The green fluorescent protein targets secretory proteins to the yeast vacuole

Optogenetic spatial patterning of cooperation in yeast populations

Microbial communities are shaped by complex metabolic interactions such as cooperation and competition for resources. Methods to control such interactions could lead to major advances in our ability to better engineer microbial consortia for synthetic biology applications. Here, we use optogenetics to control SUC2 invertase production in yeast, thereby shaping spatial assortment of cooperator and cheater cells. Yeast cells behave as cooperators (i.e., transform sucrose into hexose, a public good) upon blue light illumination or cheaters (i.e., consume hexose produced by cooperators to grow) in the dark. We show that cooperators benefit best from the hexoses they produce when their domain size is constrained between two cut-off length-scales. From an engineering point of view, the system behaves as a bandpass filter. The lower limit is the trace of cheaters' competition for hexoses, while the upper limit is defined by cooperators' competition for sucrose. Cooperation mostly occurs at the frontiers with cheater cells, which not only compete for hexoses but also cooperate passively by letting sucrose reach cooperators. We anticipate that this optogenetic method could be applied to shape metabolic interactions in a variety of microbial ecosystems.

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

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

A Protein-Engineered, Enhanced Yeast Display Platform for Rapid Evolution of Challenging Targets

Here, we enhanced the popular yeast display method by multiple rounds of DNA and protein engineering. We introduced surface exposure-tailored reporters, eUnaG2 and DnbALFA, creating a new platform of C and N terminal fusion vectors. The optimization of eUnaG2 resulted in five times brighter fluorescence and 10 °C increased thermostability than UnaG. The optimized DnbALFA has 10-fold the level of expression of the starting protein. Following this, different plasmids were developed to create a complex platform allowing a broad range of protein expression organizations and labeling strategies. Our platform showed up to five times better separation between nonexpressing and expressing cells compared with traditional pCTcon2 and c-myc labeling, allowing for fewer rounds of selection and achieving higher binding affinities. Testing 16 different proteins, the enhanced system showed consistently stronger expression signals over c-myc labeling. In addition to gains in simplicity, speed, and cost-effectiveness, new applications were introduced to monitor protein surface exposure and protein retention in the secretion pathway that enabled successful protein engineering of hard-to-express proteins. As an example, we show how we optimized the WD40 domain of the ATG16L1 protein for yeast surface and soluble bacterial expression, starting from a nonexpressing protein. As a second example, we show how using the here-presented enhanced yeast display method we rapidly selected high-affinity binders toward two protein targets, demonstrating the simplicity of generating new protein-protein interactions. While the methodological changes are incremental, it results in a qualitative enhancement in the applicability of yeast display for many applications.

Real-time measurement of the intracellular pH of yeast cells during glucose metabolism using ratiometric fluorescent nanosensors

Intracellular pH is a key parameter that influences many biochemical and metabolic pathways that can also be used as an indirect marker to monitor metabolic and intracellular processes. Herein, we utilise ratiometric fluorescent pH-sensitive nanosensors with an extended dynamic pH range to measure the intracellular pH of yeast (Saccharomyces cerevisiae) during glucose metabolism in real-time. Ratiometric fluorescent pH-sensitive nanosensors consisting of a polyacrylamide nanoparticle matrix covalently linked to two pH-sensitive fluorophores, Oregon green (OG) and 5(6)carboxyfluorescein (FAM), and a reference pH-insensitive fluorophore, 5(6)carboxytetramethylrhodamine (TAMRA), were synthesised. Nanosensors were functionalised with acrylamidopropyltrimethyl ammonium hydrochloride (ACTA) to confer a positive charge to the nanoparticle surfaces that facilitated nanosensor delivery to yeast cells, negating the need to use stress inducing techniques. The results showed that under glucose-starved conditions the intracellular pH of yeast population (n ≈ 200) was 4.67 ± 0.15. Upon addition of d-(+)-glucose (10 mM), this pH value decreased to pH 3.86 ± 0.13 over a period of 10 minutes followed by a gradual rise to a maximal pH of 5.21 ± 0.26, 25 minutes after glucose addition. 45 minutes after the addition of glucose, the intracellular pH of yeast cells returned to that of the glucose starved conditions. This study advances our understanding of the interplay between glucose metabolism and pH regulation in yeast cells, and indicates that the intracellular pH homestasis in yeast is highly regulated and demonstrates the utility of nanosensors for real-time intracellular pH measurements.

Secretion of a foreign protein from budding yeasts is enhanced by cotranslational translocation and by suppression of vacuolar targeting

BACKGROUND

Budding yeasts are often used to secrete foreign proteins, but the efficiency is variable. To identify roadblocks in the yeast secretory pathway, we used a monomeric superfolder GFP (msGFP) as a visual tracer in Saccharomyces cerevisiae and Pichia pastoris.

