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 (\u3b22m) whose early oligomers are still a matter of debate. An intermolecular interaction between D strands of facing \u3b22m molecules was repeatedly observed, suggesting that such interface may be relevant for \u3b22m dimerization. In this study, by mutating Ser33 to Cys, and assembling the disulphide-stabilized \u3b22m homodimer (DimC33), such DD strand interface was locked. Although the isolated DimC33 display a stability similar to wt \u3b22m 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 \u3b22m aggregation. The present data provide new insight into \u3b22m early steps of amyloid aggregation