Expression, purification and crystallization of fully active, glycosylated human interleukin-5

A sensitive and versatile bioassay for ligands that signal through receptor clustering

The induced expression of xanthine-guanine phosphoribosyl transferase (XGPRT) by low concentrations (-2 pg/ml) of interferon-alpha (IFN-alpha) or IFN-beta, in the 2fTPGH cell line caused a 50% cytotoxicity when these cells were grown in medium containing 6-thioguanine. We extended the application of this sensitive, reliable, and easy bioassay to other members of the cytokine family. To activate the IFN signaling pathway, we made receptor chimeras, consisting of the IFN type I receptor intracellular and transmembrane domains, fused to either the interleukin-5 (IL-5) receptors or erythropoietin (Epo) receptor extracellular domains as model systems. 2fTGH cells, stably transfected with these receptor chimeras, responded to very low concentrations of IL-5 or Epo (IC50 values of approximately 15 pg and 3 pg/ml, respectively) and thus can be used as a very sensitive bioassay for both ligands. Background activity of IL-5, Epo, tumor necrosis factor (TNF), IL-6, or leptin on cells that did not carry the receptor chimeras was very low. This methodology can in principle be extended to any ligand that acts via clustering of its receptors.

The cell surface expression level of the human interleukin-5 receptor alpha subunit determines the agonistic/antagonistic balance of the human interleukin-5 E13Q mutein

The human interleukin-5 (IL-5) receptor consists of an alpha-chain that specifically binds the ligand with intermediate affinity, and a beta c-chain, that associates with the IL-5/IL-5R alpha complex, leading to a high-affinity, signal transducing receptor complex. Structure-function studies showed that modification of the putative beta c-chain binding site in IL-5 (E13Q mutein) converted the molecule into an antagonist. However, analysis of the effect of this mutant IL-5 on COS-1 cells transfected with both receptor subunits, did not show reduced interaction with the beta c subunit [Tavernier, J., Tuypens, T., Verhee, A., Plaetinck, G., Devos, R., Van der Heyden, J., Guisez, Y. & Oefner, C. (1995) Proc. Natl Acad. Sci. USA 89, 7041-7045]. To gain more insight into the mechanism of IL-5 antagonism by E13Q, we tested its biological activity on two FDC-P1 subclones that express clearly different numbers of alpha-subunits yet an almost constant number of murine beta c-subunits. Here we show that E13Q has a biological activity comparable to wild-type IL-5 only when a high number of alpha-chains is present on the cells. Confirming the critical role of the IL5R alpha cell-surface expression level, treatment with suboptimal doses of a neutralising anti-IL-5R alpha antibody results in reduced activity of the mutant but not of wild-type IL-5.

Analysis of three human interleukin 5 structures suggests a possible receptor binding mechanism

We compared three crystal structures of human interleukin 5 (hIL5) expressed in either E. coli (hIL5E.coli), Sf9 cells (hIL5sf9) or Drosophila cells (hIL5Drosophila). The dimeric hIL5 structures show subtle but significant conformational differences which are probably a consequence of the different crystallization conditions trapping this protein into one of two states. We refer to these two distinct conformations as the 'open' and 'tight' state, according to the packing around the cleft between the two subunits. We hypothesize that these two stable conformational states reflect the structure of the free or receptor bound hIL5.

ESM-1 is a novel human endothelial cell-specific molecule expressed in lung and regulated by cytokines

We here report the identification of a novel human endothelial cell-specific molecule (called ESM-1) cloned from a human umbilical vein endothelial cell (HUVEC) cDNA library. Constitutive ESM-1 gene expression (as demonstrated by Northern blot and reverse transcription-polymerase chain reaction analysis) was found in HUVECs but not in the other human cell lines tested. The cDNA sequence contains an open reading frame of 552 nucleotides and a 1398-nucleotide 3'-untranslated region including several domains involved in mRNA instability and five putative polyadenylation consensus sequences. The deduced 184-amino acid sequence defines a cysteine-rich protein with a functional NH2-terminal hydrophobic signal sequence. Searches in several data bases confirmed the unique identity of this sequence. A rabbit immune serum raised against the 14-kDa COOH-terminal peptide of ESM-1 immunoprecipitated a 20-kDa protein only in ESM-1-transfected COS cells. Immunoblotting and immunoprecipitation of HUVEC lysates revealed a specific 20-kDa band corresponding to ESM-1. In addition, constitutive ESM-1 gene expression was shown to be tissue-restricted to the human lung. Southern blot analysis suggests that a single gene encodes ESM-1. A time-dependent up-regulation of ESM-1 mRNA was seen after addition of tumor necrosis factor alpha (TNFalpha) or interleukin (IL)-1beta but not with IL-4 or interferon gamma (IFNgamma) alone. In addition, when IFNgamma was combined with TNFalpha, IFNgamma inhibited the TNFalpha-induced increase of ESM-1 mRNA level. These data suggest that ESM-1 may have potent implications in the areas of vascular cell biology and human lung physiology.

