Disruption of genes involved in CORVET complex leads to enhanced secretion of heterologous carboxylesterase only in protease deficient Pichia pastoris

Divergent roles of ADP-ribosylation factor GTPase-activating proteins in lignocellulose utilization of Trichoderma guizhouense NJAU4742

BACKGROUND

The ability of lignocellulose degradation for filamentous fungi is always attributed to their efficient CAZymes system with broader applications in bioenergy development. ADP-ribosylation factor GTPase-activating proteins (Arf-GAPs), pivotal in fungal morphogenesis, lack comprehensive studies on their regulatory mechanisms in lignocellulose utilization.

RESULTS

Here, the orthologs (TgGlo3 and TgGcs1) of Arf-GAPs in S. cerevisiae were characterized in Trichoderma guizhouense NJAU4742. The results indicated that overexpression of Tggcs1 (OE-Tggcs1) enhanced the lignocellulose utilization, whereas increased expression of Tgglo3 (OE-Tgglo3) elicited antithetical responses. On the fourth day of fermentation with rice straw as the sole carbon source, the activities of endoglucanase, cellobiohydrolase, xylanase, and filter paper of the wild-type strain (WT) reached 8.20 U mL-1, 4.42 U mL-1, 14.10 U mL-1, and 3.56 U mL-1, respectively. Compared to WT, the four enzymes activities of OE-Tggcs1 increased by 7.93%, 6.11%, 9.08%, and 12.92%, respectively, while those decreased to varying degrees of OE-Tgglo3. During the nutritional growth, OE-Tgglo3 resulted in the hyphal morphology characterized by sparsity and constriction, while OE-Tggcs1 led to a notable increase in vacuole volume. In addition, OE-Tggcs1 exhibited higher transport efficiencies for glucose and cellobiose thereby sustaining robust cellular metabolic rates. Further investigations revealed that Tgglo3 and Tggcs1 differentially regulated the transcription level of a dynamin-like GTPase gene (Tggtp), eliciting distinct redox states and apoptotic reaction, thus orchestrating the cellular response to lignocellulose utilization.

CONCLUSIONS

Overall, these findings underscored the significance of TgArf-GAPs as pivotal regulators in lignocellulose utilization and provided initial insights into their differential modulation of downstream targets.

Komagataella phaffii as a Platform for Heterologous Expression of Enzymes Used for Industry

In the 1980s, Escherichia coli was the preferred host for heterologous protein expression owing to its capacity for rapid growth in complex media; well-studied genetics; rapid and direct transformation with foreign DNA; and easily scalable fermentation. Despite the relative ease of use of E. coli for achieving the high expression of many recombinant proteins, for some proteins, e.g., membrane proteins or proteins of eukaryotic origin, this approach can be rather ineffective. Another microorganism long-used and popular as an expression system is baker's yeast, Saccharomyces cerevisiae. In spite of a number of obvious advantages of these yeasts as host cells, there are some limitations on their use as expression systems, for example, inefficient secretion, misfolding, hyperglycosylation, and aberrant proteolytic processing of proteins. Over the past decade, nontraditional yeast species have been adapted to the role of alternative hosts for the production of recombinant proteins, e.g., Komagataella phaffii, Yarrowia lipolytica, and Schizosaccharomyces pombe. These yeast species' several physiological characteristics (that are different from those of S. cerevisiae), such as faster growth on cheap carbon sources and higher secretion capacity, make them practical alternative hosts for biotechnological purposes. Currently, the K. phaffii-based expression system is one of the most popular for the production of heterologous proteins. Along with the low secretion of endogenous proteins, K. phaffii efficiently produces and secretes heterologous proteins in high yields, thereby reducing the cost of purifying the latter. This review will discuss practical approaches and technological solutions for the efficient expression of recombinant proteins in K. phaffii, mainly based on the example of enzymes used for the feed industry.

Customizing amino acid metabolism of Pichia pastoris for recombinant protein production

Amino acids are the building blocks of proteins. In this respect, a reciprocal effect of recombinant protein production on amino acid biosynthesis as well as the impact of the availability of free amino acids on protein production can be anticipated. In this study, the impact of engineering the amino acid metabolism on the production of recombinant proteins was investigated in the yeast Pichia pastoris (syn Komagataella phaffii). Based on comprehensive systems-level analyses of the metabolomes and transcriptomes of different P. pastoris strains secreting antibody fragments, cell engineering targets were selected. Our working hypothesis that increasing intracellular amino acid levels could help unburden cellular metabolism and improve recombinant protein production was examined by constitutive overexpression of genes related to amino acid metabolism. In addition to 12 genes involved in specific amino acid biosynthetic pathways, the transcription factor GCN4 responsible for regulation of amino acid biosynthetic genes was overexpressed. The production of the used model protein, a secreted carboxylesterase (CES) from Sphingopyxis macrogoltabida, was increased by overexpression of pathway genes for alanine and for aromatic amino acids, and most pronounced, when overexpressing the regulator GCN4. The analysis of intracellular amino acid levels of selected clones indicated a direct linkage of improved recombinant protein production to the increased availability of intracellular amino acids. Finally, fed batch cultures showed that overexpression of GCN4 increased CES titers 2.6-fold, while the positive effect of other amino acid synthesis genes could not be transferred from screening to bioreactor cultures.

