From the Department of Biochemistry and Cell Biology,
the
Department of Chemistry, and the ** Institute for Cell and
Developmental Biology, State University of New York,
Stony Brook, New York 11794-5215
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ABSTRACT |
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Glycosylphosphatidylinositol (GPI)-anchored proteins can be isolated from both cells and sphingolipid and cholesterol-rich liposomes (SCRLs) in association with detergent-insoluble membranes. We found previously that detergent insolubility of lipids was characteristic of phases in which lipid acyl chains are ordered. We presented evidence that GPI-anchored proteins are insoluble because they associate with cholesterol and sphingolipid-rich lipid domains with properties similar to the liquid-ordered phase. Here, this model was tested by a variety of approaches. First, we demonstrated that saponin, which removes cholesterol from cell membranes and allows solubilization of GPI-anchored proteins by Triton X-100, had the same effect on the GPI-anchored protein alkaline phosphatase (PLAP) in SCRLs of appropriate lipid composition. The similarity of saponin action in cells and simple liposomes suggests that the compound disrupts protein-lipid interactions. However, direct interactions between PLAP and cholesterol were not needed for insolubility, because the protein was also insoluble in cholesterol-free liposomes containing lipid in an ordered phase. Instead, cholesterol acted by greatly enhancing the formation of a detergent-insoluble phase by sphingolipids, which have a tendency to form ordered phases. We propose that saponin solubilizes GPI-anchored proteins because the lipid composition of cell membranes (and the SCRLs used above) supports ordered phase formation in the presence but not the absence of cholesterol. Supporting this model, saponin did not promote Triton X-100 solubilization of PLAP in SCRLs with sphingolipid levels high enough to allow ordered phase formation in the absence of cholesterol. We also showed that two additional GPI-anchored proteins are detergent-insoluble in SCRLs and that detergent does not artifactually create ordered domains or cause components of solubilized membranes to associate with detergent-resistant membranes present in separate bilayers in the same lysate. We conclude that the ordered domain model explains the behavior of detergent-resistant membranes in liposomes and cells.
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INTRODUCTION |
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A hallmark of GPI1-anchored proteins in eukaryotic cells is their insolubility in non-ionic detergents (1-3). This property is surprising because GPI-anchored proteins are restricted to the extracellular leaflet of the lipid bilayer, and it has been generally presumed that most detergent-insoluble membrane proteins bind to the cytoskeleton. We showed earlier that GPI-anchored proteins are present in detergent-resistant membrane (DRM) fragments that can be isolated from cell lysates after extraction (3). A number of other groups have replicated this finding in a wide variety of cell types (4-9). These findings suggested that direct interactions between GPI-anchored proteins and lipids in detergent-insoluble membrane domains might lead to detergent insolubility. Further support for this proposal came from studies in which a GPI-anchored protein, PLAP, was incorporated into sphingolipid and cholesterol-rich liposomes (SCRLs), which have a lipid composition similar to that of detergent-insoluble membrane domains isolated from cells (10). The protein associated with detergent-insoluble membranes derived from SCRLs after extraction. This result showed directly that interactions between PLAP and lipids were sufficient for detergent insolubility of the protein; no other proteins were required. Based on further liposome studies on the lipid requirements for detergent insolubility, we proposed that DRMs are present in cell membranes as domains in a sphingolipid and cholesterol-rich ordered phase.
DRMs are similar in several ways to the liquid-ordered (lo) phase previously described in model membranes (11-15). lo phase domains can co-exist in bilayers with domains in the more familiar gel or liquid crystalline (lc) phases (14). Pure phospholipids exist in a solid, ordered gel phase at low temperatures. Above a melting temperature (Tm) that is characteristic of each lipid, pure lipids form the fluid, disordered lc phase. Most of the phospholipids in cell membranes have low Tm values and are believed to be in the lc phase. The lo phase, which requires sterol to form, has properties intermediate between gel and lc phases. It has been observed in mixtures of cholesterol with lipids whose highly saturated acyl chains gives them a high Tm in the pure state (11-15). Both saturated chain phosphatidylcholines and sphingomyelin can form the lo phase when mixed with cholesterol (13).
DRMs have several properties in common with the lo phase. Both are rich in cholesterol, are detergent-resistant, and contain lipids with similar acyl chain mobility (10). Because of the complexity of cell membranes, further characterization of the physical properties of cellular DRMs is difficult. As a useful working model, we have proposed that DRMs arise from domains in cell membranes that are in the lo phase or a phase similar to it. It is likely that the saturated acyl chains generally found in GPI-anchored proteins (16) mediate the association of these proteins with detergent-insoluble domains.
