Nucleation-dependent conformational conversion of the Y145Stop variant of human prion protein: structural clues for prion propagation

Dynamics of a truncated prion protein, PrP(113-231), from (15)N NMR relaxation: order parameters calculated and slow conformational fluctuations localized to a distinct region

Prion diseases are associated with the misfolding of the prion protein (PrP(C)) from a largely alpha-helical isoform to a beta-sheet rich oligomer (PrP(Sc)). Flexibility of the polypeptide could contribute to the ability of PrP(C) to undergo the conformational rearrangement during PrP(C)-PrP(Sc) interactions, which then leads to the misfolded isoform. We have therefore examined the molecular motions of mouse PrP(C), residues 113-231, in solution, using (15)N NMR relaxation measurements. A truncated fragment has been used to eliminate the effect of the 90-residue unstructured tail of PrP(C) so the dynamics of the structured domain can be studied in isolation. (15)N longitudinal (T(1)) and transverse relaxation (T(2)) times as well as the proton-nitrogen nuclear Overhauser effects have been used to calculate the spectral density at three frequencies, 0, omega(N,) and 0.87omega(H). Spectral densities at each residue indicate various time-scale motions of the main-chain. Even within the structured domain of PrP(C), a diverse range of motions are observed. We find that removal of the tail increases T(2) relaxation times significantly indicating that the tail is responsible for shortening of T(2) times in full-length PrP(C). The truncated fragment of PrP has facilitated the determination of meaningful order parameters (S(2)) from the relaxation data and shows for the first time that all three helices in PrP(C) have similar rigidity. Slow conformational fluctuations of mouse PrP(C) are localized to a distinct region that involves residues 171 and 172. Interestingly, residues 170-175 have been identified as a segment within PrP that will form a steric zipper, believed to be the fundamental amyloid unit. The flexibility within these residues could facilitate the PrP(C)-PrP(Sc) recognition process during fibril elongation.

The consequences of pathogenic mutations to the human prion protein

Prion diseases, in which the conformational transition of the native prion protein (PrP) to a misfolded form causes aggregation and subsequent neurodegeneration, have fascinated the scientific community as this transmissible disease appears to be purely protein-based. Disease can arise due to genetic factors only. At least 30 single point mutations have been indicated to cause disease in humans. Somehow, these mutations must influence the stability, processing and/or cellular interactions of PrP, such that aggregation can occur and disease develops. In this review, the current evidence for such effects of single point mutations is discussed, indicating that PrP can be affected in many different ways, although questions remain about the mechanism by which mutations cause disease.

Prion protein misfolding and disease

Transmissible spongiform encephalopathies (TSEs or prion diseases) are a rare group of invariably fatal neurodegenerative disorders that affect humans and other mammals. TSEs are protein misfolding diseases that involve the accumulation of an abnormally aggregated form of the normal host prion protein (PrP). They are unique among protein misfolding disorders in that they are transmissible and have different strains of infectious agents that are associated with unique phenotypes in vivo. A wealth of biological and biophysical evidence now suggests that the molecular basis for prion diseases may be encoded by protein conformation. The purpose of this review is to provide an overview of the existing structural information for PrP within the context of what is known about the biology of prion disease.

Amyloidogenesis of natively unfolded proteins

Aggregation and subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. The accumulated data support the model where protein fibrillogenesis proceeds via the formation of a relatively unfolded amyloidogenic conformation, which shares many structural properties with the pre-molten globule state, a partially folded intermediate first found during the equilibrium and kinetic (un)folding studies of several globular proteins and later described as one of the structural forms of natively unfolded proteins. The flexibility of this structural form is essential for the conformational rearrangements driving the formation of the core cross-beta structure of the amyloid fibril. Obviously, molecular mechanisms describing amyloidogenesis of ordered and natively unfolded proteins are different. For ordered protein to fibrillate, its unique and rigid structure has to be destabilized and partially unfolded. On the other hand, fibrillogenesis of a natively unfolded protein involves the formation of partially folded conformation; i.e., partial folding rather than unfolding. In this review recent findings are surveyed to illustrate some unique features of the natively unfolded proteins amyloidogenesis.

