Two distinct regions in the model protein Peb1 are critical for its heterologous transport out of Escherichia coli

Engineering the flagellar type III secretion system: improving capacity for secretion of recombinant protein

BACKGROUND

Many valuable biopharmaceutical and biotechnological proteins have been produced in Escherichia coli, however these proteins are almost exclusively localised in the cytoplasm or periplasm. This presents challenges for purification, i.e. the removal of contaminating cellular constituents. One solution is secretion directly into the surrounding media, which we achieved via the 'hijack' of the flagellar type III secretion system (FT3SS). Ordinarily flagellar subunits are exported through the centre of the growing flagellum, before assembly at the tip. However, we exploit the fact that in the absence of certain flagellar components (e.g. cap proteins), monomeric flagellar proteins are secreted into the supernatant.

RESULTS

We report the creation and iterative improvement of an E. coli strain, by means of a modified FT3SS and a modular plasmid system, for secretion of exemplar proteins. We show that removal of the flagellin and HAP proteins (FliC and FlgKL) resulted in an optimal prototype. We next developed a high-throughput enzymatic secretion assay based on cutinase. This indicated that removal of the flagellar motor proteins, motAB (to reduce metabolic burden) and protein degradation machinery, clpX (to boost FT3SS levels intracellularly), result in high capacity secretion. We also show that a secretion construct comprising the 5'UTR and first 47 amino acidsof FliC from E. coli (but no 3'UTR) achieved the highest levels of secretion. Upon combination, we show a 24-fold improvement in secretion of a heterologous (cutinase) enzyme over the original strain. This improved strain could export a range of pharmaceutically relevant heterologous proteins [hGH, TrxA, ScFv (CH₂)], achieving secreted yields of up to 0.29 mg L^-1, in low cell density culture.

CONCLUSIONS

We have engineered an E. coli which secretes a range of recombinant proteins, through the FT3SS, to the extracellular media. With further developments, including cell culture process strategies, we envision further improvement to the secreted titre of recombinant protein, with the potential application for protein production for biotechnological purposes.

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.

Unconventional microbial systems for the cost-efficient production of high-quality protein therapeutics

Both conventional and innovative biomedical approaches require cost-effective protein drugs with high therapeutic potency, improved bioavailability, biocompatibility, stability and pharmacokinetics. The growing longevity of the human population, the increasing incidence and prevalence of age-related diseases and the better comprehension of genetic-linked disorders prompt to develop natural and engineered drugs addressed to fulfill emerging therapeutic demands. Conventional microbial systems have been for long time exploited to produce biotherapeutics, competing with animal cells due to easier operation and lower process costs. However, both biological platforms exhibit important drawbacks (mainly associated to intracellular retention of the product, lack of post-translational modifications and conformational stresses), that cannot be overcome through further strain optimization merely due to physiological constraints. The metabolic diversity among microorganisms offers a spectrum of unconventional hosts, that, being able to bypass some of these weaknesses, are under progressive incorporation into production pipelines. In this review we describe the main biological traits and potentials of emerging bacterial, yeast, fungal and microalgae systems, by comparing selected leading species with well established conventional organisms with a long run in protein drug production.

Recent contributions in the field of the recombinant expression of disulfide bonded proteins in bacteria

The production of heterologous disulfide bonded proteins in bacteria remains a biotechnological challenge. A rapid literature survey results in the identification of some interesting proposals, such as the option of producing functional proteins in the cytoplasm in the presence of sulfhydryl oxidases and isomerases. Furthermore, an ever-increasing number of applications refers to recombinant proteins displayed at the bacterial surface. Time will tell whether these developments will lead to universally accepted laboratory protocols.

Molecular mechanisms and biological role of Campylobacter jejuni attachment to host cells

Adhesion to host cells is an important step in pathogenesis of Campylobacter jejuni, which is the most prevalent bacterial cause of human gastroenteritis worldwide. In contrast to other bacteria such as E. coli and Salmonella, adherence of C. jejuni is not mediated by fimbria or pili. A number of C. jejuni adhesion-related factors have been described. However, the results obtained by different researchers in different laboratories are often contradictory and inconclusive, with only some of the factors described being confirmed as true adhesins. In this review, we present the current state of studies on the mechanisms of attachment of C. jejuni to host cells.

Extracellular secretion of a recombinant therapeutic peptide by Bacillus halodurans utilizing a modified flagellin type III secretion system

Background

Through modification of the flagellin type III secretion pathway of Bacillus halodurans heterologous peptides could be secreted into the medium as flagellin fusion monomers. The stability of the secreted monomers was significantly enhanced through gene-targeted inactivation of host cell extracellular proteases. In evaluating the biotechnological potential of this extracellular secretion system an anti-viral therapeutic peptide, Enfuvirtide, was chosen. Currently, Enfuvirtide is synthesised utilizing 106 chemical steps. We used Enfuvirtide as a model system in an effort to develop a more cost-effective biological process for therapeutic peptide production.

Results

An attempt was made to increase the levels of the fusion peptide by two strategies, namely strain improvement through gene-targeted knock-outs, as well as vector and cassette optimization. Both approaches proved to be successful. Through chromosomal inactivation of the spo0A, lytC and lytE genes, giving rise to strain B. halodurans BhFDL05S, the secretion of recombinant peptide fusions was increased 10-fold. Cassette optimization, incorporating an expression vector pNW33N and the N- and C-terminal regions of the flagellin monomer as an in-frame peptide fusion, resulted in a further 3.5-fold increase in the secretion of recombinant peptide fusions.

Conclusions

The type III flagellar secretion system of B. halodurans has been shown to successfully secrete a therapeutic peptide as a heterologous flagellin fusion. Improvements to both the strain and expression cassette led to increased levels of recombinant peptide, showing promise for a biotechnological application.