Biomedical subjects

C A van den Hondel

Publications and source records attributed to C A van den Hondel.

At least 19 recordsLinked to original sources

PubMed2001

C-terminal propeptide of the Caldariomyces fumago chloroperoxidase: an intramolecular chaperone?

The Caldariomyces fumago chloroperoxidase (CPO) is synthesised as a 372-aa precursor which undergoes two proteolytic processing events: removal of a 21-aa N-terminal signal peptide and of a 52-aa C-terminal propeptide. The Aspergillus niger expression system developed for CPO was used to get insight into the function of this C-terminal propeptide. A. niger transformants expressing a CPO protein from which the C-terminal propeptide was deleted failed in producing any extracellular CPO activity, although the CPO polypeptide was synthesised. Expression of the full-length gene in an A. niger strain lacking the KEX2-like protease PclA also resulted in the production of CPO cross-reactive material into the culture medium, but no CPO activity. Based on these results, a function of the C-terminal propeptide in CPO maturation is indicated.

Amino Acid Sequence↗

PubMed2001

Production of tissue plasminogen activator (t-PA) in Aspergillus niger.

A protease-deficient strain of Aspergillus niger has been used as a host for the production of human tissue plasminogen activator (t-PA). In defined medium, up to 0.07 mg t-PA (g biomass)(-1) was produced in batch and fed-batch cultures and production was increased two- to threefold in two-phase batch cultures in which additional glucose was provided as a single pulse at the end of the first batch growth phase. Production was increased [up to 1.9 mg t-PA (g biomass)(-1)] by the addition of soy peptone to the defined medium. The rate of t-PA production in batch cultures supplemented with soy peptone (0.2 to 0.6 mg t-PA L(-1) h(-1)) was comparable to rates observed previously in high-producing mammalian or insect cell cultures. In glucose-limited chemostat culture supplemented with soy peptone, t-PA was produced at a rate of 0.7 mg t-PA L(-1) h(-1). Expression of t-PA in A. niger resulted in increased expression of genes (bipA, pdiA, and cypB) involved in the unfolded protein response (UPR). However, when cypB was overexpressed in a t-PA-producing strain, t-PA production was not increased. The t-PA produced in A. niger was cleaved into two chains of similar molecular weight to two-chain human melanoma t-PA. The two chains appeared to be stable for at least 16 h in culture supernatant of the host strain. However, in general, <1% of the t-PA produced in A. niger was active, and active t-PA disappeared from the culture supernatant during the stationary phase of batch cultures, suggesting that the two-chain t-PA may have been incorrectly processed or that initial proteolytic cleavage occurred within the proteolytic domain of the protein. Total t-PA (detected by enzyme-linked immunoassay) also eventually disappeared from culture supernatants, confirming significant extracellular proteolytic activity, even though the host strain was protease-deficient.

Aspergillus niger↗

PubMed2001

The secretion pathway in filamentous fungi: a biotechnological view.

The high capacity of the secretion machinery of filamentous fungi has been widely exploited for the production of homologous and heterologous proteins; however, our knowledge of the fungal secretion pathway is still at an early stage. Most of the knowledge comes from models developed in yeast and higher eukaryotes, which have served as reference for the studies on fungal species. In this review we compile the data accumulated in recent years on the molecular basis of fungal secretion, emphasizing the relevance of these data for the biotechnological use of the fungal cell and indicating how this information has been applied in attempts to create improved production strains. We also present recent emerging approaches that promise to provide answers to fundamental questions on the molecular genetics of the fungal secretory pathway.

Animals↗

PubMed2001

Identification and characterization of a family of secretion-related small GTPase-encoding genes from the filamentous fungus Aspergillus niger: a putative SEC4 homologue is not essential for growth.

