ABSTRACT There exist many phenotypically-varied prion strains, like viruses, despite the absence of conventional genetic material which codes the phenotypic information. As prion is composed solely of the pathological isoform (PrP Sc ) of prion protein (PrP), the strain-specific traits are hypothesized to be enciphered in the structural details of PrP Sc . Identification of the structures of PrP Sc is therefore vital for the understanding of prion biology, though they remain unidentified due to the incompatibility of PrP Sc with conventional high-resolution structural analyses. Based on our previous hypothesis that the region between the first and the second α-helix (H1∼H2) and the distal region of the third helix (Ctrm) of the cellular isoform of PrP (PrP C ) have important roles for efficient interactions with PrP Sc , we created series of mutant PrPs with two cysteine substitutions (C;C-PrP) which were systematically designed to form an intramolecular disulfide crosslink between H1∼H2 and Ctrm and assessed their conformational changes by prions: Specifically, a cysteine substitution in H1∼H2 from 165 to 169 was combined with cysteine-scanning along Ctrm from 220 to 229. C;C-PrPs with the crosslinks were expressed normally with the similar glycosylation patterns and subcellular localization as the wild-type PrP albeit with varied expression levels. Interestingly, some of the C;C-PrPs converted to the protease-resistant isoforms in the N2a cells persistently infected with 22L prion strain, whereas the same mutants did not convert in the cells infected with another prion strain Fukuoka1, indicating that local structures of PrP Sc in these regions vary among prion strains and contribute to prion-strain diversity. Moreover, patterns of the crosslinks of the convertible C;C-PrPs implied drastic changes in positional relations of H1∼H2 and Ctrm in the PrP Sc -induced conformational changes by 22L prion. Thus, disulfide-crosslink scanning is a useful approach for investigation of strain-specific structures of PrP Sc , and would be applicable to other types of amyloids as well.
