Baculovirus-directed expression of the human insulin receptor and an insulin-binding ectodomain

Expression and purification of functional insulin and insulin-like growth factor 1 holoreceptors from mammalian cells

The insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) are receptor tyrosine kinases (RTKs) involved in the regulation of many important cellular processes. The current proposed models of activation are derived from structural studies using soluble extracellular domains and cytoplasmic tyrosine kinase domains. Preparations of full length IR and IGF1R have been hampered by the need for unconventional affinity chromatography resins and/or harsh eluting conditions. Here, we present a purification protocol to obtain full-length, detergent solubilized IR and IGF1R at quantities suitable for biochemical and structural characterization. We screened a panel of 24 structurally diverse detergents for optimal ligand activation. The receptors purified in n-dodecyl-β-D-maltoside showed ligand-stimulated autophosphorylation and kinase activity, suggesting an intact transmembrane signaling mechanism. This convenient purification protocol can be used to produce high quantities of IR, IGF1R, or other RTKs, and can be adapted for other challenging membrane proteins.

Chaperone and foldase coexpression in the baculovirus-insect cell expression system

CONCLUSIONS

The BEVS has become widely utilized for production of recombinant proteins. However, protein aggregation and inefficient processing often limit yields, especially for secreted and membrane proteins. Since many proteins of pharmaceutical interest require similar posttranslational processing steps, engineering the folding, assembly, and secretion pathway may enhance the production of a wide variety of valuable complex proteins. Efforts should be undertaken to coexpress the relevant chaperones or foldases at low levels in concert with the final product to ensure the ideal folding and assembly environment. In the future, expression of oligosaccharide modifying enzymes and secretion factors may further improve secretion rates of assembled proteins and provide heterologous proteins with altered glycoforms. Also significant is the use of BEVS as an in vivo eucaryotic laboratory to study the fundamental roles of differnt chaperones, foldases, and secretion factors. The coexpression of chaperones and foldases will complement other approaches such as the development of alternative insect cell lines, promoters, and signal peptides to optimize the baculovirus-insect cell expression system for generating high yields of valuable proteins.

The A-chain of Insulin Contacts the Insert Domain of the Insulin Receptor

The contribution of the insulin A-chain to receptor binding is investigated by photo-cross-linking and nonstandard mutagenesis. Studies focus on the role of Val(A3), which projects within a crevice between the A- and B-chains. Engineered receptor alpha-subunits containing specific protease sites ("midi-receptors") are employed to map the site of photo-cross-linking by an analog containing a photoactivable A3 side chain (para-azido-Phe (Pap)). The probe cross-links to a C-terminal peptide (residues 703-719 of the receptor A isoform, KTFEDYLHNVVFVPRPS) containing side chains critical for hormone binding (underlined); the corresponding segment of the holoreceptor was shown previously to cross-link to a Pap(B25)-insulin analog. Because Pap is larger than Val and so may protrude beyond the A3-associated crevice, we investigated analogs containing A3 substitutions comparable in size to Val as follows: Thr, allo-Thr, and alpha-aminobutyric acid (Aba). Substitutions were introduced within an engineered monomer. Whereas previous studies of smaller substitutions (Gly(A3) and Ser(A3)) encountered nonlocal conformational perturbations, NMR structures of the present analogs are similar to wild-type insulin; the variant side chains are accommodated within a native-like crevice with minimal distortion. Receptor binding activities of Aba(A3) and allo-Thr(A3) analogs are reduced at least 10-fold; the activity of Thr(A3)-DKP-insulin is reduced 5-fold. The hormone-receptor interface is presumably destabilized either by a packing defect (Aba(A3)) or by altered polarity (allo-Thr(A3) and Thr(A3)). Our results provide evidence that Val(A3), a site of mutation causing diabetes mellitus, contacts the insert domain-derived tail of the alpha-subunit in a hormone-receptor complex.

