Acetamide selection of Kluyveromyces lactis cells transformed with an integrative vector leads to high-frequency formation of multicopy strains

CRISPR/Cas9-Based Genome Editing for Protein Expression and Secretion in Kluyveromyces lactis

The budding yeast Kluyveromyces lactis has emerged as a promising microbial chassis in industrial biotechnology. However, a lack of efficient molecular genetic manipulation tools and strategies has hindered the development of K. lactis as a biomanufacturing platform. In this study, we developed and applied a CRISPR/Cas9-based genome editing method to K. lactis. Single-gene editing efficiency was increased to 80% by disrupting the nonhomologous end-joining-related gene KU80 and performing a series of process optimizations. Subsequently, the CRISPR/Cas9 system was explored based on different sgRNA delivery modes for simultaneous multigene editing. With the aid of the color indicator, the editing efficiencies of two and three genes reached 73.3 and 36%, respectively, in the KlΔKU80 strain. Furthermore, the CRISPR/Cas9 system was used for multisite integration to enhance lactase production and combinatorial knockout of TMED10 and HSP90 to characterize the extracellular secretion of lactase in K. lactis. Generally, genome editing is a powerful tool for constructing K. lactis cell factories for protein and chemical production.

Kluyveromyces as promising yeast cell factories for industrial bioproduction: From bio-functional design to applications

As the two most widely used Kluyveromyces yeast, Kluyveromyces marxianus and K. lactis have gained increasing attention as microbial chassis in biocatalysts, biomanufacturing and the utilization of low-cost raw materials owing to their high suitability to these applications. However, due to slow progress in the development of molecular genetic manipulation tools and synthetic biology strategies, Kluyveromyces yeast cell factories as biological manufacturing platforms have not been fully developed. In this review, we provide a comprehensive overview of the attractive characteristics and applications of Kluyveromyces cell factories, with special emphasis on the development of molecular genetic manipulation tools and systems engineering strategies for synthetic biology. In addition, future avenues in the development of Kluyveromyces cell factories for the utilization of simple carbon compounds as substrates, the dynamic regulation of metabolic pathways, and for rapid directed evolution of robust strains are proposed. We expect that more synthetic systems, synthetic biology tools and metabolic engineering strategies will adapt to and optimize for Kluyveromyces cell factories to achieve green biofabrication of multiple products with higher efficiency.

Identification of a Yarrowia lipolytica acetamidase and its use as a yeast genetic marker

BACKGROUND

Yarrowia lipolytica is an oleaginous yeast that can be genetically engineered to produce lipid and non-lipid biochemicals from a variety of feedstocks. Metabolic engineering of this organism usually requires genetic markers in order to select for modified cells. The potential to combine multiple genetic manipulations depends on the availability of multiple or recyclable selectable markers.

RESULTS

We found that Y. lipolytica has the ability to utilize acetamide as the sole nitrogen source suggesting that the genome contains an acetamidase gene. Two potential Y. lipolytica acetamidase gene candidates were identified by homology to the A. nidulans acetamidase amdS. These genes were deleted in the wild-type Y. lipolytica strain YB-392, and deletion strains were evaluated for acetamide utilization. One deletion strain was unable to grow on acetamide and a putative acetamidase gene YlAMD1 was identified. Transformation of YlAMD1 followed by selection on acetamide media and counterselection on fluoroacetamide media showed that YlAMD1 can be used as a recyclable genetic marker in Saccharomyces cerevisiae and Ylamd1Δ Y. lipolytica.

CONCLUSIONS

These findings add to our understanding of Y. lipolytica nitrogen utilization and expand the set of genetic tools available for engineering this organism, as well as S. cerevisiae.

