Conformational Dynamics of Amyloid β-Protein Assembly Probed Using Intrinsic Fluorescence

Abstract

Formation of toxic oligomeric and fibrillar structures by the amyloid β-protein (Aβ) is linked to Alzheimer's disease (AD). To facilitate the targeting and design of assembly inhibitors, intrinsic fluorescence was used to probe assembly-dependent changes in Aβ conformation. To do so, Tyr was substituted in Aβ40 or Aβ42 at position 1, 10 (wild type), 20, 30, 40, or 42. Fluorescence then was monitored periodically during peptide monomer folding and assembly. Electron microscopy revealed that all peptides assembled readily into amyloid fibrils. Conformational differences between Aβ40 and Aβ42 were observed in the central hydrophobic cluster (CHC) region, Leu17−Ala21. Tyr20 was partially quenched in unassembled Aβ40 but displayed a significant and rapid increase in intensity coincident with the maturation of an oligomeric, α-helix-containing intermediate into amyloid fibrils. This process was not observed during Aβ42 assembly, during which small decreases in fluorescence intensity were observed in the CHC. These data suggest that the structure of the CHC in Aβ42 is relatively constant within unassembled peptide and during the self-association process. Solvent accessibility of the Tyr ring was studied using a mixed solvent (dimethyl sulfoxide/water) system. [Tyr40]Aβ40, [Tyr30]Aβ42, and [Tyr42]Aβ42 all were relatively shielded from solvent. Analysis of the assembly dependence of the site-specific intrinsic fluorescence data suggests that the CHC is particularly important in controlling Aβ40 assembly, whereas the C-terminus plays the more significant role in Aβ42 assembly.