Molecular conformation and dynamics of the Y145Stop variant of human prion protein in amyloid fibrils

A C-terminally truncated Y145Stop variant of the human prion protein (huPrP23-144) is associated with a hereditary amyloid disease known as PrP cerebral amyloid angiopathy. Previous studies have shown that recombinant huPrP23-144 can be efficiently converted in vitro to the fibrillar amyloid state, and that residues 138 and 139 play a critical role in the amyloidogenic properties of this protein. Here, we have used magic-angle spinning solid-state NMR spectroscopy to provide high-resolution insight into the protein backbone conformation and dynamics in fibrils formed by (13)C,(15)N-labeled huPrP23-144. Surprisingly, we find that signals from approximately 100 residues (i.e., approximately 80% of the sequence) are not detected above approximately -20 degrees C in conventional solid-state NMR spectra. Sequential resonance assignments revealed that signals, which are observed, arise exclusively from residues in the region 112-141. These resonances are remarkably narrow, exhibiting average (13)C and (15)N linewidths of approximately 0.6 and 1 ppm, respectively. Altogether, the present findings indicate the existence of a compact, highly ordered core of huPrP23-144 amyloid encompassing residues 112-141. Analysis of (13)C secondary chemical shifts identified likely beta-strand segments within this core region, including beta-strand 130-139 containing critical residues 138 and 139. In contrast to this relatively rigid, beta-sheet-rich amyloid core, the remaining residues in huPrP23-144 amyloid fibrils under physiologically relevant conditions are largely unordered, displaying significant conformational dynamics.

Formation of amyloid fibrils by bovine carbonic anhydrase

Amyloids are typically characterized by extensive aggregation of proteins where the participating polypeptides are involved in formation of intermolecular cross beta-sheet structures. Alternate structure attainment and amyloid formation has been hypothesized to be a generic property of a polypeptide, the propensities of which vary widely depending on the polypeptide involved and the physicochemical conditions it encounters. Many proteins that exist in the normal form in-vivo have been shown to form amyloid when incubated in partially denaturing conditions. The protein bovine carbonic anhydrase II (BCA II) when incubated in mildly denaturing conditions showed that the partially unfolded conformers assemble together and form ordered amyloid aggregates. The properties of these aggregates were tested using the traditional Congo-Red (CR) and Thioflavin-T (ThT) assays along with fluorescence microscopy, transmission electron microscopy (TEM), and circular dichroism (CD) spectroscopy. The aggregates were found to possess most of the characteristics ascribed to amyloid fibers. Thus, we report here that the single-domain globular protein, BCA II, is capable of forming amyloid fibrils. The primary sequence of BCA II was also analyzed using recurrence quantification analysis in order to suggest the probable residues responsible for amyloid formation.

Formation and reversible dissociation of coiled coil of peptide to the C-terminus of the HSV B5 protein: a time-resolved spectroscopic analysis

An understanding of the molecular mechanisms of the newly characterized herpes simplex virus (HSV) B5 protein is important to further elucidate the HSV cell entry and infection. The synthetic peptide of B5 (wtB5) was functionalized with the nonlinear optical chromophore cascade yellow and its molecular dynamics was probed at physiological and endosomal pH (pH 7.4 and 5.5, respectively). Steady-state CD spectroscopy was utilized to characterize the peptides at different pH. These spectra showed structural changes in the peptide with time measured over several days. Nonlinear optical measurements were carried out to probe the interactions and local environment of the labeled peptide, and the increase in the two-photon cross section of this system suggests an increase in chromophore-peptide interactions. Time-resolved fluorescence upconversion measurements reflected changes in the hydrophilic and hydrophobic local environments of the labeled peptide-chromophore system. Ultrafast depolarization measurements gave rotational correlation times indicative of a reversible change in the size of the peptide. The time-resolved results provide compelling evidence of a reversible dissociation of the coiled coils of the wtB5 peptide. This process was found to be pH-insensitive. The data from this unique combination of techniques provide an initial step to understanding the molecular dynamics of B5 and a framework for the development of novel imaging methods based on two-photon emission, as well as new therapeutics for HSV.

