Correct targeting of a vacuolar tobacco chitinase in Saccharomyces cerevisiae--post-translational modifications are dependent on the host strain

Characterization of Catechol-1,2-Dioxygenase (Acdo1p) From Blastobotrys raffinosifermentans and Investigation of Its Role in the Catabolism of Aromatic Compounds

Gallic acid, protocatechuic acid, catechol, and pyrogallol are only a few examples of industrially relevant aromatics. Today much attention is paid to the development of new microbial factories for the environmentally friendly biosynthesis of industrially relevant chemicals with renewable resources or organic pollutants as the starting material. The non–conventional yeast, Blastobotrys raffinosifermentans, possesses attractive properties for industrial bio-production processes such as thermo- and osmotolerance. An additional advantage is its broad substrate spectrum, with tannins at the forefront. The present study is dedicated to the characterization of catechol-1,2-dioxygenase (Acdo1p) and the analysis of its function in B. raffinosifermentans tannic acid catabolism. Acdo1p is a dimeric protein with higher affinity for catechol (K_M = 0.004 ± 0.001 mM, k_cat = 15.6 ± 0.4 s^–1) than to pyrogallol (K_M = 0.1 ± 0.02 mM, k_cat = 10.6 ± 0.4 s^–1). It is an intradiol dioxygenase and its reaction product with catechol as the substrate is cis,cis-muconic acid. B. raffinosifermentans G1212/YIC102-AYNI1-ACDO1-6H, which expresses the ACDO1 gene under the control of the strong nitrate-inducible AYNI1 promoter, achieved a maximum catechol-1,2-dioxygenase activity of 280.6 U/L and 26.9 U/g of dry cell weight in yeast grown in minimal medium with nitrate as the nitrogen source and 1.5% glucose as the carbon source. In the same medium with glucose as the carbon source, catechol-1,2-dioxygenase activity was not detected for the control strain G1212/YIC102 with ACDO1 expression under the regulation of its respective endogenous promoter. Gene expression analysis showed that ACDO1 is induced by gallic acid and protocatechuic acid. In contrast to the wild-type strain, the B. raffinosifermentans strain with a deletion of the ACDO1 gene was unable to grow on medium supplemented with gallic acid or protocatechuic acid as the sole carbon source. In summary, we propose that due to its substrate specificity, its thermal stability, and its ability to undergo long-term storage without significant loss of activity, B. raffinosifermentans catechol-1,2-dioxygenase (Acdo1p) is a promising enzyme candidate for industrial applications.

A new lipase (Alip2) with high potential for enzymatic hydrolysis of the diester diethyladipate to the monoester monoethyladipate

Several putative lipase genes from the genome of the yeast Blastobotrys (Arxula) raffinosifermentans (adeninivorans) LS3 were overexpressed in the yeast itself and screened for the desymmetrization of the dicarboxylic acid diester diethyl adipate (DEA) into the monoester monoethyl adipate (MEA). MEA can serve as a monomeric spacer group for functional polymers used in medical chemistry and dental applications. The selected lipase Alip2-c6hp was intracellularly located. After overexpression of the corresponding gene, it was purified and biochemically characterized using p-nitrophenyl butyrate as the substrate for standard activity tests. In fed-batch cultivation with constructed yeast strain B. raffinosifermentans G1212/YRC102-Alip2-c6h for large scale production of the Alip2-c6hp biocatalyst enzyme activities up to 674 U L^-1 were reached. Several tested diesters were hydrolyzed selectively to monoesters. Under optimized conditions, the purified enzyme Alip2p-c6h converted 96 % of the substrate DEA to MEA within 30 min incubation, whereby only 1.6 % of the unwanted side-product adipic acid (AA) was formed. At room temperature the dicarboxylic acid esters diethyl malonate (DEM), diethyl succinate (DES), dimethyl adipate (DMA) and dimethyl suberate (DMSub) were completely hydrolyzed to their corresponding monoesters. A high yield of 87 % and 25 % could also be achieved with the dioldiesters 1,4-diacetoxybutane (DAB) and diacetoxyhexane (DAH). In conclusion the potential of the lipase Alip2-c6hp expressed in B. raffinosifermentans is very promising for selective hydrolysis of DEA to MEA as well as for the production of other monoesters.

