A Simulated Intermediate State for Folding and Aggregation Provides Insights into ΔN6 β₂-Microglobulin Amyloidogenic Behavior

A major component of ex vivo amyloid plaques of patients with dialysis-related amyloidosis (DRA) is a cleaved variant of β2-microglobulin (ΔN6) lacking the first six N-terminal residues. Here we perform a computational study on ΔN6, which provides clues to understand the amyloidogenicity of the full-length β2-microglobulin. Contrary to the wild-type form, ΔN6 is able to efficiently nucleate fibrillogenesis in vitro at physiological pH. This behavior is enhanced by a mild acidification of the medium such as that occurring in the synovial fluid of DRA patients. Results reported in this work, based on molecular simulations, indicate that deletion of the N-terminal hexapeptide triggers the formation of an intermediate state for folding and aggregation with an unstructured strand A and a native-like core. Strand A plays a pivotal role in aggregation by acting as a sticky hook in dimer assembly. This study further predicts that the detachment of strand A from the core is maximized at pH 6.2 resulting into higher aggregation efficiency. The structural mapping of the dimerization interface suggests that Tyr10, His13, Phe30 and His84 are hot-spot residues in ΔN6 amyloidogenesis.透析相关淀粉样变性（dialysis-related amyloidosis, DRA）患者离体淀粉样斑块的主要组成成分，为缺失前6个N端残基的β2微球蛋白（β2-microglobulin）截短变体ΔN6。本研究针对ΔN6开展计算研究，为解析全长β2微球蛋白的淀粉样生成能力提供了重要线索。与野生型形式不同，ΔN6可在生理pH条件下高效成核启动纤维形成。该聚集行为可因培养基轻度酸化（如DRA患者滑液中发生的酸化过程）而得到增强。本研究基于分子模拟得到的结果显示，N端六肽的缺失会触发折叠与聚集中间态的形成，该中间态包含无序的A链以及类天然核心结构。A链在聚集过程中发挥关键作用，可作为粘性挂钩参与二聚体组装。本研究进一步预测，当pH为6.2时，A链从核心结构上的脱离程度达到峰值，进而使聚集效率进一步提升。对二聚化界面的结构分析表明，Tyr10、His13、Phe30及His84是ΔN6淀粉样生成过程中的热点残基。