Identification of genes that enhance cellulase protein production in yeast

Designing strong inducible synthetic promoters in yeasts

Inducible promoters are essential for precise control of target gene expression in synthetic biological systems. However, engineering eukaryotic promoters is often more challenging than engineering prokaryotic promoters due to their greater mechanistic complexity. In this study, we describe a simple and reliable approach for constructing strongly inducible synthetic promoters with minimum leakiness in yeasts. The results indicate that the leakiness of yeast-inducible synthetic promoters is primarily the result of cryptic transcriptional activation of heterologous sequences that may be avoided by appropriate insulation and operator mutagenesis. Our promoter design approach has successfully generated robust, inducible promoters that achieve a > 103-fold induction in reporter gene expression. The utility of these promoters is demonstrated by using them to produce various biologics with titers up to 2 g/L, including antigens designed to raise specific antibodies against a SARS-CoV-2 omicron variant through chicken immunization.

Genome-wide screen identifies new set of genes for improved heterologous laccase expression in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is widely used as a host cell for recombinant protein production due to its fast growth, cost-effective culturing, and ability to secrete large and complex proteins. However, one major drawback is the relatively low yield of produced proteins compared to other host systems. To address this issue, we developed an overlay assay to screen the yeast knockout collection and identify mutants that enhance recombinant protein production, specifically focusing on the secretion of the Trametes trogii fungal laccase enzyme. Gene ontology analysis of these mutants revealed an enrichment of processes including vacuolar targeting, vesicle trafficking, proteolysis, and glycolipid metabolism. We confirmed that a significant portion of these mutants also showed increased activity of the secreted laccase when grown in liquid culture. Notably, we found that the combination of deletions of OCA6, a tyrosine phosphatase gene, along with PMT1 or PMT2, two genes encoding ER membrane protein-O-mannosyltransferases involved in ER quality control, and SKI3, which encode for a component of the SKI complex responsible for mRNA degradation, further increased secreted laccase activity. Conversely, we also identified over 200 gene deletions that resulted in decreased secreted laccase activity, including many genes that encode for mitochondrial proteins and components of the ER-associated degradation pathway. Intriguingly, the deletion of the ER DNAJ co-chaperone gene SCJ1 led to almost no secreted laccase activity. When we expressed SCJ1 from a low-copy plasmid, laccase secretion was restored. However, overexpression of SCJ1 had a detrimental effect, indicating that precise dosing of key chaperone proteins is crucial for optimal recombinant protein expression. This study offers potential strategies for enhancing the overall yield of recombinant proteins and provides new avenues for further research in optimizing protein production systems.

CRISPR/Cas9-mediated point mutations improve α-amylase secretion in Saccharomyces cerevisiae

The rapid expansion of the application of pharmaceutical proteins and industrial enzymes requires robust microbial workhorses for high protein production. The budding yeast Saccharomyces cerevisiae is an attractive cell factory due to its ability to perform eukaryotic post-translational modifications and to secrete proteins. Many strategies have been used to engineer yeast platform strains for higher protein secretion capacity. Herein, we investigated a line of strains that have previously been selected after UV random mutagenesis for improved α-amylase secretion. A total of 42 amino acid altering point mutations identified in this strain line were reintroduced into the parental strain AAC to study their individual effects on protein secretion. These point mutations included missense mutations (amino acid substitution), nonsense mutations (stop codon generation), and frameshift mutations. For comparison, single gene deletions for the corresponding target genes were also performed in this study. A total of 11 point mutations and seven gene deletions were found to effectively improve α-amylase secretion. These targets were involved in several bioprocesses, including cellular stresses, protein degradation, transportation, mRNA processing and export, DNA replication, and repair, which indicates that the improved protein secretion capacity in the evolved strains is the result of the interaction of multiple intracellular processes. Our findings will contribute to the construction of novel cell factories for recombinant protein secretion.

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.

Modulation of heterologous protein secretion in the thermotolerant methylotrophic yeast Ogataea thermomethanolica TBRC 656 by CRISPR-Cas9 system

The thermotolerant methylotrophic yeast Ogataea thermomethanolica TBRC 656 is a potential host strain for industrial protein production. Heterologous proteins are often retained intracellularly in yeast resulting in endoplasmic reticulum (ER) stress and poor secretion, and despite efforts to engineer protein secretory pathways, heterologous protein production is often lower than expected. We hypothesized that activation of genes involved in the secretory pathway could mitigate ER stress. In this study, we created mutants defective in protein secretory-related functions using clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) tools. Secretion of the model protein xylanase was significantly decreased in loss of function mutants for oxidative stress (sod1Δ) and vacuolar and protein sorting (vps1Δ and ypt7Δ) genes. However, xylanase secretion was unaffected in an autophagy related atg12Δ mutant. Then, we developed a system for sequence-specific activation of target gene expression (CRISPRa) in O. thermomethanolica and used it to activate SOD1, VPS1 and YPT7 genes. Production of both non-glycosylated xylanase and glycosylated phytase was enhanced in the gene activated mutants, demonstrating that CRISPR-Cas9 systems can be used as tools for understanding O. thermomethanolica genes involved in protein secretion, which could be applied for increasing heterologous protein secretion in this yeast.