Constructing a yeast to express the largest cellulosome complex on the cell surface

Cellulosomes, which are multienzyme complexes from anaerobic bacteria, are considered nature's finest cellulolytic machinery. Thus, constructing a cellulosome in an industrial yeast has long been a goal pursued by scientists. However, it remains highly challenging due to the size and complexity of cellulosomal genes. Here, we overcame the difficulties by synthesizing the Clostridium thermocellum scaffoldin gene (CipA) and the anchoring protein gene (OlpB) using advanced synthetic biology techniques. The engineered Kluyveromyces marxianus, a probiotic yeast, secreted a mixture of dockerin-fused fungal cellulases, including an endoglucanase (TrEgIII), exoglucanase (CBHII), β-glucosidase (NpaBGS), and cellulase boosters (TaLPMO and MtCDH). The confocal microscopy results confirmed the cell-surface display of OlpB-ScGPI and fluorescence-activated cell sorting analysis results revealed that almost 81% of yeast cells displayed OlpB-ScGPI. We have also demonstrated the cellulosome complex formation using purified and crude cellulosomal proteins. Native polyacrylamide gel electrophoresis and mass spectrometric analysis further confirmed the cellulosome complex formation. Our engineered cellulosome can accommodate up to 63 enzymes, whereas the largest engineered cellulosome reported thus far could accommodate only 12 enzymes and was expressed by a plasmid instead of chromosomal integration. Interestingly, CipA 2B9C (with two cellulose binding modules, CBM) released significantly higher quantities of reducing sugars compared with other CipA variants, thus confirming the importance of cohesin numbers and CBM domain on cellulosome complex. The engineered yeast host efficiently degraded cellulosic substrates and released 3.09 g/L and 8.61 g/L of ethanol from avicel and phosphoric acid-swollen cellulose, respectively, which is higher than any previously constructed yeast cellulosome.

A Novel Regulated Hybrid Promoter That Permits Autoinduction of Heterologous Protein Expression in Kluyveromyces lactis

The yeast Kluyveromyces lactis is an important host for the expression of recombinant proteins at both laboratory and industrial scales. However, the system lacks a tightly regulated promoter that permits controlled expression of heterologous proteins. In this study, we report the engineering of a highly regulated strong hybrid promoter (termed P₃₅₀) for use in K. lactis. P₃₅₀ is tightly repressed by glucose or glycerol in the medium but strongly promotes gene expression once the carbon source has been consumed by the cells. This feature permits heterologous protein expression to be “autoinduced” at any scale without the addition of a gratuitous inducer molecule or changing feed solutions.

The yeast Kluyveromyces lactis has been a successful host for the production of heterologous proteins for over 30 years. Currently, the galactose-/lactose-inducible and glucose-repressible LAC4 promoter (P_LAC4) is the most widely used promoter to drive recombinant protein expression in K. lactis. However, P_LAC4 is not fully repressed in the presence of glucose and significant protein expression still occurs. Thus, P_LAC4 is not suitable in processes where tight regulation of heterologous gene expression is required. In this study, we devised a novel K. lactis promoter system that is both strong and tightly controllable. We first tested several different endogenous K. lactis promoters for their ability to express recombinant proteins. A novel hybrid promoter (termed P₃₅₀) was created by combining segments of two K. lactis promoters, namely, the strong constitutive P_GAP1 promoter and the carbon source-sensitive P_ICL1 promoter. We demonstrate that P₃₅₀ is tightly repressed in the presence of glucose or glycerol and becomes derepressed upon depletion of these compounds by the growing cells. We further illustrate the utility of P₃₅₀-controlled protein expression in shake flask and high-cell-density bioreactor cultivation strategies. The P₃₅₀ hybrid promoter is a strong derepressible promoter for use in autoinduction of one-step fermentation processes for the production of heterologous proteins in K. lactis.

IMPORTANCE The yeast Kluyveromyces lactis is an important host for the expression of recombinant proteins at both laboratory and industrial scales. However, the system lacks a tightly regulated promoter that permits controlled expression of heterologous proteins. In this study, we report the engineering of a highly regulated strong hybrid promoter (termed P₃₅₀) for use in K. lactis. P₃₅₀ is tightly repressed by glucose or glycerol in the medium but strongly promotes gene expression once the carbon source has been consumed by the cells. This feature permits heterologous protein expression to be “autoinduced” at any scale without the addition of a gratuitous inducer molecule or changing feed solutions.

