Domain Structure and Lipid Interaction in Human Apolipoproteins A-I and E, a General Model

Abstract

Detailed structural information on human exchangeable apolipoproteins (apo) is required to understand their functions in lipid transport. Using a series of deletion mutants that progressively lacked different regions along the molecule, we probed the structural organization of lipid-free human apoA-I and the role of different domains in lipid binding, making comparisons to apoE, which is a member of the same gene family and known to have two structural domains. Measurements of ␣-helix content by CD in conjunction with tryptophan and 8-anilino-1-naphthalenesulfonic acid fluorescence data demonstrated that deletion of the amino-terminal or central regions disrupts the tertiary organization, whereas deletion of the carboxyl terminus has no effect on stability and induces a more cooperative structure. These data are consistent with the lipid-free apoA-I molecule being organized into two structural domains similar to apoE; the amino-terminal and central parts form a helix bundle, whereas the carboxyl-terminal ␣-helices form a separate, less organized structure. The binding of the apoA-I variants to lipid emulsions is modulated by reorganization of the helix bundle structure, because the rate of release of heat on binding is inversely correlated with the stability of the helix bundle. Based on these observations, we propose that there is a two-step mechanism for lipid binding of apoA-I: apoA-I initially binds to a lipid surface through amphipathic ␣-helices in the carboxyl-terminal domain, followed by opening of the helix bundle in the amino-terminal domain. Because apoE behaves similarly, this mechanism is probably a general feature for lipid interaction of other exchangeable apolipoproteins, such as apoA-IV.

Effects of Apolipoprotein A-I on ATP-binding Cassette Transporter A1-mediated Efflux of Macrophage Phospholipid and Cholesterol

“…Thus, increasing the FC content decreases the affinity of apoA-I binding to PL particles (60), and removal of the C-terminal ␣-helix of apoA-I does the same thing (61). This finding of a link between apolipoprotein/lipid affinity and nascent HDL particle size implies that apoA-I/ABCA1 interaction (54) does not have a direct effect on the size of the HDL particles formed.…”

ApoA1 and ApoA1-specific self-antibodies in cardiovascular disease

“…In the lipid particle, the N-terminal helix ApoA1 bundle is then subjected to conformational opening thereby scheduling from hydrophobic helix-helix interactions to helix-lipid interactions. 33 Decrease in helix bundle stability facilitates opening and increases interaction with lipids. 34 There is no unique mechanism of ApoA1 helix bundle opening within the lipid particle.…”

Conformational Adaptation of Apolipoprotein A-I to Discretely Sized Phospholipid Complexes

“…The structure of the N-and C-terminal regions have been studied extensively and shown to be mainly non-helical in the lipid-free state but take on some alpha-helical character upon lipid binding (17)(18)(19)(20)(21). Although the N-and C-terminal domains both appear to be involved in maintaining the stability of the lipid-free conformation and binding to lipids, these domains also appear to have distinct functions (5,22,23).Functionally, the C-terminus has been implicated in the initiation of binding to ABCA1 and/ or lipid surfaces (18,24), while the N-terminus has been linked to LCAT activation and HDL maturation (25). Studies show that the Δ43 mutant form of apoA-I exhibits similar lipid binding ability to full length apoA-I (21).…”

“…The structure of the N-and C-terminal regions have been studied extensively and shown to be mainly non-helical in the lipid-free state but take on some alpha-helical character upon lipid binding (17)(18)(19)(20)(21). Although the N-and C-terminal domains both appear to be involved in maintaining the stability of the lipid-free conformation and binding to lipids, these domains also appear to have distinct functions (5,22,23).…”