Transportome-wide engineering of Saccharomyces cerevisiae

Synthetic biology enables the production of small molecules by recombinant microbes for pharma, food, and materials applications. The secretion of products reduces the cost of separation and purification, but it is challenging to engineer due to the limited understanding of the transporter proteins' functions. Here we describe a method for genome-wide transporter disruption that, in combination with a metabolite biosensor, enables the identification of transporters impacting the production of a given target metabolite in yeast Saccharomyces cerevisiae. We applied the method to study the transport of xenobiotic compounds, cis,cis-muconic acid (CCM), protocatechuic acid (PCA), and betaxanthins. We found 22 transporters that influenced the production of CCM or PCA. The transporter of the 12-spanner drug:H(+) antiporter (DHA1) family Tpo2p was further confirmed to import CCM and PCA in Xenopus expression assays. We also identified three transporter proteins (Qdr1p, Qdr2p, and Apl1p) involved in betaxanthins transport. In summary, the described method enables high-throughput transporter identification for small molecules in cell factories.

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.

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.

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.

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.

High-Throughput Microfluidics for the Screening of Yeast Libraries

Cell factory development is critically important for efficient biological production of chemicals, biofuels, and pharmaceuticals. Many rounds of the Design-Build-Test-Learn cycles may be required before an engineered strain meeting specific metrics required for industrial application. The bioindustry prefer products in secreted form (secreted products or extracellular metabolites) as it can lower the cost of downstream processing, reduce metabolic burden to cell hosts, and allow necessary modification on the final products , such as biopharmaceuticals. Yet, products in secreted form result in the disconnection of phenotype from genotype, which may have limited throughput in the Test step for identification of desired variants from large libraries of mutant strains. In droplet microfluidic screening, single cells are encapsulated in individual droplet and enable high-throughput processing and sorting of single cells or clones. Encapsulation in droplets allows this technology to overcome the throughput limitations present in traditional methods for screening by extracellular phenotypes. In this chapter, we describe a protocol/guideline for high-throughput droplet microfluidics screening of yeast libraries for higher protein secretion . This protocol can be adapted to screening by a range of other extracellular products from yeast or other hosts.

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

The methylotrophic yeast Pichia pastoris (Komagataella spp.) is a popular microbial host for the production of recombinant proteins. Previous studies have shown that mis-sorting to the vacuole can be a bottleneck during production of recombinant secretory proteins in yeast, however, no information was available for P. pastoris. In this work the authors have therefore generated vps (vacuolar protein sorting) mutant strains disrupted in genes involved in the CORVET (class C core vacuole/endosome tethering) complex at the early stages of endosomal sorting. Both Δvps8 and Δvps21 strains contained lower extracellular amounts of heterologous carboxylesterase (CES) compared to the control strain, which could be attributed to a high proteolytic activity present in the supernatants of CORVET engineered strains due to rerouting of vacuolar proteases. Serine proteases were identified to be responsible for this proteolytic degradation by liquid chromatography-mass spectrometry and protease inhibitor assays. Deletion of the major cellular serine protease Prb1 in Δvps8 and Δvps21 strains did not only rescue the extracellular CES levels, but even outperformed the parental CES strain (56 and 80% higher yields, respectively). Further deletion of Ybr139W, another serine protease, did not show a further increase in secretion levels. Higher extracellular CES activity and low proteolytic activity were detected also in fed batch cultivation of Δvps21Δprb1 strains, thus confirming that modifying early steps in the vacuolar pathway has a positive impact on heterologous protein secretion.

Combinatorial Screening for Transgenic Yeasts with High Cellulase Activities in Combination with a Tunable Expression System

Combinatorial screening used together with a broad library of gene expression cassettes is expected to produce a powerful tool for the optimization of the simultaneous expression of multiple enzymes. Recently, we proposed a highly tunable protein expression system that utilized multiple genome-integrated target genes to fine-tune enzyme expression in yeast cells. This tunable system included a library of expression cassettes each composed of three gene-expression control elements that in different combinations produced a wide range of protein expression levels. In this study, four gene expression cassettes with graded protein expression levels were applied to the expression of three cellulases: cellobiohydrolase 1, cellobiohydrolase 2, and endoglucanase 2. After combinatorial screening for transgenic yeasts simultaneously secreting these three cellulases, we obtained strains with higher cellulase expressions than a strain harboring three cellulase-expression constructs within one high-performance gene expression cassette. These results show that our method will be of broad use throughout the field of metabolic engineering.

Engineering of protein folding and secretion-strategies to overcome bottlenecks for efficient production of recombinant proteins

SIGNIFICANCE

Recombinant protein production has developed into a huge market with enormous positive implications for human health and for the future direction of a biobased economy. Limitations in the economic and technical feasibility of production processes are often related to bottlenecks of in vivo protein folding.

RECENT ADVANCES

Based on cell biological knowledge, some major bottlenecks have been overcome by the overexpression of molecular chaperones and other folding related proteins, or by the deletion of deleterious pathways that may lead to misfolding, mistargeting, or degradation.

