STAT3 activation by type I interferons is dependent on specific tyrosines located in the cytoplasmic domain of interferon receptor chain 2c. Activation of multiple STATS proceeds through the redundant usage of two tyrosine residues

Human type I interferons (IFNs) play an important role in the regulation of antiviral defense mechanisms, immunomodulatory activities, and growth control. Recent efforts have demonstrated the importance of IFNs in the activation of signal transducers and activators of transcription (STATs). The role of STAT1 and STAT2 in IFN-dependent JAK-STAT signaling is well established; however, the role of STAT3 and its activation by IFNs remains unclear. Understanding the IFN-dependent regulation of STAT3 is of increasing interest because recent studies have demonstrated that STAT3 may play a role in cancer. Studies have revealed that STAT3 is constitutively active in a number of cancer cell lines and that overexpression of an active form of STAT3 transforms normal fibroblasts. Therefore, STAT3 exhibits properties indicative of known oncogenes. In this report, we define the role of the type I IFN receptor in STAT3 activation and identify for the first time tyrosine residues present in the cytoplasmic domain of IFNAR2c that are critical for STAT3 activation. The regulation of STAT3 activation by IFNs was measured in a human lung fibrosarcoma cell line lacking IFNAR2c but stably expressing various IFNAR2c tyrosine mutants. We show here that in addition to IFN-dependent tyrosine phosphorylation of STAT3, activation using a STAT3-dependent electrophoretic mobility shift assay and a STAT3-specific reporter can also be demonstrated. Furthermore, we demonstrate that type I IFN-dependent activation of STAT3 proceeds through a novel mechanism that is dependent on two tyrosines, Tyr(337) and Tyr(512), present in IFNAR2c and contained within a conserved six-amino acid residue motif, GxGYxM. Surprisingly, both tyrosines were previously shown to be required for type I IFN-dependent STAT1 and STAT2 activation. Our results reveal that type I IFNs activate multiple STATs via the overlapping usage of two tyrosine residues located in the cytoplasmic domain of IFNAR2c.

A study of the vaccinia virus interferon-gamma receptor and its contribution to virus virulence

Vaccinia virus (VV) strain Western Reserve gene B8R encodes a 43 kDa glycoprotein that is secreted from infected cells early in infection as a homodimer. This protein has amino acid similarity with the extracellular domain of cellular IFN-gamma receptor (IFN-gammaR) and binds and inhibits IFN-gamma from a wide range of species. Here we demonstrate that the B8R protein also inhibits equine IFN-gamma. The 5' end of the B8R mRNA has been mapped by primer extension analysis and the contribution of IFN-gammaRs to VV virulence was studied by the construction of a deletion mutant lacking the B8R gene (vDeltaB8R) and a revertant virus (vB8R-R) in which the B8R gene was re-inserted into the deletion mutant. A recombinant virus that expressed a soluble form of the mouse IFN-gammaR was also constructed and studied. The virulence of these viruses was tested in rodent models of infection. In mice, the loss of the VV IFN-gammaR did not affect virulence compared with WT and revertant viruses, consistent with the low affinity of the VV IFN-gammaR for mouse IFN-gamma. However, expression of the mouse soluble IFN-gammaR increased virus virulence slightly. In rabbit skin, loss of the VV IFN-gammaR produced lesions with histological differences compared with WT and revertant viruses. Lastly, the affinity constants of the VV IFN-gammaR for human and mouse IFN-gamma were determined by surface plasmon resonance.

Two distinct domains within the N-terminal region of Janus kinase 1 interact with cytokine receptors

The interaction between receptors and kinases of the Janus kinase (Jak) family is critical for signaling by growth factors, cytokines, and IFNs. Therefore, the characterization of the domains involved in these interactions is pivotal not only in understanding kinase activation but also in the development of drugs that mimic or inhibit signaling. In this report, we have characterized the domains of Jak1 required to associate with distinct cytokine receptor subunits: IFN-alpha R beta L, IFN-gamma R alpha, IL-10R alpha, IL-2R beta, and IL-4R alpha. We demonstrate that two regions of Jak1 are necessary for the interaction with cytokine receptors. First, a common N-terminal region that includes Jak homology (JH) domain 7 and the first 19 aa of JH6, and, second, a C-terminal region (JH6-3) that was different for distinct receptors. The contribution of the two different regions of Jak1 to cytokine receptor binding was also variable. Deletion of JH7-6 impaired the association of IL-2R beta and IL-4R alpha chains with Jak1 but did not have a major impact on the binding of Jak1 to IFN-alpha R beta L or IL-10R alpha. Interestingly, regardless of the effect on receptor binding, removal of JH7-6 completely abrogated kinase activation, indicating that this domain is required for ligand-driven kinase activation and, thus, for proper signaling through cytokine receptors.