Quantifying the sequence-dependent species barrier between hamster and mouse prions

Prion diseases are transmissible neurodegenerative disorders. It is widely accepted that prions are the infectious agents responsible for disease transmission, and the sequence homology between the infectious prion and the host prion protein determines its transmission efficiency across species. However, previous studies have often reported different results regarding seeding efficiency, the efficiency of initiating amyloid propagation by adding pre-existing amyloid fibrils as seed. In the present study, we used synthetic peptides as a simple system to determine the sequence-dependent transmission barrier between hamster and mouse. We found that the heterologous seeding efficiency of hamster and mouse prion peptides was 4 times less than that of homologous seeding. Moreover, residue 139 was not the only residue in determining seeding efficiency. When the seed had Ile at this position, the homology at this position between seed and monomer determined the seeding efficiency. When the seed had Met at this position, homology at residues 109 and 112 determined the seeding efficiency.

Prion protein helix1 promotes aggregation but is not converted into beta-sheet

Prion diseases are caused by the aggregation of the native alpha-helical prion protein PrP(C) into its pathological beta-sheet-rich isoform PrP(Sc). In current models of PrP(Sc), helix1 is assumed to be preferentially converted into beta-sheet during aggregation of PrP(C). This was supported by the NMR structure of PrP(C) since, in contrast to the isolated helix1, helix2 and helix3 are connected by a small loop and are additionally stabilized by an interhelical disulfide bond. However, helix1 is extremely hydrophilic and has a high helix propensity. This prompted us to investigate the role of helix1 in prion aggregation using humPrP(23-159) including helix1 (144-156) compared with the C-terminal-truncated isoform humPrP(23-144) corresponding to the pathological human stop mutations Q160Stop and Y145Stop, respectively. Most unexpectedly, humPrP(23-159) aggregated significantly faster compared with the truncated fragment humPrP(23-144), clearly demonstrating that helix1 is involved in the aggregation process. However, helix1 is not resistant to digestion with proteinase K in fibrillar humPrP(23-159), suggesting that helix1 is not converted to beta-sheet. This is confirmed by Fourier transformation infrared spectroscopy since there is almost no difference in beta-sheet content of humPrP(23-159) fibrils compared with humPrP(23-144). In conclusion, we provide strong direct evidence that in contrast to earlier assumptions helix1 is not converted into beta-sheet during aggregation of PrP(C) to PrP(Sc).

Combination of NADPH and copper ions generates proteinase K-resistant aggregates from recombinant prion protein

Recent studies have demonstrated that the octapeptide repeats of the N-terminal region of prion protein may be responsible for de novo generation of infectious prions in the absence of template. Here we demonstrate that PrP-(23-98), an N-terminal portion of PrP, is converted to aggregates upon incubation with NADPH and copper ions. Other pyridine nucleotides possessing a phosphate group on the adenine-linked ribose moiety (the reduced form of nicotinamide adenine dinucleotide 3'-phosphate, nicotinic acid adenine dinucleotide phosphate, and NADP) were also effective in promoting aggregation, but NADH and NAD had no effect. The aggregation was attenuated by the metal chelator EDTA or by modification of histidyl residues with diethyl pyrocarbonate. The aggregates are amyloid-like as judged by the binding of thioflavin T, a fluorescent probe for amyloid, but do not exhibit fibrillar structures according to electron micrography. Interestingly the aggregates were resistant to proteinase K digestion. Likewise NADPH and zinc ions caused aggregation of PrP-(23-98), but the resulting aggregates were susceptible to degradation by proteinase K. Upon incubation with NADPH and copper ions, the full-length molecule PrP-(23-231) also formed proteinase K-resistant amyloid-like aggregates. Because it is possible that PrP, NADPH, and copper ions could associate in certain tissues, the aggregation observed in this study may be involved in prion initiation especially in the nonfamilial types of prion diseases.

