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To whom correspondence should be addressed: Dept. of Cell Biology, University of Massachusetts Medical School, Biotech 4, Ste. 306, 377 Plantation St., Worcester, MA 01605. Tel.: 508-856-8661; Fax: 508-856-8774; E-mail:
* This work was supported by Grant GM33048 from the National Institutes of Health.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. The on-line version of this article (available at http://www.jbc.org ) contains information for Fig. 4. § Present address: Scripps Research Institute, 10550 N. Torrey Pines Rd., La Jolla, CA 92037.
Plasma membranes are organized into functional domains both by liquid-ordered packing into “lipid rafts,” structures that resist Triton extraction, and by attachments to underlying cytoskeletal proteins in assemblies called “membrane skeletons.” Although the actin cytoskeleton is implicated in many lipid raft-mediated signaling processes, little is known about the biochemical basis for actin involvement. We show here that a subset of plasma membrane skeleton proteins from bovine neutrophils co-isolates with cholesterol-rich, detergent-resistant membrane fragments (DRMs) that exhibit a relatively high buoyant density in sucrose (DRM-H;d ∼1.16 g/ml). By using matrix-assisted laser desorption/ionization time of flight and tandem mass spectrometry, we identified 19 major DRM-H proteins. Membrane skeleton proteins include fodrin (nonerythroid spectrin), myosin-IIA, myosin-IG, α-actinin 1, α-actinin 4, vimentin, and the F-actin-binding protein, supervillin. Other DRM-H components include lipid raft-associated integral membrane proteins (stomatin, flotillin 1, and flotillin 2), extracellular surface-bound and glycophosphatidylinositol-anchored proteins (IgM, membrane-type 6 matrix metalloproteinase), and intracellular dually acylated signaling proteins (Lyn kinase, Gαi-2). Consistent with cytoskeletal association, most DRM-H-associated flotillin 2, Lyn, and Gαi-2 also resist extraction with 0.1 m octyl glucoside. Supervillin, myosin-IG, and myosin-IIA resist extraction with 0.1 m sodium carbonate, a treatment that removes all detectable actin, suggesting that these cytoskeletal proteins are proximal to the DRM-H bilayer. Binding of supervillin to the DRM-H fragments is confirmed by co-immunoaffinity purification. In spreading neutrophils, supervillin localizes with F-actin in cell extensions and in discrete basal puncta that partially overlap with Gαi staining. We suggest that the DRM-H fraction represents a membrane skeleton-associated subset of leukocyte signaling domains.
detergent-resistant membrane fragments
DRMs with higher buoyant densities (1.15–1.18 g/ml)
DRMs with lower buoyant densities (1.09–1.13 g/ml)
antibodies against the amino-terminal 340 amino acids of human supervillin
the amino-terminal 340 amino acids of human supervillin
matrix-assisted laser desorption/ionization time of flight
tandem mass spectrometry
membrane-type 6 matrix metalloproteinase
Plasma membranes in eukaryotic cells are organized into domains of specialized function. Many of these domains are relatively stable on the cell surface and involve a series of linkages between cytoplasmic cytoskeletal proteins, integral membrane proteins, and extracellular attachments to the substrate or to other cells. For example, intermediate filament anchorages to membranes at desmosomes and hemidesmosomes distribute mechanical stresses over epithelial surfaces and maintain the integrity of skin and other tissues (
). In each of these domains, interactions between cytoskeletal proteins provide resistance against disruption by nonionic detergents, such as Triton X-100 and octyl glucoside, permitting the transmembrane protein assembly to be isolated as a “membrane skeleton” (
Lipid-lipid interactions also can organize membranes into detergent-resistant domains. Such lipid “rafts” are stabilized against disruption by cold Triton X-100 through liquid-ordered packing of component cholesterol, (glyco)sphingolipids, and gel-phase glycerolipids (
). When analyzed in vivo by spectroscopic methods, plasma membrane lipid rafts consist of clusters of ∼20 proteins and ∼3500 sphingolipid molecules and are ∼50 nm in diameter, a size well below the resolution limit of light microscopy (
). These dimensions can be increased into the submicron to micron range through aggregation of raft components, e.g. by receptor activation or addition of bivalent antibodies against raft-associated surface antigens (
). After treatment of cells or membranes with cold Triton X-100, the coalescing detergent-resistant remnants of the lipid rafts are called detergent-resistant membranes (DRMs).1 DRMs, which represent a subset of the endogenous raft lipids and proteins (
). Selective recruitment of proteins from the cytoplasm or protein diffusion within the plane of the membrane can regulate signaling events, such as those downstream of growth factor addition, ion channel activation, or changes in cell adhesion. Because many signaling proteins, including heterotrimeric Gα proteins and members of the Src family of protein tyrosine kinases, are dually acylated, lipid rafts have been proposed as platforms for the controlled interaction of substrates and signaling enzymes (
). Compartmentalized signaling at lipid rafts may be especially important for hematopoietic cells, which form dynamic attachments to substrates and other cells and lack many types of stable cell surface adhesions. Although signaling downstream of lipid raft-associated adhesive stimuli requires filamentous actin (
). They engulf microbes coated with complement proteins and/or immunoglobulins and kill the ingested microorganisms by releasing proteases, antimicrobial proteins, and oxygen metabolites into phagolysosomal compartments. Although polarity is maintained by cytoplasmic microtubules (
Despite its importance in early immune defense, the neutrophil membrane skeleton is largely uncharacterized. In agreement with previous observations of a spectrin-, actin-, and band 4.1-containing membrane skeleton in neutrophils (
). We also have used F-actin blot overlays and immunoblotting to show that neutrophil plasma membranes contain ezrin, moesin, and a 205-kDa F-actin-binding protein that we named supervillin. Molecular cloning of supervillin cDNA indicated that this protein contains a carboxyl terminus with predicted similarities to villin and a novel amino terminus (
), implying a role during physiological activation of neutrophils.