Biosynthesis of High-Active Hemoproteins by the Efficient Heme-Supply Pichia Pastoris Chassis

Microbial synthesis of valuable hemoproteins has become a popular research topic, and Pichia pastoris is a versatile platform for the industrial production of recombinant proteins. However, the inadequate supply of heme limits the synthesis of high-active hemoproteins. Here a strategy for enhancing intracellular heme biosynthesis to improve the titers and functional activities of hemoproteins is reported. After selecting a suitable expressional strategy for globins, the efficient heme-supply P. pastoris chassis is established by removing the spatial segregation during heme biosynthesis, optimizing precursor synthesis, assembling rate-limiting enzymes using protein scaffolds, and inhibiting heme degradation. This robust chassis produces several highly active hemoproteins, including porcine myoglobin, soy hemoglobin, Vitreoscilla hemoglobin, and P450-BM3, which can be used in the development of artificial meat, high-cell-density fermentation, and whole-cell catalytic synthesis of high-value-added compounds. Furthermore, the engineered chassis strain has great potential for producing and applying other hemoproteins with high activities in various fields.

Alleviating vacuolar transport improves cellulase production in Trichoderma reesei

Increasing cellulase production in cellulolytic fungus Trichoderma reesei is of interest for biofuels and biorefineries. Previous studies indicated that secreted protein was occasionally accumulated in vacuoles; this phenomenon has also been reported in T. reesei. Therefore, alleviating vacuolar transport seems to be a promising strategy for improving cellulase production in T. reesei. Herein, we found that knockout of vps10, vps13, and vps21, among 11 vacuolar protein sorting factors, improved cellulase production in T. reesei. The filter paper activity in Δvps10, Δvps13, and Δvps21 increased by 1.28-, 2.45-, and 2.11-fold than that of the parent strain. Moreover, the β-glucosidase activity in Δvps13 and Δvps21 increased by 3.22- and 3.56-fold after 6 days of fermentation. Furthermore, we also found that the vacuolar trafficking towards vacuoles was partially impaired in three knockout mutants, and disruption of vps13 alleviated the autophagy process. These results indicated that alleviated transport and degradation towards vacuole in Δvps10, Δvps13, and Δvps21 might improve cellulase production. Of note, the expression of cellulase genes in Δvps13 and Δvps21 was dramatically increased in the late induction phase compared to the parent. These results suggested that Vps13 and Vps21 might influence the cellulase production at transcription level. And further transcriptome analysis indicated that increased cellulase gene expression might be attributed to the differential expression of sugar transporters. Our study unravels the effect of alleviating vacuolar transport through knockout vps10, vps13, and vps21 for efficient cellulase secretion, providing new clues for higher cellulase production in T. reesei. KEY POINTS: • Disruption of vps10, vps13 or vps21 improves cellulase production • Vacuolar transport is impaired in three vps KO mutants • Deletion of vps13 or vps21 increases the transcript of cellulase genes in late stage.

Effect of N-acetyl-l-cysteine on Cell Phenotype and Autophagy in Pichia pastoris Expressing Human Serum Albumin and Porcine Follicle-Stimulating Hormone Fusion Protein

Pichia pastoris is widely used for the production of recombinant proteins, but the low secretion efficiency hinders its wide application in biopharmaceuticals. Our previous study had shown that N-acetyl-l-cysteine (NAC) promotes human serum albumin and porcine follicle-stimulating hormone fusion protein (HSA-pFSHβ) secretion by increasing intracellular GSH levels, but the downstream impact mechanism is not clear. In this study, we investigated the roles of autophagy as well as cell phenotype in NAC promoting HSA-pFSHβ secretion. Our results showed that NAC slowed down the cell growth rate, and its effects were unaffected by Congo Red and Calcofluor White. Moreover, NAC affected cell wall composition by increasing chitin content and decreasing β-1,3-glucan content. In addition, the expressions of vesicular pathway and autophagy-related genes were significantly decreased after NAC treatment. Further studies revealed that autophagy, especially the cytoplasm-to-vacuole targeting (Cvt) pathway, mitophagy and pexophagy, was significantly increased with time, and NAC has a promoting effect on autophagy, especially at 48 h and 72 h of NAC treatment. However, the disruption of mitophagy receptor Atg32, but not pexophagy receptor Atg30, inhibited HSA-pFSHβ production, and neither of them inhibited the NAC-promoted effect of HSA-pFSHβ. In conclusion, vesicular transport, autophagy and cell wall are all involved in the NAC-promoted HSA-pFSHβ secretion and that disruption of the autophagy receptor alone does not inhibit the effect of NAC.

Synthetic activation of yeast stress response improves secretion of recombinant proteins

Yeasts, such as Pichia pastoris (syn Komagataella spp.), are particularly suitable expression systems for emerging classes of recombinant proteins. Among them, recombinant antibody fragments, such as single-chain variable fragments (scFv) and single-domain antibodies (VHH), are credible alternatives to monoclonal antibodies. The availability of powerful genetic engineering and synthetic biology tools has facilitated improvement of this cell factory to overcome certain limitations. However, cell engineering to improve secretion often remains a trial-and-error approach and improvements are often specific to the protein produced. Where multiple genetic interventions are needed to remove bottlenecks in the process of recombinant protein secretion, this leads to a high number of combinatorial possibilities for creation of new production strains. Therefore, our aim was to exploit whole transcriptional programs (stress response pathways) in order to simplify the strain engineering of new production strains. Indeed, the artificial activation of the general stress response transcription factor Msn4, as well as synthetic versions thereof, could replace the secretion enhancing effect of several cytosolic chaperones. Greater than 4-fold improvements in recombinant protein secretion were achieved by overexpression of MSN4 or synMSN4, either alone or in combination with Hac1 or ER chaperones. With this concept we were able to successfully engineer strains reaching titers of more than 2.5 g/L scFv and 8 g/L VHH in bioreactor cultivations. This increased secretion capacity of different industrially relevant model proteins indicates that MSN4 overexpression most likely represents a general concept to improve recombinant protein production in yeast.

