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.