Structural instability and fibrillar aggregation of non-expanded human ataxin-3 revealed under high pressure and temperature

Mechanistic Insight into the Suppression of Polyglutamine Aggregation by SRCP1

Protein aggregation is a hallmark of the polyglutamine diseases. One potential treatment for these diseases is suppression of polyglutamine aggregation. Previous work identified the cellular slime mold Dictyostelium discoideum as being naturally resistant to polyglutamine aggregation. Further work identified serine-rich chaperone protein 1 (SRCP1) as a protein that is both necessary in Dictyostelium and sufficient in human cells to suppress polyglutamine aggregation. Therefore, understanding how SRCP1 suppresses aggregation may be useful for developing therapeutics for the polyglutamine diseases. Here we utilized a de novo protein modeling approach to generate predictions of SRCP1's structure. Using our best-fit model, we generated mutants that were predicted to alter the stability of SRCP1 and tested these mutants' stability in cells. Using these data, we identified top models of SRCP1's structure that are consistent with the C-terminal region of SRCP1 forming a β-hairpin with a highly dynamic N-terminal region. We next generated a series of peptides that mimic the predicted β-hairpin and validated that they inhibit aggregation of a polyglutamine-expanded mutant huntingtin exon 1 fragment in vitro. To further assess mechanistic details of how SRCP1 inhibits polyglutamine aggregation, we utilized biochemical assays to determine that SRCP1 inhibits secondary nucleation in a manner dependent upon the regions flanking the polyglutamine tract. Finally, to determine if SRCP1 more could generally suppress protein aggregation, we confirmed that it was sufficient to inhibit aggregation of polyglutamine-expanded ataxin-3. Together these studies provide details into the structural and mechanistic basis of the inhibition of protein aggregation by SRCP1.

Pre-Molten, Wet, and Dry Molten Globules en Route to the Functional State of Proteins

Transitions between the unfolded and native states of the ordered globular proteins are accompanied by the accumulation of several intermediates, such as pre-molten globules, wet molten globules, and dry molten globules. Structurally equivalent conformations can serve as native functional states of intrinsically disordered proteins. This overview captures the characteristics and importance of these molten globules in both structured and intrinsically disordered proteins. It also discusses examples of engineered molten globules. The formation of these intermediates under conditions of macromolecular crowding and their interactions with nanomaterials are also reviewed.

The intrinsic amyloidogenic propensity of cofilin-1 is aggravated by Cys-80 oxidation: A possible link with neurodegenerative diseases

Cofilin-1, an actin dynamizing protein, forms actin-cofilin rods, which is one of the major events that exacerbates the pathophysiology of amyloidogenic diseases. Cysteine oxidation in cofilin-1 under oxidative stress plays a crucial role in the formation of these rods. Others and we have reported that cofilin-1 possesses a self-oligomerization property in vitro and in vivo under physiological conditions. However, it remains elusive if cofilin-1 itself forms amyloid-like structures. We, therefore, hypothesized that cofilin-1 might form amyloid-like assemblies, with a potential to intensify the pathophysiology of amyloid-linked diseases. We used various in silico and in vitro techniques and examined the amyloid-forming propensity of cofilin-1. The study confirms that cofilin-1 possesses an intrinsic tendency of aggregation and forms amyloid-like structures in vitro. Further, we studied the effect of cysteine oxidation on the stability and structural features of cofilin-1. Our data show that oxidation at Cys-80 renders cofilin-1 unstable, leading to a partial loss of protein structure. The results substantiate our hypothesis and establish a strong possibility that cofilin-1 aggregation might play a role in cofilin-mediated pathology and the progression of several amyloid-linked diseases.

Observation of liquid-liquid phase separation of ataxin-3 and quantitative evaluation of its concentration in a single droplet using Raman microscopy

Liquid-liquid phase separation (LLPS) plays an important role in a variety of biological processes and is also associated with protein aggregation in neurodegenerative diseases. Quantification of LLPS is necessary to elucidate the mechanism of LLPS and the subsequent aggregation process. In this study, we showed that ataxin-3, which is associated with Machado-Joseph disease, exhibits LLPS in an intracellular crowding environment mimicked by biopolymers, and proposed that a single droplet formed in LLPS can be quantified using Raman microscopy in a label-free manner. We succeeded in evaluating the protein concentration and identifying the components present inside and outside a droplet using the O-H stretching band of water as an internal intensity standard. Only water and protein were detected to be present inside droplets with crowding agents remaining outside. The protein concentration in a droplet was dependent on the crowding environment, indicating that the protein concentration and intracellular environment should be considered when investigating LLPS. Raman microscopy has the potential to become a powerful technique for clarifying the chemical nature of LLPS and its relationship with protein aggregation.

Polyglutamine-Independent Features in Ataxin-3 Aggregation and Pathogenesis of Machado-Joseph Disease

The expansion of a trinucleotide (CAG) repeat, translated into a polyglutamine expanded sequence in the protein encoded by the MJD gene, was identified over 20 years ago as the causative mutation in a severe neurodegenerative disorder originally diagnosed in individuals of Portuguese ancestry. This incapacitating disease, called Machado-Joseph disease or spinocebellar ataxia type 3, is integrated into a larger group of neurodegenerative disorders-the polyglutamine expansion disorders-caused by extension of a CAG repeat in the coding sequence of otherwise unrelated genes. These diseases are generally linked with the appearance of intracellular inclusions , which despite having a controversial role in disease appearance and development represent a characteristic common fingerprint in all polyglutamine-related disorders. Although polyglutamine expansion is an obvious trigger for neuronal dysfunction, the role of the different domains of these complex proteins in the function and aggregation properties of the carrier proteins is being uncovered in recent studies. In this review the current knowledge about the structural and functional features of full-length ataxin-3 protein will be discussed. The intrinsic conformational dynamics and interplay between the globular and intrinsically disordered regions of ataxin-3 will be highlighted, and a perspective picture of the role of known ataxin-3 post-translational modifications on regulating ataxin-3 aggregation and function will be drawn.

Coupled effects of salt and pressure on catalytic ability of Rhizopus chinensis lipase

BACKGROUND

Both high pressure and environmental factors could influence the catalytic abilities of enzymes. This work investigated coupled effects of pressure and salts on Rhizopus chinensis lipase (RCL) to provide significant information for its further applications.

