Expression of the alpha-galactosidase from Cyamopsis tetragonoloba (guar) by Hansenula polymorpha

Design of ancestral mammalian alkaline phosphatase bearing high stability and productivity

Mammalian alkaline phosphatase (AP) is widely used in diagnostics and molecular biology but its widespread use is impaired because it is difficult to express in Escherichia coli and has low thermostability. To overcome these challenges, we employed sequence-based protein redesign methods, specifically full consensus design (FCD) and ancestral sequence reconstruction (ASR), to create APs with enhanced properties. Biochemical analyses revealed that these newly designed APs exhibited improved levels of expression in their active form and increased thermostability compared to bovine intestinal AP isozyme II (bIAPII), without impeding enzymatic activity. Notably, the activity in culture broth of the designed APs was an order of magnitude higher than that of bIAPII, and their thermal stability increased by 13°C-17°C (measured as T50). We also assessed 28 single-point mutants of bIAPII to identify regions influencing thermostability and expression level; these mutations were common in the engineered APs but not in bIAPII. Specific mutations, such as T413E and G402S, enhanced thermostability, whereas increasing the expression level required multiple mutations. This suggests that a synergistic effect is required to enhance the expression level. Mutations enhancing thermostability were located in the crown domain, while those improving expression were close to the dimer interface, indicating distinct mechanisms underpinning these enhancements.

IMPORTANCE

Sequence-based protein redesign methods, such as full consensus design (FCD) and ancestral sequence reconstruction (ASR), have the potential to construct new enzymes utilizing protein sequence data registered in a rapidly expanding sequence database. The high thermostability of these enzymes would expand their application in diagnostics and molecular biology. These enzymes have also demonstrated a high level of active expression by Escherichia coli. These characteristics make these APs attractive candidates for industrial application. In addition, different amino acid residues are primarily responsible for thermal stability and active expression, suggesting important implications for strategies for designing enzymes by FCD and ASR.

Design of vectors for efficient integration and transformation in Hansenula polymorpha

Four vectors were constructed for high-level expression of heterologous proteins with high copy number and mitotic stability in Hansenula polymorpha. All of them contained the conserved H. polymorpha-derived ribosomal DNA (rDNA) sequence for targeting and the geneticin (G418) resistance gene as a selection marker. A strong inducible promoter, formate dehydrogenase (FMD) promoter from H. polymorpha, was used to drive the expression of heterologous genes; the formate dehydrogenase terminator of H. polymorpha was used as the transcription termination region. A modified green fluorescent protein (mGFP) and firefly luciferase protein (Luc) were used as the marker to evaluate the efficacy of these vectors. Using Southern blotting analysis, 2-30 copies of these vectors were integrated into rDNA loci. These results demonstrated that all the four vectors could be used as candidates for expression of desired proteins in H. polymorpha.

A novel enzymatic technique for limiting drug mobility in a hydrogel matrix

An oral colon specific drug delivery platform has been developed to facilitate targetted release of therapeutic proteins as well as small molecule drugs. A simple enzymatic procedure is used to modify the molecular architecture of a lightly chemically crosslinked galactomannan hydrogel as well as a model drug-galactomannan oligomer conjugate, fluoroisocynate (FITC) tagged guar oligomer, to entrap the model drug. The enzyme-modified hydrogel retains the drug until it reaches the colonic environment where bacteria secrete enzymes (namely beta-mannanase) to degrade the gel and release the drug molecule. Laser scanning confocal microscopy combined with fluorescence recovery after photobleaching is used to quantify the diffusion of the drug conjugate. The diffusion coefficient of solutes in the lightly crosslinked galactomannan hydrogel is approximately equal to the diffusion coefficient in the guar solution for simple diffusional drug loading. After drug loading, alpha-galactosidase treatment generates additional physical crosslinks in the hydrogel matrix as well as between the drug-oligomer conjugate and the hydrogel, which reduces diffusion of the drug-oligomer conjugate significantly. Degradation of the hydrogel by beta-mannanase results in a slow and controlled rate of FITC-guar oligomer diffusion, which generates an extended release profile for the model drug.

