The predominant roles of the sequence periodicity in the self‐assembly of collagen‐mimetic mini‐fibrils

Collagen fibrils represent a unique case of protein folding and self‐association. We have recently successfully developed triple‐helical peptides that can further self‐assemble into collagen‐mimetic mini‐fibrils. The 35 nm axially repeating structure of the mini‐fibrils, which is designated the d‐period, is highly reminiscent of the well‐known 67 nm D‐period of native collagens when examined using TEM and atomic force spectroscopy. We postulate that it is the pseudo‐identical repeating sequence units in the primary structure of the designed peptides that give rise to the d‐period of the quaternary structure of the mini‐fibrils. In this work, we characterize the self‐assembly of two additional designed peptides: peptide Col877 and peptide Col108rr. The triple‐helix domain of Col877 consists of three pseudo‐identical amino acid sequence units arranged in tandem, whereas that of Col108rr consists of three sequence units identical in amino acid composition but different in sequence. Both peptides form stable collagen triple helices, but only triple helices Col877 self‐associate laterally under fibril forming conditions to form mini‐fibrils having the predicted d‐period. The Co108rr triple helices, however, only form nonspecific aggregates having no identifiable structural features. These results further accentuate the critical involvement of the repeating sequence units in the self‐assembly of collagen mini‐fibrils; the actual amino acid sequence of each unit has only secondary effects. Collagen is essential for tissue development and function. This novel approach to creating collagen‐mimetic fibrils can potentially impact fundamental research and have a wide range of biomedical and industrial applications.     

To achieve self‐assembled collagen mimetic fibrils using designed peptides

It has proven challenging to obtain collagen‐mimetic fibrils by protein design. We recently reported the self‐assembly of a mini‐fibril showing a 35 nm, D‐period like, axially repeating structure using the designed triple helix Col108. Peptide Col108 was made by bacterial expression using a synthetic gene; its triple helix domain consists of three pseudo‐identical units of amino acid sequence arranged in tandem. It was postulated that the 35 nm d‐period of Col108 mini‐fibrils originates from the periodicity of the Col108 primary structure. A mutual staggering of one sequence unit of the associating Col108 triple helices can maximize the inter‐helical interactions and produce the observed 35 nm d‐period. Based on this unit‐staggered model, a triple helix consisting of only two sequence units is expected to have the potential to form the same d‐periodic mini‐fibrils. Indeed, when such a peptide, peptide 2U108, was made it was found to self‐assemble into mini‐fibrils having the same d‐period of 35 nm. In contrast, no d‐periodic mini‐fibrils were observed for peptide 1U108, which does not have long‐range repeating sequences in its primary structure. The findings of the periodic mini‐fibrils of Col108 and 2U108 suggest a way forward to create collagen‐mimetic fibrils for biomedical and industrial applications.     

How changes in the sequence of the peptide CLPFFD-NH2 can modify the conjugation and stability of gold nanoparticles and their affinity for beta-amyloid fibrils

In a previous work, we studied the interaction of beta-amyloid fibrils (Abeta) with gold nanoparticles (AuNP) conjugated with the peptide CLPFFD-NH2. Here, we studied the effect of changing the residue sequence of the peptide CLPFFD-NH2 on the efficiency of conjugation to AuNP, the stability of the conjugates, and the affinity of the conjugates to the Abeta fibrils. We conjugated the AuNP with CLPFFD-NH 2 isomeric peptides (CDLPFF-NH2 and CLPDFF-NH2) and characterized the resulting conjugates with different techniques including UV-Vis, TEM, EELS, XPS, analysis of amino acids, agarose gel electrophoresis, and CD. In addition, we determined the proportion of AuNP bonded to the Abeta fibrils by ICP-MS. AuNP-CLPFFD-NH2 was the most stable of the conjugates and presented more affinity for Abeta fibrils with respect to the other conjugates and bare AuNP. These findings help to better understand the way peptide sequences affect conjugation and stability of AuNP and their interaction with Abeta fibrils. The peptide sequence, the steric effects, and the charge and disposition of hydrophilic and hydrophobic residues are crucial parameters when considering the design of AuNP peptide conjugates for biomedical applications.     

