Role of the C-terminus in the activity, conformation, and stability of interleukin-6

Establishment of a novel cell-based assay for screening small molecule antagonists of human interleukin-6 receptor

AIM

Blockade of interleukin-6 (IL-6) or its receptor (IL-6R) is effective in preventing the progression of autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis. In the present study, we established a novel cell-based assay for identifying small molecule IL-6R antagonists.

METHODS

HEK293A cells were transfected with recombinant plasmids pTaglite-SNAP-IL6R and pABhFc-IL6 to obtain membrane-bound IL-6R and recombinant human IL-6 coupled with human Fc fragment (rhIL-6), respectively. A novel screening assay based on the interaction between IL-6R and rhIL-6 was established, optimized and validated. The stability of the assay was also assessed by calculating the Z'-factor.

RESULTS

RhIL-6 dose-dependently bound to IL-6R expressed at HEK293A cell surface. The IC50 value of the known antagonist ab47215 was 0.38±0.08 μg/mL, which was consistent with that obtained using the traditional method (0.36±0.14 μg/mL). The value of Z'-factor was 0.68, suggesting that the novel assay was stable for high throughput screening. A total of 474 compounds were screened using the novel screening assay, and 3 compounds exhibited antagonistic activities (IC50=8.73±0.28, 32.32±9.08, 57.83±4.24 μg/mL). Furthermore, the active compounds dose-dependently inhibited IL-6-induced proliferation of 7TD1 cells, and reduced IL-6-induced STAT3 phosphorylation in U937 cells.

CONCLUSION

A novel cell-based screening assay for identifying small molecule IL-6R antagonists was established, which simplifies the procedures in traditional cellular ELISA screening and profiling and reduces the costs.

Meprin metalloproteases inactivate interleukin 6

Meprins have been implicated in the pathogenesis of several inflammatory diseases, including inflammatory bowel disease, in which the cytokine IL-6 is a prominent effector molecule. Because IL-6 levels are elevated markedly in meprin α and α/β knockout mice in an experimental model of inflammatory bowel disease, the interaction between meprins and IL-6 was studied. The results demonstrate that rodent and human meprin A and B cleave IL-6 to a smaller product and, subsequently, are capable of extensive degradation of the cytokine. Analysis of the limited degradation product formed by meprin A indicated that three to five amino acids are removed from the C terminus of the cytokine. Meprin A and meprin B cleaved IL-6 with micromolar affinities (Km of 4.7 and 12.0 μM, respectively) and with high efficiencies (kcat/Km of 0.2 and 2.5 (M(-1)/s(-1)) × 10(6), respectively). These efficiency constants are among the highest for known meprin substrates. Madin-Darby canine kidney cells transiently transfected with meprin α or meprin β constructs also cleave exogenous IL-6. Both human and murine IL-6 cleaved by meprin A or B are inactivated, as demonstrated by their decreased capability to stimulate proliferation of B9 cells. These results are consistent with the proposition that one function of meprin metalloproteases is to modulate inflammation by inactivating IL-6.

Interleukin-6 N-terminal peptides modulate the expression of junB protooncogene and the production of fibrinogen in HepG2 cells

Interleukin-6 (IL-6) is a helical cytokine exerting pleiotropic activities including the regulation of hematopoiesis, B cell activation and acute-phase reaction. The structure-function relationship of the molecule is the subject of intensive investigation using point and deletion mutants. Our objective was to analyse the role of the N-terminal 18-46 region in IL-6-mediated expression of junB protooncogene and fibrinogen production, reflecting the acute phase response, with synthetic overlapping peptides. mRNA expression of junB was monitored by competitive RT-PCR, while sandwich ELISA was used for the detection of fibrinogen in the supernatant of HepG2 human hepatoma cells. We found that even short synthetic octapeptides can be stimulatory (in the absence of IL-6) or inhibitory (in the presence of IL-6) in both assays. To establish the molecular mechanism by which synthetic peptides exert their biological effects electromobility shift assay was carried out using HepG2 nuclear extracts. Peptides inducing junB expression initiate gel shifts of STAT3/DNA complexes, which may indicate the involvement of this signal transduction pathway. Circular dicroism spectroscopy data suggest that 8-11-mer peptides representing different parts of the 18-46 region have a marked tendency to adopt ordered conformations in a water/trifluoroethanol (1:1 v/v) mixture. Competition studies with rhIL-6 and selected fluorophore-labelled peptides indicate the presence of more than one binding site on soluble IL-6 receptor. Considering the possible multiple etiologic role of IL-6 in the pathogenesis of various diseases, these peptides could be useful for dissection of IL-6 related biological effects.