Structure and function of recombinant cobra venom factor

Cobra venom factor (CVF) is the complement-activating protein from cobra venom. It is a structural and functional analog of complement component C3. CVF functionally resembles C3b, the activated form of C3. Like C3b, CVF binds factor B, which is subsequently cleaved by factor D to form the bimolecular complex CVF,Bb. CVF,Bb is a C3/C5 convertase that cleaves both complement components C3 and C5. CVF is a three-chain protein that structurally resembles the C3b degradation product C3c, which is unable to form a C3/C5 convertase. Both C3 and CVF are synthesized as single-chain prepro-proteins. This study reports the recombinant expression of pro-CVF in two insect cell expression systems (baculovirus-infected Sf9 Spodoptera frugiperda cells and stably transfected S2 Drosophila melanogaster cells). In both expression systems pro-CVF is synthesized initially as a single-chain pro-CVF molecule that is subsequently proteolytically processed into a two-chain form of pro-CVF that structurally resembles C3. The C3-like form of pro-CVF can be further proteolytically processed into another two-chain form of pro-CVF that structurally resembles C3b. Unexpectedly, all three forms of pro-CVF exhibit functional activity of mature, natural CVF. Recombinant pro-CVF supports the activation of factor B in the presence of factor D and Mg2+ and depletes serum complement activity like natural CVF. The bimolecular convertase pro-CVF,Bb exhibits both C3 cleaving and C5 cleaving activity. The activity of pro-CVF and the resulting C3/C5 convertase is indistinguishable from CVF and the CVF,Bb convertase. The ability to produce active forms of pro-CVF recombinantly ensures the continued availability of an important research reagent for complement depletion because cobra venom as the source for natural CVF will be increasingly difficult to obtain as the Indian cobra is on the list of endangered species. Experimental systems to express pro-CVF recombinantly will also be invaluable for studies to delineate the structure and function relationship of CVF and its differences from C3 as well as to generate human C3 derivatives with CVF-like function for therapeutic complement depletion ("humanized CVF").

A new secreted insect protein belonging to the immunoglobulin superfamily binds insulin and related peptides and inhibits their activities

Insulin and related peptides are key hormones for the regulation of growth and metabolism. Here we describe a novel high affinity insulin-related peptide-binding protein (IBP) secreted from cells of the insect Spodoptera frugiperda. This IBP is composed of two Ig-like C2 domains, has a molecular mass of 27 kDa, binds human insulin with an affinity of 70 pm, and inhibits insulin signaling through the insulin receptor. The binding protein also binds insulin-like growth factors I and II, proinsulin, mini-proinsulin, and an insulin analog lacking the last 8 amino acids of the B-chain (des-octa peptide insulin) with high affinity, whereas an insulin analog with a Asp-B10 mutation bound with only 1% of the affinity of human insulin. This binding profile suggests that IBP recognizes a region that is highly conserved in the insulin superfamily but distinct from the classical insulin receptor binding site. The closest homologue of the Spodoptera frugiperda binding protein is the essential gene product IMP-L2, found in Drosophila, where it is implicated in neural and ectodermal development (Garbe, J. C., Yang, E., and Fristrom, J. W. (1993) Development 119, 1237-1250). Here we show that the IMP-L2 protein also binds insulin and related peptides, offering a possible functional explanation to the IMP-L2 null lethality.

The integral membrane S-locus receptor kinase of Brassica has serine/threonine kinase activity in a membranous environment and spontaneously forms oligomers in planta

To gain further insight into the mode of action of S-locus receptor kinase (SRK), a receptor-like kinase involved in the self-incompatibility response in Brassica, different recombinant SRK proteins have been expressed in a membranous environment using the insect cell/baculovirus system. Recombinant SRK proteins exhibited properties close to those of the endogenous stigmatic SRK protein and were found to autophosphorylate on serine and threonine residues in insect cell microsomes. Autophosphorylation was constitutive because it did not require the presence of pollen or stigma extracts in the phosphorylation buffer. Phosphorylation was shown to occur in trans, suggesting the existence of constitutive homooligomers of membrane-anchored recombinant SRK. To investigate the physiological relevance of these results, we have examined the oligomeric status of SRK in planta in cross-linking experiments and by velocity sedimentation on sucrose gradients. Our data strongly suggest that SRK is associated both with other SRK molecules and other stigma proteins in nonpollinated flowers. These findings may have important implications for our understanding of self-pollen signaling.

