A multistrategy approach for improving the expression of E. coli phytase in Pichia pastoris

Improving phytase production in Pichia pastoris fermentations through de-repression and methanol induction optimization

Pichia pastoris (Komagataella phaffii) is a fast-growing methylotrophic yeast with the ability to assimilate several carbon sources such as methanol, glucose, or glycerol. It has been shown to have outstanding secretion capability with a variety of heterologous proteins. In previous studies, we engineered P. pastoris to co-express Escherichia coli AppA phytase and the HAC1 transcriptional activator using a bidirectional promoter. Phytase production was characterized in shake flasks and did not reflect industrial conditions. In the present study, phytase expression was explored and optimized using instrumented fermenters in continuous and fed-batch modes. First, the production of phytase was investigated under glucose de-repression in continuous culture at three dilution factors, 0.5 d-1 , 1 d-1 , and 1.5 d-1 . The fermenter parameters of these cultures were used to inform a kinetic model in batch and fed-batch modes for growth and phytase production. The kinetic model developed aided to design the glucose-feeding profile of a fed-batch culture. Kinetic model simulations under glucose de-repression and fed-batch conditions identified optimal phytase productivity at the specific growth rate of 0.041 h-1 . Validation of the model simulation with experimental data confirmed the feasibility of the model to predict phytase production in our newly engineered strain. Methanol was used only to induce the expression of phytase at high cell densities. Our results showed that high phytase production required two stages, the first stage used glucose under de-repression conditions to generate biomass while expressing phytase, and stage two used methanol to induce phytase expression. The production of phytase was improved 3.5-fold by methanol induction compared to the expression with glucose alone under de-repression conditions to a final phytase activity of 12.65 MU/L. This final volumetric phytase production represented an approximate 36-fold change compared to the flask fermentations. Finally, the phytase protein produced was assayed to confirm its molecular weight, and pH and temperature profiles. This study highlights the importance of optimizing protein production in P. pastoris when using novel promoters and presents a general approach to performing bioprocess optimization in this important production host.

Microbial engineering for the production and application of phytases to the treatment of the toxic pollutants: A review

Phytases are a group of digestive enzymes which are commonly used as feed enzymes. These enzymes are used exogenously in the feeds of monogastric animals thereby it improves the digestibility of phosphorous and thus reduces the negative impact of inorganic P excretion on the environment. Even though these enzymes are widely distributed in many life forms, microorganisms are the most preferred and potential source of phytase. Despite the extensive availability of the phytase-producing microbial consortia, only a few microorganisms have been known to be exploited at industrial level. The high costs of the enzyme along with the incapability to survive high temperatures followed by the poor storage stability are noted to be the bottleneck in the commercialization of enzymes. For this reason, besides the conventional fermentation approaches, the applicability of cloning, expression studies and genetic engineering has been implemented for the past few years to accomplish the abovesaid benefits. The site-directed mutagenesis as well as knocking out have also validated their prominent role in microbe-based phytase production with enhanced levels. The present review provides detailed information on recent insights on the modification of phytases through heterologous expression and protein engineering to make thermostable and protease-resistant phytases.

Pathway engineering facilitates efficient protein expression in Pichia pastoris

Pichia pastoris has been recognized as an important platform for the production of various heterologous proteins in recent years. The strong promoter AOX1, induced by methanol, with the help of the α-pre-pro signal sequence, can lead to a high expression level of extracellular protein. However, this combination was not always efficient, as protein secretion in P. pastoris involves numerous procedures mediated by several cellular proteins, including folding assisted by endoplasmic reticulum (ER) molecular chaperones, degradation through ubiquitination, and an efficient vesicular transport system. Efficient protein expression requires the cooperation of various intracellular pathways. This article summarizes the process of protein secretion, modification, and transportation in P. pastoris. In addition, the roles played by the key proteins in these processes and the corresponding co-expression effects are also listed. It is expected to lay the foundation for the industrial protein production of P. pastoris. KEY POINTS: • Mechanisms of chaperones in protein folding and their co-expression effects are summarized. • Protein glycosylation modifications are comprehensively reviewed. • Current dilemmas in the overall protein secretion pathway of Pichia pastoris and corresponding solutions are demonstrated.

Increased expression of recombinant chitosanase by co-expression of Hac1p in the yeast Pichia pastoris

BACKGROUND

Pichia pastoris is one of the most popular eukaryotic hosts for producing heterologous proteins, while increasing secretion of target proteins is still a top priority for their application in industrial fields. Recently, the research effort to enhance protein production therein has focused on up-regulating the unfolded protein response (UPR).

