Identification and characterization of tilapia CRFB1, CRFB2 and CRFB5 reveals preferential receptor usage of three IFN subtypes in perciform fishes

Type I interferons are a subset of cytokines playing central roles in host antiviral defense, and their effects depend on the interaction with the heterodimeric receptor complex. Surprisingly, two pairs of the receptor subunits, CRFB1 and CRFB5, and CRFB2 and CRFB5, have been identified in fish, but the studies about preferential receptor usage of different fish IFN subtypes are rather limited. In this study, the three receptor chains of type I IFNs named as On-CRFB1, On-CRFB2 and On-CRFB5 were identified in Nile tilapia, Oreochromis niloticus. These three genes were constitutively expressed in all tissues examined, with the highest expression level observed in muscle and liver, and were rapidly induced in liver following the stimulation of poly(I:C). Interestingly, it is possible that all three subtypes of tilapia IFNs are able to signal through two pairs of the receptor subunits, On-CRFB1 and On-CRFB5, and On-CRFB2 and On-CRFB5. More importantly, tilapia group I IFNs (On-IFNd and On-IFNh) preferentially signal through a receptor complex composed of On-CRFB1 and On-CRFB5, and group II IFNs (On-IFNc) preferentially signal through a receptor complex comprised of On-CRFB2 and On-CRFB5. The present study thus provides new insights into the receptor usage of group I and group II IFNs in fish.

Identification of type I IFNs and their receptors in a cyprinid fish, the topmouth culter Culter alburnus

In fish, type I IFNs are classified into three groups, i.e. group one, group two and group three, and further separated into seven subgroups based on the number of conserved cysteines and phylogenetic relationships. In the present study, four type I IFNs, named as IFNϕ1, IFNϕ2, IFNϕ3, IFNϕ4, as reported in zebrafish, were identified in a cyprinid, the topmouth culter, Culter alburnus, a species introduced recently into China's aquaculture. These IFNs may be classified as IFNa, IFNc, IFNc and IFNd in a recent nomenclature, with IFNa and IFNd having two cysteines in group one, and IFNc four cysteines in group two. These IFNs, together with their possible receptors, IFNϕ1, IFNϕ2, IFNϕ3, IFNϕ4, and CRFB1, CRFB2 and CRFB5 have an open reading frame (ORF) of 540, 552, 567, 516 bp, and 1572, 1392, 1125 bp, respectively. These IFNs have high amino acid sequence identities, being 91.1-93.6% and 66.9-77.3%, with those in grass carp and zebrafish, respectively, and are expressed constitutively in organs/tissues examined in the fish. The expression of these IFNs can be further induced following poly (I:C) stimulation. However, the possible function of these IFNs and their signalling pathway are of interest for further research.

Functional Characterization of an Interferon Gamma Receptor-Like Protein on Entamoeba histolytica

Entamoeba histolytica is an anaerobic parasitic protozoan and the causative agent of amoebiasis. E. histolytica expresses proteins that are structurally homologous to human proteins and uses them as virulence factors. We have previously shown that E. histolytica binds exogenous interferon gamma (IFN-γ) on its surface, and in this study, we explored whether exogenous IFN-γ could modulate parasite virulence. We identified an IFN-γ receptor-like protein on the surface of E. histolytica trophozoites by using anti-IFN-γ receptor 1 (IFN-γR1) antibody and performing immunofluorescence, Western blot, protein sequencing, and in silico analyses. Coupling of human IFN-γ to the IFN-γ receptor-like protein on live E. histolytica trophozoites significantly upregulated the expression of E. histolytica cysteine protease A1 (EhCP-A1), EhCP-A2, EhCP-A4, EhCP-A5, amebapore A (APA), cyclooxygenase 1 (Cox-1), Gal-lectin (Hgl), and peroxiredoxin (Prx) in a time-dependent fashion. IFN-γ signaling via the IFN-γ receptor-like protein enhanced E. histolytica's erythrophagocytosis of human red blood cells, which was abrogated by the STAT1 inhibitor fludarabine. Exogenous IFN-γ enhanced chemotaxis of E. histolytica, its killing of Caco-2 colonic and Hep G2 liver cells, and amebic liver abscess formation in hamsters. These results demonstrate that E. histolytica expresses a surface IFN-γ receptor-like protein that is functional and may play a role in disease pathogenesis and/or immune evasion.

Interferon γ and Its Important Roles in Promoting and Inhibiting Spontaneous and Therapeutic Cancer Immunity

Originally identified in studies of cellular resistance to viral infection, interferon (IFN)-γ is now known to represent a distinct member of the IFN family and plays critical roles not only in orchestrating both innate and adaptive immune responses against viruses, bacteria, and tumors, but also in promoting pathologic inflammatory processes. IFN-γ production is largely restricted to T lymphocytes and natural killer (NK) cells and can ultimately lead to the generation of a polarized immune response composed of T helper (Th)1 CD4+ T cells and CD8+ cytolytic T cells. In contrast, the temporally distinct elaboration of IFN-γ in progressively growing tumors also promotes a state of adaptive resistance caused by the up-regulation of inhibitory molecules, such as programmed-death ligand 1 (PD-L1) on tumor cell targets, and additional host cells within the tumor microenvironment. This review focuses on the diverse positive and negative roles of IFN-γ in immune cell activation and differentiation leading to protective immune responses, as well as the paradoxical effects of IFN-γ within the tumor microenvironment that determine the ultimate fate of that tumor in a cancer-bearing individual.

Structure of the IFNγ receptor complex guides design of biased agonists

The cytokine interferon-γ (IFNγ) is a central coordinator of innate and adaptive immunity, but its highly pleiotropic actions have diminished its prospects for use as an immunotherapeutic agent. Here, we took a structure-based approach to decoupling IFNγ pleiotropy. We engineered an affinity-enhanced variant of the ligand-binding chain of the IFNγ receptor IFNγR1, which enabled us to determine the crystal structure of the complete hexameric (2:2:2) IFNγ-IFNγR1-IFNγR2 signalling complex at 3.25 Å resolution. The structure reveals the mechanism underlying deficits in IFNγ responsiveness in mycobacterial disease syndrome resulting from a T168N mutation in IFNγR2, which impairs assembly of the full signalling complex. The topology of the hexameric complex offers a blueprint for engineering IFNγ variants to tune IFNγ receptor signalling output. Unexpectedly, we found that several partial IFNγ agonists exhibited biased gene-expression profiles. These biased agonists retained the ability to induce upregulation of major histocompatibility complex class I antigen expression, but exhibited impaired induction of programmed death-ligand 1 expression in a wide range of human cancer cell lines, offering a route to decoupling immunostimulatory and immunosuppressive functions of IFNγ for therapeutic applications.

Post-translational modification of the interferon-gamma receptor alters its stability and signaling

The IFN gamma receptor 1 (IFNGR1) binds IFN-γ and activates gene transcription pathways crucial for controlling bacterial and viral infections. Although decreases in IFNGR1 surface levels have been demonstrated to inhibit IFN-γ signaling, little is known regarding the molecular mechanisms controlling receptor stability. Here, we show in epithelial and monocytic cell lines that IFNGR1 displays K48 polyubiquitination, is proteasomally degraded, and harbors three ubiquitin acceptor sites at K277, K279, and K285. Inhibition of glycogen synthase kinase 3 beta (GSK3β) destabilized IFNGR1 while overexpression of GSK3β increased receptor stability. We identified critical serine and threonine residues juxtaposed to ubiquitin acceptor sites that impacted IFNGR1 stability. In CRISPR-Cas9 IFNGR1 generated knockout cell lines, cellular expression of IFNGR1 plasmids encoding ubiquitin acceptor site mutations demonstrated significantly impaired STAT1 phosphorylation and decreased STAT1-dependent gene induction. Thus, IFNGR1 undergoes rapid site-specific polyubiquitination, a process modulated by GSK3β. Ubiquitination appears to be necessary for efficient IFNGR1-dependent gamma gene induction and represents a relatively uncharacterized regulatory mechanism for this receptor.