The emerging principles of mammalian prion propagation and transmissibility barriers: Insight from studies in vitro

Self-perpetuating conformational conversion of the cellular prion protein PrP(C) into the beta-sheet-rich "scrapie" conformer (PrP(Sc)) is believed to be the central molecular event in pathogenesis of a group of diseases known as transmissible spongiform encephalopathies. Recent advances provide growing support for the notion that a misfolded protein alone might act as an infectious agent. Furthermore, findings regarding the mechanism of prion protein structural rearrangement, the role of folding intermediates in conformational conversion, and "conformational adaptability" in the propagation of prion amyloids in vitro yield molecular-level insight into such phenomena as inherited prion diseases, prion transmission barriers, and prion strains.

Putative aggregation initiation sites in prion protein

Misfolded prion protein, PrPSc, is believed to be the pathogenic agens in transmissible spongiform encephalopathies. Little is known about the autocatalytic misfolding process. Looking at the intrinsic properties of short sequence stretches, such as conformational flexibility and the tendency to populate extended conformers, we have examined the aggregation behaviour of various peptides within the region 106-157 of the sequence of human prion protein. We observed fast aggregation for the peptide containing residues I138-I-H-F141. This sequence, which is presented at the surface of cellular prion protein, PrPC, in an almost beta-sheet-like conformation, is therefore an ideal anchor-point for initial intermolecular contacts leading to oligomerization. We further report that the aggregation propensity of the neurotoxic peptide 106-126 appears to be centred in its termini and not in the central, alanine-rich sequence (A113-G-AAAA-G-A120).

Role of N-terminal Familial Mutations in Prion Protein Fibrillization and Prion Amyloid Propagation in Vitro*

A self-perpetuating conformational conversion of the prion protein (PrP) is believed to underlie pathology and transmission of prion diseases. Here we explore the effects of N-terminal pathogenic mutations (P102L, P105L, A117V) and the residue 129 polymorphism on amyloid fibril formation by the human PrP fragment 23-144, an in vitro conversion model that can reproduce certain characteristics of prion replication such as strains and species barriers. We find that these amino acid substitutions neither affect PrP23-144 amyloidogenicity nor introduce barriers to cross-seeding of soluble protein. However, the polymorphism strongly influences the conformation of the amyloid fibrils, as determined by infrared spectroscopy. Intriguingly, unlike conformational features governed by the critical amyloidogenic region of PrP23-144 (residues 138-139), the structural features distinguishing Met-129 and Val-129 PrP23-144 amyloid fibrils are not transmissible by cross-seeding. While based only on in vitro data, these findings provide fundamental insight into the mechanism of prion-based conformational transmission, indicating that only conformational features controlling seeding specificity (e.g. those in critical intermolecular contact sites of amyloid fibrils) are necessarily transmissible by cross-seeding; conformational traits in other parts of the PrP molecule may not be "heritable" from the amyloid template.

Octapeptide repeat insertions increase the rate of protease-resistant prion protein formation

A central feature of transmissible spongiform encephalopathies (TSE or prion diseases) involves the conversion of a normal, protease-sensitive glycoprotein termed prion protein (PrP-sen) into a pro-tease-resistant form, termed PrP-res. The N terminus of PrP-sen has five copies of a repeating eight amino acid sequence (octapeptide repeat). The presence of one to nine extra copies of this motif is associated with a heritable form of Creutzfeld-Jakob disease (CJD) in humans. An increasing number of octapeptide repeats correlates with earlier CJD onset, suggesting that the rate at which PrP-sen misfolds into PrP-res may be influenced by these mutations. In order to determine if octapeptide repeat insertions influence the rate at which PrP-res is formed, we used a hamster PrP amyloid-forming peptide (residues 23-144) into which two to 10 extra octapeptide repeats were inserted. The spontaneous formation of protease-resistant PrP amyloid from these peptides was more rapid in response to an increased number of octapeptide repeats. Furthermore, experiments using full-length glycosylated hamster PrP-sen demonstrated that PrP-res formation also occurred more rapidly from PrP-sen molecules expressing 10 extra copies of the octapeptide repeat. The rate increase for PrP-res formation did not appear to be due to any influence of the octapeptide repeat region on PrP structure, but rather to more rapid binding between PrP molecules. Our data from both models support the hypothesis that extra octapeptide repeats in PrP increase the rate at which protease resistant PrP is formed which in turn may affect the rate of disease onset in familial forms of CJD.