RESULTS

The maximum activity of RCL was observed under 200 MPa at 40 °C. The highest activity was achieved at concentrations of 0.06-0.1 mol L^-1 for tested salts. The effect of monovalent cations on RCL activity followed the Hofmeister series (K⁺ > Na⁺ > Li⁺ ) at 0.1 MPa but the order of Na⁺ and K⁺ was changed under 200 MPa. Meanwhile, the effects of anions did not follow the Hofmeister series. KCl slightly improved the thermostability of RCL at moderate concentration. At 60 °C, LiCl only stabilised RCL at 0.1 mol L^-1 . The pre-transition unfolding point was shifted from 4.5 to 3.5 mol L^-1 with pressure increasing from 0.1 to 600 MPa. In addition, KCl could not change the lipase's extrinsic fluorescence evolution versus pressure.

CONCLUSION

Pressure and salts could improve catalytic ability and stability of RCL under appropriate conditions. The effect of high pressure on RCL was influenced by salts. Meanwhile salts cannot prevent high pressure-induced damage to RCL. © 2017 Society of Chemical Industry.

Modeling the Effect of Monomer Conformational Change on the Early Stage of Protein Self-Assembly into Fibrils

Filamentous self-assembly of proteins is an important process implicated in a plethora of human diseases and of interest for nanotechnology. Using rate equations, we analyze the early stage of the process in solutions that initially contain fibrillation-passive protein monomers and in which the nascent fibrils are practically insoluble. The analysis is based on a model accounting for the conformational and/or other changes the passive monomers experience to transform themselves into fibrillation-active monomers and thus become fibril nuclei. The model allows exact, comprehensive, and simple mathematical description of the early stage of fibrillation, which reveals the usually neglected role of the nucleation nonstationarity in this stage of fibrillation. We obtain exact and user-friendly expressions for experimentally accessible quantities such as the size distribution of fibrils, their number and mass concentrations, the rate and nonstationary period of fibril nucleation, and the delay time of fibril formation. Analyzing available experimental data, we find that the theory successfully describes the fibrillation time course of pathological and nonpathological ataxin-3, a protein involved in the neurodegenerative disorder spinocerebellar ataxia type-3. The analysis provides mechanistic insight into the reason for the higher fibril nucleation and elongation rates of the pathological ataxin-3.

Josephin Domain Structural Conformations Explored by Metadynamics in Essential Coordinates

The Josephin Domain (JD), i.e. the N-terminal domain of Ataxin 3 (At3) protein, is an interesting example of competition between physiological function and aggregation risk. In fact, the fibrillogenesis of Ataxin 3, responsible for the spinocerebbellar ataxia 3, is strictly related to the JD thermodynamic stability. Whereas recent NMR studies have demonstrated that different JD conformations exist, the likelihood of JD achievable conformational states in solution is still an open issue. Marked differences in the available NMR models are located in the hairpin region, supporting the idea that JD has a flexible hairpin in dynamic equilibrium between open and closed states. In this work we have carried out an investigation on the JD conformational arrangement by means of both classical molecular dynamics (MD) and Metadynamics employing essential coordinates as collective variables. We provide a representation of the free energy landscape characterizing the transition pathway from a JD open-like structure to a closed-like conformation. Findings of our in silico study strongly point to the closed-like conformation as the most likely for a Josephin Domain in water.

SUMOylation of the brain-predominant Ataxin-3 isoform modulates its interaction with p97

BACKGROUND

Machado-Joseph Disease (MJD), a form of dominantly inherited ataxia belonging to the group of polyQ expansion neurodegenerative disorders, occurs when a threshold value for the number of glutamines in Ataxin-3 (Atx3) polyglutamine region is exceeded. As a result of its modular multidomain architecture, Atx3 is known to engage in multiple macromolecular interactions, which might be unbalanced when the polyQ tract is expanded, culminating in the aggregation and formation of intracellular inclusions, a unifying fingerprint of this group of neurodegenerative disorders. Since aggregation is specific to certain brain regions, localization-dependent posttranslational modifications that differentially affect Atx3 might also contribute for MJD.

METHODS

We combined in vitro and cellular approaches to address SUMOylation in the brain-predominant Atx3 isoform and assessed the impact of this posttranslational modification on Atx3 self-assembly and interaction with its native partner, p97.

RESULTS

We demonstrate that Atx3 is SUMOylated at K356 both in vitro and in cells, which contributes for decreased formation of amyloid fibrils and for increased affinity towards p97.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These findings highlight the role of SUMOylation as a regulator of Atx3 function, with implications on Atx3 protein interaction network and self-assembly, with potential impact for further understanding the molecular mechanisms underlying MJD pathogenesis.

Investigation of the Josephin Domain protein-protein interaction by molecular dynamics

Spinocerebellar ataxia (SCA) 3, the most common form of SCA, is a neurodegenerative rare disease characterized by polyglutamine tract expansion and self-assembly of Ataxin3 (At3) misfolded proteins into highly organized fibrillar aggregates. The At3 N-terminal Josephin Domain (JD) has been suggested as being responsible for mediating the initial phase of the At3 double-step fibrillogenesis. Several issues concerning the residues involved in the JD's aggregation and, more generally, the JD clumping mechanism have not been clarified yet. In this paper we present an investigation focusing on the JD protein-protein interaction by means of molecular modeling. Our results suggest possible aminoacids involved in JD contact together with local and non-local effects following JD dimerization. Surprisingly, JD conformational changes following the binding may involve ubiquitin binding sites and hairpin region even though they do not pertain to the JD interaction surfaces. Moreover, the JD binding event has been found to alter the hairpin open-like conformation toward a closed-like arrangement over the simulated timescale. Finally, our results suggest that the JD aggregation might be a multi-step process, with an initial fast JD-JD binding mainly driven by Arg101, followed by slower structural global rearrangements involving the exposure to the solvent of Leu84-Trp87, which might play a role in a second step of JD aggregation.

High-pressure fluorescence applications

Fluorescence is the most widely used technique to study the effect of pressure on biochemical systems. The use of pressure as a physical variable sheds light into volumetric characteristics of reactions. Here we focus on the effect of pressure on protein solutions using a simple unfolding example in order to illustrate the applications of the methodology. Topics covered in this review include the relationships between practical aspects and technical limitations; the effect of pressure and the study of protein cavities; the interpretation of thermodynamic and relaxation kinetics; and the study of relaxation amplitudes. Finally, we discuss the insights available from the combination of fluorescence and other methods adapted to high pressure, such as SAXS or NMR. Because of the simplicity and accessibility of high-pressure fluorescence, the technique is a starting point that complements appropriately multi-methodological approaches related to understanding protein function, disfunction, and folding from the volumetric point of view.

