Analysis of a soluble mutant des-methionine interleukin-2 receptor alpha chain (Tac protein) produced by transfected mammalian cells

Overexpression, purification, and use of a soluble human interleukin-4 receptor alpha-chain/Ig gamma 1 fusion protein for ligand binding studies. Characterization of ligand binding to soluble IL-4 receptor alpha-chain by surface plasmon resonance measurements and by microtiter-plate-based ELISA with biotinylated IL-4

The pleiotropic cytokine IL-4 transmits cellular signals mainly via the IL-4 receptor complex, with the alpha-chain as the high affinity binding subunit. Here we describe the overexpression of a soluble IL-4R alpha-chain (sIL-4R) as a fusion to immunoglobulin gamma 1 heavy chain, consisting of the H-CH2-CH3 domains, in baby hamster kidney cells. The dimeric fusion protein named sIL-4R:E gamma 1 was purified from culture supernatant by protein-A affinity chromatography, yielding up to 10 mg/l homogenous protein which was highly stable. The antibody-like features of the sIL-4R:E gamma 1 fusion protein allowed immobilization on a biosensor matrix for surface plasmon resonance measurements by direct amine coupling as well as immobilization on microtiter plates coated with protein A for displacement binding. Kinetic parameters (kon and koff) for binding of IL-4 or the antagonistic mutant IL-4(Y124D) to the sIL-4R:E gamma 1 fusion protein on the chip as determined with the BIAcore instrument showed a high affinity binding with KD = 239 +/- 35 pM and KD=148 +/- 33 pM, respectively. The extremely high kon rate and the relatively slow koff rate for both ligands highlighted the limits of the BIAcore technology. The binding affinity as calculated in displacement binding studies with biotinylated IL-4 was similar for IL-4 and IL-4(Y124D) (IC50=1.1nM), thus offering a simple alternative for initial characterization of IL-4 mutants.

Isolation and refolding of a mutant methionine-free interleukin-2-receptor alpha chain synthesized as a fusion protein in Escherichia coli

A soluble domain of the interleukin(IL)-2 receptor, the alpha chain synthesized in Escherichia coli, was employed to study expression and refolding of the protein. The results showed that it is possible to obtain biologically active synthetic methionine-free IL-2 receptor alpha chain (synIL-2R alpha) after BrCN cleavage and renaturation of the crude cleavage material, although the alpha chain is expressed as a deglycosylated, methionine-free protein. The soluble receptor comprises amino acids 1-219 and forms 5 disulfide bonds in its biologically active state. Biological activity has been analysed by affinity chromatography and ELISA with mutant [Ala125]IL-2 and monoclonal antibodies as ligands. Renaturation yield is limited mainly by the high aggregation rate of incorrectly folded protein. Aggregation could be limited by varying the oxidation conditions. The deletion of a non-bridging cysteine at position 192 in the synIL-2R alpha did not affect the renaturation yield of the receptor protein. Additionally a cysteine-free and methionine-free beta-galactosidase derivative was fused to the soluble synIL-2R alpha derivatives to prevent reoxidation of incorrect disulfide bonds in the crude BrCN-cleavage material. It is suggested that cysteine impurities from cyanogen-bromide-cleaved peptides might interfere seriously with the refolding process of the synthetic IL-2 receptor alpha-subunit.

Overexpression in Escherichia coli of a methionine-free designed interleukin-2 receptor (Tac protein) based on a chemically cleavable fusion protein

Several fusion proteins of our previously chemically synthesized gene encoding the interleukin-2-receptor alpha subunit (IL-2R alpha or Tac protein) were constructed. They were designed in order to be cleavable by cyanogen bromide. Thus, the original internal methionines of the IL-2R alpha were replaced by either alanine, valine, leucine or isoleucine, based on secondary structure predictions. Additionally, aspartate at position 6 was substituted for glutamate in order to stabilize the acid-labile Asp-Pro bond. Direct C-terminal fusion of total beta-galactosidase and portions thereof did not result in substantial amounts of the expected construct. Ternary fusions consisting of beta-galactosidase domains N- and C-terminally fused to the mutant synthetic methionine-free interleukin-2 receptor alpha subunit (synIL-2R alpha) yielded inclusion bodies amounting to 4-7% of the total protein. This first overexpression of a type I membrane receptor can be rationalized by the known beta-galactosidase structure models. The fusion protein can be cleaved with cyanogen bromide, isolated and the resulting synIL-2R alpha detected by Western blot analysis.