C-terminal peptides of interleukin-6 modulate the expression of junB protooncogene and the production of fibrinogen by HepG2 cells

Interleukin-6 (IL-6) is a 185 amino acid residue helical cytokine with various biological activities (e. g. B cell development, acute phase reaction). We have investigated the role of the 168-185 C-terminal region of IL-6 in the induction of fibrinogen synthesis and expression of junB mRNA using synthetic peptides corresponding to this region. Circular dichroism spectroscopy data suggest that even truncated peptides have a strong tendency to adopt an ordered conformation. Peptides were tested alone or in combination with recombinant hIL-6 on an IL-6 responsive human hepatoma HepG2 cell line. The expression of the protooncogene junB monitored by competitive RT-PCR represents an early, while the fibrinogen production detected by sandwich ELISA a late, marker of IL-6 initiated events. We found that peptides--depending on their structure--modulate spontaneous as well as IL-6 induced fibrinogen production and/or mRNA expression of junB by exhibiting inhibition (in the presence of IL-6) or stimulation (in the absence of IL-6).

The single mutation Phe173 --> Ala induces a molten globule-like state in murine interleukin-6

A series of three aromatic to alanine mutants of recombinant murine interleukin-6 lacking the 22 N-terminal residues (DeltaN22mIL-6) were constructed to investigate the role of these residues in the structure and function of mIL-6. While Y78A and Y97A have activities similar to that of DeltaN22mIL-6, F173A lacks biological activity. F173A retains high levels of secondary structure, as determined by far-UV circular dichroism (CD), but has substantially reduced levels of tertiary structure, as determined by near-UV CD and (1)H NMR spectroscopy. F173A also binds the hydrophobic dye 1-anilino-8-naphthalenesulfonic acid (ANS) over a range of pH values and exhibits noncooperative equilibrium unfolding (as judged by the noncoincidence of monophasic unfolding transitions monitored by far-UV CD and lambda(max), with midpoints of unfolding at 2.6 +/- 0. 1 and 3.5 +/- 0.3 M urea, respectively, and the lack of an observable thermal unfolding transition). These are all properties of molten globule states, suggesting that the loss of activity of F173A results from the disruption of the fine structure of the protein, rather than from the loss of a side chain that is important for ligand-receptor interactions. Surprisingly, under some conditions, this loosened conformation is no more susceptible to proteolytic attack than the parent protein. By analogy with human IL-6, Phe173 in DeltaN22mIL-6 makes multiple interhelical interactions, the removal of which appear to be sufficient to induce a molten globule-like conformation.

Recombinant Soluble Interleukin-11 (IL-11) Receptor α-Chain Can Act as an IL-11 Antagonist

We have expressed a soluble N-glycosylated form of the murine interleukin-11 (IL-11) receptor α-chain (sIL-11R) and examined signaling in cells expressing the gp130 molecule. In the presence of gp130 but not the transmembrane IL-11R, the sIL-11R mediated IL-11–dependent differentiation of M1 leukemic cells and proliferation in Ba/F3 cells. Early intracellular events stimulated by the sIL-11R including phosphorylation of gp130, STAT 3, and SHP-2 were similar to signaling through the transmembrane IL-11R. IL-11 bound to sIL-11R with low affinity (kd 10 to 50 nmol/L). Binding of sIL-11R to gp130 was IL-11 dependent with intermediate affinity (kd 1.5 to 3.0 nmol/L). However, the concentration of IL-11 required for signaling through the sIL-11R was 10- to 20-fold greater than that required for cells expressing the transmembrane IL-11R and gp130 in the absence of sIL-11R. Furthermore, the sIL-11R was capable of antagonizing the activity of IL-11 when tested on cells expressing the transmembrane IL-11R and gp130. We propose that the observed IL-11 antagonism by the sIL-11R may depend on limiting numbers of gp130 molecules on cells already expressing the transmembrane IL-11R.

