Fezoui Youcef, Teplow David B
Department of Neurology, Harvard Medical School, Boston, Massachusetts 02115, USA.
J Biol Chem. 2002 Oct 4;277(40):36948-54. doi: 10.1074/jbc.M204168200. Epub 2002 Jul 30.
Amyloid beta-protein (Abeta) fibril assembly is a defining characteristic of Alzheimer's disease. Fibril formation is a complex nucleation-dependent polymerization process characterized in vitro by an initial lag phase. To a significant degree, this phase is a consequence of the energy barrier that must be overcome in order for Abeta monomers to fold and oligomerize into fibril nuclei. Here we show that low concentrations of 2,2,2-trifluoroethanol (TFE) convert predominately unstructured Abeta monomers into partially ordered, quasistable conformers. Surprisingly, this results in a temporal decrease in the lag phase for fibril formation and a significant increase in the rate of fibril elongation. The TFE effect is concentration dependent and is maximal at approximately 20% (v/v). In the presence of low concentrations of TFE, fibril formation is observed in Abeta samples at nanomolar concentration, well below the critical concentration for Abeta fibril formation in the absence of TFE. As the amount of TFE is increased above 20%, helix content progressively rises to approximately 80%, a change paralleled first by a decrease in elongation rate and then by a complete cessation of fibril growth. These findings are consistent with the hypothesis that a partially folded helix-containing conformer is an intermediate in Abeta fibril assembly. The requirement that Abeta partially folds in order to assemble into fibrils contrasts with the mechanism of amyloidogenesis of natively folded proteins such as transthyretin and lysozyme, in which partial unfolding is a prerequisite. Our results suggest that in vivo, factors that affect helix formation and stability will have significant effects on the kinetics of Abeta fibril formation.
β-淀粉样蛋白（Aβ）原纤维组装是阿尔茨海默病的一个决定性特征。原纤维形成是一个复杂的依赖成核的聚合过程，在体外其特征为有一个初始延迟期。在很大程度上，这个阶段是Aβ单体折叠并寡聚成原纤维核时必须克服的能量屏障的结果。在这里，我们表明低浓度的2,2,2-三氟乙醇（TFE）能将主要为无结构的Aβ单体转化为部分有序的准稳定构象体。令人惊讶的是，这导致原纤维形成的延迟期出现暂时缩短，且原纤维伸长速率显著增加。TFE的作用具有浓度依赖性，在约20%（v/v）时达到最大值。在低浓度TFE存在的情况下，在纳摩尔浓度的Aβ样品中就可观察到原纤维形成，这远低于无TFE时Aβ原纤维形成的临界浓度。当TFE的量增加到超过20%时，螺旋含量逐渐上升至约80%，这一变化首先伴随着伸长速率的降低，随后是原纤维生长完全停止。这些发现与以下假设一致，即含有部分折叠螺旋的构象体是Aβ原纤维组装的中间体。Aβ需要部分折叠才能组装成原纤维，这与诸如转甲状腺素蛋白和溶菌酶等天然折叠蛋白质的淀粉样变机制形成对比，在后者中部分去折叠是一个先决条件。我们的结果表明，在体内，影响螺旋形成和稳定性的因素将对Aβ原纤维形成的动力学产生显著影响。