Constitutive expression of recombinant proteins in the methylotrophic yeast Hansenula polymorpha using the PMA1 promoter

The methylotrophic yeast H. polymorpha is a popular system for the expression of recombinant proteins using the strong and regulatable methanol oxidase (MOX) promoter. Here we show that the constitutive PMA1 promoter can programme the expression of two heterologous proteins, glucose oxidase and human serum albumin. A constitutive promoter provides a useful additional facility to the H. polymorpha expression system because it allows a simplified fermentation regime, avoids the use of methanol, which is both toxic and an explosive hazard, and allows more flexibility for ectopic gene expression during the course of academic studies. A fragment previously isolated in a promoter screen, using glucose oxidase (GOD) as a reporter gene, was shown to consist of the promoter region and the first 659 bp of the H. polymorpha PMA1 gene, encoding the plasma membrane H(+)-ATPase. When the PMA1 promoter was optimally aligned with the GOD coding region, it produced 185 mg/l glucose oxidase in high cell density fed batch fermentations, whereas in previous experiments using the MOX promoter, a yield of 500 mg/l was recovered. The PMA1 promoter was also used to express recombinant human serum albumin (rHA) in H. polymorpha. In high cell density fermentations the PMA1 promoter produced 460 mg/l rHA, whereas 280 mg/l rHA was obtained using the MOX promoter. Taken together, these experiments show that the HpPMA1 programmes the constitutive expression of recombinant proteins and provides a yield comparable to that from the MOX promoter.

The methylotrophic yeast Hansenula polymorpha: a versatile cell factory

The development of heterologous overexpression systems for soluble proteins has greatly advanced the study of the structure/function relationships of these proteins and their biotechnological and pharmaceutical applications. In this paper we present an overview on several aspects of the use of the methylotrophic yeast Hansenula polymorpha as a host for heterologous gene expression. H. polymorpha has been successfully exploited as a cell factory for the large-scale production of such components. Stable, engineered strains can be obtained by site-directed integration of expression cassettes into the genome, for which various constitutive and inducible promoters are available to control the expression of the foreign genes. New developments have now opened the way to additional applications of H. polymorpha, which are unprecedented for other organisms. Most importantly, it may be the organism of choice for reliable, large-scale production of heterologous membrane proteins, using inducible intracellular membranes and targeting sequences to specifically insert these proteins stably into these membranes. Furthermore, the use of H. polymorpha offers the possibility to accumulate the produced components into specific compartments, namely peroxisomes. These organelles are massively induced during growth of the organism on methanol and may occupy up to 80% of the cell volume. Accumulation inside peroxisomes prevents undesired modifications (e.g. proteolytic processing or glycosylation) and is also in particular advantageous when proteins are produced which are toxic or harmful for the host.

Vectors for rapid selection of integrants with different plasmid copy numbers in the yeast Hansenula polymorpha DL1

Plasmids with different selectable markers were constructed and used to transform the Hansenula polymorpha strain DL1. It was shown that, depending on the host mutant strain, the use of these plasmids enables rapid selection of transformants with plasmids integrated in low (1-2), moderate (6-9) or high (up to 100) copy numbers. The vectors and mutant described are potentially useful for the construction of efficient producers of heterologous proteins in H. polymorpha.

Cloning and high-level expression of alpha-galactosidase cDNA from Penicillium purpurogenum

The cDNA coding for Penicillium purpurogenum alpha-galactosidase (alphaGal) was cloned and sequenced. The deduced amino acid sequence of the alpha-Gal cDNA showed that the mature enzyme consisted of 419 amino acid residues with a molecular mass of 46,334 Da. The derived amino acid sequence of the enzyme showed similarity to eukaryotic alphaGals from plants, animals, yeasts, and filamentous fungi. The highest similarity observed (57% identity) was to Trichoderma reesei AGLI. The cDNA was expressed in Saccharomyces cerevisiae under the control of the yeast GAL10 promoter. Almost all of the enzyme produced was secreted into the culture medium, and the expression level reached was approximately 0.2 g/liter. The recombinant enzyme purified to homogeneity was highly glycosylated, showed slightly higher specific activity, and exhibited properties almost identical to those of the native enzyme from P. purpurogenum in terms of the N-terminal amino acid sequence, thermoactivity, pH profile, and mode of action on galacto-oligosaccharides.

Application of yeasts in gene expression studies: a comparison of Saccharomyces cerevisiae, Hansenula polymorpha and Kluyveromyces lactis -- a review

From the onset of gene technology yeasts have been among the most commonly used host cells for the production of heterologous proteins. At the beginning of this new development the attention in molecular biology and biotechnology focused on the use of the best characterized species, Saccharomyces cerevisiae, leading to an increasing number of production systems for recombinant compounds. In recent years alternative yeasts became accessible for the techniques of modern molecular genetics and, thereby, for potential applications in biotechnology. In this respect Kluyveromyces lactis, and the methylotrophs Hansenula polymorpha and Pichia pastoris have been proven to offer significant advantages over the traditional baker's yeast for the production of certain proteins. In the following article, the present status of the various yeast systems is discussed.