Current prospects of type II interferon γ signaling and autoimmunity

Interferon γ (IFNγ) is a pleiotropic protein secreted by immune cells. IFNγ signals through the IFNγ receptor, a protein complex that mediates downstream signaling events. Studies into IFNγ signaling have provided insight into the general concepts of receptor signaling, receptor internalization, regulation of distinct signaling pathways, and transcriptional regulation. Although IFNγ is the central mediator of the adaptive immune response to pathogens, it has been shown to be involved in several non-infectious physiological processes. This review will provide an introduction into IFNγ signaling biology and the functional roles of IFNγ in the autoimmune response.

Crystal structure of human interferon-γ receptor 2 reveals the structural basis for receptor specificity

Interferon-γ receptor 2 is a cell-surface receptor that is required for interferon-γ signalling and therefore plays a critical immunoregulatory role in innate and adaptive immunity against viral and also bacterial and protozoal infections. A crystal structure of the extracellular part of human interferon-γ receptor 2 (IFNγR2) was solved by molecular replacement at 1.8 Å resolution. Similar to other class 2 receptors, IFNγR2 has two fibronectin type III domains. The characteristic structural features of IFNγR2 are concentrated in its N-terminal domain: an extensive π-cation motif of stacked residues KWRWRH, a NAG-W-NAG sandwich (where NAG stands for N-acetyl-D-glucosamine) and finally a helix formed by residues 78-85, which is unique among class 2 receptors. Mass spectrometry and mutational analyses showed the importance of N-linked glycosylation to the stability of the protein and confirmed the presence of two disulfide bonds. Structure-based bioinformatic analysis revealed independent evolutionary behaviour of both receptor domains and, together with multiple sequence alignment, identified putative binding sites for interferon-γ and receptor 1, the ligands of IFNγR2.

Redesigning protein cavities as a strategy for increasing affinity in protein-protein interaction: interferon- γ receptor 1 as a model

Combining computational and experimental tools, we present a new strategy for designing high affinity variants of a binding protein. The affinity is increased by mutating residues not at the interface, but at positions lining internal cavities of one of the interacting molecules. Filling the cavities lowers flexibility of the binding protein, possibly reducing entropic penalty of binding. The approach was tested using the interferon-γ receptor 1 (IFNγR1) complex with IFNγ as a model. Mutations were selected from 52 amino acid positions lining the IFNγR1 internal cavities by using a protocol based on FoldX prediction of free energy changes. The final four mutations filling the IFNγR1 cavities and potentially improving the affinity to IFNγ were expressed, purified, and refolded, and their affinity towards IFNγ was measured by SPR. While individual cavity mutations yielded receptor constructs exhibiting only slight increase of affinity compared to WT, combinations of these mutations with previously characterized variant N96W led to a significant sevenfold increase. The affinity increase in the high affinity receptor variant N96W+V35L is linked to the restriction of its molecular fluctuations in the unbound state. The results demonstrate that mutating cavity residues is a viable strategy for designing protein variants with increased affinity.

Interferons and Their Receptors in Birds: A Comparison of Gene Structure, Phylogenetic Analysis, and Cross Modulation

Interferon may be thought of as a key, with the interferon receptor as the signal lock: Crosstalk between them maintains their balance during viral infection. In this review, the protein structure of avian interferon and the interferon receptor are discussed, indicating remarkable similarity between different species. However, the structures of the interferon receptors are more sophisticated than those of the interferons, suggesting that the interferon receptor is a more complicated signal lock system and has considerable diversity in subtypes or structures. Preliminary evolutionary analysis showed that the subunits of the interferon receptor formed a distinct clade, and the orthologs may be derived from the same ancestor. Furthermore, the development of interferons and interferon receptors in birds may be related to an animal’s age and the maintenance of a balanced state. In addition, the equilibrium between interferon and its receptor during pathological and physiological states revealed that the virus and the host influence this equilibrium. Birds could represent an important model for studies on interferon’s antiviral activities and may provide the basis for new antiviral strategies.

Crystal structure of the interferon gamma receptor alpha chain from chicken reveals an undetected extra helix compared with the human counterparts

Interferon gamma (IFN-γ) is an important cytokine that induces antiviral, antiproliferative, and immunomodulatory effects on target cells, and is also crucial in the early defense against intracellular parasites, such as Listeria monocytogenes and Toxoplasma gondii. The biological activity of IFN-γ relies upon the formation of a complex with its 2 receptors, the interferon gamma alpha chain (IFNGR1) and beta chain (IFNGR2), which are type II cytokine receptors. Structural models of ligand-receptor interaction and complex structure of chicken IFNs with their receptors have remained elusive. Here we report the first structure of Gallus gallus (chicken) IFNGR1 (chIFNGR1) at 2.0 Å by molecule replacement according to the structure of selenomethionine substituted chIFNGR1. The structural comparison reveals its structural similarities with other class II cytokine receptors, despite divergent primary sequences. We further investigate the ligand-receptor interaction properties of chicken IFN-γ (chIFN-γ) and chIFNGR1 using size-exclusion chromatography and surface plasmon resonance techniques. These data aid in the understanding of the interaction of chicken (avian) IFN-γ with its receptors and its signal transduction.

Increasing affinity of interferon-γ receptor 1 to interferon-γ by computer-aided design

We describe a computer-based protocol to design protein mutations increasing binding affinity between ligand and its receptor. The method was applied to mutate interferon-γ receptor 1 (IFN-γ-Rx) to increase its affinity to natural ligand IFN-γ, protein important for innate immunity. We analyzed all four available crystal structures of the IFN-γ-Rx/IFN-γ complex to identify 40 receptor residues forming the interface with IFN-γ. For these 40 residues, we performed computational mutation analysis by substituting each of the interface receptor residues by the remaining standard amino acids. The corresponding changes of the free energy were calculated by a protocol consisting of FoldX and molecular dynamics calculations. Based on the computed changes of the free energy and on sequence conservation criteria obtained by the analysis of 32 receptor sequences from 19 different species, we selected 14 receptor variants predicted to increase the receptor affinity to IFN-γ. These variants were expressed as recombinant proteins in Escherichia coli, and their affinities to IFN-γ were determined experimentally by surface plasmon resonance (SPR). The SPR measurements showed that the simple computational protocol succeeded in finding two receptor variants with affinity to IFN-γ increased about fivefold compared to the wild-type receptor.

A novel internalization motif regulates human IFN-γ R1 endocytosis

This study tested the hypothesis that the IFN-γ R1 287-YVSLI-91 intracellular motif regulates its endocytosis. IFN-γ exerts its biological activities by interacting with a specific cell-surface RC composed of two IFN-γ R1 and two IFN-γ R2 chains. Following IFN-γ binding and along with the initiation of signal transduction, the ligand and IFN-γ R1 are internalized. Two major types of consensus-sorting signals are described in receptors, which are rapidly internalized from the plasma membrane to intracellular compartments: tyrosine-based and dileucine-based internalization motifs. Transfection of HEK 293 cells and IFN-γ R1-deficient fibroblasts with WT and site-directed, mutagenesis-generated mutant IFN-γ R1 expression vectors helped us to identify region IFN-γ R1 287-YVSLI-291 as the critical domain required for IFN-γ-induced IFN-γ R1 internalization and Y287 and LI290-291 as part of a common structure essential for receptor endocytosis and function. This new endocytosis motif, YxxLI, shares characteristics of tyrosine-based and dileucine-based internalization motifs and is highly conserved in IFN-γ Rs across species. The IFN-γ R1 270-LI-271 dileucine motif, previously thought to be involved in this receptor endocytosis, showed to be unnecessary for receptor endocytosis.

