Modulating heterologous protein production in yeast: the applicability of truncated auxotrophic markers

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.

Cross-feeding promotes heterogeneity within yeast cell populations

Cellular heterogeneity in cell populations of isogenic origin is driven by intrinsic factors such as stochastic gene expression, as well as external factors like nutrient availability and interactions with neighbouring cells. Heterogeneity promotes population fitness and thus has important implications in antimicrobial and anticancer treatments, where stress tolerance plays a significant role. Here, we study plasmid retention dynamics within a population of plasmid-complemented ura3∆0 yeast cells, and show that the exchange of complementary metabolites between plasmid-carrying prototrophs and plasmid-free auxotrophs allows the latter to survive and proliferate in selective environments. This process also affects plasmid copy number in plasmid-carrying prototrophs, further promoting cellular functional heterogeneity. Finally, we show that targeted genetic engineering can be used to suppress cross-feeding and reduce the frequency of plasmid-free auxotrophs, or to exploit it for intentional population diversification and division of labour in co-culture systems.

High-level heterologous protein production using an attenuated selection marker in Aspergillus sojae

Filamentous fungi, including Aspergillus sojae, are essential for the industrial production of enzymes. Although multi-copy introduction of a gene encoding the protein of interest is useful for increasing protein production, this method has not been established in the case of filamentous fungi. In this study, we aimed to establish an efficient system for multi-copy chromosomal integration and high-level expression of a heterologous gene in A. sojae using an attenuated selectable marker. Consequently, by truncating the promoter region of selectable markers, we efficiently introduced multiple copies of a heterologous gene and enhanced the rate of high-level protein-production in the strains. Since the multi-copy strains obtained in this study maintained high productivity even in a non-selective medium, this system could be applicable for industrial protein production.

Reciprocal relation between reporter gene transcription and translation efficiency in fission yeast

The fission yeast, Schizosaccharomyces pombe, is an excellent model for basic research but is not useful for commercial scale protein expression due to lack of strong expression vectors. Earlier, we showed that the lsd90 promoter elicited significantly greater GFP expression level than the adh1 and nmt1 promoters, albeit in different vector backbones. Here, we have systematically investigated the contribution of selectable markers, LEU2 and URA3m to GFP expression: while LEU2 elicited very low expression, the URA3m gene, with truncated promoter, elicited much greater GFP expression level with all promoters. Paradoxically, an inverse correlation was observed between the GFP transcription and translation efficiency. This system can be useful for understanding the factors governing recombinant gene expression and optimization of protein production.

Increase of Candida antarctica lipase B production under PGK promoter in Pichia pastoris: effect of multicopies

The effect of gene dosage on the production of Candida antarctica lipase B (CalB) in the methylotrophic yeast Komagataella phaffii, at high densities in a simple medium containing crude glycerin as the sole carbon source, is described. The use of crude glycerin, the main by-product of biodiesel production from vegetable oils, will reduce the production cost of the bioprocess. Two K. phaffii strains were constructed with one or three copies of LipB, an optimized version of the gene encoding CalB under the control of the constitutive P_PGK1 promoter. These two constructs were tested and compared on batches using minimal-salts medium with crude glycerin. The strain with three copies achieved a higher enzyme yield (48,760 U/L, 2.3-fold higher than the one-copy strain), with 42 g/L biomass, with no effects on growth.

Adaptive response of yeast cells to triggered toxicity of phosphoribulokinase

Adjustment of plasmid copy number resulting from the balance between positive and negative impacts of borne synthetic genes, plays a critical role in the global efficiency of multistep metabolic engineering. Differential expression of co-expressed engineered genes is frequently observed depending on growth phases, metabolic status and triggered adjustments of plasmid copy numbers, constituting a dynamic process contributing to minimize global engineering burden. A yeast model involving plasmid based expression of phosphoribulokinase (PRKp), a key enzyme for the reconstruction of synthetic Calvin cycle, was designed to gain further insights into such a mechanism. A conditional PRK expression cassette was cloned either onto a low (ARS-CEN based) or a high (2-micron origin based) copy number plasmid using complementation of a trp1 genomic mutation as constant positive selection. Evolution of plasmid copy numbers, PRKp expressions, and cell growth rates were dynamically monitored following gene de-repression through external doxycycline concentration shifts. In the absence of RubisCO encoding gene permitting metabolic recycling, PRKp expression that led to depletion of ribulose phosphate, a critical metabolite for aromatic amino-acids biosynthesis, and accumulation of the dead-end diphosphate product contribute to toxicity. Triggered copy number adjustment was found to be a dynamic process depending both on plasmid types and levels of PRK induction. With the ARS-CEN plasmid, cell growth was abruptly affected only when level PRKp expression exceeded a threshold value. In contrast, a proportional relationship was observed with the 2-micron plasmid consistent with large copy number adjustments. Micro-compartment partitioning of bulk cultures by embedding individual cells into inverse culture medium/oil droplets, revealed the presence of slow and fast growing subpopulations that differ in relative proportions for low and high copy number plasmids.

