Screening for Glycosylphosphatidylinositol-Modified Cell Wall Proteins in Pichia pastoris and Their Recombinant Expression on the Cell Surface

Glycosylphosphatidylinositol (GPI)-anchored glycoproteins have various intrinsic functions in yeasts and different uses in vitro. In the present study, the genome of Pichia pastoris GS115 was screened for potential GPI-modified cell wall proteins. Fifty putative GPI-anchored proteins were selected on the basis of (i) the presence of a C-terminal GPI attachment signal sequence, (ii) the presence of an N-terminal signal sequence for secretion, and (iii) the absence of transmembrane domains in mature protein. The predicted GPI-anchored proteins were fused to an alpha-factor secretion signal as a substitute for their own N-terminal signal peptides and tagged with the chimeric reporters FLAG tag and mature Candida antarctica lipase B (CALB). The expression of fusion proteins on the cell surface of P. pastoris GS115 was determined by whole-cell flow cytometry and immunoblotting analysis of the cell wall extracts obtained by β-1,3-glucanase digestion. CALB displayed on the cell surface of P. pastoris GS115 with the predicted GPI-anchored proteins was examined on the basis of potential hydrolysis of p-nitrophenyl butyrate. Finally, 13 proteins were confirmed to be GPI-modified cell wall proteins in P. pastoris GS115, which can be used to display heterologous proteins on the yeast cell surface.     

Quantitative evaluation of Candia antarctica lipase B displayed on the cell surface of a Pichia pastoris based on an FS anchor system

A new approach is described to quantify the number of enzyme molecules, such as Candia antarctica lipase B, that are displayed on the cell surface of Pichia pastoris. Enhanced green fluorescent protein (EGFP) and Candida antarctica lipase B (CALB) were fused and displayed on the surface of P. pastoris by linking to the anchor flocculation functional domain of FLO1p from Saccharomyces cerevisiae. Confocal laser scanning microscopy, flow cytometry, and fluorescence spectrophotometry were used to monitor the fluorescence intensity of fused EGFP. Combined with the corresponding protein concentration detected in the medium, a standard curve describing the relationship between the fusion protein concentration and fluorescence intensity were obtained and could be used to number CALB displayed on the cell surface. The results showed that approx. 10⁴ molecules of CALB molecules were immobilized on the single P. pastoris cell wall based on FS anchor system.     

Hydrophobin signal sequence mediates efficient secretion of recombinant proteins in <Emphasis Type="Italic">Pichia pastoris</Emphasis>

Pichia pastoris is an important eukaryotic organism for the expression, processing, and secretion of recombinant proteins. Here, the secretion of enhanced green fluorescent protein (EGFP) in P. pastoris by using three novel secretion signals originating from the HFBI and HFBII class 2 hydrophobins of Trichoderma reesei was investigated. EGFP was fused to the carboxyl terminus of hydrophobin secretion signals and expressed under the control of the constitutive GAP promoter. In every case, recombinant EGFP entered the secretory pathway of P. pastoris. SDS-polyacrylamide gel electrophoresis, Western blot analysis of the cells' supernatant, and fluorescence measurements on single-cell level via flow cytometry confirmed the efficient secretion of EGFP mediated by the novel secretion sequences. In conclusion, the data clearly show that the secretion sequences derived from HFBI and HFBII of T. reesei have the potential to achieve an efficient secretion of heterologous proteins in P. pastoris. Due to the small size of the hydrophobin-derived secretion signals, their coding sequence can be easily introduced to the gene of interest by PCR.     

Expression in Pichia pastoris of Candida antarctica Lipase B and Lipase B Fused to a Cellulose-Binding Domain

Candida antarctica lipase B (CALB) and C. antarctica lipase B fused to a cellulose-binding domain (CBD-CALB) were expressed functionally in the methylotrophic yeast Pichia pastoris. The cellulose-binding domain originates from cellulase A of the anaerobic rumen fungus Neocallimastix patriciarum. The genes were fused to the α-factor secretion signal sequence of Saccharomyces cerevisiae and placed under the control of the alcohol oxidase gene (AOX1) promoter. The recombinant proteins were secreted into the culture medium reaching levels of approximately 25 mg/L. The proteins were purified using hydrophobic interaction chromatography and gel filtration with an overall yield of 69%. Results from endoglycosidase H digestion of the proteins showed that CALB and CBD-CALB were N-glycosylated. The specific hydrolytic activities of recombinant CALB and CBD-CALB were identical to that reported for CALB isolated from its native source. The fusion of the CBD to the lipase resulted in a greatly enhanced binding toward cellulose for CBD-CALB compared with that for CALB.     

