Recombinant interleukin-2 analogs. Dynamic probes for receptor structure

Structural insights into the common γ-chain family of cytokines and receptors from the interleukin-7 pathway

Over the past 13 years, numerous crystal structures of complexes of the common γ-chain (γ(c)) cytokine receptors and their cytokines have been solved. Even with the remarkable progress in the structural biology of γ(c) receptors and their cytokines or interleukins, there are valuable lessons to be learned from the structural and biophysical studies of interleukin-7 (IL-7) and its α-receptor (IL-7Rα) and comparisons with other γ(c) family members. The structure of the IL-7/IL-7Rα complex teaches that interfaces between the γ(c) interleukins and their receptors can vary in size, polarity, and specificity, and that significant conformational changes might be necessary for complexes of interleukins and their receptors to bind the shared, activating γ(c) receptor. Binding, kinetic, and thermodynamic studies of IL-7 and IL-7Rα show that glycosylation and electrostatics can be important to interactions between interleukins and their receptor, even where the glycans and charged residues are distant from the interface. The structure of the IL-7Rα homodimer is a reminder that often-ignored non-activating complexes likely perform roles just as important to signaling as activating complexes. And last but not least, the structural and biophysical studies help explain and potentially treat the diseases caused by aberrant IL-7 signaling.

Modeling the role of IL2 in the interplay between CD4+ helper and regulatory T cells: studying the impact of IL2 modulation therapies

Several reports in the literature have drawn a complex picture of the effect of treatments aiming to modulate IL2 activity in vivo. They seem to promote indistinctly immunity or tolerance, probably depending on the specific context, dose and timing of their application. Such complexity might derives from the dual role of IL2 on T-cell dynamics. To theoretically address the latter possibility, we develop a mathematical model for helper, regulatory and memory T-cells dynamics, which account for most well-known facts relative to their relationship with IL2. We simulate the effect of three types of therapies: IL2 injections, IL2 depletion using anti-IL2 antibodies and IL2/anti-IL2 immune complexes injection. We focus in the qualitative and quantitative conditions of dose and timing for these treatments which allow them to potentate either immunity or tolerance. Our results provide reasonable explanations for the existent pre-clinical and clinical data and further provide interesting practical guidelines to optimize the future application of these types of treatments. Particularly, our results predict that: (i) Immune complexes IL2/anti-IL2 mAbs, using mAbs which block the interaction of IL2 and CD25 (the alpha chain of IL2 receptor), is the best option to potentate immunity alone or in combination with vaccines. These complexes are optimal when a 1:2 molar ratio of mAb:IL2 is used and the mAbs have the largest possible affinity; (ii) Immune complexes IL2/anti-IL2 mAbs, using mAbs which block the interaction of IL2 and CD122 (the beta chain of IL2 receptor), are the best option to reinforce preexistent natural tolerance, for instance to prevent allograft rejection. These complexes are optimal when a 1:2 molar ratio of mAb:IL2 is used and the mAbs have intermediate affinities; (iii) mAbs anti-IL2 can be successfully used alone to treat an ongoing autoimmune disorder, promoting the re-induction of tolerance. The best strategy in this therapy is to start treatment with an initially high dose of the mAbs (one capable to induce some immune suppression) and then scales down slowly the dose of mAb in subsequent applications.

Compensatory energetic mechanisms mediating the assembly of signaling complexes between interleukin-2 and its alpha, beta, and gamma(c) receptors

