Aluminum-Induced Structural Alterations of the Precursor of the Non-Aβ Component of Alzheimerʹs Disease Amyloid

The precursor of the non-Aβ component of Alzheimerʹs disease amyloid (NACP) is a presynaptic protein whose function has been suspected to be tightly involved in neuronal biogenesis including synaptic regulations. NACP was suggested to seed the neuritic plaque formation in the presence of Aβ during the development of Alzheimerʹs disease (AD). Recombinant NACP purified through heat treatment, DEAE-Sephacel anion-exchange, Sephacryl S-200 size-exclusion, and S-Sepharose cation-exchange chromatography steps appeared as a single band on SDS–PAGE withMrof 19 kDa. Its N-terminal amino acid sequence clearly confirmed that the protein was NACP. Interestingly, however, the protein was split into a doublet on a nondenaturing (ND)–PAGE with equal intensities. The doublet was located slightly above a 45-kDa marker protein on a 12.5% ND–PAGE. In addition, the size of NACP was more carefully estimated as 53 kDa with high-performance gel-permeation chromatography using a TSK G3000sw size-exclusion column. Recently, Lansbury and his colleagues (Biochemisty35, 13709–13715) have reported that NACP exists as an elongated “natively unfolded” structure which would make the protein more actively involved in protein–protein interactions and Kim (Mol. Cells7, 78–83) has also shown that the natively unfolded protein is extremely sensitive to proteases. Here, we report that the structure of NACP could be altered by certain environmental factors. Aluminum, a suspected risk factor for AD, converged the doublet of NACP into a singlet with slightly lower mobility on ND–PAGE. Spectroscopic analysis employing uv absorption, intrinsic fluorescence, and circular dichroism indicated that NACP experienced the structural alterations in the presence of aluminum such as the secondary structure transition to generate about 33% α-helix. This altered structure of NACP became resistant to proteases such as trypsin, α-chymotrypsin, and calpain. Therefore, it is suggested that aluminum, which influences two pathologically critical processes in AD such as the protein turnover and the protein aggregation via the structural modificaitons, could participate in the disease.