Periplasmic targeting of recombinant proteins in Escherichia coli

Strategies for Successful Over-Expression of Human Membrane Transport Systems Using Bacterial Hosts: Future Perspectives

Ten percent of human genes encode for membrane transport systems, which are key components in maintaining cell homeostasis. They are involved in the transport of nutrients, catabolites, vitamins, and ions, allowing the absorption and distribution of these compounds to the various body regions. In addition, roughly 60% of FDA-approved drugs interact with membrane proteins, among which are transporters, often responsible for pharmacokinetics and side effects. Defects of membrane transport systems can cause diseases; however, knowledge of the structure/function relationships of transporters is still limited. Among the expression of hosts that produce human membrane transport systems, E. coli is one of the most favorable for its low cultivation costs, fast growth, handiness, and extensive knowledge of its genetics and molecular mechanisms. However, the expression in E. coli of human membrane proteins is often toxic due to the hydrophobicity of these proteins and the diversity in structure with respect to their bacterial counterparts. Moreover, differences in codon usage between humans and bacteria hamper translation. This review summarizes the many strategies exploited to achieve the expression of human transport systems in bacteria, providing a guide to help people who want to deal with this topic.

Highly Expressed Soluble Recombinant Anti-GFP VHHs in Escherichia coli via Optimized Signal Peptides, Strains, and Inducers

Antigen-binding variable domains of the H chain of heavy-chain antibodies (VHHs), also known as nanobodies (Nbs), are of great interest in imaging technique, disease prevention, diagnosis, and therapy. High-level expression of soluble Nbs is very important for its industrial production. In this study, we optimized the expression system of anti-green fluorescent protein (GFP) VHHs with three different signal peptides (SPs), outer-membrane protein A (OmpA), pectate lyase B (PelB), and L-asparaginase II SP (L-AsPsII), in different Escherichia coli strains via isopropyl β-D-thiogalactoside (IPTG) induction and auto-induction, respectively. The solubility of recombinant anti-GFP VHHs with PelB or OmpA was significantly enhanced to the same extent by IPTG induction and auto-induction in BL21 (DE3) E. coli strain and the maximum yield of target protein reached approximately 0.4 mg/l in a shake flask. The binding activity of recombinant anti-GFP VHHs was also confirmed to be retained by native-polyacrylamide gel electrophoresis (PAGE). These results suggest that SPs like OmpA and PelB could efficiently improve the recombinant anti-GFP VHH solubility without changing its bioactivity, providing a novel strategy to optimize the E. coli expression system of soluble VHHs, and lay the foundation for the industrial production of soluble recombinant anti-GFP VHHs and the research of other VHHs in the future.

Inhibition of E. coli Host RNA Polymerase Allows Efficient Extracellular Recombinant Protein Production by Enhancing Outer Membrane Leakiness

With the growing interest in continuous cultivation of Escherichia coli, secretion of product to the medium is not only a benefit, but a necessity in future bioprocessing. In this study, it is shown that induced decoupling of growth and heterologous gene expression in the E. coli X-press strain (derived from BL21(DE3)) facilitates extracellular recombinant protein production. The effect of the process parameters temperature and specific glucose consumption rate (q_S ) on growth, productivity, lysis and leakiness, is investigated, to find the parameter space allowing extracellular protein production. Two model proteins are used, Protein A (SpA) and a heavy-chain single-domain antibody (VHH), and performance is compared to the industrial standard strain BL21(DE3). It is shown that inducible growth repression in the X-press strain greatly mitigates the effect of metabolic burden under different process conditions. Furthermore, temperature and q_S are used to control productivity and leakiness. In the X-press strain, extracellular SpA and VHH titer reach up to 349 and 19.6 mg g^-1 , respectively, comprising up to 90% of the total soluble product, while keeping cell lysis at a minimum. The findings demonstrate that the X-press strain constitutes a valuable host for extracellular production of recombinant protein with E. coli.

Secretion of recombinant proteins from E. coli

The microorganism Escherichia coli is commonly used for recombinant protein production. Despite several advantageous characteristics like fast growth and high protein yields, its inability to easily secrete recombinant proteins into the extracellular medium remains a drawback for industrial production processes. To overcome this limitation, a multitude of approaches to enhance the extracellular yield and the secretion efficiency of recombinant proteins have been developed in recent years. Here, a comprehensive overview of secretion mechanisms for recombinant proteins from E. coli is given and divided into three main sections. First, the structure of the E. coli cell envelope and the known natural secretion systems are described. Second, the use and optimization of different one- or two-step secretion systems for recombinant protein production, as well as further permeabilization methods are discussed. Finally, the often-overlooked role of cell lysis in secretion studies and its analysis are addressed. So far, effective approaches for increasing the extracellular protein concentration to more than 10 g/L and almost 100% secretion efficiency exist, however, the large range of optimization methods and their combinations suggests that the potential for secretory protein production from E. coli has not yet been fully realized.

Enhancement of malate production through engineering of the periplasmic rTCA pathway in Escherichia coli

The compartmentalization of enzymes into organelles is a promising strategy for limiting metabolic crosstalk and improving pathway efficiency; however, prokaryotes are unicellular organisms that lack membrane-bound organelles. To mimic this natural compartmentalization, we present here the targeting of the reductive tricarboxylic acid (rTCA) pathway to the periplasm to enhance the production of malate. A multigene combination knockout strategy was used to construct a phosphoenolpyruvate (PEP) pool. Then, the genes encoding phosphoenolpyruvate carboxykinase and malate dehydrogenase were combinatorially overexpressed to construct a cytoplasmic rTCA pathway for malate biosynthesis; however, the efficiency of malate production was low. To further enhance malate production, the rTCA pathway was targeted to the periplasm, which led to a 100% increase in malate production to 18.8 mM. Next, dual metabolic engineering regulation was adopted to balance the cytoplasmic and periplasmic pathways, leading to an increase in malate production to 58.8 mM. The final engineered strain, GL2306, produced 193 mM malate with a yield of 0.53 mol/mol in 5 L of pH-stat fed-batch culture. The strategy described here paves the way for the development of metabolic engineering and synthetic biology in the microbial production of chemicals.