Stat2 binding to the interferon-alpha receptor 2 subunit is not required for interferon-alpha signaling

The interferon-alpha (IFNalpha) receptor consists of two subunits, the IFNalpha receptor 1 (IFNaR1) and 2 (IFNaR2) chains. Following ligand binding, IFNaR1 is phosphorylated on tyrosine 466, and this site recruits Stat2 via its SH2 domain. In contrast, IFNaR2 binds Stat2 constitutively. In this study we have characterized the Stat2-IFNaR2 interaction and examined its role in IFNalpha signaling. Stat2 binds the major IFNaR2 protein but not a variant containing a shorter cytoplasmic domain. The interaction does not require a STAT SH2 domain. Both tyrosine-phosphorylated and non-phosphorylated Stat2 bind IFNaR2 in vitro; however, relatively little phosphorylated Stat2 associates with IFNaR2 in vivo. In vitro binding assays defined IFNaR2 residues 418-444 as the minimal interaction domain and site-specific mutation of conserved acidic residues within this domain disrupted in vitro and in vivo binding. An IFNaR2 construct carrying these mutations was either (i) overexpressed in 293T cells or (ii) used to complement IFNaR2-deficient U5A cells. Unexpectedly, the activity of an IFNalpha-dependent reporter gene was not reduced but, instead, was enhanced up to 2-fold. This suggests that this particular IFNaR2-Stat2 interaction is not required for IFNalpha signaling, but might act to negatively inhibit signaling. Finally, a doubly truncated recombinant fragment of Stat2, spanning residues 136-702, associated with IFNaR2 in vitro, indicating that the interaction with IFNaR2 is direct and occurs in a central region of Stat2 marked by a hydrophobic core.

The human interferon receptor: NMR-based modeling, mapping of the IFN-alpha 2 binding site, and observed ligand-induced tightening

The human interferon receptor (IFNAR) mediates the antiviral and antiproliferative activities of type I interferons (IFNs). This receptor is comprised of subunits IFNAR1 and IFNAR2, the latter exhibiting nanomolar affinity for IFNs. Here the extracellular domain of IFNAR2 (IFNAR2-EC), a soluble 25 kDa IFN-binding polypeptide, and its complex with IFN-alpha 2 were studied using multidimensional NMR. IFNAR2-EC is comprised of two fibronectin-III (FN-III) domains connected by a helical hinge region. The deduced global fold was utilized to improve the alignment of IFNAR2-EC against structurally related receptors and to model its structure. A striking feature of IFNAR2-EC is the limited and localized deviations in chemical shifts exhibited upon ligand binding, observed for only 15% of its backbone (1)H and (15)N nuclei. Analysis of these deviations maps the IFN-alpha 2 binding site upon IFNAR2-EC to a contiguous surface on the N-terminal domain, including the S3-S4 loop (residues 44-53), the S5-S6 loop and S6 beta-strand (residues 74-82), and the S7 beta-strand and the hinge region (residues 95-105). The C-terminal domain contributes only marginally to ligand binding, and no change in the hypothesized interdomain interface is observed. The proposed binding domain encompasses all residues implicated by mutagenesis studies in IFN binding, and suggests adjacent residues cooperate in forming the binding surface. D(2)O-exchange experiments indicate that binding of IFN-alpha2 induces tightening of the N-terminal domain of IFNAR2-EC. This increase in receptor rigidity may play an important role in initiating the intracellular stage of the IFN signaling cascade.

The vaccinia virus soluble interferon-gamma receptor is a homodimer

The vaccinia virus (VV) interferon (IFN)-gamma receptor (IFN-gammaR) is a 43 kDa soluble glycoprotein that is secreted from infected cells early during infection. Here we demonstrate that the IFN-gammaR from VV, cowpox virus and camelpox virus exists naturally as a homodimer, whereas the cellular IFN-gammaR dimerizes only upon binding the homodimeric IFN-gamma. The existence of the virus protein as a dimer in the absence of ligand may provide an advantage to the virus in efficient binding and inhibition of IFN-gamma in solution.

