Immune labeling and purification of a 71-kDa glutamate-binding protein from brain synaptic membranes. Possible relationship of this protein to physiologic glutamate receptors

Immunoblot studies of synaptic membranes isolated from rat brain using antibodies raised against a previously purified glutamate-binding protein (GBP) indicated labeling of an approximately 70-kDa protein band. Since the antibodies used were raised against a 14-kDa GBP, the present studies were undertaken to explore the possibility that the 14-kDa protein may have been a proteolytic fragment of a larger Mr protein in synaptic membranes. Protease activity during protein purification was prevented by introducing five protease inhibitors, and a three-step purification procedure was developed that yielded a high degree of purification of glutamate-binding proteins. The major protein enriched in the most highly purified fractions was a 71-kDa glycoprotein, but a 63-kDa protein was co-purified during most steps of the isolation procedure. The glutamate-binding characteristics of these isolated protein fractions were very similar to those previously described for the 14-kDa GBP, including estimated dissociation constants for L-glutamate binding of 0.25 and 1 microM, inhibition of glutamate binding by azide and cyanide, and a selectivity of the ligand binding site for L-glutamate and L-aspartate. The neuroexcitatory analogs of L-glutamate and L-aspartate, ibotenate, quisqualate, and D-glutamate, inhibited L-[3H]glutamate binding to the isolated proteins, as did the antagonist of L-glutamate-induced neuronal excitation, L-glutamate diethylester. On the basis of the lack of any detectable glutamate-related enzyme activity associated with the isolated proteins and the presence of distinguishing sensitivities to analogs that inhibit glutamate transport carriers in synaptic membranes, it is proposed that the 71-kDa protein may be a component of a physiologic glutamate receptor complex in neuronal membranes.     

Solubilization, partial purification, and reconstitution of glutamate- and N-methyl-D-aspartate-activated cation channels from brain synaptic membranes.

L-Glutamate-activated cation channel proteins from rat brain synaptic membranes were solubilized, partially purified, and reconstituted into liposomes. Optimal conditions for solubilization and reconstitution included treatment of the membranes with nonionic detergents in the presence of neutral phospholipids plus glycerol. The affinity batch chromatography procedure described previously [Chen et al. (1988) J. Biol. Chem. 263, 417-427] was used to obtain a fraction enriched in glutamate-binding proteins. Quench-flow procedures were developed to characterize the rapid kinetics of ion flux induced by receptor agonists. [14C]Methylamine, a cation that permeates through the open channel of both vertebrate and invertebrate glutamate receptors, was used to measure the activity of glutamate receptor-ion channel complexes in reconstituted liposomes. L-Glutamate caused an increase in the rate of [14C]methylamine influx into liposomes reconstituted with either solubilized membrane proteins or partially purified glutamate-binding proteins. The increase in methylamine influx was dependent on the concentration of L-glutamic acid with an estimated Kact for L-glutamate equal to 0.2 microM for synaptic membrane proteins and 0.32 microM for purified proteins. Of the major glutamate receptor agonists, only N-methyl-D-aspartate activated cation fluxes in liposomes reconstituted with glutamate-binding proteins. Glutamate-activated methylamine flux was completely inhibited by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid. In liposomes reconstituted with glutamate-binding proteins, N-methyl-D-aspartate- or glutamate-induced influx of Na+ led to a transient increase in the influx of the lipid-permeable anion probe S14CN-. Electrophoretic analysis of partially purified proteins reconstituted in liposomes indicated enrichment of several bands, the most prominent being those of molecular size equal to approximately 69, 60, 35, and 25 kDa. Antibodies raised against the purified 71- and 63-kDa glutamate-binding proteins reacted strongly with the approximately 69-kDa band of reconstituted proteins and markedly decreased the initial rate of glutamate-activated cation flux. These results indicate the functional reconstitution of N-methyl-D-aspartate-sensitive glutamate receptors and the role of the approximately 69-kDa protein in the function of these ion channels.     

