Expression and characterization of soluble human erythropoietin receptor made in Streptomyces lividans 66

Streptomycetes: Attractive Hosts for Recombinant Protein Production

Enzymes are increasingly applied as biocatalysts for fulfilling industrial needs in a variety of applications and there is a bursting of interest for novel therapeutic proteins. Consequently, developing appropriate expression platforms for efficiently producing such recombinant proteins represents a crucial challenge. It is nowadays widely accepted that an ideal ‘universal microbial host’ for heterologous protein expression does not exist. Indeed, the first-choice microbes, as Escherichia coli or yeasts, possess known intrinsic limitations that inevitably restrict their applications. In this scenario, bacteria belonging to the Streptomyces genus need to be considered with more attention as promising, alternative, and versatile platforms for recombinant protein production. This is due to their peculiar features, first-of-all their natural attitude to secrete proteins in the extracellular milieu. Additionally, streptomycetes are considered robust and scalable industrial strains and a wide range of tools for their genetic manipulation is nowadays available. This mini-review includes an overview of recombinant protein production in streptomycetes, covering nearly 100 cases of heterologous proteins expressed in these Gram-positives from the 1980s to December 2019. We investigated homologous sources, heterologous hosts, and molecular tools (promoters/vectors/signal peptides) used for the expression of these recombinant proteins. We reported on their final cellular localization and yield. Thus, this analysis might represent a useful source of information, showing pros and cons of using streptomycetes as platform for recombinant protein production and paving the way for their more extensive use in future as alternative heterologous hosts.

Streptomyces coelicolor sRNA scr5239 inhibits agarase expression by direct base pairing to the dagA coding region

Transcriptional regulation of primary and secondary metabolism is well-studied in Streptomyces coelicolor, a model organism for antibiotic production and cell differentiation. In contrast, little is known about post-transcriptional regulation and the potential functions of small non-coding RNAs (sRNAs) in this Gram-positive, GC-rich soil bacterium. Here, we report the identification and characterization of scr5239, an sRNA highly conserved in the genus Streptomyces. The sRNA is 159 nt long, composed of five stem-loops, and encoded in the intergenic region between SCO5238 and SCO5239. scr5239 expression is constitutive under several stress and growth conditions but dependent on the nitrogen supply. scr5239 decreases the production of the antibiotic actinorhodin, and represses expression of the extracellular agarase dagA at the post-transcriptional level by direct base pairing to the coding region 33 nt downstream of the ribosome-binding site.

Deep sequencing-based identification of small non-coding RNAs in Streptomyces coelicolor

Streptomyces coelicolor is considered the model organism among Gram positive, GC rich bacteria. Its genome has been sequenced but little is known about the occurrence and distribution of small non-coding RNAs in this biotechnologically relevant organism. Using deep sequencing we analyzed the transcriptome at the end of exponential growth, which corresponds to the onset of secondary metabolism. We mapped 193 transcriptional start sites of mRNA genes and identified putative new and alternative open reading frames. We identified 63 non-coding RNAs including 29 cis encoded antisense RNAs, and confirmed expression for 11, most of them being growth-phase dependent. A comparison between the sequencing results and bioinformatic sRNA predictions using Dynalign and RNAz revealed only a small overlap between the different approaches.

Secretory production of recombinant proteins by Streptomyces

Bacterial systems are widely applied as production platforms for proteins of biopharmaceutical or therapeutic interest and industrial enzymes. Among these prokaryotic systems, streptomycetes are attractive host cells because several strains of these Gram-positive bacteria have a high innate secretion capacity and extensive knowledge on their fermentation is available. A survey of the literature and our own experience suggests that several proteins are secreted to commercially acceptable levels. However, many heterologous proteins, most often of eukaryotic origin, are currently only poorly secreted by this host, indicating the need for further optimization of Streptomyces as a production host. In this review, the considerable efforts and strategies made in recent years aimed at improving streptomycetes as a host for the production of recombinant proteins will be discussed.

Functional large-scale production of a novel Jonesia sp. xyloglucanase by heterologous secretion from Streptomyces lividans

The gene encoding a novel xyloglucanase (Xeg) belonging to family 74 glycoside hydrolases was isolated from a Jonesia sp. strain through functional screening in Escherichia coli. The encoded xyloglucanase is a protein of 972 aminoacyl residues with a 23 residue aminoterminal signal peptide. Over-expression of Xeg in B. subtilis or E. coli failed. In contrast, Xeg was successfully over-expressed and secreted in Streptomyces lividans TK24. To this end Xeg was fused C-terminally to the secretory signal peptide of the subtilisin inhibitor protein (vsi) from Streptomyces venezuelae. The native Xeg signal peptide derived from Jonesia sp. is only poorly functional in S. lividans. Under optimal growth conditions, significant amounts of mature Xeg (100-150 mg/l) are secreted in the spent growth media. A protocol to rapidly purify Xeg to homogeneity from culture supernatants was developed. Biophysical and biochemical analyses indicate that the enzyme is intact, stable and fully functional. Xeg is the longest heterologous polypeptide shown to be secreted from S. lividans. This study further validates use of S. lividans for production of active heterologous proteins and demonstrates that heterologous polypeptides of up to 100 kDa are also tractable by this system.