Reagents and Antibodies
The protease inhibitors, 4-(2-aminoethyl)benzenesulfonyl fluoride, leupeptin, and bestatin, were purchased from Calbiochem-Novabiochem. Density gradient centrifugation media, PercollTM and OptiprepTM, were from Amersham Biosciences and Nycomed Pharma AS (Oslo, NOR), respectively. Tosyl-activated or protein A-conjugated Dynabeads M-280 were from Dynal Inc. (Lake Success, NY). Nonspecific control rabbit IgG and mouse monoclonal primary antibodies against α-actinin, β-actin, and vimentin were purchased from Sigma. Monoclonal anti-Lyn and anti-Gαi-2 antibodies were from Santa Cruz Biotechnology (Santa Cruz, CA). Monoclonal anti-flotillin 2 was from BD Biosciences; monoclonal anti-α-fodrin was from ICN Biomedicals Inc. (Aurora, OH), and polyclonal rabbit anti-non-muscle myosin-II IgG was from Biomedical Technologies Inc. (Stoughton, MA).
Myosin-IG was stained by a monoclonal antibody raised against chicken brush-border myosin (myosin-IA) that recognizes the head domain of multiple myosin-I isoforms (
). These antibodies were the kind gifts of Dr. Mark Mooseker (Yale University, New Haven, CT) and Dr. Martin Bähler (Westfälische Wilhelms-Universität, Münster, Germany), respectively. Horseradish peroxidase (HRP)-conjugated sheep IgG against bovine IgM was from Serotech Ltd. (Oxford, UK). HRP-conjugated, as well as alkaline phosphatase-conjugated, goat anti-rabbit IgG and goat anti-mouse IgG were purchased from Jackson ImmunoResearch. AlexaFluor 488 goat F(ab′)2 anti-rabbit IgG and AlexaFluor 568 goat F(ab′)2 anti-mouse IgG were purchased from Molecular Probes (Eugene, OR). All other chemicals were purchased from Sigma.
) containing human supervillin cDNA sequences (GenBankTM accession number AF051851) was digested with EcoRI and NotI and ligated into the pGEMEX-1 vector (Promega Corp., Madison, WI). A chimeric fusion protein consisting of 260 amino acids of T7 gene 10 and linker sequence plus the first 340 amino acids of human supervillin (H340) was expressed in BL21(DE3) bacteria after induction with isopropyl β-d-thiogalactopyranoside. The fusion protein was purified as inclusion bodies and used as an immunogen for the production of rabbit polyclonal antisera (Research Genetics, Huntsville, AL). Antibodies against the amino-terminal 340 amino acids of human supervillin (αH340) were affinity-purified against a chimeric fusion protein of H340 and glutathioneS-transferase (GST). The GST-H340 protein was expressed in bacteria from sequences encoded by the pGEX-6P-1 vector and HSV41 cDNA and was purified on glutathione-SepharoseTM, as described by the manufacturer (Amersham Biosciences). All manipulations were carried out at 4 °C, unless stated otherwise. Purified GST-H340 (∼12 mg) was dialyzed against 20 mm sodium phosphate buffer (pH 7.6) and coupled to ∼1.5 ml of CNBr-activated Sepharose 6MB (Sigma) for 24 h. The column was blocked with 0.2m glycine ethyl ester (pH 8.0) for 24 h, washed with 40 ml of sodium phosphate buffer (pH 7.6), 2 ml of 4.5 mMgCl2, 10 ml TBS (25 mm Tris-HCl, 1 mm EDTA (pH 7.4)), and stored in TBS containing 0.1% thimerosal. The GST-H340 Sepharose beads were rotated overnight with 10-ml aliquots of specific rabbit antisera and washed sequentially at room temperature with the following: 1) ∼150 ml TBS; 2) 6 ml of 1m NaCl in TBS; 3) 4 ml of 2 m urea in TBS; and 4) 2 ml of 10 mm Tris-HCl (pH 7.5). High affinity antibodies were eluted with 4.5 m MgCl2, 72.5 mm Tris (pH 7.0) and collected in 1-ml fractions with equivalent volumes of 10 mm Tris-HCl (pH 7.5). Fractions containing affinity-purified IgG were pooled, repeatedly diluted with 10 mm Tris-HCl (pH 7.5), and re-concentrated to ∼0.3 ml by centrifugation at 2000 × g using Ultra-Free-10TM ultrafiltrate concentrators (Millipore Corp., Bedford, MA). Concentrated antibody was clarified by centrifugation at 13,000 × g for 5 min and stored in aliquots at −20 °C with ∼1 mg/ml bovine serum albumin and 50% (v/v) sterile glycerol. The specificity of αH340 antibody was confirmed by immunoblots of whole neutrophil extracts (Fig.1A), and the suitability for staining of paraformaldehyde-fixed samples was demonstrated using immunolabeling experiments with COS-7 cells expressing a GFP-tagged bovine supervillin (not shown). H340 antigen was purified by cleavage from GST-H340 and chromatography on glutathioneTM-Sepharose and DEAE-Sepharose, as recommended by the manufacturer (AmershamBiosciences).