Conformational behavior and aggregation of ataxin-3 in SDS

Spinocerebellar ataxia type 3 (SCA3) is one of nine polyglutamine (polyQ) diseases all characterized by the presence of intraneuronal inclusions that contain aggregated protein. Aggregation of ataxin-3, the causative protein of SCA3, has been well characterized in vitro, with both pathogenic and non-pathogenic length ataxin-3 undergoing fibrillogenesis. However, only ataxin-3 containing an expanded polyQ tract leads to SCA3. Therefore other cellular factors, not present in previous in vitro studies, may modulate aggregation during disease. The interactions between fibrillar species and cell membranes have been characterized in a number of amyloid diseases, including Huntington’s Disease, and these interactions affect aggregation and toxicity. We have characterized the effects of the membrane mimetic sodium dodecyl sulfate (SDS) on ataxin-3 structure and aggregation, to show that both micellar and non-micellar SDS have differing effects on the two stages of ataxin-3 aggregation. We also demonstrate that fibrillar ataxin-3 binds phospholipids, in particular phosphorylated phosphotidylinositols. These results highlight the effect of intracellular factors on the ataxin-3 misfolding landscape and their implications in SCA3 and polyQ diseases in general are discussed.

Cyclophilin-mediated reactivation pathway of inactive adenosine kinase aggregates

Monomeric adenosine kinase (AdK), a pivotal salvage enzyme of the purine auxotrophic parasite, Leishmania donovani, tends to aggregate naturally or selectively in presence of ADP, leading to inactivation. A cyclophilin (LdCyP) from the parasite reactivated the enzyme by disaggregating it. We studied the aggregation pathway of AdK with or without ADP. Transmission electron microscopy revealed that ADP-induced aggregates, as opposed to annular or torus-shaped natural aggregates, were mostly amorphous with protofibril-like structures. Interestingly, only the natural aggregates bound thioflavin T with a KD of 3.33 μM, indicating cross β-sheet structure. Dynamic light scattering experiments indicated that monomers formed aggregates either upon prolonged storage or ADP exposure. ADP-aggregates were disaggregated by LdCyP with concomitant reactivation of the enzyme. The activity revived with decrease in the aggregate size. Displacement of ADP from the ADP-aggregated enzyme by LdCyP resulted in reactivation. CD-spectral studies suggested that, like the natural aggregates, ADP induced formation of β-sheet structure in the ADP-aggregates. However, unlike the natural aggregate, it could be reconverted to α-helical conformation upon addition of LdCyP. Based on the results, a regulatory mechanism through interplay of ADP and/or LdCyP interaction with the enzyme is envisaged and a pathway of AdK reactivation by LdCyP-chaperone is proposed.

Trinucleotide repeats: a structural perspective

Trinucleotide repeat (TNR) expansions are present in a wide range of genes involved in several neurological disorders, being directly involved in the molecular mechanisms underlying pathogenesis through modulation of gene expression and/or the function of the RNA or protein it encodes. Structural and functional information on the role of TNR sequences in RNA and protein is crucial to understand the effect of TNR expansions in neurodegeneration. Therefore, this review intends to provide to the reader a structural and functional view of TNR and encoded homopeptide expansions, with a particular emphasis on polyQ expansions and its role at inducing the self-assembly, aggregation and functional alterations of the carrier protein, which culminates in neuronal toxicity and cell death. Detail will be given to the Machado-Joseph Disease-causative and polyQ-containing protein, ataxin-3, providing clues for the impact of polyQ expansion and its flanking regions in the modulation of ataxin-3 molecular interactions, function, and aggregation.

Concentration and temperature dependences of polyglutamine aggregation by multiscale coarse-graining molecular dynamics simulations

The solvent-free multiscale coarse-graining model of polyglutamine was employed to study polyglutamine aggregation at different concentrations and temperatures by means of molecular dynamics simulation. The heterogeneity order parameter (HOP) was used to quantify the polyglutamine aggregation. Our simulation results demonstrate that polyglutamine aggregation is sensitive to concentration and temperature changes. In equilibrium states, polyglutamine molecules fluctuate between aggregating tightly and distributing uniformly. The degree of aggregation monotonically increases with decreasing temperature, but the fluctuation of HOP reaches its maximum at an intermediate temperature. With increasing concentration, the distribution of polyglutamines first changes from more uniform to more nonuniform and then changes back to be more uniform, and the HOP has the widest distribution at the turning point. Simulations with different system sizes indicate that the finite-size effect is trivial and do not change the conclusions drawn for the polyglutamine system. In addition, the composition of the potential energies has been analyzed to confirm that the nonbonded interactions dominate the aggregation of polyglutamines. These results can be thermodynamically understood by considering the competition between the system entropy and molecular interactions, and a statistical model based on HOP has been developed to explain the microscopic mechanism of polyglutamine aggregation.

The Josephin domain determines the morphological and mechanical properties of ataxin-3 fibrils

Fibrillar aggregation of the protein ataxin-3 is linked to the inherited neurodegenerative disorder Spinocerebellar ataxia type 3, a member of the polyQ expansion disease family. We previously reported that aggregation and stability of the nonpathological form of ataxin-3, carrying an unexpanded polyQ tract, are modulated by its N-terminal Josephin domain. It was also shown that expanded ataxin-3 aggregates via a two-stage mechanism initially involving Josephin self-association, followed by a polyQ-dependent step. Despite this recent progress, however, the exact mechanism of ataxin-3 fibrilization remains elusive. Here, we have used electron microscopy, atomic force microscopy, and other biophysical techniques to characterize the morphological and mechanical properties of nonexpanded ataxin-3 fibrils. By comparing aggregates of ataxin-3 and of the isolated Josephin domain, we show that the two proteins self-assemble into fibrils with markedly similar features over the temperature range 37-50°C. Estimates of persistence length and Young's modulus of the fibrils reveal a great flexibility. Our data indicate that, under physiological conditions, during early aggregation Josephin retains a nativelike secondary structure but loses its enzymatic activity. The results suggest a key role of Josephin in ataxin-3 fibrillar aggregation.

