Purification and renaturation of recombinant human interleukin-2

Soluble expression of recombinant human interleukin-2 in Escherichia coli and its facile production

Recombinant human interleukin-2 (rhIL-2) represents one of the most difficult-to-produce cytokines in E. coli due to its extreme hydrophobicity and high tendency to formation of inclusion bodies. Refolding of rhIL-2 inclusion bodies always represents cumbersome downstream processes and low production efficiency. Herein, we disclosed a fusion strategy for efficiently soluble expression and facile production of rhIL-2 in E. coli Origami B (DE3) host. A two-tandem SUMO fusion partner (His-2SUMO) with a unique SUMO protease cleavage site at C-terminus was devised to fuse with the N-terminus of rhIL-2 and the fusion protein (His-2SUMO-rhIL-2) was almost completely expressed in a soluble from. The fusion partner could be efficiently removed by Ulp1 cleavage and the rhIL-2 was simply produced by a two-step Ni-NTA affinity chromatography with a considerable purity and whole recovery. The eventually obtained rhIL-2 was well-characterized and the results showed that the purified rhIL-2 exhibits a compact and ordered structure. Although the finally obtained rhIL-2 exists in a soluble aggregates form and the aggregation probably has been occurred during expression stage, the soluble rhIL-2 aggregates remain exhibit comparable bioactivity with the commercially available rhIL-2 drug formulation.

Purification and Conformational Characterization of a Novel Interleukin-2 Mutein

Toxicity of high-dose IL-2-based therapies have motivated the development of the IL-2 mutein, which has low expansion properties for regulatory T lymphocytes. The development of two variants (A and B) for the IL-2 mutein purification as well as a conformational comparative study by Circular dichroism (CD) and fluorescence spectroscopy of these products were evaluated. For the first time, in our center, were used of DTT and 2% SDS in the solubilization step to decrease the aggregates on intermediate product, which favors that disulfide bridges are correctly formed during re-folding. A molecular weight of 18 kDa to the monomeric form and of 25-37 kDa to the oligomeric species were estimated by SDS-PAGE. IL-2 mutein showed similar far-UV CD spectral characteristic typical of cytokines with 41% of α-helix content. Batches obtained by Process B showed similar conformational features according near-UV CD and FS studies. However, those obtained by Process A differed in their folding. IL-2 mutein showed that conformational features by near-UV CD were affected by 2% SDS, no variations on secondary structure were observed. Melting temperature values by far-UV CD were higher than 95 °C, indicating a high thermal stability. Finally, the drug product obtained by Process B showed similar conformational characteristics by near-UV CD and FS, and higher biological activity values (7.0 × 10³ ng/mL) in the cell proliferation assay with respect to Process A. Also, the recovery was 15% higher than in the Process A and exhibited a 78.48% of purity. Indeed, Process B was selected for the purification.

Directed evolution of super-secreted variants from phage-displayed human Interleukin-2

Selection from a phage display library derived from human Interleukin-2 (IL-2) yielded mutated variants with greatly enhanced display levels of the functional cytokine on filamentous phages. Introduction of a single amino acid replacement selected that way (K35E) increased the secretion levels of IL-2-containing fusion proteins from human transfected host cells up to 20-fold. Super-secreted (K35E) IL-2/Fc is biologically active in vitro and in vivo, has anti-tumor activity and exhibits a remarkable reduction in its aggregation propensity- the major manufacturability issue limiting IL-2 usefulness up to now. Improvement of secretion was also shown for a panel of IL-2-engineered variants with altered receptor binding properties, including a selective agonist and a super agonist that kept their unique properties. Our findings will improve developability of the growing family of IL-2-derived immunotherapeutic agents and could have a broader impact on the engineering of structurally related four-alpha-helix bundle cytokines.