Agdc1p - a Gallic Acid Decarboxylase Involved in the Degradation of Tannic Acid in the Yeast Blastobotrys (Arxula) adeninivorans

Tannins and hydroxylated aromatic acids, such as gallic acid (3,4,5-trihydroxybenzoic acid), are plant secondary metabolites which protect plants against herbivores and plant-associated microorganisms. Some microbes, such as the yeast Arxula adeninivorans are resistant to these antimicrobial substances and are able to use tannins and gallic acid as carbon sources. In this study, the Arxula gallic acid decarboxylase (Agdc1p) which degrades gallic acid to pyrogallol was characterized and its function in tannin catabolism analyzed. The enzyme has a higher affinity for gallic acid (Km -0.7 ± 0.2 mM, kcat -42.0 ± 8.2 s-1) than to protocatechuic acid (3,4-dihydroxybenzoic acid) (Km -3.2 ± 0.2 mM, kcat -44.0 ± 3.2 s-1). Other hydroxylated aromatic acids, such as 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 2,3-dihydroxybenzoic acid, 2,4-dihydroxybenzoic acid and 2,5-dihydroxybenzoic acid are not gallic acid decarboxylase substrates. A. adeninivorans G1212/YRC102-AYNI1-AGDC1, which expresses the AGDC1 gene under the control of the strong nitrate inducible AYNI1 promoter achieved a maximum gallic acid decarboxylase activity of 1064.4 U/l and 97.5 U/g of dry cell weight in yeast grown in minimal medium with nitrate as nitrogen source and glucose as carbon source. In the same medium, gallic acid decarboxylase activity was not detected for the control strain G1212/YRC102 with AGDC1 expression under the control of the endogenous promoter. Gene expression analysis showed that AGDC1 is induced by gallic acid and protocatechuic acid. In contrast to G1212/YRC102-AYNI1-AGDC1 and G1212/YRC102, A. adeninivorans G1234 [Δagdc1] is not able to grow on medium with gallic acid as carbon source but can grow in presence of protocatechuic acid. This confirms that Agdc1p plays an essential role in the tannic acid catabolism and could be useful in the production of catechol and cis,cis-muconic acid. However, the protocatechuic acid catabolism via Agdc1p to catechol seems to be not the only degradation pathway.

Proteomic Characterization of Differential Abundant Proteins Accumulated between Lower and Upper Epidermises of Fleshy Scales in Onion (Allium cepa L.) Bulbs

The onion (Allium cepa L.) is widely planted worldwide as a valuable vegetable crop. The scales of an onion bulb are a modified type of leaf. The one-layer-cell epidermis of onion scales is commonly used as a model experimental material in botany and molecular biology. The lower epidermis (LE) and upper epidermis (UE) of onion scales display obvious differences in microscopic structure, cell differentiation and pigment synthesis; however, associated proteomic differences are unclear. LE and UE can be easily sampled as single-layer-cell tissues for comparative proteomic analysis. In this study, a proteomic approach based on 2-DE and mass spectrometry (MS) was applied to compare LE and UE of fleshy scales from yellow and red onions. We identified 47 differential abundant protein spots (representing 31 unique proteins) between LE and UE in red and yellow onions. These proteins are mainly involved in pigment synthesis, stress response, and cell division. Particularly, the differentially accumulated chalcone-flavanone isomerase and flavone O-methyltransferase 1-like in LE may result in the differences in the onion scale color between red and yellow onions. Moreover, stress-related proteins abundantly accumulated in both LE and UE. In addition, the differential accumulation of UDP-arabinopyranose mutase 1-like protein and β-1,3-glucanase in the LE may be related to the different cell sizes between LE and UE of the two types of onion. The data derived from this study provides new insight into the differences in differentiation and developmental processes between onion epidermises. This study may also make a contribution to onion breeding, such as improving resistances and changing colors.

Aadh2p: an Arxula adeninivorans alcohol dehydrogenase involved in the first step of the 1-butanol degradation pathway

Background

The non-conventional yeast Arxula adeninivorans uses 1-butanol as a carbon source and has recently attracted attention as a promising organism for 1-butanol production. Alcohol dehydrogenases (adhp) are important catalysts in 1-butanol metabolism, but only Aadh1p from Arxula has been characterized. This enzyme is involved in ethanol synthesis but has a low impact on 1-butanol degradation.