Cloning, expression, and purification of soluble human interleukin-4 receptor in Streptomyces

Interleukin-4 (IL-4) is a pleiotropic cytokine which plays a pivotal role in shaping immune responses and mediating important proinflammatory functions in asthma. This cytokine exerts its biological effects through binding to its receptor (IL-4R) complex, with the alpha chain as the high-affinity binding subunit. Soluble IL-4R (sIL-4R) lacks the transmembrane and cytoplasmic domains, so it cannot induce cellular activation. By acting as a decoy to circulating IL-4 and neutralizing its activity, the high specificity and affinity of sIL-4R make it ideal as an IL-4 antagonist. In this study, a sIL-4R cDNA encoding the extracellular domain of IL-4R alpha chain was cloned into a Streptomyces-Escherichia coli shuttle plasmid pSGLgpp and expressed secretly in Streptomyces lividans TK 24. On SDS-PAGE gel, the expressed sIL-4R protein showed a Mw of 24 kDa, agreeable with the predicted size. The N-terminal sequence of the protein was also determined, confirming its identity and indicating that no degradation occurred at the N-terminus. With DEAE-Sepharose Fast Flow and Superdex HR 75 columns, the protein was purified and used on HPLC analysis, the purity reaching about 90%. The results of the ligand-binding blot and ELISA showed that such protein has biological activity of binding with ligand IL-4.

Purification and characterization of recombinant Streptomyces griseus aminopeptidase

Recombinant Streptomyces griseus aminopeptidase (SGAP) was produced using Cangene's expression system, CANGENUS. This heat-stable aminopeptidase with an N-terminal Ala-Pro-Asp-Ile-Pro-Leu-Ala-Asn-Val-Lys-Ala sequence was purified from 16L of Streptomyces lividans fermentation supernatant with high purity and 19.5% recovery rate. This was achieved by the combination of hydrophobic-interaction and size-exclusion chromatographic procedures. The calcium-activated zinc metalloprotein demonstrated no loss of activity at -20 degrees C for at least 8 weeks in both liquid and freeze-dried formulations. The recombinant SGAP showed an apparent molecular mass of 31 kDa by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and 26.8 kDa by gel filtration. The simple, high-yield, inexpensive purification method with few intermediate steps provides a novel and practical procedure for large-scale production of active recombinant S. griseus aminopeptidase.

Osmotic regulation of the Streptomyces lividans thiostrepton-inducible promoter, ptipA

Transcriptional activation of the thiostrepton-inducible promoter, ptipA, in Streptomyces lividans is mediated by TipAL. This transcriptional activator belongs to the MerR/SoxR family that characteristically binds an operator sequence located between the -10 and -35 hexamers normally occupied by RNA polymerase. As for the Escherichia coli merT promoter, the ptipA hexamers are separated by a long 19 bp spacer and hence a topological transition of the DNA is likely to be a requisite for alignment with RNA polymerase. Growth conditions that could facilitate this conformational change were investigated using transcriptional fusions of ptipA with reporter genes. Adjustment of growth medium osmolarity led to increased and prolonged TipAL-dependent expression, both with and without the inducer, thiostrepton. These effects correlated with increases in negative DNA supercoiling. Moreover, an inability to induce the promoter with thiostrepton in strain TK64 was corrected by increasing the concentration of osmolyte, compensating for an apparent reduced level of negative DNA supercoiling in the strain. Prolonging the time of activation of tipA in the wild-type by manipulating growth conditions revealed that mycelial autolysis could be induced by thiostrepton in 4-d-old cultures.

A high-throughput assay to identify compounds that can induce dimerization of the erythropoietin receptor

Erythropoietin induces dimerization of the erythropoietin receptor on the surface of erythroid progenitor cells, promoting the differentiation of these cells into mature red blood cells. To facilitate screening of large chemical collections for identification of compounds that can dimerize erythropoietin receptor, we have developed a novel, high-throughput in vitro assay to detect compounds that can cause dimerization of the erythropoietin receptor in solution. To develop this assay, amino acid sequences corresponding to the extracellular domain of erythropoietin receptor were expressed in Escherichia coli as erythropoietin-binding protein (rEBP). A modified version of this protein ((33)P-rEBP) containing a protein kinase A substrate site incorporated into the rEBP was also expressed in E. coli and labeled in vitro using protein kinase A and ¿gamma-(33)PATP. An erythropoietin mimetic peptide (EMP-1), that induces dimerization of rEBP in solution was used to demonstrate dimerization of (33)P-rEBP and rEBP in a 96-well microtiter plate format. EMP-1 induced dimerization of rEBP in this assay with an EC(50) of approximately 245 nM and had a maximal effect at 0.5-2 microM and required the presence of rEBP immobilized on the plate capable of binding EMP-1. EMP-1-induced dimerization of (33)P-rEBP and rEBP was reversed by excess unlabeled rEBP and was not masked by complex mixtures such as whole cell fungal extracts. These data demonstrate the ability of (33)P-rEBP to dimerize with rEBP in vitro in a format that is fully compatible with high-throughput screening.