Cerneus et al. (17) showed that after treatment of mammalian cells with saponin, GPI-anchored proteins could readily be solubilized by Triton X-100. Saponin, like other cardiac glycosides, complexes with membrane cholesterol, sequestering this lipid away from other interactions (18). This result suggested that the Triton insolubility of GPI-anchored proteins requires cholesterol. Similarly, Hanada et al. (19) found that the Triton X-100 resistance of PLAP depended on the presence of cholesterol, as well as of sphingolipids. Neither study provided a mechanism for this phenomenon. Based on our model, we propose that the sphingolipid and cholesterol dependence of GPI-anchored protein insolubility does not depend on a specific interaction between the proteins and these lipids. Rather, sphingolipids and cholesterol are required to maintain detergent-insoluble ordered domains in the membranes.
In this paper, we used model membranes to test this and other predictions of our model of detergent insolubility. First, we examined the mechanism underlying the requirement of sphingolipids and cholesterol for detergent insolubility of GPI-anchored proteins. We also examined the detergent insolubility of two other GPI-anchored proteins and a transmembrane protein in SCRLs. In addition, we determined how well detergent insolubility reflects membrane properties prior to detergent addition. The results strongly suggest that detergent insolubility is a useful method for investigating cell membrane structure.
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EXPERIMENTAL PROCEDURES |
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Materials--
Brain phosphatidylcholine (brain PC), bovine
liver phosphatidylcholine (liver PC), dioleoyl phosphatidylcholine
(DOPC), dipalmitoyl phosphatidylcholine (DPPC), brain sphingomyelin,
and brain cerebrosides were from Avanti Polar Lipids (Alabaster, AL).
Cholesterol was from Avanti or Sigma. [3H]Sphingomyelin
(80 Ci/mmol; 1 mCi/ml) was from American Radiolabeled Chemicals (St.
Louis, MO). [7-3H]Cholesterol (5-15 Ci/mmol;
[3H]cholesterol) and
L-3-phosphatidyl[N-methyl-3H]choline,
1,2-dipalmitoyl (40-85 Ci/mmol; [3H]DPPC) were from
Amersham Corp. N-Octyl--D-glucopyranoside
(octyl glucoside) was from Jersey Lab and Glove Supply (Livingston,
NJ). CHAPS, saponin, and all other chemicals were from Sigma.
Acrylamide and bisacrylamide were from Boehringer Mannheim.
Nitrocellulose and pre-stained molecular weight markers were from
Bio-Rad. The enhanced chemiluminescence reagent (ECL) was from Amersham
Corp., and the Renaissance reagent was from NEN Life Science Products. Rabbit anti-PLAP (IgG fraction, 5 mg/ml) was from Dako (Carpinteria, CA).
Liposome Preparation-- Except as noted, all protein-free liposomes were multilamellar vesicles (MLVs). To prepare these, lipids (amounts given in figure legends) were dried under nitrogen gas, redissolved in a small volume of CHCl3, dried under nitrogen and by lyophilization, and resuspended in 2 ml of phosphate-buffered saline (PBS; 150 mM NaCl, 20 mM phosphate buffer, pH 7.4) at 60 °C (85 °C for liposomes containing cerebrosides). Lipids were vortexed for 30 s and incubated for 30 min at 60 or 85 °C as appropriate. They were vortexed again for 30 s, incubated for 15 min at 60 or 85 °C, and then allowed to cool to room temperature (at least 30 min.) Where noted, liposomes contained 0.1 µCi of [3H]sphingomyelin, [3H]cholesterol, or [3H]DPPC. SCRLs contained 2:1:1:2 (mole ratio) liver or brain PC:sphingomyelin:cerebrosides:cholesterol. "Low sphingolipid SCRLs" (Fig. 1A) contained the same lipids in a 56.7:5:5:33.3 mole ratio. All protein-containing liposomes were large unilamellar vesicles (LUVs). LUVs containing GPI-anchored proteins were constructed by sonication and freeze-thaw as described (10). LUVs containing bacteriorhodopsin were made by co-dialysis of lipids and purified protein as described below.
