DNA converts cellular prion protein into the beta-sheet conformation and inhibits prion peptide aggregation

Double-stranded DNA stimulates the fibrillation of alpha-synuclein in vitro and is associated with the mature fibrils: an electron microscopy study

Filamentous aggregates formed by alpha-synuclein are a prominent and presumably key etiological factor in Parkinson's and other neurodegenerative diseases characterized by motor disorders. Numerous studies have demonstrated that various environmental and intracellular factors affect the fibrillation properties of alpha-synuclein, e.g. by accelerating the process of assembly. Histones, the major component and constituent of chromatin, interact specifically with alpha-synuclein and enhance its fibrillation significantly. Here, we report that another component of chromatin, double-stranded DNA (dsDNA), either linear or supercoiled, also interacts with wild-type alpha-synuclein, leading to a significant stimulation of alpha-synuclein assembly into mature fibrils characterized by a reduced lag phase. In general, the morphology of the fibrils remains unchanged in the presence of linear dsDNA. Electron microscopy reveals that DNA forms various types of complexes upon association with the fibrils at their surface without distortion of the double-helical structure. The existence of these complexes was confirmed by the electrophoresis, which also demonstrated that a fraction of the associated DNA was resistant to digestion by restriction endonucleases. Fibrils assembled from the alpha-synuclein mutants A30P and A53T and the C-terminally truncated variants (encoding amino acid residues 1-108 or 1-124) also form complexes with linear dsDNA. Possible mechanisms and implications of dsDNA-alpha-synuclein interactions are discussed.

The cellular prion protein modulates phagocytosis and inflammatory response

The cellular prion protein (PrPc) is a glycoprotein anchored by glycosylphosphatidylinositol (GPI) to the cell surface and is abundantly expressed in the central nervous system. It is also expressed in a variety of cell types of the immune system. We investigated the role of PrPc in the phagocytosis of apoptotic cells and other particles. Macrophages from mice with deletion of the Prnp gene showed higher rates of phagocytosis than wild-type macrophages in in vitro assays. The elimination of GPI-anchored proteins from the cell surface of macrophages from wild-type mice rendered these cells as efficient as macrophages derived from knockout mice. In situ detection of phagocytosis of apoptotic bodies within the retina indicated augmented phagocytotic activity in knockout mice. In an in vivo assay of acute peritonitis, knockout mice showed more efficient phagocytosis of zymosan particles than wild-type mice. In addition, leukocyte recruitment was altered in knockout mice, as compared with wild type. The data show that PrPc modulates phagocytosis in vitro and in vivo. This activity is described for the first time and may be important for normal macrophage functions as well as for the pathogenesis of prion diseases.

Nucleic Acid and Prion Protein Interaction Produces Spherical Amyloids which can Function in vivo as Coats of Spongiform Encephalopathy Agent

The infectious agent of transmissible spongiform encephalopathies (TSE) has been considered to be PrP(SC), a structural isoform of cellular prion protein PrP(C). PrP(SC) can exist as oligomers and/or as amyloid polymers. Nucleic acids induce structural conversion of recombinant prion protein PrP and PrP(C) to PrP(SC) form in solution and in vitro. Here, we report that nucleic acids, by interacting with PrP in solution, produce amyloid fibril and fibres of different morphologies, similar to those identified in the diseased brains. In addition, the same interaction produces polymer lattices and spherical amyloids of different dimensions (15-150 nm in diameters). The polymer lattices show apparent morphological similarity to the two-dimensional amyloid crystals obtained from linear amyloids isolated in vivo. The spherical amyloids structurally resemble "spherical particles" observed in natural spongiform encephalopathy (SE) and in scrapie-infected brains (TSE). We suggest that spherical amyloids, PrP(SC)-amylospheroids, are probable constituents of the coat of the spherical particles found in vivo and the latter can act as protective coats of the SE and TSE agents in vivo.

