Apolipoprotein A-I conformation in discoidal particles: evidence for alternate structures

Molecular determinants of plasma cholesteryl ester transfer protein binding to high density lipoproteins

The plasma cholesteryl ester transfer protein (CETP) mediates the transfer of neutral lipids between lipoproteins and is associated with high density lipoproteins (HDL). To understand the mechanism of interaction of CETP with HDL, we studied the binding of pure recombinant CETP to 1-palmitoyl-2-oleoylphosphatidylcholine (POPC)/apoA-I discoidal particles. Separating bound from free CETP using native gradient gel electrophoresis, complexes of CETP with 10-nm hydrodynamic diameter discoidal particles migrated with a diameter of 12-16 nm, compared with approximately 7.5 nm for CETP. At lower ratios of CETP to discs, CETP bound to discs without displacement of apoA-I. CETP alone was unable to generate discoidal complexes. Cross-linking and fluorescence resonance energy transfer experiments indicated that CETP bound to discs as monomers. Cross-linking of CETP to apoA-I in discs suggested proximity of apoA-I and CETP. By negative-stain electron microscopy, discoidal complexes containing CETP and CETP monoclonal antibody showed localization of antibody molecules to the disc edge, suggesting that CETP was bound to the disc edge. The binding of CETP to discs of different composition or size was studied. Discs (10-nm Stokes diameter) prepared with either apoA-I or apoA-II had a similar Kd (120 nM). Inclusion of 1 mol % cholesteryl oleate, 5 mol % cholesterol, or 6 mol % phosphatidylinositol increased the binding affinity of CETP 3-10 times (20-30 nM). In comparison, plasma HDL3 had a Kd of approximately 450 nM. For POPC/apoA-I discs, 10-nm discs bound CETP with much higher affinity than smaller 7.8-nm discs (Kd = 1-2 microM). 7.7-nm hydrodynamic diameter POPC/apoA-I spherical particles containing either triolein or cholesteryl oleate in their core bound CETP with higher affinity (Kd = 50-100 nM) than 7.8-nm POPC/apoA-I discs. Thus, CETP appears to bind to the perimeter of discoidal particles, possibly in a process in which flexible segments in apoA-I or apoA-II accommodate CETP at the disc edge. The binding of CETP to HDL is markedly influenced by overall particle size and shape and by lipid composition, and the increased binding affinity for cholesterol- and cholesteryl ester-containing discs suggests a higher affinity of CETP for nascent than mature HDL.

Role of apolipoprotein A-I in cholesterol transfer between lipoproteins. Evidence for involvement of specific apoA-I domains

A series of monoclonal antibodies against epitopes spanning different domains of apoA-I have been tested for their effects on unesterified cholesterol transfer between low density lipoprotein (LDL) and well-defined homogenous lipoproteins reconstituted with phosphatidylcholine, cholesterol, and apoA-I (LpA-I). Antibodies 2G11 (reacting between residues 25 and 110), A05 (residues 25-82), A03 (residues 135-140), A44 and r5G9 (residues 149-186), and 4A12 (residues 173-205) significantly inhibit cholesterol transfer from LDL to Lp2A-I while they enhance transfer in the opposite direction, thus causing an increased net transfer to LDL. Most of these monoclonal antibodies (mAbs) also enhance phospholipid transfer to LDL but in a lesser and variable proportion relative to cholesterol. Their epitopes are mainly contained within domains that are predicted to be amphipathic alpha-helices. In contrast, mAbs 4H1 (residues 2-8), 3G10 (residues 96-121), and 5F6 (residues 116-141) have little or no effect on either cholesterol or phospholipid transfer, and the epitopes for these three mAbs have been shown in earlier studies to be structurally and functionally related. Their immunoreactivity responds similarly to variation in lipoprotein cholesterol content, and the antibodies binding to these sites compete with one another and have similar effects on the cholesterol esterification reaction. Thus, the current results are compatible with the hypothesis that they form an integrated domain with a common function in cholesterol metabolism, possibly as part of a hinge domain. Most mAbs were found to increase significantly the alpha-helicity of apoA-I in the Lp2A-I immunecomplexes, suggesting that they may increase the stability of the lipid-bound apoA-I. However, not unexpectedly, there is no correlation between the effects of mAbs on alpha-helicity and their effects on cholesterol or phospholipid transfer since each mAb has a discrete effect on these transfers. These studies demonstrate the specificity of LpA-I particles in cholesterol transport and document the existence of apoA-I domains with different functions in cholesterol transport.

