Evidence for two functionally different fibrinogen receptors on hemopoietic cells: the glycoprotein IIb-IIIa and the mitogenic fibrinogen receptor

We have previously established that the mitogenic effect of fibrinogen on hemopoietic cell lines Raji and JM is mediated via a specific receptor (Levesque, J.-P. et al.: Proc. Natl. Acad. Sci. USA 83:6494-6498, 1986). In this study, we have further characterized the fibrinogen domain involved in the binding to the mitogenic receptor. This binding was not inhibited either by a monoclonal antibody against the C-terminal sequence of the fibrinogen gamma chains or by synthetic peptides containing the Arg-Gly-Asp sequence. Such inhibition is specific of the platelet fibrinogen receptor, the glycoprotein IIb-IIIa complex. Fragments containing the fibrinogen D domain were the only plasmin degradation products of fibrinogen which were mitogenic. These fragments acted via direct binding on the mitogenic receptor with a Kd of 2.24 X 10(-6) M. This value was similar to the KI value of unlabeled fragments D (2.47 X 10(-6) M). Our results suggest the presence of two different functional types of fibrinogen receptors: the glycoprotein IIb-IIIa receptor responsible both for platelet aggregation and leukocyte adhesion and killing, and the mitogenic receptor involved in proliferation control of hemopoietic cells.     

The down-regulation of the mitogenic fibrinogen receptor (MFR) in serum-containing medium does not occur in defined medium

Normal human hemopoietic cells such as early bone marrow progenitors, or lymphoma-derived cell lines such as Raji or JM cells, possess a low-affinity receptor specific for fibrinogen. This receptor triggers a mitogenic effect. It differs from the glycoprotein IIb-IIIa which is involved in fibrinogen-induced platelet aggregation. We demonstrate here that this mitogenic fibrinogen receptor (MFR) can be internalized or reexpressed, depending on culture conditions. Internalization was temperature-dependent. At 37 degrees C in the presence of cycloheximide or actinomycin D, the half-life of cell surface MFRs was 2 h, independent of receptor occupancy. Binding of fibrinogen to the MFR resulted in a down-regulation which was fibrinogen dose-dependent. This occurred in serum-supplemented medium but not in defined medium supplemented with fatty acids. Reexpression of MFRs could be induced in 28 to 42 h by serum removal. The down-regulation of mitogenic receptors in plasma or serum could explain why normal cells do not proliferate in the peripheral blood.     

The characterization of the receptor for endothelial cell growth factor by covalent ligand attachment

Cellular receptors for endothelial cell growth factor (ECGF) have been demonstrated on several cell types by binding of 125I-ECGF in a specific and saturable manner (Schreiber, A. B., Kennedy, J., Kowalski, J., Friesel, R., Mehlman, T., and Maciag, T. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 6138-6142). Here we report the covalent cross-linking of 125I-ECGF to a polypeptide present on the surface of the plasma membrane of murine lung capillary endothelial cells by the homobifunctional reagent, disuccinimidyl suberate. Cross-linking of cell surface associated 125I-ECGF yields a major polypeptide with an apparent molecular weight of 150,000. Experiments demonstrated that the cross-linked polypeptide complex represents 125I-ECGF covalently bound specifically to a cell surface receptor because: covalent modification of the polypeptide was inhibited by excess, unlabeled ECGF; preincubation of cells with unlabeled ECGF at 37 degrees C significantly reduced cross-linking while incubation at 4 degrees C did not; other polypeptide growth factors do not compete with 125I-ECGF for cross-linking to the ECGF receptor; labeling of the polypeptide did not take place in the absence of DSS; and cells previously shown to have a paucity of ECGF receptors did not yield a cross-linked labeled receptor. These data suggest that the mitogenic events mediated by ECGF occur after occupancy of the specific cell surface polypeptide and suggest that these events are relevant to ECGF-induced signal transduction across the endothelial cell plasma membrane.     

Molecular interactions between human lactotransferrin and the phytohemagglutinin-activated human lymphocyte lactotransferrin receptor lie in two loop-containing regions of the N-terminal domain I of human lactotransferrin.

