THE N-TERMINAL REGION OF NON-A-BETA COMPONENT OF ALZHEIMERS-DISEASE AMYLOID IS RESPONSIBLE FOR ITS TENDENCY TO ASSUME BETA-SHEET AND AGGREGATE TO FORM FIBRILS

Examination of the N-terminal sequence of non-A beta component of Alzh eimer's Disease amyloid (NAC) revealed a degree of similarity to regio ns crucial for aggregation and toxicity of three other amyloidogenic p roteins, namely amyloid beta peptide (A beta), prion protein (PrP) and islet amyloid polypeptide (IAPP), leading us to believe that this mig ht be the part of the molecule responsible for causing aggregation. Se condary structure prediction analysis of NAC indicated that the N-term inal half was likely to form a beta-structure whereas the C-terminal h alf was likely to form an alpha-helix. NAC in solution altered from ra ndom coil to beta-sheet structure upon ageing, a process that has prev iously been shown to lead to fibril formation. To delineate the region of NAC responsible for aggregation we synthesised two fragments, NAC- (1-18)-peptide and NAC-(19-35)-peptide, and examined their physicochem ical properties. Upon incubation, solutions of NAC-(1-18)-peptide beca me congophilic and aggregated to form fibrils of diameter 5-10 nm, whe reas NAC-(19-35)-peptide did not bind Congo Red and remained in soluti on. Circular dichroism spectroscopy was used to study the secondary st ructure of NAC and the two fragments. In trifluaroethanol/water mixtur es, NAC and NAC-(19-35)-peptide adopted a-helical structure but NAC(1- 18)-peptide did not. NAC-(1-18)-peptide and NAC formed beta-sheet in a cetonitrile/water mixtures more readily than did NAC-(19-35)-peptide. CD spectra of NAC or NAC-(1-18)-peptide in aqueous solution indicate t he formation of beta-sheet on ageing. We propose that the N-terminal r egion of NAC is the principal determinant of aggregation. Our results indicate that NAC resembles A beta, and other amyloidogenic proteins, in that aggregation is dependent upon beta-sheet development. These re sults lend support to a role for NAC in the development of neurodegene rative disease.