Protein Purification-- PLAP was purified from a partially pure preparation from Sigma as described (10). Human and bovine erythrocyte acetylcholinesterase were purified as described (20). The peak fraction of the human enzyme eluted from the ligand-linked (acridinium) affinity resin was dialyzed against 5 mM sodium phosphate, pH 7.0, to remove the eluting ligand (decamethonium) but contained 0.1% Triton X-100. The peak fraction of the bovine enzyme was not dialyzed and contained 2% Triton X-114. The specific activity of both preparations was about 5,000 units/mg, where 1 unit = 1 µmol of acetylcholine hydrolyzed per min. Purple membranes isolated from Halobacterium halobium (21) were the kind gift of H. G. Khorana (Massachusetts Institute of Technology, Cambridge, MA). Bacteriorhodopsin, essentially the only protein in the membranes, was solubilized in octyl glucoside as described in Ref. 22 just before incorporation into liposomes. Insoluble bacteriorhodopsin was removed from lysates by pelleting at top speed in an Airfuge for 30 min.
Incorporation of Proteins into Liposomes and Detergent Extraction-- PLAP was incorporated as described (10). Except as noted, each sample contained 2 mg of lipid and about 100 ng of protein (assuming 50% incorporation of protein into liposomes). For acetylcholinesterase, just before incorporation into liposomes, 7.4 µg of human or 11.6 µg of bovine enzyme were pelleted for 10 min at top speed in an Airfuge to pellet any aggregated protein. The supernatant was mixed with liposomes containing 2 mg of total lipid, in a final volume of 550 µl of PBS with a final concentration of 0.18% CHAPS. The mixture was dialyzed against PBS for 2-3 days at 4 °C. Human acetylcholinesterase was incorporated into SCRLs or control liver PC/cholesterol 7:1 liposomes. The bovine protein was incorporated only in SCRLs and not into PC/cholesterol liposomes, as the Triton X-114 in the purified enzyme sample was sufficient to solubilize the latter. Each sample was then subjected to centrifugation in a Sorvall RC M120 ultracentrifuge at 200,000 × g for 1 h. Pelleted liposomes were resuspended in 2 ml of PBS and washed two more times under the same conditions. After the final wash, liposomes were resuspended in 0.7 ml of PBS. For bacteriorhodopsin incorporation, the solubilized bacteriorhodopsin from purple membranes containing 10 µg of protein was added to 2 mg of lipids in a final volume of 1 ml of PBS containing 120 mM octyl glucoside. The mixture was dialyzed against PBS for 2 days. Liposomes were washed once as for acetylcholinesterase and resuspended in 600 µl of PBS. Except where noted, one-half of each of these samples was extracted with detergent as detailed below; the other half served as an unextracted control.
For detergent extraction, except as noted, liposomes were extracted with 0.35-1% Triton X-100 for 20-30 min on ice. Except as noted, lysates or control samples were then subjected to ultracentrifugation for 1-2 h at 200,000 × g, and supernatant and pellet fractions were separated.Polyacrylamide Gel Electrophoresis and Immunoblotting-- Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), transfer to nitrocellulose, and Western blotting were as described (23), except that 16 × 16-cm gels were sometimes run instead of mini-gels, and the Renaissance and ECL reagents were used interchangeably. Ammoniacal silver staining was used (see Ref. 24), except that gels were not fixed or treated with glutaraldehyde, but were only incubated for 1 h in 50% methanol before addition of the silver/ammonia solution.
Analysis of Triton X-100 Insolubility-- Bacteriorhodopsin from supernatant and pellet fractions was precipitated with 10% trichloroacetic acid in the presence of 10 µg of ovalbumin as carrier, resuspended in gel loading buffer, and subjected to SDS-PAGE. PLAP was either analyzed by the same method or was harvested from supernatant and pellet (after solubilizing in 1 ml of PBS containing 1% Triton X-100 with warming to 37 °C for 5 min) by immunoprecipitation as from cell lysates (25) before SDS-PAGE. PLAP was detected by Western blotting using 1-2 µg/ml antibodies and ECL. Bacteriorhodopsin was detected by silver staining. All bands were quantitated using a Bio-Rad GS-670 scanning densitometer.