Enhancement of HSA-pFSHβ production by disrupting YPS1 and supplementing N-acetyl-L-cysteine in Pichia pastoris

Introduction

Pichia pastoris is widely used for the production of recombinant proteins, but the low production efficiency hinders its wide application in biopharmaceuticals. Moreover, many biopharmaceutical-like proteins are accompanied by degradation during secretory expression in P. pastoris.

Objective

In this study, we used human serum albumin and porcine follicle-stimulating hormone β (HSA-pFSHβ) fusion protein as a model protein to investigate whether YPS1 and YPT7 gene disruption and N-acetyl-L-cysteine (NAC) supplementation have synergistic effects to inhibit the degradation of recombinant proteins.

Results and discussion

Our results showed that YPS1 gene disruption reduced the degradation of intact HSA-pFSHβ and increased the yield of intact protein in the culture medium and cells without affecting the integrity of the cell wall. Moreover, the beneficial effects of YPS1 gene disruption were associated with the upregulation of the MAPK signaling pathway and maintenance of redox homeostasis. YPS1 gene disruption and NAC supplementation had synergistic effects on HSA-pFSHβ production. In addition, disruption of vacuolar morphology by YPT7 gene disruption or NH₄Cl treatment affected the production of recombinant HSA-pFSHβ protein. Furthermore, YPT7 gene disruption inhibited the processing of signal peptide in high-level produced HSA-pFSHβ strain. In conclusion, our results demonstrated that YPS1 disruption could reduce the degradation of intact HSA-pFSHβ proteins, and synergistically increase the yield of intact HSA-pFSHβ with NAC supplementation. This study provided a valuable reference for reducing recombinant protein degradation and therefore improving the yield of recombinant proteins in P. pastoris.

Current advances of Pichia pastoris as cell factories for production of recombinant proteins

Pichia pastoris (syn. Komagataella spp.) has attracted extensive attention as an efficient platform for recombinant protein (RP) production. For obtaining a higher protein titer, many researchers have put lots of effort into different areas and made some progress. Here, we summarized the most recent advances of the last 5 years to get a better understanding of its future direction of development. The appearance of innovative genetic tools and methodologies like the CRISPR/Cas9 gene-editing system eases the manipulation of gene expression systems and greatly improves the efficiency of exploring gene functions. The integration of novel pathways in microorganisms has raised more ideas of metabolic engineering for enhancing RP production. In addition, some new opportunities for the manufacture of proteins have been created by the application of novel mathematical models coupled with high-throughput screening to have a better overview of bottlenecks in the biosynthetic process.

Pushing and pulling proteins into the yeast secretory pathway enhances recombinant protein secretion

Yeasts and especially Pichia pastoris (syn Komagataella spp.) are popular microbial expression systems for the production of recombinant proteins. One of the key advantages of yeast host systems is their ability to secrete the recombinant protein into the culture media. However, secretion of some recombinant proteins is less efficient. These proteins include antibody fragments such as Fabs or scFvs. We have recently identified translocation of nascent Fab fragments from the cytosol into the endoplasmic reticulum (ER) as one major bottleneck. Conceptually, this bottleneck requires engineering to increase the flux of recombinant proteins at the translocation step by pushing on the cytosolic side and pulling on the ER side. This engineering strategy is well-known in the field of metabolic engineering. To apply the push-and-pull strategy to recombinant protein secretion, we chose to modulate the cytosolic and ER Hsp70 cycles, which have a key impact on the translocation process. After identifying the relevant candidate factors of the Hsp70 cycles, we combined the push-and-pull factors in a single strain and achieved synergistic effects for antibody fragment secretion. With this concept we were able to successfully engineer strains and improve protein secretion up to 5-fold for different model protein classes. Overall, titers of more than 1.3 g/L Fab and scFv were reached in bioreactor cultivations.

A streamlined strain engineering workflow with genome-wide screening detects enhanced protein secretion in Komagataella phaffii

Expression of secreted recombinant proteins burdens the protein secretion machinery, limiting production. Here, we describe an approach to improving protein production by the non-conventional yeast Komagataella phaffii comprised of genome-wide screening for effective gene disruptions, combining them in a single strain, and recovering growth reduction by adaptive evolution. For the screen, we designed a multiwell-formatted, streamlined workflow to high-throughput assay of secretion of a single-chain small antibody, which is cumbersome to detect but serves as a good model of proteins that are difficult to secrete. Using the consolidated screening system, we evaluated >19,000 mutant strains from a mutant library prepared by a modified random gene-disruption method, and identified six factors for which disruption led to increased antibody production. We then combined the disruptions, up to quadruple gene knockouts, which appeared to contribute independently, in a single strain and observed an additive effect. Target protein and promoter were basically interchangeable for the effects of knockout genes screened. We finally used adaptive evolution to recover reduced cell growth by multiple gene knockouts and examine the possibility for further enhancing protein secretion. Our successful, three-part approach holds promise as a method for improving protein production by non-conventional microorganisms.