Fluorescein isothiocyanate derivatization of the human lactotransferrin on Lys-264 inhibits the binding of the protein of human PHA-activated lymphocytes [Legrand, D., Mazurier, J., Maes, P., Rochard, E., Montreuil, J., & Spik, G. (1991) Biochem. J. 276, 733-738], indicating that part of the receptor-binding site is located in the N-terminal domain I of lactotransferrin. In the present study, a 6-kDa peptide (residues 4-52) was isolated from the N-terminal lobe of human lactotransferrin which inhibited the binding of the protein to its cell receptor. In addition, lactotransferrin was derivatized using sulfosuccinimidyl 2-(p-azidosalicylamido)ethyl-1,3'-dithiopropionate (SASD) and sulfosuccinimidyl 6-((4'-azido-2'-nitrophenyl)amino)hexanoate (sulfo-SANPAH), two heterobifunctional reagents generally used for receptor-ligand cross-linking. The azide group of these two reagents was inactivated by photolysis, and only the succinimidyl ester group was allowed to react with lysine residues of the protein. The binding of the derivatized lactotransferrins to the human lymphocyte receptor was assayed. SASD, which binds to Lys-74, was able to inhibit the binding of lactotransferrin to the cell receptor, in contrast to Lys-281-binding sulfo-SANPAH. Molecular modeling showed the position of SASD, sulfo-SANPAH, and fluorescein molecules at the surface of the protein and suggested that SASD and fluorescein could mask residues 4-6 and two loop-containing regions of human lactotransferrin (residues 28-34 and 38-45). The comparison of the primary and tertiary structures of human lactotransferrin and serotransferrin, which bind to specific cell receptors, shows that the above-mentioned regions, which are likely involved in protein-receptor interactions, possess specific structural features.     

Receptor-induced phosphorylation by mammary-derived growth factor 1 in mammary epithelial cell lines.

Previous work has shown that a mammary-derived growth factor (MDGF1), a human milk-derived, acidic, 62-kDa, N-glycosylated growth factor binds to cell surface receptors and stimulates proliferation of mammary epithelial cells. An 18-amino acid N-terminal partial sequence of the factor did not show any homology to other known growth factors or proteins. Using polyclonal antiserum raised against the synthetic peptide, we demonstrated that conditioned medium prepared from human breast cancer cell lines contains the factor. The antibody could adsorb the biological activity of the factor present in the conditioned medium. Earlier experiments on receptor cross-linking indicated that the receptor was approximately 120-140 kDa. Since tyrosine phosphorylation plays a crucial role in cell proliferation and cell transformation, experiments were conducted to find out whether MDGF1 induces the appearance of phosphotyrosine in MDGF1-receptor-positive MDA-MB 468, MCF-7, and 184A1N4 cell lines compared to receptor-negative lines. Western blot analysis using monoclonal antiphosphotyrosine indicated that MDGF1 induces phosphotyrosine in a 180-185-kDa protein in MDGF1 receptor-positive cell lines. Phosphorylation was not blocked and phosphorylated proteins were not immunoprecipitated by an antibody directed against the binding site of the EGF receptor. Cell membrane fractionation demonstrated that phosphorylation induced by MDGF1 was membrane-associated. The nature of this 180-185-kDa protein and its possible relationship to the MDGF1 receptor are under investigation.     

The human cytomegalovirus receptor on fibroblasts is a 30-kilodalton membrane protein.

Previous studies have demonstrated that human cytomegalovirus (HCMV) binding to human foreskin fibroblasts (HFF) is mediated by a single type of molecule, likely a glycoprotein, which serves as a specific receptor for the virus. In the present experiments, HCMV was found to bind to an HFF membrane protein with an approximate molecular mass of 30 kilodaltons (kDa); weak binding to 28- and 92-kDa membrane components was also observed. Binding was specific, as it was inhibited by excess unlabeled HCMV. Radiolabeled HCMV also bound selectively to Raji and Daudi lymphoblastoid cell membrane proteins of the same molecular masses. The 30-kDa radiolabeled HFF membrane protein bound to HCMV in solution; this binding was also specific, as it was blocked by an excess of HCMV. These data suggest that a membrane protein with a molecular mass of approximately 30 kDa mediates HCMV binding to several cell types.     