Polymorphonuclear leukocytes were routinely isolated from 8–10 liters of bovine blood provided by Research 87 Inc. (Marlborough, MA). Peripheral blood was collected from a single animal, immediately mixed 9:1 (v/v) with anti-coagulant solution (3.8% (w/v) sodium citrate, 2.5 mm EDTA, 50,000 units of heparin), and stored on ice overnight. Neutrophils (∼95% pure) were obtained by hypo-osmotic lysis of erythrocytes, followed by fractionation on preformed gradients of isotonic PercollTM(
) (10 mmPIPES, 100 mm KCl, 3 mm NaCl, 3.5 mm MgCl2, 1 mm ATP, 1 mm EGTA, pH 7.3) containing a mix of protease inhibitors (1 mm phenylmethylsulfonyl fluoride, 0.4 μmaprotinin, 10 μm E-64, 20 μm leupeptin, 10 μm bestatin, 5 μmN-acetyl-Leu-Leu-Met-al, 2 μm pepstatin), and disrupted by nitrogen cavitation. All manipulations were carried out at 0–4 °C. Cavitates were centrifuged at 1000 × g for 10 min to remove nuclei, and 10-ml aliquots of postnuclear supernatant were loaded onto 28 ml of 1.06 g/ml PercollTM suspension in relaxation buffer. Initially, plasma membranes were separated from secretory vesicles, granules, and cytosol using PercollTMstep gradients, as described previously (
). As a faster alternative to manual pipetting, PercollTM density gradients were generated by centrifugation. Density gradients were generated by centrifugation at 50,000 × g for 30 min in a Beckman VTi50 vertical rotor. Neutrophil plasma membranes corresponded to the “γ fraction” (∼1.04 g/ml) from these gradients. This fraction was enriched ∼10-fold in cell surface (
) markers. After centrifugation at 140,000 × g for 2 h to remove residual PercollTM particles, plasma membranes were resuspended in relaxation buffer containing protease inhibitors, disrupted with 10–20 strokes of a tight-fitting Teflon Dounce homogenizer, and stored in aliquots at −80 °C.
Neutrophils suspended in HBSS were seeded onto coverslips that were coated with poly-l-lysine or zymosan-activated plasma for 15 min at 22 °C (
). Zymosan-activated plasma was prepared by incubating bovine plasma (from the same batch of peripheral blood used to isolate the neutrophils) with 5 mg/ml zymosan at 37 °C for 60 min and clarified by centrifugation at 100,000 × g for 60 min. Non-adherent cells were removed, and adherent unactivated (poly-l-lysine) or activated (zymosan-activated plasma) cells were incubated at 37 °C in 5% CO2 for 30–60 min in 10 mm potassium phosphate, 130 mm NaCl, 0.6 mm CaCl2, 1 mm MgCl2, 2.7 mm KCl, 0.18% glucose, pH 7.4. Neutrophils were fixed with 4% paraformaldehyde in PBS (pH 7.4) for 10 min, washed three times in PBS, and then permeabilized using 0.5% Triton X-100 for 10 min at 22 °C. Exposed cytoplasmic surfaces of neutrophil plasma membranes were obtained by disrupting cells with a focused jet of 60 mm PIPES, 25 mm HEPES, 10 mm EGTA, 2 mm MgCl2 (pH 6.9) (
), delivered from a syringe with a 26-gauge needle at an oblique angle across the surface of the coverslip. Sheared cells were immediately fixed in 2% paraformaldehyde in the same buffer for ∼15 min at 22 °C. Fixed samples were washed 3 times for 5 min in PBS and blocked at 4 °C overnight, or for up to 3 days, with 10% bovine serum, 0.02% sodium azide, 0.02% thimerosal in PBS (pH 7.4). Samples then were incubated 2–4 h with 3 μg/ml affinity-purified αH340 or nonspecific rabbit IgG alone or together with mouse monoclonal anti-Gαi-2antibody at 2 μg/ml in blocking solution. As an additional specificity control, aliquots of affinity-purified αH340 were incubated for 2 h with 10 μg/ml purified H340 antigen before addition to coverslips. After three washes in PBS, samples were incubated with blocking solution containing 2 μg/ml AlexaFluor 488-labeled F(ab′)2 fragment of goat anti-rabbit IgG and either 2 μg/ml AlexaFluor 568-labeled F(ab′)2 fragment of goat anti-mouse IgG or 50 nm rhodamine phalloidin. At these antibody concentrations, no significant background was observed in the absence of primary antibody (not shown). After final washes, samples were mounted with VectaShieldTM mounting medium (Vector Laboratories Inc., Burlingame, CA) and observed using an MC100 Axioskop epifluorescence microscope with filters for Texas Red and fluorescein (Carl Zeiss Inc., Thornwood, NY). Phase contrast and fluorescence micrographs were acquired with a model Retiga-1300 QImaging Camera and processed using Openlab imaging software (Improvision Inc., Lexington, MA). Confocal immunofluorescence images were recorded on a Bio-Rad MRC 1024 laser scanning confocal microscope equipped with a krypton/argon laser and LaserSharp version 3.2 software (Bio-Rad). No bleed-through fluorescence between different channels was detected in double-label experiments.
