Niju Narayanan

Arabinose-Induction of lac-Derived Promoter Systems for Penicillin Acylase Production in Escherichia coli

Arabinose was shown to serve as an effective inducer for induction of the lac‐derived promoters in Escherichia coli using penicillin acylase (PAC) as a model protein. Upon the induction with a conventional inducer, isopropyl‐β‐d‐thiogalactopyranoside (IPTG), for pac overexpression, which is regulated by thetrc or (DE3)/T7 promoter, the production of PAC was limited by the accumulation of PAC precursors (proPAC) as inclusion bodies. Negative cellular responses, such as growth inhibition and cell lysis, were frequently observed, resulting in a low pac expression level and poor culture performance. Interestingly, these technical hurdles can be overcome simply through the use of arabinose as an inducer. The results indicate that arabinose not only induced the lac‐derived promoter systems (i.e., trc and (DE3)/T7) for pac (or LLpac) overexpression but also facilitated the posttranslational processing of proPAC for maturation. However, the arabinose‐inducibility appears to be host‐dependent and becomes less observable in the strains with a mutation in the ara operon. The arabinose‐inducibility was also investigated in the expression system with the coexistence of the trc promoter system regulating pac expression and another arabinose‐inducible promoter system of araB regulating degP coexpression.

Chaperone-mediated folding and maturation of the penicillin acylase precursor in the cytoplasm of-Escherichia coli

ABSTRACT Expression of the leaderless pac gene (LL pac), which lacks the coding region for the signal peptide of penicillin acylase (PAC), in Escherichia coli was conducted. It was demonstrated that the PAC precursor, proPAC, can be produced and even processed to form mature PAC in the cytoplasm, indicating that the posttranslational processing steps for PAC maturation can occur in both the periplasm and the cytoplasm of E. coli. The outcome of proPAC folding and PAC maturation could be affected by several factors, such as inducer type, proPAC formation rate, and chaperone availability. Misfolding of proPAC in the cytoplasm could be partially resolved through the coexpression of cytoplasmic chaperones, such as trigger factor, GroEL/ES, or DnaK/J-GrpE. The three chaperones tested showed different extents of the effect on proPAC solublization and PAC maturation, and trigger factor had the most prominent one. However, the chaperone-mediated solublization of proPAC did not guarantee its maturation, which is usually limited by the first autoproteolytic step. It was observed that arabinose could act as an effective inducer for the induction of LL pac expression regulated by the lac-derived promoter system of trc. In addition, PAC maturation could be highly facilitated by arabinose supplementation and coexpression of trigger factor, suggesting that the coordination of chaperone systems with proper culture conditions could dramatically impact recombinant protein production. This study suggests that folding/misfolding of proPAC could be a major step limiting the overproduction of PAC in E. coli and that the problem could be resolved through the search for appropriate chaperones for coexpression. It also demonstrates the analogy in the issues of proPAC misfolding as well as the expression bottleneck occurring in the cytoplasm (i.e., LL pac expression) and those occurring in the periplasm (i.e., wild-type pac expression).

Physiological Improvement to Enhance Escherichia coli Cell-Surface Display via Reducing Extracytoplasmic Stress

Cell physiology was impaired when enhanced yellow fluorescence protein (EYFP) was displayed on the Escherichia coli cell surface, resulting in growth arrest and poor display performance. Coexpression of Skp, a periplasmic chaperone known to interact with several outer membrane proteins for their transport and insertion in the outer membrane, was demonstrated to be effective to restore cell physiology. When Skp was coexpressed with EYFP display, host cells became less sensitive to ethylenediaminetetraacetic acid and sodium dodecyl sulfate, implying that cell physiology was improved. Most importantly, the display performance was highly enhanced as a result of the increased specific fluorescence intensity without growth arrest. The results of transmission electron microscopy indicate that the density of surface‐displayed EYFP was highly increased upon Skp coexpression. Cells with EYFP display experienced extracytoplasmic stress, as reflected by the induced promoter activities of three stress‐responsive genes, degP, cpxP, and rpoH. The extracytoplasmic stress reflected by the degP promoter activity appears to be consistent with the cell physiology observed phenotypically under various culture conditions for cell‐surface display. Therefore, the PdegP:: lacZ allele was proposed to be a suitable “sensor” for monitoring the extracytoplasmic stress and cell physiology during the course of E. coli cell‐surface display.

Alleviation of Proteolytic Sensitivity To Enhance Recombinant Lipase Production in Escherichia coli

ABSTRACT Two amino acids, Leu149 and Val223, were identified as proteolytically sensitive when Pseudozyma antarctica lipase (PalB) was heterologously expressed in Escherichia coli. The functional expression was enhanced using the double mutant for cultivation. However, the recombinant protein production was still limited by PalB misfolding, which was resolved by DsbA coexpression.

Cytoplasmic overexpression, folding, and processing of penicillin acylase precursor in Escherichia coli.