Membrane proteins involved in pollen-pistil interactions

Self-incompatibility (SI) is a widespread mechanism in angiosperms which prevents self-fertilization. This mechanism relies on cell-cell interactions between pollen and pistil. Among the different SI systems that have been reported, two have been particularly investigated: the gametophytic system of Solanaceae and the sporophytic system of Brassicaceae. In these two families, although the molecular bases of SI response are different, secreted and/or membrane-anchored proteins are required for self-pollen rejection. Interestingly, these proteins exhibit two functions: recognition and a catalytic activity. In this review article, we present recent advances which permit a better understanding of how these proteins control the male/female recognition event associated with the SI response.

Insulin-mediated secretion of ecdysteroids from mosquito ovaries and molecular cloning of the insulin receptor homologue from ovaries of bloodfed Aedes aegypti

The reproductive cycle of female mosquitoes is activated by ingestion of blood from vertebrate hosts. Shortly after feeding, neurohormones are released from the brain neurosecretory system and stimulate the ovaries to secrete ecdysteroids, which are necessary for vitellogenesis by the fat body. Because bombyxins, which are insulin-like peptides, stimulate ecdysteriodgenesis in silkworm larvae, we tested porcine insulin and found that it activates ecdysteroidogenesis and protein synthesis in ovaries isolated from unfed mosquitoes. To further characterize the regulation of ecdysteroidogenesis in female mosquitoes, we cloned the mosquito insulin receptor (MIR) homologue from ovarian mRNA. The sequence of the extracellular domain shows moderate homologies with vertebrate and Drosophila insulin receptor homologues, as well as with the insulin receptor-related receptor, the latter being an "orphan" receptor with an unknown function. In the intracellular domain, high homologies are observed, particularly in those subdomains that are responsible for ATP binding and kinase activity. Northern blot analysis of MIR demonstrated a highly specific expression in ovaries, and cloning experiments indicated its presence in the brain. Recombinant MIR extracts from a baculovirus expression system contained high constitutive kinase activity in the presence of manganese or magnesium. Activation was independent of a ligand. SDS-gel analysis suggested that the recombinant receptor was not post-translationally processed into an alpha- and beta-subunit as was expected from a putative cleavage signal. Enzymatic properties of the proreceptor are presented: the Km for ATP was between 15 and 50 microM in the presence of a synthetic substrate: maximal kinase activity to 100-fold over basic activity was reached in the presence of 1 mM manganese. Stimulation of key oogenic processes by porcine insulin and identification of a MIR indicate that insulin-like neurohormones may have an important regulatory role in mosquito oogenesis.

Fusion of insulin receptor ectodomains to immunoglobulin constant domains reproduces high-affinity insulin binding in vitro

A unique feature of the insulin receptor is that it is dimeric in the absence of ligand. Dimerization of two adjacent transmembrane domain (TMD) alpha helices has been shown to be critical in receptor kinase activation. Moreover, previous work has suggested that the TMD is involved in stabilizing the high-affinity binding site; soluble receptors expressed after simple truncation at the ectodomain-TMD junction have reduced affinity for insulin. To further examine this issue, we have replaced the TMD and intracellular domain of the soluble human insulin receptor (HIRs) with constant domains from immunoglobulin Fc and lambda subunits (HIRs-Fc and HIRs-lambda). Studies of receptor biosynthesis and binding characteristics were performed following transient transfection of receptor cDNAs into human embryonal kidney 293 cells. Each hybrid receptor was initially synthesized as a single chain proreceptor, followed by cleavage into alpha- and beta-Fc or beta-lambda subunits. The majority of secreted protein appeared in the cell medium as fully processed heterotetramer. Fc fragments released from HIRs-Fc by papain digestion and analyzed by nonreducing SDS-polyacrylamide gel electrophoresis were dimeric. Furthermore, dissociation constants for both chimeras were similar to those for the full-length holoreceptor (wild-type receptor, Kd1 = 200 pM and Kd2 = 2 nM; HIRs-Fc, Kd1 = 200 pM and Kd2 = 40 nM; and HIRs-lambda, Kd1 = 200 pM and Kd2 = 5 nM). These results extend previous observations that dimerization of the membrane-proximal ectodomain is necessary to maintain an intact high-affinity insulin-binding site.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Expression of the extracellular domain of the thyrotropin receptor in the baculovirus system using a promoter active earlier than the polyhedrin promoter. Implications for the expression of functional highly glycosylated proteins