Molecular dissection of the interactions of an antitumor interleukin-2-derived mutein on a phage display-based platform

A mutein with stronger antitumor activity and lower toxicity than wild-type human interleukin-2 (IL-2) has been recently described. The rationale behind its design was to reinforce the immunostimulatory potential through the introduction of four mutations that would selectively disrupt the interaction with the IL-2 receptor alpha chain (thought to be critical for both IL-2-driven expansion of T regulatory cells and IL-2-mediated toxic effects). Despite the successful results of the mutein in several tumor models, characterization of its interactions was still to be performed. The current work, based on phage display of IL-2-derived variants, showed the individual contribution of each mutation to the impairment of alpha chain binding. A more sensitive assay, based on the ability of phage-displayed IL-2 variants to induce proliferation of the IL-2-dependent CTLL-2 cell line, revealed differences between the mutated variants. The results validated the mutein design, highlighting the importance of the combined effects of the four mutations. The developed phage display-based platform is robust and sensitive, allows a fast comparative evaluation of multiple variants, and could be broadly used to engineer IL-2 and related cytokines, accelerating the development of cytokine-derived therapeutics.

Folding and Purification of Insoluble (Inclusion Body) Proteins from Escherichia coli

Heterologous expression of recombinant proteins in E. coli often results in the formation of insoluble and inactive protein aggregates, commonly referred to as inclusion bodies. To obtain the native (i.e., correctly folded) and hence active form of the protein from such aggregates, four steps are usually followed: (1) the cells are lysed, (2) the cell wall and outer membrane components are removed, (3) the aggregates are solubilized (or extracted) with strong protein denaturants, and (4) the solubilized, denatured proteins are folded with concomitant oxidation of reduced cysteine residues into the correct disulfide bonds to obtain the native protein. This unit features three different approaches to the final step of protein folding and purification. In the first, guanidine·HCl is used as the denaturant, after which the solubilized protein is folded (before purification) in an "oxido-shuffling" buffer system to increase the rate of protein oxidation. In the second, acetic acid is used to solubilize the protein, which is then partially purified by gel filtration before folding; the protein is then folded and oxidized by simple dialysis against water. Thirdly, folding and purification of a fusion protein using metal-chelate affinity chromatography are described.

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

Enhancing enzymatic activity of penicillin G acylase by coexpressing pcm gene

Penicillin G acylase (PGA; E.C. 3.5.1.11) is an important enzyme which has broad applications in industries of beta-lactim antibiotics production. In this study, a promising PGA gene from Alcaligenes faecalis (afpga) and another pcm gene encoding protein isoaspartate methyltransferase (PIMT) were constructed into pET43.1a((+)) and pET28a((+)), respectively. The recombinant plasmids pETAFPGA and pETPCM were transformed into the same host cell Escherichia coli BL21 (DE3). Results suggested that the two plasmids could peacefully exist in the host cell and the two genes could be efficiently expressed after induction. The product of pcm gene could function as a helper molecule for enzyme AFPGA. PIMT increased the enzymatic activities in supernatant of ferment broth (1.6 folds) and cell lysate (1.8 folds), while it did not significantly affect the expression level of penicillin G acylase.

Increase of soluble expression in Escherichia coli cytoplasm by a protein disulfide isomerase gene fusion system

Human protein disulfide isomerase (PDI) was selected as a fusion partner to construct a gene expression system to enhance the solubility of recombinant protein in Escherichia coli. DREBIII-1, a plant specific transcriptional factor, was found to mainly form inclusion bodies when expressed in either His-tagged or GST-fusion systems in E. coli. In contrast, when fused with PDI, the expressed DREBIII-1 was in a highly soluble and biologically active form. Two fusion proteins, HDP and HPD, were generated by positioning DREBIII-1 at the N-terminal and C-terminal of PDI, respectively. After purification, HDP exhibited a higher stability and showed only one band on SDS-PAGE, while HPD degraded as several bands. HDP was verified to have the biological function of PDI by isomerase activity assay; meanwhile, it also presented the DNA binding and transcriptional activation characteristic of DREBIII-1 in fluorescence quenching and yeast one-hybrid experiments. The PDI fusion expression system was demonstrated to be highly efficient in generating not only soluble but functional desired proteins.