Results

In this study, we identified and characterized a second adhp from A. adeninivorans (Aadh2p). Compared to Saccharomyces cerevisiae ADHs’ (ScAdh) protein sequences it originates from the same ancestral node as ScAdh6p, 7p and 4p. It is also localized in the cytoplasm and uses NAD(H) as cofactor. The enzyme has its highest activity with medium chain-length alcohols and maximum activity with 1-butanol with the catalytic efficiency of the purified enzyme being 42 and 43,000 times higher than with ethanol and acetaldehyde, respectively. Arxula adeninivorans strain G1212/YRC102-AADH2, which expresses the AADH2 gene under the control of the strong constitutive TEF1 promoter was constructed. It achieved an ADH activity of up to 8000 U/L and 500 U/g dry cell weight (dcw) which is in contrast to the control strain G1212/YRC102 which had an ADH activity of up to 1400 U/L and 200 U/g dcw. Gene expression analysis showed that AADH2 derepression or induction using non-fermentable carbon-sources such as ethanol, pyruvate, glycerol or 1-butanol did occur. Compared to G1212/YRC102 AADH2 knock-out strain had a slower growth rate and lower 1-butanol consumption if 1-butanol was used as sole carbon source and AADH2-transformants did not grow at all in the same conditions. However, addition of the branched-chain amino acids leucine, isoleucine and valine allowed the transformants to use 1-butanol as carbon source. The addition of these amino acids to the control strain and Δaadh2 mutant cultures had the effect of accelerating 1-butanol consumption.

Conclusions

Our results confirm that Aadh2p plays a major role in A. adeninivorans 1-butanol metabolism. It is upregulated by up to 60-fold when the cells grow on 1-butanol, whereas only minor changes were found in the relative expression level for Aadh1p. Thus the constitutive overexpression of the AADH2 gene could be useful in the production of 1-butanol by A. adeninivorans, although it is likely that other ADHs will have to be knocked-out to prevent 1-butanol oxidation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0573-9) contains supplementary material, which is available to authorized users.

Characterization of an Arxula adeninivorans alcohol dehydrogenase involved in the metabolism of ethanol and 1-butanol

In this study, alcohol dehydrogenase 1 from Arxula adeninivorans (Aadh1p) was identified and characterized. Aadh1p showed activity with short and medium chain length primary alcohols in the forward reaction and their aldehydes in the reverse reaction. Aadh1p has 64% identity with Saccharomyces cerevisiae Adh1p, is localized in the cytoplasm and uses NAD(+) as cofactor. Gene expression analysis showed a low level increase in AADH1 gene expression with ethanol, pyruvate or xylose as the carbon source. Deletion of the AADH1 gene affects growth of the cells with 1-butanol, ethanol and glucose as the carbon source, and a strain which overexpressed the AADH1 gene metabolized 1-butanol more rapidly. An ADH activity assay indicated that Aadh1p is a major enzyme for the synthesis of ethanol and the degradation of 1-butanol in A. adeninivorans.

Arxula adeninivorans recombinant adenine deaminase and its application in the production of food with low purine content

AIMS

Construction of a transgenic Arxula adeninivorans strain that produces a high concentration of adenine deaminase and investigation into the application of the enzyme in the production of food with low purine content.

METHODS AND RESULTS

The A. adeninivorans AADA gene, encoding adenine deaminase, was expressed in this yeast under the control of the strong inducible nitrite reductase promoter using the Xplor(®) 2 transformation/expression platform. The recombinant enzyme was biochemically characterized and was found to have a pH range of 5.5-7.5 and temperature range of 34-46 °C with medium thermostability. A beef broth was treated with the purified enzyme resulting in the concentration of adenine decreasing from 70.4 to 0.4 mg l(-1).

CONCLUSIONS

It was shown that the production of adenine deaminase by A. adeninivorans can be increased and that the recombinant adenine deaminase can be used to lower the adenine content in the food.

SIGNIFICANCE AND IMPACT OF THE STUDY

Adenine deaminase is one component of an enzymatic system that can reduce the production of uric acid from food constituents. This study gives details on the expression, characterization and application of the enzyme and thus provides evidence that supports the further development of the system.

