Second-Generation Escherichia coli SuptoxR Strains for High-Level Recombinant Membrane Protein Production

Semiautomated design and soluble expression of a chimeric antigen TbpAB01 from Glaesserella parasuis

Pathogenic bacterial membrane proteins (MPs) are a class of vaccine and antibiotic development targets with widespread clinical application. However, the inherent hydrophobicity of MPs poses a challenge to fold correctly in living cells. Herein, we present a comprehensive method to improve the soluble form of MP antigen by rationally designing multi-epitope chimeric antigen (ChA) and screening two classes of protein-assisting folding element. The study uses a homologous protein antigen as a functional scaffold to generate a ChA possessing four epitopes from transferrin-binding protein A of Glaesserella parasuis. Our engineered strain, which co-expresses P17 tagged-ChA and endogenous chaperones groEL-ES, yields a 0.346 g/L highly soluble ChA with the property of HPS-positive serum reaction. Moreover, the protein titer of ChA reaches 4.27 g/L with >90% soluble proportion in 5-L bioreactor, which is the highest titer reported so far. The results highlight a timely approach to design and improve the soluble expression of MP antigen in industrially viable applications.

Escherichia coli strains with precise domain deletions in the ribonuclease RNase E can achieve greatly enhanced levels of membrane protein production

Escherichia coli is one of the most widely utilized hosts for production of recombinant membrane proteins (MPs). Bacterial MP production, however, is usually accompanied by severe toxicity and low-level volumetric accumulation. In previous work, we had discovered that co-expression of RraA, an inhibitor of the RNA-degrading activity of RNase E, can efficiently suppress the cytotoxicity associated with the MP overexpression process and, simultaneously, enhance significantly the cellular accumulation of membrane-incorporated recombinant MPs in bacteria. Based on this, we constructed the specialized MP-producing E. coli strain SuptoxR, which can achieve dramatically enhanced volumetric yields of well-folded recombinant MPs. Ιn the present work, we have investigated whether domain deletions in the E. coli RNase E, which exhibit reduced ribonucleolytic activity, can result in suppressed MP-induced toxicity and enhanced recombinant MP production, in a manner resembling the conditions of rraA overexpression in E. coli SuptoxR. We have found that some strains encoding specific RNase E truncation variants can achieve significantly enhanced levels of recombinant MP production. Among these, we have found a single RNase E variant strain, which can efficiently suppress MP-induced toxicity and achieve greatly enhanced levels of recombinant MP production for proteins of both prokaryotic and eukaryotic origin. Based on its properties, and in analogy to the original SuptoxR strain, we have termed this strain SuptoxRNE22. E. coli SuptoxRNE22 can perform better than commercially available bacterial strains, which are frequently utilized for recombinant MP production. We anticipate that SuptoxRNE22 will become a widely utilized host for recombinant MP production in bacteria.

Improving the soluble expression of difficult-to-express proteins in prokaryotic expression system via protein engineering and synthetic biology strategies

Solubility and folding stability are key concerns for difficult-to-express proteins (DEPs) restricted by amino acid sequences and superarchitecture, resolved by the precise distribution of amino acids and molecular interactions as well as the assistance of the expression system. Therefore, an increasing number of tools are available to achieve efficient expression of DEPs, including directed evolution, solubilization partners, chaperones, and affluent expression hosts, among others. Furthermore, genome editing tools, such as transposons and CRISPR Cas9/dCas9, have been developed and expanded to construct engineered expression hosts capable of efficient expression ability of soluble proteins. Accounting for the accumulated knowledge of the pivotal factors in the solubility and folding stability of proteins, this review focuses on advanced technologies and tools of protein engineering, protein quality control systems, and the redesign of expression platforms in prokaryotic expression systems, as well as advances of the cell-free expression technologies for membrane proteins production.