Molecular organization of glutamate-sensitive chemo-stimulated nerve cell membranes. Interaction of monoclonal antibodies with glutamate-binding membrane proteins in the rat brain, human neuroblastoma and molluscan neurons

Hybrid cells obtained by fusion of myeloma PX63-Ag8-653 with immune splenocytes of BALB/c mice were found to produce monoclonal antibodies with a high degree of specificity to rat and human brain. The kinetics of specific IgG binding to purified fractions of glutamate-binding membrane proteins from rat and human brain were analyzed in Scatchard plots. The presence of a single type of binding sites with Kd = 100 nM was demonstrated. The monoclonal antibodies were shown to inhibit the specific binding of tritium-labeled L-glutamate to different brain synaptic membranes. Addition of monoclonal antibodies to the incubation medium induced a modulating effect of physiological responses to L-glutamate in Planorbarius corneus neurons. The possible use of specific antibodies to glutamate-binding proteins as immunochemical markers for the study of glutamate receptor topography on membrane surface was demonstrated with the aid of neuroblastoma cells N18 Tg2a and rat brain tissue slices. An analysis of glutamate receptor binding sites with the use of monoclonal antibodies revealed that these antibodies specifically recognize the active center in the receptor molecules which have identical antigen determinant sites in different biological systems.     

Correlation of glutamate binding activity with glutamate-binding protein immunoreactivity in the brain of control and alcohol-treated rats

Specific antibodies raised against a glutamate binding protein purified from bovine brain were used to trace the immunoreactivity of this protein in rat brain subcellular fractions. In the subcellular fractions obtained from whole brain homogenates, the synaptic membranes had the highest immunochemical reactivity towards the anti-glutamate-binding protein antibodies. The combination of measurements of glutamate binding activity and glutamate-binding protein immunoreactivity indicated that in brain synaptic membranes from control animals the highest activity in these two measures was associated with a synaptic plasma membrane subfraction that was enriched with synaptic junctions. In animals treated with ethanol for 14 days, there was a significant increase in the density of synaptic membrane glutamate binding sites. This increase in glutamate binding capacity was correlated with a greater than two-fold increase in the glutamate binding activity and binding protein immunoreactivity of the light synaptic membrane subfraction, a subfraction which does not contain many recognizable synaptic junctions. Acute administration of ethanol to rats produced a moderate but non-significant decrease in glutamate binding capacity of synaptic membranes. The increase in the number of glutamate binding protein subunits in brain plasma membranes may be an adaptive response of central nervous system neurons to the acute effects of ethanol on glutamate synaptic transmission.     

Glutamate-binding membrane proteins from human platelets]

Solubilization of the total membrane fraction of human platelets in a 2% solution of sodium deoxycholate and subsequent affinity chromatography on glutamate agarose resulted in two protein fractions possessing a glutamate-binding activity. As can be evidenced from radioligand binding data, the first fraction contains two types of binding sites (Kd1 = 1 microM, Bmax 1 = 100 pmol/mg of protein; Kd2 = 9.3 microMm Bmax2 = 395 pmol/mg of protein). The second fraction has only one type of binding sites (Kd = 1 microM, Bmax = = 110 pmol/mg of protein). SDS-PAAG electrophoresis revealed the presence in the first fraction of proteins with Mr of 14, 24, 56 and 155 kDa, whereas the second fraction was found to contain 14, 46, 71 and 155 kDa proteins. Solid phase immunoenzymatic analysis using poly- and monoclonal specific antibodies against mammalian brain glutamate-binding proteins revealed a marked immunochemical similarity of the isolated protein fractions with human brain synaptic membrane glutamate-binding proteins.     

A chloride- and calcium-dependent glutamate-binding protein from rat brain. Identification as a ubiquitous constituent of the inner mitochondrial membrane

We have recently solubilized and enriched a chloride- and calcium-dependent glutamate-binding protein from rat brain (Brose, N., Halpain, S., Suchanek, C., and Jahn, R. (1989) J. Biol. Chem. 264, 9619-9625). The partially purified protein fraction, containing two major protein components of 51,000 Da and 105,000 Da, was used to generate a rabbit antiserum. This serum quantitatively precipitated the binding activity from membrane extracts. Small amounts of the antiserum inhibited glutamate binding when chloride was absent from the incubation medium. Three protein bands were labeled by the serum on immunoblots. From the affinity purified antibody fractions contained in the serum, only the antibodies directed against a 51,000-Da protein were able to immunoprecipitate the binding activity, indicating that this protein is an essential component of the binding site. A survey of a variety of rat tissues by immunoblot analysis revealed a ubiquitous distribution of the protein. After subcellular fractionation of liver and brain, the 51,000-Da protein copurified with mitochondrial markers. Furthermore, exclusive labeling of mitochondria was observed by light and electron microscopy immunocytochemistry. Subfractionation of purified liver mitochondria resulted in a selective association of the protein with inner mitochondrial membranes. Pharmacological characterization of glutamate binding to liver mitochondrial membranes revealed a pattern almost identical to that of the chloride- and calcium-dependent glutamate-binding site in rat brain.     

