Apolipoprotein E is a plasma protein that serves as a ligand for low density lipoprotein receptors and, through its interaction with these receptors, participates in the transport of cholesterol and other lipids among various cells of the body A mutant form of apolipoprotein E that is defective in binding to low density lipoprotein receptors is associated with familial type III hyperlipoproteinemia, a genetic disorder characterized by elevated plasma cholesterol levels and accelerated coronary artery disease Apolipoprotein E is synthesized in various organs, including liver, brain, spleen, and kidney, and is present in high concentrations in interstitial fluid, where it appears to participate in cholesterol redistribution from cells with excess cholesterol to those requiring cholesterol Apolipo-protein E also appears to be involved in the repair response to tissue injury; for example, markedly increased amounts of apolipoprotein E are found at sites of peripheral nerve injury and regeneration Other functions of apolipoprotein E, unrelated to lipid transport, are becoming known, including immunoregulation and modulation of cell growth and differentiation

https://science.sciencemag.org/content/sci/240/4852/622.full.pdf

Apolipoprotein E: cholesterol transport protein with expanding role in cell biology.

Molecular physiology of reverse cholesterol transport.

High density lipoprotein metabolism.

Plasma lipoproteins: apolipoprotein structure and function.

Publisher Summary This chapter focuses on the role of apoE in lipoprotein metabolism and the most completely described function of the protein. Apolipoprotein E (apoE) is one of the best characterized in terms of its structural and functional properties. Plasma apolipoproteins regulate lipoprotein metabolism and control the transport and redistribution of lipids among tissues and cells. Apolipoproteins can perform one of three major roles because of their ability to bind lipid. First, apolipoproteins stabilize the pseudomicellar structure of lipoprotein particles. Second, apolipoproteins can act as cofactors or activators of various enzymes or lipid transfer proteins that participate in the metabolism of “remodeling” of lipoproteins as they circulate in plasma. A third function of plasma apolipoproteins-one that is restricted to apoB100 and ApoE- is to serve as a ligand for cell surface lipoprotein receptors. The three-dimensional structure of a 22-kDa fragment of human apoE is solved by X-ray crystallography; the relation of this structure to the role of apoE in lipoprotein metabolism is discussed, together with a critical and extensive examination of the chemistry and biology of this apolipoprotein, which plays such a central role in lipoprotein metabolism. Apolipoprotein E has three major isoform in the human population, which affect lipoprotein metabolism differently, resulting in different levels of the plasma lipoproteins. The role of apoE as a cofactor in lipolytic processing of triglyceride-rich lipoproteins, and the role of apoE in the reverse cholesterol transport process are described.

Apolipoprotein E: Structure-Function Relationships

Synthesis, intracellular processing, and signal peptide of human apolipoprotein E.

We have recently proposed that the major secreted isoprotein form of human apolipoprotein A-I (designated apo A-I2) is modified extracellularly to become the predominant apo A-I form seen in plasma (designated apo A-I4). In the current report we demonstrate that the primary translation product of human apo A-I (designated apo A-I2p) has a 24-amino-acid NH2-terminal extension with a sequence of Met-Lys-Ala-Ala-Val-Leu-Thr-Leu-Ala-Val-Leu-Phe- Leu-Thr-Gly-Ser-Gln-Ala-Arg-His-Phe-Trp-Gln-Gln. The first 18 amino acids of this NH2-terminal extension are cleaved intracellularly by the signal peptidase, resulting in the formation of apo A-I2, which is the secreted form of apo A-I. Sequence analysis of apo A-I2 confirmed that it contains a hexapeptide extension at its NH2 terminus compared to apo A-I4. This observation demonstrates that apo A-I2 is a propeptide and that the apo A-I2 to apo A-I4 conversion involves the removal of the NH2-terminal hexapeptide of apo A-I2 by a protease in plasma, lymph, or both. Our findings indicate that apo A-I is synthesized as a prepropeptide, which undergoes intracellular and extracellular proteolysis to attain the major plasma apo A-I4 isoprotein form.

https://www.pnas.org/content/pnas/80/9/2574.full.pdf

G Goldberger

Papers

Synthesis, intracellular processing, and signal peptide of human apolipoprotein E.

Intracellular and extracellular processing of human apolipoprotein A-I: secreted apolipoprotein A-I isoprotein 2 is a propeptide