Characterization of a leukocidin identified in Staphylococcus pseudintermedius

Bacterial infections from Staphylococcus pseudintermedius are the most common cause of skin infections (pyoderma) affecting dogs. Two component pore-forming leukocidins are a family of potent toxins secreted by staphylococci and consist of S (slow) and F (fast) components. They impair the innate immune system, the first line of defense against these pathogens. Seven different leukocidins have been characterized in Staphylococcus aureus, some of which are host and cell specific. Through genome sequencing and analysis of the S. pseudintermedius secretome using liquid chromatography mass spectrometry we identified two proteins, named "LukS-I" and "LukF-I", encoded on a degenerate prophage contained in the genome of S. pseudintermedius isolates. Phylogenetic analysis of LukS-I components in comparison to the rest of the leukocidin family showed that LukS-I was most closely related to S. intermedius LukS-I, S. aureus LukE and LukP, whereas LukF-I was most similar to S. intermedius LukF-I S. aureus gamma hemolysin subunit B. The killing effect of recombinant S. pseudintermedius LukS-I and LukF-I on canine polymorphonuclear leukocytes was determined using a flow cytometry cell permeability assay. The cytotoxic effect occurred only when the two recombinant proteins were combined. Engineered mutant versions of the two-component pore-forming leukocidins, produced through amino acids substitutions at selected points, were not cytotoxic. Anti-Luk-I produced in dogs against attenuated proteins reduced the cytotoxic effect of native canine leukotoxin which highlights the importance of Luk-I as a promising component in a vaccine against canine S. pseudintermedius infections.

Immunological Comparison of Native and Recombinant Hen's Egg Yolk Allergen, Chicken Serum Albumin (Gal d 5), Produced in Kluveromyces lactis

Chicken serum albumin (CSA) is a hen’s egg yolk allergen causing IgE-mediated allergy. The objective of this study was to produce a recombinant version of CSA and compare its IgE reactivity to natural CSA (nCSA). CSA was cloned and expressed as a soluble fraction in the yeast Kluyveromyces lactis (K. lactis) protein expression system. The gene encoding CSA was amplified with a C-terminal hemagglutinin epitope tag by polymerase chain reaction (PCR) and cloned into the pKLAC2 expression vector prior to transforming into K. lactis. Recombinant CSA (rCSA) was purified by immunoprecipitation. Twenty-one patients allergic to hen’s egg white were examined for sensitisation against nCSA. 38% of patients were found to be sensitised to CSA based on Western immunoassay. Immunoglobulin E (IgE) binding capacity of rCSA and nCSA was analysed by ELISA using sera from patients sensitised to CSA. Levels of IgE-binding were similar for both the recombinant and the natural CSA, indicating the existence of similar epitopes. rCSA produced in this study is a potential candidate to be used in component-resolved diagnosis (CRD) of egg yolk allergy. The usefulness of rCSA in CRD of egg yolk allergy warrants further characterisation using sera from patients with allergy to hen’s egg yolk in future studies.

Acetamidase as a dominant recyclable marker for Komagataella phaffii strain engineering

We have investigated the use of the gene coding for acetamidase (amdS) as a recyclable dominant marker for the methylotrophic yeast Komagataella phaffii in order to broaden its genetic toolbox. First, the endogenous constitutive AMD2 gene (a putative acetamidase) was deleted generating strain LA1. A cassette (amdSloxP) was constructed bearing a codon-optimized version of the Aspergillus nidulans amdS gene flanked by loxP sites for marker excision with Cre recombinase. This cassette was successfully tested as a dominant selection marker for transformation of the LA1 strain after selection on plates containing acetamide as a sole nitrogen source. Finally, amdSloxP was used to sequentially disrupt the K. phaffii ADE2 and URA5 genes. After each disruption event, a Cre-mediated marker recycling step was performed by plating cells on medium containing fluoroacetamide. In conclusion, amdS proved to be a suitable tool for K. phaffii transformation and marker recycling thus providing a new antibiotic-free system for genetic manipulation of this yeast.

