Structural and functional properties of full-length and truncated human proapolipoprotein AI expressed in escherichia coli

Artificial High Density Lipoprotein Nanoparticles in Cardiovascular Research

Lipoproteins are endogenous nanoparticles which are the major transporter of fats and cholesterol in the human body. They play a key role in the regulatory mechanisms of cardiovascular events. Lipoproteins can be modified and manipulated to act as drug delivery systems or nanocarriers for contrast agents. In particular, high density lipoproteins (HDL), which are the smallest class of lipoproteins, can be synthetically engineered either as nascent HDL nanodiscs or spherical HDL nanoparticles. Reconstituted HDL (rHDL) particles are formed by self-assembly of various lipids and apolipoprotein AI (apo-AI). A variety of substances including drugs, nucleic acids, signal emitting molecules, or dyes can be loaded, making them efficient nanocarriers for therapeutic applications or medical diagnostics. This review provides an overview about synthesis techniques, physicochemical properties of rHDL nanoparticles, and structural determinants for rHDL function. We discuss recent developments utilizing either apo-AI or apo-AI mimetic peptides for the design of pharmaceutical rHDL formulations. Advantages, limitations, challenges, and prospects for clinical translation are evaluated with a special focus on promising strategies for the treatment and diagnosis of atherosclerosis and cardiovascular diseases.

Lipid-free apoA-I structure - Origins of model diversity

Apolipoprotein A-I (apoA-I) is a prominent member of the exchangeable apolipoprotein class of proteins, capable of transitioning between lipid-bound and lipid-free states. It is the primary structural and functional protein of high density lipoprotein (HDL). Lipid-free apoA-I is critical to de novo HDL formation as it is the preferred substrate of the lipid transporter, ATP Binding Cassette Transporter A1 (ABCA1) Remaley et al. (2001) [1]. Lipid-free apoA-I is an important element in reverse cholesterol transport and comprehension of its structure is a core issue in our understanding of cholesterol metabolism. However, lipid-free apoA-I is highly conformationally dynamic making it a challenging subject for structural analysis. Over the past 20years there have been significant advances in overcoming the dynamic nature of lipid-free apoA-I, which have resulted in a multitude of proposed conformational models.

The unsolved mystery of apoA-I recycling in adipocyte

As the major storage site for triglycerides and free cholesterol, adipose tissue plays a central role in energy metabolism. ApoA-I is the main constituent of HDL and plays an important role in removal of excess cholesterol from peripheral tissues. Recently, multiple studies have shown beneficial effects of apoA-I on adipose metabolism and function. ApoA-I was reported to improve insulin sensitivity and exert anti-inflammatory, anti-obesity effect in animal studies. Interestingly, Uptake and resecretion of apoA-I by adipocytes has been detected. However, the significance of apoA-I recycling by adipocytes is still not clear. This article reviewed methods used to study cellular recycling of apoA-I and summarized the current knowledge on the mechanisms involved in apoA-I uptake by adipocytes. Since the main function of apoA-I is to mediate reverse cholesterol transport from peripheral tissues, the role of apoA-I internalization and re-secretion by adipocytes in intracellular cholesterol transport under physiological and pathological conditions were discussed. In addition, findings on the correlation between apoA-I recycling and obesity were discussed. Finally, it was proposed that during intracellular transport, apoA-I-protein complex may acquire cargoes other than lipids and deliver regulatory information when they were resecreted into the plasma. Although apoA-I recycling by adipocytes is still an unsolved mystery, it's likely that it is more than a redundant pathway especially under pathological conditions.

Preparation and characterization of a lovastatin-loaded protein-free nanostructured lipid carrier resembling high-density lipoprotein and evaluation of its targeting to foam cells

This study was designed to investigate whether a non-protein nanostructured lipid carrier (NLC) resembling high-density lipoprotein (HDL) could deliver a hydrophobic anti-atherogenic drug, lovastatin, to foam cells. Lovastatin-loaded NLC (LT-NLC) was prepared by a nanoprecipitation/solvent diffusion method. The LT-NLC-apoprotein (LT-NLC-apo) was prepared by incubating LT-NLC with native HDL. The physicochemical parameters of LT-NLC were characterized in terms of particle size, zeta potential, morphology, entrapment efficiency, and crystallization behavior. Targeting behavior and mechanism were demonstrated by the incubation of LT-NLC-apo with a RAW 264.7 macrophage-derived foam cell model in the presence or absence of very-low-density lipoprotein (VLDL) and lipase. The results showed that LT-NLC was solid spherical or oval in shape with an average diameter of 13.8 ± 2.2 nm, zeta potential of −29.3 ± 0.2 mV and entrapment efficiency of 96.2 ± 1.3%. Phagocytosis studies showed that uptake of LT-NLC-apo by macrophages was significantly lower than LT-NLC (p < 0.01), suggesting that LT-NLC-apo could possibly escape recognition from macrophages in vivo. The uptake was increased twofold when LT-NLC-apo was incubated with transfected foam cells containing VLDL and lipase. These results indicated that non-protein NLC resembling HDL could be a useful tool to deliver lipophilic anti-atherogenic drugs to foam cells, and that uptake could be enhanced by the VLDL receptor pathway.