Strains and Molecular Tools for Recombinant Protein Production in Pichia pastoris

Within the last two decades, the methylotrophic yeast Pichia pastoris (Komagataella phaffii) has become an important alternative to E. coli or mammalian cell lines for the production of recombinant proteins. Easy handling, strong promoters, and high cell density cultivations as well as the capability of posttranslational modifications are some of the major benefits of this yeast. The high secretion capacity and low level of endogenously secreted proteins further promoted the rapid development of a versatile Pichia pastoris toolbox. This chapter reviews common and new "Pichia tools" and their specific features. Special focus is given to expression strains, such as different methanol utilization, protease-deficient or glycoengineered strains, combined with application highlights. Different promoters and signal sequences are also discussed.

The Nma1 protein promotes long distance transport mediated by early endosomes in Ustilago maydis

Early endosomes (EEs) are part of the endocytic transport pathway and resemble the earliest class of transport vesicles between the internalization of extracellular material, their cellular distribution or vacuolar degradation. In filamentous fungi, EEs fulfill important functions in long distance transport of cargoes as mRNAs, ribosomes, and peroxisomes. Formation and maturation of early endosomes is controlled by the specific membrane-bound Rab-GTPase Rab5 and tethering complexes as CORVET (class C core vacuole/endosome tethering). In the basidiomycete Ustilago maydis, Rab5a is the prominent GTPase to recruit CORVET to EEs; in rab5a deletion strains, this function is maintained by the second EE-associated GTPase Rab5b. The tethering- and core-subunits of CORVET are essential, buttressing a central role for EE transport in U. maydis. The function of EEs in long distance transport is supported by the Nma1 protein that interacts with the Vps3 subunit of CORVET. The interaction stabilizes the binding of Vps3 to the CORVET core complex that is recruited to Rab5a via Vps8. Deletion of nma1 leads to a significantly reduced number of EEs, and an increased conversion rate of EEs to late endosomes. Thus, Nma1 modulates the lifespan of EEs to ensure their availability for the various long distance transport processes.

Heterologous production of cellulose- and starch-degrading hydrolases to expand Saccharomyces cerevisiae substrate utilization: Lessons learnt

Selected strains of Saccharomyces cerevisiae are used for commercial bioethanol production from cellulose and starch, but the high cost of exogenous enzymes for substrate hydrolysis remains a challenge. This can be addressed through consolidated bioprocessing (CBP) where S. cerevisiae strains are engineered to express recombinant glycoside hydrolases during fermentation. Looking back at numerous strategies undertaken over the past four decades to improve recombinant protein production in S. cerevisiae, it is evident that various steps in the protein production "pipeline" can be manipulated depending on the protein of interest and its anticipated application. In this review, we briefly introduce some of the strategies and highlight lessons learned with regards to improved transcription, translation, post-translational modification and protein secretion of heterologous hydrolases. We examine how host strain selection and modification, as well as enzyme compatibility, are crucial determinants for overall success. Finally, we discuss how lessons from heterologous hydrolase expression can inform modern synthetic biology and genome editing tools to provide process-ready yeast strains in future. However, it is clear that the successful expression of any particular enzyme is still unpredictable and requires a trial-and-error approach.

The mechanism of N-acetyl-l-cysteine in improving the secretion of porcine follicle-stimulating hormone in Pichia pastoris

Our previous study revealed that N-acetyl-l-cysteine (NAC) could enhance the secretion of recombinant proteins by Pichia pastoris, but the corresponding molecular mechanisms are still unclear. In the present study, we explored whether other thiols have a similar action on the secretion of recombinant human serum albumin and porcine follicle-stimulating hormone fusion protein (HSA-pFSHβ), to reveal the mechanism of NAC on HSA-pFSHβ secretion. Transcriptome analysis showed that genes involved in oxidoreductase activity and oxidation-reduction process were upregulated in cells supplemented with NAC. The other three thiol-reducing regents including dimercaptopropanol (DT), thioglycolic acid, and mercaptolactic acid could improve HSA-pFSHβ production in the culture supernatant. Among them, only DT had similar effect as NAC on HSA-pFSHβ secretion and the increase of GSH content. Moreover, 1-20 mM GSH, 1-10 mM cysteine, or 1-20 mM N-acetyl-d-cysteine supplementation could improve the secretion of HSA-pFSHβ. Furthermore, 0.4-3.2 mM ethacrynic acid, rather than 1-16 mM BSO could inhibit the effect of NAC on the production of HSA-pFSHβ. These results indicated that NAC improved the secretion of HSA-pFSHβ by increasing the intracellular GSH content through its thiol activity rather than as a precursor for GSH synthesis. In conclusion, our results demonstrate, for the first time, that the secretion of recombinant HSA-pFSHβ in Pichia pastoris could be improved through thiol-reducing agent supplementation, and the mechanism of the effect NAC has on HSA-pFSHβ secretion is associated with improving the intracellular GSH content.

