A population shift between sparsely populated folding intermediates determines amyloidogenicity

The balance between protein folding and misfolding is a crucial determinant of amyloid assembly. Transient intermediates that are sparsely populated during protein folding have been identified as key players in amyloid aggregation. However, due to their ephemeral nature, structural characterization of these species remains challenging. Here, using the power of non-uniformly sampled NMR methods we investigate the folding pathway of amyloidogenic and non-amyloidogenic variants of ?2-microglobulin (?2m) in atomic detail. Despite folding via common intermediate states, we show that the decreased population of the ITrans state and population of a less stable, more dynamic species ablates amyloid formation by increasing the energy barrier for amyloid assembly. The results show that subtle changes in conformational dynamics can have a dramatic effect in determining whether a protein is amyloidogenic, without perturbation of the mechanism of protein folding.