High-level secretion of recombinant full-length streptavidin in Pichia pastoris and its application to enantioselective catalysis

Removal and monitoring of residual nucleic acids from core streptavidin inclusion bodies for increased refolding yield

Commercial production of recombinant streptavidin (SAV) using soluble expression route is cost-prohibitive, resulting from its inherent toxicity toward commercially available Escherichia coli hosts (such as BL21) and low productivity of existing manufacturing processes. Quality challenges can also result from binding of streptavidin in the host cells. One way to overcome these challenges is to allow formation of inclusion bodies (IBs). Nevertheless, carried-over cellular contaminants during IBs preparation can hinder protein refolding and application of SAV in nucleic acid-based applications. Hence, removing associated contaminants in recombinant IBs is imperative for maximum product outcomes. In this study, the IBs isolation method from our group was improved to remove residual DNA found in refolded core SAV (cSAV). The improvements were attained by incorporating quantitative real-time polymerase chain reactions (qPCR) for residual DNA monitoring. We attained 99 % cellular DNA removal from cSAV IBs via additional wash and sonication steps, and the addition of benzonase nuclease during lysis. A 10 % increment of cSAV refolding yield (72 %) and 83 % reduction of residual DNA from refolding of 1 mg cSAV IBs were observed under extensive sonication. Refolding of cSAV was not affected and its activity was not compromised. The optimized process reported here highlights the importance of obtaining cSAV IBs with minimal contaminants prior to refolding to increase product yield, and the usefulness of the qPCR method to monitor nucleic acid removed from each step of the process.

Artificial Metalloenzymes on the Verge of New-to-Nature Metabolism

Residing at the interface of chemistry and biotechnology, artificial metalloenzymes (ArMs) offer an attractive technology to combine the versatile reaction repertoire of transition metal catalysts with the exquisite catalytic features of enzymes. While earlier efforts in this field predominantly comprised studies in well-defined test-tube environments, a trend towards exploiting ArMs in more complex environments has recently emerged. Integration of these artificial biocatalysts in enzymatic cascades and using them in whole-cell biotransformations and in vivo opens up entirely novel prospects for both preparative chemistry and synthetic biology. We highlight selected recent developments with a particular focus on challenges and opportunities in the in vivo application of ArMs.

RNA-aptamers-in-droplets (RAPID) high-throughput screening for secretory phenotypes

Synthetic biology and metabolic engineering seek to re-engineer microbes into "living foundries" for the production of high value chemicals. Through a "design-build-test" cycle paradigm, massive libraries of genetically engineered microbes can be constructed and tested for metabolite overproduction and secretion. However, library generation capacity outpaces the rate of high-throughput testing and screening. Well plate assays are flexible but with limited throughput, whereas droplet microfluidic techniques are ultrahigh-throughput but require a custom assay for each target. Here we present RNA-aptamers-in-droplets (RAPID), a method that greatly expands the generality of ultrahigh-throughput microfluidic screening. Using aptamers, we transduce extracellular product titer into fluorescence, allowing ultrahigh-throughput screening of millions of variants. We demonstrate the RAPID approach by enhancing production of tyrosine and secretion of a recombinant protein in Saccharomyces cerevisiae by up to 28- and 3-fold, respectively. Aptamers-in-droplets affords a general approach for evolving microbes to synthesize and secrete value-added chemicals.Screening libraries of genetically engineered microbes for secreted products is limited by the available assay throughput. Here the authors combine aptamer-based fluorescent detection with droplet microfluidics to achieve high throughput screening of yeast strains engineered for enhanced tyrosine or streptavidin production.

Engineering of Harobin for enhanced fibrinolytic activity obtained by random and site-directed mutagenesis

We have previously published a report on the cloning and characterization of Harobin, a fibrinolytic serine protease. However, the broad application of this fibrinolytic enzyme is limited by its low expression level that was achieved in Pichia pastoris. To counteract this shortcoming, random and site-directed mutagenesis have been combined in order to improve functional expression and activity of Harobin. By screening 400 clones from random mutant libraries for enhanced fibrinolytic activity, two mutants were obtained: N111R, R230G. By performing site-directed mutagenesis, a Harobin double mutant, N111R/R230G, was constructed and can be functionally expressed at higher level than the wild type enzyme. In addition, it possessed much higher fibrinolytic and amidolytic activity than the wild type enzyme and other single mutants. The N111R/R230G expressed in basal salts medium was purified by a three step purification procedure. At pH of 6.0-9.0, and the temperature range of 40-90 °C, N111R/R230G was more active and more heat resistant. The fibrinolytic activities of Harobin mutants were completely inhibited by PMSF and SBTI, but not by EDTA, EGTA, DTT, indicating that Harobin is a serine protease. N111R/R230G showed much better anti-thrombosis effect than wild type Harobin and single mutants, and could significantly increase bleeding and clotting time. Intravenous injection of N111R/R230G in spontaneous hypertensive rats (SHR) led to a significant reduction in systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP) (p < 0.01), while heart rate (HR) was not affected. The in vitro and in vivo results of the present study revealed that Harobin double mutant N111R/R230G is an appropriate candidate for biotechnological applications due to its high expression level and high activity in area of thrombosis and hypertension.

Secretory production of tetrameric native full-length streptavidin with thermostability using Streptomyces lividans as a host

Background

Streptavidin is a tetrameric protein derived from Streptomyces avidinii, and has tight and specific biotin binding affinity. Applications of the streptavidin-biotin system have been widely studied. Streptavidin is generally produced using protein expression in Escherichia coli. In the present study, the secretory production of streptavidin was carried out using Streptomyces lividans as a host.

Results

In this study, we used the gene encoding native full-length streptavidin, whereas the core region is generally used for streptavidin production in E. coli. Tetrameric streptavidin composed of native full-length streptavidin monomers was successfully secreted in the culture supernatant of S. lividans transformants, and had specific biotin binding affinity as strong as streptavidin produced by E. coli. The amount of Sav using S. lividans was about 9 times higher than using E. coli. Surprisingly, streptavidin produced by S. lividans exhibited affinity to biotin after boiling, despite the fact that tetrameric streptavidin is known to lose its biotin binding ability after brief boiling.

Conclusion

We successfully produced a large amount of tetrameric streptavidin as a secretory-form protein with unique thermotolerance.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0188-y) contains supplementary material, which is available to authorized users.