Interferon signaling is dependent on specific tyrosines located within the intracellular domain of IFNAR2c. Expression of IFNAR2c tyrosine mutants in U5A cells

Type I interferons (IFNs) are cytokines that play a central role in mediating antiviral, antiproliferative, and immunomodulatory activities in virtually all cells. These activities are entirely dependent on the interaction of IFNs with their particular cell surface receptor. In this report, we identify two specific tyrosine residues located within the cytoplasmic domain of IFNAR2c that are obligatory for IFN-dependent signaling. Various IFNAR2c tyrosine mutants were expressed in a human lung fibroscarcoma cell line lacking IFNAR2c (U5A). Stable clones expressing these mutants were analyzed for their ability to induce STAT1 and STAT2 activation, ISGF3 transcriptional complex formation, gene expression, and cell growth regulation in response to stimulation with type I IFNs. The replacement of all seven cytoplasmic tyrosine residues of IFNAR2c with phenylalanine resulted in a receptor unable to respond to IFN stimulation. Substitution of single tyrosines at amino acid residue 269, 316, 318, 337, or 512 with phenylalanine had no effect on IFN-dependent signaling, suggesting that no single tyrosine is essential for IFN receptor-mediated signaling. In addition, IFNAR2c retaining five proximal tyrosines residues (269, 306, 316, 318, and 337) or either two distal tyrosine residues (411 or 512) continued to be responsive to IFN stimulation. Surprisingly, the presence of only a single tyrosine at either position 337 or 512 was sufficient to restore a complete IFN response. These results indicate that IFN-dependent signaling proceeds through the redundant usage of two tyrosine residues in the cytoplasmic domain of IFNAR2c.

The Stat1 binding motif of the interferon-gamma receptor is sufficient to mediate Stat5 activation and its repression by SOCS3

Signal transduction via the interferon-gamma (IFN-gamma) receptor requires the tyrosine phosphorylation of signal transducers and activators of transcription (Stats). Whereas tyrosine phosphorylation of Stat1 occurs in all cells, activation of Stat5 by IFN-gamma is cell type-restricted. Here we investigated the mechanism of Stat5 activation by the IFN-gamma receptor. In transfection assays both Stat5 isoforms, Stat5a and Stat5b, were phosphorylated on tyrosine in response to IFN-gamma. Stat5 activation required the presence of tyrosine 420 (Tyr-420) in the murine IFNGR1 receptor chain, which also serves as the Stat1 binding site. Moreover, a peptide including Tyr-440, the Stat1 binding site of the human IFNGR1 chain, conferred the ability upon a synthetic receptor to activate Stat5. Suppressor of cytokine signaling 3 (SOCS3) inhibited the activation of Stat5 by the IFN-gamma receptor, and the Tyr-440-containing peptide stretch was sufficient for repression. SOCS3 expression had little effect on the activity of Jak kinases not associated with cytokine receptors. In IFN-gamma-treated, Stat1-deficient fibroblasts Stat5 was inefficient in inducing transcription of a Stat-dependent reporter gene, suggesting it does not per se make a major contribution to the expression of IFN-gamma-responsive genes.

Characterization of ATI, TK and IFN-alpha/betaR genes in the genome of the BeAn 58058 virus, a naturally attenuated wild Orthopoxvirus

The lack of knowledge about the natural host of Vaccinia virus (VV) along with the description of human infections caused by poxviruses after smallpox eradication has increased the need to characterize poxviruses isolated from the wild. Moreover, in the past years poxviruses have been widely studied as potential vaccination tools, with the discovery of several genes implicated in the evasion of the host immune response involved in virus pathogenesis. Among them, an Interferon (IFN)-binding protein was identified in the supernatant of VV strain WR infected cells coded by the B18R gene. It was shown that many other Orthopoxviruses also encode and express this soluble receptor although some VV strains such as Lister and modified Ankara, which were less reactogenic vaccines, do not. The BeAn 58058 virus (BAV) has been recently characterized and proposed to be an Orthopoxvirus. BAV was also shown to be less virulent in animal models than VV Lister. Here we report the identification of an IFN-alpha/betaR gene in the BAV genome with 99% of sequence identity with the VVWR B18R gene. The identified gene encodes a B18R-like IFN binding protein as demonstrated by its capacity to inhibit the IFN-mediated protection of VERO cells against EMC virus. In order to better characterize the virus we have searched for the A type inclusion body (ATI) gene currently used in the classification of Orthopoxviruses but did not detect it in the BAV genome. We have also sequenced the BAV thymidine kinase (TK) gene, a poxvirus-conserved gene, which, as expected, showed high homology with the TK gene of other poxviruses. Phylogenetic trees were constructed based on sequences of the IFN-alpha/betaR and TK genes from several poxviruses and in both cases BAV was placed in the same cluster as other VV strains. These observations strengthened the hypothesis that this virus is a variant of the VV vaccine used in Brazil. However the explanation for the BAV lack of virulence remains to be discovered.