Carboxyl-terminal domain truncation alters apolipoprotein A-I in vivo catabolism

Apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins, facilitates reverse cholesterol transport from peripheral tissue to liver. To determine the structural motifs important for modulating the in vivo catabolism of human apoA-I (h-apoA-I), we generated carboxyl-terminal truncation mutants at residues 201 (apoA-I201), 217 (apoA-I217), and 226 (apoA-I226) by site-directed mutagenesis. ApoA-I was expressed in Escherichia coli as a fusion protein with the maltose binding protein, which was removed by factor Xa cleavage. The in vivo kinetic analysis of the radioiodinated apoA-I in normolipemic rabbits revealed a markedly increased rate of catabolism for the truncated forms of apoA-I. The fractional catabolic rates (FCR) of 9.10 +/- 1.28/day (+/- S.D.) for apoA-I201, 6.34 +/- 0.81/day for apoA-I217, and 4.42 +/- 0.51/day for apoA-I226 were much faster than the FCR of recombinant intact apoA-I (r-apoA-I, 0.93 +/- 0.07/day) and h-apoA-I (0.91 +/- 0.34/day). All the truncated forms of apoA-I were associated with very high density lipoproteins, whereas the intact recombinant apoA-I (r-apoA-I) and h-apoA-I associated with HDL2 and HDL3. Gel filtration chromatography revealed that in contrast to r-apoA-I, the mutant apoA-I201 associated with a phospholipid-rich rabbit apoA-I containing particle. Analysis by agarose gel electrophoresis demonstrated that the same mutant migrated in the pre-beta position, but not within the alpha position as did r-apoA-I. These results indicate that the carboxyl-terminal region (residue 227-243) of apoA-I is critical in modulating the association of apoA-I with lipoproteins and in vivo metabolism of apoA-I.

Effect of LpA-I composition and structure on cholesterol transfer between lipoproteins

The effect of high density lipoprotein composition on the rates of unesterified cholesterol exchange between low density lipoproteins (LDL) and well-defined homogeneous discoidal lipoproteins (LpA-I) reconstituted with phosphatidylcholine, cholesterol, and apolipoprotein A-I (apoA-I) has been investigated. LpA-I containing cholesterol and 2, 3, and 4 apoA-I molecules per particle differed in their ability to accept or donate cholesterol. A significant cholesterol exchange occurs between LDL and Lp2A-I (7.8 and 9.6 nm), while there is little or no cholesterol exchange detectable between LDL and Lp3A-I (10.8 and 13.4 nm) and Lp4A-I (17.0 nm) complexes. The cholesterol transfer from LDL to the cholesterol-free Lp2A-I (9.6 nm), Lp3A-I (13.4 nm), and Lp4A-I (17.0 nm) particles also shows significant cholesterol transfer to Lp2A-I, while there is no detectable transfer to Lp3- and 4A-I particles. The rates of cholesterol transfer to cholesterol-free and cholesterol-containing Lp2A-I appear to differ significantly. Cholesterol transfer from LDL to cholesterol-free Lp2A-I is zero order with respect to acceptor concentrations when the Lp2A-I/LDL ratio is above 10. Transfer rates from LDL to cholesterol-free Lp2A-I are faster for the smaller Lp2A-I (8.5 nm) than to the larger Lp2A-I (9.7 nm) and exhibit half-times (t1/2) at 25 degrees C of 4.0 and 5.3 h, respectively. In contrast, cholesterol transfer from LDL to cholesterol-containing Lp2A-I remains dependent upon acceptor concentrations to an acceptor/donor particle ratio of 80. In addition, transfer from LDL to cholesterol-containing Lp2A-I is faster to the 9.6 nm than to 7.8 nm particles, with t1/2 of 1.4 and 2.3 h, respectively. The rates of cholesterol transfer from Lp2A-I to LDL are higher than in the opposite direction, in particular for the small Lp2A-I (7.8 nm), which has a t1/2 of approximately 50 min. The results show that changes in the composition and structure of apoA-I-containing particles have a significant effect on inter-lipoprotein exchange of cholesterol. This suggests that the kinetics of cholesterol transfer to and from reconstituted discoidal LpA-I particles cannot be fully explained by passive aqueous diffusion.

Structural domain of apolipoprotein A-I involved in its interaction with cells

Apolipoprotein A-I (apo A-I) is the major protein constituent of high-density lipoprotein (HDL), the lipoprotein fraction which mediates the reverse cholesterol transport. This apolipoprotein plays an important role in the binding of HDL to cells and participates in the efflux of cellular cholesterol. We have recently compared six different genetic variants of apo A-I and found that the apo A-I (Pro 165-->Arg) mutant is defective in promoting cellular cholesterol efflux from murine adipocytes and peritoneal macrophages and we have proposed that this region of apo A-I may be involved in their interaction with cells. To confirm this hypothesis, four monoclonal antibodies (mAbs) specific for apo A-I were used to study the inhibition of the interaction of palmitoyloleoylphosphatidylcholine (POPC): apoA-I complexes with HeLa cells and adipocytes. Among these antibodies, the apo A-I epitope recognized by the A44 mAb lies in the COOH terminal region (amino acid residues 149-186) including the proposed region. The antibodies A05, and A03 react with residues 25-82, 135-140, respectively and the A11 mAb corresponds to a discontinuous epitope at residues 99-105 and 126-132. Our results show clearly that the A44 and A05 mAbs reduce both the binding to HeLa cells and the cholesterol efflux from adipocytes. The inhibition of POPC: apoA-I complexes binding to both cell types is more strictly observed with the Fab fragments of monoclonal antibodies A44 and A05. Partial cotitration curves of these mAbs in a solid phase assay (RIA), indicated partial competition between these two antibodies. We propose a structural model for the POPC: apoA-I complexes where the N-terminal domain of one apo A-I molecule is in close spatial relationship with the C-terminal domain of the adjacent apo A-I molecule. We therefore suggest that the domain around amino acid 165 of apo A-I and which is recognized by mAb A44 (149-186) forms or contains some specific regions which mediate selectively the interaction with the binding site of cells and is involved in the efflux of cellular cholesterol.