Interleukin-2 is a key immuno-regulatory cytokine whose actions are mediated by three different cell surface receptors: the alpha, beta and the "common gamma" (gamma(c)) chains. We have undertaken a complete thermodynamic characterization of the stepwise assembly cycle for multiple possible combinations of the receptor-ligand, and receptor-receptor interactions that are necessary for formation of the high-affinity IL-2/alphabetagamma(c) signaling complex. We find an entropically favorable high affinity interaction between IL-2 and its alpha receptor, a moderately entropically favorable low affinity interaction between IL-2 and its beta receptor, and no interaction between IL-2 and the shared receptor, gamma(c). Formation of the stable intermediate trimolecular complexes of IL-2 with alpha and beta receptors, as well as IL-2 with beta and gamma(c) receptors proceeds through enthalpy-entropy compensation mechanisms. Surprisingly, we see a moderate affinity interaction between the unliganded receptor alpha and beta chains, suggesting that a preformed alphabeta complex may serve as the initial interaction complex for IL-2. Reconstitution of the IL-2/Ralphabetagamma(c) high-affinity quaternary signaling complex shows it to be assembled through cooperative energetics to form a 1:1:1:1 assembly. Collectively, the favorable entropy of the bimolecular interactions appears to be offset by the loss in rigid body entropy of the receptor components in the higher-order complexes, but overcome by the formation of increasingly enthalpically favorable composite interfaces. This enthalpic mechanism utilized by gamma(c) contrasts with the favorable entropic mechanism utilized by gp130 for degenerate cytokine interaction. In conclusion, we find that several energetically redundant pathways exist for formation of IL-2 receptor signaling complexes, suggesting a more complex equilibrium on the cell surface than has been previously appreciated.

Possible mechanism for the alpha subunit of the interleukin-2 receptor (CD25) to influence interleukin-2 receptor signal transduction

The receptors for interleukin 2 (IL-2) and interleukin 15 (IL-15) in T cells share the IL-2R beta subunit (CD122) and gamma(C) subunit but have private alpha subunits. Despite utilizing the same receptor chains known to be necessary and sufficient to transduce IL-2 signals the two cytokines manifest different cellular effects. It is commonly held that the alpha subunit of the IL-2R (CD25) is involved solely in the generation of a high affinity receptor complex. This is questioned by the development of autoimmune diseases in instances where the expression of CD25 is absent. The timely expression of CD25 in the thymus has been linked with clonal deletion. Evidence from peripheral T cells indicates that survival signals arising from the intermediate affinity IL-2R (lacking CD25) do not require the activation of Janus kinase 3 (Jak3) but do require the presence of the membrane proximal region of the gamma(C) chain. This particular signalling pathway is not observed in the high affinity receptor complex where Jak3 is activated. Recent data point to CD25 having a surface distribution consistent with it being localized within membrane microdomains. Here we suggest that in the absence of CD25 expression, IL-2R activation occurs within the soluble membrane fraction. This membrane environment and the absence of CD25 promotes Jak3 independent signal transduction and induction of antiapoptotic mechanisms. T cell antigen receptor (TCR) signalling leads to the induction of CD25 expression, which localizes to membrane microdomains. There is a dynamic pre-association of CD25 and CD122 leading to the loose association of the heterodimer with membrane microdomains. High affinity IL-2R signalling in the context of CD25 and the microdomain environment is characterized by Jak3 activation. The relative levels of high to intermediate affinity receptor signalling determines whether a cell proliferates or undergoes activation induced cell death dependent upon cell status.

Analysis of the role of the interleukin-2 receptor gamma chain in ligand binding

Interleukin-2 is the primary T cell growth factor secreted by activated T cells. IL-2 is an alpha-helical cytokine that binds to a multisubunit receptor expressed on the surface of a variety of cell types. IL-2Ralpha, IL-2Rbeta, and IL-2Rgammac receptor subunits expressed on the surface of cells may aggregate to form distinct binding sites of differing affinities. IL-2Rgammac was the last receptor subunit to be identified. It has since been shown to be shared by at least five other cytokine receptors. In this study, we have probed the role of IL-2Rgammac in the assembly of IL-2R complexes and in ligand binding. We demonstrate that in the absence of ligand IL-2Rgammac does not possess detectable affinity for IL-2Ralpha, IL-2Rbeta, or the pseudo-high-affinity binding site composed of preformed IL-2Ralpha/beta. We also demonstrate that IL-2Rgammac possesses an IL-2-dependent affinity for IL-2Rbeta and IL-2Ralpha/beta. We performed a detailed biosensor analysis to examine the interaction of soluble IL-2Rgammac with IL-2-bound IL-2Rbeta and IL-2-bound IL-2Ralpha/beta. The kinetic and equilibrium constants for sIL-2Rgammac binding to these two different liganded complexes were similar, indicating that IL-2Ralpha does not play a role in recruitment of IL-2Rgammac. We also determined that the binding of IL-2 to the isolated IL-2Rgammac was very weak (approximate K(D) = 0.7 mM). The experimental methodologies and principles derived from these studies can be extended to at least five other cytokines that share IL-2Rgammac as a receptor subunit.

