What makes Komagataella phaffii non-conventional?

The important industrial protein production host Komagataella phaffii (syn Pichia pastoris) is classified as a non-conventional yeast. But what exactly makes K. phaffii non-conventional? In this review, we set out to address the main differences to the 'conventional' yeast Saccharomyces cerevisiae, but also pinpoint differences to other non-conventional yeasts used in biotechnology. Apart from its methylotrophic lifestyle, K. phaffii is a Crabtree-negative yeast species. But even within the methylotrophs, K. phaffii possesses distinct regulatory features such as glycerol-repression of the methanol-utilization pathway or the lack of nitrate assimilation. Rewiring of the transcriptional networks regulating carbon (and nitrogen) source utilization clearly contributes to our understanding of genetic events occurring during evolution of yeast species. The mechanisms of mating-type switching and the triggers of morphogenic phenotypes represent further examples for how K. phaffii is distinguished from the model yeast S. cerevisiae. With respect to heterologous protein production, K. phaffii features high secretory capacity but secretes only low amounts of endogenous proteins. Different to S. cerevisiae, the Golgi apparatus of K. phaffii is stacked like in mammals. While it is tempting to speculate that Golgi architecture is correlated to the high secretion levels or the different N-glycan structures observed in K. phaffii, there is recent evidence against this. We conclude that K. phaffii is a yeast with unique features that has a lot of potential to explore both fundamental research questions and industrial applications.

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

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

Polymorphic distribution of proteins in solution by mass spectrometry: The analysis of insulin analogues

The characterization of conformational and oligomeric distribution of proteins is of paramount importance for the understanding of the correlation between structure and function. Among the bioanalytical approaches currently available, the electrospray ionization-mass spectrometry (ESI-MS) coupled to ion mobility spectrometry (IMS) is the best suited for high resolution identification with high sensitivity, allowing the in situ separation of oligomeric and conformational species. We tested the performance of the ESI-MS technique along with the IMS separation approach on a broad variety of insulin and insulin analogues with distinct oligomeric distribution pattern. The measurement of commercial insulin allowed the identification of species ranging from monomers to hexamers and their complexes with zinc ions. Dissimilar distribution profile for regular insulin as a function of formulation component and among the insulin analogues were observed by ESI-IMS-MS but not by ESI-MS along, crystallographic assays or size-exclusion chromatography. These data suggest the additional suitability of ESI-IMS-MS in conformational and oligomeric profiling of biomacromolecules and biopharmaceuticals. The easiness of the technique provides further motivation for its application in the characterization of both raw and formulated protein biopharmaceuticals in routine and comparability exercises.

Comparative Secretome Analysis Reveals Perturbation of Host Secretion Pathways by a Hypovirus

To understand the impact of a hypovirus infection on the secretome of the chestnut blight fungus, Cryphonectria parasitica, a phytopathogenic filamentous fungus, two-dimensional electrophoresis (2-DE) and isobaric tag for relative and absolute quantitation (iTRAQ) technology were employed to identify and quantify the secreted proteins. A total of 403 unique proteins were identified from the secretome of the wild type virus-free strain EP155. Of these proteins, 329 were predicted to be involved in known secretory pathways and they are primarily composed of metabolic enzymes, biological regulators, responders to stimulus and components involved in plant-pathogen interactions. When infected with the hypovirus CHV1-EP713, 99 proteins were found to be differentially expressed as compared to the wild type strain EP155. These proteins were mainly related to plant cell wall degradation, response to host defense, fungal virulence and intracellular structure. The effects of CHV1 on secreted proteins may reveal a relationship between physiological pathways and hypovirulence.

Identification of Secreted Candida Proteins Using Mass Spectrometry

Analysis of fungal secretomes using mass spectrometry is a useful technique in cell biology. Knowledge of the secretome of a human fungal pathogen may yield important information of host-pathogen interactions and may be useful for identifying vaccines candidates or diagnostic markers for antifungal strategies. In this chapter, with a main focus on sample preparation aspects, we describe the methodology that we apply for gel-independent batch identification and quantification of proteins that are secreted during growth in liquid cultures. Using these techniques with Candida and other yeast species, the majority of the identified proteins are classical secretory proteins and cell wall proteins containing N-terminal signal peptides for secretion, although dependent on sample preparation quality and the mass spectrometric analysis also usually, a number of nonsecretory proteins are identified.

