Structure of Interacting Segments in the Growing Amyloid Fibril of β2-Microglobulin Probed with IR Spectroscopy

Inter-segmental interaction at the growing tip of the amyloid fibril of β2-microglobulin (β2m) was investigated using IR microscopy. Cross-seeded fibril formation was implemented, in which the amyloid fibril of the #21-31 fragment of β2m (fA[#21-31]) was generated on the β2m amyloid fibril (fA[β2m]) as a seed. Differences between the IR spectra of the cross-seeded fibril and those of the seed were attributed to the contribution from the tip, whose structure is discussed. The results indicated that 6.5 ± 1.0 out of 11 residues of the fA[#21-31] tip on fA[β2m] are contained in a β-sheet at pH 2.5, which was smaller than the corresponding value (7.5 ± 1.1 residues) of the spontaneous fA[#21-31] at pH 2.5. The tip was suggested to have a planar structure, indicating the planarity of the interacting segment. The N-terminal region of fA[#21-31] in the fibril is more exposed to the solvent than that in the tip, and vice versa for the C-terminal region. This is consistent with the different protonation levels of these regions, and the direction of peptide in the fibrils is determined from these results.