Assessment of Schwanniomyces occidentalis as a host for protein production using the wide-range Xplor2 expression platform

The wide-range transformation/expression platform, Xplor2, was employed for the assessment of Schwanniomyces occidentalis as a potential producer of the recombinant proteins human IFNα2a (IFNα2a) and S. occidentalis fructofuranosidase (SFfase), and its efficiency was compared to that of Arxula adeninivorans. ADE2 and URA3 genes from both yeast species were isolated, characterized and used as selection markers in combination with the IFNα2a and SFfase expression modules, which used the strong constitutive A. adeninivorans-derived TEF1 promoter. Yeast rDNA integrative expression cassettes and yeast integrative expression cassettes equipped with a selection marker and expression modules were transformed into auxotrophic S. occidentalis and A. adeninivorans strains and a quantitative comparison of the expression efficiency was made. Whilst IFNα2a was mainly accumulated extracellularly (>95 %) in A. adeninivorans, extracellular SFfase (>90 %) was detected in both yeast species. The DNA composition of the selection marker modules and expression modules, especially their open reading frame codon usage, affects auxotrophy recovery as well as protein expression. Auxotrophy recovery was only achieved with selection marker modules of the homologous gene donor yeast. The concentration of recombinant IFNα2a was fivefold higher in A. adeninivorans (1 mg L(-1)), whereas S. occidentalis accumulated 1.5- to 2-fold more SFfase (0.5 Units ml(-1)). These results demonstrate the extension of the use of the wide-range expression platform Xplor2 to another yeast species of biotechnological interest.

Role of the AFRD1-encoded fumarate reductase in hypoxia and osmotolerance in Arxula adeninivorans

Fumarate reductase is an enzyme involved in maintaining redox balance through regeneration of reduced cofactors during oxygen deficiency conditions. This work reports the identification and characterization of the gene and its promoter and terminator elements that encodes cytosolic fumarate reductase enzyme in the nonconventional yeast, Arxula adeninivorans. The gene harbours an ORF of 1446 bp, encoding a 482-amino acid protein. The deduced amino acid sequence is similar to those of fumarate reductases from other yeast and fungi, such as the two fumarate reductases of Saccharomyces cerevisiae, Frd1p (44%) and Osm1p (41%). This enzyme is located in the cytosol and has a pH optimum of ca. 7.5 and a Michaelis constant (K(M)) of 2.9 mM with fumarate as the substrate. Expression of AFRD1 is regulated by the cultivation conditions. A shift from NaCl-free to NaCl-supplemented media and aerobic to hypoxic growth conditions leads to reduced AFRD1 transcription levels, but not to alteration in the concentration of Afrd1p. The functional analyses of Afrd1p were performed in A. adeninivorans and S. cerevisiae disruption mutants. The A. adeninivorans fumarate reductase is capable of functional complementation of the missing S. cerevisiae genes during anoxia; however, it is not involved in yeast growth under osmotic stress.

Production of a thermostable alcohol dehydrogenase from Rhodococcus ruber in three different yeast species using the Xplor®2 transformation/expression platform

The Xplor®2 transformation/expression platform was employed for comparative assessment of three different yeast species as hosts for synthesis of a thermostable nicotinamide adenine dinucleotide (NAD+)-dependent medium-chain alcohol dehydrogenase from Rhodococcus ruber strain 219. Using yeast ribosomal DNA (rDNA) integrative expression cassettes (YRCs) and yeast integrative expression cassettes (YICs) equipped with a selection-marker module and one, two or four expression modules for transformation of auxotrophic Arxula adeninivorans, Hansenula polymorpha, and Saccharomyces cerevisiae strains, quantitative comparison of the yield of recombinant alcohol dehydrogenase RR-ADH6Hp in all three species was carried out. In all cases, the RR-ADH6H gene was expressed under the control of the strong constitutive A. adeninivorans-derived TEF1 promoter, which functions in all yeast species analyzed. Recombinant RR-ADH6Hp accumulated intracellularly in all strains tested. The best yields of active enzyme were obtained from A. adeninivorans, with S. cerevisiae producing intermediate amounts. Although H. polymorpha was the least efficient producer overall, the product obtained was most similar to the enzyme synthesized by R. ruber 219 with respect to its thermostability.

GiFRD encodes a protein involved in anaerobic growth in the arbuscular mycorrhizal fungus Glomus intraradices

Fumarate reductase is a protein involved in the maintenance of redox balance during oxygen deficiency. This enzyme irreversibly catalyzes the reduction of fumarate to succinate and requires flavin cofactors as electron donors. Two examples are the soluble mitochondrial and the cytosolic fumarate reductases of Saccharomyces cerevisiae encoded by the OSM1 and FRDS1 genes, respectively. This work reports the identification and characterization of the gene encoding cytosolic fumarate reductase enzyme in the arbuscular mycorrhizal fungus, Glomus intraradices and the establishment of its physiological role. Using a yeast expression system, we demonstrate that G. intraradices GiFRD encodes a protein that has fumarate reductase activity which can functionally substitute for the S. cerevisiae fumarate reductases. Additionally, we showed that GiFRD transformants are not affected by presence of salt in medium, indicating that the presence of this gene has no effect on yeast behavior under osmotic stress. The fact that GiFRD expression and enzymatic activity was present only in asymbiotic stage confirmed existence of at least one anaerobic metabolic pathway in this phase of fungus life cycle. This suggests that the AMF behave as facultative anaerobes in the asymbiotic stage.