Glucoamylase gene fusions alleviate limitations for protein production in Aspergillus awamori at the transcriptional and (post) translational levels

In this study we have analyzed the effects of a glucoamylase gene fusion on the mRNA levels and protein levels for the human interleukin-6 gene (hil6) and the guar alpha-galactosidase gene (aglA). Previously it was shown that production of nonfused alpha-galactosidase and hIL-6 in Aspergillus awamori was limited at transcriptional and (post)translational levels, respectively (R. J. Gouka, P. J. Punt, J. G. M. Hessing, and C. A. M. J. J. van den Hondel, Appl. Environ. Microbiol. 62:1951-1957, 1996). Vectors were constructed which contained either the hil6 or aglA gene fused to the Aspergillus niger glucoamylase gene (glaA) under control of the efficient 1,4-beta-endoxylanase A promoter and transcription terminator. For comparison, the vectors were integrated in a single copy at the pyrG locus of A. awamori. A glaA fusion to the 5' end of the hil6 gene resulted in a large increase in hIL-6 yield, whereas with a glaA fusion to the 3' end of the hil6 gene, almost no protein was produced. Nevertheless, the steady-state mRNA levels of both fusions were very similar and not clearly increased compared to those of a strain expressing nonfused hIL-6. Fusions of glaA to the 5' end of the wild-type guar aglA gene resulted in truncated mRNA lacking almost 900 bases (> 80%) of the aglA sequence. When the coding sequence of the wild-type aglA gene was replaced by a synthetic aglA gene with optimized Saccharomyces cerevisiae codon usage, full-length mRNA was obtained. Compared to a nonfused synthetic aglA gene, a glaA fusion with the synthetic aglA gene resulted in a 25-fold increase in the mRNA level and, as a consequence, a similar increase in the alpha-galactosidase protein level. The truncated transcripts derived from the wild-type aglA gene were further analyzed by nuclear run-on transcription assays. These experiments indicated that transcription elongation in the nucleus proceeded at least 400 bases downstream of the site where the truncation was determined, indicating that transcription elongation or premature termination was not the reason for the generation of truncated mRNAs. As the truncated mRNA also contained a poly(A) tail, truncation most likely occurs by incorrect processing of the aglA mRNA in the nucleus.

Analysis of heterologous protein production in defined recombinant Aspergillus awamori strains

A study was carried out to obtain more insight into the parameters that determine the secretion of heterologous proteins from filamentous fungi. A strategy was chosen in which the mRNA levels and protein levels of a number of heterologous genes of different origins were compared. All genes were under control of the Aspergillus awamori 1,4-beta-endoxylanase A (exlA) expression signals and were integrated in a single copy at the A. awamori pyrG locus. A Northern (RNA) analysis showed that large differences occurred in the steady-state mRNA levels obtained with the various genes; those levels varied from high values for genes of fungal origin (A. awamori 1,4-beta-endoxylanase A, Aspergillus niger glucoamylase, and Thermomyces lanuginosa lipase) to low values for genes of nonfungal origin (human interleukin 6 and Cyamopsis tetragonoloba [guar] alpha-galactosidase). With the C. tetragonoloba alpha-galactosidase wild-type gene full-length mRNA was even undetectable. Surprisingly, small amounts of full-length mRNA could be detected when a C. tetragonoloba alpha-galactosidase gene with an optimized Saccharomyces cerevisiae codon preference was expressed. In all cases except human interleukin 6, the protein levels corresponded to the amounts expected on basis of the mRNA levels. For human interleukin 6, very low protein levels were observed, whereas relatively high steady-state mRNA levels were obtained. Our data suggest that intracellular protein degradation is the most likely explanation for the low levels of secreted human interleukin 6.