Predicting alternate structure attainment and amyloidogenesis: a nonlinear signal analysis approach

Chain hydrophobicity values have been used in prediction of alternate structure attainment by a polypeptide. Nonlinear signal analysis on the hydrophobicity values gives important clues about the propensities of particular stretches of a protein to form local or nonlocal contacts. These contacts determine the folding behavior of a polypeptide and helps in predicting the final structure that can be attained. A nonlinear signal analysis called the recurrent quantification analysis has been carried out using the hydrophobicity values on a wide range of proteins obtained from human, plant, and fungal sources. Here, we show that such an analysis gives us an easy handle in determining sequences within the proteins that may be important in beta-sheet formation leading to amyloidosis.

Oxidative refolding of amyloidogenic variants of human lysozyme

The oxidative refolding of human lysozyme and its two best characterised amyloidogenic variants, Ile56Thr and Asp67His, has been investigated in vitro by means of the concerted application of a range of biophysical techniques. The results show that in each case the ensemble of reduced denatured conformers initially collapses into a large number of unstructured intermediates with one or two disulphide bonds, the majority of which then fold to form the native-like three-disulphide intermediate, des-[77-95]. The slow step in the overall folding reaction involves the rearrangement of the latter to the fully oxidised native protein containing four disulphide bonds. The Ile56Thr and Asp67His variants were found to fold faster than the wild-type protein by a factor of 2 and 3 respectively, an observation that can be attributed primarily to the reduction in the barriers to conformational rearrangements that results from both the mutations. The efficient folding of these variants despite their enhanced propensities to aggregate when compared to the wild-type protein is consistent with their ability to be secreted in sufficient quantities to give rise to the systemic amyloidoses with which they are associated.

Prion protein with Y145STOP mutation induces mitochondria-mediated apoptosis and PrP-containing deposits in vitro

A pathogenic truncation of an amber mutation at codon 145 (Y145STOP) in Gerstmann-Straussler-Scheinker disease (GSS) was investigated through the real-time imaging in living cells, by utilizing GFP-PrP constructs. GFP-PrP(1-144) exhibited an aberrant localization to mitochondria in mouse neuroblastoma neuro2a (N2a) and HpL3-4 cells, a hippocampal cell line established from prnp gene-ablated mice, whereas full-length GFP-PrP did not. The aberrant mitochondrial localization was also confirmed by Western blot analysis. Since GFP-PrP(1-121), as previously reported, and full-length GFP-PrP do not exhibit such mitochondrial localization, the mitochondrial localization of GFP-PrP(1-144) requires not only PrP residues 121-144 (in human sequence) but also COOH-terminal truncation in the current experimental condition. Subsequently, the GFP-PrP(1-144) induced a change in the mitochondrial innermembrane potential (DeltaPsi(m)), release of cytochrome c from the intermembrane space into the cytosol, and DNA fragmentation in these cells. Non-fluorescent PrP(1-144) also induced the DNA fragmentation in N2a and HpL3-4 cells after the proteasomal inhibition. These data may provide clues as to the molecular mechanism of the neurotoxic property of Y145STOP mutation. Furthermore, immunoelectron microscopy revealed numerous electron-dense deposits in mitochondria clusters of GFP-PrP(1-144)-transfected N2a cells, whereas no deposit was detected in the cells transfected with full-length GFP-PrP. Co-localization of GFP/PrP-immunogold particles with porin-immunogold particles as a mitochondrial marker was observed in such electron-dense vesicular foci, resembling those found in autophagic vacuoles forming secondary lysosomes. Whether such electron-dense deposits may serve as a seed for the growth of amyloid plaques, a characteristic feature of GSS with Y145STOP, awaits further investigations.