Cube-DB: detection of functional divergence in human protein families

Cube-DB is a database of pre-evaluated results for detection of functional divergence in human/vertebrate protein families. The analysis is organized around the nomenclature associated with the human proteins, but based on all currently available vertebrate genomes. Using full genomes enables us, through a mutual-best-hit strategy, to construct comparable taxonomical samples for all paralogues under consideration. Functional specialization is scored on the residue level according to two models of behavior after divergence: heterotachy and homotachy. In the first case, the positions on the protein sequence are scored highly if they are conserved in the reference group of orthologs, and overlap poorly with the residue type choice in the paralogs groups (such positions will also be termed functional determinants). The second model additionally requires conservation within each group of paralogs (functional discriminants). The scoring functions are phylogeny independent, but sensitive to the residue type similarity. The results are presented as a table of per-residue scores, and mapped onto related structure (when available) via browser-embedded visualization tool. They can also be downloaded as a spreadsheet table, and sessions for two additional molecular visualization tools. The database interface is available at http://epsf.bmad.bii.a-star.edu.sg/cube/db/html/home.html.

Protein expression, crystallization and preliminary X-ray crystallographic analysis of chicken interferon-γ receptor α chain

The activity of interferon-γ (IFN-γ) relies on signal transduction, which is triggered by combination with the receptors interferon-γ receptor α chain (IFNGR1) and β chain (IFNGR2). Native recombinant chicken IFNGR1 (chIFNGR1; residues 25-237) was overexpressed in Escherichia coli, purified by refolding and crystallized using the vapour-diffusion technique. The crystals belonged to space group P6(5)22, with unit-cell parameters a = b = 64.1, c = 216.3 Å, α = β = 90, γ = 120°. The Matthews coefficient and solvent content were calculated as 2.67 Å(3) Da(-1) and 53.97%, respectively. X-ray diffraction data for chIFNGR1 were collected to 2.0 Å resolution at a synchrotron source.

Type III IFN receptor expression and functional characterisation in the pteropid bat, Pteropus alecto

Bats are rich reservoir hosts for a variety of viruses, many of which are capable of spillover to other susceptible mammals with lethal consequences. The ability of bats to remain asymptomatic to viral infection may be due to the rapid control of viral replication very early in the immune response through innate antiviral mechanisms. Type I and III interferons (IFNs) represent the first line of defence against viral infection in mammals, with both families of IFNs present in pteropid bats. To obtain further insight into the type III IFN system in bats, we describe the characterization of the type III IFN receptor (IFNλR) in the black flying fox, P. alecto with the characterization of IFNλR1 and IL10R2 genes that make up the type III IFN receptor complex. The bat IFNλR complex has a wide tissue distribution and at the cellular level, both epithelial and immune cells are responsive to IFN-λ treatment. Furthermore, we demonstrate that the bat IFNλR1 chain acts as a functional receptor. To our knowledge, this report represents the first description of an IFN receptor in any species of bat. The responsiveness of bat cells to IFN-λ support a role for the type III IFN system by epithelial and immune cells in bats.

Crystal structure of human interferon-λ1 in complex with its high-affinity receptor interferon-λR1

Interferon (IFN)-λ1 [also known as interleukin (IL)-29] belongs to the recently discovered group of type III IFNs. All type III IFNs initiate signaling processes through formation of specific heterodimeric receptor complexes consisting of IFN-λR1 and IL-10R2. We have determined the structure of human IFN-λ1 complexed with human IFN-λR1, a receptor unique to type III IFNs. The overall structure of IFN-λ1 is topologically similar to the structure of IL-10 and other members of the IL-10 family of cytokines. IFN-λR1 consists of two distinct domains having fibronectin type III topology. The ligand-receptor interface includes helix A, loop AB, and helix F on the IFN site, as well as loops primarily from the N-terminal domain and inter-domain hinge region of IFN-λR1. Composition and architecture of the interface that includes only a few direct hydrogen bonds support an idea that long-range ionic interactions between ligand and receptor govern the process of initial recognition of the molecules while hydrophobic interactions finalize it.

Genomic structure and characterization of mRNA expression pattern of porcine interferon gamma receptor 1 gene

Interferon gamma receptor (IFNGR) plays an important role in the biological effects of IFN-γ. In this study, porcine IFNGR1 cDNA was cloned and two transcripts both having a coding region of 1413 bp were identified. Porcine IFNGR1 cDNA shares 62.95%, 63.73%, 72.90% and 81.10% identity in nucleotide sequence; and 45.64%, 46.69%, 58.04% and 72.55% homology in amino acid sequence to those of rat, mouse, human and cattle, respectively. The porcine IFNGR1 genomic structure consists of seven exons and six introns and is located on porcine chromosome 1. The mRNA expression of porcine IFNGR1 gene is detected in all tissues examined, with strong expression in spleen and liver tissues and weak expression in cerebrum, cerebellum and uterus tissues, respectively. A different developmental pattern in IFNGR1 mRNA expression between Laiwu and Duroc breeds was revealed by real-time quantitative RT-PCR: in Duroc pigs, a significantly higher expression was found in the tissues of heart (P<0.05), liver (P<0.01), kidney (P<0.01) and skeletal muscle (P<0.05) of adult pigs compared to piglets. In porcine reproductive and respiratory syndrome virus (PRRSV)-infected Dapulian pigs, compared to the uninfected ones, the expression level of IFNGR1 mRNA in spleen was significantly up-regulated (P<0.05), whereas its expression in the lymph node was significantly down-regulated (P<0.05); in PRRSV-infected Duroc × Yorkshire × Landrace commercial pigs, however, the differences both in spleen and lymph node tissues were not significant.

Purification, crystallization and preliminary crystallographic studies of the complex of interferon-lambda1 with its receptor

Human interferon-lambda1 (IFN-lambda1(Ins)) and the extracellular domain of interferon-lambda1 receptor (IFN-lambda1R1) were expressed in Drosophila S2 cells and purified to homogeneity. Both IFN-lambda1(Ins) and interferon-lambda1 produced from Escherichia coli (IFN-lambda1(Bac)) were coupled with IFN-lambda1R1 at room temperature and the complexes were purified by gel filtration. Both complexes were crystallized; the crystals were flash-frozen at 100 K and diffraction data were collected to 2.16 and 2.1 A, respectively. Although the IFN-lambda1(Bac)-IFN-lambda1R1 and IFN-lambda1(Ins)-IFN-lambda1R1 complexes differed only in the nature of the expression system used for the ligand, their crystallization conditions and crystal forms were quite different. A search for heavy-atom derivatives as well as molecular-replacement trials are in progress.

Molecular characterization of novel interferon gamma receptor 1 isoforms in zebrafish (Danio rerio) and goldfish (Carassius auratus L.)