For acetylcholinesterase, supernatants after ultracentrifugation were adjusted to 1% Triton X-100. Pellets were resuspended in 500 µl of PBS containing 1% Triton X-100. All samples were incubated at 37 °C for 20 min (26) and then concentrated using Centricon 50 concentrators (Amicon, Beverly, MA) to a final volume of 50 µl. Enzyme activity was assayed using a previously described modification (27) of the colorimetric method of Ellman (28). 1 unit of enzyme activity (1 µmol of acetylcholine hydrolyzed per min) corresponds to 3.67Light Scattering Measurements-- DPPC/cholesterol MLVs were prepared and treated with Triton X-100 as described above. The light scattering of each sample before and after Triton X-100 treatment was measured in a 1-cm excitation and 4-mm emission path length quartz cuvette in a Spex 212 Fluorolog spectrofluorimeter, using 0.25-mm excitation and emission slits. Both excitation and emission wavelengths were 550 nm. The cuvette was maintained at 45 °C in a Neslab RTE 100 temperature bath. Samples were incubated in the spectrofluorimeter in the dark for 2 min. Three individual light scattering readings of 1 s each were then made and averaged. The ratio of post-Triton X-100 to pre-Triton X-100 light scattering was calculated.
Concentration of solutions is expressed as percent weight/volume for solids and percent volume/volume for liquids. ![]() |
RESULTS |
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Solubilization of PLAP from Sphingolipid and Cholesterol Containing Liposomes after Saponin Treatment-- Cerneus et al. (17) showed that pretreatment of BeWo cells with a low concentration of saponin, which complexes with membrane cholesterol, resulted in solubilization of PLAP upon addition of Triton X-100. We confirmed this finding in Madin-Darby canine kidney cells (26). Here, we asked whether saponin had a similar effect on PLAP in vitro. We have previously shown that PLAP is insoluble in Triton X-100 when incorporated into SCRLs (10). For reasons that will be discussed below, in this experiment we used "low sphingolipid" SCRLs that contained only 10% total sphingolipid. Liposomes were treated with or without saponin and then extracted with Triton X-100. Soluble and insoluble fractions were separated by ultracentrifugation. Proteins isolated from each fraction were analyzed by SDS-PAGE and Western blotting. As shown in Fig. 1A, saponin pretreatment greatly increased Triton X-100 solubilization of PLAP from these liposomes, as it does from cells. (Average of two experiments: 91% insoluble without saponin and 2% insoluble with saponin.) Fig. 1B will be discussed below.
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Cholesterol Is Not Required for Detergent Resistance of PLAP: Insolubility in Gel Phase Bilayers-- The role of cholesterol in the detergent insolubility of GPI-anchored proteins is not known. According to our model, PLAP is Triton X-100-insoluble because it associates with cholesterol-rich ordered domains. Thus, we predict that the role of cholesterol in insolubility is to maintain lipids in the ordered state. Alternatively, one might imagine that GPI-anchored proteins must interact directly and specifically with cholesterol to be insoluble.
Acyl chains in both lo and gel phase lipids are more ordered than those in the fluid lc phase. At least some lipids (including DPPC) are detergent-insoluble when present in the gel phase (10, 29), as is true for the lo phase (10). Thus, if GPI-anchored proteins partition into an ordered, detergent-insoluble environment, as we proposed, then they should associate with cholesterol-free gel phase domains as well as cholesterol-rich ordered domains. Alternatively, if cholesterol is absolutely required for insolubility, then Triton X-100 should solubilize PLAP from gel phase membranes. Gel phase DPPC liposomes containing PLAP were subjected to detergent extraction. As shown in Fig. 2A, the protein was completely insoluble. However, it seemed possible that the protein might be "forced" into the gel phase as it was the only phase available. To test this possibility, we incorporated PLAP into DPPC/DOPC 1:1 liposomes. As the detergent extraction was performed on ice, DPPC (Tm = 41 °C) was largely present in the gel phase, and DOPC (Tm =
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Cholesterol Enhances GPI-anchored Protein Insolubility by Promoting
Sphingolipid Insolubility--
Our previous results (10) and Fig. 1
(Sap) show that PLAP is Triton X-100-insoluble when it is
present in detergent-insoluble cholesterol and sphingolipid-rich
membranes. The results in Fig. 2 show that the protein is also
insoluble when it is in liposomes containing lipids in the gel phase.
Lipids with high Tm, such as sphingolipids, can form
a gel phase at or near physiological temperatures (30). Alternatively,
they can form an lo phase in the presence of cholesterol
(13). Thus, we might expect that after removal of cholesterol with
saponin, sphingolipids in cell membranes or SCRLs would form a
detergent-insoluble gel phase and that PLAP could associate with this
phase. Why then was PLAP solubilized from SCRLs after saponin treatment
(Fig. 1A, +Sap)?