Structure of the interferon-receptor complex determined by distance constraints from double-mutant cycles and flexible docking

The pleiotropic activity of type I interferons has been attributed to the specific interaction of IFN with the cell-surface receptor components ifnar1 and ifnar2. To date, the structure of IFN has been solved, but not that of the receptor or the complex. In this study, the structure of the IFN-alpha 2-ifnar2 complex was generated with a docking procedure, using nuclear Overhauser effect-like distance constraints obtained from double-mutant cycle experiments. The interaction free energy between 13 residues of the ligand and 11 of the receptor was measured by double-mutant cycles. Of the 100 pairwise interactions probed, five pairs of residues were found to interact. These five interactions were incorporated as distance constraints into the flexible docking program prodock by using fixed and movable energy-gradient grids attached to the receptor and ligand, respectively. Multistart minimization and Monte Carlo minimization docking of IFN-alpha 2 onto ifnar2 converged to a well-defined average structure, with the five distance constraints being satisfied. Furthermore, no structural artifacts or intraloop energy strain were observed. The mutual binding sites on IFN-alpha 2 and ifnar2 predicted from the model showed an almost complete superposition with the ones determined from mutagenesis studies. Based on this structure, differences in IFN-alpha 2 versus IFN-beta binding are discussed.

Mouse oocytes and preimplantation embryos bear the two sub-units of interferon-gamma receptor

Cytokines and growth factors play important roles in implantation and maintenance of pregnancy, but also during early development. Among them interferon-gamma (IFNgamma) is highly expressed by mammalian trophoblast cells during implantation and seems to be involved in some cases of pregnancy loss. In the present study we investigated the possible presence of IFNgamma receptors (IFNGR) on mouse oocytes and preimplantation embryos. The two receptor chains IFNgammaRalpha (IFNGR-1) and IFNgammaRbeta (IFNGR-2) have been detected by indirect immunofluorescence at the surface of mouse oocytes (in germinal vesicle and metaphase II stages), as well as at all stages of in vitro embryo development from the one-cell to blastocyst stage. IFNGR appeared to colocalize partly with ganglioside GM1 at the cell surface of oocytes and embryos, indicating a possible preferential localization of this receptor in "rafts" microdomains. This was analyzed in more detail using software developed in the laboratory. IFNgamma was found to bind to its receptor at all stages analyzed. RT-PCR and Southern blot experiments confirmed the presence of the transcriptionally regulated IFNGR-2 chain mRNA, in mouse oocytes and preimplantation embryos. These results show, for the first time, that mouse oocytes and preimplantation embryos bear a complete and theoretically functional IFNGR, suggesting that this cytokine could play a role during early development.

Crystal structure of the IL-10/IL-10R1 complex reveals a shared receptor binding site

Interleukin 10 (IL-10) is a dimeric cytokine that plays a central role in suppressing inflammatory responses. These activities are dependent on the interaction of IL-10 with its high-affinity receptor (IL-10R1). This intermediate complex must subsequently recruit the low-affinity IL-10R2 chain before cell signaling can occur. Here we report the 2.9 A crystal structure of IL-10 bound to a soluble form of IL-10R1 (sIL-10R1). The complex consists of two IL-10s and four sIL-10R1 molecules. Several residues in the IL-10/sIL-10R1 interface are conserved in all IL-10 homologs and their receptors. The data suggests that formation of the active IL-10 signaling complex occurs by a novel molecular recognition paradigm where IL-10R1 and IL-10R2 both recognize the same binding site on IL-10.

Identification of critical residues in bovine IFNAR-1 responsible for interferon binding

Interferons have antiviral, antigrowth and immunomodulatory effects. The human type I interferons, IFN-alpha, IFN-beta, and IFN-omega, induce somewhat different cellular effects but act through a common receptor complex, IFNAR, composed of subunits IFNAR-1 and IFNAR-2. Human IFNAR-2 binds all type I IFNs but with lower affinity and different specificity than the IFNAR complex. Human IFNAR-1 has low intrinsic binding of human IFNs but strongly affects the affinity and differential ligand specificity of the IFNAR complex. Understanding IFNAR-1 interactions with the interferons is critical to elucidating the differential ligand specificity and activation by type I IFNs. However, studies of ligand interactions with human IFNAR-1 are compromised by its low affinity. The homologous bovine IFNAR-1 serendipitously binds human IFN-alphas with nanomolar affinity. Exploiting its strong binding of human IFN-alpha2, we have identified residues important for ligand binding. Mutagenesis of any of five aromatic residues of bovine IFNAR-1 caused strong decreases in ligand binding, whereas mutagenesis of proximal neutral or charged residues had smaller effects. These residues were mapped onto a homology model of IFNAR-1 to identify the ligand-binding face of IFNAR-1, which is consistent with previous structure/function studies of human IFNAR-1. The topology of IFNAR-1/IFN interactions appears novel when compared with previously studied cytokine receptors.