Impact of self-association on function of apolipoprotein A-I

Self-association is an inherent property of the lipid-free forms of several exchangeable apolipoproteins, including apolipoprotein A-I (apoA-I), the main protein component of high density lipoproteins (HDL) and an established antiatherogenic factor. Monomeric lipid-free apoA-I is believed to be the biologically active species, but abnormal conditions, such as specific natural mutations or oxidation, produce an altered state of self-association that may contribute to apoA-I dysfunction. Replacement of the tryptophans of apoA-I with phenylalanines (ΔW-apoA-I) leads to unusually large and stable self-associated species. We took advantage of this unique solution property of ΔW-apoA-I to analyze the role of self-association in determining the structure and lipid-binding properties of apoA-I as well as ATP-binding cassette A1 (ABCA1)-mediated cellular lipid release, a relevant pathway in atherosclerosis. Monomeric ΔW-apoA-I and wild-type apoA-I activated ABCA1-mediated cellular lipid release with similar efficiencies, whereas the efficiency of high order self-associated species was reduced to less than 50%. Analysis of specific self-associated subclasses revealed that different factors influence the rate of HDL formation in vitro and ABCA1-mediated lipid release efficiency. The α-helix-forming ability of apoA-I is the main determinant of in vitro lipid solubilization rates, whereas loss of cellular lipid release efficiency is mainly caused by reduced structural flexibility by formation of stable quaternary interactions. Thus, stabilization of self-associated species impairs apoA-I biological activity through an ABCA1-mediated mechanism. These results afford mechanistic insights into the ABCA1 reaction and suggest self-association as a functional feature of apoA-I. Physiologic mechanisms may alter the native self-association state and contribute to apoA-I dysfunction.

Nanostructured lipid carriers constituted from high-density lipoprotein components for delivery of a lipophilic cardiovascular drug

To investigate the possibility of reconstituted protein-free high-density lipoprotein (HDL) being a carrier for delivering a lipophilic cardiovascular drug, Tanshinone IIA-loaded HDL-like nanostructured lipid carriers (TA-NLC) were prepared by a nanoprecipitation/solvent diffusion method. The physicochemical parameters of TA-NLC were characterized in terms of particle size, zeta potential, morphology, entrapment efficiency, differential scanning calorimetry (DSC) and stability. A novel two-step method has been employed to determine entrapment efficiency of TA-NLC. The binding properties of TA-NLC to apolipoproteins were investigated by in vitro incubation competition assay in the presence of native HDL and electrophoresis test. Phagocytosis and cytotoxicity was evaluated using mouse macrophage cell line RAW 264.7 with TA-NLC and incubated TA-NLC with native HDL (TA-NLC-apo). The results showed that TA-NLC had an average diameter of 8.0+/-1.2 nm, zeta potential of -29.0+/-0.0 mV, drug loading of 6.17+/-0.3% and entrapment efficiency of 97.84+/-1.2%. TA-NLC were demonstrated spheres with drug incorporated in lipid core forming a shell-core structure. DSC analysis showed that TA was dispersed in NLC in an amorphous state. The incorporation of glycerol trioleate to NLC led to crystal order disturbance. Agarose gel electrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-SPAGE) patterns indicated that TA-NLC could bind to apolipoprotein A-I (apoA-I) specifically in vitro. Phagocytosis studies showed significant differences in uptake between TA-NLC and TA-NLC-apo and demonstrated that TA-NLC incubated with native HDL could turn endogenous by association to apolipoproteins, which cannot trigger immunological responses and could escape from recognition by macrophages.