Comparative analysis of GFPUV-β1,3-N-acetylglucosaminyltransferase 2 production in two insect-cell-based expression systems

Active beta1,3-N-acetylglucosaminyltransferase 2 (beta3GnT2) was produced in the baculovirus expression system (BES) and in stably transformed insect Tn-5B1-4 cells. beta3GnT2 was expressed as a secreted fusion protein with GFP(UV) with three different types of signal sequence to enhance the secretion of the fusion protein. In the stably transformed cells, the maximal beta3GnT2 activity differed between isolates, but their secretion efficiencies were similar. The difference between the maximal beta3GnT activities of the isolates studied was considered to be due to the presence of a copy number of the fusion gene, as determined on the basis of the results of Southern blot analysis. The beta3GnT activities of the culture supernatant in BES (Tn-5B1-4 cells) without or with the addition of the protease inhibitor, leupeptin, were 0.68 and 2.01 mU/ml, respectively. The stably transformed Tn-5B1-4 cells (Tn-pXme11) exhibited a beta3GnT activity of 6.83 mU/ml, which was 3.4-fold higher than that observed for BES with the leupeptin addition. The purity of fusion protein purified from the culture supernatant of the Tn-pXme11 was higher than 95% on SDS-PAGE, in contrast with that purified from the culture supernatant of the baculovirus-infected cells which contained low-molecular-weight fragments of the fusion protein. The stably transformed cell line is more suitable than BES for the efficient production of the secretory protein, beta3GnT2.

Reverse micellar extraction for downstream processing of proteins/enzymes

New developments in the area of downstream processing are, hopefully, to fulfill the promises of modern biotechnology. The traditional separation processes such as chromatography or electrophoresis can become prohibitively expensive unless the product is of high value. Hence, there is a need to develop efficient and cost-effective downstream processing methods. Reverse micellar extraction is one such potential and a promising liquid-liquid extraction technique, which has received immense attention for isolation and purification of proteins/enzymes in the recent times. This technique is easy to scale-up and offers continuous operation. This review, besides briefly considering important physico-chemical and biological aspects, highlights the engineering aspects including mass transfer, mathematical modeling, and technology development. It also discusses recent developments in reverse micellar extraction such as affinity based separations, enzymatic reactions in reverse micelles coupled with membrane processes, reverse micellar extraction in hollow fibers, etc. Special emphasis has been given to some recent applications of this technique.

Overexpression, oxidative refolding, and zinc binding of recombinant forms of the murine S100 protein MRP14 (S100A9)

Recombinant murine MRP14 (mMRP14) was produced in Escherichia coli using the pGEX expression system. The mass of fusion protein, by electrospray ionization-mass spectrometry (ESI/MS), was 39,213 Da which compares well with the theoretical mass (39,210.4 Da). Thrombin digestion of fusion protein was expected at a cloned thrombin consensus sequence (. LVPRGS. ) located between glutathione S-transferase and mMRP14. Analysis of products of digestion by C4 reverse-phase HPLC and SDS-PAGE/Western blotting revealed two immunoreactive cleavage products with molecular weights around 13, 000. Masses of the two proteins determined by ESI/MS were 13,062 and 11,919 Da. The larger product corresponded to the expected mass of recombinant mMRP14 (13,061.9 Da). Analysis of the protein sequence of recombinant mMRP14 revealed a thrombin-like consensus sequence (. NNPRGH. ) located close to the C-terminus. The smaller protein corresponded to a truncated form of rec mMRP14 (rec MRP141-102) with a calculated mass of 11,918.6 Da. Optimization of the cleavage conditions resulted in >95% full-length rec mMRP14. Native mMRP14 contains one intramolecular disulfide bond between Cys79 and Cys90. The full-length recombinant protein was renatured and oxidized in ammonium acetate (pH approximately 7) for 96 h and formed >95% of the native intramolecular disulfide-bonded form. MRP141-102 bound substantially less 65Zn2+ compared to native mMRP14 or rec mMRP14 after transfer to polyvinylidene difluoride and incubation with 65ZnCl2, implicating the His residues located within the C-terminal domain in Zn2+ binding.

Improved folding yields of a model protein using protein disulfide isomerase

PURPOSE

To study the effects of recombinant human protein disulfide isomerase (rhPDI) concentration, reduced glutathione:oxidized glutathione ratio (GSH:GSSG) and temperature on the efficiency of oxidative folding of a model protein, recombinant human interleukin 2 (C125A mutation) (C125A rhIL-2).