Mutational analysis of chicken interleukin 2

Chicken interleukin 2 (chIL-2) has low, but significant, homology to both mammalian IL-2 and mammalian IL-15. In view of its unique phylogenetic position and potential use as a vaccine adjuvant, a detailed mutational analysis for critical functional sites was undertaken. It was found that Asp17 is a critical N terminal contact site for binding to the putative chIL-2 receptor, which is similar to results obtained for mammalian IL-2 and IL-15. Analysis of the C terminus did not reveal a single critical amino acid. However, deletion mutant studies demonstrated that removal of C terminal amino acids yielded proteins with decreased bioactivity and that this decrease was a function of the number and kind of amino acids removed. This study is the first non-mammalian IL-2 mutational analysis and proposes a model for the interaction between chIL-2 and its receptor.

Biosensor analysis of the interleukin-2 receptor complex

Surface plasmon resonance (SPR) biosensor technology has been a significant addition to the evolution and refinement of methods to study macromolecular interactions. Prior to the advent of SPR, we employed a variety of biochemical and biological techniques to study the interleukin-2/interleukin-2 receptor system (IL-2/IL-2R). By combining site-directed mutagenesis, equilibrium and kinetic radioligand binding, and competitive biological assays, we and others had begun to understand many aspects of the structure-activity relationships of the IL-2/IL-2R system. Due to the complexity of the IL-2R, cell-based assays proved limited in their ability to provide quantitative information on the binding characteristics of subclasses of the IL-2 receptor. SPR technology promised to be a new and powerful approach to the quantitative analysis of complex receptor systems. To demonstrate the feasibility of this technology, we employed Biacore analysis to investigate the ligand binding characteristics of novel, pre-assembled, IL-2R coiled-coil complexes. The results of these studies, although limited by instrumentation and data analysis, clearly established the utility of this method. Subsequently, by incorporating advancements in both of these areas, we have been able to carry out detailed kinetic analyses of the binding properties of individual IL-2R subunits as well as heteromeric complexes on the surface of a biosensor. Therefore, SPR biosensor analysis combined with other established analytical methods has proven to be a powerful tool for the analysis of complex hematopoietic receptor systems. Published in 1999 by John Wiley & Sons, Ltd.

Solution assembly of the pseudo-high affinity and intermediate affinity interleukin-2 receptor complexes

The high affinity interleukin-2 receptor is composed of three cell surface subunits, IL-2Ralpha, IL-2Rbeta, and IL-2Rgamma. Functional forms of the IL-2 receptor exist, however, that enlist only two of the three subunits. On activated T-cells, the alpha- and beta-subunits combine as a preformed heterodimer (the pseudo-high affinity receptor) that serves to capture IL-2. On a subpopulation of natural killer cells, the beta- and gamma-subunits interact in a ligand-dependent manner to form the intermediate affinity receptor site. Previously, we have demonstrated the feasibility of employing coiled-coil molecular recognition for the solution assembly of a heteromeric IL-2 receptor complex. In that study, although the receptor was functional, the coiled-coil complex was a trimer rather than the desired heterodimer. We have now redesigned the hydrophobic heptad sequences of the coiled-coils to generate soluble forms of both the pseudo-high affinity and the intermediate affinity heterodimeric IL-2 receptors. The properties of these complexes were examined and their relevance to the physiological IL-2 receptor mechanism is discussed.