Time-course and regional analyses of the physiopathological changes induced after cerebral injection of an amyloid β fragment in rats

Alzheimer's disease (AD) is a neurodegenerative pathology characterized by the presence of senile plaques and neurofibrillary tangles, accompanied by synaptic and neuronal loss. The major component of senile plaques is an amyloid β protein (Aβ) formed by pathological processing of the Aβ precursor protein. We assessed the time-course and regional effects of a single intracerebroventricular injection of aggregated Aβ fragment 25-35 (Aβ(25-35)) in rats. Using a combined biochemical, behavioral, and morphological approach, we analyzed the peptide effects after 1, 2, and 3 weeks in the hippocampus, cortex, amygdala, and hypothalamus. The scrambled Aβ(25-35) peptide was used as negative control. The aggregated forms of Aβ peptides were first characterized using electron microscopy, infrared spectroscopy, and Congo Red staining. Intracerebroventricular injection of Aβ(25-35) decreased body weight, induced short- and long-term memory impairments, increased endocrine stress, cerebral oxidative and cellular stress, neuroinflammation, and neuroprotective reactions, and modified endogenous amyloid processing, with specific time-course and regional responses. Moreover, Aβ(25-35), the presence of which was shown in the different brain structures and over 3 weeks, provoked a rapid glial activation, acetylcholine homeostasis perturbation, and hippocampal morphological alterations. In conclusion, the acute intracerebroventricular Aβ(25-35) injection induced substantial central modifications in rats, highly reminiscent of the human physiopathology, that could contribute to physiological and cognitive deficits observed in AD.

High pressure macromolecular crystallography for structural biology: a review

In recent years, significant progress in high pressure macromolecular crystallography has been observed. It can be attributed both to the developments in experimental techniques, as well as to recognition of importance of high pressure protein studies in biochemistry and biophysics. The number of protein structures determined at pressure up to 1 GPa is growing. The unique advantages of this method can greatly improve the investigation of higher energy conformers of functional significance and our understanding of functionally important conformers, protein folding processes and the structural base of conformational diseases.

Functional interactions as a survival strategy against abnormal aggregation

Protein aggregation is under intense scrutiny because of its role in human disease. Although increasing evidence indicates that protein native states are highly protected against aggregation, the specific protection mechanisms are poorly understood. Insight into such mechanisms can be gained through study of the relatively few proteins that aggregate under native conditions. Ataxin-3, the protein responsible for Spinocerebellar ataxia type 3, a polyglutamine expansion disease, represents one of such examples. Polyglutamine expansion is central for determining solubility and aggregation rates of ataxin-3, but these properties are profoundly modulated by its N-terminal Josephin domain. This work aims at identifying the regions that promote Josephin fibrillogenesis and rationalizing the mechanisms that protect Josephin and nonexpanded ataxin-3 from aberrant aggregation. Using different biophysical techniques, aggregation propensity predictions and rational design of amino acid substitutions, we show that Josephin has an intrinsic tendency to fibrillize under native conditions and that fibrillization is promoted by two solvent-exposed patches, which are also involved in recognition of natural substrates, such as ubiquitin. Indeed, designed mutations at these patches or substrate binding significantly reduce Josephin aggregation kinetics. Our results provide evidence that protein nonpathologic function can play an active role in preventing aberrant fibrillization and suggest the molecular mechanism whereby this occurs in ataxin-3.—Masino, L., Nicastro, G., Calder, L., Vendruscolo, M., Pastore, A. Functional interactions as a survival strategy against abnormal aggregation.

CK2 and GSK3 phosphorylation on S29 controls wild-type ATXN3 nuclear uptake

In the present work we show that murine ATXN3 (ATXN3Q6) nuclear uptake is promoted by phosphorylation on serine 29, a highly conserved residue inside the Josephin domain. Both casein kinase 2 (CK2) and glycogen synthase kinase 3 (GSK3) are able to carry out phosphorylation on this residue. S29 phosphorylation was initially assessed in vitro on purified ATXN3Q6, and subsequently confirmed in transfected COS-7 cells, by MS analysis. Site-directed mutagenesis of S29 to an alanine was shown to strongly reduce nuclear uptake, in COS-7 transiently transfected cells overexpressing ATXN3Q6, while substitution with phospho-mimic aspartic acid restored the wild-type phenotype. Finally, treatment with CK2 and GSK3 inhibitors prevented S29 phosphorylation and strongly inhibited nuclear uptake, showing that both kinases are involved in ATXN3Q6 subcellular sorting. Although other authors have previously addressed this issue, we show for the first time that ATXN3 is phosphorylated inside the Josephin domain and that S29 phosphorylation is involved in nuclear uptake of ATXN3.

Molecular evolution of the hyperthermophilic archaea of the Pyrococcus genus: analysis of adaptation to different environmental conditions

Background

Prokaryotic microorganisms are able to survive and proliferate in severe environmental conditions. The increasing number of complete sequences of prokaryotic genomes has provided the basis for studying the molecular mechanisms of their adaptation at the genomic level. We apply here a computer-based approach to compare the genomes and proteomes from P. furiosus, P. horikoshii, and P. abyssi to identify features of their molecular evolution related to adaptation strategy to diverse environmental conditions.

Results

Phylogenetic analysis of rRNA genes from 26 Pyrococcus strains suggested that the divergence of P. furiosus, P. horikoshii and P. abyssi might have occurred from ancestral deep-sea organisms. It was demonstrated that the function of genes that have been subject to positive Darwinian selection is closely related to abiotic and biotic conditions to which archaea managed to become adapted. Divergence of the P. furiosus archaea might have been due to loss of some genes involved in cell motility or signal transduction, and/or to evolution under positive selection of the genes for translation machinery. In the course of P. horikoshii divergence, positive selection was found to operate mainly on the transcription machinery; divergence of P. abyssi was related with positive selection for the genes mainly involved in inorganic ion transport. Analysis of radical amino acid replacement rate in evolving P. furiosus, P. horikoshii and P. abyssi showed that the fixation rate was higher for radical substitutions relative to the volume of amino acid side-chain.

Conclusions

The current results give due credit to the important role of hydrostatic pressure as a cause of variability in the P. furiosus, P. horikoshii and P. abyssi genomes evolving in different habitats. Nevertheless, adaptation to pressure does not appear to be the sole factor ensuring adaptation to environment. For example, at the stage of the divergence of P. horikoshii and P. abyssi, an essential evolutionary role may be assigned to changes in the trophic chain, namely, acquisition of a consumer status at a high (P. horikoshii) or low level (P. abyssi).

