A covalent homodimer probing early oligomers along amyloid aggregation

Early oligomers are crucial in amyloid aggregation; however, due to their transient nature they are among the least structurally characterized species. We focused on the amyloidogenic protein beta2-microglobulin (beta2m) whose early oligomers are still a matter of debate. An intermolecular interaction between D strands of facing beta2m molecules was repeatedly observed, suggesting that such interface may be relevant for beta2m dimerization. In this study, by mutating Ser33 to Cys, and assembling the disulphide-stabilized beta2m homodimer (DimC33), such DD strand interface was locked. Although the isolated DimC33 display a stability similar to wt beta2m under native conditions, it shows enhanced amyloid aggregation propensity. Three distinct crystal structures of DimC33 suggest that dimerization through the DD interface is instrumental for enhancing DimC33 aggregation propensity. Furthermore, the crystal structure of DimC33 in complex with the amyloid-specific dye Thioflavin-T pinpoints a second interface, which likely participates in the first steps of beta2m aggregation. The present data provide new insight into beta2m early steps of amyloid aggregation.