Oligomeric assembly of native-like precursors precedes amyloid formation by β-2 microglobulin

The deposition of beta-2-microglobulin (beta2m) as amyloid fibers results in debilitating complications for renal failure patients who are treated by hemodialysis. In vitro, wild-type beta2m can be converted to amyloid under physiological conditions by exposure to biomedically relevant concentrations of Cu2+. in this work, we have made comparative measurements of the structural and oligomeric changes in beta2m at time points preceding fibrillogenesis. Our results show Cu2+ mediates the formation of a monomeric, activated state followed by the formation of a discrete dimeric intermediate. The dimeric intermediates then assemble into tetra- and hexameric forms which display little additional oligomerization on the time scales of their own formation (< 1 h). Amyloid fiber formation progresses from these intermediate states but on much longer time scales (> 1 week). Although Cu2+ is necessary for the generation and stabilization of these intermediates, it is not required for the stability of mature amyloid fibers. This suggests that Cu2+ acts as an initiating factor of amyloidosis by inducing oligomer formation. H-1 NMR and near-UV circular dichroism are used to establish that oligomeric intermediates are native-like in structure. The native-like structure and discrete oligomeric size of beta2m amyloid intermediates suggest that this protein forms fibrils by structural domain swapping.