RESULTS

One roadblock for msGFP secretion is translocation into the ER. Foreign proteins are typically fused to the bipartite α-factor secretion signal, which consists of the signal sequence followed by the pro region. The α-factor signal sequence directs posttranslational translocation. For msGFP, posttranslational translocation is inefficient with the α-factor signal sequence alone but is stimulated by the pro region. This requirement for the pro region can be bypassed by using the Ost1 signal sequence, which has been shown to direct cotranslational translocation. A hybrid secretion signal consisting of the Ost1 signal sequence followed by the α-factor pro region drives efficient translocation followed by rapid ER export. A second roadblock for msGFP secretion in S. cerevisiae occurs during exit from the Golgi, when some of the msGFP molecules are diverted to the vacuole. Deletion of the sorting receptor Vps10 prevents vacuolar targeting of msGFP at the expense of missorting vacuolar hydrolases such as carboxypeptidase Y (CPY) to the culture medium. However, a truncation of Vps10 blocks vacuolar targeting of msGFP while permitting CPY to be sorted normally.

CONCLUSIONS

With budding yeasts, if the secretion or processing of a foreign protein is poor, we recommend two options. First, use the Ost1 signal sequence to achieve efficient entry into the secretory pathway while avoiding the processing issues associated with the α-factor pro region. Second, truncate Vps10 to suppress diversion to the vacuole. These insights obtained with msGFP highlight the value of applying cell biological methods to study yeast secretion.

Endogenous signal peptides efficiently mediate the secretion of recombinant proteins in Pichia pastoris

By predicting the potential signal peptides from proteins that are naturally secreted by Pichia pastoris, we identified three possible endogenous signal peptides: Scw, Dse and Exg. We compared their capability to mediate the secretion of enhanced green fluorescent protein (EGFP) and Candida antarctica lipase B (CALB) with that of the Saccharomyces cerevisiae α-factor prepro-signal. EGFP entered the secretory pathway of P. pastoris and was efficiently secreted into the culture medium by all three endogenous peptides. Further, these three putative endogenous signal peptides were also effective in secreting CALB. These endogenous signal peptides thus have the potential to mediate the efficient secretion of heterologous proteins in P. pastoris.

Identification of vacuole defects in fungi

Fungal vacuoles are involved in a diverse range of cellular functions, participating in cellular homeostasis, degradation of intracellular components, and storage of ions and molecules. In recent years there has been a significant increase in the number of studies linking these organelles with the regulation of growth and control of cellular morphology, particularly in those fungal species able to undergo yeast-hypha morphogenetic transitions. This has contributed to the refinement of previously published protocols and the development of new techniques, particularly in the area of live-cell imaging of membrane trafficking events and vacuolar dynamics. The current review outlines recent advances in the imaging of fungal vacuoles and assays for characterization of trafficking pathways, and other physiological activities of this important cell organelle.

Testosterone and imipramine have antidepressant effects in socially isolated male but not female rats

RATIONALE

Affective disorders are twice as likely to occur in women as they are in men suggesting a critical role for gonadal hormones in their etiology. In particular, testosterone has been shown to have protective effects in men.

OBJECTIVE

To investigate antidepressant effects and interactions between testosterone and imipramine in socially isolated male and female rats.

METHODS

A chronic social isolation model was used to induce an anxiety and depressive-like state in adult gonadectomized (Gnx) male and ovariectomized (Ovx) female rats receiving chronic testosterone and imipramine treatments. Their anxiety and depression-like behaviors were examined using the light-dark box, elevated plus maze, open field, sucrose preference and novelty induced hypophagia tests.

RESULTS

In socially isolated rats, the anxiolytic and antidepressant effects of testosterone and imipramine were limited to male rats. Additionally, testosterone enhanced the neurogenic effect of imipramine on hippocampal cell proliferation in male rats. Although female rats exhibited signs of anxiety and depressive-like behaviors following social isolation, testosterone and/or imipramine administration had no anxiolytic or antidepressant effects in Ovx females.

CONCLUSIONS

Testosterone and imipramine had anxiolytic and antidepressant effects in socially isolated male, but not female rats. Testosterone enhanced the effect of imipramine on cell proliferation in the hippocampus of male rats.