METHODS

C 125A rhIL-2 inclusion bodies were reduced and denatured by guanidium hydrochloride (Gdm.Cl) and 100 mM GSH. The solution was diluted 10 times into folding buffer, allowing C125A rhIL-2 to fold either in the absence or presence of rhPDI. The renatured and unfolded C125A rhIL-2 species were quantitated by reversed phase-HPLC.

RESULTS

The initial folding rate of C125A rhIL-2 linearly increased with rhPDI:C125A rhIL-2 molar ratio in the first 2.5 minutes, and reached the highest rate when the rhPDI:C125A rhIL-2 ratio was 1:1. The oxidative folding of C125A rhIL-2 linearly increased as the GSH:GSSG molar ratio decreased from 10:0 to 10:3. The folding of C125A rhIL-2 was also dependent on temperature, and optimum folding was realized at 23 degrees C.

CONCLUSIONS

These results demonstrate that under optimal redox potential and temperature, rhPDI enhances the oxidative folding of C125A rhIL-2. In the oxidative folding of C125A rhIL-2, rhPDI exerts its effect on folding by the acceleration of thiol/disulfide interchange.

Inclusion bodies and purification of proteins in biologically active forms

Even though recombinant DNA technology has made possible the production of valuable therapeutic proteins, its accumulation in the host cell as inclusion body poses serious problems in the recovery of functionally active proteins. In the last twenty years, alternative techniques have been evolved to purify biologically active proteins from inclusion bodies. Most of these remain only as inventions and very few are commercially exploited. This review summarizes the developments in isolation, refolding and purification of proteins from inclusion bodies that could be used for vaccine and non-vaccine applications. The second section involves a discussion on inclusion bodies, how they are formed, and their physicochemical properties. In vivo protein folding in Escherichia coli and kinetics of in vitro protein folding are the subjects of the third and fourth sections respectively. The next section covers the recovery of bioactive protein from inclusion bodies: it includes isolation of inclusion body from host cell debris, purification in denatured state alternate refolding techniques, and final purification of active molecules. Since purity and safety are two important issues in therapeutic grade proteins, the following three sections are devoted to immunological and biological characterization of biomolecules, nature, and type of impurities normally encountered, and their detection. Lastly, two case studies are discussed to demonstrate the sequence of process steps involved.

Uncoupling of stem cell inhibition from monocyte chemoattraction in MIP-1alpha by mutagenesis of the proteoglycan binding site

We have studied the role of proteoglycans in the function of Macrophage Inflammatory Protein-1 alpha (MIP-1alpha), a member of the proteoglycan binding chemokine family. Sequence and peptide analysis has identified a basic region within MIP-1alpha which appears to be the major determinant of proteoglycan binding and we have now produced a mutant of MIP-1alpha lacking the basic charges on two of the amino acids within this proteoglycan binding site. This mutant (Hep Mut) appears to have lost the ability to bind to proteoglycans. Bioassay of Hep Mut indicates that it has retained stem cell inhibitory properties but has a compromised activity as a monocyte chemoattractant, thus suggesting uncoupling of these two properties of MIP-1alpha. Receptor studies have indicated that the inactivity of Hep Mut on human monocytes correlates with its inability to bind to CCR1, a cloned human MIP-1alpha receptor. In addition, studies using proteoglycan deficient cells transfected with CCR1 have indicated that the proteoglycan binding site in MIP-1alpha is a site that is also involved in the docking of MIP-1alpha to the monocyte receptor. The site for interaction with the stem cell receptor must therefore be distinct, suggesting that MIP-1alpha utilizes different receptors for these two different biological processes.

