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.

Protein disulfide bond formation contributes to the folding and activity of many exported proteins in bacteria. However, information about disulfide bond formation is limited to only a few bacterial species. We used a multifaceted bioinformatic approach to assess the capacity for disulfide bond formation across this biologically diverse group of organisms. We combined data from a cysteine counting method, in which a significant bias for even numbers of cysteine in proteins is taken as an indicator of disulfide bond formation, with data on the presence of homologs of known disulfide bond formation enzymes. These combined data enabled us to make predictions about disulfide bond formation in the cell envelope across bacterial species. Our bioinformatic and experimental results suggest that many bacteria may not generally oxidatively fold proteins, and implicate the bacterial homolog of the enzyme vitamin K epoxide reductase, a protein required for blood clotting in humans, as part of a disulfide bond formation pathway present in several major bacterial phyla.cysteine ͉ genomics ͉ protein folding ͉ vitamin K epoxide reductase

The Escherichia coli cytoplasmic protein thioredoxin 1 can be efficiently exported to the periplasmic space by the signal sequence of the DsbA protein (DsbAss) but not by the signal sequence of alkaline phosphatase (PhoA) or maltose binding protein (MBP). Using mutations of the signal recognition particle (SRP) pathway, we found that DsbAss directs thioredoxin 1 to the SRP export pathway. When DsbAss is fused to MBP, MBP also is directed to the SRP pathway. We show directly that the DsbAss-promoted export of MBP is largely cotranslational, in contrast to the mode of MBP export when the native signal sequence is utilized. However, both the export of thioredoxin 1 by DsbAss and the export of DsbA itself are quite sensitive to even the slight inhibition of SecA. These results suggest that SecA may be essential for both the slow posttranslational pathway and the SRP-dependent cotranslational pathway. Finally, probably because of its rapid folding in the cytoplasm, thioredoxin provides, along with gene fusion approaches, a sensitive assay system for signal sequences that utilize the SRP pathway.

Disulfide bonds are covalent bonds formed post-translationally by the oxidation of a pair of cysteines. A disulfide bond can serve structural, catalytic, and signaling roles. However, there is an inherent problem to the process of disulfide bond formation: mis-pairing of cysteines can cause misfolding, aggregation and ultimately result in low yields during protein production. Recent developments in the understanding of the mechanisms involved in the formation of disulfide bonds have allowed the research community to engineer and develop methods to produce multi-disulfide-bonded proteins to high yields. This review attempts to highlight the mechanisms responsible for disulfide bond formation in Escherichia coli, both in its native periplasmic compartment in wild-type strains and in the genetically modified cytoplasm of engineered strains. The purpose of this review is to familiarize the researcher with the biological principles involved in the formation of disulfide-bonded proteins with the hope of guiding the scientist in choosing the optimum expression system.

The formation of protein disulfide bonds in the Escherichia coli periplasm by the enzyme DsbA is an inaccurate process. Many eukaryotic proteins with nonconsecutive disulfide bonds expressed in E. coli require an additional protein for proper folding, the disulfide bond isomerase DsbC. Here we report studies on a native E. coli periplasmic acid phosphatase, phytase (AppA), which contains three consecutive and one nonconsecutive disulfide bonds. We show that AppA requires DsbC for its folding. However, the activity of an AppA mutant lacking its nonconsecutive disulfide bond is DsbC-independent. An AppA homolog, Agp, a periplasmic acid phosphatase with similar structure, lacks the nonconsecutive disulfide bond but has the three consecutive disulfide bonds found in AppA. The consecutively disulfide-bonded Agp is not dependent on DsbC but is rendered dependent by engineering into it the conserved nonconsecutive disulfide bond of AppA. Taken together, these results provide support for the proposal that proteins with nonconsecutive disulfide bonds require DsbC for full activity and that disulfide bonds are formed predominantly during translocation across the cytoplasmic membrane.Disulfide bonds between cysteine residues make an important contribution to the folding pathway and stability of many proteins. Early in vitro studies on protein folding showed that the protein RNase when denatured and its disulfide bonds reduced could refold into an active enzyme with the correct disulfide bonds formed (1). Although these results indicated that the information necessary for the proper folding of proteins was intrinsic to the amino acid sequence, the rate of disulfide bond formation in these experiments was much slower than the rates observed in living cells, and the yield of active enzyme was low. The low yields of properly folded enzyme were attributed to the formation of incorrect disulfide bonds in many of the assembled protein molecules. Thus, the in vitro studies, although remarkably successful, were unable to come close to replicating the rapid kinetics and the accuracy of in vivo disulfide bond formation. To explain this inaccuracy observed in biochemical experiments, Anfinsen and co-workers