Molecular organization of glutamate-sensitive chemoexcitable membranes of nerve cells. Function of glutamate-binding proteins of the central nervous system when incorporated into liposomes

The functioning of the glutamate-binding protein of rat brain cortex synaptic membranes was studied by its incorporation into liposomes. The optimal conditions for the receptor protein incorporation were established and the kinetics of 22Na+ and 86Rb+ incorporation into the liposomes in the presence of L-glutamate were analyzed. Modelling of the CNS glutamate receptor functions was found to be dependent on the lipid composition and amount of the incorporated membrane protein. The selective transport of 22Na+ into the liposomes was stimulated in the presence of 10(-4) M glutamate. Addition of monoclonal antibodies against glutamate-binding proteins blocked the incorporation of Na+ into the liposomes. The experimental results are suggestive of the nativity of the liposome-incorporated membrane protein, which is capable of binding glutamate and regulating selective transport of Na+. It was assumed that the glutamate receptor macromolecule represents an integral complex made up of several low molecular weight subunits of glucoprotein nature that form a selective ionic channel.     

A Similar Calmodulin-Binding Protein Expressed in Chromaffin, Synaptic, and Neurohypophyseal Secretory Vesicles

The presence of calmodulin-binding proteins in three neurosecretory vesicles (bovine adrenal chromaffin granules, bovine posterior pituitary secretory granules, and rat brain synaptic vesicles) was investigated. When detergent-solubilized membrane proteins from each type of secretory organelle were applied to calmodulin-affinity columns in the presence of calcium, several calmodulin-binding proteins were retained and these were eluted by EGTA from the columns. In all three membranes, a 65-kilodalton (63 kilodaltons in rat brain synaptic vesicles) and a 53-kilodalton protein were found consistently in the EGTA eluate. 125I-Calmodulin overlay tests on nitrocellulose sheets containing transferred chromaffin and posterior pituitary secretory granule membrane proteins showed a similarity in the protein bands labeled with radioactive calmodulin. In the presence of 10(-4) M calcium, eight major protein bands (240, 180, 145, 125, 65, 60, 53, and 49 kilodaltons) were labeled with 125I-calmodulin. The presence of 10 microM trifluoperazine (a calmodulin antagonist) significantly reduced this labeling, while no labeling was seen in the presence of 1 mM EGTA. Two monoclonal antibodies (mAb 30, mAb 48), previously shown to react with a cholinergic synaptic vesicle membrane protein of approximate molecular mass of 65 kilodaltons, were tested on total membrane proteins from the three different secretory vesicles and on calmodulin-binding proteins isolated from these membranes using calmodulin-affinity chromatography. Both monoclonal antibodies reacted with a 65-kilodalton protein present in membranes from chromaffin and posterior pituitary secretory granules and with a 63-kilodalton protein present in rat brain synaptic vesicle membranes. When the immunoblotting was repeated on secretory vesicle membrane calmodulin-binding proteins isolated by calmodulin-affinity chromatography, an identical staining pattern was obtained. These results clearly indicate that an immunologically identical calmodulin-binding protein is expressed in at least three different neurosecretory vesicle types, thus suggesting a common role for this protein in secretory vesicle function.     

Immunochemical detection of adenine nucleotide-binding proteins with antibodies to 5'-p-fluorosulfonylbenzoyladenosine

5'-p-Fluorosulfonylbenzoyladenosine (FSBA) is a useful reagent for the affinity labeling of adenine nucleotide binding proteins. We have developed an immunochemical approach to the detection of proteins that have been covalently modified with FSBA, which provides an alternative to the use of a radiolabeled ligand. Antibodies have been prepared against FSBA-modified glutamate dehydrogenase and purified by chromatography on ATP-agarose. The resulting affinity-purified antibodies react on Western blots only with proteins that have been labeled previously with the affinity reagent. The degree of immunoreactivity on Western blots correlates well with the extent of covalent modification as shown by studies on the modification and inhibition of the catalytic subunit of cAMP-dependent protein kinase. In crude cellular extracts, numerous proteins can be labeled with FSBA and then detected by using this approach. The labeling and subsequent detection of these proteins can be blocked by including an excess of MgATP, which competes with FSBA for nucleotide-binding sites. The labeling of specific proteins in crude mixtures is saturable, as shown by labeling studies of p56lck, a protein-tyrosine kinase that is abundantly expressed in membranes from the T lymphoma cell line LSTRA.     