Free energy landscapes for amyloidogenic tetrapeptides dimerization

The oligomerization of four peptide sequences, KFFE, KVVE, KLLE, and KAAE is studied using replica-exchange molecular dynamics simulations with an atomically detailed peptide model. Previous experimental studies reported that of these four peptides, only those containing phenylalanine and valine residues form fibrils. We show that the fibrillogenic propensities of these peptides can be rationalized in terms of the equilibrium thermodynamics of their early oligomers. Thermodynamic stability of dimers, as measured by the temperature of monomer association, is seen to be higher for those peptides that are able to form fibrils. Although the relative high and low stabilities of the KFFE and KAAE dimers arise from their respective high and low interpeptide interaction energies, the higher stability of the KVVE dimer over the KLLE system results from the smaller loss of configurational entropy accompanying the dimerization of KVVE. Free energy landscapes for dimerization are found to be strongly sequence-dependent, with a high free energy barrier separating the monomeric and dimeric states for KVVE, KLLE, and KAAE sequences. In contrast, the most fibrillogenic peptide, KFFE, displayed downhill assembly, indicating enhanced kinetic accessibility of its dimeric states. The dimeric phase for all peptide sequences is found to be heterogeneous, containing both antiparallel beta-sheet structures that can grow into full fibrils as well as disordered dimers acting as on- or off-pathway intermediates for fibrillation.     

Transformation of the amyloidogenic peptide amylin(20-29) into its corresponding peptoid and retropeptoid: access to both an amyloid inhibitor and template for self-assembled supramolecular tapes

The highly amyloidogenic peptide sequence of amylin(20-29) was transformed into its corresponding peptoid and retropeptoid sequences to design a novel class of beta-sheet breaker peptides as amyloid inhibitors. This report describes the synthesis of the chiral peptoid building block of L-isoleucine, the solid phase synthesis of the peptoid and retropeptoid sequences of amylin(20-29), and the structural analysis of these amylin derivatives in solution by infrared spectroscopy, circular dichroism, and transmission electron microscopy. It was found that the peptoid sequence did not form amyloid fibrils or any other secondary structures and was able to inhibit amyloid formation of native amylin(20-29). Although the retropeptoid did not form amyloid fibrils it had only modest amyloid inhibitor properties since supramolecular tapes were formed.     

Interruptions in the collagen repeating tripeptide pattern can promote supramolecular association

The standard collagen triple‐helix requires a perfect (Gly‐Xaa‐Yaa)_n sequence, yet all nonfibrillar collagens contain interruptions in this tripeptide repeating pattern. Defining the structural consequences of disruptions in the sequence pattern may shed light on the biological role of sequence interruptions, which have been suggested to play a role in molecular flexibility, collagen degradation, and ligand binding. Previous studies on model peptides with 1‐ and 4‐residue interruptions showed a localized perturbation within the triple‐helix, and this work is extended to introduce natural collagen interruptions up to nine residue in length within a fixed (Gly‐Pro‐Hyp)_n peptide context. All peptides in this set show decreases in triple‐helix content and stability, with greater conformational perturbations for the interruptions longer than five residue. The most stable and least perturbed structure is seen for the 5‐residue interruption peptide, whose sequence corresponds to a Gly to Ala missense mutation, such as those leading to collagen genetic diseases. The triple‐helix peptides containing 8‐ and 9‐residue interruptions exhibit a strong propensity for self‐association to fibrous structures. In addition, a small peptide modeling only the 9‐residue sequence within the interruption aggregates to form amyloid‐like fibrils with antiparallel β‐sheet structure. The 8‐ and 9‐residue interruption sequences studied here are predicted to have significant cross‐β aggregation potential, and a similar propensity is reported for ～10% of other naturally occurring interruptions. The presence of amyloidogenic sequences within or between triple‐helix domains may play a role in molecular association to normal tissue structures and could participate in observed interactions between collagen and amyloid.     