Natural mutations of apolipoprotein A-I impairing activation of lecithin:cholesterol acyltransferase

Five natural mutations of apolipoprotein A-I (apoA-I), apoA-I(A95D), apoA-I(Y100H), apoA-I(E110K), apoA-I(V156E) and apoA-I(H162Q), were studied for their ability to activate lecithin:cholesterol acyltransferase (LCAT). Mutants apoA-I(E110K), apoA-I(V156E) and apoA-I(H162Q) had an impaired ability to activate LCAT. Combined with data on other apoA-I mutants this finding is consistent with the idea that the central region between amino acids 110 and 160 is likely to be the "active site" of apoA-I involved in the interaction with LCAT and that a specific sequence of apoA-I is required for activation of the enzyme.

Bacterial expression and characterization of mature apolipoprotein A-I

Plasma levels of apolipoprotein A-I (apoA-I) are correlated with reduced incidence of heart disease due to the critical role of this protein in reverse cholesterol transport. Because of its diversity of function and poorly understood structure, much research has sought to understand how the structure of apoA-I facilitates its function. A popular approach has been the use of site-directed mutagenesis followed by structural and functional studies. There are a wide variety of expression systems available to produce these mutant proteins including eukaryotic cell lines and prokaryotic cells such as Escherichia coli. Expression in a bacterial system is generally favorable because it can produce large amounts of pure protein quickly and economically through the use of affinity tags on the expressed protein. Unfortunately, many of these systems are not ideal for the production of apolipoproteins because, in many cases, the proteolytic digestion required to remove the affinity tag also cleaves the target protein. Here we describe a method that produces large amounts of recombinant protein that is easily purified using a histidine (His) affinity tag that is cleaved with IgA protease from Neisseria gonorrhoeae. This enzyme does not cleave the wild type apoA-I sequence, leaving intact, mature apoA-I (containing a Thr-Pro- on the N-terminus). We show that this recombinant protein is similar to wild type protein in structure and function using circular dichroism analysis, lipid clearance assays, recombinant particle formation and cholesterol efflux assays. This system is particularly useful for the bacterial production of apolipoproteins because of the extreme specificity of IgA protease for its target cleavage site.

Recombinant lipoproteins: lipoprotein-like lipid particles for drug targeting

Lipoproteins are endogenous particles that transport lipids through the blood to various cell types, where they are recognised and taken up via specific receptors. These particles are, therefore, excellent candidates for the targeted delivery of drugs to various tissues. For example, the remnant receptor and the asialoglycoprotein receptor (ASGPr), which are uniquely localised on hepatocytes, recognise chylomicrons and lactosylated high density lipopoteins (HDL), respectively. In addition, tumour cells of various origins overexpress the low density lipoprotein (LDL) receptor that recognises apolipoprotein E (apoE) on small triglyceride-rich particles and apoB-100 on LDL. Being endogenous, lipoproteins are biodegradable, do not trigger immune reactions, and are not recognised by the reticuloendothelial system (RES). However, their endogenous nature also hampers large-scale pharmaceutical application. In the past two decades, various research groups have successfully synthesised recombinant lipoproteins from commercially available natural and synthetic lipids and serum-derived or recombinant apolipoproteins, which closely mimic the metabolic behaviour of their native counterparts in animal models as well as humans. In this paper, we will summarise the studies that led to the development of these recombinant lipoproteins, and we will address the possibility of using these lipidic particles to selectively deliver a wide range of lipophilic, amphiphilic, and polyanionic compounds to hepatocytes and tumour cells. In addition, the intrinsic therapeutic activities of recombinant chylomicrons and HDL in sepsis and atherosclerosis will be discussed.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.

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.

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.

