BackgroundProduction of correctly disulfide bonded proteins to high yields remains a challenge. Recombinant protein expression in Escherichia coli is the popular choice, especially within the research community. While there is an ever growing demand for new expression strains, few strains are dedicated to post-translational modifications, such as disulfide bond formation. Thus, new protein expression strains must be engineered and the parameters involved in producing disulfide bonded proteins must be understood.ResultsWe have engineered a new E. coli protein expression strain named SHuffle, dedicated to producing correctly disulfide bonded active proteins to high yields within its cytoplasm. This strain is based on the trxB gor suppressor strain SMG96 where its cytoplasmic reductive pathways have been diminished, allowing for the formation of disulfide bonds in the cytoplasm. We have further engineered a major improvement by integrating into its chromosome a signal sequenceless disulfide bond isomerase, DsbC. We probed the redox state of DsbC in the oxidizing cytoplasm and evaluated its role in assisting the formation of correctly folded multi-disulfide bonded proteins. We optimized protein expression conditions, varying temperature, induction conditions, strain background and the co-expression of various helper proteins. We found that temperature has the biggest impact on improving yields and that the E. coli B strain background of this strain was superior to the K12 version. We also discovered that auto-expression of substrate target proteins using this strain resulted in higher yields of active pure protein. Finally, we found that co-expression of mutant thioredoxins and PDI homologs improved yields of various substrate proteins.ConclusionsThis work is the first extensive characterization of the trxB gor suppressor strain. The results presented should help researchers design the appropriate protein expression conditions using SHuffle strains.

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Genetic screening and selection procedures employing a secA-lacZ fusion strain repeatedly have yielded mutations in four genes affecting the protein export pathway of Escherichia coil These genes are secA, secD, prlA/secY, and secE. We discuss the significance of the failure to find new sec genes after extensive use of this approach. One of the genes, secE, has been characterized in some detail. From the DNA sequence of the gene and analysis of alkaline phosphatase fusions to the SecE protein, we propose that it is a 13,600-dalton integral cytoplasmic membrane protein. The data presented here and in the accompanying paper strongly suggest that secE has an important role in E. coli protein export.

The role of particular residues of the PvuII endonuclease in DNA binding and cleavage was studied by mutational analysis using a number of in vivo and in vitro approaches. While confirming the importance of residues predicted to be involved directly in function by the crystal structure, the analysis led to several striking results. Aspartate 34, which contacts the central base pair of the PvuII site (5 -CAGCTG-3 ) through the minor groove, plays a critical role in binding specificity. A D34G mutant binds with high affinity to any of the sequences in the set CANNTG, although its low level of cleavage activity acts only on the wild-type site. In addition, a His to Ala mutation at the residue that contacts the central G and is predicted to be blocked by PvuII methylation still requires the PvuII methylase to be maintained in vivo, arguing against this hypothesis as the only mechanism for methylation protection. Finally, four of the five mutations that reduce cleavage activity while still exhibiting binding in the gel shift assay are at residues that form DNA-or subunit-subunit contacts rather than in the catalytic center. This provides further evidence for a strong linkage between specific binding and catalysis.The structures of the five type II endonucleases determined by x-ray diffraction, EcoRI (1), BamHI (2, 3), EcoRV (4, 5), PvuII (6, 7) and Cfr10I (8), share substantial elements of similarity. Analysis of the enzymes complexed to DNA, where known, suggests a preliminary classification into two different groups (9). The endonucleases that produce 5Ј-overhanging ends, EcoRI and BamHI, approach the DNA from the major groove, where most of their base-specific contacts are made. The endonucleases that produce blunt-ended DNA products, EcoRV and PvuII, approach the DNA from the minor groove side and establish contacts in the major groove by wrapping around the DNA. Structure-function studies have been limited to a small number of type II enzymes. Years of studies have produced an extensive amount of information about EcoRI (10 -18), EcoRV (18 -24), and NaeI (25) and, more recently, BamHI (26,27). These studies have focused on the identification and analysis of residues involved in catalysis, testing alternative mechanisms of catalysis, and understanding how a specific DNA sequence is recognized. Attempts to alter the sequence specificity have proven to be difficult, with successful examples limited to mutants showing relaxed specificity (10,11,28) or displaying activity toward DNA substituted with unnatural bases (15,29). One explanation for this difficulty is that a change in specificity not only requires recognition of a different DNA sequence, but also requires that the linkage between recognition and catalysis be retained. A class of mutations that might illuminate this linkage is the catalytic mutations, where specific DNA binding is retained but catalysis is reduced (26, 30). In particular, mutants in this class that are outside the catalytic center might be deficient in the linkage between recognition and ca...

Current methods for producing immunoglobulin G (IgG) antibodies in engineered cells often require refolding steps or secretion across one or more biological membranes. Here, we describe a robust expression platform for biosynthesis of full-length IgG antibodies in the Escherichia coli cytoplasm. Synthetic heavy and light chains, both lacking canonical export signals, are expressed in specially engineered E. coli strains that permit formation of stable disulfide bonds within the cytoplasm. IgGs with clinically relevant antigen- and effector-binding activities are readily produced in the E. coli cytoplasm by grafting antigen-specific variable heavy and light domains into a cytoplasmically stable framework and remodelling the fragment crystallizable domain with amino-acid substitutions that promote binding to Fcγ receptors. The resulting cytoplasmic IgGs—named ‘cyclonals'—effectively bypass the potentially rate-limiting steps of membrane translocation and glycosylation.