Formation of amyloid fibrils by peptides derived from the bacterial cold shock protein CspB.

Three peptides covering the sequence regions corresponding to the first two (CspB-1), the first three (CspB-2), and the last two (CspB-3) beta-strands of CspB, the major cold shock protein of Bacillus subtilis, have been synthesized and analyzed for their conformations in solution and for their precipitation behavior. The peptides are nearly insoluble in water, but highly soluble in aqueous solutions containing 50% acetonitrile (pH 4.0). Upon shifts of the solvent condition toward lower or higher acetonitrile concentrations, the peptides all form fibrils resembling those observed in amyloid associated diseases. These fibrils have been identified and characterized by electron microscopy, binding of the dye congo red, and X-ray fiber diffraction. Characterization of the peptides in solution by circular dichroism and NMR spectroscopy shows that the formation of these fibrils does not require specific preformed secondary structure in the solution state species. While the majority of the soluble fraction of each peptide is monomeric and unstructured, different types of structures including alpha-helical, beta-sheet, and random coil conformations are observed under conditions that eventually lead to fibril formation. We conclude that the absence of tertiary contacts under solution conditions where binding interactions between peptide units are still favorable is a crucial requirement for amyloid formation. Thus, fragmentation of a sequence, like partial chemical denaturation or mutation, can enhance the capacity of specific protein sequences to form such fibrils.     

A designed system for assessing how sequence affects alpha to beta conformational transitions in proteins.

The role of amino acid sequence in conformational switching observed in prions and proteins associated with amyloid diseases is not well understood. To study alpha to beta conformational transitions, we designed a series of peptides with structural duality; namely, peptides with sequence features of both an alpha-helical leucine zipper and a beta-hairpin. The parent peptide, Template-alpha, was designed to be a canonical leucine-zipper motif and was confirmed as such using circular dichroism spectroscopy and analytical ultracentrifugation. To introduce beta-structure character into the peptide, glutamine residues at sites away from the leucine-zipper dimer interface were replaced by threonine to give Template-alphaT. Unlike the parent peptide, Template-alphaT underwent a heat-inducible switch to beta-structure, which reversibly formed gels containing amyloid-like fibrils. In contrast to certain other natural proteins where destabilization of the native states facilitate transitions to amyloid, destabilization of the leucine-zipper form of Template-alphaT did not promote a transformation. Cross-linking the termini of the peptides compatible with the alternative beta-hairpin design, however, did promote the change. Furthermore, despite screening various conditions, only the internally cross-linked form of the parent, Template-alpha, peptide formed amyloid-like fibrils. These findings demonstrate that, in addition to general properties of the polypeptide backbone, specific residue placements that favor beta-structure promote amyloid formation.     

Charge effects on the fibril-forming peptide KTVIIE: a two-dimensional replica exchange simulation study

The assembly of peptides into ordered nanostructures is increasingly recognized as both a bioengineering tool for generating new materials and a critical aspect of aggregation processes that underlie neurological diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. There is a major problem in understanding how extremely subtle sequence changes can lead to profound and often unexpected differences in self-assembly behavior. To better delineate the complex interplay of different microscopic driving forces in such cases, we develop a methodology to quantify and compare the propensity of different peptide sequences to form small oligomers during early self-assembly stages. This umbrella-sampling replica exchange molecular dynamics method performs a replica exchange molecular dynamics simulation along peptide association reaction coordinates using umbrella restraints. With this method, we study a set of sequence-similar peptides that differ in net charge: K(+)TVIIE(-), K(+)TVIIE, and (+)K(+)TVIIE. Interestingly, experiments show that only the monovalent peptide, K(+)TVIIE, forms fibrils, whereas the others do not. We examine dimer, trimer, and tetramer formation processes of these peptides, and compute high-accuracy potential of mean force association curves. The potential of mean forces recapitulate a higher stability and equilibrium constant of the fibril-forming peptide, similar to experiment, but reveal that entropic contributions to association free energies can play a surprisingly significant role. The simulations also show behavior reminiscent of experimental aggregate polymorphism, revealed in multiple stable conformational states and association pathways. Our results suggest that sequence changes can have significant effects on self-assembly through not only direct peptide-peptide interactions but conformational entropies and degeneracies as well.     