Global transcriptome profiling reveals genes responding to overproduction of a small secretory, a high cysteine- and a high glycosylation-bearing protein in Yarrowia lipolytica

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Secretion of a protein results in > 10-fold higher titer compared to its retention.

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Overproduction of rs-Prots induces oxidative stress and detoxification response.

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Excessive vacuolar protein degradation limits rs-Prot production in Y. lipolytica.

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Non-classical export protein NCE102 is upregulated upon rs-Prot overloading.

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Downregulation of cyclin CLN1 marks growth arrest in G1 under rs-Prot synthesis.

Investigation of the yeast cell’s response to recombinant secretory protein (rs-Prot) overproduction is relevant for both basic and applied research. Imbalance, overloading or stress within this process impacts the whole cell. In the present study, by using steady-state cultures and transcriptomics, we investigated the cellular response of Yarrowia lipolytica challenged with high-level expression of genes encoding proteins with significantly different biochemical characteristics: a small protein retained within the cell i) or secreted ii), a medium size secretory protein with a high number of disulfide bonds iii), or glycosylation sites iv). Extensive analysis of omics data, supported by careful manual curation, led to some anticipated observations on oxidative and unfolded protein stress (CTT1, PXMP2/4, HAC1), glycosylation (ALGs, KTRs, MNTs, MNNs), folding and translocation (SSAs, SSEs) but also generated new exciting knowledge on non-conventional protein secretion (NCE102), transcriptional regulators (FLO11, MHY1, D01353 g, RSFA, E23925g or MAF1), vacuolar proteolysis targets in Y. lipolytica (ATGs, VPSs, HSE1, PRB1, PRC1, PEP4) or growth arrest (CLN1) upon rs-Prots overproduction.

Molecular engineering improves antigen quality and enables integrated manufacturing of a trivalent subunit vaccine candidate for rotavirus

BACKGROUND

Vaccines comprising recombinant subunit proteins are well-suited to low-cost and high-volume production for global use. The design of manufacturing processes to produce subunit vaccines depends, however, on the inherent biophysical traits presented by an individual antigen of interest. New candidate antigens typically require developing custom processes for each one and may require unique steps to ensure sufficient yields without product-related variants.

RESULTS

We describe a holistic approach for the molecular design of recombinant protein antigens-considering both their manufacturability and antigenicity-informed by bioinformatic analyses such as RNA-seq, ribosome profiling, and sequence-based prediction tools. We demonstrate this approach by engineering the product sequences of a trivalent non-replicating rotavirus vaccine (NRRV) candidate to improve titers and mitigate product variants caused by N-terminal truncation, hypermannosylation, and aggregation. The three engineered NRRV antigens retained their original antigenicity and immunogenicity, while their improved manufacturability enabled concomitant production and purification of all three serotypes in a single, end-to-end perfusion-based process using the biotechnical yeast Komagataella phaffii.

CONCLUSIONS

This study demonstrates that molecular engineering of subunit antigens using advanced genomic methods can facilitate their manufacturing in continuous production. Such capabilities have potential to lower the cost and volumetric requirements in manufacturing vaccines based on recombinant protein subunits.

A subcellular proteome atlas of the yeast Komagataella phaffii

The compartmentalization of metabolic and regulatory pathways is a common pattern of living organisms. Eukaryotic cells are subdivided into several organelles enclosed by lipid membranes. Organelle proteomes define their functions. Yeasts, as simple eukaryotic single cell organisms, are valuable models for higher eukaryotes and frequently used for biotechnological applications. While the subcellular distribution of proteins is well studied in Saccharomyces cerevisiae, this is not the case for other yeasts like Komagataella phaffii (syn. Pichia pastoris). Different to most well-studied yeasts, K. phaffii can grow on methanol, which provides specific features for production of heterologous proteins and as a model for peroxisome biology. We isolated microsomes, very early Golgi, early Golgi, plasma membrane, vacuole, cytosol, peroxisomes and mitochondria of K. phaffii from glucose- and methanol-grown cultures, quantified their proteomes by liquid chromatography-electrospray ionization-mass spectrometry of either unlabeled or tandem mass tag-labeled samples. Classification of the proteins by their relative enrichment, allowed the separation of enriched proteins from potential contaminants in all cellular compartments except the peroxisomes. We discuss differences to S. cerevisiae, outline organelle specific findings and the major metabolic pathways and provide an interactive map of the subcellular localization of proteins in K. phaffii.

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

Phytase is an additive in animal feed that degrades phytic acid in plant material, reducing feeding costs, and pollution from fecal phosphorus excretion. A multistrategy approach was adopted to improve the expression of E. coli phytase in Pichia pastoris. We determined that the most suitable signal peptide for phytase secretion was an α-factor secretion signal with an initial enzyme activity of 153.51 U/mL. Increasing the copy number of this gene to four increased phytase enzyme activity by 234.35%. PDI overexpression and Pep4 gene knockout increased extracellular phytase production by 35.33% and 26.64%, respectively. By combining favorable factors affecting phytase expression and secretion, the enzyme activity of the phytase-engineered strain was amplified 384.60% compared with that of the original strain. We also evaluated the potential for the industrial production of the engineered strain using a 50-L fed-batch fermenter and achieved a total activity of 30,246 U/mL after 180 h of fermentation.