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The Saponin Effect Can Be Overcome at High Sphingolipid Concentrations-- These results support the prediction of our model that saponin facilitates Triton X-100 solubilization of PLAP by destroying the detergent-insoluble lipid phase with which insoluble PLAP associates. They also predict that the saponin effect could be overcome if the amount of sphingolipid were high enough to form a detergent-insoluble phase after cholesterol removal. The insoluble lipid observed at higher sphingolipid concentrations in the absence of cholesterol in Fig. 3A, circles, is probably in the gel phase, as gel phase separation can be detected by other methods in similar lipid mixtures (31). To test this prediction, we examined the effect of saponin treatment on Triton X-100-solubilization of PLAP from our usual SCRLs, which contain a relatively high sphingolipid concentration of 33 mol %. As shown in Fig. 1B, 83% of PLAP remained detergent-insoluble even after saponin treatment, whereas 99% of the protein was insoluble in the absence of saponin (average of five experiments). As we know from Fig. 3 and the experiments described above that the lipid insolubility of these saponin-treated liposomes is very similar to that of liposomes that lack cholesterol, we presume that PLAP associated with detergent-insoluble membranes, presumably in the gel phase, that remained after cholesterol removal.
The experiments presented provide a molecular explanation for the effect of saponin on solubilization of PLAP and provide an example of how our model for the structure of DRMs fits data obtained from cells. In the studies described next, we performed important controls that further establish the validity of our in vitro system.Both Bovine and Human Acetylcholinesterase in SCRLs Are Triton X-100-insoluble-- As PLAP is the only GPI-anchored protein we have studied so far, we next examined the detergent extractability of two other GPI-anchored proteins, bovine and human acetylcholinesterase, in SCRLs. These proteins were incorporated separately into preformed SCRLs by dialysis, as described for PLAP (10). Liposomes were then subjected to detergent extraction. Soluble and insoluble fractions were separated by centrifugation and concentrated, and acetylcholinesterase enzyme activity in both pools was measured (27). To ensure that pelletable protein was not simply aggregated, human acetylcholinesterase was also incorporated into liposomes containing liver PC and cholesterol in a 7:1 ratio. Lipids in these liposomes are fully solubilized by Triton X-100 (10). Due to the high detergent concentration in the purified bovine acetylcholinesterase, it could not be incorporated into liver PC/cholesterol 7:1 liposomes. For this reason, to control for aggregation of this protein, it was incorporated into SCRLs, and the liposomes were extracted in Triton X-100 at 37 °C. After this treatment, GPI-anchored proteins from DRMs from model membranes cells (10) and cells (26, 32) remain in the supernatant after high speed centrifugation. Thus, the amount of bovine acetylcholinesterase that could be pelleted after this treatment was taken as a measure of aggregation. Results are shown in Table I. Significant amounts of both proteins pelleted after Triton X-100 extraction of SCRLs. A fraction of both proteins also pelleted under control conditions, when there should have been no association with DRMs. This was especially true for bovine acetylcholinesterase, which thus appeared to be somewhat aggregated. However, it was clear that a substantially higher fraction of both proteins pelleted after incorporation into SCRLs and extraction on ice than under control conditions. We conclude that both proteins can associate with DRMs.
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The Transmembrane Protein Bacteriorhodopsin Is Triton X-100-soluble in SCRLs-- We wanted to ensure that not all proteins were resistant to Triton X-100 solubilization when incorporated into SCRLs. Most transmembrane proteins in cells are solubilized by Triton X-100 (26). The multiple membrane-spanning protein bacteriorhodopsin was incorporated into SCRLs. After exposure to Triton X-100 or mock extraction, solubilized and insoluble material were separated by ultracentrifugation. Protein was collected by trichloroacetic acid precipitation from supernatant and pellet fractions, analyzed by SDS-PAGE, and detected by silver staining. Protein in each fraction was quantitated by scanning densitometry of the gel. As shown in Table I, although the protein could be pelleted when liposomes were not extracted with detergent, the protein was efficiently solubilized by Triton X-100. Thus, the detergent insolubility of bovine and human acetylcholinesterase and PLAP must reflect a specific association with insoluble membrane domains.