Conventional baculovirus vectors that utilize the very late polyhedrin promoter have not proved successful for expressing a thyrotropin (TSH) receptor capable of ligand and Graves' disease autoantibody binding comparable to the receptor produced in mammalian cells. Because of the clinical importance of high level expression of this protein, we reassessed the baculovirus system using a new transfer vector (pAcMP3) containing the late basic protein promoter, which functions earlier than the classical polyhedrin promoter. Maximal synthesis of the [35S]methionine-labeled TSH receptor extracellular domain, affinity-purified using a 6-histidine tag, occurred earlier (1 day after insect cell infection) than with a vector (pVL1393) containing the polyhedrin promoter. The pAcMP3-derived TSH receptor extracellular domain was larger (approximately 68 kDa) than the pVL1393-derived protein (approximately 63 kDa). Only the 68-kDa product was secreted, albeit in trace amounts detectable only by precursor labeling. Enzymatic deglycosylation reduced both 68- and 63-kDa cellular proteins to approximately 54 kDa, indicating that the pAcMP3 vector generated a protein with greater carbohydrate content. However, despite its greater degree of glycosylation, most of the 68-kDa protein remained within the cell, almost entirely in the particulate fraction. Remarkably, the trace amounts of 68-kDa receptor protein affinity-purified from the soluble cytosolic fraction of infected insect cells completely neutralized TSH receptor autoantibodies in patients' sera and partly inhibited TSH binding. In conclusion, a baculovirus vector with a promoter active earlier than the conventional polyhedrin promoter generates a more glycosylated and functional TSH receptor extracellular domain protein, albeit at low levels. These data carry important implications for the expression by baculovirus vectors of functional, highly glycosylated proteins.

Processing of the precursors of the human thyroid-stimulating hormone receptor in various eukaryotic cells (human thyrocytes, transfected L cells and baculovirus-infected insect cells)

The complementary DNA for human thyroid-stimulating hormone (TSH) receptor encodes a single protein with a deduced molecular mass of 84.5 kDa. This protein is cleaved during its maturation in the human thyroid since the receptor protein has been shown to be composed of two subunits (alpha subunit of approximately 53 kDa and beta subunit of approximately 38 kDa) held together by disulfide bridges [Loosfelt, H., Pichon, C., Jolivet, A., Misrahi, M., Caillou, B., Jamous, M., Vannier, B. & Miligrom, E. (1992) Proc. Natl Acad. Sci. USA 89, 3765-3769]. A similar processing occurs in an L cell line permanently expressing the human TSH receptor. The processing is however incomplete, resulting in a permanent accumulation of a 95-kDa high-mannose precursor which is present only in trace amounts in the thyroid. Pulse-chase experiments show the successive appearance in the L cells of two precursors: initially the approximately 95-kDa high-mannose glycoprotein followed by a approximately 120-kDa species containing mature oligosaccharides. This latter precursor is then processed into the alpha and beta subunits. In primary cultures of human thyrocytes precursors of similar size are detected. Spodoptera frugiperda insect cells (Sf9 and Sf21) infected with a recombinant baculovirus encoding the human TSH receptor synthesize a monomeric protein of about 90 kDa soluble only in denaturing conditions. Comparison with the product of in vitro transcription-translation experiments (approximately 80 kDa), suggests that it may be incompletely or improperly glycosylated. The TSH receptor expressed in these cells is unable to bind the hormone. Immunoelectron microscopy studies show that in human thyrocytes most of the receptor is present on the cell surface; in L cells the receptor is detected on the cell surface, as well as in the endoplasmic reticulum and in the Golgi apparatus (this intracellular pool of receptor molecules probably corresponding to the high-mannose precursor); in insect cells nearly all the receptor molecules are trapped in the endoplasmic reticulum. These differences in receptor distribution are concordant with the differences observed for receptor processing.