Role of the C-terminal chain in human interferongamma stability: an electrostatic study

Electrostatic interactions in two structures of human interferon gamma (hIFNgamma), corresponding to interferon molecule alone and bound to its receptor, were analyzed on the basis of a continuum dielectric model. It was found that a number of titratable groups, mainly basic, show large pK shifts and remain in their neutral forms at physiologically relevant pH. The fact that these groups are largely common to both structures and that most of them belong to the set of most conserved sites suggests that this is a property inherent to the hIFNgamma molecule rather than an artifact of the crystal packing. His111 was also found deprotonated at neutral pH. It was concluded that receptor recognition involving His111 is driven by aromatic coupling of His111 and Tyr52 from the receptor rather than by electrostatic interactions. The structure corresponding to hIFNgamma in complex with its receptor shows a reduction in number and in degree of desolvation of the buried titratable sites. This finding suggested that on receptor binding, hIFNgamma adopts energetically more favorable, relaxed, conformation. It was experimentally shown that in contrast to the full-size hIFNgamma, the construct having 21 amino acid residues deleted from the C-terminus is soluble. The hydrophobicity profile analysis suggested that factors other than the exposure of hydrophobic parts of the molecule are responsible for the low stability and propensity for aggregation. On the basis of these results, it was assumed that the electrostatic influence of the C-terminal part contributes particularly to the low solvent exposure of the titratable groups, and hence to the low structural stability and propensity for aggregation of the recombinant hIFNgamma. Proteins 2001;43:125-133.

Regulation of interferon-gamma receptor (INF-gammaR) chains: a peculiar way to rule the life and death of human lymphocytes

Interferon-gamma (IFN-gamma) is a lymphokine produced by activated T lymphocytes and NK cells, that plays an important role in host defense mechanisms by exerting pleiotropic activities on a wide range of cell types. Cellular responses to IFN-gamma are mediated by its heterodimeric cell surface receptor (IFN-gammaR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. Several observations suggest that the signals resulting from the binding of IFN-gamma to its receptor depend on the number of surface receptors transducing the IFN-gamma signal. This review summarizes recent advances in the understanding of the fine regulation of the response of human lymphocytes to IFN-gamma through an interplay between surface expression of IFN-gammaR and a variety of environmental factors that combine to control their fate.

The structure and activity of a monomeric interferon-gamma:alpha-chain receptor signaling complex

BACKGROUND

Interferon-gamma (IFN-gamma) is a homodimeric cytokine that exerts its various activities by inducing the aggregation of two different receptors. The alpha chain receptor (IFN-gammaRalpha) is a high affinity receptor that binds to IFN-gamma in a symmetric bivalent manner to form a stable, intermediate 1:2 complex. This intermediate forms a binding template for the subsequent binding of two copies of the second receptor, beta chain receptor (IFN-gammaRbeta), producing the active 1:2:2 signaling complex.

RESULTS

A single chain monovalent variant of IFN-gamma (scIFN-gamma) was constructed and complexed to one copy of the extracellular domain (ECD) of IFN-gammaRalpha. The structure of this 1:1 complex was determined and the hormone-receptor interface shown to be characterized by a number of hydrophilic interactions mediated by several highly ordered water networks. The scIFN-gamma interface consists of segments from each of the monomer chains of the homodimer. The principal hydrophobic contact of the receptor involves a tripeptide segment of the receptor having an unusual and high energy conformation. Despite containing only one binding site for IFN-gammaRalpha, the monovalent scIFN-gamma molecule has significant activity in antiviral biological assays.

CONCLUSIONS

ScIFN-gamma binds the ECD of IFN-gammaRalpha through a highly hydrated interface with an important set of hormone-receptor contacts mediated through structured waters. Although the interface is highly hydrated, it supports tight binding and has a considerable degree of specificity. The biological activity of scIFN-gamma confirms that the scIFN-gamma:IFN-gammaRalpha complex represents a productive intermediate and that it can effectively recruit the other required component, IFN-gammaRbeta, to signal based on the 1:1:1 complex.

Antibodies against IFN gamma-binding proteins recognize a member of IFN alpha R complex

Recombinant human IFN alpha 2b coupled to a silica support was used for the purification of the IFN alpha-binding proteins from placental cell membrane extracts. The 100-kDa (p100) and 64-kDa (p64) proteins, which bind preferentially to an IFN alpha 2b-silica matrix, were identified. Using a monoclonal antibody (A6) against IFN-gammaR1, it was able to isolate p100 and p70, but only if IFN alpha 2b was present during chromatography. Similar interactions were observed using polyclonal antibody anti-IFN gamma binding proteins, as assayed in Western blot. These interactions were identified as conformation dependent. We speculate that IFN alpha 2b receptor complex shares an IFN gamma receptor complex epitope.