Structure of the murine erythropoietin receptor complex. Characterization of the erythropoietin cross-linked proteins.

The structure of the murine erythropoietin receptor was studied using antibodies against the intracellular part of the cloned erythropoietin receptor chain. These antibodies precipitated erythropoietin-receptor complexes from Triton X-100-solubilized cells. When the complexes were cross-linked by disuccinimidyl suberate, the 85- and 100-kDa erythropoietin-cross-linked proteins previously described were immunoprecipitated. However, these proteins were not precipitated when the complexes were denatured and reduced before immunoprecipitation. Using 1-ethyl 3-(3-dimethylaminopropyl)carbodiimide, we observed erythropoietin cross-linking with a protein of 66 kDa in addition to the 100- and 85-kDa proteins. Only the 66-kDa erythropoietin-cross-linked protein was immunoprecipitated by anti-receptor antibodies after denaturation and reduction of the complex. Thus, our results suggest that the 85- and 100-kDa proteins previously evidenced by cross-linking are associated with the cloned chain of the receptor to form a multimeric complex but these proteins seem immunologically unrelated to the cloned chain. We observed that reducing the length of molecules able to cross-link amino groups decreased the efficiency of cross-linking with the 100-kDa protein and only the 85-kDa protein was cross-linked with erythropoietin using 1,5-difluoro-2,4-dinitrobenzene. These results suggest that the 85- and 100-kDa proteins occupate slightly different positions relative to the erythropoietin molecule bound to the receptor.     

The two proteins of the erythropoietin receptor are structurally similar

The structure of the erythropoietin receptor has been identified in this laboratory as two proteins of 100 and 85 kDa by cross-linking 125I-erythropoietin (125I-EP) to the surface of erythroid cells purified from the spleens of mice infected with the anemia strain of Friend virus. This study investigates the relatedness of these two proteins and the possibility that these proteins are subunits of the functional receptor for EP. Other workers have claimed that the 100- and 85-kDa proteins are bridged by disulfide bonds. This most likely is an artifact due to the insolubility of the cross-linked membrane. Proteolytic digestion by the method of Cleveland (Cleveland, D. W., Fischer, S. G., Kirschner, M. W., and Laemmli, U. K. (1977) J. Biol. Chem. 252, 1102-1106) resulted in identical fragments from the 100- and 85-kDa proteins, which strongly suggests that the primary amino acid sequence of these two proteins is similar if not identical. Increasing the number of protease inhibitors during the preparation of membranes and the binding and cross-linking steps increased the ratio of 100-kDa protein labeled compared to the 85-kDa protein. Together these results suggest that the 85-kDa protein is derived by proteolytic cleavage of the 100-kDa receptor for EP. It is not clear whether the 100-kDa protein can bind EP in the absence of the 85-kDa protein.     

B lymphocyte activation is accompanied by phosphorylation of a 72-kDa protein-tyrosine kinase

The cross-linking of membrane IgM on the surface of splenic B lymphocytes or WEHI 231 cells leads to the rapid phosphorylation on tyrosine of a 72-kDa protein as detected in Western blotting experiments using anti-phosphotyrosine antibodies. The 72-kDa phosphoprotein detected in this manner comigrates, in both one- and two-dimensional polyacrylamide gel electrophoresis systems, with PTK72, a 72-kDa protein-tyrosine kinase characterized previously in this laboratory (Zioncheck, T. F., Harrison, M. L., Isaacson, C. C., and Geahlen, R. L. (1988) J. Biol. Chem. 263, 19195-19202). Anti-phosphotyrosine antibodies and anti-PTK72 antibodies immunoprecipitate the same protein-tyrosine kinase from extracts of anti-IgM-activated cells as determined by immune complex kinase assays and one-dimensional phosphopeptide mapping. These results indicate that the tyrosine phosphorylation of a 72-kDa protein-tyrosine kinase is an early event in the activation of B lymphocytes via the antigen receptor.     