Exploiting strain diversity and rational engineering strategies to enhance recombinant cellulase secretion by Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of lignocellulosic material into bioethanol has progressed in the past decades; however, several challenges still exist which impede the industrial application of this technology. Identifying the challenges that exist in all unit operations is crucial and needs to be optimised, but only the barriers related to the secretion of recombinant cellulolytic enzymes in Saccharomyces cerevisiae will be addressed in this review. Fundamental principles surrounding CBP as a biomass conversion platform have been established through the successful expression of core cellulolytic enzymes, namely β-glucosidases, endoglucanases, and exoglucanases (cellobiohydrolases) in S. cerevisiae. This review will briefly address the challenges involved in the construction of an efficient cellulolytic yeast, with particular focus on the secretion efficiency of cellulases from this host. Additionally, strategies for studying enhanced cellulolytic enzyme secretion, which include both rational and reverse engineering approaches, will be discussed. One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development. Furthermore, with the advancement in next-generation sequencing, studies that utilise this method of exploiting intra-strain diversity for industrially relevant traits will be reviewed. Finally, future prospects are discussed for the creation of ideal CBP strains with high enzyme production levels.Key Points• Several challenges are involved in the construction of efficient cellulolytic yeast, in particular, the secretion efficiency of cellulases from the hosts.• Strategies for enhancing cellulolytic enzyme secretion, a core requirement for CBP host microorganism development, include both rational and reverse engineering approaches.• One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development.

Yeast synthetic biology for designed cell factories producing secretory recombinant proteins

Yeasts are prominent hosts for the production of recombinant proteins from industrial enzymes to therapeutic proteins. Particularly, the similarity of protein secretion pathways between these unicellular eukaryotic microorganisms and higher eukaryotic organisms has made them a preferential host to produce secretory recombinant proteins. However, there are several bottlenecks, in terms of quality and quantity, restricting their use as secretory recombinant protein production hosts. In this mini-review, we discuss recent developments in synthetic biology approaches to constructing yeast cell factories endowed with enhanced capacities of protein folding and secretion as well as designed targeted post-translational modification process functions. We focus on the new genetic tools for optimizing secretory protein expression, such as codon-optimized synthetic genes, combinatory synthetic signal peptides and copy number-controllable integration systems, and the advanced cellular engineering strategies, including endoplasmic reticulum and protein trafficking pathway engineering, synthetic glycosylation, and cell wall engineering, for improving the quality and yield of secretory recombinant proteins.

Role of BGS13 in the Secretory Mechanism of Pichia pastoris

The methylotrophic yeast Pichia pastoris has been utilized for heterologous protein expression for over 30 years. Because P. pastoris secretes few of its own proteins, the exported recombinant protein is the major polypeptide in the extracellular medium, making purification relatively easy. Unfortunately, some recombinant proteins intended for secretion are retained within the cell. A mutant strain isolated in our laboratory, containing a disruption of the BGS13 gene, displayed elevated levels of secretion for a variety of reporter proteins. The Bgs13 peptide (Bgs13p) is similar to the Saccharomyces cerevisiae protein kinase C 1 protein (Pkc1p), but its specific mode of action is currently unclear. To illuminate differences in the secretion mechanism between the wild-type (wt) strain and the bgs13 strain, we determined that the disrupted bgs13 gene expressed a truncated protein that had reduced protein kinase C activity and a different location in the cell, compared to the wt protein. Because the Pkc1p of baker's yeast plays a significant role in cell wall integrity, we investigated the sensitivity of the mutant strain's cell wall to growth antagonists and extraction by dithiothreitol, determining that the bgs13 strain cell wall suffered from inherent structural problems although its porosity was normal. A proteomic investigation of the bgs13 strain secretome and cell wall-extracted peptides demonstrated that, compared to its wt parent, the bgs13 strain also displayed increased release of an array of normally secreted, endogenous proteins, as well as endoplasmic reticulum-resident chaperone proteins, suggesting that Bgs13p helps regulate the unfolded protein response and protein sorting on a global scale.IMPORTANCE The yeast Pichia pastoris is used as a host system for the expression of recombinant proteins. Many of these products, including antibodies, vaccine antigens, and therapeutic proteins such as insulin, are currently on the market or in late stages of development. However, one major weakness is that sometimes these proteins are not secreted from the yeast cell efficiently, which impedes and raises the cost of purification of these vital proteins. Our laboratory has isolated a mutant strain of Pichia pastoris that shows enhanced secretion of many proteins. The mutant produces a modified version of Bgs13p. Our goal is to understand how the change in the Bgs13p function leads to improved secretion. Once the Bgs13p mechanism is illuminated, we should be able to apply this understanding to engineer new P. pastoris strains that efficiently produce and secrete life-saving recombinant proteins, providing medical and economic benefits.

