Prion diseases are rare and obligatory fatal neurodegenerative disorders caused 
by the accumulation of a misfolded isoform (PrPSc) of the host-encoded prion 
protein (PrPc). Prophylactic and therapeutic regimens against prion diseases are 
very limited. To extend such strategies we selected peptide aptamers binding to 
PrP from a combinatorial peptide library presented on the Escherichia coli 
thioredoxin A (trxA) protein as a scaffold. In a yeast two-hybrid screen 
employing full-length murine PrP (aa 23-231) as a bait we identified three 
peptide aptamers that reproducibly bind to PrP. Treatment of prion-infected 
cells with recombinantly expressed aptamers added to the culture medium 
abolished PrPSc conversion with an IC50 between 350 and 700 nM. For expression 
in eukaryotic cells, peptide aptamers were fused to an N-terminal signal peptide 
for entry of the secretory pathway. The C terminus was modified by a 
glycosyl-phosphatidyl-inositol-(GPI) anchoring signal, a KDEL retention motif 
and the transmembrane and cytosolic domain of LAMP-I, respectively. These 
peptide aptamers retained their binding properties to PrPc and, depending on 
peptide sequence and C-terminal modification, interfered with endogenous PrPSc 
conversion upon expression in prion-infected cells. Notably, infection of cell 
cultures could be prevented by expression of KDEL peptide aptamers. For the 
first time, we show that trxA-based peptide aptamers can be targeted to the 
secretory pathway, thereby not losing the affinity for their target protein. 
Beside their inhibitory effect on prion conversion, these molecules could be 
used as fundament for rational drug design.