Recombinant Soluble Interleukin-11 (IL-11) Receptor α-Chain Can Act as an IL-11 Antagonist

We have expressed a soluble N-glycosylated form of the murine interleukin-11 (IL-11) receptor α-chain (sIL-11R) and examined signaling in cells expressing the gp130 molecule. In the presence of gp130 but not the transmembrane IL-11R, the sIL-11R mediated IL-11–dependent differentiation of M1 leukemic cells and proliferation in Ba/F3 cells. Early intracellular events stimulated by the sIL-11R including phosphorylation of gp130, STAT 3, and SHP-2 were similar to signaling through the transmembrane IL-11R. IL-11 bound to sIL-11R with low affinity (kd 10 to 50 nmol/L). Binding of sIL-11R to gp130 was IL-11 dependent with intermediate affinity (kd 1.5 to 3.0 nmol/L). However, the concentration of IL-11 required for signaling through the sIL-11R was 10- to 20-fold greater than that required for cells expressing the transmembrane IL-11R and gp130 in the absence of sIL-11R. Furthermore, the sIL-11R was capable of antagonizing the activity of IL-11 when tested on cells expressing the transmembrane IL-11R and gp130. We propose that the observed IL-11 antagonism by the sIL-11R may depend on limiting numbers of gp130 molecules on cells already expressing the transmembrane IL-11R.

Identification and characterization of recombinant murine interleukin-6 with a C-terminal pentapeptide extension using capillary reversed phase HPLC-MS and edman degradation

We have identified a preparation of recombinant murine interleukin-6 (mIL-6) that, in addition to the anticipated product, also contained approximately equal amounts of mIL-6 with a C-terminal pentapeptide extension. The extension mutant was generated by readthrough of the stopcodon, and termination at a second in-frame stopcodon 12 base pairs 3' in the expression vector. Aliquots of the preparation were subjected to proteolytic digestion with Asp-N and Lys-C-endopeptidase. The resultant peptides were separated by reversed-phase capillary HPLC, and analysed using a combination of mass spectrometry and N-terminal sequence analysis. These data revealed a C-terminal pentapeptide (Gln-Gly-Ser-Val-Asp) extension, with the authentic stopcodon being translated as glutamine. The extension mutant was isolated by reversed-phase HPLC and shown to have similar mitogenic activity to mIL-6 on murine hybridoma 7TD1 cells.

Roles of histidine 31 and tryptophan 34 in the structure, self-association, and folding of murine interleukin-6

Interleukin-6 (IL-6) is a multifunctional cytokine which is involved in a broad spectrum of activities such as immune defense, hematopoiesis, and the acute phase response, as well as in the pathogenesis of multiple myeloma. A series of murine IL-6 (mIL-6) mutants, H31A, W34A, and H31A/W34A, were constructed to investigate the roles of His31 and Trp34 in the structure, conformational stability, time-dependent aggregation, folding, and spectral properties of mIL-6. The characteristic pH-dependent quenching of fluorescence of mIL-6 at low pH was shown to be caused by an interaction between Trp34 and protonated His31 at low pH and not associated with Trp157. Denaturant-induced equilibrium unfolding experiments monitored by fluorescence and far-UV CD showed that the increased quantum yield and blue shift of the wavelength of the emission maximum observed for mIL-6 at moderate denaturant concentrations were also associated with Trp34, rather than Trp157. The tendency to form aggregation-prone unfolding intermediates, as judged by poor fits to a two-state unfolding mechanism, low m values (slopes of the unfolding curve in the transition region), and the range of denaturant concentrations over which these intermediates formed, was shown to be higher for H31A than mIL-6 but significantly lower for W34A and H31A/W34A. These differences were most pronounced at pH 7.4 and correlated with the tendencies of the proteins to aggregate at high protein concentrations in the absence of denaturant. As judged by the 1H NMR chemical shifts of the aromatic residues, the global conformations of H31A and W34A were not significantly different from that of mIL-6. Nuclear Overhauser effects (NOE) between the side chains of His31 and Trp34 were consistent with the indole side chain of Trp34 being oriented toward the face of the imidazolium side chain of His31, an arrangement consistent with our estimates of a low interaction energy (0.4-0.6 kcal/mol) between these side chains. A shift in the pKa of the His31 side chain in W34A (+0.3 unit) suggested that, in the absence of Trp34, His31 could interact with other residues. Further mutations in this region should yield forms of mIL-6, even less prone to aggregation, which would be more suitable for NMR studies. Mutation of His31 and Trp34 to alanine did not significantly alter the mitogenic activity of the mutants on mouse hybridoma 7TD1 cells, even though the corresponding region of human IL-6 has been shown to be important for biological activity.

