Studies of synthetic peptides of human apolipoprotein A-I containing tandem amphipathic alpha-helixes

Secondary structure of human apolipoprotein A-I(1-186) in lipid-mimetic solution

The solution structure of an apoA-I deletion mutant, apoA-I(1-186) was determined by the chemical shift index (CSI) method and the torsion angle likelihood obtained from shift and sequence similarity (TALOS) method, using heteronuclear multidimensional NMR spectra of [u-(13)C, u-(15)N, u-50% (2)H]apoA-I(1-186) in the presence of sodium dodecyl sulfate (SDS). The backbone resonances were assigned from a combination of triple-resonance data (HNCO, HNCA, HN(CO)CA, HN(CA)CO and HN(COCA)HA), and intraresidue and sequential NOEs (three-dimensional (3D) and four-dimensional (4D) 13C- and 15N-edited NOESY). Analysis of the NOEs, H(alpha), C(alpha) and C' chemical shifts shows that apoA-I(1-186) in lipid-mimetic solution is composed of alpha-helices (which include the residues 8-32, 45-64, 67-77, 83-87, 90-97, 100-140, 146-162, and 166-181), interrupted by short irregular segments. There is one relatively long, irregular and mostly flexible region (residues 33-44), that separates the N-terminal domain (residues 1-32) from the main body of protein. In addition, we report, for the first time, the structure of the N-terminal domain of apoA-I in a lipid-mimetic environment. Its structure (alpha-helix 8-32 and flexible linker 33-44) would suggest that this domain is structurally, and possibly functionally, separated from the other part of the molecule.

Identification of a sequence of apolipoprotein A-I associated with the activation of Lecithin:Cholesterol acyltransferase

We aimed to distinguish between the effects of mutations in apoA-I on the requirements for the secondary structure and a specific amino acid sequence for lecithin:cholesterol acyltransferase (LCAT) activation. Several mutants were constructed targeting region 140-150: (i) two mutations affecting alpha-helical structure, deletion of amino acids 140-150 and substitution of Ala(143) for proline; (ii) two mutations not affecting alpha-helical structure, substitution of Val(149) for arginine and substitution of amino acids 63-73 for sequence 140-150; and (iii) a mutation in a similar region away from the target area, deletion of amino acids 63-73. All mutations affecting region 140-150 resulted in a 4-42-fold reduction in LCAT activation. Three mutations, apoA-I(Delta140-150), apoA-I(P143A), and apoA-I(140-150 --> 63-73), affected both the apparent V(max) and K(m), whereas the mutation apoA-I(R149V) affected only the V(max). The mutation apoA-I(Delta63-73) caused only a 5-fold increase in the K(m). All mutants, except apoA-I(P143A) and apoA-I(Delta63-73), were active in phospholipid binding assay. All mutants, except apoA-I(P143A), formed normal discoidal complexes with phospholipid. The mutation apoA-I(Delta63-73) caused a significant reduction in the stability of apoA-I.phospholipid complexes in denaturation experiments. Combined, our results strongly suggest that although the correct conformation and orientation of apoA-I in the complex with lipids are crucial for activation of LCAT, when these conditions are fulfilled, activation also strongly depends on the sequence that includes amino acids 140-150.

Single repeat deletion in ApoA-I blocks cholesterol esterification and results in rapid catabolism of delta6 and wild-type ApoA-I in transgenic mice

The deletion mutation Delta6 apolipoprotein A-I lacks residues 143-164 or repeat 6 in the mature apoA-I protein. In vitro studies show this mutation dramatically reduces the rate of lecithin:cholesterol acyltransferase (LCAT) catalyzed cholesterol esterification. The present study was initiated to investigate the effect of this mutation on in vivo high density lipoprotein (HDL) cholesterol esterification and metabolism. Transgenic mice expressing human Delta6 apoA-I (TgDelta6 +/+) were created and then crossed with apoA-I knockout mice (-/-) to generate mice expressing only human Delta6 apoA-I (TgDelta6 -/-). Human Delta6 apoA-I was associated with homogeneous sized alpha-HDL, when wild-type mouse apoA-I was present (in TgDelta6 +/+ and +/- mice). However, in the absence of endogenous mouse apoA-I, Delta6 apoA-I was found exclusively in cholesterol ester-poor HDL, and lipid-free HDL fractions. This observation coincides with the 6-fold lower cholesterol ester mass in TgDelta6 -/- mouse plasma compared with control. Structural studies show that despite the structural perturbation of a domain extending from repeat 5 to repeat 8 (137-178), Delta6 apoA-I binds to spherical unilamellar vesicles with only 2-fold less binding affinity. In summary, these data show a domain corresponding to apoA-I repeat 6 is responsible for providing an essential conformation for LCAT catalyzed generation of cholesterol esters. Deletion of apoA-I repeat 6 not only blocks normal levels of cholesterol esterification but also exerts a dominant inhibition on the ability of wild-type apoA-I to activate LCAT in vivo.

Structure and function of apolipoprotein A-I and high-density lipoprotein

Structural biology and molecular modeling have provided intriguing insights into the atomic details of the lipid-associated structure of the major protein component of HDL, apo A-I. For the first time, an atomic resolution map is available for future studies of the molecular interactions of HDL in such biological processes as ABC1-regulated HDL assembly, LCAT activation, receptor binding, reverse lipid transport and HDL heterogeneity. Within the context of this paradigm, the current review summarizes the state of HDL research.