Improving the Secretion of a Methyl Parathion Hydrolase in Pichia pastoris by Modifying Its N-Terminal Sequence

Pichia pastoris is commonly used to express and secrete target proteins, although not all recombinant proteins can be successfully produced. In this study, we used methyl parathion hydrolase (MPH) from Ochrobactrum sp. M231 as a model to study the importance of the N-terminus of the protein for its secretion. While MPH can be efficiently expressed intracellularly in P. pastoris, it is not secreted into the extracellular environment. Three MPH mutants (N₆₆-MPH, D₁₀-MPH, and N₉-MPH) were constructed through modification of its N-terminus, and the secretion of each by P. pastoris was improved when compared to wild-type MPH. The level of secreted D₁₀-MPH was increased to 0.21 U/mL, while that of N₉-MPH was enhanced to 0.16 U/mL. Although N₆₆-MPH was not enzymatically active, it was secreted efficiently, and was identified by SDS-PAGE. These results demonstrate that the secretion of heterologous proteins in P. pastoris may be improved by modifying their N-terminal structures.     

Display of Candida antarctica lipase B on Pichia pastoris and its application to flavor ester synthesis

Two alternative cell-surface display systems were developed in Pichia pastoris using the α-agglutinin and Flo1p (FS) anchor systems, respectively. Both the anchor cell wall proteins were obtained originally from Saccharomyces cerevisiae. Candida antarctica lipase B (CALB) was displayed functionally on the cell surface of P. pastoris using the anchor proteins α-agglutinin and FS. The activity of CALB displayed on P. pastoris was tenfold higher than that of S. cerevisiae. The hydrolytic and synthetic activities of CALB fused with α-agglutinin and FS anchored on P. pastoris were investigated. The hydrolytic activities of both lipases displayed on yeast cells surface were more than 200 U/g dry cell after 120 h of culture (200 and 270 U/g dry cell, respectively). However, the synthetic activity of CALB fused with α-agglutinin on P. pastoris was threefold higher than that of the FS fusion protein when applied to the synthesis of ethyl caproate. Similarly, the CALB displayed on P. pastoris using α-agglutinin had a higher catalytic efficiency with respect to the synthesis of other short-chain flavor esters than that displayed using the FS anchor. Interestingly, for some short-chain esters, the synthetic activity of displaying CALB fused with α-agglutinin on P. pastoris was even higher than that of the commercial CALB Novozyme 435.     

Presep: Predicting the Propensity of a Protein Being Secreted into the Supernatant when Expressed in Pichia pastoris

Pichia pastoris is commonly used for the production of recombinant proteins due to its preferential secretion of recombinant proteins, resulting in lower production costs and increased yields of target proteins. However, not all recombinant proteins can be successfully secreted in P. pastoris. A computational method that predicts the likelihood of a protein being secreted into the supernatant would be of considerable value; however, to the best of our knowledge, no such tool has yet been developed. We present a machine-learning approach called Presep to assess the likelihood of a recombinant protein being secreted by P. pastoris based on its pseudo amino acid composition (PseAA). Using a 20-fold cross validation, Presep demonstrated a high degree of accuracy, with Matthews correlation coefficient (MCC) and overall accuracy (Q2) scores of 0.78 and 95%, respectively. Computational results were validated experimentally, with six β-galactosidase genes expressed in P. pastoris strain GS115 to verify Presep model predictions. A strong correlation (R² = 0.967) was observed between Presep prediction secretion propensity and the experimental secretion percentage. Together, these results demonstrate the ability of the Presep model for predicting the secretion propensity of P. pastoris for a given protein. This model may serve as a valuable tool for determining the utility of P. pastoris as a host organism prior to initiating biological experiments. The Presep prediction tool can be freely downloaded at http://www.mobioinfor.cn/Presep.     