New Insights into the Mechanisms of Protein Misfolding and Aggregation in Amyloidogenic Diseases Derived from Pressure Studies†

Hydrostatic pressure is a robust tool for studying the thermodynamics of protein folding and protein interactions, as well as the dynamics and structure of folding intermediates. One of the main innovations obtained from using high pressure is the stabilization of folding intermediates such as molten-globule conformations, thus providing a unique opportunity for characterizing their structure and dynamics. Equally important is the prospect of understanding protein misfolding diseases by using pressure to populate partially folded intermediates at the junction between productive and off-pathway folding, which may give rise to misfolded proteins, aggregates, and amyloids. High hydrostatic pressure (HHP) has also been used to dissociate nonamyloid aggregates and inclusion bodies. In many proteins, the competition between correct folding and misfolding can lead to formation of insoluble aggregates, an important problem for the biotechnology industry and for human pathologies such as amyloidosis, Alzheimer's, Parkinson's, prion's, and tumor diseases. The diversity of diseases that result from protein misfolding has made this theme an important research focus for pharmaceutical and biotechnology companies. The use of high-pressure promises to contribute to the identification of the mechanisms behind these defects and creation of therapies against these diseases.

High pressure induces scrapie-like prion protein misfolding and amyloid fibril formation

Our understanding of conformational conversion of proteins in diseases is essential for any diagnostic and therapeutic approach. Although not fully understood, misfolding of the prion protein (PrP) is implicated in the pathogenesis of prion diseases. Despite several efforts to produce the pathologically misfolded conformation in vitro from a recombinant PrP, no positive result has yet been obtained. Within the "protein-only hypothesis", the reason for this hindrance may be that the experimental conditions used did not allow selection of the pathway adopted in vivo resulting in conversion into the infectious form. Here, using a pressure perturbation approach, we show that recombinant PrP is converted to a novel misfolded conformer, which is prone to aggregate and ultimately form amyloid fibrils. A short incubation at high pressure (600 MPa) of the truncated form of hamster prion protein (SHaPrP(90-231)) resulted in the formation of pre-amyloid structures. The mostly globular aggregates were characterized by ThT and ANS binding, and by a beta-sheet-rich secondary structure. After overnight incubation at 600 MPa, amyloid fibrils were formed. In contrast to pre-amyloid structures, they showed birefringency of polarized light after Congo red staining and a strongly decreased ANS binding capacity, but enhanced ThT binding. Both aggregate types were resistant to digestion by PK, and can be considered as potential scrapie-like forms or precursors. These results may be useful for the search for compounds preventing pathogenic PrP misfolding and aggregation.

Reversible aggregation plays a crucial role on the folding landscape of p53 core domain

The role of tumor suppressor protein p53 in cell cycle control depends on its flexible and partially unstructured conformation, which makes it crucial to understand its folding landscape. Here we report an intermediate structure of the core domain of the tumor suppressor protein p53 (p53C) during equilibrium and kinetic folding/unfolding transitions induced by guanidinium chloride. This partially folded structure was undetectable when investigated by intrinsic fluorescence. Indeed, the fluorescence data showed a simple two-state transition. On the other hand, analysis of far ultraviolet circular dichroism in 1.0 M guanidinium chloride demonstrated a high content of secondary structure, and the use of an extrinsic fluorescent probe, 4,4'-dianilino-1,1' binaphthyl-5,5'-disulfonic acid, indicated an increase in exposure of the hydrophobic core at 1 M guanidinium chloride. This partially folded conformation of p53C was plagued by aggregation, as suggested by one-dimensional NMR and demonstrated by light-scattering and gel-filtration chromatography. Dissociation by high pressure of these aggregates reveals the reversibility of the process and that the aggregates have water-excluded cavities. Kinetic measurements show that the intermediate formed in a parallel reaction between unfolded and folded structures and that it is under fine energetic control. They are not only crucial to the folding pathway of p53C but may explain as well the vulnerability of p53C to undergo departure of the native to an inactive state, which makes the cell susceptible to malignant transformation.