Interferon gamma (IFNgamma) is a highly pleotropic pro-inflammatory and anti-viral cytokine that mediates its effects by binding to a receptor complex composed of interferon gamma receptors 1 and 2 (IFNGR1 and IFNGR2). Using gene synteny analysis, we identified a distinct isoform of the zebrafish IFNGR1. The two zebrafish IFNGR1 called here IFNGR1-1 and IFNGR1-2 were used to identify the respective cDNA sequences of the goldfish IFNGR1-1 and IFNGR1-2. Analysis of protein sequences revealed that all fish IFNGR1 species have potential JAK1 and STAT1 docking sites. Phylogenetically, teleost IFNGR1 proteins grouped separately from those of higher vertebrates. Q-PCR analysis revealed that while the constitutive mRNA levels of the two zebrafish IFNGR1 isoforms were comparable in different tissues examined, the goldfish IFNGR1-1 tissue expression was substantially higher than that of IFNGR1-2. Q-PCR analysis of goldfish immune cell populations revealed highest expression of both receptor isoforms in monocytes. Incubation of goldfish macrophages with recombinant goldfish IFNgamma2 (rgIFNgamma2) up-regulated expression of both IFNGR1-1 and IFNGR1-2, while treatment of cells with rgTNFalpha2 only increased the expression of IFNGR1-1. Treatment with rgTGFbeta resulted in more modest increases in expression of both receptor isoforms only after prolonged treatment. In vitro binding studies indicated that rgIFNGR1-1 bound to rgIFNgamma1 but not rgIFNgamma2, while the rgIFNGR1-2 bound to rgIFNgamma2. Thus, unlike mammals that have a single IFNGR1, cyprinid fish have two distinct IFNGR1 isoforms that preferentially bind corresponding ligands, IFNgamma1 and IFNgamma2, respectively, suggesting that the type II interferon system of these fish species is distinct from that of higher vertebrates.

[Gamma interferon: basics aspects, clinic significance and terapeutic uses]

Interferons are a family of pleiotropic cytokines, their name was assigned because of their anti-replicative viral activity. IFNgamma or immune type II interferon does not share receptors with the type I interferon, its structure is different and its gene is located in different chromosome, although its biologic effects are similar. Along of several years of research, it has been found that IFNgamma enhances the transcription of genes involved in immunomodulation, antiviral responses and antitumoral activities. Regarding to the immune system, IFNgamma increases the cytotoxic and phagocytic activity of macrophages and upregulates the expression of major histocompatibility complex (MHC) class I and class II molecules in dendritics cells and other antigen presenting cells. IFNgamma also promotes the development and differentiation of naive CD4+ T lymphocytes to Th1 helper subset. Indeed, this cytokine has a key role in the control of bacterial, micotic, viral and parasitic infections. Depending of the micro-environment, IFNgamma has a dual role as pro or anti inflammatory cytokine. Novel therapeutic strategies are currently being developed with the aim to enhance the immune response or replace IFNgamma gene abnormal expression with beneficial results in humans, being recombinant IFNgamma safe and well tolerated.

Molecular cloning and characterization of chicken interferon-gamma receptor alpha-chain

In this study, a cDNA sequence of Huiyang chicken interferon-gamma (IFN-gamma) receptor alpha-chain (chIFNGR-1) gene wasgenerated using rapid amplification of cDNA ends (RACE) method for the first time. The predicted 422 amino acids showed approximately 25%-29% sequence identity and 53%-55% similarity to mammalian homologues. There are two fibronectin type-III (FN-III) domains of about 110 residues in the extracellular domain, and LPKS and YDKPH motifs in the intracellular domain, which are conserved in the mammalian IFNGR-1 as the binding sites of JAK1 and STAT1. Expression analysis by Northern blot revealed that the chIFNGR-1 was highly expressed in spleen, thymus, peripheral blood lymphocytes (PBLs), lung, cecum tonsil, and liver. The extracellular region of chIFNGR-1 (chIFNGR-1EC) was expressed in Escherichia coli and purified. The purified IFNGR-1EC was further characterized by mass spectroscopy and circular dichroism (CD) spectroscopy. The molecular weight of the recombinant chIFNGR-1EC (rchIFNGR-1EC) was measured as 24 364 Da, and its secondary structure contained 17.6% alpha-helix, 36.4% beta-sheet, 17.2% turn, and 28.8% random coil. Furthermore, three-dimensional modeling presented the most probable structure of chIFNGR-1EC. These * ndings show that the identified chicken cDNA sequence encodes an IFNGR1 homologue, and the chIFNGR-1EC resembles the similar structure with other IFN receptors.

Differential receptor subunit affinities of type I interferons govern differential signal activation

Type I interferons (IFNs) elicit antiviral, antiproliferative and immunmodulatory responses by binding to a shared cell surface receptor comprising the transmembrane proteins ifnar1 and ifnar2. Activation of differential response patterns by IFNs has been observed, suggesting that members of the family play different roles in innate immunity. The molecular basis for differential signaling has not been identified yet. Here, we have investigated the recognition of various IFNs including several human IFNalpha species, human IFNomega and human IFNbeta as well as ovine IFNtau2 by the receptor subunits in detail. Binding to the extracellular domains of ifnar1 (ifnar1-EC) and ifnar2 (ifnar2-EC) was monitored in real time by reflectance interference and total internal reflection fluorescence spectroscopy. For all IFNs investigated, competitive 1:1 interaction not only with ifnar2-EC but also with ifnar1-EC was shown. Furthermore, ternary complex formation was studied with ifnar1-EC and ifnar2-EC tethered onto solid-supported membranes. These analyses confirmed that the signaling complexes recruited by IFNs have very similar architectures. However, differences in rate and affinity constants over several orders of magnitude were observed for both the interactions with ifnar1-EC and ifnar2-EC. These data were correlated with the potencies of ISGF3 activation, antiviral and anti-proliferative activity on 2fTGH cells. The ISGF3 formation and antiviral activity correlated very well with the binding affinity towards ifnar2. In contrast, the affinity towards ifnar1 played a key role for antiproliferative activity. A striking correlation was observed for relative binding affinities towards ifnar1 and ifnar2 with the differential antiproliferative potency. This correlation was confirmed by systematically engineering IFNalpha2 mutants with very high differential antiproliferative potency.

A novel gene of beta chain of the IFN-gamma receptor of Huiyang chicken: cloning, distribution, and CD assay

The beta chain of the interferon-gamma receptor (IFNGR-2) plays a critical role in signal transmission to the nucleus by IFN-gamma. Here, we cloned the full-length cDNA of IFNGR-2 of Huiyang chicken using RACE. mRNA transcripts of IFNGR-2 were detected in peripheral blood leukocytes (PBL) and various organs using Northern blot analysis. The extracellular region of IFNGR-2 (IFNGR-2EC) was expressed in Pichia pastoris, and its secondary structure was investigated by circular dichroism (CD). The Huiyang chicken IFNGR-2 gene is 2221 bp with a polyA+ tail, and it encodes 334 amino acids sharing 30%-33% identity with that of rat, mouse, and human IFNGR-2. IFNGR-2 is localized on chromosome 1 of chicken in tandem with IFNAR-1, interleukin- 10 receptor (IL-10R-2), and IFNAR-2. IFNGR-2 was highly expressed in PBL, muscle, spleen, thymus, and cecal tonsil, whereas its expression in cardiac muscle, cloacal bursa, liver, and kidney was comparatively low. Recombinant protein of IFNGR-2EC expressed in P. pastoris formed the secondary structure including 19.8% alpha-helix, 29.6% beta-sheet, 19.7% turn, and 30.9% random. The data show that Huiyang chicken IFNGR-2 shares properties of the IFN receptor family in gene structure and distribution in multiple tissues and PBL. CD analysis indicated that the recombinant protein of IFNGR-2EC resembles the known structure of human IFN receptors.