Interleukin-6: structure-function relationships

Interleukin-6 (IL-6) is a multifunctional cytokine that plays a central role in host defense due to its wide range of immune and hematopoietic activities and its potent ability to induce the acute phase response. Overexpression of IL-6 has been implicated in the pathology of a number of diseases including multiple myeloma, rheumatoid arthritis, Castleman's disease, psoriasis, and post-menopausal osteoporosis. Hence, selective antagonists of IL-6 action may offer therapeutic benefits. IL-6 is a member of the family of cytokines that includes interleukin-11, leukemia inhibitory factor, oncostatin M, cardiotrophin-1, and ciliary neurotrophic factor. Like the other members of this family, IL-6 induces growth or differentiation via a receptor-system that involves a specific receptor and the use of a shared signaling subunit, gp130. Identification of the regions of IL-6 that are involved in the interactions with the IL-6 receptor, and gp130 is an important first step in the rational manipulation of the effects of this cytokine for therapeutic benefit. In this review, we focus on the sites on IL-6 which interact with its low-affinity specific receptor, the IL-6 receptor, and the high-affinity converter gp130. A tentative model for the IL-6 hexameric receptor ligand complex is presented and discussed with respect to the mechanism of action of the other members of the IL-6 family of cytokines.

Disruption of the disulfide bonds of recombinant murine interleukin-6 induces formation of a partially unfolded state

A chemical modification approach was used to investigate the role of the two disulfide bonds of recombinant murine interleukin-6 (mIL-6) in terms of biological activity and conformational stability. Disruption of the disulfide bonds of mIL-6 by treatment with iodoacetic acid (IAA-IL-6) or iodoacetamide (IAM-IL-6) reduced the biological activity, in the murine hybridoma growth factor assay, by 500- and 200-fold, respectively. Both alkylated derivatives as well as the fully reduced (but not modified) molecule (DTT-IL-6) retained a high degree of alpha-helical structure as measured by far-UV CD (37-51%) when compared to the mIL-6 (59%). However, the intensity of the near-UV CD signal of the S-alkylated derivatives was very low relative to that of mIL-6, suggesting a reduction in fixed tertiary interactions. Both IAA-IL-6 and IAM-IL-6 exhibit native-like unfolding properties at pH 4.0, characteristic of a two-state unfolding mechanism, and are destabilized relative to mIL-6, by 0.3 +/- 1.6 and 2.4 +/- 1.2 kcal/mol, respectively. At pH 7.4, however, both modified proteins display stable unfolding intermediates. These intermediates are stable over a wide range of GdnHCl concentrations (0.5-2 M) and are characterized by increased fluorescence quantum yield and a blue shift of lambda(max) from 345 nm, for wild-type recombinant mIL-6, to 335 nm. These properties were identical to those observed for DTT-IL-6 in the absence of denaturant. DTT-IL-6 appears to form a partially unfolded and highly aggregated conformation under all conditions studied, as showed by a high propensity to self-associate (demonstrated using a biosensor employing surface plasmon resonance), and an increased ability to bind the hydrophobic probe 8-anilino-1-naphthalenesulfonic acid. The observed protein concentration dependence of the fluorescence characteristics of these mIL-6 derivatives is consistent with the aggregation of partially folded forms of DTT-IL-6, IAM-IL-6, and IAA-IL-6 during denaturant-induced unfolding. For all forms of the protein studied here, the aggregated intermediates unfold at similar denaturant concentrations (2.1-2.9 M GdnHCl), suggesting that the alpha-helical structure and nonspecific hydrophobic interprotein interactions are of similar strength in all cases.