Xplor 2--an optimized transformation/expression system for recombinant protein production in the yeast Arxula adeninivorans

Combining ease of genetic manipulation and fermentation with the ability to secrete and to glycosylate proteins in the basic eukaryotic manner, Arxula adeninivorans provides an attractive expression platform. Based on a redesign of the basic vector, a new Arxula vector system, Xplor 2, for heterologous gene expression was established, which allows (1) the construction of expression plasmids for supertransformation of A. adeninivorans strains secreting target proteins of biotechnological interest and (2) the integration of small vector cassettes consisting of yeast DNA sequences only. For this purpose, a set of modules including the ATRP1m selection-marker module, expression modules for constitutive expression of the genes phyK (Klebsiella-derived phytase) and IFNalpha2a (human interferon alpha), the HARS (Hansenula polymorpha autonomous replication sequence) for autonomous replication and the chaperone module AHSB4 promoter -HpCNE1 gene (calnexin) -PHO5 terminator to improve secretion efficiency were constructed and integrated in various combinations in the basic vector Xplor 2. After removal of the complete Escherichia coli-based plasmid parts (resistance marker, ColE1 ori and f1(-) origin), the remaining yeast-based linear vector fragment with or without rDNA targeting sequences were transformed as yeast rDNA integrative expression cassettes and yeast integrative expression cassettes (YICs), respectively, and the resulting strains were tested for their capacity to secrete PhyK or IFNalpha2a. Maximal expression levels were consistently obtained using YICs for transformation irrespective of whether or not they carry HARS and/or calnexin modules. It is recommended that at least 50 such transformants be analyzed to ensure selection of the best transformants.

Application of a wide-range yeast vector (CoMed) system to recombinant protein production in dimorphic Arxula adeninivorans, methylotrophic Hansenula polymorpha and other yeasts

BACKGROUND

Yeasts provide attractive expression platforms in combining ease of genetic manipulation and fermentation of a microbial organism with the capability to secrete and to modify proteins according to a general eukaryotic scheme. However, early restriction to a single yeast platform can result in costly and time-consuming failures. It is therefore advisable to assess several selected systems in parallel for the capability to produce a particular protein in desired amounts and quality. A suitable vector must contain a targeting sequence, a promoter element and a selection marker that function in all selected organisms. These criteria are fulfilled by a wide-range integrative yeast expression vector (CoMed) system based on A. adeninivorans- and H. polymorpha-derived elements that can be introduced in a modular way.

RESULTS

The vector system and a selection of modular elements for vector design are presented. Individual single vector constructs were used to transform a range of yeast species. Various successful examples are described. A vector with a combination of an rDNA sequence for genomic targeting, the E. coli-derived hph gene for selection and the A. adeninivorans-derived TEF1 promoter for expression control of a GFP (green fluorescent protein) gene was employed in a first example to transform eight different species including Hansenula polymorpha, Arxula adeninivorans and others. In a second example, a vector for the secretion of IL-6 was constructed, now using an A. adeninivorans-derived LEU2 gene for selection of recombinants in a range of auxotrophic hosts. In this example, differences in precursor processing were observed: only in A. adeninivorans processing of a MFalpha1/IL-6 fusion was performed in a faithful way.

CONCLUSION

rDNA targeting provides a tool to co-integrate up to 3 different expression plasmids by a single transformation step. Thus, a versatile system is at hand that allows a comparative assessment of newly introduced metabolic pathways in several organisms or a comparative co-expression of bottleneck genes in cases where production or secretion of a certain product is impaired.

Role of cell cycle-regulated expression in the localized incorporation of cell wall proteins in yeast

The yeast cell wall is an essential organelle that protects the cell from mechanical damage and antimicrobial peptides, participates in cell recognition and adhesion, and is important for the generation and maintenance of normal cell shape. We studied the localization of three covalently bound cell wall proteins in Saccharomyces cerevisiae. Tip1p was found only in mother cells, whereas Cwp2p was incorporated in small-to-medium-sized buds. When the promoter regions of TIP1 and CWP2 (responsible for transcription in early G1 and S/G2 phases, respectively) were exchanged, the localization patterns of Tip1p and Cwp2p were reversed, indicating that the localization of cell wall proteins can be completely determined by the timing of transcription during the cell cycle. The third protein, Cwp1p, was incorporated into the birth scar, where it remained for several generations. However, we could not detect any role of Cwp1p in strengthening the birth scar wall or any functional interaction with the proteins that mark the birth scar pole as a potential future budding site. Promoter-exchange experiments showed that expression in S/G2 phase is necessary but not sufficient for the normal localization of Cwp1p. Studies of mutants in which septum formation is perturbed indicate that the normal asymmetric localization of Cwp1p also depends on the normal timing of septum formation, composition of the septum, or both.

Comparison of different signal peptides for secretion of heterologous proteins in fission yeast

In the fission yeast Schizosaccharomyces pombe, there are relatively few signal peptides available and most reports of their activity have not been comparative. Using sequence information from the S. pombe genome database we have identified three putative signal peptides, designated Cpy, Amy and Dpp, and compared their ability to support secretion of green fluorescent protein (GFP). In the comparison we also included the two well-described secretion signals derived from the precursors of, respectively, the Saccharomyces cerevisiae alpha-factor and the S. pombe P-factor. The capability of the tested signal peptides to direct secretion of GFP varied greatly. The alpha-factor signal did not confer secretion to GFP and all the produced GFP was trapped intracellular. In contrast, the Cpy signal peptide supported efficient secretion of GFP with yields approximating 10 mg/L. We also found that the use of an attenuated version of the S. cerevisiae URA3 marker substantially increases vector copy number and expression yield in fission yeast.