New structural and functional aspects of the type I interferon-receptor interaction revealed by comprehensive mutational analysis of the binding interface

Type I interferons bind to two cell surface receptors, ifnar1 and ifnar2, as the first step in the activation of several signal transduction pathways that elicit an anti-viral state and an anti-proliferative response. Here, we quantitatively mapped the complete binding region of ifnar2 on interferon (IFN)alpha2 by 35 individual mutations to alanine and isosteric residues. Of the six "hot-spot" residues identified (Leu-30, Arg-33, Arg-144, Ala-145, Met-148, and Arg-149), four are located on the E-helix, which is located at the center of the binding site flanked by residues on the A-helix and the AB-loop. The contribution of residues of the D-helix, which have been previously implicated in binding, proved to be marginal for the interaction with the extracellular domain of ifnar2. Interestingly, the ifnar2 binding site overlaps the largest continuous hydrophobic patch on IFNalpha2. Thus, hydrophobic interactions seem to play a significant role stabilizing this interaction, with the charged residues contributing toward the rapid association of the complex. Relating the anti-viral and anti-proliferative activity of the various interferon mutants with their affinity toward ifnar2 results in linear function over the whole range of affinities investigated, suggesting that ifnar2 binding is the rate-determining step in cellular activation. Dose-time analysis of the anti-viral response revealed that shortening the incubation time of low-level activation cannot be compensated by higher IFN doses. Considering the strict dependence of the cellular response on affinity, these results suggest that for maintaining transcription of IFN-responsive genes over a longer time period, low but continuous signaling through the IFN receptor is essential.

Observation of an unexpected third receptor molecule in the crystal structure of human interferon-gamma receptor complex

BACKGROUND

Molecular interactions among cytokines and cytokine receptors form the basis of many cell-signaling pathways relevant to immune function. Interferon-gamma (IFN-gamma) signals through a multimeric receptor complex consisting of two different but structurally related transmembrane chains: the high-affinity receptor-binding subunit (IFN-gammaRalpha) and a species-specific accessory factor (AF-1 or IFN-gammaRbeta). In the signaling complex, the two receptors probably interact with one another through their extracellular domains. Understanding the atomic interactions of signaling complexes enhances the ability to control and alter cell signaling and also provides a greater understanding of basic biochemical processes.

RESULTS

The crystal structure of the complex of human IFN-gamma with the soluble, glycosylated extracellular part of IFN-gammaRalpha has been determined at 2.9 A resolution using multiwavelength anomalous diffraction methods. In addition to the expected 2:1 complex, the crystal structure reveals the presence of a third receptor molecule not directly associated with the IFN-gamma dimer. Two distinct intermolecular contacts, involving the edge strands of the C-terminal domains, are observed between this extra receptor and the 2:1 receptor-ligand complex thereby forming a 3:1 complex.

CONCLUSIONS

The observed interactions in the 2:1 complex of the high-affinity cell-surface receptor with the IFN-gamma cytokine are similar to those seen in a previously reported structure where the receptor chains were not glycosylated. The formation of beta-sheet packing interactions between pairs of IFN-gammaRalpha receptors in these crystals suggests a possible model for receptor oligomerization of Ralpha and the structurally homologous Rbeta receptors in the fully active IFN-gamma signaling complex.

Structural characterization of oligosaccharides in recombinant soluble human interferon receptor 2 using fluorophore-assisted carbohydrate electrophoresis

The N-linked oligosaccharide profiles (banding patterns in gels) and structures of recombinant soluble human interferon receptor 2 (r-shIFNAR2) were determined using fluorophore-assisted carbohydrate electrophoresis (FACE, Glyko, Novato, CA). The method involves releasing N-linked oligosaccharide moieties from a glycoprotein by digestion with peptide-N glycanase (PNGase F), labeling the released oligosaccharides with the fluorescent dye 8-aminonaphthalene-1,3,6-trisulfonate (ANTS), and separating the labeled oligosaccharides by gel electrophoresis. The isolated oligosaccharides in the bands from the profiling gels can then be sequenced using exoglycosidases to reveal the oligosaccharide structures. The oligosaccharide profile of r-shIFNAR2 consists of at least nine oligosaccharide bands. The relative amount of oligosaccharide in each band can vary, depending on the culture conditions of the source cells. FACE structural analysis shows that r-shIFNAR2 contains only core-fucosylated N-linked oligosaccharides, most of which are fully sialylated (approximately 92%). The major types and relative amounts of the oligosaccharides from a representative sample are: disialylated, galactosylated, biantennary (15%); trisialylated, galactosylated, triantennary (19%), tetrasialylated, galactosylated, tetraantennary (30%), and N-acetyllactosamine-containing higher-order oligosaccharides including tri-, tetra-, and pentaantennary (28%). The remaining oligosaccharides are not fully sialylated and/or not fully galactosylated di-, tri-, and tetraantennary structures (approximately 5%) and unidentified structures (approximately 3%). A method for determining the types and structures of the N-acetyllactosamine containing oligosaccharides is also reported in this study.