Detergent-resistant membranes were prepared as shown (Fig. 2A and Fig. 3A). Pelleted neutrophil plasma membranes (Fig. 3A, fraction 1) were resuspended to 1–2 mg of membrane protein per ml of Triton X-100 extraction buffer (TEB: 1% Triton X-100, 25 mm Tris, 250 mm NaCl, 2.5 mm MgCl2, 1 mm EGTA, 1 mm ATP) containing protease inhibitors and were incubated for 60 min on ice. Samples (6 × 2.7 ml) were underlayered with 0.3 ml of 48% (v/v) OptiprepTMin TEB and centrifuged at 100,000 × g for 60 min at 4 °C in a Beckman TLA 100.3 rotor. Supernatant was discarded. The sharp opalescent band at the top of the OptiprepTM cushion, which represented the Triton X-100 extracted plasma membranes (Fig.3A, fraction 2), was collected and adjusted to 30% (v/v) with TEB and/or 60% OptiprepTM. Centrifugation of aliquots (8 × 500 μl) at 350,000 × g for 2 h at 4 °C in a Beckman TLA120.1 rotor generated gradients of OptiprepTM with detergent-resistant membrane fragments (DRMs) floating at low buoyant density (Fig. 2A). Fractions (50 μl) were collected and analyzed for total protein concentration using BCA assay reagent (Pierce), for total cholesterol concentration using Infinity cholesterol assay reagent (Sigma), and for refractive indices using a model 334610 refractometer (Bausch & Lomb, Rochester, NY), according to manufacturer's instructions. Cholesterol-rich DRM fractions were then pooled (total DRMs; Fig. 3A,fraction 4), mixed with an equal volume of TEB, and centrifuged into linear 20–45% (w/v) sucrose density gradients at 200,000 × g for 18 h at 4 °C in a Beckman SW50.1 rotor. Single gradients were fractionated (300 μl) at a rate of 0.5 ml/min using a model 640 density gradient fractionator (Isco Inc., Lincoln, NE). Aliquots were assayed for total protein and density and analyzed by SDS-PAGE, silver staining, and Western blotting, as described in the figure legends. Fractions containing lighter (DRM-L, d ∼1.09–1.13 g/ml; Fig. 3A,fraction 5) and denser (DRM-H, d ∼1.15–1.18 g/ml; Fig. 3A,fraction 6) DRMs were identified, pooled, and stored at −80 °C until use.
Transmission electron microscopy of plasma membrane thin sections was carried out as detailed earlier (
), with minor modifications. Briefly, DRM-H fractions were deposited onto EM grids that had been coated with carbon and glow-discharged on the day of the experiment. Samples were allowed to adhere for 30–60 s and stained for 30 s with 2% OsO4 in 0.1 m cacodylate buffer (pH 7.4) and for 30 s with 2% uranyl acetate in PBS (pH 7.4). After each treatment, grids were rinsed with 6–10 drops of PBS and drained using filter paper. Stained samples were air-dried and examined on a model EM-301 transmission electron microscope (Philips Electron Optics Inc., Rahway, NJ) at an accelerating voltage of 60 kV.
Octyl Glucoside and Sodium Carbonate Extractions
DRM-H preparations (600 μg) were diluted 1:1 with relaxation buffer and pelleted by ultracentrifugation (100,000 × g, 60 min, 4 °C) using a Beckman TLA120.2 rotor. Pellets (100 μg) were resuspended in 200 μl of TEB, or 0.1 m octyl glucoside extraction buffer (0.1 m octyl glucoside, 25 mmTris, 250 mm NaCl, 2.5 mm MgCl2, 1 mm EGTA, 1 mm ATP), or 0.1 mNa2CO3 and extracted for 60 min on ice. As a positive control, the pellet was completely solubilized by incubation in 200 μl of SDS at 70 °C for 15 min. Samples were then re-centrifuged as above using a Beckman TLA120.1 rotor, and the pellets were resuspended to 200 μl with TEB. Both supernatants (S) and resuspended pellets (P) were adjusted to final concentrations of 1% Triton X-100, 250 mm NaCl, 50 mm Tris-HCl (pH 7.5). Volumes equivalent to 100 μl of the initial assay mixture were analyzed by SDS-PAGE, silver staining, and Western blotting.
Gel Electrophoresis and Immunoblotting
Protein or volume equivalents of OptiprepTM/sucrose density gradient fractions or DRM-H isolates/extracts were separated at 4 °C on high resolution 100 × 140 × 1.5-mm 5–15% gradient polyacrylamide SDS gels (
). Molecular mass standards were prestained full range Rainbow markers (AmershamBiosciences). Separated proteins were documented by digitally scanning gels using a model GS-700 imaging densitometer (Bio-Rad). For immunoblotting, gels were electrotransferred to nitrocellulose membranes (Schleicher & Schuell) at 4 °C. Blots were 1) blocked with 5% (w/v) nonfat dry milk in TBST (TBS containing 0.05% Tween 20) for 60 min, 2) incubated with the appropriate antibody for 2–4 h in this blocking solution, 3) washed 3 times for 5 min in TBST, 4) incubated for 1–2 h with alkaline phosphatase- or HRP-conjugated goat anti-rabbit or anti-mouse IgG in blocking solution, 5) washed 3 times for 5 min in TBST, and 6) visualized with BCIP/NBT Membrane Phosphatase Substrate (Kirkegaard & Perry Laboratories, Gaithersburg, MD) or RenaissanceTM Western blot Chemiluminescence Reagent Plus (PerkinElmer Life Sciences), as appropriate. Results were quantified after exposure to BioMax MR Scientific Imaging Film (Eastman Kodak Co.) and/or digitally scanned, as above.