Detergent-resistant Membranes Are Not Formed from Model Bilayers That Do Not Contain lo Phase Domains-- According to our model, DRMs originate from domains in an ordered state that co-exist with fluid lc phase domains in cell membranes prior to detergent extraction. We were concerned by the alternative possibility that separate phases might not exist in membranes until detergent exposure. DRMs might only be formed after selective solubilization of certain lipids. To test this possibility, we determined whether DRMs could be isolated from liposomes that do not contain ordered domains but that contain lipids with the potential to form such domains. For this experiment, we used a system in which formation of lo phase domains has been well characterized: liposomes containing mixtures of DPPC and cholesterol. The phase diagram showing the conditions of temperature and lipid composition under which lo phase domains form in these mixtures has been derived (14). Above the Tm of pure DPPC (41 °C), a uniform lc phase is present at low concentrations of cholesterol. Above a threshold cholesterol concentration, a separate lo phase begins to form. (The fact that a threshold lipid concentration of either lipid in a binary mixture is required for phase separation is a well known feature of lipid phase behavior.)
We reasoned that if detergent selectively solubilized some of the DPPC in lc phase DPPC/cholesterol mixtures, then the cholesterol concentration remaining in the bilayer would rise, leading to the formation of detergent-resistant lo phase domains. Alternatively, if such detergent-induced reorganization did not occur, then DRMs should only be recovered from liposomes in which lo phase domains were present before detergent extraction. To test this idea, a series of MLVs containing DPPC and cholesterol in varying ratios was constructed. A small amount of [3H]DPPC or [3H]cholesterol was incorporated into these liposomes as a tracer. (Because both tracers gave very similar results, only those using [3H]DPPC are shown.) Liposomes were subjected to detergent extraction at 45 °C (above the Tm of pure DPPC) and then to ultracentrifugation. Supernatant and pellet fractions were separated, and the radioactivity in each was measured. Results are shown in Fig. 4A (squares). The fraction of the lipid in each set of liposomes that was present in the lo phase, as determined from the phase diagram (14), is shown in the same figure (diamonds). Formation of the lo phase occurs at a threshold cholesterol concentration of about 5 mol %. Above about 30 mol % cholesterol, the liposomes are entirely in the lo phase (14). Comparing the two curves showed that insolubility paralleled lo phase formation, although the former was shifted slightly to the right (i.e. to higher cholesterol concentrations) of the latter. Thus, in this system insoluble membranes were only obtained from liposomes that contained lipids in the lo phase before extraction, and detergent did not create insoluble membranes where lo phase domains did not exist.
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PLAP Must Be in a Bilayer That Contains Detergent-resistant Domains to be Detergent-resistant-- It seemed possible that detergent extraction might artifactually induce GPI-anchored proteins to associate with lo phase domains. To test this possibility, we asked whether GPI-anchored proteins must be present in bilayers that contain detergent-insoluble domains to associate with DRMs during solubilization. If detergent extraction drives proteins into DRMs during solubilization, then they might be expected to "hop" into DRMs present in separate bilayers in the same lysate. We performed a mixing experiment to address this question. PLAP was incorporated into LUVs containing liver PC and cholesterol in a 7:1 ratio. These liposomes are fully solubilized by Triton X-100 (10). The PLAP-containing 7:1 liposomes were mixed with protein-free PC/cholesterol 7:1 LUVs (Fig. 5, left) or with SCRLs (Fig. 5, right), and the mixtures were subjected to detergent extraction and ultracentrifugation. As expected, no detectable pellet was observed after extraction and ultracentrifugation of PC/cholesterol 7:1 liposomes. A pellet of the expected size was observed in the tube containing SCRLs mixed with PLAP-containing PC/cholesterol 7:1 after detergent extraction and ultracentrifugation, indicating that DRMs derived from the SCRLs were present. Nevertheless, as shown in Fig. 5, the protein was almost completely solubilized in both cases. Thus, PLAP must be in a bilayer that contains detergent-resistant domains during solubilization to be detergent-resistant.
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DISCUSSION |
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We have proposed that DRMs exist in cell membranes as lipid domains that are more ordered than the conventional, fluid state. We have also suggested that GPI-anchored proteins are detergent-resistant because their acyl chains interact preferentially with these domains. The data provided here, including a straightforward molecular explanation for the importance of cholesterol and sphingolipid in the detergent resistance of GPI-anchored proteins in cells, provide strong additional support for several aspects of our model.
