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.

A novel quinone-forming monooxygenase family involved in modification of aromatic polyketides

RppA is a type III polyketide synthase (PKS) that catalyzes condensation of five molecules of malonyl-CoA to form 1,3,6,8-tetrahydroxynaphthalene (THN). In Streptomyces antibioticus IFO13271 and several other Streptomyces species, an open reading frame, named momA, is present as a neighbor of rppA. MomA belonged to the "cupin" superfamily because it contained a set of two motifs that is responsible for binding one equivalent of metal ions. MomA catalyzed monooxygenation of the THN produced from malonyl-CoA by the action of RppA to form flaviolin. In addition, it used several polyketides as substrates and formed the corresponding quinones. MomA required redox-active transition metal ions (Ni(2+), Cu(2+), Fe(3+), Fe(2+), Mn(2+), and Co(2+)) for its activity, whereas it was inhibited by a redox-inert transition metal ion (Zn(2+)). MomA neither possessed any flavin prosthetic group nor required nicotinamide cofactors for monooxygenation, which shows that MomA as a member of the cupin superfamily is a novel monooxygenase. Consistent with the catalytic property of MomA, WhiE-ORFII showing similarity in amino acid sequence to MomA and containing a cupin domain also catalyzed monooxygenation of THN. whiE-ORFII is located immediately upstream of the "minimal PKS" gene within the whiE type II PKS gene cluster for biosynthesis of a gray spore pigment in Streptomyces coelicolor A3(2), and a number of whiE-ORFII homologues are present in the biosynthetic gene cluster for polyketides of type II in various Streptomyces species. These findings show that a novel class of quinone-forming monooxygenases is involved in modification of aromatic polyketides synthesized by PKSs of types II and III.

Cloning, characterization and phylogenetic relationships of cel5B, a new endoglucanase encoding gene from Thermobifida fusca

Thermobifida fusca, a thermophilic, aerobic, cellulolytic bacterium has a highly complex cellulase system comprising three endoglucanases, two exoglucanases and one processive endoglucanase. Zymogram analysis indicated that additional cellulases may exist in T. fusca strain TM51, therefore a TM51 expression library was prepared in Streptomyces lividans TK24 and screened for hydrolases. A new endoglucanase gene, named Tf cel5B, was identified. Heterologous Cel5B, produced in S. lividans, had temperature and pH optima of 77 degrees C and 8.2, respectively and retained more than 60% of its activity after 24 h incubation at 60 degrees C. Domain analysis revealed an N-terminal catalytic domain with homology to known endoglucanases in family GH5 and a C-terminal cellulose binding module III domain (CBD). Comparing the domain structures of all seven known T. fusca cellulases showed, that the cellulase system of this organism consists of pairs of enzymes from the same GH family, including Cel5A--Cel5B, Cel6A--Cel6B and Cel9A--Cel9B plus a single family GH48 enzyme (Cel48A). Furthermore, the catalytic and substrate binding domains of enzymes, belonging to the same GH family were arranged in opposite orientations. Phylogenetic comparisons of the catalytic domain sequences of the T. fusca cellulases to other family GH5, GH6, GH9 and GH48 cellulases of bacterial origin revealed that the enzyme pairs in the same GH family are not closely related to each other, instead they showed significant similarities to various cellulase enzymes from taxonomically distinct organisms. Therefore, the complex and highly efficient cellulase system of T. fusca seems to be evolved as a result of horizontal gene transfers rather than gene duplication events.