The secretome of Pichia pastoris in fed-batch cultivations is largely independent of the carbon source but changes quantitatively over cultivation time

The quantitative changes of the secretome of recombinant Pichia pastoris (Komagataella phaffii) CBS7435 over the time-course of methanol- or glucose-limited fed-batch cultures were investigated by LC-ESI-MS/MS to define the carbon source-specific secretomes under controlled bioreactor conditions. In both set-ups, no indication for elevated cell lysis was found. The quantitative data revealed that intact and viable P. pastoris cells secrete only a low number of endogenous proteins (in total 51), even during high cell density cultivation. Interestingly, no marked differences in the functional composition of the P. pastoris secretome between methanol- and glucose-grown cultures were observed with only few proteins being specifically affected by the carbon source. The 'core secretome' of 22 proteins present in all analysed carbon sources (glycerol, glucose and methanol) consists mainly of cell wall proteins. The quantitative analysis additionally revealed that most secretome proteins were already present after the batch phase, and depletion rather than accumulation occurred during the fed-batch processes. Among the changes over cultivation time, the depletion of both the extracellularly detected chaperones and the only two identified proteases (Pep4 and Yps1-1) during the methanol- or glucose-feed phase appear as most prominent.

Disruption of vacuolar protein sorting components of the HOPS complex leads to enhanced secretion of recombinant proteins in Pichia pastoris

Background

The yeast Pichia pastoris is a widely used host for the secretion of heterologous proteins. Despite being an efficient producer, we observed previously that certain recombinant proteins were mistargeted to the vacuole on their route to secretion. Simultaneous disruption of one vacuolar sorting pathway together with vacuolar proteases prevented this mis-sorting and resulted in higher levels of secreted heterologous protein. Inspired by the positive results, we now set out to investigate the influence of further parts of the vacuolar pathway, namely the Cvt-pathway and the homotypic fusion and protein sorting (HOPS) complex.

Results

Strains impaired in the Cvt pathway (∆atg11, ∆atg8) had no effect on secretion of the model protein carboxylesterase (CES), but resulted in lower secretion levels of the antibody fragment HyHEL-Fab. Disruption of genes involved in the HOPS complex led to vacuole-like compartments of the B category of vps mutants, which are characteristic for the deleted genes YPT7, VPS41 and VAM6. In particular ∆ypt7 and ∆vam6 strains showed an improvement in secreting the model proteins HyHEL-Fab and CES. Additional disruption of the vacuolar protease Pep4 and the potential protease Vps70 led to even further enhanced secretion in ∆ypt7 and ∆vam6 strains. Nevertheless, intracellular product accumulation was still observed. Therefore, the secretory route was strengthened by overexpression of early or late secretory genes in the vacuolar sorting mutants. Thereby, overexpression of Sbh1, a subunit of the ER translocation pore, significantly increased HyHEL-Fab secretion, leading up to fourfold higher extracellular Fab levels in the ∆ypt7 strain. The beneficial impact on protein secretion and the suitability of these strains for industrial applicability was confirmed in fed-batch cultivations.

Conclusions

Disruption of genes involved in the HOPS complex, especially YPT7, has a great influence on the secretion of the two different model proteins HyHEL-Fab and CES. Therefore, disruption of HOPS genes shows a high potential to increase secretion of other recombinant proteins as well. Secretion of HyHEL-Fab was further enhanced when overexpressing secretion enhancing factors. As the positive effect was also present in fed-batch cultivations, these modifications likely have promising industrial relevance.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1155-4) contains supplementary material, which is available to authorized users.

RNAi expression tuning, microfluidic screening, and genome recombineering for improved protein production in Saccharomyces cerevisiae

The cellular machinery that supports protein synthesis and secretion lies at the foundation of cell factory-centered protein production. Due to the complexity of such cellular machinery, the challenge in generating a superior cell factory is to fully exploit the production potential by finding beneficial targets for optimized strains, which ideally could be used for improved secretion of other proteins. We focused on an approach in the yeast Saccharomyces cerevisiae that allows for attenuation of gene expression, using RNAi combined with high-throughput microfluidic single-cell screening for cells with improved protein secretion. Using direct experimental validation or enrichment analysis-assisted characterization of systematically introduced RNAi perturbations, we could identify targets that improve protein secretion. We found that genes with functions in cellular metabolism (YDC1, AAD4, ADE8, and SDH1), protein modification and degradation (VPS73, KTR2, CNL1, and SSA1), and cell cycle (CDC39), can all impact recombinant protein production when expressed at differentially down-regulated levels. By establishing a workflow that incorporates Cas9-mediated recombineering, we demonstrated how we could tune the expression of the identified gene targets for further improved protein production for specific proteins. Our findings offer a high throughput and semirational platform design, which will improve not only the production of a desired protein but even more importantly, shed additional light on connections between protein production and other cellular processes.

High-Throughput Screening and Selection of Pichia pastoris Strains

Clone screening procedures for Pichia pastoris expression strain comparison rely on the availability of a cultivation environment that ensures equal growth and production capabilities for all assessed transformants. As clonal variation in such experiments is caused by diverging numbers and possibly also genomic locations of integrated (linearized) expression constructs, the productivity assessment of a larger number of strains is mandatory for selecting a set of strains for follow-up bioreactor cultivations in order to define the best-producing clone. Microscale cultivation provides the means to reliably compare growth and productivity of a large number of transformants and by that narrows down the amount of selected strains for scaling up.

Detection and Elimination of Cellular Bottlenecks in Protein-Producing Yeasts

Yeasts are efficient cell factories and are commonly used for the production of recombinant proteins for biopharmaceutical and industrial purposes. For such products high levels of correctly folded proteins are needed, which sometimes requires improvement and engineering of the expression system. The article summarizes major breakthroughs that led to the efficient use of yeasts as production platforms and reviews bottlenecks occurring during protein production. Special focus is given to the metabolic impact of protein production. Furthermore, strategies that were shown to enhance secretion of recombinant proteins in different yeast species are presented.

