Amyloid-induced aggregation and precipitation of soluble proteins: an electrostatic contribution of the Alzheimer's b (25-35) amyloid fibril
Biochemistry (American Chemical Society) 40(7): 48-54
Amyloid-induced aggregation and precipitation of soluble proteins were investigated in vitro using the amyloid fibrils of the b(25-35) peptide, a cytotoxic fragment of the Alzheimer's b-peptide at positions 25-35. The aggregation rate of firefly luciferase was found to be modulated by both a chaperone molecule DnaK and the b(25-35) amyloid, but their effects were opposite in direction. The amyloid fibril drastically facilitated the luciferase aggregation, which may define a kind of anti-chaperone activity. The effect of the b(25-35) amyloid to promote protein aggregation and precipitation was further demonstrated for a wide variety of target proteins. The amount of coprecipitation was well correlated with the predicted isoelectric point of the target proteins, indicating that the interaction between the b(25-35) amyloid and the target was driven by an electrostatic force between them. This view was confirmed by the experiments using an electrically neutral mutant peptide, b(25-35)KA. It was also found that clustering of the b(25-35) peptide to form amyloid and the conformation of the target protein are additional factors that determine the strength of the amyloid--protein interaction. Spectroscopic and electron microscopic methods have revealed that the proteins coprecipitated with the b(25-35) amyloid formed amorphous aggregates deposited together with the amyloid fibrils. The conformation of protein molecules left in the residual soluble fraction was also damaged in the amyloid-containing solution. As a summary, this study has proposed a scheme for events related to the nonspecific amyloid--protein interaction, which may play substantial roles in in vivo conditions. Reprinted by permission of the publisher.