Comparison of the Sec and Tat secretion pathways for heterologous protein production by Streptomyces lividans

Streptomyces is an interesting host for the secretory production of recombinant proteins because of its natural ability to secrete high levels of active proteins into the culture broth and the availability of extensive fermentation knowledge. In bacterial expression systems, heterologous protein secretion has, so far, almost exclusively been investigated using signal peptides that direct the secretion to the Sec pathway. In this study, we assessed the possibility of the Streptomyces lividans twin-arginine translocation (Tat) pathway to secrete the human proteins tumor necrosis factor (TNF) alpha and interleukin (IL) 10 by fusing the coding sequences of mature hTNFalpha and hIL10 to the twin-arginine signal peptides of S. lividans xylanase C (XlnC) and Streptomyces antibioticus tyrosinase. Both proteins were secreted and this secretion was blocked in the DeltatatB and DeltatatC single mutants, indicating that the transport of hTNFalpha and hIL10 could be directed through the Tat pathway. Secretion levels of hTNFalpha and hIL10, however, were lower for Tat-dependent than for Sec-dependent transport using the Sec-dependent signal peptide of the Streptomyces venezuelae subtilisin inhibitor. Surprisingly, Sec-dependent transport was enhanced in the tatB deletion strain. This was especially interesting in the case of hIL10, where Sec-dependent transport of hIL10 was at least 15 times higher in the DeltatatB mutant than in the wild-type strain.

Functional cross-talk between fatty acid synthesis and nonribosomal peptide synthesis in quinoxaline antibiotic-producing streptomycetes

Quinoxaline antibiotics are chromopeptide lactones embracing the two families of triostins and quinomycins, each having characteristic sulfur-containing cross-bridges. Interest in these compounds stems from their antineoplastic activities and their specific binding to DNA via bifunctional intercalation of the twin chromophores represented by quinoxaline-2-carboxylic acid (QA). Enzymatic analysis of triostin A-producing Streptomyces triostinicus and quinomycin A-producing Streptomyces echinatus revealed four nonribosomal peptide synthetase modules for the assembly of the quinoxalinoyl tetrapeptide backbone of the quinoxaline antibiotics. The modules were contained in three protein fractions, referred to as triostin synthetases (TrsII, III, and IV). TrsII is a 245-kDa bimodular nonribosomal peptide synthetase activating as thioesters for both serine and alanine, the first two amino acids of the quinoxalinoyl tetrapeptide chain. TrsIII, represented by a protein of 250 kDa, activates cysteine as a thioester. TrsIV, an unstable protein of apparent Mr about 280,000, was identified by its ability to activate and N-methylate valine, the last amino acid. QA, the chromophore, was shown to be recruited by a free-standing adenylation domain, TrsI, in conjunction with a QA-binding protein, AcpPSE. Cloning of the gene for the QA-binding protein revealed that it is the fatty acyl carrier protein, AcpPSE, of the fatty acid synthase of S. echinatus and S. triostinicus. Analysis of the acylation reaction of AcpPSE by TrsI along with other A-domains and the aroyl carrier protein AcmACP from actinomycin biosynthesis revealed a specific requirement for AcpPSE in the activation and also in the condensation of QA with serine in the initiation step of QA tetrapeptide assembly on TrsII. These data show for the first time a functional interaction between nonribosomal peptide synthesis and fatty acid synthesis.

Isolation ofmak1 fromActinoplanes missouriensis and evidence that Pep2 fromStreptomyces coelicolor is a maltokinase

The gene mak1FN coding for maltokinase from Actinoplanes missouriensis is located in a cluster similar to glycogen metabolism clusters identified in Streptomyces coelicolor. Sequence comparisons demonstrate that mak1-related genes coding for homologous proteins are present in many bacterial genomes including taxonomic distantly related groups such as Rhodospirillales or green sulfur bacteria. More than 50% of the aligned sequences are longer than the mak1 gene from A. missouriensis, and the N-terminal portion of these putative maltokinases exhibit high sequence homologies with trehalose synthases. A more detailed sequence comparison indicates a relationship of maltokinases to aminoglycoside phospho-transferases and protein kinases. Transformation of S. lividans with plasmid vectors containing either the mak1 gene from A. missouriensis or the pep2 gene from S. coelicolor resulted in recombinant strains, which produced measurable amounts of maltokinase activity. The proteins Pep2 and Mak1 were over expressed with Streptomyces lividans 66 as a heterologous host and further characterized. The possible physiological function of maltokinases is discussed.