Impact of plasmid architecture on stability and yEGFP3 reporter gene expression in a set of isomeric multicopy vectors in yeast

Multicopy episomal plasmids in yeast, used whenever elevated levels of foreign or homologous gene expression are necessary, are known to be less stable compared to the endogenous 2-μm plasmid they are based on, at least without selective pressure. Considering that rich medium favors growth rate and, simultaneously, is less expensive than selective medium, enhancing stability in non-selective medium is extremely desirable. In this study, we changed the architecture of a multicopy model expression plasmid, creating six isoforms (same size, same DNA content but different positions and orientations of the expression block) and studied mitotic stability, copy number, as well as reporter yEGFP3 expression between isoforms. With one isoform being significantly more stable than the others and another one exhibiting elevated plasmid copy numbers in rich medium, we show that consideration of the arrangement of the plasmid elements might be crucial for productivity employing Saccharomyces cerevisiae as a host. We strongly believe that the ideal architecture has to be assessed for each case and assembly strategy has to begin by evaluating the stability of the vector backbone before insertion of the desired gene. For the plasmid set studied, yEGFP3 reporter production depends more on mitotic stability than on elevated plasmid copy numbers in a small number of cells retaining the plasmid under non-selective conditions.

Multicopy plasmid integration in Komagataella phaffii mediated by a defective auxotrophic marker

Background

A commonly used approach to improve recombinant protein production is to increase the levels of expression by providing extra-copies of a heterologous gene. In Komagataella phaffii (Pichia pastoris) this is usually accomplished by transforming cells with an expression vector carrying a drug-resistance marker following a screening for multicopy clones on plates with increasingly higher concentrations of an antibiotic. Alternatively, defective auxotrophic markers can be used for the same purpose. These markers are generally transcriptionally impaired genes lacking most of the promoter region. Among the defective markers commonly used in Saccharomyces cerevisiae is leu2-d, an allele of LEU2 which is involved in leucine metabolism. Cells transformed with this marker can recover prototrophy when they carry multiple copies of leu2-d in order to compensate the poor transcription from this defective allele.

Results

A K. phaffii strain auxotrophic for leucine (M12) was constructed by disrupting endogenous LEU2. The resulting strain was successfully transformed with a vector carrying leu2-d and an EGFP (enhanced green fluorescent protein) reporter gene. Vector copy numbers were determined from selected clones which grew to different colony sizes on transformation plates. A direct correlation was observed between colony size, number of integrated vectors and EGFP production. By using this approach we were able to isolate genetically stable clones bearing as many as 20 integrated copies of the vector and with no significant effects on cell growth.

Conclusions

In this work we have successfully developed a genetic system based on a defective auxotrophic which can be applied to improve heterologous protein production in K. phaffii. The system comprises a K. phaffii leu2 strain and an expression vector carrying the defective leu2-d marker which allowed the isolation of multicopy clones after a single transformation step. Because a linear correlation was observed between copy number and heterologous protein production, this system may provide a simple approach to improve recombinant protein productivity in K. phaffii.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0715-8) contains supplementary material, which is available to authorized users.

Molecular strategies to increase the levels of heterologous transcripts in Komagataella phaffii for protein production

Komagataella phaffii (formerly Pichia pastoris) is a well-known fungal system for heterologous protein production in the context of modern biotechnology. To obtain higher protein titers in this system many researchers have sought to optimize gene expression by increasing the levels of transcription of the heterologous gene. This has been typically achieved by manipulating promoter sequences or by generating clones bearing multiple copies of the desired gene. The aim of this work is to describe how these different molecular strategies have been applied in K. phaffii presenting their advantages and drawbacks.

Expression and purification of FGF21 in Pichia pastoris and its effect on fibroblast-cell migration

Fibroblast growth factor (FGF)21 functions in the maintenance of glucose homeostasis and exerts protective effects on the liver, heat and kidneys. However, the roles of FGF21 in other tissue types are yet to be fully elucidated. The present study detected elevated expression levels of FGF21 in skin tissue. Furthermore, it was revealed that FGF21 expression in the skin was induced upon wounding. In addition, β‑klotho expression was detected in the skin tissue. To examine the role of FGF21 in the wound healing process, recombinant human (h)FGF21 was expressed in a the yeast strain Pichia (P.) pastoris, a well‑known system for recombinant protein production. Based on the sequence of hFGF21 and the optimal codon of P. pastoris, codon‑optimized FGF21 open reading frame sequences were obtained using seven pairs of 55‑59‑nt primers with seven rounds of PCR. The recombinant FGF21 was purified and its function was examined in human fibroblast cells using a wound healing cell migration assay. Treatment with FGF21 promoted cell migration, which is an important step in wound healing. Furthermore, FGF21 treatment enhanced the activity of c‑Jun N‑terminal kinase, a key regulator in fibroblast‑cell migration. In conclusion, FGF21 is induced after wounding and FGF21 expressed and purified from yeast markedly accelerates wound healing. The present study was the first to elucidate the function of FGF21 in skin tissues and provided a theoretical basis for the use of FGF21 in the treatment of skin wounds.