Synthesis and application of fluorescent ras proteins for live-cell imaging

Semisynthetic Ras proteins are efficient probes for cell-biology experiments. With a Bodipy FL fluorophore introduced at an appropriate site on the Ras peptide by solid-phase synthesis, the resulting Ras chimera is processed by the cellular machinery and the intracellular localization of the protein can then be visualized by means of confocal laser fluorescence microscopy at relatively low concentrations. The absence of a large N-terminal protein tag overcomes possible interferences in the interaction with cellular partner proteins. The fluorescence emission from Bodipy FL is continuous and disappears only after irreversible bleaching. These characteristics make Ras proteins with nonprotein fluorophores suitable for biophysical analysis. The easy accessibility of the lipopeptide moiety by chemical synthesis opens up numerous options for further biological investigations.

Secretion and surface display of green fluorescent protein using the yeast Saccharomyces cerevisiae

Green fluorescent protein (GFP) continues to be a very useful tool in biotechnology, but soluble production of GFP and GFP-protein fusions has been difficult. In this study, we have produced yeast-enhanced green fluorescent protein (yEGFP) in Saccharomyces cerevisiae as a soluble, secreted product with a purified level of 6 mg/L. Expression was directed by the inducible GAL1-10 promoter and synthetic prepro leader sequence. The secretion of yEGFP by yeast was strongly dependent on temperature, with 20 degrees C induction being optimal. Use of 2 micro multicopy expression constructs elevated yields over a low-copy CEN-based system by approximately 2-fold. Yeast-enhanced GFP was also expressed as a fusion to the Aga2p mating agglutinin in order to test the secretory processing fidelity of yEGFP-protein fusions. When the cell surface anchoring protein, Aga1p, was co-overexpressed with the Aga2p-yEGFP fusion, the Aga2p-yEGFP protein was tethered to the yeast cell surface. Flow cytometry and fluorescence microscopy analysis indicated that the fusion was displayed on the yeast cell surface at high levels. In the absence of high level Aga1p expression, the Aga2p-yEGFP fusion protein was instead secreted in its entirety with no detectable surface display. These findings reveal that yeast is a suitable host for secretion of GFP and GFP-protein fusions and thus could enable a wide range of biochemistry and biotechnology applications.

A Rab-E GTPase mutant acts downstream of the Rab-D subclass in biosynthetic membrane traffic to the plasma membrane in tobacco leaf epidermis

The function of the Rab-E subclass of plant Rab GTPases in membrane traffic was investigated using a dominant-inhibitory mutant (RAB-E1(d)[NI]) of Arabidopsis thaliana RAB-E1(d) and in vivo imaging approaches that have been used to characterize similar mutants in the plant Rab-D2 and Rab-F2 subclasses. RAB-E1(d)[NI] inhibited the transport of a secreted green fluorescent protein marker, secGFP, but in contrast with dominant-inhibitory RAB-D2 or RAB-F2 mutants, it did not affect the transport of Golgi or vacuolar markers. Quantitative imaging revealed that RAB-E1(d)[NI] caused less intracellular secGFP accumulation than RAB-D2(a)[NI], a dominant-inhibitory mutant of a member of the Arabidopsis Rab-D2 subclass. Furthermore, whereas RAB-D2(a)[NI] caused secGFP to accumulate exclusively in the endoplasmic reticulum, RAB-E1(d)[NI] caused secGFP to accumulate additionally in the Golgi apparatus and a prevacuolar compartment that could be labeled by FM4-64 and yellow fluorescent protein (YFP)-tagged Arabidopsis RAB-F2(b). Using the vacuolar protease inhibitor E64-d, it was shown that some secGFP was transported to the vacuole in control cells and in the presence of RAB-E1(d)[NI]. Consistent with the hypothesis that secGFP carries a weak vacuolar-sorting determinant, it was shown that a secreted form of DsRed reaches the apoplast without appearing in the prevacuolar compartment. When fused to RAB-E1(d), YFP was targeted specifically to the Golgi via a saturable nucleotide- and prenylation-dependent mechanism but was never observed on the prevacuolar compartment. We propose that RAB-E1(d)[NI] inhibits the secretory pathway at or after the Golgi, causing an accumulation of secGFP in the upstream compartments and an increase in the quantity of secGFP that enters the vacuolar pathway.