Arrangement of the subunits of the nicotinic acetylcholine receptor of Torpedo californica as determined by alpha-neurotoxin cross-linking

[3H]Methyl-alpha-neurotoxin prereacted with dithiobis(succinimidyl propionate) (DTSP) can be covalently linked to each of the subunits of the nicotinic acetylcholine receptor in membranes from the electric tissue of Torpedo californica. Pronounced changes in the cross-linking pattern are observed upon prior incubation with receptor specific ligands and upon reduction and/or alkylation of the receptor. d-Tubocurarine has been shown to bind to two different sites in receptor-rich membranes. These sites are present in equal numbers but have different affinities [Neubig, R. R., & Cohen, J. B. (1979) Biochemistry 18, 5464-5475; Sine, S., & Taylor, P. (1981) J. Biol. Chem. 256, 6692-6699]. Using d-tubocurarine inhibition of [3H]-methyl-alpha-neurotoxin binding, we demonstrate two inhibitory constants for d-tubocurarine of 67 +/- 21 nM and 4.9 +/- 1.7 microM in unreduced membranes. We utilize the large difference in Ki's to preferentially block toxin cross-linking at the high affinity site for d-tubocurarine. Low concentrations of this competitive antagonist selectively block the cross-linking of toxin to the beta and gamma subunits of the receptor, suggesting that these subunits are located close to the toxin binding site which is also the high-affinity binding site for d-tubocurarine. Reduction of disulfide bonds alters the affinity of the receptor for alpha-neurotoxin. Alterations are also seen in the cross-linking pattern of DTSP-activated [3H]methyl-alpha-neurotoxin to reduced and alkylated membranes in the presence of tubocurarine. The constants for d-tubocurarine inhibition of [3H]methyl-alpha-neurotoxin binding to reduced and alkylated membranes are 172 +/- 52 nM and 2.4 +/- 0.4 microM. The effects of bromoacetylcholine, carbamoylcholine, gallamine, and procaine on the cross-linking pattern are also examined. Our observations are consistent with an arrangement of the subunits in the membrane of alpha beta alpha gamma delta.     

Identification of a novel mitochondrial complex containing mitofusin 2 and stomatin-like protein 2

A reverse genetics approach was utilized to discover new proteins that interact with the mitochondrial fusion mediator mitofusin 2 (Mfn2) and that may participate in mitochondrial fusion. In particular, in vivo formaldehyde cross-linking of whole HeLa cells and immunoprecipitation with purified Mfn2 antibodies of SDS cell lysates were used to detect an approximately 42-kDa protein. This protein was identified by liquid chromatography and tandem mass spectrometry as stomatin-like protein 2 (Stoml2), previously described as a peripheral plasma membrane protein of unknown function associated with the cytoskeleton of erythrocytes (Wang, Y., and Morrow, J. S. (2000) J. Biol. Chem. 275, 8062-8071). Immunoblot analysis with anti-Stoml2 antibodies showed that Stoml2 could be immunoprecipitated specifically with Mfn2 antibody either from formaldehyde-cross-linked and SDS-lysed cells or from cells lysed with digitonin. Subsequent immunocytochemistry and cell fractionation experiments fully supported the conclusion that Stoml2 is indeed a mitochondrial protein. Furthermore, demonstration of mitochondrial membrane potential-dependent import of Stoml2 accompanied by proteolytic processing, together with the results of sublocalization experiments, suggested that Stoml2 is associated with the inner mitochondrial membrane and faces the intermembrane space. Notably, formaldehyde cross-linking revealed a "ladder" of high molecular weight protein species, indicating the presence of high molecular weight Stoml2-Mfn2 hetero-oligomers. Knockdown of Stoml2 by the short interfering RNA approach showed a reduction of the mitochondrial membrane potential, without, however, any obvious changes in mitochondrial morphology.     

Characterization of a plasma retinol-binding protein membrane receptor expressed in the retinal pigment epithelium.