Insolubility of Proteins in Liposomes Mimics That in Cell Membranes-- Phase behavior of membrane lipids is much better understood in model membranes than in cells. This is because many of the techniques used to study phase separation in liposomes are not feasible in cells, due to the greater complexity of biological membranes. Detergent insolubility is an exception. In this and another study (31), we have found a good correlation between detergent insolubility and the presence of ordered domains. This suggests that detergent insolubility provides a valuable tool for studying phase behavior in membranes. In this study, we found striking parallels between the insolubility behavior of both GPI-anchored proteins and lipids in cells in our model system and that previously reported for cells. Insolubility was observed both in vivo and in vitro. In addition, in both cases, insolubility depends on membrane sphingolipid and cholesterol. Saponin treatment abolishes Triton X-100 insolubility of both lipids and GPI-anchored proteins in model membranes, as shown here, and in cells (17, 26).
Clearly, however, liposomes model cell membranes only imprecisely. The asymmetry of the lipid bilayer and the presence of proteins in cell membranes are probably among the most important differences between the two systems. In addition, it is possible that some biological phospholipids may participate in the formation of ordered domains. Phospholipids containing highly unsaturated acyl chains would be unlikely to do so. However, there is evidence that palmitoyloleoyl-PC, which is one of the most abundant phospholipids in mammalian cell membranes, may sometimes form an lo-like phase with cholesterol (15). This may explain why some insoluble membranes were isolated in mixtures of bovine PC and cholesterol, even in the absence of sphingolipids (Fig. 3A), although no insoluble membranes could be isolated from similar mixtures of DOPC and cholesterol (10). For these reasons, although our liposome system is useful in providing a model for the phase behavior of lipids in cell membranes, it cannot be used to predict the exact lipid composition required for formation of ordered domains in vivo. Nevertheless, the fact that saponin abolishes Triton X-100 insolubility in cells suggests that they, like low sphingolipid SCRLs, have a lipid composition that is sufficient for formation of ordered domains in the presence but not the absence of cholesterol.Cholesterol Promotes Formation of a Detergent-insoluble Phase, Explaining the Cholesterol Requirement for Triton X-100 Insolubility of GPI-anchored Proteins-- There has been no explanation for the observation that cholesterol is required for the Triton X-100 insolubility of GPI-anchored proteins in cells (17, 19). Our finding that PLAP was insoluble in gel phase liposomes showed that insolubility does not strictly require cholesterol and suggests that the role of cholesterol in insolubility is to promote DRM formation. The fact that saponin treatment did not lead to complete Triton X-100 solubilization of PLAP or lipids in SCRLs that contained high amounts of sphingolipids is consistent with this model.
These findings can be explained by the phase behavior of phospholipid, sphingolipid, and cholesterol mixtures. In another study (31), we analyzed phase separation using a detergent-free fluorescence quenching assay. As expected, in mixtures of low Tm phospholipid and sphingolipid, phase separation and formation of a gel phase occurred above a threshold concentration of sphingolipid. Similarly, an lo phase formed above a threshold concentration of sphingolipid in ternary mixtures containing phospholipids, sphingolipids, and a constant amount of cholesterol. However, phase separation occurred more readily in the presence of cholesterol. Thus, cholesterol was required for phase separation at low but not at high sphingolipid concentrations. These results agree with the recent demonstration by Silvius et al. (35) that cholesterol can promote phase separation in ternary mixtures of low and high Tm PC and cholesterol. Because the effects of saponin are so similar in low sphingolipid SCRLs and in cells, our finding that cholesterol promotes the formation of insoluble domains is very likely to be true in cells as well as liposomes. Together, our data suggest that the plasma membranes of eukaryotic cells contain enough sphingolipids to form ordered domains in the presence of sterols but not enough to form gel phase domains in the absence of sterols. This is likely to be an important evolutionary adaptation, as the presence of gel phase domains in membranes can greatly slow the lateral diffusion of membrane proteins and can even lead to a loss of permeability barrier function. It is interesting to note from comparing Figs. 1 and 3A that PLAP is more insoluble than [3H]sphingomyelin. This is most noticeable in "high sphingolipid" SCRLs after saponin treatment; 83% of the PLAP, but only about 25% of the [3H]sphingomyelin, was insoluble. This difference probably stems from structural differences between the proteins and lipids, which allow the protein to partition more completely into the ordered domains than the lipid. This will be a very interesting area for future examination.Insolubility Behavior of Other Proteins Fits the Model-- In addition to PLAP, two other GPI-anchored proteins, human and bovine erythrocyte acetylcholinesterase, were insoluble in Triton X-100 when incorporated into SCRLs. As detergent insolubility is a general feature of cellular GPI-anchored proteins, this finding provides further evidence that SCRLs are a good model for DRMs in cell membranes. Bacteriorhodopsin, a transmembrane protein, was almost fully solubilized from SCRLs by Triton X-100. This reflects the behavior of most transmembrane proteins in cells and shows that detergent insolubility of the three GPI-anchored proteins we have examined is specific.