Recombinant soluble beta-1,4-galactosyltransferases expressed in Saccharomyces cerevisiae. Purification, characterization and comparison with human enzyme

beta-1,4-Galactosyltransferase (Gal-T, EC 2.4.1.38) transfers galactose (Gal) from UDP-Gal to N-acetyl-D-glucosamine or a derivative GlcNAc-R. Soluble Gal-T, purified from human breast milk, was shown to be very heterogeneous by isoelectric focusing (IEF). In order to produce sufficient homogeneous enzyme for three-dimensional analysis, the human enzyme (hGal-T) has been expressed in Saccharomyces cerevisiae, production scaled up to 187 U recombinant Gal-T (rGal-T) and purified. The purification protocol was based on chromatography on concanavalin-A-Sepharose followed by affinity chromatographies on GlcNAc-Sepharose and alpha-lactalbumin-Sepharose. Analysis by SDS/PAGE revealed hyperglycosylation at the single N-glycosylation site, preventing recognition by antibodies. Analysis by IEF revealed considerable heterogeneity of rGal-T. The N-glycan could be removed by treatment with endoglycosidase H (endo H). The N-deglycosylated form of rGal-T retained full activity and showed only three isoforms by IEF analysis. Then we abolished the single N-glycosylation consensus sequence by site-directed mutagenesis changing Asn69-->Asp. The soluble mutated enzyme (N-deglycosylated rGal-T) was expressed in S. cerevisiae and its production scaled up to 60 U.N-deglycosylated rGal-T was purified to electrophoretic homogeneity. When analyzed by IEF, N-deglycosylated rGal-T was resolved in two bands. The O-glycans could be removed by jack bean alpha-mannosidase treatment and the completely deglycosylated Gal-T appeared homogeneous by IEF. The kinetic parameters of N-deglycosylated rGal-T were shown not to differ to any significant extent from those of the hGal-T. No significant changes in CD spectra were observed between hGal-T and N-deglycosylated rGal-T. Light-scattering analysis revealed dimerization of both enzymes. These data indicate that N-deglycosylated rGal-T was correctly folded, homogeneous and thus suitable for crystallization experiments.

Identification of receptor-binding domains on human interleukin 5 and design of an interleukin 5-derived receptor antagonist

A detailed structure-function analysis of human interleukin 5 (hIL5) has been performed. The hIL5 receptor is composed of two different polypeptide chains, the alpha and beta subunits. The alpha subunit alone is sufficient for ligand binding, but association with the beta subunit leads to a 2- to 3-fold increase in binding affinity. The beta chain is shared with the receptors for IL3 and granulocyte/macrophage-colony-stimulating factor--hence the descriptor beta C (C for common). All hIL5 mutants were analyzed in a solid-phase binding assay for hIL5R alpha interaction and in a proliferation assay using IL5-dependent cell lines for receptor-complex activation. Most residues affecting binding to the receptor alpha subunit were clustered in a loop connecting beta-strand 1 and helix B (mutants H38A, K39A, and H41A), in beta-strand 2 (E89A and R91A; weaker effect for E90A) and close to the C terminus (T109A, E110A, W111S, and I112A). Mutations at one position, E13 (Glu13), caused a reduced activation of the hIL5 receptor complex. In the case of E13Q, only 0.05% bioactivity was detected on a hIL5-responsive subclone of the mouse promyelocytic cell line FDC-P1. Moreover, on hIL5-responsive TF1 cells, the same mutant was completely inactive and proved to have antagonistic properties. Interactions of this mutant with both receptor subunits were nevertheless indistinguishable from those of nonmutated hIL5 by crosslinking and Scatchard plot analysis of transfected COS-1 cells.

Structure/function analysis of human interleukin 5 and its receptor

We have performed a detailed structure-function analysis of human interleukin 5 (hIL5) and its receptor. By testing a hIL5 mutant panel in a solid phase binding assay and a proliferation assay using hIL5 dependent cell-lines, areas on hIL5 involved in either the receptor alpha-subunit interaction or in receptor activation were identified. Epitope mapping data of a neutralizing and a non-neutralizing monoclonal antibody were in agreement with the mutant analysis. hIL5 binding areas on the IL5R alpha-subunit were identified by interspecies chimaera analysis. Finally, hIL5 mutants with reduced receptor activation potential have antagonistic properties.