Interactions among the components of the interleukin-10 receptor complex

We used fluorescence resonance energy transfer previously to show that the interferon-gamma (IFN-gamma) receptor complex is a preformed entity mediated by constitutive interactions between the IFN-gammaR2 and IFN-gammaR1 chains, and that this preassembled entity changes its structure after the treatment of cells with IFN-gamma. We applied this technique to determine the structure of the interleukin-10 (IL-10) receptor complex and whether it undergoes a similar conformational change after treatment of cells with IL-10. We report that, like the IFN-gamma receptor complex, the IL-10 receptor complex is preassembled: constitutive but weaker interactions occur between the IL-10R1 and IL-10R2 chains, and between two IL-10R2 chains. The IL-10 receptor complex undergoes no major conformational changes when cells are treated with cellular or Epstein-Barr viral IL-10. Receptor complex preassembly may be an inherent feature of Class 2 cytokine receptor complexes.

Specific and Stable Fluorescence Labeling of Histidine-Tagged Proteins for Dissecting Multi-Protein Complex Formation

Labeling of proteins with fluorescent dyes offers powerful means for monitoring protein interactions in vitro and in live cells. Only a few techniques for noncovalent fluorescence labeling with well-defined localization of the attached dye are currently available. Here, we present an efficient method for site-specific and stable noncovalent fluorescence labeling of histidine-tagged proteins. Different fluorophores were conjugated to a chemical recognition unit bearing three NTA moieties (tris-NTA). In contrast to the transient binding of conventional mono-NTA, the multivalent interaction of tris-NTA conjugated fluorophores with oligohistidine-tagged proteins resulted in complex lifetimes of more than an hour. The high selectivity of tris-NTA toward cumulated histidines enabled selective labeling of proteins in cell lysates and on the surface of live cells. Fluorescence labeling by tris-NTA conjugates was applied for the analysis of a ternary protein complex in solution and on surfaces. Formation of the complex and its stoichiometry was studied by analytical size exclusion chromatography and fluorescence quenching. The individual interactions were dissected on solid supports by using simultaneous mass-sensitive and multicolor fluorescence detection. Using these techniques, formation of a 1:1:1 stoichiometry by independent interactions of the receptor subunits with the ligand was shown. The incorporation of transition metal ions into the labeled proteins upon labeling with tris-NTA fluorophore conjugates provided an additional sensitive spectroscopic reporter for detecting and monitoring protein-protein interactions in real time. A broad application of these fluorescence conjugates for protein interaction analysis can be envisaged.

Preassembly and ligand-induced restructuring of the chains of the IFN-gamma receptor complex: the roles of Jak kinases, Stat1 and the receptor chains

We previously demonstrated using noninvasive technologies that the interferon-gamma (IFN-gamma) receptor complex is preassembled (1). In this report we determined how the receptor complex is preassembled and how the ligand-mediated conformational changes occur. The interaction of Stat1 with IFN-gammaR1 results in a conformational change localized to IFN-gammaR1. Jak1 but not Jak2 is required for the two chains of the IFN-gamma receptor complex (IFN-gammaR1 and IFN-gammaR2) to interact; however, the presence of both Jak1 and Jak2 is required to see any ligand-dependant conformational change. Two IFN-gammaR2 chains interact through species-specific determinants in their extracellular domains. Finally, these determinants also participate in the interaction of IFN-gammaR2 with IFN-gammaR1. These results agree with a detailed model of the IFN-gamma receptor that requires the receptor chains to be pre-associated constitutively for the receptor to be active.

Monitoring the Dynamics of Ligand−Receptor Complexes on Model Membranes

Ligand-induced cross-linking of cell surface receptors is a basic paradigm of signal activation by many transmembrane receptors. After ligand binding, the receptor complexes formed on the membrane are dynamically maintained by two-dimensional protein-protein interactions on the membrane. The biophysical principles governing the dynamics of such interactions have not been understood, mainly because the measurement of lateral interactions on membranes so far has not been experimentally addressed. Here, we describe a generic approach for measuring two-dimensional dissociation rate constants in vitro using a novel high-affinity chelator lipid for reconstituting a ternary cytokine-receptor complex on solid-supported membranes. While monitoring the interaction between the ligand and one of the receptor subunits on the membrane by fluorescence resonance energy transfer, the equilibrium on the surface was perturbed by rapidly tethering a large excess of the unlabeled receptor subunit. Displacement of labeled by unlabeled protein in the ternary complex was detected as a recovery of the donor quenching. Since the dissociation of the ligand-receptor complex in plane of the membrane was the rate-limiting step under these conditions, the two-dimensional rate constant of this process was determined. Strikingly, the two-dimensional dissociation was much slower than ligand dissociation into solution, suggesting that membrane tethering significantly affects the dissociation process. This result highlights the importance of studying ligand-receptor complexes tethered to membranes for understanding the principles governing signal activation by ligand-induced receptor assembling.

Mutational analysis of the IFNAR1 binding site on IFNalpha2 reveals the architecture of a weak ligand-receptor binding-site

Type I interferons activate cellular responses by forming a ternary complex with two receptor components, IFNAR1 and IFNAR2. While the binding of the IFNAR2 receptor to interferon is of high affinity and well characterized, the binding to IFNAR1 is weak, transient, and poorly understood. Here, we mapped the complete binding region of IFNAR1 on IFNalpha2 by creating a panel of 21 single alanine mutant proteins, and determined their binding affinities. The IFNAR1 binding site on IFNalpha2 maps to the center of the B and C helices, opposite to the binding site for IFNAR2. No hot spots for binding were found in the interface, with individual mutations having an up to fivefold effect on binding. Of the nine residues that affected binding, three adjacent conserved residues, located on the B helix, conferred an increase in the binding affinity to IFNAR1, as well as an increase in the biological activity of the interferon mutant. This suggests that binding of alpha interferons to the IFNAR1 receptor is sub-optimal. A correlation between binding affinity and biological activity was found, albeit not across the whole range of affinities. In WISH cells, but not DAUDI cells, the anti-proliferative activity was markedly affected by fluctuations in the IFNalpha2 affinity towards the IFNAR1 receptor. On the other hand, the antiviral activity of interferons on WISH cells seems to change in accordance to the binding affinity towards IFNAR1 only as long as the binding affinity is not beyond twofold of the wild-type. In accordance, the biological roles of the two interferon-receptor subunits are discussed.

High-affinity adaptors for switchable recognition of histidine-tagged proteins

We aspired to create chemical recognition units, which bind oligohistidine tags with high affinity and stability, as tools for selectively attaching spectroscopic probes and other functional elements to recombinant proteins. Several supramolecular entities containing 2-4 nitrilotriacetic acid (NTA) moieties were synthesized, which additionally contained an amino group, to which fluorescein was coupled as a sensitive reporter probe. These multivalent chelator heads (MCH) (termed bis-, tris-, and tetrakis-NTA) were characterized with respect to their interaction with hexahistidine (H6)- and decahistidine (H10)-tagged targets. Substantially increased binding stability with increasing number of NTA moieties was observed by analytical size exclusion chromatography. The binding enthalpies as determined by isothermal titration calorimetry increased nearly additively with the number of possible coordinative bonds between chelator heads and tags. Yet, a substantial excess of histidines in the oligohistidine tag was required for obtaining fully additive binding enthalpies. Dissociation kinetics of MCH/oligohistidine complexes measured by fluorescence dequenching showed an increase in stability by 4 orders of magnitude compared to that of mono-NTA, and subnanomolar affinity was reached for tris-NTA. The gain in free energy with increasing multivalency was accompanied by an increasing loss of entropy, which was ascribed to the high flexibility of the binding partners. Numerous applications of these MCHs for noncovalent, high affinity, yet reversible tethering of spectroscopic probes and other functional elements to the recombinant proteins can be envisioned.