MALDI-TOF and MS/MS via MALDI Post Source Decay
For identification of DRM-H associated proteins, protein bands were excised from Coomassie Blue-stained gels with clean razor blades. Gel pieces (∼1.5 × 1.5 × 6 mm) were rinsed 3 times for 5 min in deionized water. In-gel tryptic digests were performed according to published protocols (
) with minor modifications, e.g. the addition of 0.01%n-octyl glucopyranoside in the digest buffer. Peptide digests were concentrated using ZipTip C18 micropipette tips (Millipore Corp.) according to recommended protocols, with the exception that the ion-pairing agent trifluoroacetic acid was replaced by 1% formic acid. MALDI-TOF and Post-Source-Decay (PSD) data were obtained on an AXIMA-CFR mass spectrometer using software version 2.0.1 (Kratos Analytical, Manchester, UK). Typically, mass spectrometric spectra were obtained at a laser power near threshold with pulsed extraction optimized for 2,000 Da; mass resolutions varied from 6,000 to 9,000 across the tryptic peptide mass range. PSD data were obtained at a laser power ∼15% higher than threshold. Samples were co-crystallized with α-cyano-4-hydroxycinnamic acid (10 mg/ml in 1% formic acid, 50% acetonitrile) via the dried drop method. External calibration in MS mode was accomplished by a 3-point peptide mixture that covered the mass range typical of tryptic digests (600–2,500 Da) and spotted locally to the sample of interest. PSD fragment ions were fitted to a generated curve, which was calibrated with PSD fragments from the synthetic peptide, P14R. Data base searches were performed using Protein Prospector version 3.4.1 (prospector.ucsf.edu). Monoisotopic peptide masses were searched against the NCBI nonredundant data base using the MS-Fit program and 50–100 ppm mass tolerances. PSD fragments were searched against the NCBI nonredundant data base using MS-Tag and 0.2 Da as the parent tolerance and 0.5 Da as the fragment ion tolerance.
Preparative immunoisolations of supervillin DRM complexes were performed as described previously (
) and confirmed by analytical scale experiments, using M280 tosyl-activated or protein A-conjugated Dynabeads. Affinity-purified rabbit αH340 IgG and nonspecific control rabbit IgG (0.5-mg aliquots) in PBS (pH 7.5) were coupled to washed Dynabeads by end-over-end rotation overnight at 37 °C (tosyl-activated) or 4 °C (protein A-conjugated). Protein A-bound IgG was cross-linked using 20 mm dimethylpimelimidate in 0.2 m sodium borate (pH 8.5) for 45 min at room temperature. Unreacted sites on protein beads were blocked in 0.2 m glycine ethyl ester (pH 8.2) overnight at 4 °C. Cross-linked antibody beads were washed 3 times for 10 min in borate buffer (pH 8.2), 1 time for 10 min in 0.1m sodium acetate (pH 4.5), and 3 times for 10 min in TEB (pH 7.4). In analytical experiments, aliquots of neutrophil plasma membranes (250 μg of protein) were incubated for 1 h at 4 °C in 0.5 ml of extraction buffer containing 1% Triton X-100 or 0.1m octyl glucoside. The resulting detergent extracts were incubated for an additional 4 h at 4 °C with Dynabeads (∼5 × 107 beads/tube) containing covalently bound supervillin-specific αH340 or nonspecific rabbit IgG plus 1% bovine serum albumin (for silver staining) or 5% goat serum (for immunoblot analyses). Unbound material, including bovine serum albumin and goat serum proteins, was removed by washing the beads 3 times for 10 min in extraction buffer with 1% Triton X-100 or 0.1 m octyl glucoside, respectively. Immunoadsorbed material was magnetically separated, eluted in SDS sample buffer at 70 °C for 10 min, and analyzed by SDS-PAGE, silver staining, and immunoblotting.
Neutrophil Membrane Actin and Supervillin
By using the αH340 antibody specific for supervillin (Fig.1A), supervillin was co-localized with actin filaments at the peripheries of adherent neutrophils (Fig. 1B,e–h). Most unactivated neutrophils appeared round and roughly symmetrical, with evenly distributed cortical F-actin and supervillin (not shown). Activated neutrophils exhibited a spread, polarized morphology with supervillin and F-actin accumulations in ruffles and membrane protrusions (Fig.1B,a–h). To examine signals from proteins closely associated with the plasma membrane, the ventral membrane skeleton was exposed by mechanical shear of spreading neutrophils (
). Membrane sheets representing the cytoplasmic faces of basal membranes contained discrete actin- and supervillin-associated puncta (Fig. 1B, m–p). These puncta were concentrated at or near the leading edges of lamellipodial extensions and occasionally were observed along radially projecting actin bundles. Staining also appeared as small patches within the interiors of the adherent membranes. No patches or puncta were observed with nonspecific rabbit IgG (Fig. 1B,b) or after preadsorption of the αH340 antibody with antigen (Fig. 1B,j), indicating specificity. These results suggest that supervillin is associated with a subset of the F-actin-rich neutrophil membrane skeleton that appears as discrete membrane-associated domains.
