Abstract
BACKGROUND: The Twin-arginine translocation (Tat) pathway of Escherichia coli has great potential for the export of biopharmaceuticals to the periplasm due to its ability to transport folded proteins, and its proofreading mechanism that allows correctly folded proteins to translocate. Coupling the Tat-dependent protein secretion with the formation of disulfide bonds in the cytoplasm of E. coli CyDisCo provides a powerful platform for the production of industrially challenging proteins. In this study, we investigated the effects on the E. coli cells of exporting a folded substrate (scFv) to the periplasm using a Tat signal peptide, and the effects of expressing an export-incompetent misfolded variant. RESULTS: Cell growth is decreased when either the correctly folded or misfolded scFv is expressed with a Tat signal peptide. However, only the production of misfolded scFv leads to cell aggregation and formation of inclusion bodies. The comprehensive proteomic analysis revealed that both conditions, recombinant protein overexpression and misfolded protein accumulation, lead to downregulation of membrane transporters responsible for protein folding and insertion into the membrane while upregulating the production of chaperones and proteases involved in removing aggregates. These conditions also differentially affect the production of transcription factors and proteins involved in DNA replication. The most distinct stress response observed was the cell aggregation caused by elevated levels of antigen 43. Finally, Tat-dependent secretion causes an increase in tatA expression only after induction of protein expression, while the subsequent post-induction analysis revealed lower tatA and tatB expression levels, which correlate with lowered TatA and TatB protein abundance. CONCLUSIONS: The study identified characteristic changes occurring as a result of the production of both a folded and a misfolded protein, but also highlights an exclusive unfolded stress response. Countering and compensating for these changes may result in higher yields of pharmaceutically relevant proteins exported to the periplasm.