Functional Cartography of the Ectodomain of the Type I Interferon Receptor Subunit ifnar1

Ligand-induced cross-linking of the type I interferon (IFN) receptor subunits ifnar1 and ifnar2 induces a pleiotrophic cellular response. Several studies have suggested differential signal activation by flexible recruitment of the accessory receptor subunit ifnar1. We have characterized the roles of the four Ig-like sub-domains (SDs) of the extracellular domain of ifnar1 (ifnar1-EC) for ligand recognition and receptor assembling. Various sub-fragments of ifnar1-EC were expressed in insect cells and purified to homogeneity. Solid phase binding assays with the ligands IFN(alpha)2 and IFN(beta) revealed that all three N-terminal SDs were required and sufficient for ligand binding, and that IFN(alpha)2 and IFN(beta) compete for this binding site. Cellular binding assays with different fragments, however, highlighted the key role of the membrane-proximal SD for the formation of an in situ IFN-receptor complex. Even substitution with the corresponding SD from homologous cytokine receptors did not restore high-affinity ligand binding. Receptor assembling analysis on supported lipid bilayers in vitro revealed that the membrane-proximal SD controls appropriate orientation of the receptor on the membrane, which is required for efficient association of ifnar1 into the ternary complex.

A model of the ternary complex of interleukin-10 with its soluble receptors

BACKGROUND

Interleukin-10 (IL-10) is a cytokine whose main biological function is to suppress the immune response by induction of a signal(s) leading to inhibition of synthesis of a number of cytokines and their cellular receptors. Signal transduction is initiated upon formation of a ternary complex of IL-10 with two of its receptor chains, IL-10R1 and IL-10R2, expressed on the cell membrane. The affinity of IL-10R1 toward IL-10 is very high, which allowed determination of the crystal structure of IL-10 complexed with the extracellular/soluble domain of IL-10R1, while the affinity of IL-10R2 toward either IL-10 or IL-10/sIL-10R1 complex is quite low. This so far has prevented any attempts to obtain structural information about the ternary complex of IL-10 with its receptor chains.

RESULTS

Structures of the second soluble receptor chain of interleukin-10 (sIL-10R2) and the ternary complex of IL-10/sIL-10R1/sIL-10R2 have been generated by homology modeling, which allowed us to identify residues involved in ligand-receptor and receptor-receptor interactions.

CONCLUSION

The previously experimentally determined structure of the intermediate/binary complex IL-10/sIL-10R1 is the same in the ternary complex. There are two binding sites for the second receptor chain on the surface of the IL-10/sIL-10R1 complex, involving both IL-10 and sIL-10R1. Most of the interactions are hydrophilic in nature, although each interface includes two internal hydrophobic clusters. The distance between C-termini of the receptor chains is 25 A, which is common for known structures of ternary complexes of other cytokines. The structure is likely to represent the biologically active signaling complex of IL-10 with its receptor on the surface of the cell membrane.

Structural, Kinetic, and Thermodynamic Analysis of the Binding of the 40 kDa PEG−Interferon-α2aand Its Individual Positional Isomers to the Extracellular Domain of the Receptor IFNAR2

Type-I Interferons exert antiviral and antiproliferative activities through the binding to a common cell surface receptor comprising two subunits, IFNAR1 and IFNAR2. Human recombinant Interferon-alpha(2a) (IFNalpha(2a)) is a potent drug (Roferon-A) used to treat various cancers and viral diseases including Hepatitis B/C infections. To significantly improve the pharmacological properties of the drug, a pegylated form of IFNalpha(2a) was developed (PEGASYS). This 40 kDa PEG-conjugated IFNalpha(2a) ((40)PEG-IFNalpha(2a)) is obtained by the covalent binding of one 40 kDa branched PEG-polymer to a lysine side-chain of IFNalpha(2a). Here, we report the detailed structural, kinetic, and thermodynamic analysis of the binding to the extracellular domain of the receptor IFNAR2 of (40)PEG-IFNalpha(2a) and its isolated positional isomers modified at K31, K134, K131, K121, K164, and K70, respectively, in comparison with unmodified IFNalpha(2a). Our binding studies, using the surface plasmon resonance technique, show that the pegylation does not abolish the binding to the receptor, but significantly reduces the affinity mainly due to a change of the association rate. The results are supported by modeling and simulation of the binding, using Self-Avoiding-Walk calculations for the polymer conformations. A correlation between the structural parameters and the kinetic and thermodynamic parameters of the binding of the positional isomers could be established. For the Isomer-K31 and -K164, the PEG-polymer attachment point is located in proximity to the binding interface, and the isomers display affinity in the range 150-520 nM in an enthalpy-driven binding process. In contrast for the Isomer-K134, -K131, -K121, and -K70, the PEG-polymer is attached remotely from the binding interface, and the isomers exhibit a higher affinity (32-76 nM) in an entropy-driven binding process. This study constitutes an essential collection of knowledge on which the interaction of (40)PEG-IFNalpha(2a) and its positional isomers with its cellular receptors can be better understood.

Structural bases of unphosphorylated STAT1 association and receptor binding

The crystal structure has been determined at 3.0 A resolution for an unphosphorylated STAT1 (1-683) complexed with a phosphopeptide derived from the alpha chain of interferon gamma (IFNgamma) receptor. Two dimer interfaces are seen, one between the N domains (NDs) (amino acid residues 1-123) and the other between the core fragments (CFs) (residues 132-683). Analyses of the wild-type (wt) and mutant STAT1 proteins by static light scattering, analytical ultracentrifugation, and coimmunoprecipitation suggest that STAT1 is predominantly dimeric prior to activation, and the dimer is mediated by the ND interactions. The connecting region between the ND and the CF is flexible and allows two interconvertable orientations of the CFs, termed "antiparallel" or "parallel," as determined by SH2 domain orientations. Functional implications of these dimer conformations are discussed. Also revealed in this structure is the detailed interaction between STAT1 SH2 domain and its docking site on IFNgamma receptor.

Lateral ligand-receptor interactions on membranes probed by simultaneous fluorescence-interference detection

We describe an experimental approach for studying ligand-receptor interactions in the plane of the membrane. The extracellular domains of the type I interferon receptor subunits ifnar1-EC and ifnar2-EC were tethered in an oriented fashion onto solid-supported, fluid lipid bilayers, thus mimicking membrane anchoring and lateral diffusion of the receptor. Ligand-induced receptor assembling was investigated by simultaneous total internal reflection fluorescence spectroscopy and reflectance interferometry (RIf). Based on a rigorous characterization of the interactions of fluorescence-labeled IFNalpha2 with each of the receptor subunits, the dynamics of the ternary complex formation on the fluid lipid bilayer was addressed in further detail making use of the features of the simultaneous detection. All these measurements supported the formation of a ternary complex in two steps, i.e., association of the ligand to ifnar2-EC and subsequent recruitment of ifnar1-EC on the surface of the membrane. Based on the ability to control and quantify the receptor surface concentrations, equilibrium, and rate constants of the interaction in the plane of the membrane were determined by monitoring ligand dissociation at different receptor surface concentrations. Using mutants of IFNalpha2 binding to ifnar2-EC with different association rate constants, the key role of the association rate constants for the assembling mechanism was demonstrated.