The acyl chains of many GPI-anchored proteins are largely saturated (16). We have previously proposed that these proteins partition preferentially into ordered domains, which are rich in saturated chains, and that detergent insolubility results from this association. The structure of the lipid anchors of both human and bovine erythrocyte acetylcholinesterase proteins has been determined (36-38), allowing us to directly examine the effect of acyl chain saturation on detergent insolubility. A large fraction (69%) of bovine acetylcholinesterase molecules contains 18:0 chains at both positions (36). As we would have predicted, this protein was largely insoluble in SCRLs. Human acetylcholinesterase, in contrast, appears to be unusual among GPI-anchored proteins in that it contains predominantly not only 18:0 alkyl but also 22:4 and 22:5 acyl groups (38). However, this protein is also modified with a palmitate chain linked to the inositol ring (37). (This acyl chain is present in immature GPI anchors but is generally removed from mature GPI-anchored proteins (39).) Thus, this protein is anchored in the membrane by one unsaturated and two saturated chains. Our data suggest that this arrangement is sufficient for targeting to lo phase domains. As the shape and size of these domains is not known, it is possible that proteins such as human acetylcholinesterase are positioned at the edges of the domains, with the multiply unsaturated chain extending into the lc phase.Detergent-resistant Membranes Are Not Formed from Model Bilayers That Do Not Contain Ordered Lipid Domains-- Although there is no evidence supporting the idea, it has been suggested that the formation of DRMs may be subject to detergent-induced artifacts. We have taken several approaches to show that this is not the case. First, we have shown that genuine lo phase membranes are detergent-resistant and that DRMs from cells have several of the properties of lo phase domains (10). These data suggest that DRMs isolated from cells are actually in an ordered phase. The possibility can be raised, however, that these domains are not present in cell membranes before detergent extraction but form artifactually as detergent reorganizes lipids into separate phases. However, our results with model DPPC/cholesterol membranes suggested that although detergent extraction might underestimate the amount of membrane in the lo phase, it does not induce the formation of ordered domains artifactually. We also found a close correspondence between the amount of detergent-resistant lipid and the amount of lipid in the lo phase in ternary mixtures of phospholipids, sphingolipids, and cholesterol under similar conditions to those found in plasma membranes (31), suggesting that detergent does not artifactually induce lo phase formation in this system either. Finally, we showed that both PLAP and [3H]sphingomyelin must be in bilayers containing detergent-insoluble domains at the time of solubilization to be detergent-insoluble. They cannot move from fluid phase membranes to insoluble domains in separate membranes present in the same lysate. Together, these studies provide substantial evidence that exposure to detergent does not rearrange membrane lipids to create insoluble domains from previously uniform lc bilayers.
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ACKNOWLEDGEMENTS |
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We are grateful to Dr. T. Rosenberry for purified bovine and human acetylcholinesterase and to Drs. T. Rosenberry and R. Haltiwanger for insightful comments on the manuscript.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grants GM 47897 (to D. A. B.) and GM 48596 (to E. L.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ Present address: Dept. of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616.
¶ Present address: Dept. of Anatomy, Medical College of Yangzhou University, Yangzhou, Jiangsu, China.
To whom correspondence should be addressed. E-mail:
dbrown{at}mcbsgi.bio.sunysb.edu.
1
The abbreviations used are: GPI,
glycosylphosphatidylinositol; PLAP, human placental alkaline
phosphatase; SCRL, sphingolipid- and cholesterol-rich liposome; DRM,
detergent-resistant membrane; lo, liquid-ordered;
lc, liquid crystalline; Tm, melting temperature; PC, phosphatidylcholine; DOPC, dioleoyl-PC; DPPC, dipalmitoyl-PC; [3H]cholesterol,
[7-3H]cholesterol; [3H]DPPC,
L-3-phosphatidyl[N-methyl-3H] choline,
1,2-dipalmitoyl; octyl glucoside,
N-octyl--D-glucopyranoside; CHAPS,
3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate; LUV,
large unilamellar vesicle; MLV, multilamellar vesicle; PBS, phosphate-buffered saline; PAGE, polyacrylamide gel electrophoresis; ECL, enhanced chemiluminescence.
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REFERENCES |
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