We showed previously that Triton-insoluble remnants of the neutrophil plasma membrane contained F-actin and the actin-binding membrane skeleton proteins, fodrin, myosin-IIA, and supervillin (
). Resistance to solubilization by Triton X-100 suggested an association with the underlying cytoskeleton but was also consistent with an association with DRMs. We now report that, after Triton extraction of neutrophil plasma membranes, these membrane skeleton proteins exhibit low densities in OptiprepTM gradients and partition into fractions containing elevated ratios of cholesterol to protein (Fig.2, A and B). These fractions are also enriched in proteins immunoreactive with antibodies against Gαi-2 and Lyn kinase (Fig. 2C). All three of these properties are hallmarks of DRMs (
Further purification of this light membrane fraction (Fig.3A, fraction 4) from OptiprepTM gradients by sedimentation into linear 20–45% sucrose density gradients demonstrated that neutrophil plasma membrane DRMs contain both lower buoyant density (DRM-L; Fig.3A,fraction 5) and higher buoyant density (DRM-H; Fig. 3A,fraction 6) DRMs. DRM-L fractions exhibited densities of ∼1.09 to 1.13 g/ml, values similar to those reported for DRMs containing GPI-anchored proteins, heterotrimeric G-proteins, and Src family kinases (
). DRM-H fractions exhibited densities of 1.15 to 1.18 g/ml (Fig. 3A) and were highly enriched in polypeptides with the mobilities of fodrin and actin (Fig. 3B, arrowheads), membrane skeleton proteins reported to associate with DRMs (
). These and other polypeptides were consistently enriched (see below), along with supervillin (Fig. 3C), during the ∼10–20-fold purification of DRM-H from total Triton-insoluble neutrophil plasma membranes (Fig. 3B). Conversely, the DRM-H fraction was depleted of numerous low molecular mass membrane proteins (Fig.3B,lane 3), as well as of most glycosylated proteins recognized by the lectin, concanavalin A (data not shown).
As visualized by negative stain electron microscopy, the DRM-H fraction contained both membrane sheets and filamentous material (Fig.3D). Cytoskeletal filaments reminiscent of those associated with neutrophil plasma membranes (Fig. 3D, a) also were apparent in micrographs of the DRM-H fraction (Fig. 3D, b). These filaments were associated with membrane fragments with surface areas of 0.05–0.5 μm2 that also contained a semi-regular array of particles with diameters of ∼25 nm (Fig.3D, c). Thus, the DRM-H fraction consisted of detergent-resistant membrane microdomains tightly associated with a discrete set of membrane skeleton proteins.
Abundant DRM-H Proteins
We used a mass spectrometric approach to identify 25 polypeptides in the 23 DRM-H bands that stained most intensely with Coomassie Blue (Fig. 4). These polypeptides were representative of 19 proteins or protein complexes, many of which are characteristic of liquid-ordered microdomains. For instance, the DRM-H proteins, stomatin, flotillin 1, and flotillin 2, are oligomeric integral membrane proteins found in lipid rafts from erythrocytes, brain, lung, and numerous other tissues (
), the dually acylated non-receptor tyrosine kinase Lyn, subunits of the heterotrimeric Gi-2protein, and the GPI-anchored membrane type 6 matrix metalloproteinase (MT-6-MMP, MMP-25, leukolysin). These membrane proteins are targeted to liquid-ordered microdomains through covalent modifications with saturated fatty acyl groups (
Membrane skeleton proteins constituted a second major category of proteins in the DRM-H fraction. We found heterodimeric fodrin (nonerythrocyte spectrins αII and βII), β-actin, and other F-actin binding proteins, including supervillin, α-actinin 1, α-actinin 4, non-muscle myosin-IIA heavy and light chains, and the heavy chain and calmodulin light chain of myosin-IG, an unconventional myosin originally identified as a minor histocompatibility antigen (
). Other proteins, such as clathrin, bactenecin (a positively charged neutrophil defensin), and the G18.1b polypeptide (which is known only as expressed sequence tags and a predicted open reading frame) are less easily categorized.
To confirm that these proteins were in the same structure as supervillin, a tightly bound neutrophil membrane skeleton protein (
), we immunoisolated supervillin-containing lipoprotein complexes from unfractionated detergent extracts of neutrophil plasma membranes (Fig.5). Of the many proteins present in the total Triton X-100 extract of plasma membranes (Fig. 5A,lane a), a specific subset of at least 15 silver-stained polypeptides (Fig. 5A,lane d, arrowheads and dashes) was recovered with beads coated with an affinity-purified IgG specific for supervillin (Fig. 5A,lane b). These 15 polypeptides were not observed in experiments with nonspecific rabbit IgG (Fig. 5A, lane c), indicating specificity. The overall pattern and molecular masses of these polypeptides (Fig. 5A,arrowheads anddashes) were very similar to the SDS-gel profile and sizes of cytoskeletal and signaling proteins identified by mass spectrometric analysis (Fig. 4A). The identities of eight of these major silver-stained polypeptides were confirmed by immunoblotting (Fig. 5B), using antibodies specific for α-fodrin, non-muscle myosin-II, supervillin, Lyn kinase, flotillin 2, β-actin, Gαi-2, and myosin-I head domains. All eight proteins were specifically adsorbed to Dynabeads coated with anti-supervillin IgG (Fig. 5B,αH340), as compared with control rabbit IgG (Fig. 5B,control). The other 10 polypeptides identified in Fig. 4 also may be present in the anti-supervillin pellets but could not be assayed by immunoblot analyses due to antibody unavailability, co-migration with IgG, and/or low molecular mass. Interestingly, similar results were obtained with both 1% Triton X-100, which does not extract lipid raft proteins (Fig. 5B, TEB), and with 100 mm octyl glucoside (Fig. 5B,OG), which does (
). Thus, most or all of the proteins identified as components of the DRM-H fraction are associated in the same lipoprotein complexes as supervillin, and these complexes may contain differentially stabilized lipid raft components.