Investigation of protein secretion and secretion stress in Ashbya gossypii

Background

Ashbya gossypii is a filamentous Saccharomycete used for the industrial production of riboflavin that has been recently explored as a host system for recombinant protein production. To gain insight into the protein secretory pathway of this biotechnologically relevant fungus, we undertook genome-wide analyses to explore its secretome and its transcriptional responses to protein secretion stress.

Results

A computational pipeline was used to predict the inventory of proteins putatively secreted by A. gossypii via the general secretory pathway. The proteins actually secreted by this fungus into the supernatants of submerged cultures in minimal and rich medium were mapped by two-dimensional gel electrophoresis, revealing that most of the A. gossypii secreted proteins have an isoelectric point between 4 and 6, and a molecular mass above 25 kDa. These analyses together indicated that 1-4% of A. gossypii proteins are likely to be secreted, of which less than 33% are putative hydrolases. Furthermore, transcriptomic analyses carried out in A. gossypii cells under recombinant protein secretion conditions and dithiothreitol-induced secretion stress unexpectedly revealed that a conventional unfolded protein response (UPR) was not activated in any of the conditions, as the expression levels of several well-known UPR target genes (e.g. IRE1, KAR2, HAC1 and PDI1 homologs) remained unaffected. However, several other genes involved in protein unfolding, endoplasmatic reticulum-associated degradation, proteolysis, vesicle trafficking, vacuolar protein sorting, secretion and mRNA degradation were up-regulated by dithiothreitol-induced secretion stress. Conversely, the transcription of several genes encoding secretory proteins, such as components of the glycosylation pathway, was severely repressed by dithiothreitol

Conclusions

This study provides the first insights into the secretion stress response of A. gossypii, as well as a basic understanding of its protein secretion potential, which is more similar to that of yeast than to that of other filamentous fungi. Contrary to what has been widely described for yeast and fungi, a conventional UPR was not observed in A. gossypii, but alternative protein quality control mechanisms enabled it to cope with secretion stress. These data will help provide strategies for improving heterologous protein secretion in A. gossypii.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1137) contains supplementary material, which is available to authorized users.

Gill bacteria enable a novel digestive strategy in a wood-feeding mollusk

Bacteria play many important roles in animal digestive systems, including the provision of enzymes critical to digestion. Typically, complex communities of bacteria reside in the gut lumen in direct contact with the ingested materials they help to digest. Here, we demonstrate a previously undescribed digestive strategy in the wood-eating marine bivalve Bankia setacea, wherein digestive bacteria are housed in a location remote from the gut. These bivalves, commonly known as shipworms, lack a resident microbiota in the gut compartment where wood is digested but harbor endosymbiotic bacteria within specialized cells in their gills. We show that this comparatively simple bacterial community produces wood-degrading enzymes that are selectively translocated from gill to gut. These enzymes, which include just a small subset of the predicted wood-degrading enzymes encoded in the endosymbiont genomes, accumulate in the gut to the near exclusion of other endosymbiont-made proteins. This strategy of remote enzyme production provides the shipworm with a mechanism to capture liberated sugars from wood without competition from an endogenous gut microbiota. Because only those proteins required for wood digestion are translocated to the gut, this newly described system reveals which of many possible enzymes and enzyme combinations are minimally required for wood degradation. Thus, although it has historically had negative impacts on human welfare, the shipworm digestive process now has the potential to have a positive impact on industries that convert wood and other plant biomass to renewable fuels, fine chemicals, food, feeds, textiles, and paper products.

Glucose induces rapid changes in the secretome of Saccharomyces cerevisiae

Background

Protein secretion is a fundamental process in all living cells. Proteins can either be secreted via the classical or non-classical pathways. In Saccharomyces cerevisiae, gluconeogenic enzymes are in the extracellular fraction/periplasm when cells are grown in media containing low glucose. Following a transfer of cells to high glucose media, their levels in the extracellular fraction are reduced rapidly. We hypothesized that changes in the secretome were not restricted to gluconeogenic enzymes. The goal of the current study was to use a proteomic approach to identify extracellular proteins whose levels changed when cells were transferred from low to high glucose media.