Expression of the major bean proteins from Theobroma cacao (cocoa) in the yeasts Hansenula polymorpha and Saccharomyces cerevisiae

The production in two yeast expression systems of recombinant forms of the major proteins from the cocoa bean is described. Three major protein species are found in the cocoa bean: an albumin of molecular mass 21 kDa (p21) and two insoluble vicilin-like proteins of molecular mass 31 kDa and 47 kDa (p31 and p47, respectively). The p31 and p47 species are known to be derived from a common 67-kDa precursor (p67) by post-translational processing that includes the deletion of a hydrophilic domain located immediately after an N-terminal signal sequence. All three proteins appear to be targeted to membrane-bound storage organelles by N-terminal signal sequences. The p21 and p67 coding sequences were expressed in Hansenula polymorpha using the powerful methanol oxidase (MOX) promoter and in Saccharomyces cerevisiae using the promoter of the pyruvate kinase (PYK) gene. The expression constructs contained the native plant signal sequence, or various yeast signals. The p21 protein was successfully expressed and secreted from both yeasts. The insoluble p67 protein proved more difficult. Species of the correct molecular mass were recovered internally and small amounts of a p47 species were secreted using a yeast leader sequence. However, proteolytic cleavage, probably due to Kex2p-like processing, led to the appearance of other protein species.

Review: methylotrophic yeasts as factories for the production of foreign proteins

In this contribution we discuss the potential of methylotrophic yeasts as hosts for the high level production of valuable foreign proteins. Recent relevant achievements on the intracellular production or secretion of proteins are summarized. Special attention is paid to a specific advantage of the use of methylotrophic yeasts, namely the possibility of accumulating the foreign gene products inside peroxisomes. This approach may be of major advantage when the protein product is toxic for the host cell and, also, to protect these proteins from undesired side-effects such as proteolysis or aggregation.

A disruption-replacement approach for the targeted integration of foreign genes in Hansenula polymorpha

A system has been developed which allows the selection of integrative transformants with replacement of the Hansenula polymorpha methanol oxidase gene (MOX) with expression cassettes carrying heterologous gene under the control of the MOX promoter. The system is convenient for comparison of the expression levels of different constructs integrated into the same locus of the H. polymorpha genome. This system was used to compare the secretion levels of human urinary plasminogen activator, the secretion of which was directed by different signal sequences.

Stable multicopy integration of vector sequences in Hansenula polymorpha

Plasmids without an origin of replication, but bearing the URA3 gene of Saccharomyces cerevisiae as a selective marker for transformation, are shown to replicate autonomously in Hansenula polymorpha, indicating that parts of the S. cerevisiae URA3 gene can fulfil an autonomous replication and stabilization function in H. polymorpha. Such plasmids, replicated in low copy number in monomeric conformation, could be rescued in E. coli, and showed a low mitotic stability under selective and non-selective conditions. Selective propagation of such transformants, however, led to the integration of plasmid sequences into the H. polymorpha genome. The integration event usually occurred in high copy number (approx. 30-50) at a single non-homologous site of the genome. The plasmid sequences were found to be present in tandem array and stable under non-selective conditions. In contrast, the use of homologous URA3 gene under similar conditions led to low-copy-number transformants.

Expression and secretion of antifreeze peptides in the yeast Saccharomyces cerevisiae

The antifreeze peptide AFP6 from the polar fish Pseudopleuronectus americanus has been expressed in and secreted by the yeast Saccharomyces cerevisiae as a biologically active molecule. The gene for the 37 amino acid long peptide has been chemically synthesized using yeast preferred codons. Subsequently, the gene has been cloned into an episomal expression vector as well as in a multicopy integration vector, which is mitotically more stable. The expression is under the control of the inducible GAL7 promoter. The enzyme alpha-galactosidase has been investigated as a carrier protein to facilitate expression and secretion of AFP. In order to reach increased expression levels, tandem repeats of the AFP gene (up to eight copies) have been cloned. In most cases the genes are efficiently expressed and the products secreted. The expression level amounts to approximately 100 mg/l in the culture medium. In a number of genetic constructs the genes are directly linked and expressed as AFP multimers. In other constructs linker regions have been inserted between the AFP gene copies, that allow the peptide to be processed by specific proteinases, either from the endogenous yeast proteolytic system or from a non-yeast source. The latter requires a separate processing step after yeast cultivation to obtain mature AFP. In all these cases proteolytic processing is incomplete, generating a heterogeneous mixture of mature AFP, carrier and chimeric protein, and/or a mixture of AFP-oligomers. The antifreeze activity has been demonstrated for such mixtures as well as for AFP multimers.