To test the relative strengths of association between DRM-H proteins and lipids, we extracted the DRM-H fraction with 0.1 m octyl glucoside or 0.1 m sodium carbonate (pH 11.5). Supernatants and extracted membrane pellets were analyzed for the presence of component proteins (Fig. 6). We then organized the resulting immunoblots (Fig. 6, middle columns) from least extractable, i.e. most tightly bound (Fig. 6,top), to most extractable, i.e. most weakly bound (Fig. 6, bottom). Sedimentation of proteins in 1% Triton-extraction buffer (Fig. 6, TEB, left column) or following membrane solubilization in hot 1% SDS (Fig.6, SDS, right column) served as negative and positive controls, respectively. With octyl glucoside (Fig. 6,OG), we observed partial solubilization of Lyn and IgM, but relatively little dissociation of the raft proteins, flotillin 2 and Gαi-2. In extractions with sodium carbonate (Fig. 6,Na2CO3), the integral (flotillin 2) and lipid-anchored (Gαi-2, Lyn) membrane proteins were retained in the pellets, as expected. Extractabilities of the membrane skeleton proteins varied from completely extracted (actin and α-actinin) and ∼50% extracted (fodrin) to only ∼20–25% extracted (myosin-IIA and myosin-IG) and virtually unextracted (supervillin). Because a significant portion of supervillin can be extracted with 1 m KCl and 0.1 n NaOH (pH 13) (
), we conclude that it is a peripheral membrane protein that is very tightly associated with DRM-H lipids and/or integral membrane proteins. Surprisingly, myosin-IIA and myosin-IG are also relatively tightly associated with these membranes, remaining bound even after the removal of all detectable actin.
DRM-H Is a Subset of Neutrophil Lipid Domains
Sucrose density gradient profiles showed that DRM-H was characterized by increased amounts of membrane skeleton proteins, whereas most signaling proteins exhibited a biphasic distribution between DRM-L and DRM-H (Fig.7, A and B). As assessed from immunoblot analyses of equal protein loads, all of the assayed proteins co-isolated to some degree with both DRM-L and DRM-H (Fig. 7B). The major difference between DRM-L and DRM-H was the higher percentages of fodrin, supervillin, myosin-IG, and actin that were associated with DRM-H fractions. DRM-H also contained significant amounts of myosin-IIA, Lyn kinase, the lipid raft marker flotillin 2, and Gαi-2 immunoreactive proteins. Thus, after Triton extraction of neutrophil plasma membranes, there is both considerable overlap between the compositions of DRM-L and DRM-H and a characteristic enrichment of cytoskeletal proteins in association with DRM-H membranes. Increased association of cytoskeletal proteins correlates with increased sedimentability in sucrose density gradients (Fig. 7C).
In agreement with the observation of overlapping and distinct biochemical pools of supervillin and Gαi proteins (Fig.7B), we observed partial but significant overlap between the supervillin and Gαi immunofluorescent signals in adherent membrane fragments from lysed neutrophils (Fig. 7D). This co-localization was especially prominent in puncta at the F-actin-rich membrane peripheries of actively spreading cells (Fig.7D,a–c). By contrast, only occasional overlap of supervillin and Gαi signals was observed in the centers of the membrane remnants (Fig. 7D,d–f). The extent of co-localization observed by immunofluorescence microscopy (Fig. 7D) was roughly consistent with the degree to which supervillin and Gαi-2 immunoreactive proteins co-purified in the DRM-H sucrose density fractions (Fig. 7B). Taken together with the identification of Gαi-2 as a major co-purifying protein (Fig. 4), these observations suggest that supervillin both co-localizes and co-purifies with a subset of Gαi-2-associated signaling domains. These results further suggest that the isolated DRM-L and DRM-H fractions represent different lateral domains within the neutrophil plasma membrane.
In this paper, we present proteomic and microscopic evidence for a DRM-associated membrane skeleton from neutrophil plasma membranes (Figs. Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7). This lipoprotein complex is enriched in cholesterol (Fig.2) and exhibits a higher buoyant density than do DRMs with less associated membrane skeleton material (Figs. 3 and 7, DRM-Hversus DRM-L). Cytoskeletal proteins characteristically enriched in this fraction include fodrin, supervillin, myosin-IIA, myosin-IG, and α-actinins 1 and 4 (Figs. Figure 3, Figure 4, Figure 5, Figure 6, Figure 7). Immunoaffinity purification of supervillin-associated DRM complexes confirms that these proteins are in the same lipoprotein complexes (Fig. 5). Octyl glucoside extraction demonstrates that most of the hydrophobically associated lipid raft proteins in this complex are relatively stabilized against extraction by this detergent (Figs. 5 and 6). Carbonate extraction demonstrates that the peripheral membrane proteins, supervillin, myosin-IIA, and myosin-IG, are very tightly associated with DRM lipids and/or integral membrane proteins, remaining bound even in the absence of detectable actin (Fig. 6). As supervillin and Gαi-2 immunoreactive proteins appear to preferentially co-localize in actin-rich areas of cells engaged in active spreading (Figs. 1 and 7D), the DRM-H fraction may represent a transiently induced, membrane skeleton-associated subset of neutrophil signaling domains (Fig.8).