Truncations of the C-terminus have different effects on the conformation and activity of phosphatidylinositol transfer protein

Contributions of the C-terminus toward the conformation and activity of phosphatidylinositol transfer protein (PITP) were studied by comparing properties of the 271 amino acid, full-length protein, PITP(1-271), and two truncated species, PITP(1-259) and PITP(1-253). Using recombinant proteins and an in vitro phospholipid transfer assay with phosphatidylcholine vesicles, the activities of PITP(1-271) and PITP(1-259) were identical, while the activity of PITP(1-253) was almost totally abolished. By most physical and chemical criteria, however, PITP(1-259) and PITP(1-253) were virtually indistinguishable and differed significantly from the full-length protein. Results of second derivative analysis of absorbance spectra were consistent with an additional two Tyr residues being exposed to the solvent in PITP(1-259) and PITP(1-253) in comparison to PITP(1-271). Only one out of four Cys residues in PITP(1-271) reacted with dithiobisnitrobenzoic acid, while two Cys residues were accessible in both truncated species. Quenching of intrinsic Trp fluorescence by acrylamide demonstrated an increase in exposure of Trp residues in both PITP(1-259) and PITP(1-253); binding of the fluorescence probe 1,8-ANS to these proteins was also significantly higher compared to PITP(1-271). These results describe a more relaxed overall tertiary structure brought about by the C-terminal truncations. This altered structure did not affect the stability of the truncated proteins, as indicated by equilibrium unfolding in guanidinium chloride. Refolding of the denatured PITP(1-259), however, was considerably slower than that of full-length PITP. Our study suggests a critical role of the C-terminal residues 254-259 in transfer activity of PITP. Residues 260-271, on the other hand, appear to be more important for the rapid folding and maintenance of a compact native conformation of the protein.

The interleukin-6 (IL-6) partial antagonist (Q159E,T162P)IL-6 interacts with the IL-6 receptor and gp130 but fails to induce a stable hexameric receptor complex

The extracellular "soluble" domains of the IL-6 receptor (sIL-6R) and gp130 (sgp130) form a hexameric ternary receptor complex together with IL-6, consisting of two molecules of each component. In this report we have investigated the interactions of the partial IL-6 antagonist (Q159E,T162P)IL-6 ((QT)IL-6), with the sIL-6R and sgp130. The kinetic rate constants of the binding of sIL-6R to immobilized monomeric (QT)IL-6 or IL-6 were obtained using an optical biosensor with analysis of the primary data by linear and nonlinear regression. Both methods of analysis showed that, due to a higher off-rate, sIL-6R has lower apparent affinity for (QT)IL-6 than IL-6. The lower affinity of (QT)IL-6 was further confirmed by equilibrium binding measurements at the sensor surface and in solution. Using the biosensor it was also shown that the (QT)IL-6 complex interacts with sgp130, supporting the notion that the biological activity of (QT)IL-6 is mediated via gp130. However, the IL-6 mutant, when incubated with sIL-6R and sgp130, failed to induce a stable hexameric receptor complex, as shown by narrowbore size exclusion chromatography.

NMR studies of the solution properties of recombinant murine interleukin-6

The effects of solvent, pH and temperature on the 1H-NMR spectra of recombinant murine interleukin-6 (IL-6) are described. Assignments made from two-dimensional homonuclear spectra are presented for resonances of the fifteen aromatic amino-acid side chains. A time-dependent loss of intensity was observed for all resonances in the spectrum of IL-6, probably as a result of aggregation. This aggregation is markedly temperature-dependent. The pKa values of the four histidine residues in murine IL-6 has been measured; one has a value of 5.5, approx. one pH unit less than the value exhibited by the other three. Analysis of the NOESY spectra has allowed a preliminary characterisation of the nature of interactions among the aromatic side chains within the protein fold. 1H and 15N resonances of residues Thr-4 to Val-21 are assigned from three-dimensional 1H-15N correlated spectroscopy, and evidence is presented for these residues comprising a mobile N-terminal tail with little ordered structure. An N-terminal mutant lacking the first 22 residues of the murine IL-6 sequence and known to possess full biological activity was also examined and shown to have essentially retained the tertiary fold of the native molecule.