A molecular model of Alzheimer amyloid beta-peptide fibril formation

Polymerization of the amyloid beta (Abeta) peptide into protease-resistant fibrils is a significant step in the pathogenesis of Alzheimer's disease. It has not been possible to obtain detailed structural information about this process with conventional techniques because the peptide has limited solubility and does not form crystals. In this work, we present experimental results leading to a molecular level model for fibril formation. Systematically selected Abeta-fragments containing the Abeta16-20 sequence, previously shown essential for Abeta-Abeta binding, were incubated in a physiological buffer. Electron microscopy revealed that the shortest fibril-forming sequence was Abeta14-23. Substitutions in this decapeptide impaired fibril formation and deletion of the decapeptide from Abeta1-42 inhibited fibril formation completely. All studied peptides that formed fibrils also formed stable dimers and/or tetramers. Molecular modeling of Abeta14-23 oligomers in an antiparallel beta-sheet conformation displayed favorable hydrophobic interactions stabilized by salt bridges between all charged residues. We propose that this decapeptide sequence forms the core of Abeta-fibrils, with the hydrophobic C terminus folding over this core. The identification of this fundamental sequence and the implied molecular model could facilitate the design of potential inhibitors of amyloidogenesis.     

Amyloid fibril formation in vitro from halophilic metal binding protein: Its high solubility and reversibility minimized formation of amorphous protein aggregations

Halophilic proteins are characterized by high net negative charges and relatively small fraction of hydrophobic amino acids, rendering them aggregation resistant. These properties are also shared by histidine‐rich metal binding protein (HP) from moderate halophile, Chromohalobacter salexigens, used in this study. Here, we examined how halophilic proteins form amyloid fibrils in vitro. His‐tagged HP, incubated at pH 2.0 and 58°C, readily formed amyloid fibrils, as observed by thioflavin fluorescence, CD spectra, and transmission or atomic force microscopies. Under these low‐pH harsh conditions, however, His‐HP was promptly hydrolyzed to smaller peptides most likely responsible for rapid formation of amyloid fibril. Three major acid‐hydrolyzed peptides were isolated from fibrils and turned out to readily form fibrils. The synthetic peptides predicted to form fibrils in these peptide sequences by Waltz software also formed fibrils. Amyloid fibril was also readily formed from full‐length His‐HP when incubated with 10–20% 2,2,2‐trifluoroethanol at pH 7.8 and 25°C without peptide bond cleavage.     

Charge attraction and beta propensity are necessary for amyloid fibril formation from tetrapeptides

Amyloid fibrils in which specific proteins have polymerized into a cross-beta-sheet structure are found in about 20 diseases. In contrast to the close structural similarity of fibrils formed in different amyloid diseases, the structures of the corresponding native proteins differ widely. We show here that peptides as short as 4 residues with the sequences KFFE or KVVE can form amyloid fibrils that are practically identical to fibrils formed in association with disease, as judged by electron microscopy and Congo red staining. In contrast, KLLE or KAAE do not form fibrils. The fibril-forming KFFE and KVVE show partial beta-strand conformation in solution, whereas the non-fibril-forming KLLE and KAAE show random structure only, suggesting that inherent propensity for beta-strand conformation promotes fibril formation. The peptides KFFK or EFFE do not form fibrils on their own but do so in an equimolar mixture. Thus, intermolecular electrostatic interactions, either between charged dipolar peptides or between complementary charges of co-fibrillating peptides favor fibril formation.     