DNA fragments containing genetic information for five secretion-related small GTPases of Aspergillus niger (srgA-E) were isolated and identified as members of different Rab/Ypt subfamilies. This isolation and the search for similar sequences in fungal genomic and EST databases showed that, in contrast to Saccharomyces cerevisiae, filamentous fungi also possess homologues of mammalian Rab2 GTPases. Multiple transcripts with unusually long 5' and 3' untranslated regions were found for all srg genes. Their level of expression was independent of the type of carbon source used for growth. Although the transcripts of srgA and srgB were abundant to the same extent throughout the cultivation, that of the other genes peaked during the early growth phase and then declined. Two genes, srgA and srgB, were characterized further. The protein encoded by srgA exhibited relatively low identity (58%) to its closest S. cerevisiae homologue SEC4, whereas the protein encoded by srgB showed 73% identity with S. cerevisiae YPT1. In contrast to other SEC4 homologues, srgA was unable to complement an S. cerevisiae sec4 mutant, and its disruption was not lethal in A. niger. SrgA mutants displayed a twofold increase in their hyphal diameter, unusual apical branching and strongly reduced protein secretion during growth on glucose.

Amino Acid Sequence↗

PubMed2000

The Effect of organic nitrogen sources on recombinant glucoamylase production by Aspergillus niger in chemostat culture.

Aspergillus niger B1, a recombinant strain carrying 20 extra copies of the native glucoamylase gene, was grown in glucose-limited chemostat cultures supplemented with various organic nitrogen sources (dilution rate 0.12 +/- 0.01 h(-1), pH 5.4). In cultures supplemented with l-alanine, l-methionine, casamino acids, or peptone, specific glucoamylase (GAM) production rapidly decreased to less than 20% of the initial level. Reducing the pH of the culture to 4.0 resulted in stable GAM production for up to 400 h. Morphological mutants (a light brown and a dark brown mutant) appeared in each fermentation and generally displaced B1. Light brown mutants had higher selection coefficients relative to B1 than dark brown mutants and became the dominant strain in all fermentations except those maintained at pH 4.0. Several mutants isolated from these cultures had reduced ability to produce GAM in batch culture, although few had lost copies of the glaA gene. Some mutants had methylated DNA.

Aspergillus niger↗

PubMed2000

Regulation of expression of the Aspergillus niger benzoate para-hydroxylase cytochrome P450 system.

Cytochrome P450 enzyme systems are found throughout nature and are involved in many different, often complex, bioconversions. In the endoplasmic reticulum of the filamentous fungus Aspergillus niger a cytochrome P450 enzyme system is present that is capable of the para-hydroxylation of benzoate. The expression of the two genes encoding the components of this system, the cytochrome P450 gene encoding benzoate para-hydroxylase (bphA) and the gene encoding cytochrome P450 reductase (cprA), is inducible by benzoate. The BPH system was used as a model system to study the mechanisms that result in co-regulation of both components of an eukaryote cytochrome P450 enzyme system. Deletion analysis of the transcription control regions of cprA and bphA resulted in the identification of a region that was involved in benzoate induction of gene expression. The functional competence of the cprA Benzoate Responsive Region thus defined was demonstrated directly by cloning this fragment upstream of a constitutively expressed mini-promoter and analysing expression of the hybrid transcription control region in a lacZ reporter system. Further analysis of cprA gene expression revealed a clear quantitative discrepancy between induction at the protein level (approximately 4-fold) and at the transcription level (> 20-fold). The majority of the transcripts observed after benzoate induction (cprAbeta) were larger then the constitutively expressed cprAalpha transcript. The difference in size between the cprAalpha and cprAbeta transcript is caused by differential promoter use. As the longer cprAbeta transcript carries a small uORF we propose that post-transcriptional regulation of CPR expression underlies the discrepancy in the degree of induction at the protein and transcriptional level. Our results show that regulation of CPR expression is particularly complex, involving regulatory promoter elements, differential promoter use and regulation at the post-transcriptional level.

Amino Acid Sequence↗

PubMed2000

Studies on the production of fungal peroxidases in Aspergillus niger.

To get insight into the limiting factors existing for the efficient production of fungal peroxidase in filamentous fungi, the expression of the Phanerochaete chrysosporium lignin peroxidase H8 (lipA) and manganese peroxidase (MnP) H4 (mnp1) genes in Aspergillus niger has been studied. For this purpose, a protease-deficient A. niger strain and different expression cassettes have been used. Northern blotting experiments indicated high steady-state mRNA levels for the recombinant genes. Manganese peroxidase was secreted into the culture medium as an active protein. The recombinant protein showed specific activity and a spectrum profile similar to those of the native enzyme, was correctly processed at its N terminus, and had a slightly lower mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Recombinant MnP production could be increased up to 100 mg/liter upon hemoglobin supplementation of the culture medium. Lignin peroxidase was also secreted into the extracellular medium, although the protein was not active, presumably due to incorrect processing of the secreted enzyme. Expression of the lipA and mnp1 genes fused to the A. niger glucoamylase gene did not result in improved production yields.

