On the Aggregation of Apolipoprotein A-I

In vivo, apolipoprotein A-I (ApoA-I) is commonly found together with lipids in so-called lipoprotein particles. The protein has also been associated with several diseases—such as atherosclerosis and amyloidosis—where insoluble aggregates containing ApoA-I are deposited in various organs or arteries. The deposited ApoA-I has been found in the form of amyloid fibrils, suggesting that amyloid formation may be involved in the development of these diseases. In the present study we investigated ApoA-I aggregation into amyloid fibrils and other aggregate morphologies. We studied the aggregation of wildtype ApoA-I as well as a disease-associated mutant, ApoA-I K107Δ, under different solution conditions. The aggregation was followed using thioflavin T fluorescence intensity. For selected samples the aggregates formed were characterized in terms of size, secondary structure content, and morphology using circular dichroism spectroscopy, dynamic light scattering, atomic force microscopy and cryo transmission electron microscopy. We find that ApoA-I may form globular protein-only condensates, in which the α-helical conformation of the protein is retained. The protein in its unmodified form appears resistant to amyloid formation; however, the conversion into amyloid fibrils rich in β-sheet is facilitated by oxidation or mutation. In particular, the K107Δ mutant shows higher amyloid formation propensity, and the end state appears to be a co-existence of β-sheet rich amyloid fibrils and α-helix-rich condensates.