Atan1p-an extracellular tannase from the dimorphic yeast Arxula adeninivorans: molecular cloning of the ATAN1 gene and characterization of the recombinant enzyme

The tannase-encoding Arxula adeninivorans gene ATAN1 was isolated from genomic DNA by PCR, using as primers oligonucleotide sequences derived from peptides obtained after tryptic digestion of the purified tannase protein. The gene harbours an ORF of 1764 bp, encoding a 587-amino acid protein, preceded by an N-terminal secretion sequence comprising 28 residues. The deduced amino acid sequence was similar to those of tannases from Aspergillus oryzae (50% identity), A. niger (48%) and putative tannases from A. fumigatus (52%) and A. nidulans (50%). The sequence contains the consensus pentapeptide motif (-Gly-X-Ser-X-Gly-) which forms part of the catalytic centre of serine hydrolases. Expression of ATAN1 is regulated by the carbon source. Supplementation with tannic acid or gallic acid leads to induction of ATAN1, and accumulation of the native tannase enzyme in the medium. The enzymes recovered from both wild-type and recombinant strains were essentially indistinguishable. A molecular mass of approximately 320 kDa was determined, indicating that the native, glycosylated tannase consists of four identical subunits. The enzyme has a temperature optimum at 35-40 degrees C and a pH optimum at approximately 6.0. The enzyme is able to remove gallic acid from both condensed and hydrolysable tannins. The wild-type strain LS3 secreted amounts of tannase equivalent to 100 U/l under inducing conditions, while the transformant strain, which overexpresses the ATAN1 gene from the strong, constitutively active A. adeninivorans TEF1 promoter, produced levels of up to 400 U/l when grown in glucose medium in shake flasks.

Characterization of the Arxula adeninivorans AHOG1 gene and the encoded mitogen-activated protein kinase

Arxula adeninivorans is an osmo-resistant yeast species that can tolerate high levels of osmolytes like NaCl, PEG400 and ethylene glycol. As in other yeast species, this tolerance is elicited by components of the high osmolarity glycerol (HOG) response pathway. In the present study, we isolated and characterized as a key component of this pathway the A. adeninivorans AHOG1 gene encoding the mitogen-activated protein (MAP) kinase Ahog1p, an enzyme of 45.9 kDa. The gene includes a coding sequence of 1,203 bp disrupted by a 57-bp intron. The identity of the gene was confirmed by complementation of a hog1 mutation in a Saccharomyces cerevisiae mutant strain and the high degree of homology of the derived amino acid sequence with that of MAP kinases from other yeasts and fungi. Under stress-free conditions, the inactive Ahoglp is present in low levels. When exposed to osmotic stress, Ahoglp is rendered active by phosphorylation. In addition, AHOG1 expression is increased. Assessment of the AHOG1 promoter activity with a lacZ reporter gene confirmed its inducibility by osmolytes, a characteristic not observed in homologous HOG1 genes of other yeast species. This specific property could account for the fast adaptation and high osmo-resistance encountered in this species.

An extracellular lipase from the dimorphic yeastArxula adeninivorans: molecular cloning of theALIP1 gene and characterization of the purified recombinant enzyme

The lipase-encoding Arxula adeninivorans ALIP1 gene was isolated using fragments of lipase isolates obtained by trypsin digestion for the definition of oligonucleotide primers in a PCR screening approach. The gene harbours an ORF of 1347 bp encoding a 420 amino acid protein of some 50 kDa preceded by an N-terminal 28 prepro-secretion sequence. The deduced amino acid sequence was found to be similar to the lipases from Candida albicans and C. parapsilosis (34-38% identity) and more distantly related to other lipases. The sequence contains the consensus pentapeptide motif (-Gly-X-Ser-X-Gly-) that forms a part of the interfacial lipid recognition site in lipases. The expression of the gene is regulated by carbon source. In media supplemented with Tween 20, induction of the ALIP1 gene and accumulation of the encoded lipase in the medium is observed, thus demonstrating gene regulation by lipophilic compounds. The enzyme characteristics are analysed from isolates of native strains as well as from those of recombinant strains expressing the ALIP1 gene under control of the strong A. adeninivorans-derived TEF1 promoter. For both proteins a molecular mass of 100 kDa was determined, indicating a dimeric structure, a pH optimum at pH 7.5 and a temperature optimum at 30 degrees C. The enzyme hydrolyses all ester bonds in all triglyceride substrates tested. Middle-sized chain fatty acids are more efficiently hydrolysed than short- and long-chain fatty acids, with the highest activity on C8/C10 fatty acid esters pNP-caprylate, pNP-caprate and tricaprylin.