Aspergillus niger↗

PubMed2000

Can hTNF-alpha be successfully produced and secreted in filamentous fungus Aspergillus niger?

A gene-fusion expression strategy was applied for the heterologous expression of hTNF-alpha in A. niger AB1.13. The TNF-alpha gene was fused with the A. niger glucoamylase GII form as a carrier-gene, behind its transcription control and secretion signal. The protein was expressed in the cells in the form of a glucoamylase-fusion protein, but was not present in the culture medium. From the expression of two hTNF-alpha analogues, LK 811 (Cys95/148) and LK 802 (Cys95/148, His107/108) respectively, we concluded that oligomerisation was not the critical point for secretion of hTNF-alpha in A. niger, but more probably improper folding already at the stage of monomer formation, or even incorrect processing of the molecule during the secretion pathway.

Amino Acid Sequence↗

PubMed1999

Filamentous fungi as production organisms for glycoproteins of bio-medical interest.

Filamentous fungi are commonly used in the fermentation industry for large scale production of glycoproteins. Several of these proteins can be produced in concentrations up to 20-40 g per litre. The production of heterologous glycoproteins is at least one or two orders of magnitude lower but research is in progress to increase the production levels. In the past years the structure of protein-linked carbohydrates of a number of fungal proteins has been elucidated, showing the presence of oligo-mannosidic and high-mannose chains, sometimes with typical fungal modifications. A start has been made to engineer the glycosylation pathway in filamentous fungi to obtain strains that show a more mammalian-like type of glycosylation. This mini review aims to cover the current knowledge of glycosylation in filamentous fungi, and to show the possibilities to produce glycoproteins with these organisms with a more mammalian-like type of glycosylation for research purposes or pharmaceutical applications.

Biotechnology↗

PubMed1998

Optimization and stability of glucoamylase production by recombinant strains of Aspergillus niger in chemostat culture.

When grown on a medium containing 5 g maltodextrin L-1, Aspergillus niger transformant N402[pAB6-10]B1, which has an additional 20 copies of the glucoamylase (glaA) gene, produced 320 +/- 8 mg (mean +/- S.E.) glucoamylase (GAM) L-1 in batch culture and 373 +/- 9 mg GAM L-1 in maltodextrin-limited chemostat culture at a dilution rate of 0.13 h-1. These values correspond to specific production rates (qp) of 5.6 and 16.0 mg GAM [g biomass]-1 h-1, respectively. In maltodextrin-limited chemostat cultures grown at dilution rates from 0.06 to 0.14 h-1, GAM was produced by B1 in a growth-correlated manner, demonstrating that a continuous flow culture system operated at a high dilution rate is an efficient way of producing this enzyme. In chemostat cultures grown at high dilution rates, GAM production in chemostat cultures was repressed when the limiting nutrient was fructose or xylose, but derepressed when the limiting nutrient was glucose (qp, 12.0), potassium (6.2), ammonium (4.1), phosphate (2.0), magnesium (1.5) or sulphate (0.9). For chemostat cultures grown at a dilution rate of 0.13 h-1, the addition of 5 g mycopeptone L-1 to a glucose-mineral salts medium resulted in a 64% increase in GAM concentration (from 303 +/- 12 to 496 +/- 10 mg GAM L-1) and a 37% increase in specific production rate (from 12.0 +/- 0.4 to 16.4 +/- 1.6 mg GAM [g biomass]-1 h-1). However, although recombinant protein production was stable for at least 948 h (191 generations) when A. niger B1 was grown in chemostat culture on glucose-mineral salts medium, it was stable for less than 136 h (27 generations) on medium containing mycopeptone. The predominant morphological mutants occurring after prolonged chemostat culture were shown to have selective advantage in the chemostat over the parental strain. Compared to their parental strains, two morphological mutants had similar GAM production levels, while a third had a reduced production level. Growth tests and molecular analysis revealed that the number of glaA gene copies in this latter strain (B1-M) was reduced, which could explain its reduced GAM production. Shake-flask cultures carried out with the various morphological mutants revealed that in batch culture all three strains produced considerably less GAM than their parent strains and even less than N402. We show that physiological changes in these morphological mutants contribute to this decreased level of GAM production.