ABG1, a novel and essential Candida albicans gene encoding a vacuolar protein involved in cytokinesis and hyphal branching

Immunoscreening of a Candida albicans expression library resulted in the isolation of a novel gene encoding a 32.9-kDa polypeptide (288 amino acids), with 27.7% homology to the product of Saccharomyces cerevisiae YGR106c, a putative vacuolar protein. Heterozygous mutants in this gene displayed an altered budding growth pattern, characterized by the formation of chains of buds, decreasingly in size towards the apex, without separation of the daughter buds. Consequently, this gene was designated ABG1. A conditional mutant for ABG1 with the remaining allele under the control of the MET3 promoter did not grow in the presence of methionine and cysteine, demonstrating that ABG1 was essential for viability. Western analysis revealed the presence of a major 32.9-kDa band, mainly in a particulate fraction (P40) enriched in vacuoles, and tagging with green fluorescent protein confirmed that Abg1p localized to the vacuole. Vacuole inheritance has been linked to the regulation of branching frequency in C. albicans. Under repressing conditions, the conditional mutant had an increased frequency of branching under hyphal inducing conditions and an altered sensitivity to substances that interfered with cell wall assembly. Repression of ABG1 in the conditional mutant strain caused disturbance of normal size and number of vacuoles both in yeast and mycelial cells and also in the asymmetric vacuole inheritance associated with the characteristic pattern of germ tubes and branching in C. albicans. These observations indicate that ABG1 plays a key role in vacuole biogenesis, cytokinesis, and hyphal branching.

Viral preprotoxin signal sequence allows efficient secretion of green fluorescent protein by Candida glabrata, Pichia pastoris, Saccharomyces cerevisiae, and Schizosaccharomyces pombe

Besides its importance as model organism in eukaryotic cell biology, yeast species have also developed into an attractive host for the expression, processing, and secretion of recombinant proteins. Here we investigated foreign protein secretion in four distantly related yeasts (Candida glabrata, Pichia pastoris, Saccharomyces cerevisiae, and Schizosaccharomyces pombe) by using green fluorescent protein (GFP) as a reporter and a viral secretion signal sequence derived from the K28 preprotoxin (pptox), the precursor of the yeast K28 virus toxin. In vivo expression of GFP fused to the N-terminal pptox leader sequence and/or expression of the entire pptox gene was driven either from constitutive (PGK1 and TPI1) or from inducible and/or repressible (GAL1, AOX1, and NMT1) yeast promoters. In each case, GFP entered the secretory pathway of the corresponding host cell; confocal fluorescence microscopy as well as sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western analysis of cell-free culture supernatants confirmed that GFP was efficiently secreted into the culture medium. In addition to the results seen with GFP, the full-length viral pptox was correctly processed in all four yeast genera, leading to the secretion of a biologically active virus toxin. Taken together, our data indicate that the viral K28 pptox signal sequence has the potential for being used as a unique tool in recombinant protein production to ensure efficient protein secretion in yeast.

A wide-range integrative yeast expression vector system based on Arxula adeninivorans-derived elements

An Arxula adeninivorans integration vector was applied to a range of alternative yeast species including Saccharomyces cerevisiae, Debaryomyces hansenii, Debaryomyces polymorphus, Hansenula polymorpha and Pichia pastoris. The vector harbours a conserved A. adeninivorans-derived 25S rDNA sequence for targeting, the A. adeninivorans-derived TEF1 promoter for expression control of the reporter sequence, and the Escherichia coli-derived hph gene conferring resistance against hygromycin B for selection of recombinants. Heterologous gene expression was assessed using a green fluorescent protein (GFP) reporter gene. The plasmid was found to be integrated into the genome of the various hosts tested; recombinant strains of all species exhibited heterologous gene expressions of a similar high level.

A GFP-based assay reveals a role for RHD3 in transport between the endoplasmic reticulum and Golgi apparatus

We describe the use of a secreted form of Aequoria victoria green fluorescent protein (secGFP) in a non-invasive live cell assay of membrane traffic in Arabidopsis thaliana. We show that in comparison to GFP-HDEL, which accumulates in the endoplasmic reticulum (ER), secGFP generates a weak fluorescence signal when transported to the apoplast. The fluorescence of secGFP in the apoplast can be increased by growth of seedlings on culture medium buffered at pH 8.1, suggesting that apoplastic pH is responsible, at least in part, for the low fluorescence intensity of seedlings expressing secGFP. Inhibition of secGFP transport between the ER and plasma membrane (PM), either by Brefeldin A (BFA) treatment or by genetic intervention results in increased intracellular secGFP accumulation accompanied by an increase in the secGFP fluorescence intensity. secGFP thus provides a valuable tool for forward and reverse genetic analysis of membrane traffic and endomembrane organisation in Arabidopsis. Using this assay for quantitative sublethal perturbation of secGFP transport, we identify a role for root hair defective 3 (RHD3) in transport of secreted and Golgi markers between the ER and the Golgi apparatus.

