Zhang H, Kaneko K, Nguyen J T, Livshits T L, Baldwin M A, Cohen F E, James T L, Prusiner S B
Department of Pharmaceutical Chemistry, University of California, San Francisco 94143-0518, USA.
J Mol Biol. 1995 Jul 21;250(4):514-26. doi: 10.1006/jmbi.1995.0395.
Prions are composed largely, if not entirely, of the scrapie isoform of the prion protein (PrPSc). Conversion of the cellular isoform (PrPC) to PrPSc is accompanied by a diminution in the alpha-helical content and an increase in the beta-sheet structure. To investigate the structural basis of this transition, peptide fragments corresponding to Syrian hamster PrP residues 90 to 145 and 109 to 141, which contain the most conserved residues of the prion protein and the first two putative alpha-helical regions in a PrPC model, were studied using infrared spectroscopy and circular dichroism. The peptides could be induced to form alpha-helical structures in aqueous solutions in the presence of organic solvents, such as trifluoroethanol and hexafluoroisopropanol, or detergents, such as sodium dodecyl sulfate and dodecyl phosphocholine. NaCl at physiological concentration or acetonitrile induced the peptides to acquire substantial beta-sheet. The intermolecular nature of the beta-sheet was evident in the formation of rod-shaped polymers as detected by electron microscopy. Resistance to hydrolysis by proteinase K and epitope mapping argue that the beta-sheet structures were formed by the interaction of residues lying between 109 and 141. A similar range of residues was shown by nuclear magnetic resonance spectroscopy to be capable of forming alpha-helices. The alpha-helical structures seem to require a hydrophobic support from either intermolecular interactions or the hydrophobic environment provided by micelles, in agreement with the predicted hydrophobic nature of the packing surface among the four putative helices of PrPC and the outer surfaces of the first two helices. Our results suggest that perturbation of the packing environment of the highly conserved residues is a possible mechanism for triggering the conversion of PrPC to PrPSc where alpha-helices appear to be converted into beta-sheets.
朊病毒主要（如果不是完全）由朊病毒蛋白的瘙痒病异构体（PrPSc）组成。细胞异构体（PrPC）向PrPSc的转化伴随着α-螺旋含量的减少和β-折叠结构的增加。为了研究这种转变的结构基础，使用红外光谱和圆二色性对与叙利亚仓鼠PrP残基90至145和109至141相对应的肽片段进行了研究，这些片段包含朊病毒蛋白中最保守的残基以及PrPC模型中的前两个假定的α-螺旋区域。在有机溶剂（如三氟乙醇和六氟异丙醇）或去污剂（如十二烷基硫酸钠和十二烷基磷胆碱）存在下，这些肽可在水溶液中诱导形成α-螺旋结构。生理浓度的NaCl或乙腈可诱导这些肽获得大量的β-折叠。通过电子显微镜检测到的棒状聚合物的形成表明β-折叠具有分子间性质。对蛋白酶K水解的抗性和表位作图表明，β-折叠结构是由109至141之间的残基相互作用形成的。核磁共振光谱显示，类似范围的残基能够形成α-螺旋。α-螺旋结构似乎需要来自分子间相互作用或胶束提供的疏水环境的疏水支持，这与PrPC的四个假定螺旋之间的堆积表面以及前两个螺旋的外表面的预测疏水性质一致。我们的结果表明，高度保守残基堆积环境的扰动可能是触发PrPC向PrPSc转化的一种机制，其中α-螺旋似乎转化为β-折叠。