Results

We performed two iTRAQ experiments and identified 347 proteins that were present in the extracellular fraction including metabolic enzymes, proteins involved in oxidative stress, protein folding, and proteins with unknown functions. Most of these proteins did not contain typical ER-Golgi signal sequences. Moreover, levels of many of these proteins decreased upon a transfer of cells from media containing low to high glucose media. Using an extraction procedure and Western blotting, we confirmed that the metabolic enzymes (glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, glucose-6-phosphate dehydrogenase, pyruvate decarboxylase), proteins involved in oxidative stress (superoxide dismutase and thioredoxin), and heat shock proteins (Ssa1p, Hsc82p, and Hsp104p) were in the extracellular fraction during growth in low glucose and that the levels of these extracellular proteins were reduced when cells were transferred to media containing high glucose. These proteins were associated with membranes in vesicle-enriched fraction. We also showed that small vesicles were present in the extracellular fraction in cells grown in low glucose. Following a transfer from low to high glucose media for 30 minutes, 98% of these vesicles disappeared from the extracellular fraction.

Conclusions

Our data indicate that transferring cells from low to high glucose media induces a rapid decline in levels of a large number of extracellular proteins and the disappearance of small vesicles from the extracellular fraction. Therefore, we conclude that the secretome undergoes dynamic changes during transition from glucose-deficient to glucose-rich media. Most of these extracellular proteins do not contain typical ER signal sequences, suggesting that they are secreted via the non-classical pathway.

Reconstitution of translation from Thermus thermophilus reveals a minimal set of components sufficient for protein synthesis at high temperatures and functional conservation of modern and ancient translation components

Thermus thermophilus is a thermophilic model organism distantly related to the mesophilic model organism E. coli. We reconstituted protein translation of Thermus thermophilus in vitro from purified ribosomes, transfer ribonucleic acids (tRNAs) and 33 recombinant proteins. This reconstituted system was fully functional, capable of translating natural messenger RNA (mRNA) into active full-length proteins at temperatures up to 65°C and with yields up to 60 μg/ml. Surprisingly, the synthesis of active proteins also occurred at 37°C, a temperature well below the minimal growth temperature for T. thermophilus. A polyamine was required, with tetraamine being most effective, for translation at both high and low temperatures. Using such a defined in vitro system, we demonstrated a minimal set of components that are sufficient for synthesizing active proteins at high temperatures, the functional compatibility of key translation components between T. thermophilus and E. coli, and the functional conservation of a number of resurrected ancient elongation factors. This work sets the stage for future experiments that apply abundant structural information to biochemical characterization of protein translation and folding in T. thermophilus. Because it contains significantly reduced nucleases and proteases, this reconstituted thermostable cell-free protein synthesis system may enable in vitro engineering of proteins with improved thermostability.

Engineering Escherichia coli BL21(DE3) derivative strains to minimize E. coli protein contamination after purification by immobilized metal affinity chromatography

Recombinant His-tagged proteins expressed in Escherichia coli and purified by immobilized metal affinity chromatography (IMAC) are commonly coeluted with native E. coli proteins, especially if the recombinant protein is expressed at a low level. The E. coli contaminants display high affinity to divalent nickel or cobalt ions, mainly due to the presence of clustered histidine residues or biologically relevant metal binding sites. To improve the final purity of expressed His-tagged protein, we engineered E. coli BL21(DE3) expression strains in which the most recurring contaminants are either expressed with an alternative tag or mutated to decrease their affinity to divalent cations. The current study presents the design, engineering, and characterization of two E. coli BL21(DE3) derivatives, NiCo21(DE3) and NiCo22(DE3), which express the endogenous proteins SlyD, Can, ArnA, and (optionally) AceE fused at their C terminus to a chitin binding domain (CBD) and the protein GlmS, with six surface histidines replaced by alanines. We show that each E. coli CBD-tagged protein remains active and can be efficiently eliminated from an IMAC elution fraction using a chitin column flowthrough step, while the modification of GlmS results in loss of affinity for nickel-containing resin. The "NiCo" strains uniquely complement existing methods for improving the purity of recombinant His-tagged protein.