Structure, stability and biological properties of a N-terminally truncated form of recombinant human interleukin-6 containing a single disulfide bond

A mutant species of the 185-residue chain of human interleukin-6 lacking 22-residues at its N-terminus and with a Cys-->Ser substitution at positions 45 and 51 was produced in Escherichia coli. The 163-residue protein des-(A1-S22)-[C45S, C51S]interleukin-6, containing a single disulfide bridge, formed inclusion bodies. Mutant interleukin-6 was solubilized in 6 M guanidine hydrochloride, subjected to oxidative refolding and purified to homogeneity by ammonium sulfate precipitation and hydrophobic chromatography. The purity of the mutant species was established by electrophoresis, isoelectrofocusing and reverse-phase HPLC and its structural identity was checked by N-terminal sequencing of both the intact protein and several of its proteolytic fragments. Electrospray mass spectrometry analysis of mutant interleukin-6 gave a molecular mass of 18,695 +/- 2 Da in excellent agreement with the calculated value. Circular dichroic, fluorescence emission and second-derivative ultraviolet absorption spectra indicated that mutant interleukin-6 maintains the overall secondary and tertiary structure, as well as stability characteristics, of the recombinant wild-type human interleukin-6. The urea-induced unfolding of mutant interleukin-6, monitored by circular dichroic measurements in the far-ultraviolet region, occurs as a highly cooperative process with a midpoint of denaturation at 5.5 M urea. The data of the reversible unfolding of mutant interleukin-6 mediated by urea were used to calculate a value of 20.9 +/- 0.4 kJ.mol-1 for the thermodynamic stability of the protein at 25 degrees C in the absence of denaturant. The biological activity of mutant interleukin-6 was evaluated in vitro by the hybridoma proliferation assay, and in vivo by measuring thrombopoiesis in monkeys. Dose/response effects of the mutant were comparable or even higher than those of the wild-type protein. Overall the results of this study show that mutant interleukin-6 is a biologically active cytokine, which could find practical use as a therapeutic agent.

Structure-function relationships in human interleukin-11. Identification of regions involved in activity by chemical modification and site-directed mutagenesis

Chemical modification approaches combined with site-directed and deletion mutagenesis have been used to identify functionally critical regions/residues of recombinant human IL-11 (rhIL-11). Incubation of rhIL-11 with iodoacetic acid results in specific alkylation of a single methionine residue, Met58, and a 25-fold reduction of in vitro biological activity on mouse plasmacytoma cells. A similar decrease in activity is observed when Met58 is substituted with Ala, Leu, Gln, Glu, or Lys by site-directed mutagenesis. Treatment of rhIL-11 with succinic anhydride leads to modification of the amino-terminal amino group and partial labeling of 2 lysines, Lys41 and Lys98, and to a 3-fold decrease in activity. The activity losses can be attributed to modification of the lysine residues, since the succinyl derivative of the amino terminus is fully active. In addition, carboxyl-terminal deletion mutagenesis studies have demonstrated that removal of the last 4 residues reduces rhIL-11 activity 25-fold, whereas removal of 8 or more amino acids results in an inactive molecule. Based on secondary structure predictions and the location of exon/intron boundaries in the IL-11 genomic structure, we propose a four-helix bundle topology as a structural model for rhIL-11. This model has been tested by limited proteolysis using three side chain-specific endoproteinases. A limited number of protease-sensitive cleavage sites are present in rhIL-11, and all but two are located in the postulated helix interconnecting loops or at helix termini. alpha-Helices, which in the proposed structure form a compact core of the molecule, are inaccessible to digestion under limiting conditions. According to the model, Met58, Lys41 and Lys98 are located on the surface of the molecule, in agreement with their preferential accessibility to chemical modifications. By analogy with human growth hormone, we postulate that Met58 and the carboxyl terminus of rhIL-11 are involved in the primary receptor binding site (site I), whereas Lys41 and Lys98 may be a part of binding site II.

