Symmons M F, Buchanan S G, Clarke D T, Jones G, Gay N J
Department of Biochemistry, University of Cambridge, UK.
FEBS Lett. 1997 Jul 28;412(2):397-403. doi: 10.1016/s0014-5793(97)00809-0.
The development of neuro-degenerative diseases often involves amyloidosis, that is the formation of polymeric fibrillar structures from normal cellular proteins or peptides. For example, in Alzheimer's disease, a 42 amino acid peptide processed from the amyloid precursor protein forms filaments with a beta-sheet structure. Because of this, the structure and dynamics of polymeric peptide filaments is of considerable interest. We showed previously that a 23 amino acid peptide constituting a single leucine-rich repeat (LRRN) polymerises spontaneously in solution to form long filaments of a beta-sheet structure, a property similar to that of Alzheimer's beta-amyloid and prion peptides. Here we report that a variant of LRRN in which a highly conserved asparagine residue is replaced by aspartic acid does not form either filaments or beta structure. By contrast, a variant which replaces this asparagine residue with glutamine forms filaments ultrastructurally indistinguishable from those of LRRN. Electron micrographs of LRRN filaments show that many consist of two interleaved strands which appear to have a ribbon-like morphology. X-ray diffraction patterns from oriented LRRN fibres reveal that they are composed of long beta-sheet arrays, with the interstrand hydrogen bonding parallel to the filament axis. This 'cross-beta' structure is similar to that adopted by beta-amyloid and prion derived fibres. Taken together, these results indicate that the LRR filaments are stabilised by inter- or intra-strand hydrogen bonded interactions comparable to the asparagine ladders of beta-helix proteins or the 'glutamine zippers' of poly-glutamine peptides. We propose that similar stabilising interactions may underlie a number of characterised predispositions to neuro-degenerative diseases that are caused by mutations to amide residues. Our finding that amyloid-like filaments can form from a peptide motif not at present correlated with degenerative disease suggests that a propensity for beta-filament formation is a common feature of protein sub-domains.
神经退行性疾病的发展通常涉及淀粉样变性，即由正常细胞蛋白或肽形成聚合纤维状结构。例如，在阿尔茨海默病中，由淀粉样前体蛋白加工而成的一种42个氨基酸的肽形成具有β-折叠结构的细丝。因此，聚合肽细丝的结构和动力学备受关注。我们之前表明，构成单个富含亮氨酸重复序列（LRRN）的一种23个氨基酸的肽在溶液中自发聚合形成β-折叠结构的长细丝，这一特性与阿尔茨海默病的β-淀粉样蛋白和朊病毒肽相似。在此我们报告，LRRN的一个变体，其中一个高度保守的天冬酰胺残基被天冬氨酸取代，既不形成细丝也不形成β-结构。相比之下，将这个天冬酰胺残基替换为谷氨酰胺的一个变体形成的细丝在超微结构上与LRRN的细丝无法区分。LRRN细丝的电子显微镜照片显示，许多细丝由两条交织的链组成，看起来具有带状形态。来自取向的LRRN纤维的X射线衍射图谱表明，它们由长的β-折叠阵列组成，链间氢键平行于细丝轴。这种“交叉β”结构与β-淀粉样蛋白和朊病毒衍生纤维所采用的结构相似。综上所述，这些结果表明，LRR细丝通过链间或链内氢键相互作用得以稳定，这与β-螺旋蛋白的天冬酰胺梯子或聚谷氨酰胺肽的“谷氨酰胺拉链”相当。我们提出，类似的稳定相互作用可能是许多由酰胺残基突变引起的神经退行性疾病特征性易感性的基础。我们的发现，即淀粉样样细丝可以由目前与退行性疾病无关的肽基序形成，表明形成β-细丝的倾向是蛋白质亚结构域的一个共同特征。