Aspergillus niger↗

PubMed1998

Cytochrome P450 enzyme systems in fungi.

The involvement of cytochrome P450 enzymes in many complex fungal bioconversion processes has been characterized in recent years. Accordingly, there is now considerable scientific interest in fungal cytochrome P450 enzyme systems. In contrast to S. cerevisiae, where surprisingly few P450 genes have been identified, biochemical data suggest that many fungi possess numerous P450 genes. This review summarizes the current information pertaining to these fungal cytochrome P450 systems, with emphasis on the molecular genetics. The use of molecular techniques to improve cytochrome P450 activities in fungi is also discussed.

Cytochrome P-450 Enzyme System↗

PubMed1998

Analysis of the role of the gene bipA, encoding the major endoplasmic reticulum chaperone protein in the secretion of homologous and heterologous proteins in black Aspergilli.

The function of the endoplasmic-reticulum-localized chaperone binding protein (BiP) in relation to protein secretion in filamentous fungi was studied. It was shown that the overproduction of several homologous and heterologous recombinant proteins by Aspergillus strains induces the expression of bipA, the BiP-encoding gene from Aspergillus niger and Aspergillus awamori. As this result could imply that BiP plays a role in protein overproduction, the effect of modulation of bipA gene expression on protein secretion was studied in several recombinant strains expressing glucoamylase (glaA) fusion genes. For overproduction of BiPA in these strains, extra copies of the bipA gene under the control of an inducible promoter were introduced. To allow analysis of the effect of a decreased bipA expression level on protein secretion, replacement of the wild-type gene for a bipA gene driven by the glaA promoter was attempted. However, this endeavour failed because of the lethality of this replacement. Although the final amount of secreted recombinant protein did not change significantly in strains with increased BiPA levels, increased levels of unprocessed fusion protein were detected in the total protein extracts of these strains.

Animals↗

PubMed1998

Expression and secretion of defined cutinase variants by Aspergillus awamori.

Several cutinase variants derived by molecular modelling and site-directed mutagenesis of a cutinase gene from Fusarium solani pisi are poorly secreted by Saccharomyces cerevisiae. The majority of these variants are successfully produced by the filamentous fungus Aspergillus awamori. However, the L51S and T179Y mutations caused reductions in the levels of extracellular production of two cutinase variants by A. awamori. Metabolic labelling studies were performed to analyze the bottleneck in enzyme production by the fungus in detail. These studies showed that because of the single L51S substitution, rapid extracellular degradation of cutinase occurred. The T179Y substitution did not result in enhanced sensitivity towards extracellular proteases. Presumably, the delay in the extracellular accumulation of this cutinase variant is caused by the enhanced hydrophobicity of the molecule. Overexpression of the A. awamori gene encoding the chaperone BiP in the cutinase-producing A. awamori strains had no significant effect on the secretion efficiency of the cutinases. A cutinase variant with the amino acid changes G28A, A85F, V184I, A185L, and L189F that was known to aggregate in the endoplasmic reticulum of S. cerevisiae, resulting in low extracellular protein levels, was successfully produced by A. awamori. An initial bottleneck in secretion occurred before or during translocation into the endoplasmic reticulum but was rapidly overcome by the fungus.

Aspergillus↗

PubMed1997

The ER chaperone encoding bipA gene of black Aspergilli is induced by heat shock and unfolded proteins.

We describe the cloning and characterisation of the BiP gene homologues of the filamentous fungi Aspergillus niger and Aspergillus awamori. The BiP genes of these black Aspergilli encode an identical protein of 672 amino acids, which has a high homology with the BiP protein from Saccharomyces cerevisiae and contains a putative signal sequence of 38 amino acids. The DNA sequences of the Aspergillus BiP genes diverge in particular in the three intronic sequences and the 5'- and 3'- noncoding regions. Sequences resembling Heat Shock Elements (HSE) and Unfolded Protein Response (UPR) elements, as found in the yeast KAR2 promoter, are present in the 5' non-transcribed regions of both genes. The expression of the A. niger bipA gene is increased by heat shock and tunicamycin treatment.