Plasmid reorganization during integrative transformation in Hansenula polymorpha

During studies of integrative transformation in Hansenula polymorpha, it was found that transformants with plasmids possessing the LEU2 gene of H. polymorpha were frequently unstable and lost plasmids while growing on non-selective medium. These transformants possessed reorganized plasmids capable of replication in H. polymorpha. Two such plasmids were isolated and characterized. It was shown that they contain additional DNA segments which were not present in the original plasmid used for transformation. Southern hybridization analysis carried out with labeled DNA probes derived from these segments showed that they consisted of H. polymorpha DNA. The hybridization patterns indicated that corresponding sequences were homologous to several chromosomal regions. These chromosomal DNA segments apparently carried H. polymorpha autonomous replicating sequences (HARS), since plasmids bearing them could transform H. polymorpha with high efficiency and were maintained in transformants in an autonomous state. Sequence analysis of one such captured chromosomal fragment revealed several eight- to ten-base AT-rich blocks similar to the presumed HARS sequence defined by Roggenkamp et al. (1986). Analogous reorganization was also observed with respect to integrative plasmids carrying the TRP3 and HIS3 genes of H. polymorpha and the ADE2 gene of Saccharomyces cerevisiae as selectable markers.

Expression of an alpha-galactosidase gene under control of the homologous inulinase promoter in Kluyveromyces marxianus

For expression of the alpha-galactosidase gene from Cyamopsis tetragonoloba in Kluyveromyces marxianus CBS 6556 we have used the promoter of the homologous inulinase-encoding gene (INU1). The INU1 gene has been cloned and sequenced and the coding region shows an identity of 59% with the Saccharomyces cerevisiae invertase gene (SUC2). In the 5'-flanking region of INU1 we found a sequence (TAAATCCGGGG) that perfectly matches to the MIG1 binding consensus sequence (WWWWTSYGGGG) of the S. cerevisiae GAL1, GAL4 and SUC2 genes. Using the K. marxianus INU1 promoter and prepro-signal sequence, we obtained a high alpha-galactosidase production level (153 mg/l) and a secretion efficiency of 99%. Both the production level and the secretion efficiency were significantly reduced when the INU1 pro-peptide was deleted. With either the S. cerevisiae PGK or GAL7 promoter we could obtain only low alpha-galactosidase production levels (2 mg/l).

Expression of the glucose oxidase gene from Aspergillus niger in Hansenula polymorpha and its use as a reporter gene to isolate regulatory mutations

The glucose oxidase gene (god) from Aspergillus niger was expressed in Hansenula polymorpha using the methanol oxidase promoter and transcription termination region and the MF-alpha leader sequence from Saccharomyces cerevisiae to direct secretion. The expression cassette was cloned into the S. cerevisiae vector YEp13 and used to transform H. polymorpha strain A16. In the initial transformants plasmid replication was unstable, but was stabilized by a growth regime consisting of alternating cycles of selective and non-selective growth. The stabilized strain was grown to high cell density by fed-batch fermentation. Upon induction of the MOX promoter, glucose oxidase synthesis was initiated. At the end of the fermentation, the culture density was 76 g dry weight/1 and 108 IU/ml (0.5 g/1 or 0.65% dry weight) glucose oxidase was found in the culture medium; a further 86 IU/ml (0.43 g/1 or 0.56% dry weight) was recovered from the cell lysate. A plate assay was used to monitor glucose oxidase levels in individual colonies. This was then used to isolate mutants which showed abnormal regulation of god expression or which showed an altered pattern of secretion. One mutant, which showed increased production of glucose oxidase, was grown to high cell density by fed-batch fermentation (100.6 g/l) and produced 445 IU/ml(2.25 g/l or 2.2% dry weight) extracellularly and 76 IU/ml (0.38 g/l or 0.4% dry weight) intracellularly. The mutant thus not only increased total production but exported 83% of the total enzyme made compared to 55% in the parent strain.

Comparative study on the production of guar alpha-galactosidase by Saccharomyces cerevisiae SU50B and Hansenula polymorpha 8/2 in continuous cultures

Saccharomyces cerevisiae SU50B and Hansenula polymorpha 8/2, both carrying a multicopy integrated guar alpha-galactosidase, have been cultivated in continuous cultures, using various mixtures of carbon sources and cultivation conditions. Both S. cerevisiae SU50B and H. polymorpha 8/2 are stable and produce high levels of extracellular alpha-galactosidase in continuous cultures for more than 500 h. For these expression systems the strong inducible promoter systems GAL7 and methanol oxidase, respectively, were used. The induction of alpha-galactosidase synthesis by galactose in SU50B is limited by the low galactose uptake. Apart from that, at high dilution rates, the glucose repression is substantial, and a maximal expression level of 28.6 mg of extracellular alpha-galactosidase.g (dry weight) of biomass-1 could be obtained. In H. polymorpha, the induction of alpha-galactosidase synthesis, in addition to methanol oxidase synthesis using formaldehyde, is very effective up to 42 mg of extracellular alpha-galactosidase.g (dry weight) of biomass-1. Productivities in terms of specific production rate enable a good comparison with those of other heterologous expression systems in the literature. The productivities found with S. cerevisiae SU50B and H. polymorpha, 3.25 and 5.5 mg of alpha-galactosidase.g of biomass-1.liter-1.h-1, respectively, rank among the highest reported in the literature. Enzyme production and secretion in H. polymorpha are more complex. A two-peaked optimum is found in enzyme production. No clear explanation of this phenomenon can be given.