Effect of introducing a short amyloidogenic sequence from the Aβ peptide at the N‐terminus of 18‐residue amphipathic helical peptides

Fibril formation is the hallmark of pathogenesis in Alzheimer's disease and other amyloid disorders caused by conformational alterations leading to the aggregation of soluble monomers. Aβ40 self‐associates to form amyloid fibrils. Its central seven‐residue segment KLVFFAE (Aβ16–22), which is thought to be crucial for fibril formation of the full‐length peptide, forms fibrils even in isolation. Context‐dependent induction of amyloid formation by such sequences in peptides, which otherwise do not have that propensity, is of considerable interest. We have examined the effect of introducing the Aβ16–22 sequence at the N‐terminus of two amphipathic helical 18‐residue peptides Ac‐WYSEMKRNVQRLERAIEE‐am and Ac‐KQLIRFLKRLDRNLWGLA‐am, which have high average hydrophobic moment <μH> values but have net charges of 0 and +4, respectively, at neutral pH. Upon incubation in aqueous buffer, fibril‐like aggregates were discernible by transmission electron microscopy for the peptide with only 0 net charge, which also displayed ThT binding and β‐structure. Although both the sequences have been derived from amphipathic helical segments in globular proteins and possess high average hydrophobic moments, the +4 charge peptide lacks the ability to form fibrils, while the peptide with 0 charge has the tendency to form fibrillar structures. Variation in the net charge and the presence of several glutamic acids in the sequence of the peptide with net charge 0 appear to favor the formation of fibrils when the Aβ16–22 sequence is attached at the N‐terminus. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Identification of novel short peptides derived from the alpha 4, alpha 5, and alpha 6 fibrils of type IV collagen with anti-angiogenic properties

Angiogenesis, or neovascularization, is tightly controlled by positive and negative regulators, many of which reside in the extracellular matrix. We have now identified eight novel 19- to 20-residue peptides derived from the alpha4, alpha5, and alpha6 fibrils of type IV collagen, which we have designated tetrastatins, pentastatins, and hexastatins, respectively. We have shown that these endogenous peptides suppress the proliferation and migration of HUVECs in vitro. By performing clustering analyses of the sequences using sequence similarity criteria and of the experimental results using a hierarchical algorithm, we report that the clusters identified by the experimental results coincide with the sequence-based clusters, indicating a tight relationship between peptide sequence and anti-angiogenic potency. These peptides may have potential as anti-angiogenic therapeutic agents.     

Heterogeneous triangular structures of human islet amyloid polypeptide (amylin) with internal hydrophobic cavity and external wrapping morphology reveal the polymorphic nature of amyloid fibrils

The misfolding and self-assembly of human islet amyloid polypeptide (hIAPP or amylin) into amyloid fibrils is pathologically linked to type II diabetes. The polymorphic nature of both hIAPP oligomers and fibrils has been implicated for the molecular origin of hIAPP toxicity to islet β-cells, but little is known about the polymorphic structure and dynamics of these hIAPP oligomers/fibrils at the atomic level. Here, we model the polymorphism of full length hIAPP(1-37) oligomers based on experimental data from solid-state NMR, mass per length, and electron microscopy using all-atom molecular dynamics simulation with explicit solvent. As an alternative to steric zipper structures mostly presented in the 2-fold symmetrical fibrils, the most striking structural feature of our proposed hIAPP oligomers is the presence of 3-fold symmetry along the fibril growth axis, in which three β-sheet-layers wind around a hydrophobic core with different periodicities. These 3-fold triangular hIAPP structures dramatically differ in the details of the β-layer assembly and core-forming sequence at the cross section, but all display a high structural stability with favorable layer-to-layer interactions. The 3-fold hIAPP structures can also serve as templates to present triple-stranded helical fibrils via peptide elongation, with different widths from 8.7 to 9.9 nm, twists from 2.8° to 11.8°, and pitches from 14.5 to 61.1 nm, in reasonable agreement with available biophysical data. Because similar 3-fold Aβ oligomers are also observed by both NMR experiments and our previous simulations, the 3-fold structure could be a general conformation to a broad range of amyloid oligomers and fibrils. Most importantly, unlike the conventional stacking sandwich model, the proposed wrapping-cord structures can readily accommodate more than three β-layers via a two dimension conformation search by rotating and translating the β-layers to adopt different favorable packings, which can greatly enrich the polymorphism of amyloid oligomers and fibrils.     