The ALEU2 gene--a new component for an Arxula adeninivorans-based expression platform

The ALEU2 gene, encoding beta-isopropylmalate dehydrogenase, was isolated from the non-conventional yeast Arxula adeninivorans. The isolated gene harbours an open reading frame of 1086 bp, encoding a putative protein of 362 amino acids. The derived protein sequence shares a high degree of homology with other fungal beta-isopropylmalate dehydrogenases thus confirming the identity of the gene. The isolated ALEU2 gene was tested for its suitability to complement the auxotrophy of an A. adeninivorans aleu2 host. For this purpose the plasmid pAL-ALEU2m which contains the ALEU2 gene as a selection marker and the 25S rDNA for targeting was employed in transformation experiments. Transformants harboured a single copy of the heterologous DNA and were found to be mitotically stable. For assessment of heterologous gene expression, two model genes were incorporated into the vector: the GFP gene, encoding intracellular green fluorescent protein, and the HSA gene, encoding the secreted human serum albumin. For expression control, both gene sequences were fused to the constitutive A. adeninivorans-derived TEF1 promoter and the Saccharomyces cerevisiae-derived PHO5 terminator. In the respective recombinant strains the GFP was localised in the cytoplasm, whereas more than 95% of the HSA accumulated in the culture medium. In initial fermentation trials using a 200-ml shake flask, maximal HSA product levels were observed after 96 h of cultivation.

Calnuc, an EF-hand Ca(2+)-binding protein, is stored and processed in the Golgi and secreted by the constitutive-like pathway in AtT20 cells

Calnuc is an ubiquitous, EF-hand Ca(2+) binding protein found in the cytoplasm where it binds to Galphai3, in the Golgi lumen where it constitutes a Ca(2+) storage pool, and secreted outside the cell. Here we investigated the pathway of secretion of calnuc in AtT20 cells. We found by pulse-chase experiments that calnuc is synthesized in the endoplasmic reticulum, transported to the Golgi where it remains greater than 12 h and undergoes posttranslational modification (O-glycosylation and sulfation) followed by secretion into the culture medium. We examined if calnuc is secreted by the constitutive or regulated secretory pathway in AtT20 cells. By immunofluorescence and immunogold labeling, endogenous calnuc is found in immature secretion granules (ISG) but not mature regulated secretory granules (RSG), whereas overexpressed calnuc-green fluorescent protein (GFP) is found in both ISG and RSG, where it colocalizes with ACTH. Neither calnuc nor calnuc-GFP are released by the regulated secretory pathway, suggesting that endogenous calnuc and calnuc-GFP are progressively removed from ISG and RSG during granule maturation. We conclude that calnuc is secreted via the constitutive-like pathway and represents a useful endogenous marker for this pathway in AtT20 cells. Together, these observations indicate that calnuc has a unique itinerary as it is retained in the Golgi and is then constitutively secreted extracellularly where it may influence cell behavior via its Ca(2+)-binding properties.

Improperly folded green fluorescent protein is secreted via a non-classical pathway

The green fluorescent protein is a cytosolic protein frequently used as a molecular tag to study protein localization in intact cells. We discovered that this protein is secreted into the medium by several but not all cell lines through a non-classical secretory pathway that is insensitive to brefeldin A. Green fluorescent protein is secreted efficiently by Chinese hamster ovary cells, with 60% of synthesized proteins secreted over 8 hours. This pathway is sensitive to changes in temperature but not to factors in serum or chemicals known to affect other non-classical protein secretion pathways. Fluorescence is observed in cells expressing green fluorescent protein, indicating that some of the protein must be fully folded in the cytosol. However, secreted green fluorescent protein is not fluorescent and therefore not folded properly. Furthermore, cellular fluorescence does not change over 6 hours whereas a significant proportion of green fluorescent protein is secreted. Thus, nascent green fluorescent protein either is folded correctly or incorrectly, and the improperly folded molecules can be exported. Non-classical secretion might be a route by which cells remove an excess of improperly folded, cytosolic proteins.

Integrated bioprocessing in Saccharomyces cerevisiae using green fluorescent protein as a fusion partner

In this study, we examine the use of green fluorescent protein (GFP) for monitoring a hexokinase (HXK)-GFP fusion protein in Saccharomyces cerevisiae for various events including expression, degradation, purification, and localization. The fusion, HXK-EK-GFP-6 x His, was constructed where the histidine tag (6 x His) would allow for convenient affinity purification, and the enterokinase (EK) cleavage site would be used for separation of HXK from GFP after affinity purification. Our results showed that both HXK and GFP remained active in the fusion and, more importantly, that there was a linear correlation between HXK activity and GFP fluorescence. Enterokinase cleavage studies revealed that both GFP fluorescence intensity and HXK activity remained unchanged after separation of the fusion proteins, which indicated that fusion of GFP did not cause structural alteration of HXK and thus did not affect the enzymatic activity of HXK. We also found that degradation of the fusion protein occurred, and that degradation was limited to HXK with GFP remaining intact in the fusion. Confocal microscopy studies showed that while GFP was distributed evenly in the yeast cytosol, HXK-GFP fusion followed the correct localization of HXK, which resulted in a di-localization of both cytosol and the nucleus. GFP proved to be a useful fusion partner that may lead to the possibility of integrating the bioprocesses by quantitatively following the entire process visually.