Origin of the anomalous circular dichroism spectra of many apomyoglobin mutants

Several authors have reported that many sperm whale apomyoglobin mutants show anomalous circular dichroism spectra. These mutants have a low molar ellipticity compared to the wild-type protein but in several cases have the same stability of unfolding. A model in which native apomyoglobin is not folded in the same manner as that in other proteins and in which mutants show progressive reductions in their degree of folding has been suggested to explain this phenomenon. However, nuclear magnetic resonance of the native apomyoglobin conformation has shown that this state is folded and compact, raising the possibility that the anomalous circular dichroism spectra could have another explanation. We studied several mutants with anomalous circular dichroism spectra and found that these proteins were all contaminated with nucleic acid that contributed to the ultraviolet absorption and caused uncertainty in the determination of protein concentration. The resulting overestimation of the concentration of apomyoglobin explains the phenomenon of anomalous circular dichroism spectra. We describe a procedure to remove the contaminant nucleic acid which yields accurate protein concentration measurements and provides the normal circular dichroism spectra. Our findings support a well-structured native conformation for apomyoglobin and may also be of the interest to scientists working with the purification of recombinant proteins.

Hydration and packing effects on prion folding and beta-sheet conversion. High pressure spectroscopy and pressure perturbation calorimetry studies

The main hypothesis for prion diseases proposes that the cellular protein (PrP(C)) can be altered into a misfolded, beta-sheet-rich isoform (PrP(Sc)), which undergoes aggregation and triggers the onset of transmissible spongiform encephalopathies. Here, we compare the stability against pressure and the thermomechanical properties of the alpha-helical and beta-sheet conformations of recombinant murine prion protein, designated as alpha-rPrP and beta-rPrP, respectively. High temperature induces aggregates and a large gain in intermolecular antiparallel beta-sheet (beta-rPrP), a conformation that shares structural similarity with PrP(Sc). alpha-rPrP is highly stable, and only pressures above 5 kilobars (1 kilobar = 100 MegaPascals) cause reversible denaturation, a process that leads to a random and turnrich conformation with concomitant loss of alpha-helix, as measured by Fourier transform infrared spectroscopy. In contrast, aggregates of beta-rPrP are very sensitive to pressure, undergoing transition into a dissociated species that differs from the denatured form derived from alpha-rPrP. The higher susceptibility to pressure of beta-rPrP can be explained by its less hydrated structure. Pressure perturbation calorimetry supports the view that the accessible surface area of alpha-rPrP is much higher than that of beta-rPrP, which explains the lower degree of hydration of beta-rPrP. Our findings shed new light on the mechanism of prion conversion and show how water plays a prominent role. Our results allow us to propose a volume and free energy diagram of the different species involved in the conversion and aggregation. The existence of different folded conformations as well as different denatured states of PrP may explain the elusive character of its conversion into a pathogenic form.

Scrapie-like prion protein is translocated to the nuclei of infected cells independently of proteasome inhibition and interacts with chromatin

Prion diseases are fatal transmissible neurodegenerative disorders characterized by the accumulation of an abnormally folded isoform of the cellular prion protein (PrPC) denoted PrPSc. Recently, wild-type and pathogenic PrP mutants have been shown to be degraded by the endoplasmic reticulum-associated degradation proteasome pathway after translocation into the cytosol. We show here that a protease resistant form of PrP accumulated in the nuclei of prion-infected cells independently of proteasome activity, and that this nuclear translocation required an intact microtubule network. Moreover, our results show for the first time that nuclear PrP interacts with chromatin in vivo, which may have physiopathological consequences in prion diseases

Analysis of the interactions between HIV-1 and the cellular prion protein in a human cell line