A specific membrane receptor for plasma retinol-binding protein (RBP) is expressed in the retinal pigment epithelium (RPE). When chemically cross-linking RBP to RPE membranes, an 86-kDa RBP.RBP receptor complex is formed, and a 63-kDa protein was identified as the RBP-binding membrane protein (B?vik, C.-O., Eriksson, U., Allen, R., and Peterson, P. (1991) J. Biol. Chem. 266, 14978-14985). To explore in more detail the characteristics of this membrane receptor, we have generated a monoclonal antibody, A52, to the 63-kDa protein (p63). A52 binds the 86-kDa RBP.RBP receptor complex and p63. Several lines of evidence suggest that p63 is not a regular integral membrane protein, and it occurs in different forms. One form is firmly attached to membranes, is part of a high molecular weight complex, and is able to bind RBP. The other form of p63 can be removed from membranes by treatment with an alkaline buffer and is unable to bind RBP. Both forms of p63 contain extensive hydrophobic domains and are found in the detergent phase upon extraction with Triton X-114. The expression of p63 is restricted to RPE, and immunohistochemical localization of tissue sections from bovine retina showed highest expression in the basolateral portion of RPE cells. Immunofluorescence localization, using isolated RPE cells, showed that p63 is exposed on the cell surface of newly isolated RPE cells.     

Disulfides of the lutropin receptor

Affinity cross-linking of the lutropin receptor with 125I-human choriogonadotropin (hCG) on porcine granulosa cells produced four distinct homone-receptor complexes under reducing conditions. They contain 18-, 24-, 28-, and 34-kDa components (Ji, I., Bock, J. H., and Ji, T. H. (1985) J. Biol. Chem. 260, 12815-12821). Photoaffinity labeling and cross-linking produced 136-, 102-, and 74-kDa hCG-receptor complexes under reducing conditions and the 136-kDa complex under nonreducing conditions. In addition, the unreduced 102-kDa complex was seen in photoaffinity labeling but not in cross-linking. When the unreduced 136-kDa complex was reduced, the 102- and 74-kDa complexes were generated, indicating release of the 34- and the 28-kDa components in two steps. When the unreduced 102-kDa complex was reduced, the 74-kDa complex was produced, indicating the release of a 28-kDa component. The 74-kDa complex could not be reduced but was cleaved by alkaline treatment to produce the hCG alpha beta dimer. The results indicate that the 24-kDa component is released from the 74-kDa complex, since the apparent mass of the hCG alpha beta dimer on gels is 50 kDa. The 24-kDa component appears to be the initial site for photoaffinity labeling or cross-linking and to be disulfide linked to the 28-kDa component which is in turn disulfide linked to the 34-kDa component. These intercomponent disulfides exist in some receptors but not all. Formation of the disulfide-linked 136-kDa band required the presence of a sulfhydryl-blocking agent, N-ethylmaleimide. In particular, the 34-kDa component was vulnerable to reduction. There was no significant evidence of disulfides between the hormone and any of the receptor components.     

Structure of the glucocorticoid receptor in intact cells in the absence of hormone.

The nonactivated glucocorticoid receptor (Mr approximately 330,000, Strokes radius = 82 A) contained in cell extracts and complexed with a steroidal ligand was previously investigated by chemical cross-linking. It was identified as a heterotetramer composed of one receptor polypeptide, two molecules of the 90-kDa heat shock protein hsp90, and one 59-kDa protein subunit (Rexin, M., Busch, W., and Gehring, U. (1991) J. Biol. Chem. 266, 24601-24605). We now have used the cross-linking technique to investigate the receptor structure in intact WEHI-7 mouse lymphoma cells at 37 degrees C and under steroid-free conditions. Using immunochemical methods we show that the receptor present in whole cells likewise exists as a high molecular weight structure of Strokes radius 82 A. It has a subunit composition identical to that of the nonactivated receptor-steroid complex in cell extracts. This is the first account of a steroid hormone receptor in its native state as it is contained in target cells under physiological conditions and before a hormonal signal is received.     

