In hexameric β2m, which shows how β2m can oligomerise and aggregate, Trp60 establishes stacking interactions with Phe56. Therefore, every β2m mutant missing an aromatic residue at position 60 would lack the stabilising π–π stacking interaction during the conversion from the native fold to the amyloid aggregate. Such consideration is in keeping with the experimental results: both Trp and Phe at position 60 can successfully establish π–π stacking interactions, and indeed w.t. β2m and W60F readily aggregate into amyloid fibrils [8]. When Trp60 is substituted with a
apexbio non-aromatic residue (i.e. Val or Cys), van der Waals interactions can somehow stabilise the intermediate, but experimentally the tendency to amyloid aggregation clearly decreases [9]. The Trp60 → Gly mutation, instead, not only abolishes all hydrophobic interactions with Phe56 but also leaves an empty volume, which likely destabilises β2m intermolecular interactions observed in the hexamer. These evidences point to a crucial role that Trp60, together with Phe56 and the other neighbouring aromatic residues (residues Phe62, Tyr66, Tyr67, Phe70), may play as promoters of the transition from a native, or grossly native, fold to amyloid aggregate. This interpretation is supported not only by the experimentally determined tendency to β-aggregation at
pH 7.4 (w.t. β2m and W60F > W60V/C > W60G), but also by previous reports on hexameric β2m, and by mutational analysis of β2m aromatic residues in the E region [6] and [23] The W60V mutant and w.t. β2m show the same folding stability but the W60V mutant has lower tendency to amyloid aggregation at pH 7.4, suggesting that the differences in aggregation behaviour described here and elsewhere [7] and [9] are not (only) due to an increase/decrease in protein stability.