Growth-dependent secretome of Candida utilis

Recently, the food yeast Candida utilis has emerged as an excellent host for production of heterologous proteins. Since secretion of the recombinant product is advantageous for its purification, we characterized the secreted proteome of C. utilis. Cells were cultivated to the exponential or stationary growth phase, and the proteins in the medium were identified by MS. In parallel, a draft genome sequence of C. utilis strain DSM 2361 was determined by massively parallel sequencing. Comparisons of protein and coding sequences established that C. utilis is not a member of the CUG clade of Candida species. In total, we identified 37 proteins in the culture solution, 17 of which were exclusively present in the stationary phase, whereas three proteins were specific to the exponential growth phase. Identified proteins represented mostly carbohydrate-active enzymes associated with cell wall organization, while no proteolytic enzymes and only a few cytoplasmic proteins were detected. Remarkably, cultivation in xylose-based medium generated a protein pattern that diverged significantly from glucose-grown cells, containing the invertase Inv1 as the major extracellular protein, particularly in its highly glycosylated S-form (slow-migrating). Furthermore, cultivation without ammonium sulfate induced the secretion of the asparaginase Asp3. Comparisons of the secretome of C. utilis with those of Kluyveromyces lactis and Pichia pastoris, as well as with those of the human fungal pathogens Candida albicans and Candida glabrata, revealed a conserved set of 10 and six secretory proteins, respectively.

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.

Plant secretome: unlocking secrets of the secreted proteins

Plant secretomics is a newly emerging area of the plant proteomics field. It basically describes the global study of secreted proteins into the extracellular space of plant cell or tissue at any given time and under certain conditions through various secretory mechanisms. A combination of biochemical, proteomics and bioinformatics approaches has been developed to isolate, identify and profile secreted proteins using complementary in vitro suspension-cultured cells and in planta systems. Developed inventories of secreted proteins under normal, biotic and abiotic conditions revealed several different types of novel secreted proteins, including the leaderless secretory proteins (LSPs). On average, LSPs can account for more than 50% of the total identified secretome, supporting, as in other eukaryotes, the existence of novel secretory mechanisms independent of the classical endoplasmic reticulum-Golgi secretory pathway, and suggesting that this non-classical mechanism of protein expression is, for as yet unknown reasons, more massively used than in other eukaryotic systems. Plants LSPs, which seem to be potentially involved in the defense/stress responses, might have dual (extracellular and/or intracellular) roles as most of them have established intracellular functions, yet presently unknown extracellular functions. Evidence is emerging on the role of glycosylation in the apical sorting and trafficking of secretory proteins. These initial secretome studies in plants have considerably advanced our understanding on secretion of different types of proteins and their underlying mechanisms, and opened a door for comparative analyses of plant secretomes with those of other organisms. In this first review on plant secretomics, we summarize and discuss the secretome definition, the applied approaches for unlocking secrets of the secreted proteins in the extracellular fluid, the possible functional significance and secretory mechanisms of LSPs, as well as glycosylation of secreted proteins and challenges involved ahead. Further improvements in existing and developing strategies and techniques will continue to drive forward plant secretomics research to building comprehensive and confident data sets of secreted proteins. This will lead to an increased understanding on how cells couple the concerted action of secreted protein networks to their internal and external environments.

Constitutive secretion in Tetrahymena thermophila

The growth, survival, and life cycle progression of the freshwater ciliated protozoan Tetrahymena thermophila are responsive to protein signals thought to be released by constitutive secretion. In addition to providing insights about ciliate communication, studies of constitutive secretion are of interest for evaluating the utility of T. thermophila as a platform for the expression of secreted protein therapeutics. For these reasons, we undertook an unbiased investigation of T. thermophila secreted proteins using wild-type and secretion mutant strains. Extensive tandem mass spectrometry analyses of secretome samples were performed. We identified a total of 207 secretome proteins, most of which were not detected in a set of abundant whole-cell protein identifications. Numerous proteases and other hydrolases were secreted from cells grown in rich medium but not cells transferred to a nutrient starvation condition. On the other hand, we detected the starvation-enhanced secretion of a small number of cytosolic proteins, suggestive of an exosome-like pathway in T. thermophila. Subsets of proteins from the T. thermophila regulated secretion pathway were detected with differential representation across strains and culture conditions. Finally, many secretome proteins had a predicted N-terminal signal sequence but no other annotated characteristic or functional classification. Our work provides the first comprehensive analysis of secreted proteins in T. thermophila and establishes the groundwork for future studies of constitutive protein secretion biology and biotechnology in ciliates.

