A native to amyloidogenic transition regulated by a backbone trigger

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Abstract
Many polypeptides can self-associate into linear, aggregated assemblies termed amyloid fibers. High-resolution structural insights into the mechanism of fibrillogenesis are elusive owing to the transient and mixed oligomeric nature of assembly intermediates. Here, we report the conformational changes that initiate fiber formation by β-2-microglobulin (β2m) in dialysis-related amyloidosis. Access of β2m to amyloidogenic conformations is catalyzed by selective binding of divalent cations. The chemical basis of this process was determined to be backbone isomerization of a conserved proline. On the basis of this finding, we designed a β2m variant that closely adopts this intermediate state. The variant has kinetic, thermodynamic and catalytic properties consistent with its being a fibrillogenic intermediate of wild-type β2m. Furthermore, it is stable and folded, enabling us to unambiguously determine the initiating conformational changes for amyloid assembly at atomic resolution.