Construction of a Novel <Emphasis Type="Italic">Pichia pastoris</Emphasis> Cell-Surface Display System Based on the Cell Wall Protein Pir1

A novel system based on Pir1 from Saccharomyces cerevisiae was developed for cell-surface display of heterologous proteins in Pichia pastoris with the alpha-factor secretion signal sequence. As a model protein, enhanced green fluorescence protein (EGFP) was fused to the N-terminal of the mature peptide of Pir1 (Pir1-a). The expression of fusion protein EGFP-Pir1-a was irregular throughout the P. pastoris cell surface per detection by confocal laser scanning microscopy. A truncated sequence containing only the internal repetitive sequences of Pir1-a (Pir1-b) was used as a new anchor protein in further study. The fusion protein EGFP-Pir1-b was expressed uniformly on the cell surface. The fluorescence intensity of the whole yeast was measured by spectrofluorometer. Western blot confirmed that the fusion proteins were released from cell walls after mild alkaline treatment. The results indicate that a Pir1-based system can express proteins on the surface of P. pastoris and that the fusion proteins do not affect the manner in which Pir1 attaches to the cell wall. The repetitive sequences of Pir1 are required for cell wall retention, and the C-terminal sequence contributes to the irregular distribution of fusion proteins in P. pastoris.     

A Phytase-Based Reporter System for Identification of Functional Secretion Signals in Bifidobacteria

Health-promoting effects have been attributed to a number of Bifidobacterium sp. strains. These effects as well as the ability to colonise the host depend on secreted proteins. Moreover, rational design of protein secretion systems bears the potential for the generation of novel probiotic bifidobacteria with improved health-promoting or therapeutic properties. To date, there is only very limited data on secretion signals of bifidobacteria available. Using in silico analysis, we demonstrate that all bifidobacteria encode the major components of Sec-dependent secretion machineries but only B. longum strains harbour Tat protein translocation systems. A reporter plasmid for secretion signals in bifidobacteria was established by fusing the coding sequence of the signal peptide of a sialidase of Bifidobacterium bifidum S17 to the phytase gene appA of E. coli. The recombinant strain showed increased phytase activity in spent culture supernatants and reduced phytase levels in crude extracts compared to the control indicating efficient phytase secretion. The reporter plasmid was used to screen seven predicted signal peptides in B. bifidum S17 and B. longum E18. The tested signal peptides differed substantially in their efficacy to mediate protein secretion in different host strains. An efficient signal peptide was used for expression and secretion of a therapeutically relevant protein in B. bifidum S17. Expression of a secreted cytosine deaminase led to a 100-fold reduced sensitivity of B. bifidum S17 to 5-fluorocytosine compared to the non-secreted cytosine deaminase suggesting efficient conversion of 5-fluorocytosine to the cytotoxic cancer drug 5-fluorouracil by cytosine deaminase occurred outside the bacterial cell. Selection of appropriate signal peptides for defined protein secretion might improve therapeutic efficacy as well as probiotic properties of bifidobacteria.     

Electrospun micro- and nanofiber tubes for functional nervous regeneration in sciatic nerve transections

Background

Rhipicephalus (Boophilus) spp. ticks economically impact on cattle production in Africa and other tropical and subtropical regions of the world. Tick vaccines constitute a cost-effective and environmentally friendly alternative to tick control. The R. microplus Bm86 protective antigen has been produced by recombinant DNA technology and shown to protect cattle against tick infestations.

Results

In this study, the genes for Bm86 (R. microplus), Ba86 (R. annulatus) and Bd86 (R. decoloratus) were cloned and characterized from African or Asian tick strains and the recombinant proteins were secreted and purified from P. pastoris. The secretion of recombinant Bm86 ortholog proteins in P. pastoris allowed for a simple purification process rendering a final product with high recovery (35–42%) and purity (80–85%) and likely to result in a more reproducible conformation closely resembling the native protein. Rabbit immunization experiments with recombinant proteins showed immune cross-reactivity between Bm86 ortholog proteins.

Conclusion

These experiments support the development and testing of vaccines containing recombinant Bm86, Ba86 and Bd86 secreted in P. pastoris for the control of tick infestations in Africa.     

Protein secretion in <Emphasis Type="Italic">Pichia pastoris</Emphasis> and advances in protein production

Yeast expression systems have been successfully used for over 20 years for the production of recombinant proteins. With the growing interest in recombinant protein expression for various uses, yeast expression systems, such as the popular Pichia pastoris, are becoming increasingly important. Although P. pastoris has been successfully used in the production of many secreted and intracellular recombinant proteins, there is still room for improvement of this expression system. In particular, secretion of recombinant proteins is still one of the main reasons for using P. pastoris. Therefore, endoplasmic reticulum protein folding, correct glycosylation, vesicular transport to the plasma membrane, gene dosage, secretion signal sequences, and secretome studies are important considerations for improved recombinant protein production.     