Metabolic engineering a yeast to produce astaxanthin

In this study, an astaxanthin-biosynthesis Kluyveromyces marxianus strain Sm23 was first constructed, which could produce 31µg/g DCW astaxanthin. Then, repeated genome integration of the key astaxanthin biosynthesis genes Hpchyb and bkt was done to increase gene copy number and astaxanthin yield. Four improved strains were obtained and the yield of astaxanthin and the total yield of carotenoids in a strain increased with the copy numbers of Hpchyb and bkt. To improve the yield further, the gene Hpchyb from Haematococcus pluvialis was modified by site-directed mutagenesis to increase the enzyme efficiency or/and to prevent the heterologous protein degradation by ubiquitination. Using repeated-integration approach of bkt and the mutated Hpchyb into Sm23, the S3-2 strain was obtained and shown to produce the 3S, 3'S-astaxanthin at 9972µg/g DCW in a 5L fermentor.

Heterologous expression of an α-amylase inhibitor from common bean (Phaseolus vulgaris) in Kluyveromyces lactis and Saccharomyces cerevisiae

Background

Phaseolamin or α-amylase inhibitor 1 (αAI) is a glycoprotein from common beans (Phaseolus vulgaris L.) that inhibits some insect and mammalian α-amylases. Several clinical studies support the beneficial use of bean αAI for control of diabetes and obesity. Commercial extracts of P. vulgaris are available but their efficacy is still under question, mainly because some of these extracts contain antinutritional impurities naturally present in bean seeds and also exhibit a lower specific activity αAI. The production of recombinant αAI allows to overcome these disadvantages and provides a platform for the large-scale production of pure and functional αAI protein for biotechnological and pharmaceutical applications.

Results

A synthetic gene encoding αAI from the common bean (Phaseolus vulgaris cv. Pinto) was codon-optimised for expression in yeasts (αAI-OPT) and cloned into the protein expression vectors pKLAC2 and pYES2. The yeasts Kluyveromyces lactis GG799 (and protease deficient derivatives such as YCT390) and Saccharomyces cerevisiae YPH499 were transformed with the optimised genes and transformants were screened for expression by antibody dot blot. Recombinant colonies of K. lactis YCT390 that expressed and secreted functional αAI into the culture supernatants were selected for further analyses. Recombinant αAI from K. lactis YCT390 was purified using anion-exchange and affinity resins leading to the recovery of a functional inhibitor. The identity of the purified αAI was confirmed by mass spectrometry. Recombinant clones of S. cerevisiae YPH499 expressed functional αAI intracellularly, but did not secrete the protein.

Conclusions

This is the first report describing the heterologous expression of the α-amylase inhibitor 1 (αAI) from P. vulgaris in yeasts. We demonstrated that recombinant strains of K. lactis and S. cerevisiae expressed and processed the αAI precursor into mature and active protein and also showed that K. lactis secretes functional αAI.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0719-4) contains supplementary material, which is available to authorized users.

Development of a Candida glabrata dominant nutritional transformation marker utilizing the Aspergillus nidulans acetamidase gene (amdS)

The gene encoding Aspergillus nidulans acetamidase (amdS) was placed under control of Candida albicans ACT1 promoter and terminator sequences and then cloned into a plasmid containing C. glabrata ARS10,CEN8 or ARS10+CEN8 sequences. All plasmids transformed C. glabrata wild-type cells to acetamide+, with the ARS-only containing plasmid transforming cells at the highest frequencies (>1.0 × 10(4) transformants μg(-1)). Plasmids were rapidly lost under non-selective conditions with the frequency dependent on chromosomal element, thus recycling the acetamide- phenotype. The amdS plasmid was used to transform a set of clinical isolates resistant to a variety of antifungal drugs. All strains were successfully transformed to the acetamide+ phenotype at high frequency, confirming that this plasmid construct could be used as a simple dominant marker on virtually any strain. Gap repair experiments demonstrated that just as in Saccharomyces cerevisiae, gap repair functions efficiently inC. glabrata, suggesting that C. glabrata has numerous similarities toS. cerevisiae with regard to ease of molecular manipulation. The amdS system is inexpensive and efficient, and combined with existing C. glabrata plasmid elements, confers a high transformation frequency for C. glabrata with a phenotype that can be easily recycled.

