Apolipoprotein A-I (apoA-I) is known to be anti-atherogenic as it clears cholesterol and phospholipids from peripheral tissues to form high-density lipoprotein (HDL). Structurally, apoA-I is highly dynamic, making it well-adapted to bind various amounts of lipid molecules. However, this feature also contributes to its amyloidogenic propensity in atherosclerosis and hereditary amyloidosis. The majority of apoA-I exists in lipid-bound forms, known as discoidal and spherical HDL. Nevertheless, there is also evidence suggesting the existence of lipid-free and lipid-poor forms. In order to better understand the less characterised conformations of apoA-I, we used apoA-I interacting with various lipids at submicellar concentrations (ie. lipid monomers) as an experimental model to mimic and study the lipid-poor form. We aim to characterise this interaction using a range of biophysical techniques, as well as to determine the effects of lipid monomers and C-terminal truncation on fibril formation of methionine-oxidised (MetO) apoA-I, which is amyloidogenic itself.
The data from analytical ultracentrifugation (AUC) revealed that a vast majority of lipids stabilised a smaller, more compact form of apoA-I. Interestingly, all submicellar lipids decreased the thermostability of apoA-I based on the DSF data. ThT assay showed that some lipids completely abolished, while some only delayed fibrillogenesis of MetO apoA-I. This may reflect different lipids induce different conformations of apoA-I. Comparing to the WT, the C-terminal truncated apoA-I (apoA-I 1-184) was found to be monomeric and more thermodynamically stable. Accordingly, MetO apoA-I 1-184 formed fibrils at a slower rate than the WT.
In conclusion, we have shown that lipid monomers induce conformations of apoA-I which differ in self-association states, thermostability and fibrillogenic propensity comparing to the lipid-free apoA-I. More studies are required to structurally distinguish the difference between lipid-poor apoA-I and apoA-I with higher level of lipidation, as well as the functional implications such difference.