Unconventional secretion of Pichia pastoris Acb1 is dependent on GRASP protein,peroxisomal functions,and autophagosome formation

In contrast to the enormous advances made regarding mechanisms of conventional protein secretion, mechanistic insights into the unconventional secretion of proteins are lacking. Acyl coenzyme A (CoA)–binding protein (ACBP; AcbA in Dictyostelium discoideum), an unconventionally secreted protein, is dependent on Golgi reassembly and stacking protein (GRASP) for its secretion. We discovered, surprisingly, that the secretion, processing, and function of an AcbA-derived peptide, SDF-2, are conserved between the yeast Pichia pastoris and D. discoideum. We show that in yeast, the secretion of SDF-2–like activity is GRASP dependent, triggered by nitrogen starvation, and requires autophagy proteins as well as medium-chain fatty acyl CoA generated by peroxisomes. Additionally, a phospholipase D implicated in soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor–mediated vesicle fusion at the plasma membrane is necessary, but neither peroxisome turnover nor fusion between autophagosomes and the vacuole is essential. Moreover, yeast Acb1 and several proteins required for its secretion are necessary for sporulation in P. pastoris. Our findings implicate currently unknown, evolutionarily conserved pathways in unconventional secretion.     

The Secretion of an Intrinsically Disordered Protein with Different Secretion Signals in Bacillus subtilis

In this study, a naturally unsecretory intrinsically disordered domain of nucleoskeletal-like protein (Nsp) was attempted to be secreted with different types of secretion signals in Bacillus subtilis. The results showed that Nsp can be secreted efficiently by all selected Sec-type signal peptides. Nsp was successfully exported when fused to Tat-type signal peptides but less efficient than Sec-type. The fusion protein with the non-classical extracellular proteins can be detected in the cell and extracellular milieu. This study further demonstrated that the mature protein plays an important role in protein secretion. Moreover, these results indicated that Nsp could be a useful tool to understand the individual roles of mature proteins and signal peptide in protein secretion, to evaluate the effect of conformation of mature proteins on their export pathway when coupled with Tat-type signal peptide, and to seek the signal of non-classical secretory proteins.     

Amplification of leader proregions as a mean to increase the secretion of antibody fragments in the Pichia pastoris yeast

The dependence of secretion efficiency in Pichia pastoris cells on the copy number of proregions in leader polypeptides has been studied. The humanized light kappa-chain of the murine H3-1 antibody was used as a reporter protein. The leader pre-pro-polypeptides were composed of the signal peptide (preregion) from the α-factor precursors of Saccharomyces cerevisiae and a variable number of proregions from the prepro-precursors of the α-factor or the Hsp150p protein of S. cerevisiae or Hsp150p of P. pastoris. An increase in the proregion copy number either resulted in an almost 1.5-fold increase or a fivefold decrease in secretion depending on the proregion used. It was concluded that the enhancement of the proregion copy number could be of potential value for the intensification of protein secretion in P. pastoris.     

Avoiding entry into intracellular protein degradation pathways by signal mutations increases protein secretion in Pichia pastoris

In our previous study, we serendipitously discovered that protein secretion in the methylotrophic yeast Pichia pastoris is enhanced by a mutation (V50A) in the mating factor alpha (MFα) prepro-leader signal derived from Saccharomyces cerevisiae. In the present study, we investigated 20 single-amino-acid substitutions, including V50A, located within the MFα signal peptide, indicating that V50A and several single mutations alone provided significant increase in production of the secreted proteins. In addition to hydrophobicity index analysis, both an unfolded protein response (UPR) biosensor analysis and a microscopic observation showed a clear difference on the levels of UPR induction and mis-sorting of secretory protein into vacuoles among the wild-type and mutated MFα signal peptides. This work demonstrates the importance of avoiding entry of secretory proteins into the intracellular protein degradation pathways, an observation that is expected to contribute to the engineering of strains with increased production of recombinant secreted proteins.     

Microbial production of spider silk proteins

The remarkable properties of spider dragline silk and related protein polymers will find many applications if the materials can be produced economically. We have demonstrated the production of high molecular weight spider dragline silk analog proteins encoded by synthetic genes in several microbial systems, including Escherichia coli and Pichia pastoris. In E. coli, proteins of up to 1000 amino acids in length could be produced efficiently, but the yield and homogeneity of higher molecular weight silk proteins were found to be limited by truncated synthesis, probably as a result of ribosome termination errors. No such phenomenon was observed in the yeast P. pastoris, where higher molecular weight silk proteins could be produced without heterogeneity due to truncated synthesis. Spider dragline silk analog proteins could be secreted by P. pastoris when fused to both the signal sequence and N-terminal pro-sequence of the Saccharomyces cerevisiae alpha-mating factor gene.     