Proteomic and biochemical analyses unveil tight interaction of ataxin-3 with tubulin

Ataxin-3 consists of an N-terminal globular Josephin domain and an unstructured C-terminal region containing a stretch of consecutive glutamines that triggers an inherited neurodegenerative disorder, spinocerebellar ataxia type 3, when its length exceeds a critical threshold. The pathology results from protein misfolding and intracellular accumulation of fibrillar amyloid-like aggregates. Plenty of work has been carried out to elucidate the protein's physiological role(s), which has shown that ataxin-3 is multifunctional; it acts as a transcriptional repressor, and also has polyubiquitin-binding/ubiquitin-hydrolase activity. In addition, a recent report shows that it participates in sorting misfolded protein to aggresomes, close to the microtubule-organizing center. Since a thorough understanding of the protein's physiological role(s) requires the identification of all the molecular partners interacting with ataxin-3, we pursued this goal by taking advantage of two-dimensional chromatography coupled to tandem mass spectrometry. We found that different ataxin-3 constructs, including the sole Josephin domain, bound alpha- and beta-tubulin from soluble rat brain extracts. Coimmunoprecipitation experiments confirmed this interaction. Also, normal ataxin-3 overexpressed in COS7 cultured cells partially colocalized with microtubules, whereas an expanded variant only occasionally did so, probably due to aggregation. Furthermore, by surface plasmon resonance we determined a dissociation constant of 50-70nM between ataxin-3 and tubulin dimer, which strongly supports the hypothesis of a direct interaction of this protein with microtubules in vivo. These findings suggest an involvement of ataxin-3 in directing aggregated protein to aggresomes, and shed light on the mode of interaction among the different molecular partners participating in the process.

Asymmetric kinetics of protein structural changes

Thermodynamic and kinetic understanding of structural transformations in proteins is critical to new developments in medicine and biotechnology. These fields often require the design of mechanism-based modulators of protein function. Researchers increasingly consider these structural changes-such as folding/unfolding or shuttling between active and inactive states-within the energy landscape concept that supposes a high-dimensional, rugged conformational surface. The unevenness, or asperity, of this conformational surface results from energetic barriers and kinetic traps. However, for a large number of protein reactions, such as reversible folding/unfolding, the literature only reports simple two-state transitions, which calls into question the use of a more complex energy landscape model. The question is: are these reactions really that simple, or are we misled by a biased experimental approach? In this Account, we argue in favor of the latter possibility. Indeed, the frequently employed temperature-jump method only allows recording protein structure changes in the heating direction. Under those conditions, it might not be possible to detect other kinetic pathways that could have been taken in the cooling direction. Recently, however, we have developed bidirectional pressure- and temperature-jump methods, which can offer new insights. Here, we show the potential of these methods both for studying protein folding/unfolding reactions, taking ribonuclease A as model, and for studying functionally relevant protein conformational changes, using the open/closed allosteric transition of tryptophan synthase. For example, the heating and cooling temperature-jump induced kinetics involved in the folding/unfolding conformational surface of ribonuclease A is illustrated above. In both of our model systems, the kinetic transition states of several reaction steps were path-dependent, i.e. the rates and thermodynamic activation parameters depend on the direction of the applied pressure and temperature perturbation. This asymmetry suggests that proteins cope with external stress by adapting their structure to form different ensembles of conformational substates. These states are distinguished by their activation enthalpy and entropy barriers, which can be strongly negative in the folding direction. Based on our analysis of activation compressibility and heat capacity, hydration and packing defects of the kinetic transition states are also very important for determining the reaction path. We expect that a more generalized use of this experimental approach should allow researchers to obtain greater insight into the mechanisms of physiologically relevant protein structural changes.

Study of subcellular localization and proteolysis of ataxin-3

In this work we investigate subcellular localization and proteolytic cleavage of different forms of ataxin-3 (AT-3), the protein responsible for spinocerebellar ataxia type 3. Normal (AT-3Q6 and AT-3Q26) and pathological (AT-3Q72) ataxins-3, as well as two truncated forms lacking poly-Q, were studied. Full-length proteins were also expressed as C14A mutants, in order to assess whether AT-3 autoproteolytic activity was involved in its fragmentation. We found that both normal and pathological proteins localized in the cytoplasm and in the nucleus, as expected, but also in the mitochondria. Microsequencing showed that all ataxins-3 underwent the same proteolytic cleavage, removing the first 27 amino acids. Interestingly, while normal ataxins were further cleaved at a number of caspase sites, pathological AT-3 was proteolyzed to a much lesser extent. This may play a role in the pathogenesis, hampering degradation of aggregation-prone expanded AT-3. In addition, autolytic cleavage was apparently not involved in AT-3 proteolysis.

Canine Plasminogen: Spectral Responses to Changes in 6-Aminohexanoate and Temperature

We studied the near UV absorption spectrum of canine plasminogen. There are 19 tryptophans, 19 phenylalanines and 34 tyrosines in the protein. 4th derivative spectra optimized for either tryptophan or tyrosine give a measure of the polarity of the environments of these two aromatic amino acids. Plasminogen at temperatures between 0°C and 37°C exists as a mixture of four conformations: closed-relaxed, open-relaxed, closed-compact, and open-compact. The closed to open transition is driven by addition of ligand to a site on the protein. The relaxed to compact transition is driven by increasing temperature from 0°C to above 15-20°C.

When the conformation of plasminogen is mainly closed-relaxed, the 4th derivative spectra suggest that the average tryptophan environment is similar to a solution of 20% methanol at the same temperature. Under the same conditions, 4th derivative spectra suggest that the average tyrosine environment is similar to water. These apparent polarities change as the plasminogen is forced to assume the other conformations. We try to rationalize the information based on the known portions of the plasminogen structure.

Mechanisms of ataxin-3 misfolding and fibril formation: kinetic analysis of a disease-associated polyglutamine protein

The polyglutamine diseases are a family of nine proteins where intracellular protein misfolding and amyloid-like fibril formation are intrinsically coupled to disease. Previously, we identified a complex two-step mechanism of fibril formation of pathologically expanded ataxin-3, the causative protein of spinocerebellar ataxia type-3 (Machado-Joseph disease). Strikingly, ataxin-3 lacking a polyglutamine tract also formed fibrils, although this occurred only via a single-step that was homologous to the first step of expanded ataxin-3 fibril formation. Here, we present the first kinetic analysis of a disease-associated polyglutamine repeat protein. We show that ataxin-3 forms amyloid-like fibrils by a nucleation-dependent polymerization mechanism. We kinetically model the nucleating event in ataxin-3 fibrillogenesis to the formation of a monomeric thermodynamic nucleus. Fibril elongation then proceeds by a mechanism of monomer addition. The presence of an expanded polyglutamine tract leads subsequently to rapid inter-fibril association and formation of large, highly stable amyloid-like fibrils. These results enhance our general understanding of polyglutamine fibrillogenesis and highlights the role of non-poly(Q) domains in modulating the kinetics of misfolding in this family.