The cellular prion protein (PrP(c)) is highly conserved in mammals and expressed widely in different tissues but its physiological role remains elusive. Recently, the human PrP(c) was shown to possess nucleic acid binding and chaperoning properties similar to human immunodeficiency virus type 1 (HIV-1) nucleocapsid protein, a key viral factor in virus structure and replication. These findings prompted us to determine if PrP(c) could influence HIV-1 replication. We used the human 293T cell line as a model system, since only a very low level of PrP(c) accumulates in these cells. Expression of PrP at a high level resulted in a specific decrease of HIV-1 Env and Vpr expression. Despite similar levels of intracellular Gag, virus production was reduced by eightfold and infectivity by three- to fourfold in the presence of PrP(c). A PrP(c) mutant lacking the glycosylphosphatidylinositol (GPI) anchor peptide did not impair HIV-1 production, suggesting that PrP(c) trafficking is critical for this inhibitory effect. Coexpressing HIV-1 and PrP(c) in these cells also caused a fraction of PrP(c) to become partially proteinase K-resistant (PrP(res)), further illustrating the interactions between HIV-1 and PrP(c).

Antibody to DNA detects scrapie but not normal prion protein

Prion diseases, a group of fatal neurodegenerative disorders, are characterized by the presence of the abnormal scrapie isoform of prion protein (PrP(Sc)) in affected brains. A conformational change is believed to convert the normal cellular prion protein into PrP(Sc). Detection of PrP(Sc) for diagnosis and prophylaxis is impaired because available Abs recognizing epitopes on PrP fail to distinguish between PrP(Sc) and normal cellular prion protein. Here, we report that an anti-DNA Ab, OCD4, as well as gene 5 protein, a well established DNA-binding protein, capture PrP from brains affected by prion diseases in both humans and animals but not from unaffected controls. OCD4 appears to immunoreact with DNA (or a DNA-associated molecule) that forms a conformation-dependent complex with PrP in prion diseases. Whereas PrP immunocaptured by OCD4 is largely protease-resistant, a fraction of it remains protease-sensitive. Moreover, OCD4 detects disease-associated PrP >10 times more efficiently than a widely used Ab to PrP. Our finding that anti-DNA Abs and gene 5 protein specifically target disease-associated DNA-PrP complexes in a wide variety of species and disease phenotypes opens new avenues in the study and diagnosis of prion diseases.

Prions at the crossroads: the need to identify the active TSE agent

Structural change in the cellular prion protein, PrPC to a ProteinaseK-resistant beta-sheet-rich insoluble form PrPSC and its accumulation have been considered to be central to the pathogenesis of the prion diseases (TSE). In a recent paper, Deleault et al have shown that specific endogenous RNA molecules can induce in vitro structural conversion of endogenous PrPC to PrPSC. Small highly structured synthetic RNAs can also induce this conversion process. However, recent in vivo results show that PrPSC is not directly involved in the prion pathogenesis. It is possible, however, that nucleic-acid-induced PrPSC associated with the inducer nucleic acid could be the components of the infectious agent.

Modulation of prion protein oligomerization, aggregation, and beta-sheet conversion by 4,4'-dianilino-1,1'-binaphthyl-5,5'-sulfonate (bis-ANS)

The prion protein (PrP) is the major agent implicated in the diseases known as transmissible spongiform encephalopathies. The onset of transmissible spongiform encephalopathy is related to a change in conformation of the PrP(C), which loses most of its alpha-helical content, becoming a beta-sheet-rich protein, known as PrP(Sc). Here we have used two Syrian hamster prion domains (PrP 109-141 and PrP 109-149) and the murine recombinant PrP (rPrP 23-231) to investigate the effects of anilino-naphtalene compounds on prion oligomerization and aggregation. Aggregation in the presence of bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-sulfonate), ANS (1-anilinonaphthalene-8-sulfonate), and AmNS (1-amino-5-naphtalenesulfonate) was monitored. Bis-ANS was the most effective inhibitor of prion peptide aggregation. Bis-ANS binds strongly to rPrP 23-231 leading to a substantial increase in beta-sheet content and to limited oligomerization. More strikingly, the binding of bis-ANS to full-length rPrP is diminished by the addition of nanomolar concentrations of oligonucleotides, demonstrating that they compete for the same binding site. Thus, bis-ANS displays properties similar to those of nucleic acids, causing oligomerization and conversion to beta-sheet (Cordeiro, Y., Machado, F., Juliano, L., Juliano, M. A., Brentani, R. R., Foguel, D., and Silva, J. L. (2001) J. Biol. Chem. 276, 49400-49409). This dual effect of bis-ANS on prion protein makes this compound highly important to sequester crucial conformations of the protein, which may be useful to the understanding of the disease and to serve as a lead for the development of new therapeutic strategies.