The buoyant density of the DRM-H fraction (d ∼1.16 g/ml) is probably the reason it has not been described previously. DRM purification methods typically involve the collection of lipoprotein complexes with densities within the range of 1.02 to 1.16 (5–35% sucrose) (
). Presumably, attachment of DRM-associated proteins to the cytoskeleton causes an increased density due to the increased ratio of protein to lipid in the detergent-resistant fraction, resulting in equilibrium banding at intermediate sucrose concentrations. DRMs with densities in the range of our DRM-H fraction (d ∼ 1.15–1.18; 33–40% sucrose) are likely to sediment, or to be spread throughout, the 30–35% sucrose step gradient used in many DRM purification methods (
). Also, the isolated DRM-H fraction is not necessarily homogeneous with respect to the relative ratios of the co-isolating proteins. For instance, the small amount of DRM-H clathrin (Fig. 4) may be caused by a few contaminating coated pits or it may represent a limited association between coated membrane and a subset of DRMs. Other components of the DRM-H fraction, especially proteins present in minor amounts, remain for future analysis. The determination of which proteins interact through direct interactions and which are co-associated through a common affinity for the gel-phase DRM lipids is similarly beyond the scope of this investigation. However, the silver-stained profile of proteins that co-sediment as the DRM-H fraction (Figs. 3, 4, and 7) is similar to the pattern of proteins that are co-purified using antibodies directed against supervillin (Fig. 5). This result suggests that supervillin directly or indirectly associates with lipoprotein complexes containing the other proteins in Fig. 4 and may therefore serve as a marker protein for DRM-H rafts in neutrophil plasma membranes.
Our results are consistent with electron microscopic observations of a subplasmalemmal filament complex associated with Triton X-100-extracted neutrophil ghosts (
). In the latter studies, high resolution scanning electron micrographs revealed that the cytoplasmic surfaces of adherent, spreading neutrophils contain interlocking networks of globular centers and radiating branched filaments. Morphologically similar networks are observed in cross-sections of purified plasma membranes (Fig. 3D,a) and in negatively stained DRM-H samples (Fig. 3D,b andc). Given our observations of myosin-IIA and myosin-IG bound to DRMs lacking detectable F-actin, these domains may represent regions where actin and myosin are recruited and organized at the membrane, as originally implied (
). The partial superimposition of the supervillin and Gαi-2immunofluorescence signals (Fig. 7D) suggests that at least some of these areas of overlap may correspond to domains involved in the active signaling between chemotactic receptors and the actin cytoskeleton.
Actin, spectrin, or fodrin, and/or associated proteins have been co-purified with lipid raft components from other hematopoietic cell types. For example, erythrocyte DRMs are enriched in stomatin, flotillins 1 and 2, β-actin, and a loosely bound member of the spectrin gene family (
). Among a total of ∼70 DRM proteins, including markers for mitochondria, rough endoplasmic reticulum, the Golgi complex, and the nuclear envelope, were sequences representative of fodrin, myosin-IIA heavy and light chains, supervillin, myosin-IG (GenBankTM accession numberAI089291), vimentin, β-actin, flotillin 2, Gαi-2, and the Src-related tyrosine kinase Lck. Although it is not known whether these proteins co-associate with each other in T cells, as opposed to being present in different DRMs, it is striking that 12 of the 25 polypeptides identified in our study are also present in total Jurkat T cell DRMs. Thus, lipid raft-associated membrane skeletons similar to DRM-H may be present in hematopoietic cells of different origin.
The neutrophil DRM-H fraction also exhibits some compositional similarities to the microdomains associated with the multichain receptor for IgE (FcεR) in mast cells (
). Either type of Fcμ receptor would be hard to detect in the presence of the more abundant, co-electrophoresing IgM decamer, leaving open the question of whether the IgM that co-sediments with the neutrophil DRM-H fraction is bound through a specific Fc receptor.
Although not documented previously, the actin-independent association of myosins and other F-actin-binding proteins with DRMs is consistent with reports (
) indicating that lipid raft domains are intrinsically linked to active movements of the cortical actin cytoskeleton. After priming by receptor occupancy with IgE, for example, the FcεR is associated with Lyn in lipid rafts on mast cells (
). Following receptor cross-linking by multivalent antigen, the structure and organization of these microdomains are changed both by actin-based lateral movements of signal transduction proteins within the plane of the membrane and by recruitment of signaling proteins from cytoplasmic stores (
We hypothesize that fodrin, myosin-IIA, myosin-IG, and supervillin are components of a mobile, actin-based membrane skeleton that regulates the organization and/or transport of cholesterol-rich signaling domains at the neutrophil plasma membrane (Fig. 8). Associations with membrane skeleton components also could differentially stabilize and/or localize these domains within the membrane. Whereas the intracellular localization of myosin-IG is unknown, fodrin and myosin-II both localize throughout the cortex, as well as at the plasma membrane (
), suggesting that supervillin may be a characteristic component of this type of lipid raft-associated membrane skeleton. The tight association between supervillin and DRM-H fragments, in conjunction with the high affinity binding of supervillin for F-actin and its ability to reorganize cytoskeletal structures (
), suggests that supervillin may be involved in recruitment of the actin-based cytoskeleton to such liquid-ordered regions of the plasma membrane. The tight association of myosin motors with the DRM-H fragments suggests the possibility of a role in membrane transport or directed motility at the cell surface.
We gratefully acknowledge Dr. Gregory Hendricks for assistance with the electron microscopy, Drs. Mark Mooseker and Martin Bähler for their generous gifts of antibodies against myosin I sequences, and Dr. Silvia Corvera for providing anti-clathrin heavy chain antibody. We also thank Megan Staples for help with immunoaffinity purification, Louise Ohrn for the megaliters of solutions used for plasma membrane purification, and Ernestina Bernal and Donna Castellanos for expert glassware washing.