The turn sequence directs beta-strand alignment in designed beta-hairpins

A previous NMR investigation of model decapeptides with identical beta-strand sequences and different turn sequences demonstrated that, in these peptide systems, the turn residues played a more predominant role in defining the type of beta-hairpin adopted than cross-strand side-chain interactions. This result needed to be tested in longer beta-hairpin forming peptides, containing more potentially stabilizing cross-strand hydrogen bonds and side-chain interactions that might counterbalance the influence of the turn sequence. In that direction, we report here on the design and 1H NMR conformational study of three beta-hairpin forming pentadecapeptides. The design consists of adding two and three residues at the N- and C-termini, respectively, of the previously studied decapeptides. One of the designed pentadecapeptides includes a potentially stabilizing R-E salt bridge to investigate the influence of this interaction on beta-hairpin stability. We suggest that this peptide self-associates by forming intermolecular salt bridges. The other two pentadecapeptides behave as monomers. A conformational analysis of their 1H NMR spectra reveals that they adopt different types of beta-hairpin structure despite having identical strand sequences. Hence, the beta-turn sequence drives beta-hairpin formation in the investigated pentadecapeptides that adopt beta-hairpins that are longer than the average protein beta-hairpins. These results reinforce our previous suggestion concerning the key role played by the turn sequence in directing the kind of beta-hairpin formed by designed peptides.     

Identification of a penta- and hexapeptide of islet amyloid polypeptide (IAPP) with amyloidogenic and cytotoxic properties

Pancreatic amyloid is found in more than 95 % of type II diabetes patients. Pancreatic amyloid is formed by the aggregation of islet amyloid polypeptide (hIAPP or amylin), which is a 37-residue peptide. Because pancreatic amyloid is cytotoxic, it is believed that its formation is directly associated with the development of the disease. We recently showed that hIAPP amyloid formation follows the nucleation-dependent polymerization mechanism and proceeds via a conformational transition of soluble hIAPP into aggregated beta-sheets. Here, we report that the penta- and hexapeptide sequences, hIAPP(23-27) (FGAIL) and hIAPP(22-27) (NFGAIL) of hIAPP are sufficient for the formation of beta-sheet-containing amyloid fibrils. Although these two peptides differ by only one amino acid residue, they aggregate into completely different fibrillar assemblies. hIAPP(23-27) (FGAIL) fibrils self-assemble laterally into unusually broad ribbons, whereas hIAPP(22-27) (NFGAIL) fibrils coil around each other in a typical amyloid fibril morphology. hIAPP(20-27) (SNNFGAIL) also aggregates into beta-sheet-containing fibrils, whereas no amyloidogenicity is found for hIAPP(24-27) (GAIL), indicating that hIAPP(23-27) (FGAIL) is the shortest fibrillogenic sequence of hIAPP. Insoluble amyloid formation by the partial hIAPP sequences followed kinetics that were consistent with a nucleation-dependent polymerization mechanism. hIAPP(22-27) (NFGAIL), hIAPP(20-27) (SNNFGAIL), and also the known fibrillogenic sequence, hIAPP(20-29) (SNNFGAILSS) exhibited significantly lower kinetic and thermodynamic solubilities than the pentapeptide hIAPP(23-27) (FGAIL). Fibrils formed by all short peptide sequences and also by hIAPP(20-29) were cytotoxic towards the pancreatic cell line RIN5fm, whereas no cytotoxicity was observed for the soluble form of the peptides, a notion that is consistent with hIAPP cytotoxicity. Our results suggest that a penta- and hexapeptide sequence of an appropriate amino acid composition can be sufficient for beta-sheet and amyloid fibril formation and cytotoxicity and may assist in the rational design of inhibitors of pancreatic amyloid formation or other amyloidosis-related diseases.     

Amyloid fibril formation requires a chemically discriminating nucleation event: studies of an amyloidogenic sequence from the bacterial protein OsmB.

