Biophysical studies of the development of amyloid fibrils from a peptide fragment of cold shock protein B

Properties of a tardigrade desiccation-tolerance protein aerogel

Lyophilization is promising for tackling degradation during the drying and storage of protein-based drugs. Tardigrade cytosolically abundant heat soluble (CAHS) proteins are necessary and sufficient for desiccation-tolerance in vivo and protein protection in vitro. Hydrated CAHS proteins form coiled-coil-based fine-stranded, cold-setting hydrogels, but the dried protein remains largely uncharacterized. Here, we show that dried CAHS D gels (i.e., aerogels) retain the structural units of their hydrogels, but the details depend on prelyophilization CAHS concentrations. Low concentration samples (<10 g/L) form thin (<0.2 μm) tangled fibrils lacking regular structure on the micron scale. Upon increasing the concentration, the fibers thicken and form slabs comprising the walls of the aerogel pores. These changes in morphology are associated with a loss in disorder and an increase in large β sheets and a decrease in α helices and random coils. This disorder-to-order transition is also seen in hydrated gels as a function of concentration. These results suggest a mechanism for pore formation and indicate that using CAHS proteins as excipients will require attention to initial conditions because the starting concentration impacts the lyophilized product.

Engineering of Supramolecular β-Sheet and Nontoxic Amyloid Fibrils from Synthetic Oligopeptides Containing γ-Aminobutyric Acid as the N-Terminal Residue

Here we demonstrate that three synthetic tripeptides containing conformationally flexible γ-aminobutyric acid (γ-Abu) as the N-terminal residue form supramolecular β-sheet and nanofibrillar aggregates upon self-association in aqueous medium. Congo red and thioflavin T binding study establish that these nanofibrillar aggregates are amyloidogenic in nature. The MTT cell survival assay suggests that these amyloid-like nanofibrillar aggregates are nontoxic toward cultured Neuro 2A cells. Interestingly, none of these tripeptides bear sequence identity with any amyloid forming proteins or peptides; however upon self-association, they form supramolecular β-sheet and amyloid-like nanofibrils those are nontoxic in nature. The results highlight the self-assembling nature of the conformationally flexible peptides in aqueous environment and support the hypothesis that amyloid formation is the intrinsic property of the polypeptide chain. Also the cytotoxicity is not predictive from amyloid fibril formation alone. Such nontoxic amyloid fibrils can be exploited in future to design functional biomaterials for various biomedical applications.

Structural transitions and interactions in the early stages of human glucagon amyloid fibrillation

A mechanistic understanding of the intermolecular interactions and structural changes during fibrillation is crucial for the design of safe and efficacious glucagon formulations. Amide hydrogen/deuterium exchange with mass spectrometric analysis was used to identify the interactions and amino acids involved in the initial stages of glucagon fibril formation at acidic pH. Kinetic measurements from intrinsic and thioflavin T fluorescence showed sigmoidal behavior. Secondary structural measurement of fibrillating glucagon using far-UV circular dichroism spectroscopy showed changes in structure from random coil → α-helix → β-sheet, with increase in α-helix content during the lag phase followed by increase in β-sheet content during the growth phase. Hydrogen/deuterium exchange with mass spectrometric analysis of fibrillating glucagon suggested that C-terminal residues 22-29 are involved in interactions during the lag phase, during which N-terminal residues 1-6 showed no changes. Molecular dynamics simulations of glucagon fragments showed C-terminal to C-terminal interactions with greater α-helix content for the 20-29 fragment, with hydrophobic and aromatic residues (Phe-22, Trp-25, Val-23, and Met-27) predominantly involved. Overall, the study shows that glucagon interactions during the early phase of fibrillation are mediated through C-terminal residues, which facilitate the formation of α-helix-rich oligomers, which further undergo structural rearrangement and elongation to form β-sheet-rich mature fibrils.

Physico-chemical stress induced amyloid formation in insulin: Amyloid characterization, cytotoxicity analysis against human neuroblastoma cell lines and its prevention using black seeds (Nigella sativa)

OBJECTIVE

To investigate the aggregation and fibrillation of insulin at low pH and moderate temperature; and to further test the aggregated insulin for its cytotoxicity on human neuroblastoma (SH-SY5Y) cell line and inhibition of the cytotoxicity by black seeds (Nigella sativa) extract.