The murine anti-human common gamma chain monoclonal antibody CP.B8 blocks the second step in the formation of the intermediate affinity IL-2 receptor

A murine monoclonal antibody, CP.B8, specific for the extracellular portion of the human common gamma (gammac) chain, and its Fab fragment are shown to block the binding of IL-2 to COS-7 cells transfected with the cDNA for the full-length IL-2 receptor beta (IL-2Rbeta) and gammac chains, components which together comprise the intermediate affinity IL-2 receptor (IL-2R) expressed on the surface of resting T cells, NK cells, and on certain intestinal epithelial cells. To investigate the mechanism of this inhibition, the extracellular portions of the IL-2Rbeta and gammac chains were expressed and purified, and their interactions with each other and with IL-2 were studied by gel filtration and by surface plasmon resonance (SPR). By gel filtration, a stable ternary complex was formed by association of the three proteins, while no stable binary complexes were detected between any two of the three proteins. By SPR analysis, IL-2 was shown to associate rapidly with IL-2Rbeta, forming a binary complex with an equilibrium dissociation constant (Kd) of 800 nM, which permitted subsequent association of the gammac chain. Dissociation of the IL-2/IL-2Rbeta/gammac chain complex was significantly slower than dissociation of the IL-2/IL-2Rbeta complex. Using these model systems, we tested the ability of mAb CP.B8 to inhibit the association of the gammac chain with IL-2 and IL-2Rbeta. By gel filtration, mAb CP.B8 formed a stable complex with the gammac chain, preventing its association with IL-2 and IL-2Rbeta. MAb CP.B8 was also capable of dissociating the gammac chain already complexed with IL-2 and IL-2Rbeta. SPR analysis confirmed these findings and showed, in addition, that the Fab fragment of CP.B8 was also capable of inhibiting the association of the gammac chain with the IL-2/IL-2Rbeta complex. We conclude that mAb CP.B8 blocks the second step in the formation of the intermediate affinity IL-2R on the surface of transfected COS-7 cells by binding at or close to a region on the gammac chain that is involved in contact with IL-2 and/or IL-2Rbeta.

Structural modifications of interleukin-2 at positions 47 and 65

Interleukin-2 (IL-2) is a cytokine essential for the growth and proliferation of T-cells. The purpose of this research was to study the effects of altering the "kink" caused by Pro in an alpha-helix of the protein. The Pro-47 residue was chosen because it was originally, but mistakenly, traced to the kink of an alpha-helix [1]. Pro-65 is now recognized to be situated in the middle of helix B [2]. To study the significance of this Pro for the bioactivity and overall conformation of IL-2 it was mutated to Gly and Ala. We successfully obtained 17 different mutants at position 47 and two mutants at position 65. Certain amino acid substitutions representing different categories of amino acids, namely, acidic, neutral and helix stabilizing, were chosen for more thorough investigation. The results showed that Asn-47 and Asp-47 decreased the bioactivity of these mutants by 50- and 700-fold respectively, while the Kd to its high affinity receptors was increased 180- and 90-fold respectively, compared to IL-2. The intermediate binding affinity of Asn-47 and Asp-47 was decreased 8- and 37-fold, respectively. On the other hand, Gly-47, Gly-65 and Ala-65 showed less dramatic decreases in bioactivity and high affinity binding. The intermediate binding affinity of these mutants decreased from 5- to 3-fold and low affinity binding decreased approximately 4-fold suggesting some structural and conformational changes. From these observations, we conclude that Asn-47 or Asp-47 disrupt the hydrophobic packing of the core and thus changed the overall conformation of the protein, thereby giving rise to partial agonists. Although Pro-65 lies within the helix, it may be near the surface of the protein but may not be the actual binding site and thus any conservative mutation can be better tolerated.