Expression and biological activity of two recombinant polypeptides related to subunit 1 of the interferon-alpha receptor

Abnormal production of interferon alpha (IFN-alpha) has been found in certain autoimmune diseases and can be also observed after prolonged therapy with IFN-alpha. IFN-alpha can contribute to the pathogenesis of allograft rejection in bone marrow transplants. Therefore, the development of IFN-alpha inhibitors as a soluble receptor protein may be valuable for the therapeutic control of these diseases. We have expressed two polypeptides encoding amino acids 93-260 (P1) and 261-410 (P2) of the extracellular domain of subunit 1 of the interferon-alpha receptor (IFNAR 1-EC) in E. coli. The activities of the recombinant polypeptides and of their respective antibodies were evaluated using antiproliferative and antiviral assays. Expression of P1 and P2 polypeptides was achieved by transformation of cloned plasmid pRSET A into E. coli BL21(DE3)pLysS and by IPTG induction. P1 and P2 were purified by serial sonication steps and by gel filtration chromatography with 8 M urea and refolded by dialysis. Under reducing SDS-PAGE conditions, the molecular weight of P1 and P2 was 22 and 17 kDa, respectively. Polyclonal anti-P1 and anti-P2 antibodies were produced in mice. P1 and P2 and their respective polyclonal antibodies were able to block the antiproliferative activity of 6.25 nM IFN-alphaB on Daudi cells, but did not block IFN-alphaB activity at higher concentrations (>6. 25 nM). On the other hand, the polypeptides and their respective antibodies did not inhibit the antiviral activity of IFN-alphaB on Hep 2/c cells challenged with encephalomyocarditis virus.

Differential nuclear localization of the IFNGR-1 and IFNGR-2 subunits of the IFN-gamma receptor complex following activation by IFN-gamma

We have recently identified a nuclear localization sequence (NLS) in the C-terminus of murine type II interferon (IFN), IFN-gamma, that is responsible for the internalization and nuclear translocation of extracellularly added IFN-gamma. Because the uptake of IFN-gamma is a receptor-mediated endocytotic process, we examined in this study the fate of both the receptor subunits (IFNGR-1 and IFNGR-2) of the heterodimeric IFN-gamma receptor complex. Human IFN-gamma (HuIFN-gamma) was also found to contain a polybasic NLS in a conserved C-terminal region capable of directing its nuclear translocation. Like the ligand, the IFNGR-1 subunit of the receptor complex on WISH cells was found to be translocated to the nucleus on treatment with HuIFN-gamma. Using a combination of immunoprecipitation and immunofluorescence techniques, we found the nuclear accumulation of IFNGR-1 to be ligand dependent, and it was evident within 10-20 min after ligand stimulation. IFNGR-1 was found to colocalize, in a time-dependent and dose-dependent fashion, with the nuclear translocation of the transcription factor Stat1alpha, which is activated by this ligand-receptor system. In addition, Stat1alpha was found to be complexed with IFNGR-1 over the time period of its nuclear translocation. In marked contrast, IFNGR-2 was not transported to the nucleus. The surface immunofluorescence pattern of IFNGR-2 suggested that, following ligand stimulation, the majority of IFNGR-2 remains at the cell surface, whereas IFNGR-1 is endocytosed and targeted to the cell nucleus. These findings suggest that IFNGR-1 plays an active intracellular role in signal transduction events subsequent to the binding of ligand to the dimeric receptor complex. Furthermore, these studies provide the first example of the selective endocytosis and nuclear translocation of a subunit of a multimeric receptor complex.