Functional analysis of a vacuolar ABC transporter in wild-type Candida albicans reveals its involvement in virulence

ATP-driven transport proteins belonging to the ATP-binding cassette (ABC) superfamily perform important functions in cell metabolism and detoxification. Compounds can be actively transported across membranes, including the plasma membrane or organellar membranes. The vacuole is an important organelle in fungal cells required for compartmentalization of metabolites as well as toxic substances. Sequestration into the vacuole is often energy-dependent. We present the first isolation and molecular analysis of a vacuolar ABC transporter gene in the opportunistic fungal pathogen Candida albicans. The protein encoded by the MLT1 gene is highly similar to Multiple Drug Resistance-associated Protein (MRP)-like transporters of yeast and higher organisms that form the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)/MRP subfamily of ABC transporters, a class of proteins so far not characterized in C. albicans. MLT1 expression is extensively growth phase-regulated, and gene transcripts are inducible by metabolic poisons. Gene replacement mutants generated in wild-type C. albicans with the dominant selection marker MPAR showed a profound reduction in virulence in a mouse peritonitis model that was reversed by complementation with an intact MLT1 gene. Hence, this report provides primary evidence for the involvement of vacuolar ABC transporters in fungal virulence.

Oligomerization of green fluorescent protein in the secretory pathway of endocrine cells

Green fluorescent protein (GFP) is used extensively as a reporter protein to monitor cellular processes, including intracellular protein trafficking and secretion. In general, this approach depends on GFP acting as a passive reporter protein. However, it was recently noted that GFP oligomerizes in the secretory pathway of endocrine cells. To characterize this oligomerization and its potential role in GFP transport, cytosolic and secretory forms of enhanced GFP (EGFP) were expressed in GH4C1 and AtT-20 endocrine cells. Biochemical analysis showed that cytosolic EGFP existed as a 27 kDa monomer, whereas secretory forms of EGFP formed disulphide-linked oligomers. EGFP contains two cysteine residues (Cys(49) and Cys(71)), which could play a role in this oligomerization. Site-directed mutagenesis of Cys(49) and Cys(71) showed that both cysteine residues were involved in disulphide interactions. Substitution of either cysteine residue resulted in a reduction or loss of oligomers, although dimers of the secretory form of EGFP remained. Mutation of these residues did not adversely affect the fluorescence of EGFP. EGFP oligomers were stored in secretory granules and secreted by the regulated secretory pathway in endocrine AtT-20 cells. Similarly, the dimeric mutant forms of EGFP were still secreted via the regulated secretory pathway, indicating that the higher-order oligomers were not necessary for sorting in AtT-20 cells. These results suggest that the oligomerization of EGFP must be considered when the protein is used as a reporter molecule in the secretory pathway.

Eukaryotic peptide deformylases. Nuclear-encoded and chloroplast-targeted enzymes in Arabidopsis

Arabidopsis (ecotype Columbia-0) genes, AtDEF1 and AtDEF2, represent eukaryotic homologs of the essential prokaryotic gene encoding peptide deformylase. Both deduced proteins contain three conserved protein motifs found in the active site of all eubacterial peptide deformylases, and N-terminal extensions identifiable as chloroplast-targeting sequences. Radiolabeled full-length AtDEF1 was imported and processed by isolated pea (Pisum sativum L. Laxton's Progress No. 9) chloroplasts and AtDEF1 and 2 were immunologically detected in Arabidopsis leaf and chloroplast stromal protein extracts. The partial cDNAs encoding the processed forms of Arabidopsis peptide deformylase 1 and 2 (pAtDEF1 and 2, respectively) were expressed in Escherichia coli and purified using C-terminal hexahistidyl tags. Both recombinant Arabidopsis peptide deformylases had peptide deformylase activity with unique kinetic parameters that differed from those reported for the E. coli enzyme. Actinonin, a specific peptide deformylase inhibitor, was effective in vitro against Arabidopsis peptide deformylase 1 and 2 activity, respectively. Exposure of several plant species including Arabidopsis to actinonin resulted in chlorosis and severe reductions in plant growth and development. The results suggest an essential role for peptide deformylase in protein processing in all plant plastids.