The functional display of interleukin-2 on filamentous phage

We report the novel display of interleukin-2 (IL-2) and an IL-2 analog, D126, on the surface of filamentous bacteriophage using a phagemid vector system. A synthetic human IL-2 gene and its D126 analog were fused to the carboxyl-terminal domain of the gene III minor phage coat protein. Expression of IL-2 and D126 was verified by their reactivity with an IL-2-specific antibody. Biological response of IL-2 phage on murine CTLL-2 cells was comparable to that of recombinant soluble IL-2, while the D126 phage displayed a reduced biological response similar to that previously measured by soluble D126 protein. Biosensor surface plasmon resonance was employed to verify binding of the IL-2 and D126 phage to the IL-2 alpha beta cc receptor complex. A 41-fold enrichment of IL-2 phage over R408 helper phage was demonstrated in biopanning affinity selection studies employing biotinylated alpha beta cc receptor complex. These biopanning studies are the first reports of affinity selection of IL-2 phage and demonstrate a novel use for the alpha beta cc receptor complex. Together, these studies confirm that the structural integrity of IL-2 and D126 is maintained when they are displayed as a gIIIp fusion protein on phage particles and provide the foundation for further selection studies employing IL-2 analog phage libraries.

Kinetic analysis of ligand binding to interleukin-2 receptor complexes created on an optical biosensor surface

The interleukin-2 receptor (IL-2R) is composed of at least three cell surface subunits, IL-2R alpha, IL-2R beta, and IL-2R gamma c. On activated T-cells, the alpha- and beta-subunits exist as a preformed heterodimer that simultaneously captures the IL-2 ligand as the initial event in formation of the signaling complex. We used BIAcore to compare the binding of IL-2 to biosensor surfaces containing either the alpha-subunit, the beta-subunit, or both subunits together. The receptor ectodomains were immobilized in an oriented fashion on the dextran matrix through unique solvent-exposed thiols. Equilibrium analysis of the binding data established IL-2 dissociation constants for the individual alpha- and beta-subunits of 37 and 480 nM, respectively. Surfaces with both subunits immobilized, however, contained a receptor site of much higher affinity, suggesting the ligand was bound in a ternary complex with the alpha- and beta-subunits, similar to that reported for the pseudo-high-affinity receptor on cells. Because the binding responses had the additional complexity of being mass transport limited, obtaining accurate estimates for the kinetic rate constants required global fitting of the data sets from multiple surface densities of the receptors. A detailed kinetic analysis indicated that the higher-affinity binding sites detected on surfaces containing both alpha- and beta-subunits resulted from capture of IL-2 by a preformed complex of these subunits. Therefore, the biosensor analysis closely mimicked the recognition properties reported for these subunits on the cell surface, providing a convenient and powerful tool to assess the structure-function relationships of this and other multiple subunit receptor systems.

A point mutation in interleukin-2 that alters ligand internalization

In previous studies, we have identified an interleukin-2 (IL-2) analog containing a point mutation at position 51 (T51P) that expresses nearly wild-type bioactivity, yet has approximately 10-fold lower receptor binding affinity. Since ligand-dependent receptor internalization may be the rate-limiting step controlling the duration of IL-2 receptor signaling, a reduction in the receptor internalization rate could contribute to the observed response enhancement for this analog. To evaluate this possibility, we compared the internalization of IL-2 and T51P in three separate assays. While the internalization rate for IL-2 agreed with values determined by others, the internalization of T51P was markedly reduced. The receptor binding rate constants for this analog were only slightly different; thus, altered binding kinetics could not explain the decreased internalization rate. The effects of reduced internalization were also observable in bioassays, where T51P maintained T-cell proliferation for a longer period compared with IL-2. These results indicate that the T51P point mutation reduces the receptor internalization rate compared with IL-2 in a fashion that is independent of the dissociation rate. This analog may represent a new approach to the preparation of cytokine analogs with potentiated agonist and antagonist properties.