Protofibrils, pores, fibrils, and neurodegeneration: separating the responsible protein aggregates from the innocent bystanders

Many neurodegenerative diseases, including Alzheimer's and Parkinson's and the transmissible spongiform encephalopathies (prion diseases), are characterized at autopsy by neuronal loss and protein aggregates that are typically fibrillar. A convergence of evidence strongly suggests that protein aggregation is neurotoxic and not a product of cell death. However, the identity of the neurotoxic aggregate and the mechanism by which it disables and eventually kills a neuron are unknown. Both biophysical studies aimed at elucidating the precise mechanism of in vitro aggregation and animal modeling studies support the emerging notion that an ordered prefibrillar oligomer, or protofibril, may be responsible for cell death and that the fibrillar form that is typically observed at autopsy may actually be neuroprotective. A subpopulation of protofibrils may function as pathogenic amyloid pores. An analogous mechanism may explain the neurotoxicity of the prion protein; recent data demonstrates that the disease-associated, infectious form of the prion protein differs from the neurotoxic species. This review focuses on recent experimental studies aimed at identification and characterization of the neurotoxic protein aggregates.

RNA molecules stimulate prion protein conversion

Much evidence supports the hypothesis that the infectious agents of prion diseases are devoid of nucleic acid, and instead are composed of a specific infectious protein. This protein, PrP(Sc), seems to be generated by template-induced conformational change of a normally expressed glycoprotein, PrP(C) (ref. 2). Although numerous studies have established the conversion of PrP(C) to PrP(Sc) as the central pathogenic event of prion disease, it is unknown whether cellular factors other than PrP(C) might be required to stimulate efficient PrP(Sc) production. We investigated the biochemical amplification of protease-resistant PrP(Sc)-like protein (PrPres) using a modified version of the protein-misfolding cyclic amplification method. Here we report that stoichiometric transformation of PrP(C) to PrPres in vitro requires specific RNA molecules. Notably, whereas mammalian RNA preparations stimulate in vitro amplification of PrPres, RNA preparations from invertebrate species do not. Our findings suggest that host-encoded stimulatory RNA molecules may have a role in the pathogenesis of prion disease. They also provide a practical approach to improve the sensitivity of diagnostic techniques based on PrPres amplification.

Characterization of 2'-fluoro-RNA aptamers that bind preferentially to disease-associated conformations of prion protein and inhibit conversion

We have isolated artificial ligands or aptamers for infectious prions in order to investigate conformational aspects of prion pathogenesis. The aptamers are 2'-fluoro-modified RNA produced by in vitro selection from a large, randomized library. One of these ligands (aptamer SAF-93) had more than 10-fold higher affinity for PrPSc than for recombinant PrPC and inhibited the accumulation of PrPres in near physiological cell-free conversion assay. To understand the molecular basis of these properties and to distinguish specific from non-specific aptamer-PrP interactions, we studied deletion mutants of bovine PrP in denatured, alpha-helix-rich and beta-sheet-rich forms. We provide evidence that, like scrapie-associated fibrils (SAF), the beta-oligomer of PrP bound to SAF-93 with at least 10-fold higher affinity than did the alpha-form. This differential affinity could be explained by the existence of two binding sites within the PrP molecule. Site 1 lies within residues 23-110 in the unstructured N terminus and is a nonspecific RNA binding site found in all forms of PrP. The region between residue 90 and 110 forms a hinge region that is occluded in the alpha-rich form of PrP but becomes exposed in the denatured form of PrP. Site 2 lies in the region C-terminal of residue 110. This site is beta-sheet conformation-specific and is not recognized by control RNAs. Taken together, these data provide for the first time a specific ligand for a disease conformation-associated site in a region of PrP critical for conformational conversion. This aptamer could provide tools for the further analysis of the processes of PrP misfolding during prion disease and leads for the development of diagnostic and therapeutic approaches to TSEs.