Structure-function analyses of the kit receptor for the steel factor

Binding of the Steel factor (SLF) to the product of the c-kit proto-oncogene stimulates the receptor's intrinsic tyrosine kinase that phosphorylates a set of cytoplasmic signaling molecules. Germ-line mutations in the genes that encode the receptor or the ligand result in remarkably similar phenotypes that affect melanogenesis, erythropoiesis and gametogenesis in mice. We concentrated on the initial events of the signal transduction pathway that underlies these processes. The extracellular portion of Kit is comprised of five immunoglobulin-(Ig)-like domains. Ligand binding to this domain induces rapid and extensive dimerization of the receptor molecules in a mechanism that involves monovalent binding of the dimeric ligand, followed by an increase in receptors' affinity and gradual stabilization of the dimers. It thus appears that Kit has at least two functions: ligand binding and ligand-induced receptor dimerization, in addition to the kinase activity. Both functions are independent of the transmembrane and cytoplasmic domains, as a recombinant soluble ectodomain retained high affinity to SLF and ligand-dependent dimerization. In order to correlate these functions with specific structures, we employed ligand-competitive monoclonal antibodies, soluble deletion mutants of the ectodomain and chimeric human-mouse Kit proteins. These approaches indicated that the N-terminal three Ig-like domains constitute the binding site, whose core is the second domain. Further experiments suggested that a putative dimerization site is distinct from the binding cleft and may be located on the fourth Ig-like domain.

Expression of the full-length rabbit prolactin receptor and its specific domains in baculovirus infected insect cells

The prolactin receptor is a membrane protein mainly involved in the development of the mammary gland and in lactation in mammals. We used specific cDNA constructs and the insect/baculovirus expression system and produced independently and in large amounts several recombinant forms of the rabbit mammary gland prolactin receptor: the full-length receptor (L1, L2), a truncated membrane form (S), a secretable form of the extracellular domain (E) and two forms of the intracellular domain (I1, I2). Of these forms, the L1 and L2 are associated with the membrane fraction, the E is predominantly secreted into the medium and the I1 and I2 are expressed as soluble proteins and surprisingly, a great portion accumulates in the culture medium. The molecular mass (94 kDa) of the expressed full-length receptor corresponds to the translation product of the entire cDNA coding region. The receptor biochemically identified in the rabbit mammary gland is however much shorter. Thus, in the mammary gland, the receptor presumably undergoes post-translational modifications. The receptor forms L1, L2 and S bind prolactin with specificity and affinity similar to those reported for the native receptor. They also interact with two monoclonal antibodies, M110 and A917, specific for the native conformation of the hormone-binding site. The I1 and I2 forms do not bind prolactin, whereas the E form does. Thus, the hormone binding site is located in the extracellular domain which can function autonomously as a PRL-binding soluble protein. However, the E form binds prolactin with a higher affinity than the native receptor and it does not bind one of the two antireceptor monoclonal antibodies, known to be hormone binding-site specific. Thus, the conformation of the native receptor and that of the E form differ.

Analysis of soluble human and mouse interferon-gamma receptors expressed in eukaryotic cells

The extracellular domains of the human and mouse interferon-gamma receptors were produced in insect Spodoptera frugiperda cells infected with recombinant baculoviruses and in mammalian Chinese-hamster-ovary cells. The receptors expressed in both systems are secreted into the culture medium. Their signal peptides are cleaved off and the proteins show heterogeneity in glycosylation which, however, does not affect the capacity to bind interferon gamma or specific antibodies. The soluble mouse receptors exhibit binding capacities similar to those of cell-surface-anchored receptors, whereas the human receptors exhibit a lower binding capacity. All soluble receptors inhibit the binding of interferon gamma to cellular receptors and neutralize the antiviral activity exerted by interferon gamma. These receptors could therefore be useful for structure/function analyses and in vivo studies.

Characterization of baculovirus-expressed human alpha and beta platelet-derived growth factor receptors