Aspergillus↗

PubMed1997

An improved colony-PCR method for filamentous fungi for amplification of PCR-fragments of several kilobases.

A method was developed to perform PCR directly on mycelial pellets or colonies treated with NOVOzym 234. The method allows rapid screening of large numbers of transformants of both sporulating and non-sporulating fungi for the presence of (co)transforming plasmid copies or for specific genetic modifications such as gene disruption and site specific integration. PCR fragments of at least 3.2 kb can be obtained. Using this method the identification of specific disruption mutants from Aspergillus niger and Beauveria bassiana was carried out.

Aspergillus niger↗

PubMed1997

Isolation and analysis of functional homologues of the secretion-related SAR1 gene of Saccharomyces cerevisiae from Aspergillus niger and Trichoderma reesei.

The Aspergillus niger and Trichoderma reesei genes encoding the functional homologues of the small GTP-binding protein SAR1p, which is involved in the secretion pathway in Saccharomyces cerevisiae, have been cloned and characterised. The A. niger gene (sarA) contains five introns, whereas the T. reesei gene (sar1) has only four. In both cases the first intron is at the same position as the single S. cerevisiae SAR1 intron. The encoded proteins show 70-80% identity to the SAR1 protein. Complementation of S. cerevisiae sar1 and sec12 mutants by expression vectors carrying the A. niger sarA and T. reesei sar1 cDNA clones confirmed that the cloned genes are functional homologues of the S. cerevisiae SAR1 gene. Three mutant alleles of the A. niger sarA gene (D29G, E109K, D29G/E109K), generated by site-directed mutagenesis, revealed a thermosensitive dominant-negative phenotype in the presence of the wild-type sarA allele. This result contrasts with the situation in S. cerevisiae, where similar mutations have a thermosensitive phenotype. Taken together, our results indicate that the sarA gene is involved in an essential function in A. niger.

Alleles↗

PubMed1997

Efficient production of secreted proteins by Aspergillus: progress, limitations and prospects.

Filamentous fungi are widely used for the production of homologous and heterologous proteins but, compared to homologous proteins, the levels of production of heterologous proteins are usually low. During the last 5 years, the levels of production of heterologous proteins have been drastically improved by fusing the corresponding gene to the 3' end of a homologous gene, encoding a well-secreted protein such as glucoamylase. Nevertheless, little research has been carried out to determine the limitations that hamper heterologous protein production. Recently we have carried out a detailed analysis of the levels of production of several proteins and glucoamylase fusion proteins in defined recombinant Aspergillus awamori strains. In this review we will focus on the use of filamentous fungi for the production of heterologous, especially non-fungal proteins. In particular, the effect of gene-fusion strategies will be reviewed. Furthermore, the remaining limitations in heterologous protein production and suggestions for improvement strategies for overproduction of these protein will be discussed.

Aspergillus↗

PubMed1997

The Aspergillus niger GCN4 homologue, cpcA, is transcriptionally regulated and encodes an unusual leucine zipper.

The general control transcriptional regulator gene cpcA of Aspergillus niger was cloned by complementation of a Saccharomyces cerevisiae delta gcn4 mutant strain. The encoded protein conferred resistance to amino acid analogues when expressed in yeast. Disruption of cpcA in A. niger resulted in a strain which is sensitive towards 3-aminotriazole and fails to respond to amino acid starvation, cpcA encodes a transcript of approximately 2400 nucleotides in length that includes a 5' leader region of 900 nucleotides. The 5' leader region contains two small open reading frames, suggesting translational control of gene expression. Steady-state mRNA levels of cpcA increase by a factor of three upon amino acid starvation. The coding region of cpcA is interrupted by a 57 bp intron and the deduced amino acid sequence displays an approximately 30% overall identity to yeast GCN4p and Neurospora crassa cpc1p. Critical amino acid residues of the transcriptional activation domains of GCN4p are conserved in cpcAp. The basic DNA-binding domain shows up to 70% amino acid sequence identity to other basic zipper (bZIP)-type transcriptional activators. cpcAp binds specifically to a GCN4p recognition element in gel retardation experiments. The C-terminal dimerization domain encodes a leucine zipper with only a single leucine residue.

Amitrole↗