Effectivity of expression of mature forms of mutant human apolipoprotein A-I

In order to probe the structural and functional properties of a central region of apolipoprotein A-I (apoA-I), we engineered mutants of the mature form of the protein and expressed them using the baculovirus/insect cell expression system. The mutations which targeted the region of apoA-I between amino acids 140 and 150 included: (i) deletion of the region 140-150 (apoA-I(Delta140-150)); (ii) substitution of arginine 149 with valine (apoA-I(R149V)); (iii) substitution of proline 143 with alanine (apoA-I(P143A)); (iv) deletion of region 63-73 (apoA-I(Delta63-73)), which has structural properties similar to 140-150; and (v) a chimeric protein substituting amino acids 140-150 with amino acids 63-73 (apoA-I(140-150 --> 63-73)). The efficiencies of synthesis were vastly different for the various mutants as follows: apoA-I(R149V) > apoA-I(140-150 --> 63-73) > apoA-I(Delta63-73) > apoA-I(P143A) > apoA-I > apoA-I(Delta140-150). About 50% of the synthesized wild type and all apoA-I mutants was retained in the cells. During expression of apoA-I(R149V) an unusual spontaneous recombination occurred. In addition to the expected mutant, another form of apoA-I with an apparent M(r) of 36K was produced which consisted of a duplication of the amino-terminal end of apoA-I, from the prepeptide through to amino acid 62, linked to the original pre-apoA-I(R149V) sequence via a 4-amino-acid linker. Despite the fact that this form of apoA-I carries two prepeptides and consequently two cleavage sites, there was little, if any, cleavage at the internal cleavage site. During expression, less than 20% of this mutant was retained in the cells. These results demonstrate that at least in the model of insect cells, the efficiency of apoA-I synthesis, processing, and secretion depends on apoA-I secondary structure and/or folding.

Matrix metalloproteinases-3, -7, and -12, but not -9, reduce high density lipoprotein-induced cholesterol efflux from human macrophage foam cells by truncation of the carboxyl terminus of apolipoprotein A-I. Parallel losses of pre-beta particles and the high affinity component of efflux

Matrix metalloproteinases (MMPs) have been suggested to function in remodeling of the arterial wall, but no information is available on their possible role in early atherogenesis, when cholesterol accumulates in the cells of the arterial intima, forming foam cells. Here, we incubated the major component responsible for efflux of cholesterol from foam cells, high density lipoprotein 3 (HDL(3)), with MMP-1, -3, -7, -9, or -12 at 37 degrees C before adding it to cholesterol-loaded human monocyte-derived macrophages. After incubation with MMP-3, -7, or -12, the ability of HDL(3) to induce the high affinity component of cholesterol efflux from the macrophage foam cells was strongly reduced, whereas preincubation with MMP-1 reduced cholesterol efflux only slightly and preincubation with MMP-9 had no effect. These differential effects of the various MMPs were reflected in their differential abilities to degrade the small pre-beta migrating particles present in the HDL(3) fraction. NH(2)-terminal sequence and mass spectrometric analyses of the apolipoprotein (apo) A-I fragments generated by MMPs revealed that those MMPs that strongly reduced cholesterol efflux (MMPs-3, -7, and -12) cleaved the COOH-terminal region of apoA-I and produced a major fragment of about 22 kDa, whereas MMPs-1 and -9, which had little and no effect on cholesterol efflux, degraded apoA-I only slightly and not at all, respectively. These results show, for the first time, that some members of the MMP family can degrade the apoA-I of HDL(3), so blocking cholesterol efflux from macrophage foam cells. This expansion of the substrate repertoire of MMPs to include apoA suggests that these proteinases are directly involved in the accumulation of cholesterol in atherosclerotic lesions.

A novel mutant, ApoA-I nichinan (Glu235-->0), is associated with low HDL cholesterol levels and decreased cholesterol efflux from cells

A novel variant of apolipoprotein (apo) A-I associated with low high density lipoprotein (HDL) cholesterolemia has been identified in a Japanese family during screening for apoA-I variants by isoelectric focusing (IEF) gel analysis. ApoA-I (Glu235-->0) Nichinan was caused by a 3-bp deletion of nucleotides 1998 through 2000 in exon 4 of the apoA-I gene. Four subjects in the family were heterozygous carriers for this mutation; the mean plasma concentrations of apoA-I and HDL cholesterol of affected family members were 30% and 32% lower, respectively, than those of unaffected family members. There were no differences in the levels of very low density lipoprotein and low density lipoprotein cholesterol, triglycerides, and other apolipoproteins between the carriers and the noncarrier family members. In the proband, plasma lecithin:cholesterol acyltransferase activity was normal. Functional consequences of the mutation were examined by expressing the mutated and wild-type proapoA-I cDNAs in Escherichia coli. Cholesterol efflux to recombinant proapoA-I Nichinan from mouse peritoneal macrophages loaded with [3H]cholesterol-labeled acetylated low density lipoprotein was decreased by 54% when compared that of normal recombinant proapoA-I. In vivo turnover studies in normal rabbits demonstrated that the recombinant proapoA-I Nichinan was rapidly cleared (22% faster) compared with normal recombinant proapoA-I. We conclude that apoA-I (Glu235-->0) Nichinan induced a critical structural change in the carboxyl-terminal domain of apoA-I for cellular cholesterol efflux and increased the catabolism of apoA-I, resulting in low HDL cholesterol levels.