Small, highly structured RNAs participate in the conversion of human recombinant PrP(Sen) to PrP(Res) in vitro

We have identified a small, highly structured (shs)RNA that binds human recombinant prion protein (hrPrP) with high affinity and specificity under physiological conditions (e.g. 10% bovine calf serum (BCS), neutral pH, nanomolar concentrations of RNA and hrPrP). We also demonstrate the ability of this shsRNA to form highly stable nucleoprotein complexes with hrPrP and cellular PrP (PrP(C)) from various cell extracts and mammalian brain homogenates. The apparent mass of the nucleoprotein complex is dependent on the molar ratio of hrPrP to RNA during complex formation. The hrPrP in these complexes acquires resistance to degradation by Proteinase K (PK). Other shsRNAs, however, under identical conditions, neither form stable complexes with hrPrP nor do they induce resistance to PK digestion. We also demonstrate that the RNAs in these nucleoprotein complexes become resistant to ribonuclease A hydrolysis. These interactions between shsRNAs and hrPrP suggest possible roles of RNAs in the modulation of PrP structure and perhaps disease development. ShsRNAs that bind to hrPrP with high affinity and induce resistance to PK digestion can be used to develop molecular biology assays for the screening of compounds associated with PrP structure transformation or for drugs that inhibit this process.

Prion protein interactions with nucleic acid: possible models for prion disease and prion function

Several models for the transmission and progression of prion diseases have arisen, evolving with the acquisition of new experimental results. It is generally accepted that the PrP(Sc) protein is at least part of the infectious particle and the major protein component of the scrapie-associated fibrils (SAFs) that characterize the disease. An additional, unknown cofactor is most likely involved in transmission of the disease, perhaps by influencing the PrP(c) --> PrP(Sc) transition. This review relates experimental observations on the interactions of nucleic acids (NAs) and PrP with specific focus on alterations in structure. In particular, NAs appear to induce PrP(c) to acquire some of the structural and biochemical characteristics of PrP(Sc). An updated hypothesis is related wherein NAs, on the basis of their structure, act in the PrP(c) --> PrP(Sc) transformation by serving as catalysts and/or chaperones and not by encoding genetic information.

Towards cellular receptors for prions

Transmissible spongiform encephalopathies (TSE) are attributed to the conversion of the cellular prion protein (PrP(c)) into an abnormal isoform (PrP(sc)). This can be caused by the invasion of living organisms by infectious particles, or be inherited due to mutations on the PrP(c) gene. One of the most intriguing problems of prion biology is the inability to generate the infectious agent in vitro. This argues strongly that other cellular proteins besides those added in test tubes or found in cellular preparations are necessary for infection. Despite recent progress in the understanding of prion pathology, the subcellular compartments in which the interaction and conversion of PrP(c) into PrP(sc) take place are still controversial. PrP(c) interacts with various macromolecules at the cell membrane, in endocytic compartments and in the secretory pathway, all of which may play specific roles in the internalisation of PrP(sc) and conversion of PrP(c). A specific interacting protein required for the propagation of prions was originally proposed as a prion receptor, and later referred to as a ligand, a cofactor, protein X, or a partner. However, current studies indicate that PrP(c) associates with multi-molecular complexes, which mediate a variety of functions in distinct cellular compartments. It is proposed that a deeper understanding of the mechanics of such interactions, coupled to a better knowledge of the corresponding signalling pathways and ensuing cellular responses, will have a major impact on the prevention and treatment of TSE.

