Amyloid fibrillization is an exceedingly complex process in which incoming peptide chains bind to the fibril while concertedly folding. The coupling between folding and binding is not fully understood. We explore the molecular pathways of association of Aβ40 monomers to fibril tips by combining time-resolved in situ scanning probe microscopy with molecular modeling. The comparison between experimental and simulation results shows that a complex supported by nonnative contacts is present in the equilibrium structure of the fibril tip and impedes fibril growth in a supersaturated solution. The unraveling of this frustrated state determines the rate of fibril growth. The kinetics of growth of freshly cut fibrils, in which the bulk fibril structure persists at the tip, complemented by molecular simulations, indicate that this frustrated complex comprises three or four monomers in nonnative conformations and likely is contained on the top of a single stack of peptide chains in the fibril structure. This pathway of fibril growth strongly deviates from the common view that the conformational transformation of each captured peptide chain is templated by the previously arrived peptide. The insights into the ensemble structure of the frustrated complex may guide the search for suppressors of Aβ fibrillization. The uncovered dynamics of coupled structuring and assembly during fibril growth are more complex than during the folding of most globular proteins, as they involve the collective motions of several peptide chains that are not guided by a funneled energy landscape.

淀粉样蛋白原纤维化是一个极其复杂的过程，其中进入的肽链与原纤维结合，同时协同折叠。折叠和绑定之间的耦合尚未完全理解。我们通过将时间分辨原位扫描探针显微镜与分子建模相结合，探索 Aβ40 单体与原纤维尖端缔合的分子途径。实验和模拟结果的比较表明，在原纤维尖端的平衡结构中存在由非天然接触支持的复合物，并在过饱和溶液中阻碍原纤维的生长。这种沮丧状态的解体决定了原纤维的生长速度。新鲜切割的原纤维的生长动力学，其中大块原纤维结构在尖端持续存在，辅以分子模拟，表明这种受挫的复合物包含三个或四个非天然构象的单体，并且可能包含在原纤维结构中单叠肽链的顶部。这种原纤维生长途径强烈偏离了每个捕获的肽链的构象转化以先前到达的肽为模板的普遍观点。对受挫复合体整体结构的深入了解可能会指导寻找 Aβ 原纤维化抑制因子。在原纤维生长过程中发现的耦合结构和组装动力学比大多数球状蛋白折叠过程中更复杂，因为它们涉及几个肽链的集体运动，这些运动不受漏斗能量景观的引导。