Reconstitution of the immunopurified 49-kDa sodium-dependent bile acid transport protein derived from hepatocyte sinusoidal plasma membranes

Reconstitution, using phosphatidylcholine liposomes in conjugation with immunological purification procedures, has been used to establish directly the identity of the hepatocyte Na(+)-dependent bile acid transport protein. Octyl glucoside-solubilized sinusoidal plasma membranes were shown to form proteoliposomes exhibiting taurocholate transport properties which were similar to those of plasma membrane vesicles, namely, Na(+)-dependence and marked inhibition by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and by taurochenodeoxycholate. Proteoliposomes formed from plasma membrane proteins depleted of the putative 49-kDa bile acid transport protein by immunoprecipitation with monoclonal antibody 25D-1, which specifically recognizes this protein (Ananthanarayanan, M., von Dippe, P., and Levy, D. (1988) J. Biol. Chem. 263, 8338-8343), showed a 94% reduction in mediated transport capacity. Proteoliposomes containing total membrane protein also demonstrated Na(+)-dependent alanine transport. The addition of taurochenodeoxycholate or the removal of the 49-kDa protein by monoclonal antibody 25D-1 immunoprecipitation had no effect on the uptake of alanine, thus confirming the specificity of these procedures. When only the immunoprecipitated 48-kDa protein was used in the reconstitution system, a 2200% increase of taurocholate uptake was observed. These results definitively establish that this 49-kDa sinusoidal membrane protein is the sole essential component of the Na(+)-dependent bile acid transport system.     

Cellular distribution of type I and type II receptors for transforming growth factor-beta

Affinity labeling of target cells for transforming growth factor-beta (TGF beta) by cross-linking with 125I-TGF beta via disuccinimidyl suberate or by the photoreactive analogue 4-azidobenzoyl-125I-TGF beta has revealed the presence of multiple TGF beta receptor forms. Two distinct types of TGF beta receptors can be distinguished based on structural analysis of the 125I-TGF beta-labeled species by peptide mapping. Type I TGF beta receptors include the 280-kilodalton labeled receptor form previously found to be the subunit of a disulfide-linked TGF beta receptor complex. (Massagué, J. (1985) J. Biol. Chem. 260, 7059-7066), as well as a 65-kDa labeled receptor form present in all cell lines examined, and a 130-140-kDa labeled receptor form detected only in 3T3-L1 cells. The 280-kDa form is the major TGF beta receptor species in most cell lines examined, but is apparently absent in rat skeletal muscle myoblasts. Type I TGF beta receptors bind TGF beta with an apparent Kd of 50-500 pM. Type II TGF beta receptors include an 85-kDa labeled receptor form present in all mammalian cells examined and a 110-kDa labeled receptor form present in chick embryo fibroblasts. Type II TGF beta receptors bind TGF beta with an apparent Kd of about 50 pM. Except for the 280-kDa type I TGF beta receptor form, none of the TGF beta receptor forms described here is found as part of a disulfide-linked receptor complex. All the TGF beta receptor forms described here behave as intrinsic membrane proteins exposed on the surface of intact cells.     

Neuropeptide Y. Differential binding to rat intestinal laterobasal membranes

Neuropeptide Y (NPY), a neurotransmitter contained within intrinsic nerves of the small intestine, inhibits secretion when added to the serosal side of intestinal mucosa mounted in Ussing chambers. Using NPY radiolabeled with IODO-GEN, lactoperoxidase, or the Bolton-Hunter reagent, we have localized high affinity NPY receptors to the serosal laterobasal membranes of the rat intestinal epithelial cell, isolated according to a recently described protocol that minimizes contamination with endoplasmic reticulum, Golgi, and brush-border membranes. In addition, certain species of radiolabeled NPY, including those labeled with IODO-GEN at the tyrosine residue 36, also demonstrated an ability to bind to brush-border membranes. These receptors were specific for NPY since the homologous peptides, pancreatic polypeptide and peptide YY, were less efficient than NPY in inhibiting the membrane binding of radiolabeled NPY. By cross-linking NPY to its receptor with either disuccinimidyl suberate or dithiobis(succinimidyl propionate) and analyzing the resulting complexes on sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by radioautography, we identified two NPY receptor species with molecular sizes of 52-59 kDa and 37-39 kDa. The 37-39-kDa species further possesses a disulfide bond which may attach it to a separate approximately 5-kDa subunit, as evidenced by retarded migration in the absence of the reducing agent dithiothreitol. The intestinal NPY receptor is slightly smaller than the rat brain receptor previously characterized using similar techniques. The localization of NPY receptors on laterobasal membranes is consistent with previous anatomic and physiologic findings, and their identification by cross-linking techniques will constitute the basis for detailed characterization.     