Destabilization of non-pathological variants of ataxin-3 by metal ions results in aggregation/fibrillogenesis

Ataxin-3 (AT3), a protein that causes spinocerebellar ataxia type 3, has a C-terminus containing a polyglutamine stretch, the length of which can be expanded in its pathological variants. Here, we report on the role of Cu(2+), Mn(2+), Zn(2+) and Al(3+) in the induction of defective protein structures and subsequent aggregation/fibrillogenesis of three different non-pathological forms of AT3, i.e. murine (Q6), human non-expanded (Q26) and human moderately expanded (Q36). AT3 variants showed an intrinsic propensity to misfolding/aggregation; on the other hand, Zn(2+) and Al(3+) strongly stimulated the amplitude and kinetics of these conformational conversions. While both metal ions induced a time-dependent aggregation into amyloid-like fibrillar forms, only small oligomers and/or short protofibrillar species were detected for AT3s alone. The rate and extent of the metal-induced aggregation/fibrillogenesis processes increased with the size of the polyglutamine stretch. Mn(2+) and Cu(2+) had no effect on (Q6) or actually prevented (Q26 and Q36) the AT3 structural transitions. The observation that Zn(2+) and Al(3+) promote AT3 fibrillogenesis is consistent with similar results found for other amyloidogenic molecules, such as beta-amyloid and prion proteins. Plausibly, these metal ions are a major common factor/cofactor in the etiopathogenesis of neurodegenerative diseases. Studies of liposomes as membrane models showed dramatic changes in the structural properties of the lipid bilayer in the presence of AT3, which were enhanced after supplementing the protein with Zn(2+) and Al(3+). This suggests that cell membranes could be a potential primary target in the ataxin-3 pathogenesis and metals could be a biological factor capable of modulating their interaction with AT3.

Purification of polyglutamine proteins

The misfolding and formation of fibrillar-like aggregates by polyglutamine proteins is believed to be a key factor in the development of the neurodegenerative polyglutamine diseases; however, relatively little is known about structural and conformational aspects of polyglutamine-induced misfolding and aggregation. This is largely attributable to the fact that polyglutamine proteins have proved difficult to purify in quantities suitable for biochemical and biophysical analyses, thus limiting the extent to which the proteins can be conformationally characterized. Recent advances, however, have seen the development of a number of protocols enabling the expression and purification of these proteins in more significant quantities. In this report, we describe a purification protocol for ataxin-3, which, in its polyglutamine-expanded form, causes Machado-Joseph disease. Purification of different length ataxin-3 variants, including one of pathological length, is facilitated by an N-terminal hexa-histidine tag, which enables binding to a nickel-chelated agarose resin. A key issue that arose during purification was the undesirable proteolysis of ataxin-3 by a trace contaminant protease. We solved this problem by the addition of a benzamidine-binding step during purification, which greatly reduced the level of proteases present. We found that the inclusion of this step had a significant positive impact on the quality of the purified protein product. We also inactivated trace amounts of proteases during experiments by the addition of specific protease inhibitors. Finally, we also describe initial structural and functional analyses that confirm the integrity of the purified protein.

Pressure-jump-induced kinetics reveals a hydration dependent folding/unfolding mechanism of ribonuclease A

Pressure-jump (p-jump)-induced relaxation kinetics was used to explore the energy landscape of protein folding/unfolding of Y115W, a fluorescent variant of ribonuclease A. Pressure-jumps of 40 MPa amplitude (5 ms dead-time) were conducted both to higher (unfolding) and to lower (folding) pressure, in the range from 100 to 500 MPa, between 30 and 50 degrees C. Significant deviations from the expected symmetrical protein relaxation kinetics were observed. Whereas downward p-jumps resulted always in single exponential kinetics, the kinetics induced by upward p-jumps were biphasic in the low pressure range and monophasic at higher pressures. The relative amplitude of the slow phase decreased as a function of both pressure and temperature. At 50 degrees C, only the fast phase remained. These results can be interpreted within the framework of a two-dimensional energy surface containing a pressure- and temperature-dependent barrier between two unfolded states differing in the isomeric state of the Asn-113-Pro-114 bond. Analysis of the activation volume of the fast kinetic phase revealed a temperature-dependent shift of the unfolding transition state to a larger volume. The observed compensation of this effect by glycerol offers an explanation for its protein stabilizing effect.

The Two-stage Pathway of Ataxin-3 Fibrillogenesis Involves a Polyglutamine-independent Step

The aggregation of ataxin-3 is associated with spinocerebellar ataxia type 3, which is characterized by the formation of intraneuronal aggregates. However, the mechanism of aggregation is currently not well understood. Ataxin-3 consists of a folded Josephin domain followed by two ubiquitin-interacting motifs and a C-terminal polyglutamine tract, which in the non-pathological form is less than 45 residues in length. We demonstrate that ataxin-3 with 64 glutamines (at(Q64)) undergoes a two-stage aggregation. The first stage involves formation of SDS-soluble aggregates, and the second stage results in formation of SDS-insoluble aggregates via the poly(Q) region. Both these first and second stage aggregates display typical amyloid-like characteristics. Under the same conditions at(Q15) and at(QHQ) undergo a single step aggregation event resulting in SDS-soluble aggregates, which does not involve the polyglutamine tract. These aggregates do not convert to the SDS-insoluble form. These observations demonstrate that ataxin-3 has an inherent capacity to aggregate through its non-polyglutamine domains. However, the presence of a pathological length polyglutamine tract introduces an additional step resulting in formation of a highly stable amyloid-like aggregate.

