Structures of non-prion amyloids

肠杆菌科的细胞外卷曲蛋白形成纤维结构，参与生物膜的形成和与宿主细胞的粘附。这些卷曲原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，但它们具有蛋白质淀粉样蛋白的许多特性。我们证实，由大肠杆菌的主要卷曲蛋白CsgA和CsgB形成的原纤维具有淀粉样蛋白的许多典型特征。此外，我们证明卷曲原纤维具有区分蛋白质淀粉样蛋白的交叉β结构。然而，固体核磁共振实验表明，curli结构不是基于寄存器内平行β -片结构，这是许多人类疾病相关淀粉样蛋白和酵母朊蛋白淀粉样蛋白常见的结构。固体核磁共振和电子显微镜数据与β -螺旋状结构一致，但不足以确定这种结构。

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.