METHODS

Bovine pancreatic insulin was incubated at pH 2.0, 45 ℃ under stirring condition at 400 r/min for 24 h. Amyloids like structures in the aggregated insulin were characterized using various techniques such as thioflavin T assay (ThT), 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence, circular dichroism (CD) and dynamic light scattering (DLS). Moreover, cytotoxicity of aggregated insulin was monitored on SH-SY5Y cell line in the presence and absence of black seeds extract using standard 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide (MTT), lactate dehydrogenase (LDH) and reactive oxygen species (ROS) assay kit.

RESULTS

Our finding demonstrated that insulin under the mentioned conditions formed amyloid-like structure. ANS binding to aggregated insulin showed increase in fluorescence, suggesting structural change and increase in hydrophobicity in insulin occurring during the fibril formation. DLS measurement revealed progressive increase in hydrodynamic radius of aggregated insulin. Cytotoxicity assays illustrated aggregated insulin induced apoptosis in SH-SY5Y cell through ROS formation. Moreover, LDH measurement showed aggregated insulin triggered membrane damage in SH-SY5Y cell lines. Black seeds extract was found to inhibit amyloid formation and protected the cells against amyloid toxicity.

CONCLUSION

Insulin molded into amyloid like structure at low pH and under stirring conditions. Characterization of insulin aggregates illustrated conformational change in insulin and it experiences α-helix to β-sheet transition during the course of fibrillation. Black seeds extract inhibited amyloid progression of insulin via ROS scavenging and restrained the cytotoxicity caused by insulin fibrils suggesting black seeds containing polyphenols may serve as a lead structure to a novel anti-amyloidogenic drugs.

IL-10 alters immunoproteostasis in APP mice, increasing plaque burden and worsening cognitive behavior

Anti-inflammatory strategies are proposed to have beneficial effects in Alzheimer's disease. To explore how anti-inflammatory cytokine signaling affects Aβ pathology, we investigated the effects of adeno-associated virus (AAV2/1)-mediated expression of Interleukin (IL)-10 in the brains of APP transgenic mouse models. IL-10 expression resulted in increased Aβ accumulation and impaired memory in APP mice. A focused transcriptome analysis revealed changes consistent with enhanced IL-10 signaling and increased ApoE expression in IL-10-expressing APP mice. ApoE protein was selectively increased in the plaque-associated insoluble cellular fraction, likely because of direct interaction with aggregated Aβ in the IL-10-expressing APP mice. Ex vivo studies also show that IL-10 and ApoE can individually impair glial Aβ phagocytosis. Our observations that IL-10 has an unexpected negative effect on Aβ proteostasis and cognition in APP mouse models demonstrate the complex interplay between innate immunity and proteostasis in neurodegenerative diseases, an interaction we call immunoproteostasis.

Different morphology of amyloid fibrils originating from agitated and non-agitated conditions

In vitro amyloid formation has been suggested to be a common property of any polypeptide chain depending on particular environmental conditions although in vivo amyloid fibril formation can be promoted by point mutations or triplet expansions. Here, we explored the influence of agitation on fibril formation of amyloidogenic alanine segments fused to Cold Shock Protein B (CspB) of Bacillus subtilis. While without agitation fibril formation was clearly dependent on the presence of an amyloidogenic alanine segment, fibril formation was independent of the amyloidogenic segment under agitation. Agitation even led to fibrillation of native CspB lacking the amyloidogenic segment. Furthermore, agitation not only influenced the kinetics of fibril formation, but also resulted in completely different fibril morphologies. These results indicate that experimental conditions can alter the region that undergoes a conformational change during in vitro fibrillation. Moreover, the data show that deductions from in vitro assays on in vivo fibril formation mechanisms are afflicted with a certain degree of uncertainty and therefore need to be cautiously discussed.

Role of protein stabilizers on the conformation of the unfolded state of cytochrome c and its early folding kinetics: investigation at single molecular resolution

An insight into the conformation and dynamics of unfolded and early intermediate states of a protein is essential to understand the mechanism of its aggregation and to design potent inhibitor molecules. Fluorescence correlation spectroscopy has been used to study the effects of several model protein stabilizers on the conformation of the unfolded state and early folding dynamics of tetramethyl rhodamine-labeled cytochrome c from Saccharomyces cerevisiae at single molecular resolution. Special attention has been given to arginine, which is a widely used stabilizer for improving refolding yield of different proteins. The value of the hydrodynamic radius (r(H)) obtained by analyzing the intensity fluctuations of the diffusing molecules has been found to increase in a two-state manner as the protein is unfolded by urea. The results further show that the presence of arginine and other protein stabilizers favors a relatively structured conformation of the unfolded states (r(H) of 29 A) over an extended one (r(H) of 40 A), which forms in their absence. Also, the time constant of a kinetic component (tau(R)) of about 30 micros has been observed by analyzing the correlation functions, which represents formation of a collapsed state. This time constant varies with urea concentration representing an inverted Chevron plot that shows a roll-over and behavior in the absence of arginine. To the best of our knowledge, this is one of the first applications of fluorescence correlation spectroscopy to study direct folding kinetics of a protein.