Single-chain antibody variable region-targeted interleukin-2 stimulates T cell killing of human colorectal carcinoma cells

Recombinant DNA techniques were used to clone, construct and express the bifunctional molecule FV/IL-2. The FV/IL-2 is a single-chain 37 kDa fusion protein expressed in Escherichia coli under control of the strong T7 bacteriophage promoter in the expression vector pT7-7-FV-IL-2. The fused gene fragment FV-IL-2 contains a single-chain anti-TAG72 FV gene fragment and the human recombinant cDNA fragment of the IL-2 molecule. The renatured soluble form of FV/IL-2 was purified from E. coli inclusion bodies using hydroxylapatite chromatography. The yield of this fusion protein was estimated at 2.0 mg/L. Our data showed that the FV/IL-2 molecule retained the TAG72 antigen-binding specificity and the IL-2 activity as measured in the standard T cell proliferation as well as cytotoxicity assays. Therefore, it may prove to be useful in targeting the biological effect of IL-2 to tumour cells and stimulating its immune destruction.

Renaturation of recombinant proteins produced as inclusion bodies

Expression of recombinant proteins in Escherichia coli often results in the formation of insoluble inclusion bodies. Within the last few years specific methods and strategies have been developed to prepare active proteins from these inclusion bodies. These methods include (i) isolation of inclusion bodies after disintegration of cells by mechanical forces and purification by washing with detergent solutions or low concentrations of denaturant, (ii) solubilization of inclusion bodies with high concentrations of urea or guanidine-hydrochloride in combination with reducing reagents, and (iii) renaturation of the proteins including formation of native disulphide bonds. Renatured and native disulphide bond formation are accomplished by (a) either air oxidation, (b) glutathione reoxidation starting from reduced material, or (c) disulphide interchange starting from mixed disulphides containing peptides. The final yield of renatured proteins can be increased by adding low concentrations of denaturant during renaturation.

Secondary structure of human interleukin 2 from 3D heteronuclear NMR experiments

Recombinant 15N-labeled human interleukin 2 (IL-2) has been studied by 2D and 3D NMR using uniformly 15N-labeled protein. Assignment of the backbone resonances has enabled the secondary structure of the protein to be defined. The secondary structure was found to consist of four alpha-helical regions and a short section of antiparallel beta-sheet. This structure is more similar to recent published structures of interleukin 4 and granulocyte-macrophage colony-stimulating factor than to a structure of IL-2 previously obtained from low-resolution X-ray diffraction data.

Cloning and expression of the variable regions of mouse myeloma protein MOPC315 in E. coli: recovery of active FV fragments

Recombinant DNA techniques were used to clone and express the FV portion of MOPC315, a mouse myeloma protein with a high affinity for 2,4-dinitrophenyl (DNP). The FV fragment consists of a heterodimer of heavy and light chain variable domains (VH and VL). Two separate bacterial plasmid constructs, containing either a variable region cDNA for the light chain or a variable region synthetic gene for the heavy chain demonstrated high levels of expression (150-200 mg/L) under control of the bacteriophage T7 promoter. Recombinant chains were initially recovered as inclusion bodies and then dissolved separately in 8 M urea, combined together, and refolded by subsequent chaotrope removal. Biologically active FV was affinity purified from the chain mixture by specific binding to DNP-lysine-Sepharose. Yields of active material as high as 20% were obtained with activity confirmed by fluorescence quench analysis. The purified FV displayed a binding affinity of 4.8 +/- 0.3 x 10(-7) M which was identical to the native FV. Chimeric FVs composed of recombinant and native chain mixtures yielded similar results. Recombinant MOPC315 FV activity was also obtained using a single chain construct (sFV), in which recombinant VH and VL were linked via a (Gly4Ser)3 spacer region. Binding affinity of the sFV was shown to be the same as the recombinant and native FVs. The ease of purification and characterization of active MOPC315 as the recombinant and native FVs. The ease of purification and characterization of active MOPC315 FV makes this system useful in the study of the optimization of antibody production in bacteria.

Predicting the Solubility of Recombinant Proteins in Escherichia coli

We have studied the cause of inclusion body formation in Escherichia coli grown at 37 degrees C using statistical analysis of the composition of 81 proteins that do and do not form inclusion bodies. Six composition derived parameters were used. In declining order of their correlation with inclusion body formation, the parameters are charge average, turn forming residue fraction, cysteine fraction, proline fraction, hydrophilicity, and total number of residues. The correlation with inclusion body formation is strong for the first two parameters but weak for the last four. This correlation can be used to predict the probability that a protein will form inclusion bodies using only the protein's amino acid composition as the basis for the prediction.

