Specific cholesterol binding drives drastic structural alterations in Apolipoprotein A1

Abstract

Aberrant regulation of the cholesterol transport leads to several chronic metabolic disorders. Apolipoprotein A1 (ApoA1) is a key player involved in removing excess cholesterol from the cells, and is also a major constituent of high-density lipoprotein (HDL) that regulates lipid metabolism. While mechanisms governing HDL formation have been the focus of several investigations, structural properties of monomeric ApoA1 are poorly understood. Here, we report cholesterolfree and bound states of monomeric ApoA1 using µs-long (>50 µs in total) atomistic simulations in explicit solvent. Our results indicate that lipid-free ApoA1 exhibits spatial proximity between N-and Cterminal domains, resulting in a closed conformation. However, upon cholesterol addition, ApoA1 spontaneously forms open circular topology. Remarkably, these drastic structural perturbations are driven by specific binding sites and, high C-terminal mobility. Simulations reveal two distinct cholesterol binding sites, one at the C-terminal and a novel cholesterol binding site at the N-terminal. Furthermore, low concentration of cholesterol, leading to less N-terminal binding does not manifest in open topology, highlighting the importance of the Nterminal binding site. A population density map generated from tens of simulations with different starting configurations revealed the existence of distinct populated states, indicating stable protein regions and an obligatory intermediate with distinct N-terminal helix pairs (H1-H7;H4-H7) participating in the opening process. Collectively, this study suggests a previously unknown mechanism for sequestering of cholesterol by ApoA1 that explains its functional diversity.

Structure dynamics of ApoA-I amyloidogenic variants in small HDL increase their ability to mediate cholesterol efflux

“…Lyophilized POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, Avanti Polar Lipids) and cholesterol (FC) (Avanti Polar Lipids) were dissolved in 3:1 chloroform:methanol, and the solvent was evaporated by overnight incubation under a stream of nitrogen gas. POPC and FC were dissolved in PBS, and lipoparticles were generated by using the cholate dialysis method 13 . Details about the experimental procedure can be found in the Supplementary Information file.…”

“…These mutations are localized to two major regions of the ApoA-I structure, either within residues 25 to 75 in the N-terminal domain or within residues 170 to 178 in the central domain of ApoA-I. The reason for the high occurrence of amyloid-prone variants in these two regions is not known, but this might indicate that these regions have specific functions in lipid-association and/or in protein structure dynamics 13 . Interestingly, carriers of several ApoA-I variants, including Gly26Arg 14 , Leu75Pro 15,16 , and Leu174Ser 17,18 , have decreased blood levels of ApoA-I, yet do not have increased risk of CVD.…”