Accurate analysis of fusion expression of <Emphasis Type="Italic">Pichia pastoris</Emphasis> glycosylphosphatidylinositol-modified cell wall proteins

Glycosylphosphatidylinositol (GPI)-anchored glycoproteins have diverse intrinsic functions in yeasts, and they also have different uses in vitro. The GPI-modified cell wall proteins GCW21, GCW51, and GCW61 of Pichia pastoris were chosen as anchoring proteins to construct co-expression strains in P. pastoris GS115. The hydrolytic activity and the amount of Candida antarctica lipase B (CALB) displayed on cell surface increased significantly following optimization of the fusion gene dosage and combination of the homogeneous or heterogeneous cell wall proteins. Maximum CALB hydrolytic activity was achieved at 4920 U/g dry cell weight in strain GS115/CALB-GCW (51 + 51 + 61 + 61) after 120 h of methanol induction. Changes in structural morphology and the properties of the cell surfaces caused by co-expression of fusion proteins were observed by transmission electron microscopy (TEM) and on plates containing cell-wall-destabilizing reagent. Our results suggested that both the outer and inner cell layers were significantly altered by overexpression of GPI-modified cell wall proteins. Interestingly, quantitative analysis of the inner layer components showed an increase in β-1,3-glucan, but no obvious changes in chitin in the strains overexpressing GPI-modified cell wall proteins.     

Pichia pastoris: A highly successful expression system for optimal synthesis of heterologous proteins

One of the most important branches of genetic engineering is the expression of recombinant proteins using biological expression systems. Nowadays, different expression systems are used for the production of recombinant proteins including bacteria, yeasts, molds, mammals, plants, and insects. Yeast expression systems such as Saccharomyces cerevisiae (S. cerevisiae) and Pichia pastoris (P. pastoris) are more popular. P. pastoris expression system is one of the most popular and standard tools for the production of recombinant protein in molecular biology. Overall, the benefits of protein production by P. pastoris system include appropriate folding (in the endoplasmic reticulum) and secretion (by Kex2 as signal peptidase) of recombinant proteins to the external environment of the cell. Moreover, in the P. pastoris expression system due to its limited production of endogenous secretory proteins, the purification of recombinant protein is easy. It is also considered a unique host for the expression of subunit vaccines which could significantly affect the growing market of medical biotechnology. Although P. pastoris expression systems are impressive and easy to use with well-defined process protocols, some degree of process optimization is required to achieve maximum production of the target proteins. Methanol and sorbitol concentration, Mut forms, temperature and incubation time have to be adjusted to obtain optimal conditions, which might vary among different strains and externally expressed protein. Eventually, optimal conditions for the production of a recombinant protein in P. pastoris expression system differ according to the target protein.     

Expression of Recombinant Proteins in the Methylotrophic Yeast Pichia pastoris

Protein expression in the microbial eukaryotic host Pichia pastoris offers the possibility to generate high amounts of recombinant protein in a fast and easy to use expression system.As a single-celled microorganism P. pastoris is easy to manipulate and grows rapidly on inexpensive media at high cell densities. Being a eukaryote, P. pastoris is able to perform many of the post-translational modifications performed by higher eukaryotic cells and the obtained recombinant proteins undergo protein folding, proteolytic processing, disulfide bond formation and glycosylation [1].As a methylotrophic yeast P. pastoris is capable of metabolizing methanol as its sole carbon source. The strong promoter for alcohol oxidase, AOX1, is tightly regulated and induced by methanol and it is used for the expression of the gene of interest. Accordingly, the expression of the foreign protein can be induced by adding methanol to the growth medium [2; 3].Another important advantage is the secretion of the recombinant protein into the growth medium, using a signal sequence to target the foreign protein to the secretory pathway of P. pastoris. With only low levels of endogenous protein secreted to the media by the yeast itself and no added proteins to the media, a heterologous protein builds the majority of the total protein in the medium and facilitates following protein purification steps [3; 4].The vector used here (pPICZαA) contains the AOX1 promoter for tightly regulated, methanol-induced expression of the gene of interest; the α-factor secretion signal for secretion of the recombinant protein, a Zeocin resistance gene for selection in both E. coli and Pichia and a C-terminal peptide containing the c-myc epitope and a polyhistidine (6xHis) tag for detection and purification of a recombinant protein. We also show western blot analysis of the recombinant protein using the specific Anti-myc-HRP antibody recognizing the c-myc epitope on the parent vector.Download video file.^{(116M, mp4)}