The IFNAR1 subunit of the type I IFN receptor complex contains a functional nuclear localization sequence

A nuclear localization sequence (NLS) in the type II interferon (IFN) IFN gamma, which is responsible for the nuclear translocation of both the ligand and the alpha-subunit (IFNGR1) of the receptor complex, has previously been characterized and its role in signaling examined in detail. We have now identified an NLS in the type I IFN receptor (IFNAR) common subunit IFNAR1 from humans and show that the human IFNAR1 subunit can translocate to the nucleus following human IFN beta stimulation. An NLS in human IFNAR1 is located in the extracellular domain of IFNAR1 within the sequence (382)RKIIEKKT (numbered for the precursor form). Nuclear import by the NLS functions in a conventional fashion requiring cytosolic import factors, is energy-dependent and inhibited by the prototypical NLS of the SV40 large T-antigen. These studies provide a mechanism for nuclear import of IFNAR1, as well as for type I IFN ligands, and a starting point for studying an alternate role for IFNAR1 in nuclear signaling within the type I IFN system.

Role of tyrosine 441 of interferon-gamma receptor subunit 1 in SOCS-1-mediated attenuation of STAT1 activation

Suppressor of cytokine signaling (SOCS)-1, the key negative regulator of interferon (IFN)-gamma-dependent signaling, is induced in response to IFNgamma. SOCS-1 binds to and inhibits the IFNgamma receptor-associated kinase Janus-activated kinase (JAK) 2 and inhibits its function in vitro, but the mechanism by which SOCS-1 inhibits IFNgamma-dependent signaling in vivo is not clear. Upon stimulation, mouse IFNgamma receptor subunit 1 (IFNGR1) is phosphorylated on several cytoplasmic tyrosine residues, and Tyr(419) is required for signal transducer and activator of transcription (STAT) 1 activation in mouse embryo fibroblasts. However, the functions of the other three cytoplasmic tyrosine residues are not known. Here we show that Tyr(441) is required to attenuate STAT1 activation in response to IFNgamma. Several tyrosine to phenylalanine mutants of IFNGR1, expressed at normal levels in stable pools of IFNGR1-null cells, were analyzed for the phosphorylation of STAT1 during a 48-h period, and antiviral activity in response to IFNgamma was also measured. Stronger activation of STAT1 was observed in cells expressing all IFNGR1 variants mutated at Tyr(441), and, consistently, stronger antiviral activity was also observed in these cells. Furthermore, constitutive overexpression of SOCS-1 inhibited IFNgamma-dependent signaling only in cells expressing IFNGR1 variants that included the Tyr(441) mutation. Mutation of Tyr(441) also blocked the ability of SOCS-1 to bind to IFNGR1 and JAK2 in response to IFNgamma and the normal down-regulation of STAT1 activation and antiviral activity. These results, together with data from the literature, suggest a model in which, in response to IFNgamma, phosphorylation of Tyr(441) creates a docking site for SOCS-1, which then binds to JAK2 within the receptor-JAK complex to partially inhibit JAK2 phosphorylation. Furthermore, the virtually complete blockade of STAT1 phosphorylation by overexpressed SOCS-1 in this experiment suggests that the binding of SOCS-1 to Tyr(441) also blocks the access of STAT1 to Tyr(419) and that this effect may be the principal mechanism of inhibition of downstream signaling.

NMR backbone dynamics of the human type I interferon binding subunit, a representative cytokine receptor

The antiviral and antiproliferative activities of type I interferons (IFNs) are mediated by a common receptor, and its second subunit (IFNAR2) exhibits nanomolar affinity to both IFNalpha and IFNbeta subtypes. We have previously determined the structure of the IFN-binding extracellular domain of IFNAR2 (IFNAR2-EC) using multidimensional NMR [Chill, J. H., Quadt, S. R., Levy, R., Schreiber, G. E., and Anglister, J. (2003) Structure 11, 791-802], showing it to comprise two fibronectin domains linked by a hinge. As the first cytokine receptor structure determined in the unliganded state and in solution, IFNAR2-EC offers an opportunity to characterize the dynamics of the cytokine receptor family and their correlation to biological function. Backbone dynamics of IFNAR2-EC were investigated using 15N relaxation at 11.74 and 18.79 T, and measurements of residual dipolar couplings (RDCs). Dynamics of the binding site distinguish between rigid structural domains, which stabilize the binding site conformation, and a more flexible binding interface which interacts with the ligand. Measurements of diffusional anisotropy and RDCs and model-free analysis all show that the backbone of the hinge interdomain region of IFNAR2-EC is rigid on the picosecond to nanosecond time scale. Signal transduction in cytokines receptors is initiated by ligand-induced juxtaposition of the two receptor subunits, triggering the mutual phosphorylation of kinases associated to their cytoplasmic domains. The rigidity of the hinge ensures correct positioning of the receptor subunits in the ternary signaling complex and modulates the interaction between kinases in the cytoplasm, thereby controlling the rate and efficiency of phosphorylation.

Phosphorylation and specific ubiquitin acceptor sites are required for ubiquitination and degradation of the IFNAR1 subunit of type I interferon receptor

Ubiquitination, endocytosis, and lysosomal degradation of the IFNAR1 (interferon alpha receptor 1) subunit of the type I interferon (IFN) receptor is mediated by the SCFbeta-Trcp (Skp1-Cullin1-F-box protein beta transducin repeat-containing protein) E3 ubiquitin ligase in a phosphorylation-dependent manner. In addition, stability of IFNAR1 is regulated by its binding to Tyk2 kinase. Here we characterize the determinants of IFNAR1 ubiquitination and degradation. We found that the integrity of two Ser residues at positions 535 and 539 within the specific destruction motif present in the cytoplasmic tail of IFNAR1 is essential for the ability of IFNAR1 to recruit beta-Trcp as well as to undergo efficient ubiquitination and degradation. Using an antibody that specifically recognizes IFNAR1 phosphorylated on Ser535 we found that IFNAR1 is phosphorylated on this residue in cells. This phosphorylation is promoted by treatment of cells with IFNalpha. Although the cytoplasmic tail of IFNAR1 contains seven Lys residues that could function as potential ubiquitin acceptor sites, we found that only three (Lys501, Lys525, and Lys526), all located proximal to the destruction motif, are essential for ubiquitination and degradation of IFNAR1. Expression of Tyk2 stabilized IFNAR1 in a manner that was dependent neither on its binding to beta-Trcp nor IFNAR1 ubiquitination. We discuss the complexities and specifics of the ubiquitination and degradation of IFNAR1, which is a beta-Trcp substrate that undergoes degradation via a lysosomal pathway.

IFN-α Subtypes Differentially Affect Human T Cell Motility

The type I IFN family includes 14 closely related antiviral cytokines that are produced in response to viral infections. They bind to a common receptor, and have qualitatively similar biological activities. The physiological relevance of this redundancy is still unclear. In this study, we analyzed and compared the effects of two potent antiviral type I IFNs, IFN-alpha 2 and IFN-alpha 8, on the motility of various populations of human T lymphocytes in vitro. In this study, we show that IFN-alpha 2 induces chemokinesis of both CD4(+) and CD8(+) T cells at various stages of differentiation, and induces functional changes that result in enhanced T cell motility, including up-regulation of the integrins LFA-1 and VLA-4, and subsequently, increased ICAM-1- and fibronectin-dependent migration. In contrast, IFN-alpha 8 did not affect T cell motility, despite having similar antiviral properties and similar effects on the induction of the antiviral protein MxA. However, transcription of other IFN-stimulated genes showed that transcription of these genes is selectively activated by IFN-alpha 2, but not IFN-alpha 8, in T cells. Finally, while the antiviral activity of the two subtypes is inhibited by Abs against the two subunits of the IFN-alpha receptor, the chemokinetic effect of IFN-alpha 2 is selectively blocked by Abs against the A1 receptor subunit. These observations are consistent with the possibility that subtype-specific intracellular signaling pathways are activated by type I IFNs in T lymphocytes.