Protein folding and aggregation: Two sides of the same coin in the condensation of proteins revealed by pressure studies

Hydrostatic pressure can be considered as "thermodynamic tweezers" to approach the protein folding problem and to study the cases when folding goes wrong leading to the protein folding disorders. The main outcome of the use of high pressure in this field is the stabilization of folding intermediates such as partially folded conformations, thus allowing us to characterize their structural properties. Because partially folded intermediates are usually at the intersection between productive and off-pathway folding, they may give rise to misfolded proteins, aggregates and amyloids that are involved in many neurodegenerative diseases, such as transmissible spongiform encephalopathies, Alzheimer's disease, Parkinson's disease and Huntington's disease. Of particular interest is the use of hydrostatic pressure to unveil the structural transitions in prion conversion and to populate possible intermediates in the folding/unfolding pathway of the prion protein. The main hypothesis for prion diseases proposes that the cellular protein (PrP(C)) can be altered into a misfolded, beta-sheet-rich isoform, the PrP(Sc) (from scrapie). It has been demonstrated that hydrostatic pressure affects the balance between the different prion species. The last findings on the application of high pressure on amyloidogenic proteins will be discussed here as regards to their energetic and volumetric properties. The use of high pressure promises to contribute to the identification of the underlying mechanisms of these neurodegenerative diseases and to develop new therapeutic approaches.

Towards a structural understanding of the fibrillization pathway in Machado-Joseph's disease: trapping early oligomers of non-expanded ataxin-3

Machado-Joseph's disease is caused by a CAG trinucleotide repeat expansion that is translated into an abnormally long polyglutamine tract in the protein ataxin-3. Except for the polyglutamine region, proteins associated with polyglutamine diseases are unrelated, and for all of these diseases aggregates containing these proteins are the major components of the nuclear proteinaceous deposits found in the brain. Aggregates of the expanded proteins display amyloid-like morphological and biophysical properties. Human ataxin-3 containing a non-pathological number of glutamine residues (14Q), as well as its Caenorhabditis elegans (1Q) orthologue, showed a high tendency towards self-interaction and aggregation, under near-physiological conditions. In order to understand the discrete steps in the assembly process leading to ataxin-3 oligomerization, we have separated chromatographically high molecular mass oligomers as well as medium mass multimers of non-expanded ataxin-3. We show that: (a) oligomerization occurs independently of the poly(Q)-repeat and it is accompanied by an increase in beta-structure; and (b) the first intermediate in the oligomerization pathway is a Josephin domain-mediated dimer of ataxin-3. Furthermore, non-expanded ataxin-3 oligomers are recognized by a specific antibody that targets a conformational epitope present in soluble cytotoxic species found in the fibrillization pathway of expanded polyglutamine proteins and other amyloid-forming proteins. Imaging of the oligomeric forms of the non-pathological protein using electron microscopy reveals globular particles, as well as short chains of such particles that likely mimic the initial stages in the fibrillogenesis pathway occurring in the polyglutamine-expanded protein. Thus, they constitute potential targets for therapeutic approaches in Machado-Joseph's disease, as well as valuable diagnostic markers in disease settings.

Tuning amyloidogenic conformations through cosolvents and hydrostatic pressure: when the soft matter becomes even softer

Compact packing, burial of hydrophobic side-chains, and low free energy levels of folded conformations contribute to stability of native proteins. Essentially, the same factors are implicated in an even higher stability of mature amyloid fibrils. Although both native insulin and insulin amyloid are resistant to high pressure and influence of cosolvents, intermediate aggregation-prone conformations are susceptible to either condition. Consequently, insulin fibrillation may be tuned under hydrostatic pressure or-- through cosolvents and cosolutes-- by preferential exclusion or binding. Paradoxically, under high pressure, which generally disfavors aggregation of insulin, an alternative "low-volume" aggregation pathway, which leads to unique circular amyloid is permitted. Likewise, cosolvents are capable of preventing, or altering amyloidogenesis of insulin. As a result of cosolvent-induced perturbation, distinct conformational variants of fibrils are formed. Such variants, when used as templates for seeding daughter generations, reproduce initial folding patterns regardless of environmental biases. By the close analogy, this suggests that the "prion strains" phenomenon may mirror a generic, common feature in amyloids. The susceptibility of amyloidogenic conformations to pressure and cosolvents is likely to arise from their "frustration", as unfolding results in less-densely packed side-chains, void volumes, and exposure of hydrophobic groups. The effects of cosolvents and pressure are discussed in the context of studies on other amyloidogenic protein models, amyloid polymorphism, and "strains".

Polyglutamine is Not All: The Functional Role of the AXH Domain in the Ataxin-1 Protein

A family of neurodegenerative diseases is associated with anomalous expansion of a polyglutamine tract in the coding region of the corresponding proteins. The current working hypothesis is that polyglutamine diseases are caused by misfolding and aggregation of the proteins with a process dictated by the polyglutamine tracts, although increasing evidence suggests an involvement of the protein context in modulating these properties. Here, we show that the AXH domain of ataxin-1, the protein involved in spinocerebellar ataxia type-1, is the region responsible for the transcriptional repression activity of ataxin-1 and participates in protein aggregation. In vitro, the isolated domain undergoes a conformational transition towards a beta-enriched structure associated with aggregation and amyloid fibre formation spontaneously and without need for destabilizing conditions. Using a transfected cell line, we demonstrate that, while determined by polyglutamine expansion, ataxin-1 aggregation is noticeably reduced by deletion of AXH or by replacement with the homologous sequence from the transcription factor HBP1, which has no known tendency to aggregate. These results provide the first direct evidence of an involvement of a region other than the polyglutamine tract in polyglutamine pathologies.

The role of the 132-160 region in prion protein conformational transitions

The native conformation of host-encoded cellular prion protein (PrP(C)) is metastable. As a result of a post-translational event, PrP(C) can convert to the scrapie form (PrP(Sc)), which emerges as the essential constituent of infectious prions. Despite thorough research, the mechanism underlying this conformational transition remains unknown. However, several studies have highlighted the importance of the N-terminal region spanning residues 90-154 in PrP folding. In order to understand why PrP folds into two different conformational states exhibiting distinct secondary and tertiary structure, and to gain insight into the involvement of this particular region in PrP transconformation, we studied the pressure-induced unfolding/ refolding of recombinant Syrian hamster PrP expanding from residues 90-231, and compared it with heat unfolding. By using two intrinsic fluorescent variants of this protein (Y150W and F141W), conformational changes confined to the 132-160 segment were monitored. Multiple conformational states of the Trp variants, characterized by their spectroscopic properties (fluorescence and UV absorbance in the fourth derivative mode), were achieved by tuning the experimental conditions of pressure and temperature. Further insight into unexplored conformational states of the prion protein, likely to mimic the in vivo structural change, was obtained from pressure-assisted cold unfolding. Furthermore, salt-induced conformational changes suggested a structural stabilizing role of Tyr150 and Phe141 residues, slowing down the conversion to a beta-sheet form.