CRISPR/Cas9-Mediated Homology-Directed Genome Editing in Pichia pastoris

State-of-the-art strain engineering techniques for the methylotrophic yeast Pichia pastoris (syn. Komagataella spp.) include overexpression of endogenous and heterologous genes and deletion of host genes. For efficient gene deletion, methods such as the split-marker technique have been established. However, synthetic biology trends move toward building up large and complex reaction networks, which often require endogenous gene knockouts and simultaneous overexpression of individual genes or whole pathways. Realization of such engineering tasks by conventional approaches employing subsequent steps of transformations and marker recycling is very time- and labor-consuming. Other applications require tagging of certain genes/proteins or promoter exchange approaches, which are hard to design and construct with conventional methods. Therefore, efficient systems are required that allow precise manipulations of the P. pastoris genome, including simultaneous overexpression of multiple genes. To meet this challenge, we have developed a CRISPR/Cas9-based kit for gene insertions, deletions, and replacements, which paves the way for precise genomic modifications in P. pastoris. In this chapter, the versatile method for performing these modifications without the integration of a selection marker is described. A ready-to-use plasmid kit for performing CRISPR/Cas9-mediated genome editing in P. pastoris based on the GoldenPiCS modular cloning vectors is available at Addgene as CRISPi kit (#1000000136).

Filamentous fungi-like secretory pathway strayed in a yeast system: peculiarities of Yarrowia lipolytica secretory pathway underlying its extraordinary performance

Microbial production of secretory proteins constitutes one of the key branches of current industrial biotechnology, earning billion dollar (USD) revenues each year. That industrial branch strongly relies on fluent operation of the secretory machinery within a microbial cell. The secretory machinery, directing the nascent polypeptide to its final destination, constitutes a highly complex system located across the eukaryotic cell. Numerous molecular identities of diverse structure and function not only build the advanced network assisting folding, maturation and secretion of polypeptides but also serve as sensors and effectors of quality control points. All these events must be harmoniously orchestrated to enable fluent processing of the protein traffic. Availability of these elements is considered to be the limiting factor determining capacity of protein traffic, which is of crucial importance upon biotechnological production of secretory proteins. The main purpose of this work is to review and discuss findings concerning secretory machinery operating in a non-conventional yeast species, Yarrowia lipolytica, and to highlight peculiarities of this system prompting its use as the production host. The reviewed literature supports the thesis that secretory machinery in Y. lipolytica is characterized by significantly higher complexity than a canonical yeast protein secretion pathway, making it more similar to filamentous fungi-like systems in this regard.

The impact of ERAD on recombinant protein secretion in Pichia pastoris (syn Komagataella spp.)

The yeast Pichia pastoris (syn. Komagataella spp.) is a popular cell factory for recombinant protein production. Yeasts in general provide a good starting point for cell factory engineering. They are intrinsically robust and easy to manipulate and cultivate. However, their secretory pathway is not evolutionarily adapted to high loads of secretory protein. In particular, more complex proteins, like the antibody fragment (Fab) used in this study, overwhelm the folding and secretion capacity. This triggers cellular stress responses, which may cause excessive intracellular degradation. Previous results have shown that, in fact, about 60 % of the newly synthesized Fab is intracellularly degraded. Endoplasmic reticulum-associated protein degradation (ERAD) is one possible intracellular degradation pathway for proteins aimed for secretion. We therefore targeted ERAD for cell factory engineering and investigated the impact on recombinant protein secretion in P. pastoris. Three components of the ERAD-L complex, which is involved in the degradation of luminal proteins, and a protein involved in proteasomal degradation, were successfully disrupted in Fab-secreting P. pastoris. Contrary to expectation, the effect on secretion was marginal. In the course of more detailed investigation of the impact of ERAD, we took a closer look at the intracellular variants of the recombinant protein. This enabled us to further zero in on the issue of intracellular Fab degradation and exclude an overshooting ER quality control. We propose that a major fraction of the Fab is actually degraded before entering the secretory pathway.

Engineering strategies for enhanced production of protein and bio-products in Pichia pastoris: A review

Pichia pastoris has been recognized as one of the most industrially important hosts for heterologous protein production. Despite its high protein productivity, the optimization of P. pastoris cultivation is still imperative due to strain- and product-specific challenges such as promoter strength, methanol utilization type and oxygen demand. To address the issues, strategies involving genetic and process engineering have been employed. Optimization of codon usage and gene dosage, as well as engineering of promoters, protein secretion pathways and methanol metabolic pathways have proved beneficial to innate protein expression levels. Large-scale production of proteins via high cell density fermentation additionally relies on the optimization of process parameters including methanol feed rate, induction temperature and specific growth rate. Recent progress related to the enhanced production of proteins in P. pastoris via various genetic engineering and cultivation strategies are reviewed. Insight into the regulation of the P. pastoris alcohol oxidase 1 (AOX1) promoter and the development of methanol-free systems are highlighted. Novel cultivation strategies such as mixed substrate feeding are discussed. Recent advances regarding substrate and product monitoring techniques are also summarized. Application of P. pastoris to the production of biodiesel and other value-added products via metabolic engineering are also reviewed. P. pastoris is becoming an indispensable platform through the use of these combined engineering strategies.