DAB389 interleukin-2 receptor binding domain mutations. Cytotoxic probes for studies of ligand-receptor interactions

Site-directed mutagenesis was used to generate point mutations in the diphtheria toxin-related fusion protein, DAB389 interleukin-2 (IL-2). Thr-439, in the IL-2 receptor binding domain of the fusion toxin, was changed to a Pro residue. The resultant fusion toxin, DAB389 IL-2(T439P), was 300-fold less cytotoxic than wild type DAB389 IL-2, partially as the result of a 100-fold decrease in binding affinity for the high affinity form of the IL-2 receptor. However, DAB389 IL-2(T439P) stimulated DNA synthesis to a greater extent than expected. Studies of intoxication kinetics indicated that the increased stimulation might result from an increased contact time between the mutated IL-2 receptor binding domain and the receptor, perhaps due to a decreased internalization rate. Another mutant, DAB389 IL-2(Q514D), in which a Gln residue at position 514 was changed to an Asp, was 2000-fold less cytotoxic than wild type DAB389 IL-2. This mutant had a 50-fold decrease in binding affinity, did not stimulate DNA synthesis and also had a reduced rate of intoxication. Gln-514 appears to play a role in receptor binding and activation, whereas Thr-439 appears to be involved with receptor binding and signaling internalization of the fusion toxin-receptor complex.

Solution assembly of a soluble, heteromeric, high affinity interleukin-2 receptor complex

In this study, we report the use of coiled-coil (leucine zipper) molecular recognition for the solution assembly of stable, high affinity, heteromeric interleukin-2 receptor complexes. Co-expression of interleukin-2 receptor alpha and beta extracellular domains (ectodomains), each fused to seven coiled-coil heptad repeats, resulted in the formation of heteromeric complexes that bound interleukin-2 in a cooperative fashion and with much higher affinity than similar homomeric complexes. The dissociation constants for these solution complexes are within the range of values reported for the comparable cell surface "pseudo high affinity" interleukin-2 receptor. Ligand-induced cross-linking of homomeric or heteromeric receptor subunits is the common signal transmission mechanism employed by hematopoietin receptors. Individual receptor ectodomains, however, often do not bind ligand with measurable affinity. This is the first study to demonstrate the feasibility of coiled-coil mediated preassembly of cytokine receptor complexes.

Ligand binding analysis of soluble interleukin-2 receptor complexes by surface plasmon resonance

Knowledge of the kinetic binding characteristics is often critical to the development of ligand/receptor structure-activity relationships. To better understand the contribution of each of the subunits to ligand binding in the multimeric interleukin-2 receptor system, we have previously prepared stable solution complexes of the alpha- and beta-subunits. In this study, we have employed surface plasmon resonance biosensor methodology (BIAcore) to evaluate both the kinetic and equilibrium binding constants for these complexes. The structural nature of the complexes facilitated immobilization on the sensor surfaces in a manner that minimized interference with ligand interactions. The interleukin-2 receptor complex surfaces displayed excellent binding capacity and stability toward regeneration. In all cases where the binding constants were measurable, the values determined for interleukin-2 were in good agreement with those previously determined by other methods. When interleukin-2 analogs with receptor subunit specific mutations were employed, the binding parameters were consistent with the nature of the mutations. The combination of coiled-coil-mediated solution assembly and surface plasmon resonance analysis of ligand binding provides a powerful approach to the study of multimeric cytokine receptor systems.

Ligand binding analysis of interleukin-2 receptor complexes using surface plasmon resonance

In this study we have employed surface plasmon resonance to examine the kinetic binding constants of a new class of soluble cytokine receptor complexes. The solution assembly of both homomeric and heteromeric interleukin-2 receptor ectodomain complexes has been achieved using coiled-coil molecular recognition. These complexes were immobilized on a biosensor surface and the kinetic binding constants were determined. The values obtained for these parameters compared favorably with those reported in studies of comparable cell surface complexes. The combination of these methods provides a powerful approach to the ligand interactions of cytokine receptors.