Production of Protein Complexes in Non-methylotrophic and Methylotrophic Yeasts : Nonmethylotrophic and Methylotrophic Yeasts

Protein complexes can be produced in multimilligram quantities using nonmethylotrophic and methylotrophic yeasts such as Saccharomyces cerevisiae and Komagataella (Pichia) pastoris. Yeasts have distinct advantages as hosts for recombinant protein production owing to their cost efficiency, ease of cultivation and genetic manipulation, fast growth rates, capacity to introduce post-translational modifications, and high protein productivity (yield) of correctly folded protein products. Despite those advantages, yeasts have surprisingly lagged behind other eukaryotic hosts in their use for the production of multisubunit complexes. As our knowledge of the metabolic and genomic bottlenecks that yeast microorganisms face when overexpressing foreign proteins expands, new possibilities emerge for successfully engineering yeasts as superb expression hosts. In this chapter, we describe the current state of the art and discuss future possibilities for the development of yeast-based systems for the production of protein complexes.

An alkali-thermostable xylanase from Bacillus pumilus functionally expressed in Kluyveromyces lactis and evaluation of its deinking efficiency

This work aimed at studying the recombinant expression of an alkali- and thermo-stable xylanase from Bacillus pumilus in Kluyveromyces lactis and its use in deinking of civic paper waste. Efficient expression with a 3-fold increase in the activity than the native organism was achieved. An inducer concentration of 2.5% and medium pH of 9.0 was the best for enzyme expression. Purified enzyme showed an optimum activity at temperatures 50 and 60°C and pH 9.0 and 10.0, respectively. At pH 12.0, enzyme retained 74% and 26% activity after 2 and 3h of incubation, respectively. After incubation at 50 and 60°C for 1h, the enzyme showed 100% retention of activity, and remained active for 4h at 60°C retaining 23% residual activity. Partially purified recombinant enzyme showed higher deinking efficiency (273%) of laser print waste paper than crude xylanase from Bacillus and commercial acidic enzyme. This xylanase with superior stability characteristics could be a suitable candidate in paper and pulp industries.

Metabolic engineering of Kluyveromyces lactis for L-ascorbic acid (vitamin C) biosynthesis

BACKGROUND

L-ascorbic acid (L-AA) is naturally synthesized in plants from D-glucose by 10 steps pathway. The pathway branch to synthesize L-galactose, the key intermediate for L-ascorbic acid biosynthesis, has been recently elucidated. Budding yeast produces an 5-carbon ascorbic acid analogue Dehydro-D-arabinono 1,4-lactone (D-DAL), which is synthesized from D-arabinose. Yeast is able to synthesize L-ascorbic acid only if it is cultivated in the presence of one of its precursors: L-galactose, L-galactono 1,4-lactone, or L-gulono 1,4-lactone extracted from plants or animals. To avoid feeding the yeast culture with this "L" enantiomer, we engineered Kluyveromyces lactis with L-galactose biosynthesis pathway genes: GDP-mannose 3,5-epimerase (GME), GDP-L-galactose phosphorylase (VTC2) and L-galactose-1-phosphate phosphatase (VTC4) isolated from Arabidopsis thaliana.

RESULTS

Plasmids were constructed and modified such that the cloned plant genes were targeted to the K. lactis LAC4 Locus by homologous recombination and that the expression was associated to the growth on D-galactose or lactose. Upon K. lactis transformation, GME was under the control of the native LAC4 promoter whereas VTC2 and VTC4 were expressed from the S. cerevisiae promoters GPD1 and ADH1 respectively. The expression in K. lactis, of the L-galactose biosynthesis genes was determined by Reverse Transcriptase-PCR and western blotting. The recombinant yeasts were capable to produce about 30 mg.L(-1) of L-ascorbic acid in 48 hours of cultivation when cultured on rich medium with 2% (w/v) D-galactose. We also evaluated the L-AA production culturing recombinant recombinant strains in cheese whey, a waste product during cheese production, as an alternative source of lactose.

