Of PK and subsequently deglycosylated with PNGase F. Samples were resolved on Tricine-SDSPAGE and probed with the monoclonal antibodies, #51 (lane 1), W226 (lane 2), and R1 (lane 3). doi:10.1371/journal.pone.0050111.gZou et al. described human CJD PrPSc PK-resistant C-terminal peptides spanning from positions 154/156 and 162/167 to the Cterminus [19]. These buy RE 640 fragments are analogous to GPI- PrPSc peptides N152-S232/M153-S232 and Y162-S232, S169-S232, respectively. Zanusso et al. described two additional amino-terminally truncated human CJD PrPSc peptides (MW of 16/17 kDa) [20], analogous 25033180 to the GPI- PrPSc peptides G141-S232 and M133-S232/ S134-S232. Kocisko et al. used a C-terminal antibody (epitope 217232) to demonstrate the presence of a number of amino-terminally truncated PK-resistant species in SHaPrPSc [18]. Using synthetic mouse prions, Bocharova et al. identified the regions beginning at 138/141, 152/153, and 162, and extending to the C-terminus as being resistant to PK [21]. This suggests that synthetic prions and PrPSc share key structural elements, which 
would explain the capacity of recombinant PrP fibrils to change their conformation, via a “deformed templating” mechanism, to that of PrPSc [22]. In contrast, relatively few C-terminally truncated peptides have been described. Notari et al. reported two human CJD PrPSc peptides truncated near position 228 [23]. Stahl et al. also reported the presence of a peptide truncated at position 228 in PK-treatedSHaPrPSc [24]. The absence of such fragments in our study could be explained by slight differences in sample preparation, or perhaps by the fact that the absence of the GPI-anchor might have an effect on nearby residues. This conspicuous absence of the C-terminally truncated peptides is a reflection of the stability of the C-terminal region, in GPI2 PrPSc appears to be the most stable part of the molecule, which is inconsistent with the presence of substantial stretches of ahelical secondary structure in that region. Our results agree with Smirnovas et al., who showed the C-terminus of GPI- PrPSc to exhibit extremely low rates of H/D exchange, typical of extensive H-bonding (b-sheet) [9]. These authors showed that an FTIR absorbance band (,1,660 cm21) previously assigned to a-helical secondary structure in PrPSc is also present in the spectrum of recombinant PrP amyloid fibrils, which contain no a-helices, and therefore cannot be taken as evidence of the presence of a-helical structure. They concluded that GPI2 PrPSc consists of a series of b-sheet stretches connected by short loops and/or turns, in agreement with our conclusions. Some stretches exhibiting a somewhat higher exchange rate, suggested to overlap with loops/ turns, such as 133?48 or 81?18, are consistent with flexible stretches identified in our study, although discrepancies also exist. The limited resolution of both analytical techniques prevents a more exhaustive comparison, but overall both of them agree. GPI- PrPSc fibrils are about 3? nm wide ([25] and our unpublished results). This constraint means that each PrPSc monomer must be coiled in such a way as to fit approximately 140?45 residues (,G85 232) into this width. To do so, PrPSc monomers must necessarily adopt a multi-layer architecture, as seen in SH3 fibers [26] or the HET-s fungal prion domain [27]. The HET-s prion domain packs 70 residues into two 4 IBP b-strands alternating with turns and loops [27]. Wille et al. have suggested that PrPSc fibrils are compos.Of PK and subsequently deglycosylated with PNGase F. Samples were resolved on Tricine-SDSPAGE and probed with the monoclonal antibodies, #51 (lane 1), W226 (lane 2), and R1 (lane 3). doi:10.1371/journal.pone.0050111.gZou et al. described human CJD PrPSc PK-resistant C-terminal peptides spanning from positions 154/156 and 162/167 to the Cterminus [19]. These fragments are analogous to GPI- PrPSc peptides N152-S232/M153-S232 and Y162-S232, S169-S232, respectively. Zanusso et al. described two additional amino-terminally truncated human CJD PrPSc peptides (MW of 16/17 kDa) [20], analogous 25033180 to the GPI- PrPSc peptides G141-S232 and M133-S232/ S134-S232. Kocisko et al. used a C-terminal antibody (epitope 217232) to demonstrate the presence of a number of amino-terminally truncated PK-resistant species in SHaPrPSc [18]. Using synthetic mouse prions, Bocharova et al. identified the regions beginning at 138/141, 152/153, and 162, and extending to the C-terminus as being resistant to PK [21]. This suggests that synthetic prions and PrPSc share key structural elements, which would explain the capacity of recombinant PrP fibrils to change their conformation, via a “deformed templating” mechanism, to that of PrPSc [22]. In contrast, relatively few C-terminally truncated peptides have been described. Notari et al. reported two human CJD PrPSc peptides truncated near position 228 [23]. Stahl et al. also reported the presence of a peptide truncated at position 228 in PK-treatedSHaPrPSc [24]. The absence of such fragments in our study could be explained by slight differences in sample preparation, or perhaps by the fact that the absence of the GPI-anchor might have an effect on nearby residues. This conspicuous absence of the C-terminally truncated peptides is a reflection of the stability of the C-terminal region, in GPI2 PrPSc appears to be the most stable part of the molecule, which is inconsistent with the presence of substantial stretches of ahelical secondary structure in that region. Our results agree with Smirnovas et al., who showed the C-terminus of GPI- PrPSc to exhibit extremely low rates of H/D exchange, typical of extensive H-bonding (b-sheet) [9]. These authors showed that an FTIR absorbance band (,1,660 cm21) previously assigned to a-helical secondary structure in PrPSc is also present in the spectrum of recombinant PrP amyloid fibrils, 
which contain no a-helices, and therefore cannot be taken as evidence of the presence of a-helical structure. They concluded that GPI2 PrPSc consists of a series of b-sheet stretches connected by short loops and/or turns, in agreement with our conclusions. Some stretches exhibiting a somewhat higher exchange rate, suggested to overlap with loops/ turns, such as 133?48 or 81?18, are consistent with flexible stretches identified in our study, although discrepancies also exist. The limited resolution of both analytical techniques prevents a more exhaustive comparison, but overall both of them agree. GPI- PrPSc fibrils are about 3? nm wide ([25] and our unpublished results). This constraint means that each PrPSc monomer must be coiled in such a way as to fit approximately 140?45 residues (,G85 232) into this width. To do so, PrPSc monomers must necessarily adopt a multi-layer architecture, as seen in SH3 fibers [26] or the HET-s fungal prion domain [27]. The HET-s prion domain packs 70 residues into two b-strands alternating with turns and loops [27]. Wille et al. have suggested that PrPSc fibrils are compos.