Amyloidogenic sequences in native protein structures

Numerous short peptides have been shown to form beta-sheet amyloid aggregates in vitro. Proteins that contain such sequences are likely to be problematic for a cell, due to their potential to aggregate into toxic structures. We investigated the structures of 30 proteins containing 45 sequences known to form amyloid, to see how the proteins cope with the presence of these potentially toxic sequences, studying secondary structure, hydrogen-bonding, solvent accessible surface area and hydrophobicity. We identified two mechanisms by which proteins avoid aggregation: Firstly, amyloidogenic sequences are often found within helices, despite their inherent preference to form beta structure. Helices may offer a selective advantage, since in order to form amyloid the sequence will presumably have to first unfold and then refold into a beta structure. Secondly, amyloidogenic sequences that are found in beta structure are usually buried within the protein. Surface exposed amyloidogenic sequences are not tolerated in strands, presumably because they lead to protein aggregation via assembly of the amyloidogenic regions. The use of alpha-helices, where amyloidogenic sequences are forced into helix, despite their intrinsic preference for beta structure, is thus a widespread mechanism to avoid protein aggregation.

Monitoring Alzheimer Amyloid Peptide Aggregation by EPR

Plaques containing the aggregated beta-Amyloid (Abeta) peptide in the brain are the main indicators of Alzheimer's disease. Fibrils, the building blocks of plaques, can also be produced in vitro and consist of a regular arrangement of the peptide. The initial steps of fibril formation are not well understood and could involve smaller aggregates (oligomers) of Abeta. Such oligomers have even been implicated as the toxic agents. Here, a method to study oligomers on the time scale of aggregation is suggested. We have labeled the 40 residue Abeta peptide variant containing an N-terminal cysteine (cys-Abeta) with the MTSL [1-oxyl-2,2,5,5-tetramethyl-Delta-pyrroline-3-methyl] methanethiosulfonate spin label (SL-Abeta). Fibril formation in solutions of pure SL-Abeta and of SL-Abeta mixed with Abeta was shown by Congo-red binding and electron microscopy. Continuous-wave 9 GHz electron paramagnetic resonance reveals three fractions of different spin-label mobility: one attributed to monomeric Abeta, one to a multimer (8-15 monomers), and the last one to larger aggregates or fibrils. The approach, in principle, allows detection of oligomers on the time scale of aggregation.

Amyloid peptides and proteins in review

Amyloids are filamentous protein deposits ranging in size from nanometres to microns and composed of aggregated peptide beta-sheets formed from parallel or anti-parallel alignments of peptide beta-strands. Amyloid-forming proteins have attracted a great deal of recent attention because of their association with over 30 diseases, notably neurodegenerative conditions like Alzheimer's, Huntington's, Parkinson's, Creutzfeldt-Jacob and prion disorders, but also systemic diseases such as amyotrophic lateral sclerosis (Lou Gehrig's disease) and type II diabetes. These diseases are all thought to involve important conformational changes in proteins, sometimes termed misfolding, that usually produce beta-sheet structures with a strong tendency to aggregate into water-insoluble fibrous polymers. Reasons for such conformational changes in vivo are still unclear. Intermediate aggregated state(s), rather than precipitated insoluble polymeric aggregates, have recently been implicated in cellular toxicity and may be the source of aberrant pathology in amyloid diseases. Numerous in vitro studies of short and medium length peptides that form amyloids have provided some clues to amyloid formation, with an alpha-helix to beta-sheet folding transition sometimes implicated as an intermediary step leading to amyloid formation. More recently, quite a few non-pathological amyloidogenic proteins have also been identified and physiological properties have been ascribed, challenging previous implications that amyloids were always disease causing. This article summarises a great deal of current knowledge on the occurrence, structure, folding pathways, chemistry and biology associated with amyloidogenic peptides and proteins and highlights some key factors that have been found to influence amyloidogenesis.