Fibril Conformation as the Basis of Species- and Strain-Dependent Seeding Specificity of Mammalian Prion Amyloids

Spongiform encephalopathies are believed to be transmitted by self-perpetuating conformational conversion of the prion protein. It was shown recently that fundamental aspects of mammalian prion propagation can be reproduced in vitro in a seeded fibrillization of the recombinant prion protein variant Y145Stop (PrP23-144). Here we demonstrate that PrP23-144 amyloids from different species adopt distinct secondary structures and morphologies, and that these structural differences are controlled by one or two residues in a critical region. These sequence-specific structural characteristics correlate strictly with the seeding specificity of amyloid fibrils. However, cross-seeding of PrP23-144 from one species with preformed fibrils from another species may overcome natural sequence-based structural preferences, resulting in a new amyloid strain that inherits the secondary structure and morphology of the template. These data provide direct biophysical evidence that protein conformations are transmitted in PrP amyloid strains, establishing a foundation for a structural basis of mammalian prion transmission barriers.

Amyloidogenic domains, prions and structural inheritance: rudiments of early life or recent acquisition?

Amyloids are self-assembled fibre-like beta-rich protein aggregates. Amyloidogenic prion proteins propagate amyloid state in vivo and transmit it via infection or in cell divisions. While amyloid aggregation may occur in the absence of any other proteins, in vivo propagation of the amyloid state requires chaperone helpers. Yeast prion proteins contain prion domains which include distinct aggregation and propagation elements, responsible for these functions. Known aggregation and propagation elements are short in length and composed of relatively simple sequences, indicating possible ancient origin. Prion-like self-assembled structures could be involved in the initial steps of biological compartmentalization in early life.

Propagating Prions in Fungi and Mammals

Prions constitute a rare class of protein, which can switch to a robust amyloid form and then propagate that form in the absence of a nucleic acid determinant, thereby creating a unique, protein-only infectious agent. Details of the mechanism that drives conversion to the prion form and then subsequent propagation of that form are beginning to emerge using a range of in vivo and in vitro approaches. Recent studies on both mammalian and fungal prions are providing a greater understanding of the structural features that distinguish prions from non-transmissible amyloids.

Emerging Principles of Conformation-Based Prion Inheritance

The prion hypothesis proposes that proteins can act as infectious agents. Originally formulated to explain transmissible spongiform encephalopathies (TSEs), the prion hypothesis has been extended with the finding that several non-Mendelian traits in fungi are due to heritable changes in protein conformation, which may in some cases be beneficial. Although much remains to be learned about the specific role of cellular cofactors, mechanistic parallels between the mammalian and yeast prion phenomena point to universal features of conformation-based infection and inheritance involving propagation of ordered beta-sheet-rich protein aggregates commonly referred to as amyloid. Here we focus on two such features and discuss recent efforts to explain them in terms of the physical properties of amyloid-like aggregates. The first is prion strains, wherein chemically identical infectious particles cause distinct phenotypes. The second is barriers that often prohibit prion transmission between different species. There is increasing evidence suggesting that both of these can be manifestations of the same phenomenon: the ability of a protein to misfold into multiple self-propagating conformations. Even single mutations can change the spectrum of favored misfolded conformations. In turn, changes in amyloid conformation can shift the specificity of propagation and alter strain phenotypes. This model helps explain many common and otherwise puzzling features of prion inheritance as well as aspects of noninfectious diseases involving toxic misfolded proteins.

Molecular basis of barriers for interspecies transmissibility of mammalian prions

Spongiform encephalopathies are believed to be transmitted by a unique mechanism involving self-propagating conformational conversion of prion protein into a misfolded form. Here we demonstrate that fundamental aspects of mammalian prion propagation, including the species barrier and strain diversity, can be reproduced in vitro in a seeded fibrillization of the recombinant prion protein variant Y145Stop. Our data show that species-specific substitution of a single amino acid in a critical region completely changes the seeding specificity of prion protein fibrils. Furthermore, we demonstrate that sequence-based barriers that prevent cross-seeding between proteins from different species can be bypassed, and new barriers established, by a template-induced adaptation process that leads to the emergence of new strains of prion fibrils. Although the seeding barriers observed in this study do not fully match those seen in animals, the present findings provide fundamental insight into mechanistic principles of these barriers at a molecular level.

