Influence of N-terminal helix bundle stability on the lipid-binding properties of human apolipoprotein A-I

Abstract

As the principal component of high-density lipoprotein (HDL), apolipoprotein (apo) A-I plays essential roles in lipid transport and metabolism. Because of its intrinsic conformational plasticity and flexibility, the molecular details of the tertiary structure of lipid-free apoA-I have not been fully elucidated. Previously, we demonstrated that the stability of the N-terminal helix bundle structure is modulated by proline substitution at the most hydrophobic region (residues around Y18) in the N-terminal domain. Here we examine the effect of proline substitution at S55 located in another relatively hydrophobic region compared to most of the helix bundle domain to elucidate the influences on the helix bundle structure and lipid interaction. Fluorescence measurements revealed that the S55P mutation had a modest effect on the stability of the bundle structure, indicating that residues around S55 are not pivotally involved in the helix bundle formation, in contrast to the insertion of proline at position 18. Although truncation of the C-terminal domain (Δ190-243) diminishes the lipid binding of apoA-I molecule, the mutation S55P in addition to the C-terminal truncation (S55P/Δ190-243) restored the lipid binding, suggesting that the S55P mutation causes a partial unfolding of the helix bundle to facilitate lipid binding. Furthermore, additional proline substitution at Y18 (Y18P/S55P/Δ190-243), which leads to a drastic unfolding of the helix bundle structure, yielded a greater lipid binding ability. Thus, proline substitutions in the N-terminal domain of apoA-I that destabilized the helix bundle promoted lipid solubilization. These results suggest that not only the hydrophobic C-terminal helical domain but also the stability of the N-terminal helix bundle in apoA-I are important modulators of the spontaneous solubilization of membrane lipids by apoA-I, a process that leads to the generation of nascent HDL particles.Keywords amphipathic α-helix; apolipoprotein A-I; helix bundle structure; lipid binding; proline substitution

Binding of human apoA-I[K107del] variant to TG-rich particles: implications for mechanisms underlying hypertriglyceridemia

“…Exactly these properties (lower stability and loosely folded conformation with greater exposure of hydrophobic surfaces) were shown for the apoA-I[K107del] variant by the physicochemical analysis, supporting the concept of disruption of the four-segment bundle structure. The integrity of the four-segment bundle of apoA-I was reported to be crucial for the protein overall structure and function ( 43,45,46 ). Therefore, we surmised that the stabilizing salt bridge network involving K107 might be essential for maintaining the apoA-I conformation and regions and an extended section, one at each end of the dimer.…”

ApoA1 and ApoA1-specific self-antibodies in cardiovascular disease

“…As mentioned above, anti-ApoA1 autoantibodies belong to G class and possess pro-inflammatory properties. 94 A first evidence highlighting a possible link between ApoA1 IgG antibodies titers and atherosclerosis was obtained by Delgado Alves et al 95 who observed a reverse correlation between anti-HDL IgG and paraoxonase-1 (PON-1) activity (an enzyme responsible for antioxidant properties of HDL), and with the total antioxidant capacitance of the sera of SLE patients with atherosclerosis. In fact, this correlation can be explained by the involvement of anti-ApoA1 antibodies in the impairment of HDL function.…”

New insights into the determination of HDL structure by apolipoproteins

“…3A ) is well adapted to open readily and expose hydrophobic surfaces for interaction with PL-water surface. Reductions in helix bundle stability promote opening and enhance lipid binding ( 20,59,60 ). The precise pathway by which the helix bundle opens in apoA-I and other helix bundleforming apolipoproteins has been the subject of extensive investigation.…”