Conformational Reorganization of Apolipoprotein E Triggered by Phospholipid Assembly

Abstract

Apolipoprotein E (apoE), a major determinant protein for lipid metabolism, actively participates in lipid transport in the central nervous system via high-affinity interaction with the low-density lipoprotein receptor (LDLR). Prior evidences indicate that the phospholipids first need to assemble around apoE before the protein can recognize its receptor. However, despite multiple attempts via spectroscopic and biochemical investigations, it is unclear what are the impacts of lipid assembly on the globular structure of apoE. Here, using a combination of all-atom and coarse-grained molecular dynamics simulations, we demonstrate that an otherwise compact tertiary fold of monomeric apoE3 spontaneously unwraps in an aqueous phospholipid solution in two distinct stages. Interestingly, these structural reorganizations are triggered by an initial localized binding of lipid molecules to the C-terminal domain of the protein, which induce a rapid separation of the C-terminal domain of apoE3 from the rest of its tertiary fold. This is followed by a slow lipidinduced interhelix separation event within the N-terminal domain of the protein, as seen in an extensively long coarse-grained simulation. Remarkably, the resultant complex takes the shape of an "open conformation" of the lipid-stabilized unwrapped protein, which intriguingly coincides with an earlier proposal by a small-angle X-ray scattering (SAXS) experiment. The lipid-binding activity and the lipid-induced protein conformation are found to be robust across a monomeric mutant and wild-type sequence of apoE3. The "open" complex derived in coarse-grained simulation retains its structural morphology after reverse-mapping to the all-atom representation. Collectively, the investigation puts forward a plausible structure of currently elusive conformationally activated state of apoE3, which is primed for recognition by the lipoprotein receptor and can be exploited for eventual lipid transport.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

“…It has been proposed that only monomeric apoE can bind lipid vesicles and that phospholipids play a role in successful apoE-receptor interaction [ 127 , 128 ]. ApoE4 peptides present a high lipid-binding affinity related to a lower recycling capacity and increased intracellular cholesterol accumulation [ 129 , 130 ].…”

Relevance of lipoproteins, membranes, and extracellular vesicles in understanding C-reactive protein biochemical structure and biological activities

“…As apo E associates with lipids, the salt bridges holding its two domains together weaken, relaxing the tertiary structure of the apoprotein. Step 2 is a slower, reversible process involving relaxing the N-terminal domain into an open conformation with increased exposure of the LDL-R binding peptide ( 53 , 60 ). To provide perspective on the major conformational rearrangement of apo E in the absence and presence of lipids, Chen noted the distance between residues N-terminal domain residue C 112 and C-terminal domain residue W 264 (specifically for apo E3) was ∼28Å in the absence of lipid, but > 80 Å in its presence (see Figure 5 for perspective of these structural changes).…”

Apolipoprotein E4 has extensive conformational heterogeneity in lipid free and bound forms

“…This interpretation is fully compatible with the configurations identified by Henry et al 29 using crosslinking, mass spectrometry, and simulations of ApoE4, though our data suggests a more expanded configuration of the N-terminal tail (as measured by ApoE4 5,86 ) and a larger separation between the N- and C-terminal halves of the protein (as measured by ApoE4 86,241 ). Interestingly, previous simulations of ApoE3 identifies only a close configuration for helices H3 and H4, possibly suggesting a different structural organization of the two variants in their monomeric lipid-bound form 42 .…”