental triggers using the well established and widely exploited yeast prion model.
We are using two different yeast prions in this study, namely [PSI+]/Sup35 and [PIN+]/Rnq1, and are focussing on the role of the Tsa1/Tsa2 peroxiredoxins (Prxs). Prxs are antioxidant enzymes that have multiple functions in stress protection and, in collaboration with the laboratory of Professor Chris Grant at the University of Manchester, we have recently shown that the two major cytoplasmic Prxs (that both are ribosome-associated) suppress oxidative stress-induced de novo formation of the[PSI+] prion. Using high throughput assays for de novo formation of the [PSI+] and [PIN+] prions, we are therefore defining the role of Tsa1/Tsa2 in prion formation.
These important new findings strongly implicated oxidative damage of Sup35 as an important trigger of the formation of the heritable prion conformation in yeast. Oxidative damage has also been implicated in the de novo formation of mammalian prions and a trigger for other protein misfolding diseases of man. We have recently established that direct oxidation of the Sup35 polypeptides lead to structural transitions favouring conversion to the transmissible amyloid-like form and hence plays a role in the mechanism of induction of de novo prion formation. These studies are also being extended to other disease-associated, amyloid-forming proteins expressed in yeast including alpha-synuclein. The overall aim of this project is to define the mechanism by which oxidative stress induces yeast prion formation de novo and also the misfolding of human amyloidogenic proteins.
Researchers: Dr Gemma Staniforth (postdoc)
Collaborator: Professor Chris Grant, Dr Vicki Doronina, University of Manchester
Funded by: BBSRC
The natural history of yeast prions
The [PSI+] and [URE3] prions modify the yeast cell phenotype without apparent detriment, suggesting that prions could also represent a novel form of epigenetic inheritance. What we must now establish is whether prions exist in S. cerevisiae strains isolated from a variety of ecosystems and natural environments and are not simply an artifact of 'domestication' of this species.
The main aim of this project is to establish whether [PSI+] and [PIN+], two different and well characterised prions, are present in a wide range of wild strains of S. cerevisiae isolated from 'natural' environments. We will also establish whether the [PSI+] and [PIN+] prions from laboratory strains can replicate in such wild strains and vice versa. The hypothesis that underpins this project is that the [PSI+] and [PIN+] prions can exist in wild strains of S. cerevisiae and that apparently 'prion-free' wild strains have the molecular machinery to replicate one or both prions. Our laboratory was the first to report that a yeast prion (in this case [PIN+]) is present in some wild strains of S. cerevisiae, but in that study we only looked at relatively atypical pathogenic strains isolated from immunocompromised human patients. Although widely viewed as a domesticated organism, nevertheless S. cerevisiae has been found in a variety of ecological niches including tree bark and the surface of fruits. Each niche represents a unique environment in which the organism has to adapt and retain viability and fertility. As first suggested by Susan Lindquist and her colleagues at MIT, the ability to modulate protein function in a reversible manner without the immediate need to fix changes in DNA sequence could be of benefit to the host both in short-term adaptation and in the longer term, for the evolution of new traits.
Researchers: Professor Mick Tuite
Collaborators: Professor Susan Lindquist (MIT-WIBR)
Funded by: The Leverhulme Trust
Yeast models of protein misfolding diseases
To facilitate the analysis of the cause and consequences of disease-associated protein misfolding, a number of 'neurodegenerative disease models' have been established in the budding yeast Saccharomyces cerevisiae. S. cerevisiae is a highly tractable but simple eukaryote that nevertheless has many of the protein misfolding and quality control mechanisms that exist in higher eukaryotes. A number of the high profile human neurodegenerative diseases described to date, including Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD) are being successfully studied in yeast models. The main objective of using a yeast-based model approach to further our understanding of these diseases is to establish the molecular basis of the causation and pathology of what are a globally important group of diseases. For example, by genetically manipulating S. cerevisiae to accumulate misfolded proteins allows the molecular mechanisms important in the cellular folding machinery and the response to misfolded proteins, to be identified. The factors which trigger misfolding and the generation of the toxic entity associated with this misfolding can also be established in order to identify potential intervention strategies.
In this project we are looking at two different disease models to try and establish the associated mechanism(s) of proteotoxicity:
Amyotrophic Lateral Sclerosis (ALS), a non-transmissible neurodegenerative disease affecting the upper and lower motor neurons of the brain and spinal cord. The most common fALS-associated mutation occurs in the gene encoding superoxide dismutase 1 (SOD1), an enzyme that converts superoxide anions to hydrogen peroxide and oxygen thus protecting cells against oxidative damage. We have developed a model in which we have introduced disease-associated mutations into the yeast SOD1 protein and are using a range of cell biological and biochemical approaches to pinpoint the associated toxicity that we see in yeast.
Amyloid toxicity. Two 'model' amyloids are being used one of which (Rnq1p) is a native yeast prion protein. The other 'model' is a protein carrying a polyglutamine expansion and based on the Huntington Disease-associated huntingtin protein. By carrying out genetic and cell biological screens in yeast expressing the disease-associated forms of these proteins we are identifying how the cell responds to amyloid-induced toxicity and the cellular factors that can be manipulated to abrogate the toxicity.
Researchers: Emma Bastow (PhD student), Selena Li (PhD student)
Collaborators: Dr Campbell Gourlay (University of Kent)
Funded by: BBSRC
Heterozygous inhibition of the amyloid state
A number of single-site mutations have been identified in the yeast Sup35p prion protein that lead to a failure to propagate the [PSI+] prion. One of the interesting properties of these so-called 'PNM – [PSI+] no more - mutations is that the prion form cannot propagate in the heterozygous configuration i.e. in diploid cells expressing equal levels of both normal and mutant Sup35p. This state mimics what has been observed with CJD and other prion diseases when there is heterozygosity at the PrP-encoding locus. This is very easy to show in yeast by mating two strains with different SUP35 alleles and showing that the diploid produced is unable to propagate the [PSI+] prion state.
The amyloids formed by Aβ, tau, huntingtin, α-synuclein, serum amyloid A and numerous other proteins responsible for amyloidoses in humans may or may not be self-propagating and none has been found to have resistance polymorphisms in populations. There are good reasons for this lack which have nothing to do with whether 'resistance' mutations analogous to the SUP35-based PNM mutations, may occur. In this project we are exploring the hypothesis that resistance mutants can be found such that, as in prions, heterozygosity at certain sites may prevent the formation of a non-self-propagating amyloid state.
Researchers: Dr Brian Cox
Collaborators: Professor Mike Resnick
Funded by: The Leverhulme Trust
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