Structures of non-prion amyloids

肠杆菌科的胞外卷曲蛋白形成纤维状结构，参与生物膜的形成和对宿主细胞的粘附。这些卷曲纤维被认为是一种功能性淀粉样蛋白，因为它们不是错误折叠的结果，但它们具有淀粉样蛋白的许多特性。我们证实，原纤维形成的CsgA和CsgB，大肠杆菌的主要卷曲蛋白，具有许多典型的淀粉样蛋白的标志。此外，我们证明卷曲纤维具有交叉β结构，区分淀粉样蛋白。然而，固态NMR实验表明卷曲结构不是基于配准平行β-折叠结构，这是许多人类疾病相关淀粉样蛋白和酵母朊病毒淀粉样蛋白所共有的。固态NMR和电子显微镜数据与β-螺旋样结构一致，但不足以确定地建立这样的结构。 Pmel 17是哺乳动物中真黑素沉积所必需的黑素细胞蛋白，并且以丝状形式存在于黑素体中。人Pmel 17的管腔部分包括具有10个拷贝的部分重复序列pt.e.gttp.qv.的区域（RPT），已知在体内对于细丝形成是必需的。我们发现，人RPT区域很容易形成淀粉样蛋白在体外，但只有在温和的酸性条件下典型的溶酶体样黑素体腔，和丝迅速成为可溶性在中性pH值。在同样的温和的酸性条件下，Pmel丝促进真黑素形成。电子衍射，圆二色性，和固态核磁共振研究的Pmel 17细丝表明，该结构是丰富的β折叠。我们认为，RPT是淀粉样蛋白的核心结构域的Pmel 17丝黑色素形成的关键。

The extra-cellular curli proteins of Enterobacteriaceae form fibrous structures that are involved in biofilm formation and adhesion to host cells. These curli fibrils are considered a functional amyloid because they are not a consequence of misfolding, but they have many of the properties of protein amyloid. We confirm that fibrils formed by CsgA and CsgB, the primary curli proteins of Escherichia coli, possess many of the hallmarks typical of amyloid. Moreover, we demonstrate that curli fibrils possess the cross-beta structure that distinguishes protein amyloid. However, solid state NMR experiments indicate that curli structure is not based on an in-register parallel beta-sheet architecture, which is common to many human disease-associated amyloids and the yeast prion amyloids. Solid state NMR and electron microscopy data are consistent with a beta-helix-like structure, but are not sufficient to establish such a structure definitively. Pmel17 is a melanocyte protein necessary for eumelanin deposition in mammals and found in melanosomes in a filamentous form. The luminal part of human Pmel17 includes a region (RPT) with 10 copies of a partial repeat sequence, pt.e.gttp.qv., known to be essential in vivo for filament formation. We found that the human RPT region readily forms amyloid in vitro, but only under the mildly acidic conditions typical of the lysosome-like melanosome lumen, and the filaments quickly become soluble at neutral pH. Under the same mildly acidic conditions, the Pmel filaments promote eumelanin formation. Electron diffraction, circular dichroism, and solid-state NMR studies of Pmel17 filaments show that the structure is rich in beta sheet. We suggest that RPT is the amyloid core domain of the Pmel17 filaments so critical for melanin formation.