The powerful high pressure tool for protein conformational studies

The pressure behavior of proteins may be summarized as a the pressure-induced disordering of their structures. This thermodynamic parameter has effects on proteins that are similar but not identical to those induced by temperature, the other thermodynamic parameter. Of particular importance are the intermolecular interactions that follow partial protein unfolding and that give rise to the formation of fibrils. Because some proteins do not form fibrils under pressure, these observations can be related to the shape of the stability diagram. Weak interactions which are differently affected by hydrostatic pressure or temperature play a determinant role in protein stability. Pressure acts on the 2 degrees, 3 degrees and 4 degrees structures of proteins which are maintained by electrostatic and hydrophobic interactions and by hydrogen bonds. We present some typical examples of how pressure affects the tertiary structure of proteins (the case of prion proteins), induces unfolding (ataxin), is a convenient tool to study enzyme dissociation (enolase), and provides arguments to understand the role of the partial volume of an enzyme (butyrylcholinesterase). This approach may have important implications for the understanding of the basic mechanism of protein diseases and for the development of preventive and therapeutic measures.

Defining the role of ubiquitin-interacting motifs in the polyglutamine disease protein, ataxin-3

Polyglutamine (polyQ) expansions cause neurodegeneration that is associated with protein misfolding and influenced by functional properties of the host protein. The polyQ disease protein, ataxin-3, has predicted ubiquitin-specific protease and ubiquitin-binding domains, which suggest that ataxin-3 functions in ubiquitin-dependent protein surveillance. Here we investigate direct links between the ubiquitin-proteasome pathway and ataxin-3. In neural cells we show that, through its ubiquitin interaction motifs (UIMs), normal or expanded ataxin-3 binds a broad range of ubiquitinated proteins that accumulate when the proteasome is inhibited. The expression of a catalytically inactive ataxin-3 (normal or expanded) causes ubiquitinated proteins to accumulate in cells, even in the absence of proteasome inhibitor. This accumulation of ubiquitinated proteins occurs primarily in the cell nucleus in transfected cells and requires intact UIMs in ataxin-3. We further show that both normal and expanded ataxin-3 can undergo oligoubiquitination. Although this post-translational modification occurs in a UIM-dependent manner, it becomes independent of UIMs when the catalytic cysteine residue of ataxin-3 is mutated, suggesting that ataxin-3 ubiquitination is itself regulated in trans by its own de-ubiquitinating activity. Finally, pulse-chase labeling reveals that ataxin-3 is degraded by the proteasome, with expanded ataxin-3 being as efficiently degraded as normal ataxin-3. Mutating the UIMs does not alter degradation, suggesting that UIM-mediated oligoubiquitination of ataxin-3 modulates ataxin-3 function rather than stability. The function of ataxin-3 as a de-ubiquitinating enzyme, its post-translational modification by ubiquitin, and its degradation via the proteasome link this polyQ protein to ubiquitin-dependent pathways already implicated in disease pathogenesis.

The AXH Domain Adopts Alternative Folds

AXH is a protein module identified in two unrelated families that comprise the transcriptional repressor HBP1 and ataxin-1 (ATX1), the protein responsible for spinocerebellar ataxia type-1 (SCA1). SCA1 is a neurodegenerative disorder associated with protein misfolding and formation of toxic intranuclear aggregates. We have solved the structure in solution of monomeric AXH from HBP1. The domain adopts a nonclassical permutation of an OB fold and binds nucleic acids, a function previously unidentified for this region of HBP1. Comparison of HBP1 AXH with the crystal structure of dimeric ATX1 AXH indicates that, despite the significant sequence homology, the two proteins have different topologies, suggesting that AXH has chameleon properties. We further demonstrate that HBP1 AXH remains monomeric, whereas the ATX1 dimer spontaneously aggregates and forms fibers. Our results describe an entirely novel, to our knowledge, example of a chameleon fold and suggest a link between these properties and the SCA1 pathogenesis.

Exploring the temperature-pressure configurational landscape of biomolecules: from lipid membranes to proteins

Hydrostatic pressure has been used as a physical parameter for studying the stability and energetics of biomolecular systems, such as lipid mesophases and proteins, but also because high pressure is an important feature of certain natural membrane environments and because the high-pressure phase behaviour of biomolecules is of biotechnological interest. By using spectroscopic and scattering techniques, the temperature- and pressure-dependent structure and phase behaviour of lipid systems, differing in chain configuration, headgroup structure and concentration, and proteins have been studied and are discussed. A thermodynamic approach is presented for studying the stability of proteins as a function of both temperature and pressure. The results demonstrate that combined temperature-pressure dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins. We also introduce pressure as a kinetic variable. Applying the pressure jump relaxation technique in combination with time-resolved synchrotron X-ray diffraction and spectroscopic techniques, the kinetics of un/refolding of proteins has been studied. Finally, recent advances in using pressure for studying misfolding and aggregation of proteins will be discussed.

Characterization of the structure and the amyloidogenic properties of the Josephin domain of the polyglutamine-containing protein ataxin-3

Expansion of the polyglutamine (polyQ) region in the protein ataxin-3 is associated with spinocerebellar ataxia type 3, an inherited neurodegenerative disorder that belongs to the family of polyQ diseases. Increasing evidence indicates that protein aggregation and fibre formation play an important role in these pathologies. In a previous study, we determined the domain architecture of ataxin-3, suggesting that it comprises a globular domain, named Josephin, and a more flexible C-terminal region, that includes the polyQ tract. Here, we have characterised for the first time the biophysical properties of the isolated Josephin motif, showing that it is an autonomously folded unit and that it has no significant interactions with the C-terminal region. Study of its thermodynamic stability indicates that Josephin has an intrinsic tendency to aggregate and forms temperature-induced fibrils similar to those described for expanded ataxin-3. We show that, under destabilising conditions, the behaviours of the isolated Josephin domain and ataxin-3 are extremely similar. Our data therefore strongly suggest that the stability and aggregation properties of non-expanded ataxin-3 are determined by those of the Josephin domain, which is sufficient to reproduce the behaviour of the full-length protein. Our data support a mechanism in which the thermodynamic stability of ataxin-3 is governed by the properties of the Josephin domain, but the presence of an expanded polyQ tract increases dramatically the protein's tendency to aggregate.