DNA Converts Cellular Prion Protein into the β-Sheet Conformation and Inhibits Prion Peptide Aggregation

Abstract

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.

Nucleic acid induced unfolding of recombinant prion protein globular fragment is pH dependent

“…1 Synthetic nucleic acids, in solution and in vitro, can catalyze conversion of recombinant and cellular PrP C to PrP Sc as evidenced from secondary structural studies of the protein and proteinase K (PK) resistance properties. [4][5][6][7] Our studies also indicated that bending and unwinding of nucleic acid occurred by prion protein and natural poly amines (spermine and spermidine) can inhibit this interaction. 8,9 Highly structured small RNA binds to PrP C at neutral pH and yields Proteinase K resistant component in the presence of cellular cofactors.…”

Characterization of 2′-Fluoro-RNA Aptamers That Bind Preferentially to Disease-associated Conformations of Prion Protein and Inhibit Conversion

“…The binding of heparin and copper comap to residues 53-93 and, interestingly, heparin binding seems to be enhanced by copper ions (19,20). Large and small nucleic acids (21)(22)(23)(24) also interact with PrP. It has been shown that human, ovine, and murine PrPs possess nucleic acid binding and chaperoning activities similar to retroviral nucleocapsid protein (25,26), and these properties are associated with the unstructured N-terminal region of the protein (5,10).…”

“…Previously reported 2Ј-OH aptamers raised against recombinant hamster prion protein bound to a site between residues 23-52 within this nonspecific nucleic acid binding region (27). The physiological implications of the interactions between PrP and nucleic acid raise a number of questions that are still a matter of speculation (23)(24)(25)(26)28).…”

Antibody to DNA detects scrapie but not normal prion protein

“…Recent evidence suggests that PrP may form a macromolecular complex with nucleic acid (15). To explore whether nucleic acid represents a possible target for PrP Sc detection, we screened anti-DNA Abs for their ability to capture PrP Sc from homogenates of diseased brains.…”