The distribution of glutamate receptors in cultures of the motor area of the rat cerebral cortex studied by an immunoelectron microscopic method

The aim of the study was to analyse the distribution and localization of glutamate receptors in the cultured cells of the neonatal rat motor cortex, using immunoelectron microscopic technique, and monoclonal antibodies preliminary labeled with colloidal gold. Monoclonal antibodies against glutamate-binding proteins of the adult rat cerebral cortex were produced by means of hybridization of immune splenocytes with plasmocytoma cells. It was found that monoclonal antibodies labeled with colloidal gold could reveal selectively the localization of glutamate receptors on the membranes of neurons. Glutamate receptors were detected on differentiating neuron membranes only, being absent from the glia cell surface.     

Detection with Monoclonal Antibodies of a 15-kDa Proteolipid in Both Presynaptic Plasma Membranes and Synaptic Vesicles in Torpedo Electric Organ

A protein, the mediatophore, has been purified from Torpedo electric organ presynaptic plasma membranes. This protein mediates the release of acetylcholine through artificial membranes when activated by calcium and is made up of 15-kDa proteolipid subunits. After immunization with purified delipidated mediatophore, monoclonal antibodies binding to the 15-kDa proteolipid band on Western blots of purified mediatophore were selected. A 15-kDa proteolipid antigen was also detected in cholinergic synaptic vesicles. Using an immunological assay, it was estimated that presynaptic plasma membranes and synaptic vesicles contain similar proportions of 15-kDa proteolipid antigen. Detection by immunofluorescence in the electric organ showed that only nerve endings were labeled. In electric lobes, the staining was associated with intracellular membranes of the electroneuron cell bodies and in axons. Nerve endings at Torpedo neuromuscular junctions were also labeled with anti-15-kDa proteolipid monoclonal antibodies.     

Biochemical evidence for the presence of an amiloride binding protein in adult alveolar type II pneumocytes.

An amiloride binding protein in adult rat and rabbit alveolar type II (ATII) cells was characterized using three different antibodies against epithelial Na+ channel proteins. We found that 1) polyclonal antibodies raised against epithelial Na+ channel proteins from bovine kidney cross-react with a 135-kDa protein in ATII membrane vesicles on Western blots; 2) using the photoreactive amiloride analog, 2'-methoxy-5'-nitrobenzamil (NMBA), in combination with anti-amiloride antibodies, we found that NMBA specifically labeled the same M(r) protein; and 3) monoclonal anti-idiotypic antibodies directed against anti-amiloride antibodies also recognized this same M(r) protein on Western blots. We also demonstrated a low benzamil affinity binding site (apparent Kd = 370 nM) in rabbit ATII cell membranes and both high and low benzamil affinity binding sites (apparent Kd = 6 nM and 230 nM) in bovine kidney membranes using [3H]Br-benzamil as a ligand. Pharmacological inhibitory profiles for displacing bound [3H]Br-benzamil were also different between ATII cells and bovine kidneys. These observations indicate that adult ATII pneumocytes express a population of epithelial Na+ channels having a low affinity to benzamil and amiloride and a pharmacological inhibitory profile different from that in bovine kidney.     

Protein Methylation in Cerebellar Synaptosomes

Abstract: Synaptosomes from five regions of adult rat brain were isolated, analyzed for methyl acceptor proteins, and probed for methyltransferases by photoaffinity labeling. Methylated proteins of 17 and 35 kDa were observed in all regions, but cerebellar synaptosomes were enriched in a 21–26-kDa family of methyl acceptor proteins and contained a unique major methylated protein of 52 kDa and a protein of 50 kDa, which was methylated only in the presence of EGTA. When cerebellar and liver subcellular fractions were compared, the cytosolic fractions of each tissue contained methylated proteins of 17 and 35 kDa; liver membrane fractions contained few methylated proteins, whereas cerebellar microsomes had robust methylation of the 21–26-kDa group. Differential centrifugation of lysed cerebellar synaptosomes localized the 17- and 35-kDa methyl acceptor proteins to the synaptoplasm, the 21–26-kDa family to the synaptic membranes, and the 52-kDa to synaptic vesicles. The 21–26-kDa family was identified as GTP-binding proteins by [α-³²P]GTP overlay assay; these proteins contained a putative methylated carboxyl cysteine, based on the presence of volatile methyl esters and the inhibition of methylation by acetylfarnesylcysteine. The 52-kDa methylated protein also contained volatile methyl esters, but did not bind [α-³²P]GTP. When synaptosomes were screened for putative methyltransferases by S -adenosyl-L-[ methyl -³H]methionine photoaffinity labeling, a protein of 24 kDa was detected only in cerebellum, and this labeled protein was localized to synaptic membranes.     