Structure-function analysis of human IL-6: identification of two distinct regions that are important for receptor binding

Interleukin-6 (IL-6) is a multifunctional cytokine that plays an important role in host defense. It has been predicted that IL-6 may fold as a 4 alpha-helix bundle structure with up-up-down-down topology. Despite a high degree of sequence similarity (42%) the human and mouse IL-6 polypeptides display distinct species-specific activities. Although human IL-6 (hIL-6) is active in both human and mouse cell assays, mouse IL-6 (mIL-6) is not active on human cells. Previously, we demonstrated that the 5 C-terminal residues of mIL-6 are important for activity, conformation, and stability (Ward LD et al., 1993, Protein Sci 2:1472-1481). To further probe the structure-function relationship of this cytokine, we have constructed several human/mouse IL-6 hybrid molecules. Restriction endonuclease sites were introduced and used to ligate the human and mouse sequences at junction points situated at Leu-62 (Lys-65 in mIL-6) in the putative connecting loop AB between helices A and B, at Arg-113 (Val-117 in mIL-6) at the N-terminal end of helix C, at Lys-150 (Asp-152 in mIL-6) in the connecting loop CD between helices C and D, and at Leu-178 (Thr-180 in mIL-6) in helix D. Hybrid molecules consisting of various combinations of these fragments were constructed, expressed, and purified to homogeneity. The conformational integrity of the IL-6 hybrids was assessed by far-UV CD. Analysis of their biological activity in a human bioassay (using the HepG2 cell line), a mouse bioassay (using the 7TD1 cell line), and receptor binding properties indicates that at least 2 regions of hIL-6, residues 178-184 in helix D and residues 63-113 in the region incorporating part of the putative connecting loop AB through to the beginning of helix C, are critical for efficient binding to the human IL-6 receptor. For human IL-6, it would appear that interactions between residues Ala-180, Leu-181, and Met-184 and residues in the N-terminal region may be critical for maintaining the structure of the molecule; replacement of these residues with the corresponding 3 residues in mouse IL-6 correlated with a significant loss of alpha-helical content and a 200-fold reduction in activity in the mouse bioassay. A homology model of mIL-6 based on the X-ray structure of human granulocyte colony-stimulating factor is presented.

Solution structure of synthetic peptides corresponding to the C-terminal helix of interleukin-6

Two synthetic peptides corresponding to the C-terminal 19 residues of human and murine interleukin-6, respectively, have been synthesized and their structures in solution investigated using high-resolution 1H-NMR spectroscopy. Both peptides show a marked dependence of chemical-shift dispersion on pH, with a greater degree of structure apparent above pH 4.5, where their glutamate carboxyl groups are ionised. In purely aqueous solution, neither peptide adopts a well-defined structure, although the murine peptide has characteristics of a nascent helix. Titration of the murine peptide with trifluoroethanol produced a significant increase in structure, which was then investigated using two-dimensional NMR. In 50% (by vol.) trifluoroethanol the murine peptide consists of a well-defined central helix of 12 residues with unstructured N-terminal and C-terminal regions. These observations lend experimental support to the current model of the interleukin-6 structure, which proposes a four-helical bundle with the last helix encompassing the C-terminal 20-30 residues. Furthermore, the fact that synthetic peptides corresponding to part of the putative receptor-binding surface of interleukin-6 are able to adopt a similar conformation in solution to that proposed for the intact protein suggests that such peptide analogues should be useful starting points in the design of peptide agonists and antagonists of interleukin-6.

NMR studies of a murine-human chimera of leukaemia inhibitory factor (LIF). Comparison with human LIF

Leukaemia inhibitory factor (LIF) is a polyfunctional cytokine active on many cell types. We present here 1H NMR studies on the solution properties and stability of MH35, a chimera of murine and human LIF which can be expressed at high levels in Escherichia coli, thus enabling efficient labelling of the protein with the stable isotopes 13C and 15N. MH35 has 85% sequence identity with human LIF and similar activity in biological assays. The 1H chemical shifts of the 12 conserved aromatic residues and the pKa values of the five conserved histidine residues in MH35 and human LIF are very similar. Temperature dependence studies indicate that both proteins are stable, with significant conformational changes occurring only above 70 degrees C. These results show that these proteins have a similar overall structure and stability and that MH35 is therefore a suitable analogue of human LIF for structural studies in solution.