Penicillin acylase (PAC) precursor, proPAC, was overproduced in a soluble or insoluble form in the cytoplasm of Escherichia coli through the expression of the leader‐less pac gene (ll‐pac) devoid of the coding region for the signal peptide of PAC. Also, a portion of the overexpressed proPAC was further processed to form mature PAC, indicating that the posttranslational processing steps for PAC maturation can occur in both the periplasm and the cytoplasm of E. coli. The cultivation performance for ll‐pac expression was limited by several factors, including (1) misfolding of proPAC, resulting in the aggregation of insoluble proPAC as inclusion bodies, (2) intracellular proteolysis, leading to the degradation of the overexpressed gene products, and (3) inefficient PAC maturation, limiting the formation of active PAC. The effect of coexpression of various cytoplasmic chaperones, including trigger factor, GroEL/ES, DnaK/J‐GrpE, and their combinations, on ll‐pac expression was investigated. Intracellular proteolysis of the overexpressed gene products could be prevented by coexpression of GroEL/ES. On the other hand, coexpression of trigger factor appeared to be able to facilitate the folding of soluble proPAC and to improve PAC maturation. The roles of trigger factor and GroEL/ES could be coordinated to significantly improve ll‐pac expression performance. DnaK/J‐GrpE had an effect for solublization of proPAC and perhaps, similar to trigger factor, for improving PAC maturation. The ll‐pac expression performance was also significantly improved through the simultaneous coexpression of DnaK/J‐GrpE and GroEL/ES. The results of the study suggest that the folding and/or processing of proPAC could be a major issue limiting the overproduction of PAC in E. coli and the bottleneck could be eliminated through the coexpression of appropriate chaperone(s).

Enhancing Functional Expression of Heterologous Burkholderia Lipase in Escherichia coli

Functional expression of lipase from Burkholderia sp. C20 (Lip) in various cellular compartments of Escherichia coli was explored. The poor expression in the cytoplasm of E. coli was improved by several strategies, including coexpression of the cytoplasmic chaperone GroEL/ES, using a mutant E. coli host strain with an oxidative cytoplasm, and protein fusion technology. Fusing Lip with the N-terminal peptide tags of T7PK, DsbA, and DsbC was effective in enhancing the solubility and biological activity. Non-fused Lip or Lip fusions heterologously expressed in the periplasm of E. coli formed insoluble aggregates with a minimum activity. Biologically active and intact Lip was obtained upon the secretion into the extracellular medium using the native signal peptide and the expression performance was further improved by coexpression of the periplasmic chaperon Skp. The extracellular expression was even more effective when Lip was secreted as a Lip–HlyA fusion via the α-hemolysin transporter. Finally, Lip could be functionally displayed on the E. coli cell surface when fused with the carrier EstA.

Enhancing functional expression of heterologous lipase B in Escherichia coli by extracellular secretion

Functional expression of recombinant Pseudozyma antarctica lipase B (PalB) in Escherichia coli has been technically problematic due to protein misfolding, ineffective disulfide bond formation, and protein instability associated with intracellular proteolysis. To overcome these problems, an alternative approach was explored in this study by extracellular secretion of PalB via two Sec-independent secretion systems, i.e., the α-hemolysin (type I) and the modified flagellar (type III) secretion systems, which can export proteins of interest from the cytoplasm directly to the exterior of the cell. Both shaker flask and bioreactor cultivations were performed to characterize the developed PalB expression/secretion systems. Bioactive PalB was expressed and secreted extracellularly either as a HlyA fusion (i.e., PalB-HlyA via type I system) or an intact protein (via type III system). However, the secretion intermediates in the intracellular fraction of culture samples were non-bioactive even though they were soluble, suggesting that the extracellular secretion did mediate the development of PalB activity. Also importantly, the secretion strategy appeared to have a minimum impact on cell physiology. PalB secretion via the type I system was fast with higher specific PalB activities but poor cell growth. On the other hand, the secretion via the type III system was slow with lower specific PalB activities but effective cell growth.

High-level gene expression for recombinant penicillin acylase production using the araB promoter system in Escherichia coli

The pac gene encoding penicillin acylase (PAC) was overexpressed under the regulation of the araB promoter (ParaB, also known as PBAD) in Escherichia coli(E. coli). The current ParaB expression system exhibited minimum leaking pac expression in the absence of arabinose as well as fast and high‐level pac expression upon induction with arabinose in a wide concentration range. The production of PAC was limited by the accumulation of PAC precursors (i.e., proPAC in both soluble and insoluble forms) and various negative cellular responses, such as growth arrest and cell lysis. The culture performance could be improved by degP coexpression and the individual contribution of DegP protease and chaperone activities to the enhancement on the production of PAC was characterized. The study highlights the importance of identifying the step(s) limiting high‐level gene expression and subsequent design and construction of the host/vector system for enhancing recombinant protein production in E. coli.

Structural identification of recombinant human CD83 mutant variant as a potent therapeutic protein

The formation of aberrant disulfide bonds is a structural consideration for the manufacturing of the extracellular domain of human CD83 (hCD83ext), a potential therapeutic protein. In certain instances, hCD83ext protein products, even when stored frozen, tended to dimerize or even multimerize through the formation of aberrant intermolecular disulfide bonds. Herein, we discovered an analytical inconsistency and applied a modified sample preparation protocol for proper structural analysis of hCD83ext products which are heterologously expressed in Escherichia coli and subsequently purified. In addition, a mutant derivative with the Cys100Ser mutation was identified as an improved version which did not form dimers or multimers. The identification of this mutant variant as a more potent therapeutic protein than other hCD83ext species demonstrated that the structural variation associated with disulfide bond formation can be a critical issue for rigorous control of the quality and bioactivity of therapeutic proteins. The application of this mutant variant for protein therapeutics is currently under exploration.