Optimizing the Binding Affinity of a Carrier Protein

Prolonging the circulatory half-life of low mass protein drugs can be achieved either by administration of a pro-drug or through co-injection with a carrier protein that will slowly release the active protein. The rate of release is concentration and affinity dependent. The optimal relationship between these two in prolonging the half-life of a pro-drug is the focus of this work. Interferon (IFN) beta is one of the most widely used protein drugs in the clinic. Here, we show that the circulatory half-life of IFNbeta can be significantly extended by co-administration with the extracellular domain of the IFN receptor ifnar2 (ifnar2-EC). To investigate the concentration/affinity relation, a range of tighter binding ifnar2-EC mutants was designed that bind IFNbeta, but not IFNalpha2, up to 50-fold tighter compared with the wild-type ifnar2-EC. This increased affinity is related to a slower dissociation rate, whereas the association of IFNbeta with ifnar2-EC is already near optimum. Using the wild-type and mutant receptors, we investigated their potential in occluding IFNbeta from circulation in a tissue culture assay, as well as in rats. To determine the potential of ifnar2-EC as a carrier protein, we co-administered a mixture of IFNbeta and ifnar2-EC to rats both intravenously and subcutaneously, and followed the blood plasma concentrations of IFNbeta over time. The tighter binding ifnar2-EC mutant had a clear advantage in prolonging the half-life of IFNbeta in circulation, even when lower protein concentrations were administered. A numerical simulation of the in vivo data demonstrates that the optimal binding affinity of a carrier protein is around the concentration needed to obtain optimal activity of the ligand.

Identification of PNRC2 and TLE1 as activation function-1 cofactors of the orphan nuclear receptor ERRgamma

Estrogen-related receptor gamma (ERRgamma) is an orphan nuclear receptor highly expressed in heart, skeletal muscle, kidney, and brain. To identify activation function-1 (AF-1)-dependent cofactors involved in the transcriptional function of ERRgamma, we screened for human cDNAs coding for proteins that bind to the bacterial expressed AF-1 by biopanning of a phage display library. Phages displaying fusion proteins with full-length PNRC2 (proline-rich nuclear receptor co-regulatory protein 2), already shown to be a cofactor for other nuclear receptors, and with a polypeptide of the bHLH corepressor TLE1 bound to the AF-1 containing bait. Pull-down analyses demonstrated a direct interaction of the receptor with the newly identified full-length proteins. Surprisingly, not only PNRC2 but also the corepressor TLE1 functioned as ERRgamma coactivator in a reporter gene analysis.

The human type I interferon receptor: NMR structure reveals the molecular basis of ligand binding

The potent antiviral and antiproliferative activities of human type I interferons (IFNs) are mediated by a single receptor comprising two subunits, IFNAR1 and IFNAR2. The structure of the IFNAR2 IFN binding ectodomain (IFNAR2-EC), the first helical cytokine receptor structure determined in solution, reveals the molecular basis for IFN binding. The atypical perpendicular orientation of its two fibronectin domains explains the lack of C domain involvement in ligand binding. A model of the IFNAR2-EC/IFNalpha2 complex based on double mutant cycle-derived constraints uncovers an extensive and predominantly aliphatic hydrophobic patch on the receptor that interacts with a matching hydrophobic surface of IFNalpha2. An adjacent motif of alternating charged side chains guides the two proteins into a tight complex. The binding interface may account for crossreactivity and ligand specificity of the receptor. This molecular description of IFN binding should be invaluable for study and design of IFN-based biomedical agents.

Interferon-alpha/beta-receptor interactions: a complex story unfolding

The Type I interferons (IFN-alpha/beta) exhibit pleiotropic biological activities. Notably, the different IFN subtypes activate the same cell surface receptor complex to mediate variable responses. Accumulating evidence suggests that distinct differences in critical amino acid residues among the different IFN-alphas and IFN-beta determine the nature of the ligand-receptor interaction and the subsequent responses. This review focuses on IFN-receptor interactions, the key residues involved in this interaction and the potential for targeted modifications of the ligand to enhance bioactivity.

The human interferon system: characterization and classification after discovery of novel members

The human interferon (IFN) system is the best characterized of all animal IFN systems. Until recently it is thought that all IFNs and IFN-related genes and proteins have been discovered. However, in the last three years, the discovery and characterization of IFNs including IFN-epsilon (IFN-epsilon), IFN-kappa (IFN-kappa) and a novel IFN-lambda (IFN-lambda) family, in particular, substantially changed this opinion. In this article, we attempt to review recent developments in the field of interferon discovery and present an overview of current classification of the human IFN system. Characterization of the constituent parts of the human IFN system including ligands, receptors and players involved in the signal transduction pathway are discussed.

Contribution of the Box 1 and Box 2 motifs of cytokine receptors to Jak1 association and activation

Kinases of the Jak family (Jak1/2/3 and Tyk2) interact with the membrane proximal domain of different cytokine receptors and play a critical role in the activation of cytokine and growth factor signaling pathways. In this report we demonstrate that both the Box 1 and Box 2 motif collaborate in the association and activation of Jak1 by type I interferons. Mutational analysis of the beta chain of type I interferon receptor (IFNalphaRbetaL/IFNAR2) revealed that Box 1 plays a more significant role in activation than in the association with Jak1. On the contrary, the Box 2 motif contributes more to the association with Jak1 than to kinase activation. Additionally, the study of the Jak1 binding sites on the IL2 receptor beta (IL2Rbeta), IFNgammaRalpha/IFNGR1, and IL10Ralpha/IL10R1 chains suggests that cytokine receptors have two different kinds of interaction with Jak1. One form of interaction involves the Box 1 and the previously described Box 2 motif, which we now designate as Box 2A, characterized by the VEVI and LEVL sequences present in IFNalphaRbetaL/IFNAR2 and IL2Rbeta subunits, respectively. The second form of interaction requires a motif termed Box 2B, which is present in the IFNgammaRalpha/IFNGR1 (SILLPKS) and IL10Ralpha/IL10R1 (SVLLFKK) chains. Interestingly, Box 2B localizes close to the membrane region (8-10 amino acids from the membrane) similar to Box 1, whereas Box 2A is more distal (38-58 amino acids from the membrane).

Seeing the light: preassembly and ligand-induced changes of the interferon gamma receptor complex in cells

Our experiments were designed to test the hypothesis that the cell surface interferon gamma receptor chains are preassembled rather than associated by ligand and to assess the molecular changes on ligand binding. To accomplish this, we used fluorescence resonance energy transfer, a powerful spectroscopic technique that has been used to determine molecular interactions and distances between the donor and acceptor. However, current commercial instruments do not provide sufficient sensitivity or the full spectra to provide decisive results of interactions between proteins labeled with blue and green fluorescent proteins in living cells. In our experiments, we used the blue fluorescent protein and green fluorescent protein pair, attached a monochrometer and charge-coupled device camera to a modified confocal microscope, reduced background fluorescence with the use of two-photon excitation, and focused on regions of single cells to provide clear spectra of fluorescence resonance energy transfer. In contrast to the prevailing view, the results demonstrate that the receptor chains are preassociated and that the intracellular domains move apart on binding the ligand interferon gamma. Application of this technology should lead to new rapid methods for high throughput screening and delineation of the interactome of cells.

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.