CRITICAL ISSUES

While important success could be achieved by this strategy, the list of reported unsuccessful cases is disappointingly long and obviously dependent on the recombinant protein to be produced. Singular engineering of protein folding steps may not lead to desired results if the pathway suffers from several limitations. In particular, the connection between folding quality control and proteolytic degradation needs further attention.

FUTURE DIRECTIONS

Based on recent understanding that multiple steps in the folding and secretion pathways limit productivity, synergistic combinations of the cell engineering approaches mentioned earlier need to be explored. In addition, systems biology-based whole cell analysis that also takes energy and redox metabolism into consideration will broaden the knowledge base for future rational engineering strategies.

Systematic screening of glycosylation- and trafficking-associated gene knockouts in Saccharomyces cerevisiae identifies mutants with improved heterologous exocellulase activity and host secretion

Background

As a strong fermentator, Saccharomyces cerevisiae has the potential to be an excellent host for ethanol production by consolidated bioprocessing. For this purpose, it is necessary to transform cellulose genes into the yeast genome because it contains no cellulose genes. However, heterologous protein expression in S. cerevisiae often suffers from hyper-glycosylation and/or poor secretion. Thus, there is a need to genetically engineer the yeast to reduce its glycosylation strength and to increase its secretion ability.

Results

Saccharomyces cerevisiae gene-knockout strains were screened for improved extracellular activity of a recombinant exocellulase (PCX) from the cellulose digesting fungus Phanerochaete chrysosporium. Knockout mutants of 47 glycosylation-related genes and 10 protein-trafficking-related genes were transformed with a PCX expression construct and screened for extracellular cellulase activity. Twelve of the screened mutants were found to have a more than 2-fold increase in extracellular PCX activity in comparison with the wild type. The extracellular PCX activities in the glycosylation-related mnn10 and pmt5 null mutants were, respectively, 6 and 4 times higher than that of the wild type; and the extracellular PCX activities in 9 protein-trafficking-related mutants, especially in the chc1, clc1 and vps21 null mutants, were at least 1.5 times higher than the parental strains. Site-directed mutagenesis studies further revealed that the degree of N-glycosylation also plays an important role in heterologous cellulase activity in S. cerevisiae.

Conclusions

Systematic screening of knockout mutants of glycosylation- and protein trafficking-associated genes in S. cerevisiae revealed that: (1) blocking Golgi-to-endosome transport may force S. cerevisiae to export cellulases; and (2) both over- and under-glycosylation may alter the enzyme activity of cellulases. This systematic gene-knockout screening approach may serve as a convenient means for increasing the extracellular activities of recombinant proteins expressed in S. cerevisiae.

Genome-wide identification of the targets for genetic manipulation to improve L-lactate production by Saccharomyces cerevisiae by using a single-gene deletion strain collection

To identify genome-wide targets for gene manipulation for increasing L-lactate production in recombinant Saccharomyces cerevisiae strains, we transformed all available single-gene deletion strains of S. cerevisiae with a plasmid carrying the human L-lactate dehydrogenase gene, and examined L-lactate production in the obtained transformants. The thresholds of increased or decreased L-lactate production were determined based on L-lactate production by the standard strain in repetitive experiments. L-lactate production data for 4802 deletion strains were obtained, and deletion strains with increased or decreased L-lactate production were identified. Functional category analysis of genes whose deletion increased L-lactate production revealed that ribosome biogenesis-related genes were overrepresented. Most deletion strains for genes related to ribosome biogenesis exhibited increased L-lactate production in 200-ml batch cultures. We deleted the genes related to ribosome biogenesis in a recombinant strain of S. cerevisiae with a genetic background different from that of the above deletion strains, and examined the effect of target gene deletion on L-lactate production. We observed that deletion of genes related to ribosome biogenesis leads to increased L-lactate production by recombinant S. cerevisiae strains, and the single-gene deletion strain collection could be utilized in identifying target genes for improving L-lactate production in S. cerevisiae recombinant strains.

Understanding physiological responses to pre-treatment inhibitors in ethanologenic fermentations

Alcohol-based liquid fuels feature significantly in the political and social agendas of many countries, seeking energy sustainability. It is certain that ethanol will be the entry point for many sustainable processes. Conventional ethanol production using maize- and sugarcane-based carbohydrates with Saccharomyces cerevisiae is well established, while lignocellulose-based processes are receiving growing interest despite posing greater technical and scientific challenges. A significant challenge that arises from the chemical hydrolysis of lignocellulose is the generation of toxic compounds in parallel with the release of sugars. These compounds, collectively termed pre-treatment inhibitors, impair metabolic functionality and growth. Their removal, pre-fermentation or their abatement, via milder hydrolysis, are currently uneconomic options. It is widely acknowledged that a more cost effective strategy is to develop resistant process strains. Here we describe and classify common inhibitors and describe in detail the reported physiological responses that occur in second-generation strains, which include engineered yeast and mesophilic and thermophilic prokaryotes. It is suggested that a thorough understanding of tolerance to common pre-treatment inhibitors should be a major focus in ongoing strain engineering. This review is a useful resource for future metabolic engineering strategies.