Structure and membrane topology of the high-affinity receptor for human IFN-gamma: requirements for binding IFN-gamma. One single 90-kilodalton IFN-gamma receptor can lead to multiple cross-linked products and isolated proteins

We analyzed the high affinity receptor for IFN-gamma of Raji cells and human placenta by combining Scatchard analysis, cross-linking experiments, and receptor purification. Only one high affinity binding site was found, Kd 2.1 X 10(-10). The receptor is a 90-kDa glycoprotein. However, multiple cross-linked products of 110 kDa to about 250 kDa could be generated and proteins of 90, 70, and 50 kDa could be obtained upon purification. These proteins all contained the same 90-kDa receptor, or part of it. We suggest that extensive cross-linking and/or proteolysis may explain many of the conflicting results published thus far. The extracellular domain of the 90-kDa receptor protein was highly resistant to digestion with trypsin or proteinase K. Trypsin digestion neither affected the number of binding sites per cell, nor the Kd for IFN-gamma. A cluster of sites for different proteases was found in the intracellular domain. The 50-kDa fragment created by trypsin digestion had the same characteristics as the isolated 50-kDa receptor fragment. It contained the IFN-gamma binding site and the receptor's extracellular and amino-terminal domain. N-linked glycosylation contributed about 15 kDa to its molecular mass, of which 4 kDa were attributable to sialic acid residues. O-Linked glycosylation was not detected. The number of binding sites per cell and the Kd for IFN-gamma were not affected by the presence or absence of N-linked glycosylation. The receptor contained at least one critical disulfide bridge and the reduced receptor could be reactivated in vitro.     

Interleukin-1 receptor antagonist competitively inhibits the binding of interleukin-1 to the type II interleukin-1 receptor.

The interleukin-1 receptor antagonist (IL-1ra) inhibits the binding of interleukin-1 (IL-1) to T-cell lines possessing the type I IL-1 receptor; evidence has been published (Carter, D. B., Deibel, M. R. J., Dunn, C. J., Tomich, C. S., Laborde, A. L., Slightom, J. L., Berger, A. E., Bienkowski, M. J., Sun, F. F., McEwan, R. N., Harris, P. K. W., Yem, A. W., Waszak, G. A., Chosay, J. G., Sieu, L. C., Hardee, M. M., Zurcher-Neely, H. A., Reardon, I. M., Heinrickson, R. L., Truesdell, S. E., Shelly, J. A., Eessalu, T. E., Taylor, B. M., and Tracey, D. E. (1990) Nature 344, 633-638; Hannum, C. H., Wilcox, C. J., Arend, W. P., Joslin, F. G., Dripps, D. J., Heimdal, P. L., Armes, L. G., Sommer, A., Eisenberg, S. P., and Thompson, R. C. (1990) Nature 343, 336-340) that IL-Ira does not bind to the type II IL-1 receptor (IL-1RtII). In this study we examined the ability of human recombinant IL-1ra to block the binding of IL-1 to the IL-1RtII on human polymorphonuclear leukocytes (PMN) and Raji human B-lymphoma cells. The binding of 125I-IL-1 beta to PMN was competively inhibited by IL-1ra. IL-1 beta was more potent in inhibiting the binding of 125I-IL-1 beta than IL-1ra. Incubating PMN with 125I-IL-1ra in the presence of increasing concentrations of IL-1 beta or IL-1ra showed that IL-1 beta was an approximately 40-fold more potent inhibitor of binding of 125I-IL-1ra than unlabeled IL-1ra. The IL-1ra was approximately 500-fold less potent in inhibiting the binding of 125I-IL-1 alpha than IL-1 alpha. IL-1ra was also able to competitively inhibit binding of 125I-IL-1 beta to Raji cells. PMN or Raji cells were also incubated with 125I-IL-1 in the absence or presence of IL-1 or IL-1ra. After cross-linking of IL-1 to cells followed by specific immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a band at 85 kDa corresponding to the 68-kDa IL-1RtII. However, in the presence of an excess of either unlabeled IL-1 or IL-1ra, the 85-kDa IL-1.IL-1RtII complex was not present. These findings demonstrate that the IL-1ra recognizes and blocks IL-1 binding to the IL-1RtII.