The effect of carbon source on the secretome of Kluyveromyces lactis

A proteomic analysis was performed on spent fermentation medium following bioreactor propagation of a wild-type industrial strain to identify proteins naturally secreted by Kluyveromyces lactis cells. Here, we report changes detected in the K. lactis secretome as a result of growth in three different carbon sources: glucose, galactose and glycerol. A total of 151 secreted proteins were detected by multi-dimensional separations and reversed-phase online nanoESI-MS/MS analysis. From these, we were able to identify 63 proteins (termed the "base secretome") that were common to all three fermentation conditions. The majority of base secretome proteins, 79%, possessed general secretory pathway (GSP) sequences and were involved with cell wall structure, glycosylation, carbohydrate metabolism and proteolysis. There was little variation in the functional groupings of base secretome GSP proteins and GSP proteins that were not part of the base secretome. In contrast, the majority of non-GSP proteins detected were not part of the base secretome and the functions of these proteins varied significantly. Finally, through further identification of non-GSP proteins in carbon sources not originally tested, we have gained further evidence of a protein export mechanism separate from the GSP in K. lactis.

Hunting down fungal secretomes using liquid-phase IEF prior to high resolution 2-DE

The secreted proteins (secretome) of fungi play a key role in interactions of pathogenic and symbiotic fungi with plants. Using the plant pathogenic fungus Leptosphaeria maculans and symbiont Laccaria bicolor grown in culture, we have established a proteomic protocol for extraction, concentration and resolution of the fungal secretome. As no proteomic data were available on mycelium tissues from both L. maculans and L. bicolor, mycelial proteins were studied; they also helped verifying the purity of secretome samples. The quality of protein extracts was initially assessed by both 1-DE and 2-DE using first a broad pH range for IEF, and then narrower acidic and basic pH ranges, prior to 2-DE. Compared with the previously published protocols for which only dozens of 2-D spots were recovered from fungal secretome samples, up to approximately 2000 2-D spots were resolved by our method. MS identification of proteins along several pH gradients confirmed this high resolution, as well as the presence of major secretome markers such as endopolygalacturonases, beta-glucanosyltransferases, pectate lyases and endoglucanases. Shotgun proteomic experiments evidenced the enrichment of secreted protein within the liquid medium. This is the first description of the proteome of L. maculans and L. bicolor, and the first application of liquid-phase IEF to any fungal extracts.

The appearance of pyrrolysine in tRNAHis guanylyltransferase by neutral evolution

tRNA(His) guanylyltransferase (Thg1) post-transcriptionally adds a G (position -1) to the 5'-terminus of tRNA(His). The Methanosarcina acetivorans Thg1 (MaThg1) gene contains an in-frame TAG (amber) codon. Although a UAG codon typically directs translation termination, its presence in Methanosarcina mRNA may lead to pyrrolysine (Pyl) incorporation achieved by Pyl-tRNA(Pyl), the product of pyrrolysyl-tRNA synthetase. Sequencing of the MaThg1 gene and transcript confirmed the amber codon. Translation of MaThg1 mRNA led to a full-length, Pyl-containing, active enzyme as determined by immunoblotting, mass spectrometry, and biochemical analysis. The nature of the inserted amino acid at the position specified by UAG is not critical, as Pyl or Trp insertion yields active MaThg1 variants in M. acetivorans and equal amounts of full-length protein. These data suggest that Pyl insertion is akin to natural suppression and unlike the active stop codon reassignment that is required for selenocysteine insertion. Only three Pyl-containing proteins have been characterized previously, a set of methylamine methyltransferases in which Pyl is assumed to have specifically evolved to be a key active-site constituent. In contrast, Pyl in MaThg1 is a dispensable residue that appears to confer no selective advantage. Phylogenetic analysis suggests that Thg1 is becoming dispensable in the archaea, and furthermore supports the hypothesis that Pyl appeared in MaThg1 as the result of neutral evolution. This indicates that even the most unusual amino acid can play an ordinary role in proteins.