A signal sequence is sufficient for green fluorescent protein to be routed to regulated secretory granules

To investigate trafficking in neuroendocrine cells, green fluorescent protein (GFP) tags were fused to various portions of the preproneuropeptide Y (NPY) precursor. Two neuroendocrine cell lines, AtT-20 corticotrope tumor cells and PC-12 pheochromocytoma cells, along with primary anterior pituitary cells, were examined. Expression of chimeric constructs did not disrupt trafficking or regulated secretion of endogenous ACTH and prohormone convertase 1 in AtT-20 cells. Western blot and immunocytochemical analyses demonstrated that the chimeric constructs remained intact, as long as the Lys-Arg cleavage site within preproNPY was deleted. GFP was stored in, and released from, regulated granules in cells expressing half of the NPY precursor fused to GFP, and also in cells in which only the signal sequence of preproNPY was fused to GFP. Thus, in neuroendocrine cells, entering the lumen of the secretory pathway is sufficient to target GFP to regulated secretory granules.

The beauty of the yeast: live cell microscopy at the limits of optical resolution

The yeast Saccharomyces cerevisiae is a very powerful system for cell biological research. Recent advances in electronic light microscopy together with the application of green fluorescent protein and other in vivo staining techniques have allowed novel and exciting insights into structural organization and dynamics of cells as small as yeast. Methods for staining yeast for microscopic inspection and for introducing tags for localization studies of proteins in living or fixed cells are summarized. Electronic light microscopy, video/deconvolution methods, and confocal laser scanning microscopy as novel tools for structural analyses, and their practical applications in yeast, are discussed.

Advances in Imaging the Cell Biology of Plant-Microbe Interactions

All plant-microbe interactions are initiated at the level of the cell. Recently, the light microscope has increased in popularity as an investigative tool in plant cell biology, in part because of the parallel developments of confocal laser scanning and video microscopy, computerized image processing, and an ever-increasing array of fluorescent probes that can be applied to living cells. In addition, transgenic plants and cells can be generated in which specific components are fluorescently labeled without any invasive experimental manipulation. The application of such techniques to plant-microbe interactions has revealed microbe-induced changes in cytosolic calcium levels, the visualization of reactive oxygen species generation, cytoskeleton rearrangements, DNA cleavage, and the detailed resolution of intercellular and intracellular trafficking of viral components. These techniques, integrated with electron microscopy, molecular genetics, and other types of investigations, are likely to play an increasingly important role in future studies of plant responses to microbial pathogens or mutualists.

Functional domain mapping and subcellular distribution of Dal82p in Saccharomyces cerevisiae

Previous studies have shown that (i) Dal81p and Dal82p are required for allophanate-induced gene expression in Saccharomyces cerevisiae; (ii) the cis-acting element mediating the induced transcriptional response to allophanate is a dodecanucleotide, UIS(ALL); and (iii) Dal82p binds specifically to UIS(ALL). Here we show that Dal82p is localized to the nucleus and parallels movement of the DNA through the cell cycle. Deletion analysis of DAL82 identified and localized three functional domains. Electrophoretic mobility shift assays identified a peptide (consisting of Dal82p amino acids 1-85) that is sufficient to bind a DNA fragment containing UIS(ALL). LexA-tethering experiments demonstrated that Dal82p is capable of mediating transcriptional activation. The activation domain consists of two parts: (i) an absolutely required core region (amino acids 66-99) and (ii) less well defined regions flanking residues 66-99 that are required for full wild-type levels of activation. The Dal82p C terminus contains a predicted coiled-coil motif that down-regulates Dal82p-mediated transcriptional activation.

Ablation of a critical surfactant protein B intramolecular disulfide bond in transgenic mice

The 79-amino acid, mature SP-B peptide contains three intramolecular disulfide bonds shared by all saposin-like proteins. This study tested the hypothesis that the disulfide bond formed between cysteine residues 35 and 46 (residues 235 and 246 of the SP-B proprotein) is essential for proper function of SP-B. To test the role of this bridge in SP-B function in vivo, a construct was generated in which cysteine residues 235 and 246 of the human SP-B proprotein were mutated to serine and cloned under the control of the 3.7-kilobase hSP-C promoter (hSP-B(C235S/C246S)). In two transgenic mouse lines, expression of the mutant peptide in the wild-type murine SP-B background was invariably lethal in the neonatal period. In four additional lines, survival was inversely related to the level of transgene expression. To test the ability of the mutant peptide to functionally replace the wild-type protein, transgenic mice were crossed into the SP-B null background. No animals that expressed hSP-B(C235S/C246S) in the murine SP-B-/- background survived the neonatal period. hSP-B(C235S/C246S) proprotein accumulated in the endoplasmic reticulum and was not processed to the mature, biologically active peptide. The results of these studies demonstrate that the intramolecular bridge between residues 235 and 246 is critical for intracellular trafficking of SP-B and suggest that overexpression of mutant SP-B in the wild-type background may be lethal.