Flexible N-terminal Region of Prion Protein Influences Conformation of Protease-resistant Prion Protein Isoforms Associated with Cross-species Scrapie Infection in Vivo and in Vitro

Transmissible spongiform encephalopathy (TSE) diseases are characterized by the accumulation in brain of an abnormal protease-resistant form of the host-encoded prion protein (PrP), PrP-res. PrP-res conformation differs among TSE agents derived from various sources, and these conformational differences are thought to influence the biological characteristics of these agents. In this study, we introduced deletions into the flexible N-terminal region of PrP (residues 34-124) and investigated the effect of this region on the conformation of PrP-res generated in an in vitro cell-free conversion assay. PrP deleted from residues 34 to 99 generated 12-16-kDa protease-resistant bands with intact C termini but variable N termini. The variable N termini were the result of exposure of new protease cleavage sites in PrP-res between residues 130 and 157, suggesting that these new cleavage sites were caused by alterations in the conformation of the PrP-res generated. Similarly truncated 12-16-kDa PrP bands were also identified in brain homogenates from mice infected with mouse-passaged hamster scrapie as well as in the cell-free conversion assay using conditions that mimicked the hamster/mouse species barrier to infection. Thus, by its effects on PrP-res conformation, the flexible N-terminal region of PrP seemed to influence TSE pathogenesis and cross-species TSE transmission.

Do Amyloids Remember Their Origin? New Insights into the Prion Species Barrier

Excellent work demonstrates that, in the test tube, prion protein itself is responsible for its species specificity, although questions remain about whether this occurs in the same way in the organism.

The Effect of Disease-associated Mutations on the Folding Pathway of Human Prion Protein

Propagation of transmissible spongiform encephalopathies is believed to involve the conversion of cellular prion protein, PrP(C), into a misfolded oligomeric form, PrP(Sc). An important step toward understanding the mechanism of this conversion is to elucidate the folding pathway(s) of the prion protein. We reported recently (Apetri, A. C., and Surewicz, W. K. (2002) J. Biol. Chem. 277, 44589-44592) that the folding of wild-type prion protein can best be described by a three-state sequential model involving a partially folded intermediate. Here we have performed kinetic stopped-flow studies for a number of recombinant prion protein variants carrying mutations associated with familial forms of prion disease. Analysis of kinetic data clearly demonstrates the presence of partially structured intermediates on the refolding pathway of each PrP variant studied. In each case, the partially folded state is at least one order of magnitude more populated than the fully unfolded state. The present study also reveals that, for the majority of PrP variants tested, mutations linked to familial prion diseases result in a pronounced increase in the thermodynamic stability, and thus the population, of the folding intermediate. These data strongly suggest that partially structured intermediates of PrP may play a crucial role in prion protein conversion, serving as direct precursors of the pathogenic PrP(Sc) isoform.

Autocatalytic conversion of recombinant prion proteins displays a species barrier

The most unorthodox feature of the prion disease is the existence of an abnormal infectious isoform of the prion protein, PrP(Sc). According to the "protein-only" hypothesis, PrP(Sc) propagates its abnormal conformation in an autocatalytic manner using the normal isoform, PrP(C), as a substrate. Because autocatalytic conversion is considered a key element of prion replication, in this study I tested whether in vitro conversion of recombinant PrP into abnormal isoform displays specific features of an autocatalytic process. I found that recombinant human PrP formed two distinct beta-sheet rich isoforms, the beta-oligomer and the amyloid fibrils. The kinetics of the fibrils formation measured at different pH values were consistent with a model in which the beta-oligomer was not on the kinetic pathway to the fibrillar form. As judged by electron microscopy, an acidic pH favored to the long fibrils, whereas short fibrils morphologically similar to "prion rods" were formed at neutral pH. At neutral pH the conversion to the fibrils can be seeded with small aliquots of preformed fibrils. As small as 0.001% aliquot displayed seeding activity. The conversion of human PrP was seeded with high efficacy only with the preformed fibrils of human but not mouse PrP and vice versa. These studies illustrate that in vitro conversion of recombinant PrP displays specific features of an autocatalytic process and mimics the transmission barrier of prion propagation observed in vivo. I speculate that this model can be used as a rapid assay for assessing the intrinsic propensities of prion transmission between different species.