The main hypothesis for prion diseases proposes that the cellular protein (PrP C ) can be altered into a misfolded, ␤-sheet-rich isoform (PrP Sc ), which in most cases undergoes aggregation. In an organism infected with PrP Sc , PrP C is converted into the ␤-sheet form, generating more PrP Sc . We find that sequence-specific DNA binding to recombinant murine prion protein (mPrP-(23-231)) converts it from an ␣-helical conformation (cellular isoform) into a soluble, ␤-sheet isoform similar to that found in the fibrillar state. The recombinant murine prion protein and prion domains bind with high affinity to DNA sequences. Several double-stranded DNA sequences in molar excess above 2:1 (pH 4.0) or 0.5:1 (pH 5.0) completely inhibit aggregation of prion peptides, as measured by light scattering, fluorescence, and circular dichroism spectroscopy. However, at a high concentration, fibers (or peptide aggregates) can rescue the peptide bound to the DNA, converting it to the aggregating form. Our results indicate that a macromolecular complex of prion-DNA may act as an intermediate for the formation of the growing fiber. We propose that host nucleic acid may modulate the delicate balance between the cellular and the misfolded conformations by reducing the protein mobility and by making the protein-protein interactions more likely. In our model, the infectious material would act as a seed to rescue the protein bound to nucleic acid. Accordingly, DNA would act on the one hand as a guardian of the Sc conformation, preventing its propagation, but on the other hand may catalyze Sc conversion and aggregation if a threshold level is exceeded.

Background: p53 function is lost in more than 50% of tumors.Results: p53 aggregates into amyloid oligomers and fibrils in vitro and in breast cancer tissues; mutant p53 seeds amyloid aggregation of WT p53, a behavior typical of a prion.Conclusion: Prion-like aggregation is crucial for the negative dominance of mutant p53.Significance: The inhibition of aggregation could be a target for cancer therapy.

Alzheimer's disease, Parkinson's disease, cystic fibrosis, prion diseases, and many types of cancer are considered to be protein conformation diseases. Most of them are also known as amyloidogenic diseases due to the occurrence of pathological accumulation of insoluble aggregates with fibrillar conformation. Some neuroblastomas, carcinomas, and myelomas show an abnormal accumulation of the wild-type tumor suppressor protein p53 either in the cytoplasm or in the nucleus of the cell. Here we show that the wild-type p53 core domain (p53C) can form fibrillar aggregates after mild perturbation. Gentle denaturation of p53C by pressure induces fibrillar aggregates, as shown by electron and atomic force microscopies, by binding of thioflavin T, and by circular dichroism. On the other hand, heat denaturation produced granular-shaped aggregates. Annular aggregates similar to those found in the early aggregation stages of alpha-synuclein and amyloid-beta were also observed by atomic force microscopy immediately after pressure treatment. Annular and fibrillar aggregates of p53C were toxic to cells, as shown by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] reduction assay. Interestingly, the hot-spot mutant R248Q underwent similar aggregation behavior when perturbed by pressure or high temperature. Fibrillar aggregates of p53C contribute to the loss of function of p53 and seed the accumulation of conformationally altered protein in some cancerous cells.

Conversion of the cellular prion protein (PrP C ) into its altered conformation, PrPSc , is believed to be the major cause of prion diseases. Although PrP is the only identified agent for these diseases, there is increasing evidence that other molecules can modulate the conversion. We have found that interaction of PrP with double-stranded DNA leads to a protein with higher ␤-sheet content and characteristics similar to those of PrP Sc . RNA molecules can also interact with PrP and potentially modulate PrP C to PrP Sc conversion or even bind differentially to both PrP isoforms. Here, we investigated the interaction of recombinant murine PrP with synthetic RNA sequences and with total RNA extracted from cultured neuroblastoma cells (N2aRNA). We found that PrP interacts with N2aRNA with nanomolar affinity, aggregates upon this interaction, and forms species partially resistant to proteolysis. RNA does not bind to N-terminal deletion mutants of PrP, indicating that the N-terminal region is important for this process. Cell viability assays showed that only the N2aRNA extract induces PrP-RNA aggregates that can alter the homeostasis of cultured cells. Small RNAs bound to PrP give rise to nontoxic small oligomers. Nuclear magnetic resonance measurements of the PrP-RNA complex revealed structural changes in PrP, but most of its native fold is maintained. These results indicate that there is selectivity in the species generated by interaction with different molecules of RNA. The catalytic effect of RNA on the PrP C 3 PrP Sc conversion depends on the RNA sequence, and small RNA molecules may exert a protective effect.Prion diseases can be infectious, sporadic, or inherited (1). Regardless of their origin, they are related to modifications of a ubiquitous membrane-anchored protein, the prion protein (PrP).3 Through a poorly understood process, the cellular PrP isoform (PrP C ), an ␣-helix-rich protein, undergoes a profound conformational change, acquiring higher ␤-sheet content; the latter isoform is known as PrP Sc (Sc from scrapie) and is the only known component of the infectious prion particle (1-4).The protein-only hypothesis postulates that PrP Sc "multiplies" by catalyzing the conversion of PrP C into a likeness of itself, thus becoming responsible for its own propagation (5). This hypothesis is based strongly on the fact that PrP knock-out mice are resistant to prion infection, suggesting that endogenous PrP is necessary for prion propagation and infection (6). It was also suggested, however, that an additional unknown factor could influence the PrP C to PrP Sc conversion (7-10). This molecule would act by lowering the free energy barrier between PrP C and PrP Sc and triggering conversion (11,12). In this field, a great number of biological macromolecules have emerged as candidates for conversion catalysts. Cellular adhesion molecules, nucleic acids (NAs), basal membrane molecules, and sulfated glycans, among other biological macromolecules, have been reported to interact with PrP C and to induce its conversion in...