Monitoring of protein conformation by high-performance size-exclusion liquid chromatography and scanning diode array second-derivative UV absorption spectroscopy

Genetic methods now allow the rapid production of mutant proteins for structure-function analysis. To properly interpret any change in biologic activity resulting from modification in primary sequence, it is essential to monitor conformational changes resulting from mutations. Several methods allow low-resolution protein conformational analysis. One method, second-derivative UV absorption spectroscopy, is particularly useful for proteins containing tyrosine and/or tryptophan residues. Using high-performance size-exclusion liquid chromatography and scanning diode array detection we have demonstrated that it is possible to monitor the degree of aggregation as well as conformational perturbation for a series of interleukin-2 structural mutants. Furthermore, the combination of high-performance liquid chromatography and second-derivative UV absorption spectroscopy avoids a potential artifactual contribution in non-chromatographic analysis due to protein aggregation.

The use of sarkosyl in generating soluble protein after bacterial expression

Actin, like many other proteins, is highly insoluble after expression in Escherichia coli. In order to understand the origin of insoluble aggregates, we asked whether morphological inclusions were always correlated with insolubility. The strain expressing actin was compared to one that expresses part of the myosin tail; the latter strain yields soluble protein after various cell lysis or disruption procedures. Morphological inclusions were observed in both strains, indicating there is no obligate relationship between solubility and inclusions. Studies presented here suggest that extreme insolubility results from coaggregation of the actin with bacterial outer membrane components upon bacterial lysis. The properties of the outer membrane have been exploited in the development of nondenaturing procedures that yield soluble actin. One procedure involves the disruption of coaggregates with sarkosyl detergent (N-laurylsarcosine); another prevents the formation of coaggregates by lysing in the presence of sarkosyl. These methods may be useful for other proteins that become insoluble after bacterial expression.

Solubility as a function of protein structure and solvent components

This review deals with ways of stabilizing proteins against aggregation and with methods to determine, predict, and increase solubility. Solvent additives (osmolytes) that stabilize proteins are listed with a description of their effects on proteins and on the solvation properties of water. Special attention is given to areas where solubility limitations pose major problems, as in the preparation of highly concentrated solutions of recombinant proteins for structural determination with NMR and X-ray crystallography, refolding of inclusion body proteins, studies of membrane protein dynamics, and in the formulation of proteins for pharmaceutical use. Structural factors relating to solubility and possibilities for protein engineering are analyzed.

Interleukin 1 responsiveness and receptor expression by murine TH1 and TH2 clones

Murine Th1 and Th2 T cell lines differ in their responses to interleukin 1 (IL 1). Therefore, we examined two T-cell lines, D10.G4.1 (Th2) and MTg12B (Th1) in an attempt to correlate IL 1 receptor (IL 1R) expression with their IL 1 responsiveness. D10.G4.1 cells, which respond to IL 1, expressed two forms of the IL 1R, with molecular masses of approximately 80 kDa and approximately 60 kDa. In contrast, MTg12B cells failed to respond to IL 1 and only expressed the approximately 60 kDa receptor form. This suggests that the approximately 80 kDa receptor is essential for signaling. Expression of both IL 1R forms on D10.G4.1 cells could be inhibited by the anti-IL 4 antibody, 11B11. Antigen presentation reversibly upregulated both forms of the IL 1R, whereas stimulation with concanavalin A (ConA) and anti-CD3 only upregulated the approximately 60 kDa moiety. Upregulation of the approximately 80-kDa IL 1R by repeated antigenic stimulation resulted in a marked increase in sensitivity of D10.G4.1 cells to IL 1.