Contribution of rotational diffusion to pulsed field gradient diffusion measurements

NMR diffusion experiments employing pulsed field gradients are well established as sensitive probes of the displacement of individual nuclear spins in a sample. Conventionally such measurements are used as a measure of translational diffusion, but here we demonstrate that under certain conditions rotational motion will contribute very significantly to the experimental data. This situation occurs when at least one spatial dimension of the species under study exceeds the root mean square displacement associated with translational diffusion, and leads to anomalously large apparent diffusion coefficients when conventional analytical procedures are employed. We show that in such a situation the effective diffusion coefficient is a function of the duration of the diffusion delay used, and that this dependence provides a means of characterizing the dimensions of the species under investigation.

Probing the Mechanism of Amyloidogenesis through a Tandem Repeat of the PI3-SH3 Domain Suggests a Generic Model for Protein Aggregation and Fibril Formation

Aggregation of the SH3 domain of the PI3 kinase, both as a single domain and as a tandem repeat in which the C terminus of one domain is linked to the N terminus of another by a flexible linker of ten glycine/serine residues, has been studied under a range of conditions in order to investigate the mechanism of protein aggregation and amyloid formation. The tandem repeat was found to form amyloid fibrils much more readily than the single domain under the acidic conditions used here, and the fibrils themselves have higher morphological homogeneity. The folding-unfolding transition of the PI3-SH3 domain shows two-state behaviour and is pH dependent; at pH 3.6, which is near the pH mid-point for folding and only slightly below the isoelectric point of the protein, both the single domain and the tandem repeat spontaneously form broad distributions of soluble oligomers without requirement for nucleation. Under prolonged incubation under these conditions, the oligomers convert into thin, curly fibrils that interact with thioflavin-T, suggesting that they contain an organised beta-sheet structure. Under more acidic conditions (pH 2.0) where the proteins are fully denatured and carry a positive net charge, long, straight fibrils are formed in a process having a pronounced lag phase. The latter was found to be reduced dramatically by the addition of oligomers exceeding a critical size of approximately 20 molecules. The results suggest that the process of aggregation of these SH3 domains can take place by a variety of mechanisms, ranging from downhill formation of relatively amorphous species to nucleated formation of highly organised structures, the relative importance of which varies greatly with solution conditions. Comparison with the behaviour of other amyloidogenic systems suggests that the general mechanistic features outlined here are likely to be common to at least a wide variety of peptides and proteins.

Lattice models of peptide aggregation: Evaluation of conformational search algorithms

We present a series of conformational search calculations on the aggregation of short peptide fragments that form fibrils similar to those seen in many protein mis-folding diseases. The proteins were represented by a face-centered cubic lattice model with the conformational energies calculated using the Miyazawa-Jernigan potential. The searches were performed using algorithms based on the Metropolis Monte Carlo method, including simulated annealing and replica exchange. We also present the results of searches using the tabu search method, an algorithm that has been used for many optimization problems, but has rarely been used in protein conformational searches. The replica exchange algorithm consistently found more stable structures then the other algorithms, and was particularly effective for the octamers and larger systems.

Kinetics of fibril formation by polyalanine peptides

Ordered beta-sheet complexes, termed amyloid fibrils, are the underlying structural components of the intra- and extracellular fibrillar protein deposits that are associated with a variety of human diseases, including Alzheimer's, Parkinson's, and the prion diseases. In this work, we investigated the kinetics of fibril formation using our newly developed off-lattice intermediate resolution model, PRIME. The model is simple enough to allow the treatment of large multichain systems while maintaining a fairly realistic description of protein dynamics without built-in bias toward any conformation when used in conjunction with constant temperature discontinuous molecular dynamics, a fast alternative to conventional molecular dynamics. Simulations were performed on systems containing 48-96 model Ac-KA14K-NH2 peptides. We found that fibril formation for polyalanines incorporate features that are characteristic of three models, the templated assembly, nucleated polymerization, and nucleated conformational conversion models, but that none of them gave a completely satisfactory description of the simulation kinetics. Fibril formation was nucleation-dependent, occurring after a lag time that decreased with increasing peptide concentration and increased with increasing temperature. Fibril formation appeared to be a conformational conversion process in which small amorphous aggregates --> beta-sheets --> ordered nucleus --> subsequent rapid growth of a small stable fibril or protofilament. Fibril growth in our simulations involved both beta-sheet elongation, in which the fibril grew by adding individual peptides to the end of each beta-sheet, and lateral addition, in which the fibril grew by adding already formed beta-sheets to its side. The initial rate of fibril formation increased with increasing concentration and decreased with increasing temperature.