CONCLUSIONS

This work is the first attempt to engineer K. lactis cells for L-ascorbic acid biosynthesis by a fermentation process without any trace of "L" isomers precursors in the culture medium. We have engineered K. lactis strains capable of converting lactose and D-galactose into L-galactose, by the integration of the genes from the A. thaliana L-galactose pathway. L-galactose is a rare sugar, which is one of the main precursors for L-AA production.

amdSYM, a new dominant recyclable marker cassette for Saccharomyces cerevisiae

Despite the large collection of selectable marker genes available for Saccharomyces cerevisiae, marker availability can still present a hurdle when dozens of genetic manipulations are required. Recyclable markers, counterselectable cassettes that can be removed from the targeted genome after use, are therefore valuable assets in ambitious metabolic engineering programs. In the present work, the new recyclable dominant marker cassette amdSYM, formed by the Ashbya gossypii TEF2 promoter and terminator and a codon-optimized acetamidase gene (Aspergillus nidulans amdS), is presented. The amdSYM cassette confers S. cerevisiae the ability to use acetamide as sole nitrogen source. Direct repeats flanking the amdS gene allow for its efficient recombinative excision. As previously demonstrated in filamentous fungi, loss of the amdS marker cassette from S. cerevisiae can be rapidly selected for by growth in the presence of fluoroacetamide. The amdSYM cassette can be used in different genetic backgrounds and represents the first counterselectable dominant marker gene cassette for use in S. cerevisiae. Furthermore, using astute cassette design, amdSYM excision can be performed without leaving a scar or heterologous sequences in the targeted genome. The present work therefore demonstrates that amdSYM is a useful addition to the genetic engineering toolbox for Saccharomyces laboratory, wild, and industrial strains.

A novel, lactase-based selection and strain improvement strategy for recombinant protein expression in Kluyveromyces lactis

BACKGROUND

The Crabtree-negative yeast species Kluyveromyces lactis has been established as an attractive microbial expression system for recombinant proteins at industrial scale. Its LAC genes allow for utilization of the inexpensive sugar lactose as a sole source of carbon and energy. Lactose efficiently induces the LAC4 promoter, which can be used to drive regulated expression of heterologous genes. So far, strain manipulation of K. lactis by homologous recombination was hampered by the high rate of non-homologous end-joining.

RESULTS

Selection for growth on lactose was applied to target the insertion of heterologous genes downstream of the LAC4 promoter into the K. lactis genome and found to yield high numbers of positive transformants. Concurrent reconstitution of the β-galactosidase gene indicated the desired integration event of the expression cassette, and β-galactosidase activity measurements were used to monitor gene expression for strain improvement and fermentation optimization. The system was particularly improved by usage of a cell lysis resistant strain, VAK367-D4, which allowed for protein accumulation in long-term fermentation. Further optimization was achieved by increased gene dosage of KlGAL4 encoding the activator of lactose and galactose metabolic genes that led to elevated transcription rates. Pilot experiments were performed with strains expressing a single-chain antibody fragment (scFvox) and a viral envelope protein (BVDV-E2), respectively. scFvox was shown to be secreted into the culture medium in an active, epitope-binding form indicating correct processing and protein folding; the E2 protein could be expressed intracellularly. Further data on the influence of protein toxicity on batch fermentation and potential post-transcriptional bottlenecks in protein accumulation were obtained.

CONCLUSIONS

A novel Kluyveromyces lactis host-vector system was developed that places heterologous genes under the control of the chromosomal LAC4 promoter and that allows monitoring of its transcription rates by β-galactosidase measurement. The procedure is rapid and efficient, and the resulting recombinant strains contain no foreign genes other than the gene of interest. The recombinant strains can be grown non-selectively in rich medium and stably maintained even when the gene product exerts protein toxicity.

Isotopic labeling of heterologous proteins in the yeast Pichia pastoris and Kluyveromyces lactis

Several protein expression systems are available for the preparation of stable isotope-labeled recombinant proteins for NMR studies. Yeast expression systems have several advantages over prokaryotic systems, such as the widely used Escherichia coli expression system. Protein expression using the methylotrophic yeast Pichia pastoris is commonly employed for the preparation of isotope-labeled proteins. Recently, the hemiascomycete yeast Kluyveromyces lactis expression system was reported as being useful for preparing proteins for NMR studies. Since each yeast expression system has different features, their applications have increased in number. In this chapter, we describe procedures for the efficient production of uniformly isotope-labeled proteins using the P. pastoris and the K. lactis yeast expression systems.