Molecular size of the high-affinity glutamate-binding site on synaptic membranes from rat brain

We have used radiation inactivation as a means of determining the molecular size of the high-affinity glutamate-binding site on rat brain synaptic membranes. The molecular size was 75,000 +/- 15,000 in the absence of glutamate and 263,000 +/- 34,000 in the presence of glutamate. These data may be interpreted as suggesting that the high-affinity glutamate-binding site is comprised of a number of subunits. The minimum sub-unit size detected by this method was 75,000 +/- 15,000.     

Functional expression of NADPH oxidase components (alpha- and beta-subunits of cytochrome b558 and 45-kDa flavoprotein) by intrinsic human glomerular mesangial cells.

Recently, we have shown that human glomerular mesangial cells (HMCs) release oxygen radicals from the plasma membrane in response to cytokines. Now we have used diphenylene iodonium, a covalent binding inhibitor of activated 45-kDa flavoprotein, in neutrophils radiolabeled with 125I and could identify a 45-kDa protein band in a separated HMC plasma membrane fraction. Low temperature difference spectroscopy showed a peak absorbance at 428 and 558 nm. Direct potentiometry of HMC membranes (-340 to -160 mV) showed the presence of a low potential cytochrome (76 pmol/mg to HMC membrane protein) identified as cytochrome b558. In slot blots, mouse monoclonal antibody (mAb) 7D5, specific for the extracellular domain of the alpha-subunit, showed a positive reaction with HMCs. In Western blots, mAb 449, directed against the cytoplasmic epitope of the alpha-subunit, identified a 23-kDa protein; and mAb 48, raised against the large (beta) subunit of cytochrome b558 of human neutrophils (Verhoeven, A. J., Bolscher, B. G. J. M., Meerhof, L. J., van Zwieten, R., Keijer, J., Weening, R. S., and Roos, D. (1989) Blood 73, 1686-1694), detected a smear between 75 and 100 kDa in denatured HMC membrane protein. These data determined with HMCs, suggest for the first time the expression of three essential components of NADPH:O2- oxidoreductase in mesenchymal cells.     

Molecular organization of glutamate-sensitive, chemoexcitatory membranes of nerve cells. Physico-chemical characteristics of the glutamate-binding synaptic membrane proteins of the rat cerebral cortex

Some physico-chemical properties of glutamate-binding proteins solubilized from rat cerebral cortex synaptic membranes and purified by affinity chromatography were studied. Purified proteins were shown to be homogenous during SDS polyacrylamide gel electrophoresis (Mr 14000). The Scatchard plots for L-[3H]glutamate binding to the purified membrane proteins revealed the presence of one type of binding sites with Kd 800-1000 nM and Bmax 180-200 pmol/mg of protein. Ultracentrifugation of the glutamate-binding membrane protein in sucrose linear gradient demonstrated that the position of the protein peak depends on protein concentration, i.e. after dilution of the sample the protein peak is shifted from 28 000-30 000 to 12 000-15 000. The values of sedimentation coefficients decrease correspondingly to 2.1S. Presumably, these processes are due to dissociation of receptor macromolecules. The glutamate receptor is a glycoprotein-lipid complex made up of several low molecular weight subunits.     