Molecular dynamics simulations of spontaneous fibril formation by random-coil peptides

Assembly of normally soluble proteins into amyloid fibrils is a cause or associated symptom of numerous human disorders, including Alzheimer's and the prion diseases. We report molecular-level simulation of spontaneous fibril formation. Systems containing 12-96 model polyalanine peptides form fibrils at temperatures greater than a critical temperature that decreases with peptide concentration and exceeds the peptide's folding temperature, consistent with experimental findings. Formation of small amorphous aggregates precedes ordered nucleus formation and subsequent rapid fibril growth through addition of beta-sheets laterally and monomeric peptides at fibril ends. The fibril's structure is similar to that observed experimentally.

FTIR reveals structural differences between native beta-sheet proteins and amyloid fibrils

The presence of beta-sheets in the core of amyloid fibrils raised questions as to whether or not beta-sheet-containing proteins, such as transthyretin, are predisposed to form such fibrils. However, we show here that the molecular structure of amyloid fibrils differs more generally from the beta-sheets in native proteins. This difference is evident from the amide I region of the infrared spectrum and relates to the distribution of the phi/psi dihedral angles within the Ramachandran plot, the average number of strands per sheet, and possibly, the beta-sheet twist. These data imply that amyloid fibril formation from native beta-sheet proteins can involve a substantial structural reorganization.

Conformational constraints for amyloid fibrillation: the importance of being unfolded

Recent reports give strong support to the idea that amyloid fibril formation and the subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. In this review, recent findings are surveyed to illustrate that protein fibrillogenesis requires a partially folded conformation. This amyloidogenic conformation is relatively unfolded, and shares many structural properties with the pre-molten globule state, a partially folded intermediate frequently observed in the early stages of protein folding and under some equilibrium conditions. The inherent flexibility of such an intermediate is essential in allowing the conformational rearrangements necessary to form the core cross-beta structure of the amyloid fibril.

Small HDL form via apo A-I a complex with atrial natriuretic peptide

The goal of this study was to test the ability of small high density lipoproteins (small HDL) to bind human alpha-atrial natriuretic peptide (alpha-hANP), an amyloidogenic peptide whose involvement in cardiac pathologies is gaining increasing clinical evidence. After incubation of HDL with labeled ANP, the peptide associated to lipoprotein was detectable only in small HDL containing preparations. HDL-associated alpha-[(125)I]hANP was subjected to chromatography, electrophoresis, and autoradiography. The autoradiograph showed two radioactive bands, whose molecular weight was consistent with the chromatographic pattern. Immunoblotting showed the presence of apo A-I in both autoradiographic bands. The proteins of the main band were electroeluted, incubated with labeled ANP, and subjected to two-dimensional electrophoresis followed by autoradiography. The mass spectrometry and molecular weight analyses of the radioactive spot demonstrated the presence of an apo A-I dimer. This finding provided a novel solid evidence that small HDL via apo A-I dimer are involved in the ANP sequestration and thus may play a role in preventing amyloid fibril formation.

A general model for amyloid fibril assembly based on morphological studies using atomic force microscopy

Based on atomic force microscopy analysis of the morphology of fibrillar species formed during fibrillation of alpha-synuclein, insulin, and the B1 domain of protein G, a previously described model for the assembly of amyloid fibrils of immunoglobulin light-chain variable domains is proposed as a general model for the assembly of protein fibrils. For all of the proteins studied, we observed two or three fibrillar species that vary in diameter. The smallest, protofilaments, have a uniform height, whereas the larger species, protofibrils and fibrils, have morphologies that are indicative of multiple protofilaments intertwining. In all cases, protofilaments intertwine to form protofibrils, and protofibrils intertwine to form fibrils. We propose that the hierarchical assembly model describes a general mechanism of assembly for all amyloid fibrils.