Distinct central amphipathic alpha-helices in apolipoprotein A-I contribute to the in vivo maturation of high density lipoprotein by either activating lecithin-cholesterol acyltransferase or binding lipids

Recombinant adenoviruses with cDNAs for human apolipoprotein A-I (wild type (wt) apoA-I) and three mutants, referred to as Delta4-5A-I, Delta5-6A-I, and Delta6-7A-I, that have deletions removing regions coding for amino acids 100-143, 122-165, and 144-186, respectively, were created to study structure/function relationships of apoA-I in vivo. All mutants were expressed at lower concentrations than wt apoA-I in plasma of fasting apoA-I-deficient mice. The Delta5-6A-I mutant was found primarily in the lipid-poor high density lipoprotein (HDL) pool and at lower concentrations than Delta4-5A-I and Delta6-7A-I that formed more buoyant HDL(2/3) particles. At an elevated adenovirus dose and earlier blood sampling from fed mice, both Delta5-6A-I and Delta6-7A-I increased HDL-free cholesterol and phospholipid but not cholesteryl ester. In contrast, wt apoA-I and Delta4-5A-I produced significant increases in HDL cholesteryl ester. Further analysis showed that Delta6-7A-I and native apoA-I could bind similar amounts of phospholipid and cholesterol that were reduced slightly for Delta5-6A-I and greatly for Delta4-5A-I. We conclude from these findings that amino acids (aa) 100-143, specifically helix 4 (aa 100-121), contributes to the maturation of HDL through a role in lipid binding and that the downstream sequence (aa 144-186) centered around helix 6 (aa 144-165) is responsible for the activation of lecithin-cholesterol acyltransferase.

A single amino acid deletion in the carboxy terminal of apolipoprotein A-I impairs lipid binding and cellular interaction

The carboxy-terminal region of apolipoprotein (apo) A-I has been shown by mutagenesis or synthetic peptides to play an important role in lipid binding. However, the precise functional domain of the C-terminal remains to be defined. In this study, apoA-I Nichinan, a naturally occurring human apoA-I variant with a deletion of glutamic acid 235, was expressed in Escherichia coli to examine the effect of this mutation on the functional domain of apoA-I for lipid binding and related consequences. A dimyristoyl phosphatidylcholine binding study with recombinant (r-) proapoA-I Nichinan showed a significantly slow initial rate of lipid binding. On preincubation with human plasma lipoprotein fractions (d<1.225 g/mL) at 37 degrees C for 1 hour, (125)I-labeled normal r-proapoA-I was chromatographed as a single peak at the high density lipoprotein (HDL) fraction, whereas (125)I-labeled r-proapoA-I Nichinan was chromatographed into the HDL fraction as well as the free r-proapoA-I fraction (23% of radioactivity). Circular dichroism measurements showed that the alpha-helix content of lipid-bound r-proapoA-I Nichinan was reduced, being 62% (versus 73%) of normal r-proapoA-I. Nondenaturing gradient gel electrophoresis of reconstituted HDL particles assembled with r-proapoA-I Nichinan and normal r-proapoA-I showed similar particle size. To study cholesterol efflux, human skin fibroblasts were labeled with [(3)H]cholesterol, followed by incubation with either lipid-free r-proapoA-I or DMPC/r-proapoA-I complex. Fractional cholesterol efflux from [(3)H]cholesterol-labeled fibroblasts to lipid-free r-proapoA-I Nichinan or DMPC/r-proapoA-I Nichinan complexes was significantly reduced relative to that of normal r-proapoA-I or DMPC/r-proapoA-I during the 6-hour incubation. Binding assays of human skin fibroblasts by lipid-free r-proapoA-I showed that r-proapoA-I Nichinan was 32% less bound to fibroblasts than was normal r-proapoA-I. Our data demonstrate that the deletion of glutamic acid 235 at the C-terminus substantially reduces the lipid-binding properties of r-proapoA-I Nichinan, which may cause a reduction in its capacity to interact with plasma membranes as well as to promote cholesterol efflux from cultured fibroblasts.

Apolipoprotein-mediated plasma membrane microsolubilization. Role of lipid affinity and membrane penetration in the efflux of cellular cholesterol and phospholipid

Lipid-free apolipoprotein (apo) A-I contributes to the reverse transport of cholesterol from the periphery to the liver by solubilizing plasma membrane phospholipid and cholesterol. The features of the apolipoprotein required for this process are not understood and are addressed in the current study. Membrane microsolubilization of human fibroblasts is not specific for apo A-I; unlipidated apos A-II, C, and E incubated with the fibroblast monolayers at a saturating concentration of 50 micrograms/ml are all able to release cholesterol and phospholipid similarly. To determine the properties of the apolipoprotein that drive the process, apo A-I peptides spanning the entire sequence of the protein were utilized; the peptides correspond to the 11- and 22-residue amphipathic alpha-helical segments, as well as adjacent combinations of the helices. Of the 20 helical peptides examined, only peptides representing the N-and C-terminal portions of the protein had the ability to solubilize phospholipid and cholesterol. Cholesterol efflux to the most effective peptides, 44-65 and 209-241, was approximately 50 and 70%, respectively, of that to intact apo A-I. Deletion mutants of apo E and apo A-I were constructed that have reduced lipid binding affinities as compared with the intact molecule. The proteins, apo A-I (Delta222-243), apo A-I (Delta190-243), apo E3 (Delta192-299) and apo E4 (Delta192-299) all exhibited a decreased ability to remove cellular cholesterol and phospholipid. These decreases correlated with the reduced ability of these proteins to penetrate into a phospholipid monomolecular film. Overall, the results indicate that insertion of amphipathic alpha-helices between the plasma membrane phospholipid molecules is a required step in the mechanism of apolipoprotein-mediated cellular lipid efflux. Therefore the lipid binding ability of the apolipoprotein is critical for efficient membrane microsolubilization.