Post-translational modifications of the AFET3 gene product: a component of the iron transport system in budding cells and mycelia of the yeast Arxula adeninivorans

The yeast Arxula adeninivorans is characterized by a temperature-dependent dimorphism. A. adeninivorans grows as budding cells at temperatures up to 42 degrees C, but forms mycelia at higher temperatures. A strong correlation exists between morphological status and iron uptake, achieved by two transport systems that differ in iron affinity. In the presence of high Fe(II) concentrations (>2 microm), budding cells accumulate iron concentrations up to seven-fold higher than those observed in mycelia, while at low Fe(II) concentrations (<2 microm), both cell types accumulate similar amounts of iron. The copper-dependent Fe(II) oxidase Afet3p, composed of 615 amino acids, is a component of the high-affinity iron transport system. This protein shares a high degree of homology with other yeast iron transport proteins, namely Fet3p of Saccharomyces cerevisiae, Cafet3p of Candida albicans and Pfet3p of Pichia pastoris. Expression of the AFET3 gene is found to be strongly dependent on iron concentration but independent of the morphological stage; however, cell morphology was found to influence post-translational modifications of the gene product. O-glycosylation was observed in budding cells only, whereas N-glycosylation occurred in both cell types. The N-glycosylated 103 kDa glycoprotein matures into the 108.5 kDa form, further characterized by serine phosphorylation. Both N-glycosylation and phosphorylation occur at low iron concentrations (< or =5 microm). The mature Afet3p of 108.5 kDa is uniformly distributed within the plasma membrane in cells of both morphological stages.

The green fluorescent protein targets secretory proteins to the yeast vacuole

The green fluorescent protein (GFP) was used as a marker to study the intracellular transport of vacuolar and secretory proteins in yeast. Therefore, the following gene constructs were expressed in Saccharomyces cerevisiae under control of the GAL1 promoter: GFP N-terminally fused to the yeast secretory invertase (INV-GFP), the plant vacuolar chitinase (CHN-GFP) and its secretory derivative (CHNDeltaVTP-GFP), which did not contain the vacuolar targeting peptide (VTP), both chitinase forms (CHN and CHNDeltaVTP), GFP without any targeting information and two secretory GFP variants with and without the VTP of chitinase (N-GFP-V and N-GFP). Whereas chitinase without VTP is accumulated in the culture medium the other gene products are retained inside the cell up to 48 h of induction. Independently of a known VTP they are transported to the vacuole, so far as they contain a signal peptide for entering the endoplasmic reticulum. This was demonstrated by confocal laser scanning microscopy, immunocytochemical analysis and subcellular fractionation experiments as well. The transport of the GFP fusion proteins is temporary delayed by a transient accumulation in electron-dense structures very likely derived from the ER, because they also contain the ER chaperone Kar2p/Bip. Our results demonstrate that GFP directs secretory proteins without VTP to the yeast vacuole, possibly by the recognition of an unknown vacuolar signal and demonstrates, therefore, a first limitation for the application of GFP as a marker for the secretory pathway in yeast.

The peptide HDEF as a new retention signal is necessary and sufficient to direct proteins to the endoplasmic reticulum

The key feature of tomato RNase LX localised solely outside the vacuole is the C-terminal peptide HDEF which is very similar to known endoplasmic reticulum (ER) retention signals. For functional testing of the ER-targeting ability of HDEF, different constructs including the complete RNase LX, two truncated forms without HDEF and the truncated chitinase FB7-1deltaVTP C-terminally flanked by HDEF, were expressed in Saccharomyces cerevisiae. The majority of RNase and chitinase, both containing HDEF, accumulates within the ER. However, the truncated constructs without the peptide are released into the medium. We provide compelling evidence that peptide HDEF at the C-terminus of secretory plant proteins is a new ER retention signal in yeast and most likely in plants.