Recombinant apolipoproteins for the treatment of vascular diseases

The protein components of human lipoproteins, apolipoproteins, allow the redistribution of cholesterol from the arterial wall to other tissues and exert beneficial effects on systems involved in the development of arterial lesions, like inflammation and hemostasis. Because of these properties, the antiatherogenic apolipoproteins, particularly apo A-I and apo E, may provide an innovative approach to the management of vascular diseases. The recent availability of extractive or biosynthetic molecules is allowing a detailed overview of their therapeutic potential in a number of animal models of arterial disease. Infusions of apo E, or more dramatically, of apo A-I, both recombinant or extractive, cause a direct reduction of the atherosclerotic burden in experimental animals. Naturally, as the apo A-I(Milano) (apo A-I(M)) dimer, or engineered recombinant apolipoproteins with prolonged permanence in plasma and improved function may offer an even better approach to the therapeutic handling of arterial disease. This progress will go on in parallel with innovations in the technologies for direct, non invasive assessments of human atherosclerosis, thus allowing closer monitoring of this potential new approach to therapy.

Production of mature human apolipoprotein A-I in a baculovirus-insect cell system: propeptide is not essential for intracellular processing but may assist rapid secretion

To achieve expression of human mature apolipoprotein A-I (apoA-I) in the baculovirus-insect cell expression system, the propeptide encoding region of full-length preproapoA-I was deleted using polymerase chain reaction and the resulting cDNA was cloned into BacPak8 plasmid. After transfection into Sf21 insect cells and plaque purification, mature human apoA-I was secreted by the infected cells into the medium as determined by immunoblotting, amino-terminal sequencing, and molecular weight determination. In both monolayer cell cultures, and in suspension cell culture, maximum expression was achieved by the fifth day. For the first 4 days, 50 to 70% of the synthesized apoA-I was retained in the cells. This intracellular apoA-I was represented by mature apoA-I as shown by immunoblotting and amino-terminal sequencing. Further incubation resulted in a sharp decrease in the cell apoA-I content without a corresponding increase in protein in the medium and most likely represents intracellular degradation of the protein. We conclude that the deletion of the propeptide, while not preventing the correct cleavage of prepeptide during intracellular processing, results in reduced secretion of mature apoA-I. The baculovirus-insect cell expression system described in this study provides a useful method for producing recombinant mature apoA-I and is a potential tool for understanding the function of propeptide in intracellular transport and secretion of apoA-I from cells.

Efflux of cellular cholesterol and phospholipid to apolipoprotein A-I mutants

Human plasma apolipoprotein A-I (apoA-I) and recombinant full-length proapoA-I (apoA-I-(-6-243)) as well as four truncated forms of proapoA-I were used as acceptors to study cholesterol and phospholipid efflux from HepG2 cells. Efflux of both cholesterol and phospholipid to the lipid-free plasma apoA-I was twice that of apoA-I-(-6-243). When apoA-I was incorporated into reconstituted high density lipoprotein, cholesterol efflux increased, phospholipid efflux decreased and the difference between plasma apoA-I and apoA-I-(-6-243) disappeared. Truncation of recombinant apoA-I to residues 222 (apoA-I-(-6-222)) and 210 (apoA-I-(-6-210)) resulted in a 70-95% decrease in their ability to promote the efflux of both intracellular and plasma membrane cholesterol. Further truncation to residues 150 (apoA-I-(-6-150)) and 135 (apoA-I-(-6-135)) fully restored the ability of apoA-I to promote cholesterol efflux. Phospholipid efflux closely paralleled the efflux of cholesterol. Interaction of 125I-labeled apoA-I with the cells was similar for apoA-I-(-6-243), apoA-I-(-6-222), and apoA-I-(-6-210), but slightly higher for apoA-I-(-6-150) and apoA-I-(-6-135). When complexed with phospholipid, all forms except apoA-I-(-6-210) formed discoidal reconstituted high density lipoprotein particles. When the same amounts of free or lipid-associated apoA-I were compared, association of apoA-I with phospholipid increased cholesterol efflux and decreased phospholipid efflux, and the difference in the ability of different mutants to promote cholesterol and phospholipid efflux disappeared. We conclude that the capacity of lipid-free apoA-I to promote cholesterol efflux is related to its ability to mobilize cellular phospholipid, which apparently involves a region around residues 222-243. A second lipid-binding region is exposed when the carboxyl-terminal half of apoA-I is absent.