Unusual property of prion protein unfolding in neutral salt solution

The unfolding of cellular prion protein and its refolding to the scrapie isoform are related to prion diseases. Studies in the literature have shown that structures of proteins, either acidic or basic, are stabilized against denaturation by certain neutral salts, for example, sulfate and fluoride. Contrary to these observations, the full-length recombinant prion protein (amino acid residues 23-231) is denatured by these protein structure stabilizing salts. Under identical concentrations of salts, the structure of the sheep prion protein, which contains a greater number of glycine groups in the N-terminal unstructured segment than the mouse protein, becomes more destabilized. In contrast to the full-length protein, the C-terminal 121-231 prion protein fragment, consisting of all the structural elements of the protein, viz., three alpha-helices and two short beta-strands, is stabilized against denaturation by these salts. We suggest that an increase in the concentration of the anions on the surface of the prion protein molecule due to their preferential interaction with the glycine residues in the N-terminal segment destabilizes the structure of the prion protein by perturbing the prion helix 1 which is the most soluble of all the protein alpha-helices reported so far in the literature. The present results could be relevant to explain the observed structural conversion of the prion protein by anionic nucleic acids and sulfated glycosaminoglycans.

DNA-induced partial unfolding of prion protein leads to its polymerisation to amyloid

The full-length mouse recombinant prion protein (23-231 amino acid residues) contains all of its structural elements viz. three alpha-helices and a short two-stranded antiparallel beta-sheet in its C-terminal fragment comprising 121-231 amino acid residues. The incubated mixture of this prion protein fragment and nucleic acid results in the formation of amyloid fibres evidenced from electron microscopy, birefringence and fluorescence of the fibre bound Congo Red and Thioflavin T dyes, respectively. The secondary structure of the amyloid formed in nucleic acid solution is similar to the in vivo isolated prion protein 27-30 amyloid but unlike in it, a hydrophobic milieu is absent in the 121-231 amyloid. Thermal denaturation study demonstrates a partial unfolding of the protein fragment in nucleic acid solution. We propose that nucleic acid catalyses unfolding of prion protein helix 1 followed by a nucleation-dependent polymerisation of the protein to amyloid.

Functional interactions of nucleocapsid protein of feline immunodeficiency virus and cellular prion protein with the viral RNA

All lentiviruses and oncoretroviruses examined so far encode a major nucleic-acid binding protein (nucleocapsid or NC* protein), approximately 2500 molecules of which coat the dimeric RNA genome. Studies on HIV-1 and MoMuLV using in vitro model systems and in vivo have shown that NC protein is required to chaperone viral RNA dimerization and packaging during virus assembly, and proviral DNA synthesis by reverse transcriptase (RT) during infection. The human cellular prion protein (PrP), thought to be the major component of the agent causing transmissible spongiform encephalopathies (TSE), was recently found to possess a strong affinity for nucleic acids and to exhibit chaperone properties very similar to HIV-1 NC protein in the HIV-1 context in vitro. Tight binding of PrP to nucleic acids is proposed to participate directly in the prion disease process. To extend our understanding of lentiviruses and of the unexpected nucleic acid chaperone properties of the human prion protein, we set up an in vitro system to investigate replication of the feline immunodeficiency virus (FIV), which is functionally and phylogenetically distant from HIV-1. The results show that in the FIV model system, NC protein chaperones viral RNA dimerization, primer tRNA(Lys,3) annealing to the genomic primer-binding site (PBS) and minus strand DNA synthesis by the homologous FIV RT. FIV NC protein is able to trigger specific viral DNA synthesis by inhibiting self-priming of reverse transcription. The human prion protein was found to mimic the properties of FIV NC with respect to primer tRNA annealing to the viral RNA and chaperoning minus strand DNA synthesis.