The interleukin-2 and interleukin-4 receptors studied by molecular modelling

BACKGROUND

Interleukin-2 (IL2) and interleukin-4 (IL4) are members of the four-helix bundle family of cytokines, whose receptors show similarity to each other and to the growth hormone receptor fold. These proteins help to control, among other things, the rate of clonal expansion of lymphocytes, and thus play an important role in the regulation of the immune system. They are therefore of interest as transmembrane signalling proteins, as well as potential pharmaceutical targets.

RESULTS

We have modelled structures of the extracellular components of the IL2 and IL4 receptors based on the structure of the complex of human growth hormone with its receptor, and incorporating the recently discovered shared gamma c chain. The models provide possible explanations for several experimental observations, including those from site-directed mutagenesis around the binding sites. Receptor residues that may be close to important side chains on IL2 and IL4 are identified and possible effects of their mutation are discussed. A comparison is made between the models and the growth hormone complex, and between the gamma c chain bound to IL2 and to IL4.

CONCLUSIONS

The models offer structural explanations for observed behaviour such as the effects of mutation of the A- and D-helices of the cytokines. In addition, they may be of use in the identification of residues which may interact in the ligand-receptor interfaces, and which would therefore be worthy of further investigation.

Receptor binding properties of four-helix-bundle growth factors deduced from electrostatic analysis

Hormones of the hematopoietin class mediate signal transduction by binding to specific transmembrane receptors. Structural data show that the human growth hormone (hGH) forms a complex with a homodimeric receptor and that hGH is a member of a class of hematopoietins possessing an antiparallel 4-alpha-helix bundle fold. Mutagenesis experiments suggest that electrostatic interactions may have an important influence on hormone-receptor recognition. In order to examine the specificity of hormone-receptor complexation, an analysis was made of the electrostatic potentials of hGH, interleukin-2 (IL-2), interleukin-4 (IL-4), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and the hGH and IL-4 receptors. The binding surfaces of hGH and its receptor, and of IL-4 and its receptor, show complementary electrostatic potentials. The potentials of the hGH and its receptor display approximately 2-fold rotational symmetry because the receptor subunits are identical. In contrast, the potentials of GM-CSF and IL-2 lack such symmetry, consistent with their known high affinity for hetero-oligomeric receptors. Analysis of the electrostatic potentials supports a recently proposed hetero-oligomeric model for a high-affinity IL-4 receptor and suggests a possible new receptor binding mode for G-CSF; it also provides valuable information for guiding structural and mutagenesis studies of signal-transducing proteins and their receptors.

Expression and ligand binding characterization of the beta-subunit (p75) ectodomain of the interleukin-2 receptor

The baculovirus-mediated eukaryotic insect cell expression system was used to prepare large quantities of the beta-subunit ectodomain of the high-affinity interleukin-2 receptor (IL-2R beta x). We describe the expression, purification, and biophysical characterization of this ligand binding domain. The human cDNA encoding IL-2R beta x was inserted into baculovirus transfer vectors. High titer recombinant baculovirus was produced in Spodoptera frugiperda (Sf9) insect cells, and the viral supernatants were subsequently used to infect monolayers of Trichoplusia ni (High Five) insect cells in serum-free culture. Maximal expression of the recombinant protein excreted into the cell culture supernatants was determined by SDS/PAGE analysis, where a band migrating with an apparent molecular mass of 31 kDa was identified by immunostaining. One-step purification was achieved by affinity chromatography on either a monoclonal antibody (TIC-1) column or an IL-2 column, with a final yield of approximately 5 mg/L of culture supernatant. Interestingly, partial purification was also demonstrated using metal chelate affinity chromatography. Amino-terminal sequence analysis of the protein matched the published sequence. Both equilibrium sedimentation analysis and gel filtration chromatography indicated that IL-2R beta x remains monomeric. Deconvolution of far-UV circular dichroism (CD) spectra indicated the predominant secondary structural element to be beta-sheet, consistent with structural analysis and predictions for other members of the hematopoietic receptor family. A dissociation constant (Kd) for IL-2R beta x in solution of 5.3 x 10(-7) M was calculated from competitive receptor binding assays.(ABSTRACT TRUNCATED AT 250 WORDS)