OBJECTIVE

We evaluated the effects of activated UPR via Hac1p co-expression with the promoter AOX1 (PAOX1) or GAP (PGAP) on expression of recombinant chitosanase (rCBS) in P. pastoris.

METHOD

The DNA sequence encoding the chitosanase was chemically synthesized and cloned into pPICZαA and the resulted pPICZαA/rCBS was transformed into P. pastoris for expressing rCBS. The P. pastoris HAC1i cDNA was chemically synthesized and cloned into pPIC3.5K to give pPIC3.5K/Hac1p. The HAC1i cDNA was cloned into pGAPZB and then inserted with HIS4 gene from pAO815 to construct the vector pGAPZB/Hac1p/HIS4. For co-expression of Hac1p, the two plasmids pPIC3.5K/Hac1p and pGAPZB/Hac1p/HIS4 were transformed into P. pastoris harboring the CBS gene. The rCBS was assessed based on chitosanase activity and analyzed by SDS-PAGE. The enhanced Kar2p was detected with western blotting to evaluate UPR.

RESULTS

Hac1p co-expression with PAOX1 enhanced rCBS secretion by 41% at 28°C. Although the level of UPR resulted from Hac1p co-expression with PAOX1 was equivalent to that with PGAP in terms of the quantity of Kar2p (a hallmark of the UPR), substitution of PGAP for PAOX1 further increased rCBS production by 21%. The methanol-utilizing phenotype of P. pastoris did not affect rCBS secretion with co-expression of Hac1p or not. Finally, Hac1p co-expression with PAOX1 or PGAP promoted rCBS secretion from 22 to 30°C and raised the optimum induction temperature.

CONCLUSION

The study indicated that Hac1p co-expression with PAOX1 or PGAP is an effective strategy to trigger UPR of P. pastoris and a feasible means for improving production of rCBS therein.

Efficient Secretory Expression and Purification of Food-Grade Porcine Myoglobin in Komagataella phaffii

Myoglobin (MG) is one of the eukaryotic heme-binding proteins that is closely associated with the real color and metallic taste of meat and can be used as a color additive in artificial meat alternatives. However, the traditional extraction methods are expensive and time-consuming and the heterologous biosynthesis of MG has never been reported. Herein, we achieved the secretory expression of porcine MG by engineered Komagataella phaffii using the suitable host (X33), signal peptide (α-factor signal peptide), and modified constitutive promoter (G1 promoter). In addition, the fermentation conditions for MG production were optimized at shaking-flask level (BMGY medium with 40 mg/L of hemin, 30 °C) and at fermenter level (30% DO, feeding 150 mg/L of hemin), resulting in the highest titer of 285.42 mg/L MG in fed-batch fermentations. Furthermore, a purification method for food-grade MG was developed, which can obtain 0.22 mol of heme/mol of MG with 88.0% purity and 66.1% recovery rate.

Oxidative stress tolerance contributes to heterologous protein production in Pichia pastoris

BACKGROUND

Pichia pastoris (syn. Komagataella phaffii) is an important yeast system for heterologous protein expression. A robust P. pastoris mutant with oxidative and thermal stress cross-tolerance was acquired in our previous study. The robust mutant can express a 2.5-fold higher level of lipase than its wild type (WT) under methanol induction conditions.

RESULTS

In this study, we found that the robust mutant not only can express a high level of lipase, but also can express a high level of other heterogeneous proteins (e.g., green fluorescence protein) under methanol induction conditions. Additionally, the intracellular reactive oxygen species (ROS) levels in the robust mutant were lower than that in the WT under methanol induction conditions. To figure out the difference of cellular response to methanol between the WT and the robust mutant, RNA-seq was detected and compared. The results of RNA-seq showed that the expression levels of genes related to antioxidant, MAPK pathway, ergosterol synthesis pathway, transcription factors, and the peroxisome pathway were upregulated in the robust mutant compared to the WT. The upregulation of these key pathways can improve the oxidative stress tolerance of strains and efficiently eliminate cellular ROS. Hence, we inferred that the high heterologous protein expression efficiency in the robust mutant may be due to its enhanced oxidative stress tolerance. Promisingly, we have indeed increased the expression level of lipase up to 1.6-fold by overexpressing antioxidant genes in P. pastoris.

CONCLUSIONS

This study demonstrated the impact of methanol on the expression levels of genes in P. pastoris and emphasized the contribution of oxidative stress tolerance on heterologous protein expression in P. pastoris. Our results shed light on the understanding of protein expression mechanism in P. pastoris and provided an idea for the rational construction of robust yeast with high expression ability.