Design, characterization, and structure of a biologically active single-chain mutant of human IFN-gamma

A mutant form of human interferon-gamma (IFN-gamma SC1) that binds one IFN-gamma receptor alpha chain (IFN-gamma R alpha) has been designed and characterized. IFN-gamma SC1 was derived by linking the two peptide chains of the IFN-gamma dimer by a seven-residue linker and changing His111 in the first chain to an aspartic acid residue. Isothermal titration calorimetry shows that IFN-gamma SC1 forms a 1:1 complex with its high-affinity receptor (IFN-gamma R alpha) with an affinity of 27(+/- 9) nM. The crystal structure of IFN-gamma SC1 has been determined at 2.9 A resolution from crystals grown in 1.4 M citrate solutions at pH 7.6. Comparison of the wild-type receptor-binding domain and the Asp111-containing domain of IFN-gamma SC1 show that they are structurally equivalent but have very different electrostatic surface potentials. As a result, surface charge rather than structural changes is likely responsible for the inability of the His111-->Asp domain of to bind IFN-gamma R alpha. The AB loops of IFN-gamma SC1 adopt conformations similar to the ordered loops of IFN-gamma observed in the crystal structure of the IFN-gamma/IFN-gamma R alpha complex. Thus, IFN-gamma R alpha binding does not result in a large conformational change in the AB loop as previously suggested. The structure also reveals the final six C-terminal amino acid residues of IFN-gamma SC1 (residues 253-258) that have not been observed in any other reported IFN-gamma structures. Despite binding to only one IFN-gamma R alpha, IFN-gamma SC1 is biologically active in cell proliferation, MHC class I induction, and anti-viral assays. This suggests that one domain of IFN-gamma is sufficient to recruit IFN-gamma R alpha and IFN-gamma R beta into a complex competent for eliciting biological activity. The current data are consistent with the main role of the IFN-gamma dimer being to decrease the dissociation constant of IFN-gamma for its cellular receptors.

Jak-Stat signal transduction pathway through the eyes of cytokine class II receptor complexes

Cells of the immune system communicate with each other to initiate, establish and maintain immune responses. The communication occurs through cell-to-cell contact or through a variety of intercellular mediators that include cytokines, chemokines, growth factors and hormones. In the case of cytokines, the signal is transmitted from the outside to the inside of a cell through cell surface receptors specific for each cytokine. At this step the signal is also decoded and amplified: ligand binding causes recruitment and/or activation of numerous cytoplasmic proteins. One cytokine can activate a number of signal transduction pathways leading to regulation of a wide array of biological activities. One of these pathways, the Jak-Stat pathway, is briefly reviewed here with respect to the class II cytokine receptors. Signal transduction through receptors for interferons Type I (IFN-alpha, IFN-beta, IFN-omega) and Type II (IFN-gamma), and interleukin 10 (IL-10) is described in detail. In addition, a complex between tissue factor (TF) and coagulation factor VIIa, and two new receptors related to the class II cytokine receptor family are discussed. Oncogene (2000).

Structure-function study of the extracellular domain of the human type I interferon receptor (IFNAR)-1 subunit

Despite accumulating information about the different effector molecules and signaling cascades that are invoked on interferon-alpha (IFN-alpha) binding to the type 1 IFN receptor, little is known about the specifics of the binding interactions between the ligand and the receptor complex. The IFN-alpha/beta receptor (IFNAR)-2 subunit of the IFN receptor is considered the primary binding chain of the receptor, yet it is clear that both receptor subunits, IFNAR-1 and IFNAR-2, cooperate in the high-affinity binding of IFN to the receptor complex. Earlier results from our laboratory suggested that an association of IFNAR-1 with membrane Galalpha1-4Gal-containing glycolipids facilitates receptor-mediated signaling. The data implicated amino acid residues in the SD100 domain of IFNAR-1 in the glycosphingolipid (GSL) modification of the type 1 IFN receptor. Interestingly, the human and murine counterparts of IFNAR-1 exhibit remarkable species specificity despite their considerable amino acid sequence identity. Certainly, those amino acid residues that effect GSL modification of IFNAR-1 are conserved between species, yet specific regions of IFNAR-1 that confer species specificity have not been defined. To delineate further the role of the IFNAR-1 SD100A domain in receptor function, a chimeric cDNA was assembled, in which the SD100A domain of the murine IFNAR-1 chain was replaced with the human sequence. This construct was expressed in IFNAR-1-/- mouse embryonic fibroblasts, and stable transfectants were established. Transfectants are fully sensitive to murine IFN-alpha4 treatment with respect to the induction of IFN-stimulated gene factor 3 (ISGF3) and sis-inducing factor (SIF) signal transducer and activator of transcription factor (Stat) complexes, exhibiting comparable levels of Stat activation to those observed in IFNAR-1-/- cells reconstituted with intact MuIFNAR-1. Similar results were obtained with IFN-induced antiviral and growth inhibitory responses. Viewed together, these data suggest that the SD100A domain of IFNAR-1 does not contribute to species-specific IFN binding.