Amyloid-forming peptides selected proteolytically from phage display library

We demonstrated that amyloid-forming peptides could be selected from phage-displayed library via proteolysis-based selection protocol. The library of 28-residue peptides based on a sequence of the second zinc finger domain of Zif268, and computationally designed betabetaalpha peptide, FSD-1, was presented monovalently on the surface of M13 phage. The library coupled the infectivity of phage particles to proteolytic stability of a peptide introduced into the coat protein III linker. It was designed to include variants with a strong potential to fold into betabetaalpha motif of zinc finger domains, as expected from secondary structure propensities, but with no structure stabilization via zinc ion coordination. As our primary goal was to find novel monomeric betabetaalpha peptides, the library was selected for stable domains with the assumption that folded proteins are resistant to proteolysis. After less than four rounds of proteolytic selection with trypsin, chymotrypsin, or proteinase K, we obtained a number of proteolysis-resistant phage clones containing several potential sites for proteolytic attack with the proteinases. Eight peptides showing the highest proteolysis resistance were expressed and purified in a phage-free form. When characterized, the peptides possessed proteolytic resistance largely exceeding that of the second zinc finger domain of Zif268 and FSD-1. Six of the characterized peptides formed fibrils when solubilized at high concentrations. Three of them assembled into amyloids as determined through CD measurements, Congo red and thioflavin T binding, and transmission electron microscopy.

Stimulation and inhibition of fibril formation by a peptide in the presence of different concentrations of SDS

Sodium dodecyl sulphate (SDS), a detergent that mimics some characteristics of biological membranes, has been found to affect significantly fibril formation by a peptide from human complement receptor 1. In aqueous solution the peptide is unfolded but slowly aggregates to form fibrils. In sub-micellar concentrations of SDS the peptide is initially alpha-helical but converts rapidly to a beta-sheet structure and large quantities of fibrils form. In SDS above the critical micellar concentration the peptide adopts a stable alpha-helical structure and no fibrils are observed. These findings demonstrate the sensitivity of fibril formation to solution conditions and suggest a possible role for membrane components in amyloid fibril formation in living systems.

Amyloid beta-protein oligomerization: prenucleation interactions revealed by photo-induced cross-linking of unmodified proteins

Assembly of the amyloid beta-protein (Abeta) into neurotoxic oligomers and fibrils is a seminal event in Alzheimer's disease. Understanding the earliest phases of Abeta assembly, including prenucleation and nucleation, is essential for the development of rational therapeutic strategies. We have applied a powerful new method, photoinduced cross-linking of unmodified proteins (PICUP), to the study of Abeta oligomerization. Significant advantages of this method include an extremely short reaction time, enabling the identification and quantification of short lived metastable assemblies, and the fact that no pre facto structural modification of the native peptide is required. Using PICUP, the distribution of Abeta oligomers existing prior to assembly was defined. A rapid equilibrium was observed involving monomer, dimer, trimer, and tetramer. A similar distribution was seen in studies of an unrelated amyloidogenic peptide, whereas nonamyloidogenic peptides yielded distributions indicative of a lack of monomer preassociation. These results suggest that simple nucleation-dependent polymerization models are insufficient to describe the dynamic equilibria associated with prenucleation phases of Abeta assembly.

Conformational characterization of oligomeric intermediates and aggregates in beta-lactoglobulin heat aggregation

In one of the first studies of isolated intermediates in protein aggregation, we have used circular dichroism and fluorescence spectroscopy to characterize metastable oligomers that are formed in the early steps of beta-lactoglobulin heat aggregation. The intermediates show typical molten globule characteristics (secondary structure content similar to the native and less tight packing of the side chains), in agreement with the belief that partly folded states play a key role in protein aggregation. The far-UV CD signal bears strong resemblance to that of a known folding intermediate. Cryo-transmission electron microscopy of the aggregates reveals spherical particles with a diameter of about 50 nm and an internal threadlike structure. Isolated oligomers as well as larger aggregates bind the dye thioflavin T, usually a signature of the amyloid superstructures found in many protein aggregates. This result suggests that the structural motif recognized by thioflavin T can be formed in small oligomers.

Amyloid fibril formation by a helical cytochrome

The substitution of alanines for the two cysteines which form thioether linkages to the haem group in cytochrome c(552) from Hydogenobacter thermophilus destabilises the native protein fold. The holo form of this variant slowly converts into a partially folded apo state that over prolonged periods of time aggregates into fibrillar structures. Characterisation of these structures by electron microscopy and thioflavin-T binding assays shows that they are amyloid fibrils. The data demonstrate that when the native state of this cytochrome is destabilised by loss of haem, even this highly alpha-helical protein can form beta-sheet structures of the type most commonly associated with protein deposition diseases.