A set of aspartyl protease-deficient strains for improved expression of heterologous proteins in Kluyveromyces lactis

Secretion of recombinant proteins is a common strategy for heterologous protein expression using the yeast Kluyveromyces lactis. However, a common problem is degradation of a target recombinant protein by secretory pathway aspartyl proteases. In this study, we identified five putative pfam00026 aspartyl proteases encoded by the K. lactis genome. A set of selectable marker-free protease deletion mutants was constructed in the prototrophic K. lactis GG799 industrial expression strain background using a PCR-based dominant marker recycling method based on the Aspergillus nidulans acetamidase gene (amdS). Each mutant was assessed for its secretion of protease activity, its health and growth characteristics, and its ability to efficiently produce heterologous proteins. In particular, despite having a longer lag phase and slower growth compared with the other mutants, a Δyps1 mutant demonstrated marked improvement in both the yield and the quality of Gaussia princeps luciferase and the human chimeric interferon Hy3, two proteins that experienced significant proteolysis when secreted from the wild-type parent strain.

The Fps1p aquaglyceroporin facilitates the use of small aliphatic amides as a nitrogen source by amidase-expressing yeasts

Saccharomyces cerevisiae acquires a resistance to high, toxic levels of acetic acid by destabilizing Fps1p, the plasma membrane aquaglyceroporin through which this acid - in its undissociated state - enters the cell. In this study, Fps1p loss was shown to confer resistances to acetic acid, acrolein and allyl alcohol, not just in S. cerevisiae but also in the osmotolerant spoilage yeast Zygosaccharomyces rouxii. However, in Z. rouxii, the loss of Fps1p severely compromised the use of acetamide and several other small amides as sources of nitrogen, an indication that these amides enter the cells of this yeast by passive diffusion through the Fps1p pore. Saccharomyces cerevisiae cannot grow on acetamide, but was conferred with an ability to use this and other small amides as nitrogen sources by heterologous expression of a Z. rouxii ORF (ZrAMD1) with protein sequence identity to the amdS-encoded amidase of Aspergillus nidulans. This capacity of ZrAMD1-expressing S. cerevisiae to assimilate amide nitrogen was severely compromised by the loss of Fps1p. ZrAMD1 appears to encode the major amidase of Z. rouxii as a Zramd1Delta deletant mutant had, like the Zrfps1Delta deletant, lost the ability to assimilate small amides as sources of nitrogen.

Disruption of the OCH1 and MNN1 genes decrease N-glycosylation on glycoprotein expressed in Kluyveromyces lactis

Glycoproteins secreted by the yeast Kluyveromyces lactis are usually modified by the addition at asparagines-linked glycosylation sites of heterogeneous mannan residues. The secreted glycoproteins in K. lactis that become hypermannosylated will bear a non-human glycosylation pattern and can adversely affect the half-life, tissue distribution and immunogenicity of a therapeutic protein. Here, we describe engineering a K. lactis strain to produce non-hypermannosylated glycoprotein, decreasing the outer-chain mannose residues of N-linked oligosaccharides. We investigated and developed the method of two-step homologous recombination to knockout the OCH1 gene, encoding alpha1,6-mannosyltransferase and MNN1 gene, which is homologue of Saccharomyces cerevisiae MNN1, encoding a putative alpha1,3-mannosyltransferase. We found the Kloch1 mutant strain has a defect in hyperglycosylation, inability in adding mannose to the core oligosaccharide. The N-linked oligosaccharides assembled on a secretory glycoprotein, HSA/GM-CSF in Kloch1 mutant, contained oligosaccharide Man(13-14)GlcNAc(2), and in Kloch1 mnn1 mutant, contained oligosaccharide Man(9-11)GlcNAc(2), whereas those in the wild-type strain, consisted of oligosaccharides with heterogeneous sizes, Man(>30)GlcNAc(2). Taken together, these results indicated that KlOch1p plays a key role in the outer-chain mannosylation of N-linked oligosaccharides in K. lactis. The KlMnn1p, was proved to be certain contribution to the outer hypermannosylation, most possibly encodes alpha1,3-mannosyltransferase. Therefore, the Kloch1 and Kloch1 mnn1 mutants can be used as a foundational host to produce glycoproteins lacking the outer-chain hypermannoses and further maybe applicable to be a promising system for yeast therapeutic protein production.