The Saccharomyces cerevisiae vacuolar acid trehalase is targeted at the cell surface for its physiological function

Previous studies in the yeast Saccharomyces cerevisiae have proposed a vacuolar localization for Ath1, which is difficult to reconcile with its ability to hydrolyze exogenous trehalose. We used fluorescent microscopy to show that the red fluorescent protein mCherry fused to the C-terminus of Ath1, although mostly localized in the vacuole, was also targeted to the cell surface. Also, hybrid Ath1 truncates fused at their C-terminus with the yeast internal invertase revealed that a 131 amino acid N-terminal fragment of Ath1was sufficient to target the fusion protein to the cell surface, enabling growth of the suc2Delta mutant on sucrose. The unique transmembrane domain appeared to be indispensable for the production of a functional Ath1, and its removal abrogated invertase secretion and growth on sucrose. Finally, the physiological significance of the cell-surface localization of Ath1 was established by showing that fusion of the signal peptide of invertase to N-terminal truncated Ath1 allowed the ath1Delta mutant to grow on trehalose, whereas the signal sequence of the vacuolar-targeted Pep4 constrained Ath1 in the vacuole and prevented growth of this mutant on trehalose. Use of trafficking mutants that impaired Ath1 delivery to the vacuole abrogated neither its activity nor its growth on exogenous trehalose.

Computational analysis of the interaction between transcription factors and the predicted secreted proteome of the yeast Kluyveromyces lactis

Background

Protein secretion is a cell translocation process of major biological and technological significance. The secretion and downstream processing of proteins by recombinant cells is of great commercial interest. The yeast Kluyveromyces lactis is considered a promising host for heterologous protein production. Because yeasts naturally do not secrete as many proteins as filamentous fungi, they can produce secreted recombinant proteins with few contaminants in the medium. An ideal system to address the secretion of a desired protein could be exploited among the native proteins in certain physiological conditions. By applying algorithms to the completed K. lactis genome sequence, such a system could be selected. To this end, we predicted protein subcellular locations and correlated the resulting extracellular secretome with the transcription factors that modulate the cellular response to a particular environmental stimulus.

Results

To explore the potential Kluyveromyces lactis extracellular secretome, four computational prediction algorithms were applied to 5076 predicted K. lactis proteins from the genome database. SignalP v3 identified 418 proteins with N-terminal signal peptides. From these 418 proteins, the Phobius algorithm predicted that 176 proteins have no transmembrane domains, and the big-PI Predictor identified 150 proteins as having no glycosylphosphatidylinositol (GPI) modification sites. WoLF PSORT predicted that the K. lactis secretome consists of 109 putative proteins, excluding subcellular targeting. The transcription regulators of the putative extracellular proteins were investigated by searching for DNA binding sites in their putative promoters. The conditions to favor expression were obtained by searching Gene Ontology terms and using graph theory.

Conclusion

A public database of K. lactis secreted proteins and their transcription factors are presented. It consists of 109 ORFs and 23 transcription factors. A graph created from this database shows 134 nodes and 884 edges, suggesting a vast number of relationships to be validated experimentally. Most of the transcription factors are related to responses to stress such as drug, acid and heat resistance, as well as nitrogen limitation, and may be useful for inducing maximal expression of potential extracellular proteins.

Genome, secretome and glucose transport highlight unique features of the protein production host Pichia pastoris

BACKGROUND

Pichia pastoris is widely used as a production platform for heterologous proteins and model organism for organelle proliferation. Without a published genome sequence available, strain and process development relied mainly on analogies to other, well studied yeasts like Saccharomyces cerevisiae.

RESULTS

To investigate specific features of growth and protein secretion, we have sequenced the 9.4 Mb genome of the type strain DSMZ 70382 and analyzed the secretome and the sugar transporters. The computationally predicted secretome consists of 88 ORFs. When grown on glucose, only 20 proteins were actually secreted at detectable levels. These data highlight one major feature of P. pastoris, namely the low contamination of heterologous proteins with host cell protein, when applying glucose based expression systems. Putative sugar transporters were identified and compared to those of related yeast species. The genome comprises 2 homologs to S. cerevisiae low affinity transporters and 2 to high affinity transporters of other Crabtree negative yeasts. Contrary to other yeasts, P. pastoris possesses 4 H+/glycerol transporters.

CONCLUSION

This work highlights significant advantages of using the P. pastoris system with glucose based expression and fermentation strategies. As only few proteins and no proteases are actually secreted on glucose, it becomes evident that cell lysis is the relevant cause of proteolytic degradation of secreted proteins. The endowment with hexose transporters, dominantly of the high affinity type, limits glucose uptake rates and thus overflow metabolism as observed in S. cerevisiae. The presence of 4 genes for glycerol transporters explains the high specific growth rates on this substrate and underlines the suitability of a glycerol/glucose based fermentation strategy. Furthermore, we present an open access web based genome browser http://www.pichiagenome.org.