The synthesis of recombinant membrane proteins in yeast for structural studies

Historically, recombinant membrane protein production has been a major challenge meaning that many fewer membrane protein structures have been published than those of soluble proteins. However, there has been a recent, almost exponential increase in the number of membrane protein structures being deposited in the Protein Data Bank. This suggests that empirical methods are now available that can ensure the required protein supply for these difficult targets. This review focuses on methods that are available for protein production in yeast, which is an important source of recombinant eukaryotic membrane proteins. We provide an overview of approaches to optimize the expression plasmid, host cell and culture conditions, as well as the extraction and purification of functional protein for crystallization trials in preparation for structural studies.

Molecular basis of nitrate uptake by the plant nitrate transporter NRT1.1

The NRT1/PTR family of proton-coupled transporters are responsible for nitrogen assimilation in eukaryotes and bacteria through the uptake of peptides. However, in the majority of plant species members of this family have evolved to transport nitrate as well as additional secondary metabolites and hormones. In response to falling nitrate levels, NRT1.1 is phosphorylated on an intracellular threonine that switches the transporter from a low to high affinity state. Here we present both the apo and nitrate bound crystal structures of Arabidopsis thaliana NRT1.1, which together with in vitro binding and transport data identify a key role for His356 in nitrate binding. Our data support a model whereby phosphorylation increases structural flexibility and in turn the rate of transport. Comparison with peptide transporters further reveals how the NRT1/PTR family has evolved to recognize diverse nitrogenous ligands, whilst maintaining elements of a conserved coupling mechanism within this superfamily of nutrient transporters.

Expression, stabilization and purification of membrane proteins via diverse protein synthesis systems and detergents involving cell-free associated with self-assembly peptide surfactants

G-protein coupled receptors (GPCRs) are involved in regulating most of physiological actions and metabolism in the bodies, which have become most frequently addressed therapeutic targets for various disorders and diseases. Purified GPCR-based drug discoveries have become routine that approaches to structural study, novel biophysical and biochemical function analyses. However, several bottlenecks that GPCR-directed drugs need to conquer the problems including overexpression, solubilization, and purification as well as stabilization. The breakthroughs are to obtain efficient protein yield and stabilize their functional conformation which are both urgently requiring of effective protein synthesis system methods and optimal surfactants. Cell-free protein synthesis system is superior to the high yields and post-translation modifications, and early signs of self-assembly peptide detergents also emerged to superiority in purification of membrane proteins. We herein focus several predominant protein synthesis systems and surfactants involving the novel peptide detergents, and uncover the advantages of cell-free protein synthesis system with self-assembling peptide detergents in purification of functional GPCRs. This review is useful to further study in membrane proteins as well as the new drug exploration.

Heat shock response improves heterologous protein secretion in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a widely used platform for the production of heterologous proteins of medical or industrial interest. However, heterologous protein productivity is often low due to limitations of the host strain. Heat shock response (HSR) is an inducible, global, cellular stress response, which facilitates the cell recovery from many forms of stress, e.g., heat stress. In S. cerevisiae, HSR is regulated mainly by the transcription factor heat shock factor (Hsf1p) and many of its targets are genes coding for molecular chaperones that promote protein folding and prevent the accumulation of mis-folded or aggregated proteins. In this work, we over-expressed a mutant HSF1 gene HSF1-R206S which can constitutively activate HSR, so the heat shock response was induced at different levels, and we studied the impact of HSR on heterologous protein secretion. We found that moderate and high level over-expression of HSF1-R206S increased heterologous α-amylase yield 25 and 70 % when glucose was fully consumed, and 37 and 62 % at the end of the ethanol phase, respectively. Moderate and high level over-expression also improved endogenous invertase yield 118 and 94 %, respectively. However, human insulin precursor was only improved slightly and this only by high level over-expression of HSF1-R206S, supporting our previous findings that the production of this protein in S. cerevisiae is not limited by secretion. Our results provide an effective strategy to improve protein secretion and demonstrated an approach that can induce ER and cytosolic chaperones simultaneously.