In an effort to biochemically characterize PDGF receptors and their mechanism of activation, recombinant baculovirus vectors containing the cDNAs of the human alpha PDGF receptor or beta PDGF receptor were engineered. Characterization of recombinant PDGF receptor expression in infected Sf9 insect cells by immunoblot analysis with specific PDGF receptor peptide antisera revealed that the alpha and beta PDGF receptor gene products were translated as 160- and 165-kDa transmembrane proteins, respectively. Ligand binding analysis demonstrated saturable, high-affinity binding of either 125I-labeled PDGF AA or 125I-labeled PDGF BB to Sf9 cells expressing the recombinant alpha PDGF receptor. In contrast, recombinant beta PDGF receptor expressing Sf9 cells showed high-affinity binding only for PDGF BB. Analysis of the kinetics of PDGF receptor expression demonstrated that receptor number increased dramatically from 24- to 48-h postinfection. Early in infection, the PDGF receptors were present in low numbers, lacked tyrosine phosphorylation, and exhibited ligand-dependent tyrosine phosphorylation. However, with increasing time postinfection and increasing receptor number, the PDGF receptors became constitutively tyrosine-phosphorylated in serum-free culture medium. Cross-linking studies revealed that receptor activation involved ligand-independent receptor dimer formation at high receptor number. Thus, these results strongly suggest that PDGF stabilizes and increases the frequency of PDGF receptor interaction, which ultimately results in PDGF receptor activation and intracellular signaling.

Biosynthesis of the precursor of a soluble human insulin receptor ectodomain in insect Sf9 cells infected with a recombinant baculovirus

In contrast to transfected mammalian cells, insect Sf9 cells infected with a recombinant Baculovirus inefficiently process and secrete a soluble derivative of the extracellular domain of the human insulin receptor. The high-mannose form of the receptor precursor that accumulates intracellularly is not grossly aberrant or malfolded, as its interaction with a diverse panel of monoclonal antibodies are comparable to secreted precursor and proteolytically processed receptor, both of which bear partially trimmed oligosaccharide chains. Thus the inefficient step in the biosynthesis of this protein in Sf9 cells is either at, or just preceding, the trimming of its high-mannose oligosaccharide chains.

Binding of the integrin Mac-1 (CD11b/CD18) to the third immunoglobulin-like domain of ICAM-1 (CD54) and its regulation by glycosylation

Both the integrins LFA-1 and Mac-1 bind to ICAM-1, an immunoglobulin superfamily member. Previously, we localized the binding sites of LFA-1 and the major group of human rhinoviruses to the first NH2-terminal immunoglobulin-like domain of ICAM-1. Here, we show that the binding site on ICAM-1 for Mac-1 is unexpectedly distinct from that for LFA-1 and maps to the third NH2-terminal immunoglobulin-like domain. These findings provide a function for the tandem duplication of immunoglobulin-like domains in ICAM-1 and have implications for other immunoglobulin superfamily members. Mutations at two sites in the third domain that destroy consensus sequences for N-linked glycosylation enhance binding to purified Mac-1. Agents that interfere with carbohydrate processing provide evidence that the size of the N-linked oligosaccharide side chains on ICAM-1 affects binding to Mac-1 but not to LFA-1. Thus, we suggest that the extent of glycosylation on ICAM-1 may regulate adhesion to LFA-1 or Mac-1 in vivo.

Purification and biochemical characterization of a soluble mouse interferon-gamma receptor produced in insect cells

The extracellular domain of the mouse interferon gamma receptor comprising amino acids 17-243 of the protein was produced in Spodoptera frugiperda cells infected with a recombinant baculovirus. The receptor was mainly secreted into the culture medium and was purified to homogeneity in several hundred milligram amounts. The purification procedure involved four chromatography steps and delivered a soluble and active receptor with an overall recovery of 30%. From each purification run, two pools of soluble receptor with the same interferon gamma binding capacity were isolated. Under reducing electrophoretic conditions the protein of pool I migrates as two bands of molecular masses 32 and 34 kDa and of pool II as two bands of 30 and 32 kDa. The soluble receptor of both pools carries a heterogeneous glycosylation. After deglycosylation it appears as one protein band of 27 kDa. N-linked carbohydrates contribute about 6 kDa and O-linked carbohydrates 1 kDa to its molecular mass. The nonreduced protein specifically binds interferon gamma on ligand blots and in a solid-phase binding system and competes for the binding of radiolabeled interferon gamma to the cell surface receptor. The soluble mouse interferon gamma receptor exists as a monomer in physiological buffer and binds interferon gamma in its dimeric form. It is stable at room temperature and against tryptic digestion, but is very sensitive to proteinase K digestion. The soluble mouse interferon gamma receptor produced in the insect/baculovirus expression system may prove useful to study the function of interferon gamma receptor as an antagonist of endogenous interferon gamma in the treatment of immunological and inflammatory disorders.