Conformational analysis of apolipoprotein E3/E4 heteromerization

• Published in: The FEBS journal Vol. 286; no. 10; pp. 1986 - 1998
• Main Authors: Tu, Kai‐Han, Abhari, Devan, Narayanaswami, Vasanthy
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.05.2019
• Subjects: Alleles; Alzheimer's disease; apolipoprotein E3; Apolipoprotein E3 - chemistry; Apolipoprotein E3 - genetics; Apolipoprotein E3 - metabolism; Apolipoprotein E4; Apolipoprotein E4 - chemistry; Apolipoprotein E4 - genetics; Apolipoprotein E4 - metabolism; Apolipoproteins; Cholesterol; Circular Dichroism; Conformational analysis; Energy transfer; Fluorescence; Fluorescence Resonance Energy Transfer; FRET; heteromerization; Heterozygotes; Homeostasis; Humans; Immunoblotting; Isoforms; Lipids; lipoproteins; Neurodegenerative diseases; Phosphatidylcholines - chemistry; Population genetics; Protein Conformation; Protein Denaturation; Protein Interaction Domains and Motifs; Protein Multimerization; Proteins; apolipoprotein E3; FRET; heteromerization; apolipoprotein E4; lipoproteins
• ISSN: 1742-464X; 1742-4658
• DOI: 10.1111/febs.14794

Apolipoprotein E (apoE) is a 299 residue, exchangeable apolipoprotein that has essential roles in cholesterol homeostasis and reverse cholesterol transport. It is a two‐domain protein with the C‐terminal (CT) domain mediating protein self‐association via helix‐helix interactions. In humans, the APOE gene is polymorphic with three common alleles, ε2, ε3, and ε4, occurring in frequencies of ~ 5%, 77%, and 18%, respectively. Heterozygotes expressing apoE3 and apoE4 isoforms, which differ in residue at position 112 in the N‐terminal domain (C112 in apoE3 and R112 in apoE4), represent the highest population of ε4 carriers, an allele highly associated with Alzheimer's disease. The objective of this study was to determine if apoE3 and apoE4 have the ability to hybridize to form a heteromer in lipid‐free state. Refolding an equimolar mixture of His‐apoE3 and FLAG‐apoE4 (or vice versa) followed by pull‐down and immunoblotting indicated formation of apoE3/apoE4 heteromers. Förster resonance energy transfer between donor fluorophore on one isoform and acceptor on the other, both located in the respective CT domains, revealed a distance of separation of ~ 46 Å between the donor/acceptor pair. Similarly, a quencher placed on one was able to mediate significant quenching of fluorescence emission on the other, indicative of spatial proximity within collisional distance between the two. ApoE3/apoE4 heteromer association was also noted in lipid‐associated state in reconstituted lipoprotein particles. The possibility of heteromerization of apoE3/apoE4 bears implications in the potential mitigating role of apoE3 on the folding and physiological behavior of apoE4 and its role in maintaining cholesterol homeostasis. In humans, the APOE gene is polymorphic. Heterozygotes expressing apoE3 and apoE4 isoforms represent the largest population of ε4 carriers, an allele highly associated with Alzheimer's disease. Using fluorescence resonance energy transfer, quenching and pull‐down assays, we demonstrate that apoE3 and apoE4 can heteromerize via protein–protein interactions. This possibility bears implications for the potential mitigating role of apoE3 on the pathological behavior of apoE4.