Glutamate-Dependent Active-Site Labeling of Brain Glutamate Decarboxylase

A major regulatory feature of brain glutamate decarboxylase (GAD) is a cyclic reaction that controls the relative amounts of holoenzyme and apoenzyme [active and inactive GAD with and without bound pyridoxal 5'-phosphate (pyridoxal-P, the cofactor), respectively]. Previous studies have indicated that progression of the enzyme around the cycle should be stimulated strongly by the substrate, glutamate. To test this prediction, the effect of glutamate on the incorporation of pyridoxal-P into rat-brain GAD was studied by incubating GAD with [32P]pyridoxal-P, followed by reduction with NaBH4 to link irreversibly the cofactor to the enzyme. Adding glutamate to the reaction mixture strongly stimulated labeling of GAD, as expected. 4-Deoxypyridoxine 5'-phosphate (deoxypyridoxine-P), a close structural analogue of pyridoxal-P, was a competitive inhibitor of the activation of glutamate apodecarboxylase by pyridoxal-P (Ki = 0.27 microM) and strongly inhibited glutamate-dependent labeling of GAD. Analysis of labeled GAD by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis showed two labeled proteins with apparent molecular masses of 59 and 63 kDa. Both proteins could be purified by immunoaffinity chromatography on a column prepared with a monoclonal antibody to GAD, and both were labeled in a glutamate-dependent, deoxypyridoxine-P-sensitive manner, indicating that both were GAD. Three peaks of GAD activity (termed peaks I, II, and III) were separated by chromatography on phenyl-Sepharose, labeled with [32P]pyridoxal-P, purified by immunoaffinity chromatography, and analyzed by SDS-polyacrylamide gel electrophoresis. Peak I contained only the 59-kDa labeled protein. Peaks II and III contained the both the 59- and 63-kDa proteins, but in differing proportions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rat brain synaptic vesicles are devoid of Mg2+-ATPase activity and contain beta-amyloid precursor protein

Rat brain synaptic vesicles (SVs) isolated by gel filtration on Sephacryl S-500 had little Mg2+(H+)-ATPase activity, though it was identified by Western blots with antibodies against the H+-ATPase A-subunit and other vesicle proteins. In contrast, tyrosine hydroxylase and dopa decarboxylase activities in the SVs were substantial, suggesting that the absence of Mg2+(H+)-ATPase activity was not due to inactivation during isolation but rather to the nature of the SVs. The vesicle component reactive to H+-ATPase antibody was also identified in the synaptosomal cytosol, so the antibody for the A-subunit seemed unnecessary to detect H+-ATPase. The SVs contained beta-amyloid precursor protein of approximately 100 kDa. Based on these observations, SVs without Mg2+(H+)-ATPase seemed to play a role(s) in the delivery of cytoplasmic and plasma membrane proteins to nerve terminals as well as in neurotransmission.     

Properties of kinesin isolated from human prostatic DU 145 tumor cells and bovine brain

We have isolated and compared the 116-kilodalton (kDa) kinesin heavy chain from DU 145 human prostatic tumor cells and bovine brain. Comparative sodium dodecyl sulfate - polyacrylamide gel electrophoreses (SDS-PAGE), Western blots, and proteolytic digestion analysis all showed that the 116-kDa polypeptides from both sources were indistinguishable. Polyclonal antibodies raised against sea urchin kinesin cross-reacted with both brain and DU 145 kinesin on Western blots. SDS-PAGE and A-5m chromatographic studies indicated that kinesin forms a quarternary heteropolymer of approximately 400 kDa. DU 145 cells had three proteins of 116, 72, and 64 kDa forming the heteropolymer, in a 2:1:1 ratio, whereas brain cells appeared to have equimolar amounts of the 116-kDa heavy chain and a 64-kDa light chain.     

Acylation of cell-associated IL-1 by palmitic acid

To determine whether membrane-associated IL-1 is palmitylated, we labeled LPS-activated human monocytes with [3H]palmitic acid. The plasma membranes were isolated, and the membrane proteins extracted and analyzed simultaneously by SDS-PAGE-autoradiography and Western blot analysis from the same gel. When the monocytes were labeled with [3H]palmitate, 23- and 31-kDa bands were visualized, for membrane-associated IL-1 and its precursor, respectively. The 31- and 23-kDa bands were excised from several gels and rehydrated and analyzed again by SDS-PAGE, autoradiography, and Western blot analysis. The 23- and 31-kDa bands appeared again by both methods. To further investigate membrane-associated IL-1 acylation, human monocytes were labeled with [3H]palmitate, the plasma membranes isolated, and the membrane proteins extracted by CHAPS detergent. Immunoprecipitation of isolated membrane proteins using anti-IL-1 antibodies revealed two bands of 23 and 31 kDa after autoradiography. The studies demonstrate that both membrane-associated IL-1 and the IL-1 precursor are acylated with palmitic acid.