Effects of temperature on Escherichia coli overproducing beta-lactamase or human epidermal growth factor

The effects of temperature on strains of Escherichia coli which overproduce and excrete either beta-lactamase or human epidermal growth factor were investigated. E. coli RB791 cells containing plasmid pKN which has the tac promoter upstream of the gene for beta-lactamase were grown and induced with isopropyl-beta-D-thiogalactopyranoside in batch culture at 37, 30, 25, and 20 degrees C. The lower temperature greatly reduced the formation of periplasmic beta-lactamase inclusion bodies, increased significantly the total amount of beta-lactamase activity, and increased the purity of extracellular beta-lactamase from approximately 45 to 90%. Chemostat operation at 37 and 30 degrees C was difficult due to poor cell reproduction and beta-lactamase production. However, at 20 degrees C, continuous production and excretion of beta-lactamase were obtained for greater than 450 h (29 generations). When the same strain carried plasmid pCU encoding human epidermal growth factor, significant cell lysis was observed after induction at 31 and 37 degrees C, whereas little cell lysis was observed at 21 and 25 degrees C. Both total soluble and total human epidermal growth factor increased with decreasing temperature. These results indicate that some of the problems of instability of strains producing high levels of plasmid-encoded proteins can be mitigated by growth at lower temperatures. Further, lower temperatures can increase for at least some secreted proteins both total plasmid-encoded protein formed and the fraction that is soluble.

Purification and characterization of recombinant human interleukin 4. Biological activities, receptor binding and the generation of monoclonal antibodies

A synthetic gene coding for human interleukin 4 (IL-4) was cloned and expressed in Saccharomyces cerevisiae (baker's yeast) as a C-terminal fusion protein with the yeast prepro alpha-mating factor sequence, resulting in secretion of mature IL-4 into the culture medium (0.6-0.8 micrograms/ml). A protocol was developed for purification of this protein. Crude cell-free conditioned medium was passed over a concanavalin A-Sepharose affinity column; bound proteins were eluted and further purified by S-Sepharose Fast Flow cation exchange and C18 reverse-phase h.p.l.c. Highly purified IL-4 was obtained by this method (0.3-0.4 mg per litre of culture) with a recovery of 51%. Thermospray liquid chromatography-mass spectrometry showed the C-terminal N-glycosylation site to be largely unmodified, and also showed that the N-terminus of the purified recombinant IL-4 (rIL-4) was authentic. Thiol titration revealed no free cysteine residues, implying that there are three disulphide groups, the positions of which remain to be determined. We have characterized the biological activities of the purified rIL-4. This material is active in B-cell co-stimulator assays, T-cell proliferation assays and in the induction of cell-surface expression of CD23 (the low-affinity receptor for IgE) on tonsillar B-cells. Half-maximal biological activity of the rIL-4 was achieved at a concentration of 120 pM. We have radioiodinated rIL-4 without loss of biological activity and performed equilibrium binding studies on Raji cells, a human B-cell line. The 125I-rIL-4 bound specifically to a single class of binding studies on Raji cells, a human B-cell line. The 125I-rIL-4 bound specifically to a single class of binding site with high affinity (Kd = 100 pM) and revealed 1100 receptors per cell. Receptor-ligand cross-linking studies demonstrated a single cell-surface receptor with an apparent molecular mass of 124 kDa. Two monoclonal antibodies have been raised to the human rIL-4, one of which blocks both the biological activity of rIL-4 and binding to its receptor.

The analysis of recombinant interleukin-2 by thermospray liquid chromatography-mass spectrometry

The complete sequence of recombinant human interleukin-2 expressed in Escherichia coli has been confirmed by thermospray liquid chromatography-mass spectrometry (TS-LC-MS) of a tryptic digest derived from 100 micrograms (7 nmol) of reduced carboxymethylated interleukin-2. The preparation was shown by this method to contain predominantly unprocessed N-terminal initiator Met, with some authentic N-terminal Ala; the rest of the protein was as predicted from the DNA sequence, though some deamidated material was noted. TS-LC-MS proved to be a rapid and efficient method for surveying the protein tryptic peptide products allowing all the data to be collected in one chromatographic run; all tryptic fragments were identified by their molecular ions including those for the larger peptides (Mr 1500-3500) which, due to the presence of doubly and triply charged molecular ions, were brought within the mass range of the instrument (1800 Da). It is proposed that TS-LC-MS is a good general method for analyzing recombinant protein digests with respect to sequence confirmation, processing, and post-translational modification, and since each chromatographic peak is identified allows for subsequent monitoring of the protein by LC using uv detection. The method suffers from the disadvantages that all the sample is consumed during the experiment and that no fragment (sequence) ions are generally observed.