Heterologous protein production in yeast

The exploitation of recombinant DNA technology to engineer expression systems for heterologous proteins represented a major task within the field of biotechnology during the last decade. Yeasts attracted the attention of molecular biologists because of properties most favourable for their use as hosts in heterologous protein production. Yeasts follow the general eukaryotic posttranslational modification pattern of expressed polypeptides, exhibit the ability to secrete heterologous proteins and benefit from an established fermentation technology. Aside from the baker's yeast Saccharomyces cerevisiae, an increasing number of alternative non-Saccharomyces yeast species are used as expression systems in basic research and for an industrial application. In the following review a selection from the different yeast systems is described and compared.

Development of a strain of Hansenula polymorpha for the efficient expression of guar alpha-galactosidase

A strain of the methylotrophic yeast Hansenula polymorpha, A16, has been developed that expresses the guar alpha-galactosidase gene to 22.4 mg/g dry cell weight in chemostat cultures at a dilution rate of 0.1 h(-1). This corresponds to more than 13.1% of soluble cell protein, of which 56-62% is secreted into the medium. The alpha-galactosidase gene was flanked by the promoter and terminator sequences of the H.polymorpha mox gene, which can direct expression of the mox gene itself more than 30% of total cell protein under methanol growth. The expression cassette (pUR3510) based on the Saccharomyces cerevisiae plasmid, YEp13, was integrated into the genome. Such transformants were stable in chemostat cultures and exhibited 100% stability for both alpha-galactosidase+ and leu+ phenotypes. Chemostat cultures produced higher levels of alpha-galactosidase with higher specific productivities expressed as mg alpha-galactosidase g(-1) h(-1) compared to batch cultures.

Multiple-copy integration of the alpha-galactosidase gene from Cyamopsis tetragonoloba into the ribosomal DNA of Kluyveromyces lactis

We have developed a vector system for high-copy-number integration into the ribosomal DNA of the yeast Kluyveromyces lactis. This system is analogous to the pMIRY-system developed for Saccharomyces cerevisiae. Plasmids containing a portion of K. lactis rDNA for targeted homologous recombination, as well as the S. cerevisiae TRP1 gene with various promoter deletions, were constructed and, after transformation to K. lactis, analyzed for both copy number and stability. These plasmids were found to be present in about 60 copies per cell and were stably maintained during growth under non-selective conditions. Using this vector system, we expressed a fusion construct containing the S. cerevisiae GAL7 promoter, the SUC2 (invertase) signal sequence and the gene coding for alpha-galactosidase from the plant Cyamopsis tetragonoloba. Although the maximum copy number of these integrated plasmids was only about 15, we nevertheless obtained a high level of alpha-galactosidase production (250 mg/l) with a secretion efficiency of about 95%. When compared to extrachromosomal K. lactis vectors containing the same fusion construct, the multicopy integrants showed a much higher alpha-galactosidase production level and a considerably higher stability under non-selective conditions.

Yeast systems for the commercial production of heterologous proteins

Yeasts are attractive hosts for the production of heterologous proteins. Unlike prokaryotic systems, their eukaryotic subcellular organization enables them to carry out many of the post-translational folding, processing and modification events required to produce "authentic" and bioactive mammalian proteins. In addition, they retain the advantages of a unicellular microorganism, with respect to rapid growth and ease of genetic manipulation. The vast majority of yeast expression work has focused on the well-characterized baker's yeast Saccharomyces cerevisiae. However, with the development of DNA transformation technologies, a growing number of non-Saccharomyces yeasts are becoming available as hosts for recombinant polypeptide production. These include Hansenula polymorpha, Kluyveromyces lactis, Pichia pastoris, Schizosaccharomyces pombe, Schwanniomyces occidentalis and Yarrowia lipolytica. The performance of these alternative yeast expression systems is reviewed here relative to S. cerevisiae, and the advantages and limitations of these systems are discussed.