The sequence of the Escherichia coli OsmB protein was found to resemble that of the C-terminal region of the beta amyloid protein of Alzheimer's disease, which seems to be the major determinant of its unusual structural and solubility properties. A peptide corresponding to residues 28-44 of the OsmB protein was synthesized, and its conformational properties and aggregation behavior were analyzed. The peptide OsmB(28-44) was shown to form amyloid fibrils, as did two sequence analogs designed to test the sequence specificity of fibril formation. These fibrils bound Congo red, and two of the peptides showed birefringence. The peptide fibrils were analyzed by electron microscopy and Fourier transform infrared spectroscopy. Subtle differences were observed which were not interpretable at the molecular level. The rate of fibril formation by each peptide was followed by monitoring the turbidity of supersaturated aqueous solutions. The kinetics of aggregation were characterized by a delay period during which the solution remained clear, followed by a nucleation event which led to a growth phase, during which the solution became viscous and turbid due to the presence of insoluble fibrils. The observation of a kinetic barrier to aggregation is typical of a crystallization event. The delay period could be eliminated by seeding the supersaturated solution with previously formed fibrils. Each peptide could be nucleated by fibrils formed from that same peptide, but not by fibrils from closely related sequences, suggesting that fibril growth requires specific hydrophobic interactions. It appears likely that this repeated sequence motif, which comprises most of the OsmB protein sequence, dictates the structure and possibly the function of that protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Amyloid-forming peptides from beta2-microglobulin-Insights into the mechanism of fibril formation in vitro

Beta(2)-Microglobulin (beta(2)m) is one of over 20 proteins known to be involved in human amyloid disease. Peptides equivalent to each of the seven beta-strands of the native protein, together with an eighth peptide (corresponding to the most stable region in the amyloid precursor conformation formed at pH 3.6, that includes residues in the native strand E plus the eight succeeding residues (named peptide E')), were synthesised and their ability to form fibrils investigated. Surprisingly, only two sequences, both of which encompass the region that forms strand E in native beta(2)m, are capable of forming amyloid-like fibrils in vitro. These peptides correspond to residues 59-71 (peptide E) and 59-79 (peptide E') of intact beta(2)m. The peptides form fibrils under the acidic conditions shown previously to promote amyloid formation from the intact protein (pH <5 at low and high ionic strength), and also associate to form fibrils at neutral pH. Fibrils formed from these two peptides enhance fibrillogenesis of the intact protein. No correlation was found between secondary structure propensity, peptide length, pI or hydrophobicity and the ability of the peptides to associate into amyloid-like fibrils. However, the presence of a relatively high content of aromatic side-chains correlates with the ability of the peptides to form amyloid fibrils. On the basis of these results we propose that residues 59-71 may be important in the self-association of partially folded beta(2)m into amyloid fibrils and discuss the relevance of these results for the assembly mechanism of the intact protein in vitro.     

Perturbation of the stability of amyloid fibrils through alteration of electrostatic interactions

The self-assembly of proteins and peptides into polymeric amyloid fibrils is a process that has important implications ranging from the understanding of protein misfolding disorders to the discovery of novel nanobiomaterials. In this study, we probe the stability of fibrils prepared at pH 2.0 and composed of the protein insulin by manipulating electrostatic interactions within the fibril architecture. We demonstrate that strong electrostatic repulsion is sufficient to disrupt the hydrogen-bonded, cross-β network that links insulin molecules and ultimately results in fibril dissociation. The extent of this dissociation correlates well with predictions for colloidal models considering the net global charge of the polypeptide chain, although the kinetics of the process is regulated by the charge state of a single amino acid. We found the fibrils to be maximally stable under their formation conditions. Partial disruption of the cross-β network under conditions where the fibrils remain intact leads to a reduction in their stability. Together, these results support the contention that a major determinant of amyloid stability stems from the interactions in the structured core, and show how the control of electrostatic interactions can be used to characterize the factors that modulate fibril stability.