Prepro-alpha-factor has a cleavable signal sequence

Human lipocalins bind and export fatty acids through the secretory pathway of yeast cells

The activation of fatty acids to their acyl-CoA derivatives is a crucial step for their integration into more complex lipids or their degradation via beta-oxidation. Yeast cells employ five distinct acyl-CoA synthases to facilitate this ATP-dependent activation of acyl chains. Notably, mutant cells that are deficient in two of these fatty acid-activating (FAA) enzymes, namely, Faa1 and Faa4, do not take up free fatty acids but rather export them out of the cell. This unique fatty acid export pathway depends on small, secreted pathogenesis-related yeast proteins (Pry). In this study, we investigate whether the expression of human fatty acid-binding proteins, including Albumin, fatty acid-binding protein 4 (Fabp4), and three distinct lipocalins (ApoD, Lcn1, and Obp2a), could promote fatty acid secretion in yeast. To optimize the expression and secretion of these proteins, we systematically examined various signal sequences in both low-copy and high-copy number plasmids. Our findings reveal that directing these fatty-acid binding proteins into the secretory pathway effectively promotes fatty acid secretion from a sensitized quadruple mutant model strain (faa1∆ faa4∆ pry1∆ pry3∆). Furthermore, the level of fatty acid secretion exhibited a positive correlation with the efficiency of protein secretion. Importantly, the expression of all human lipid-binding proteins rescued Pry-dependent fatty acid secretion, resulting in the secretion of both long-chain saturated and unsaturated fatty acids. These results not only affirm the in vitro binding capabilities of lipocalins to fatty acids but also present a novel avenue for enhancing the secretion of valuable lipidic compounds. Given the growing interest in utilizing yeast as a cellular factory for producing poorly soluble compounds and the potential of lipocalins as platforms for engineering substrate-binding specificity, our model is considered as a powerful tool for promoting the secretion of high-value lipid-based molecules.

Peptide Variant Detection by a Living Yeast Biosensor via an Epitope-Selective Protease

We previously demonstrated that we could hijack the fungal pheromone signaling pathway to provide a living yeast biosensor where peptide biomarkers were recognized by G-protein-coupled receptors and engineered to transcribe a readout. Here, we demonstrated that the protease could be reintroduced to the biosensor to provide a simple mechanism for distinguishing single-amino-acid changes in peptide ligands that, otherwise, would likely be difficult to detect using binding-based assays. We characterized the dose-response curves for five fungal pheromone G-protein-coupled receptors, peptides, and proteases-Saccharomyces cerevisiae, Candida albicans, Schizosaccharomyces pombe, Schizosaccharomyces octosporus, and Schizosaccharomyces japonicus. Alanine scanning was carried out for the most selective of these-S. cerevisiae and C. albicans-with and without the protease. Two peptide variants were discovered, which showed diminished cleavage by the protease (CaPep2A and CaPep2A13A). Those peptides were then distinguished by utilizing the biosensor strains with and without the protease, which selectively cleaved and altered the apparent concentration of peptide required for half-maximal activation for 2 peptides-CaPep and CaPep13A, respectively-by more than one order of magnitude. These results support the hypothesis that the living yeast biosensor with a sequence-specific protease can translate single-amino-acid changes into more than one order of magnitude apparent shift in the concentration of peptide required for half-maximal activation. With further engineering by computational modeling and directed evolution, the biosensor could likely distinguish a wide variety of peptide sequences beyond the alanine scanning carried out here. In the future, we envision incorporating proteases into our living yeast biosensor for use as a point of care diagnostic, a scalable communication language, and other applications.

Challenges of expressing recombinant human tissue factor as a secreted protein in Pichia pastoris

Tissue factor (TF) is the core reagent in the prothrombin time (PT) assay. In this study, expression and α-factor mediated secretion of three forms of tissue factor (full-length TF (Full-TF), extracellular plus transmembrane domain (TED-TF), and only extracellular domain (ED-TF) were investigated in Pichia pastoris. The amino acid sequence of TF was obtained from the UniProt database, back-translated and codon-optimized for expression in Pichia pastoris. The Full-TF sequence was synthesized but the ED-TF, TED-TF coding fragments were extracted from the Full-TF by PCR. All the coding sequences were cloned into pPICZαA vector in-frame with the α-factor; and electroporated into KM71H. The culture supernatants and the cell lysates were analyzed using SDS-PAGE, dot-blotting, and Western-blotting for expression of TF. The Full-TF and TED-TF expression vector pPICZαA were successfully inserted into the KM71H, but the product was not detected in the SDS-PAGE analysis of the culture supernatant. However, ED-TF expression and secretion was verified by SDS-PAGE, dot blotting, and Western blotting. It seems that the TM domain in the Full-TF and TED-TF have an important role in impairing α-factor-mediated secretion of TF. Therefore, further investigation is necessary to overcome challenges of expressing Full-TF as a heterologous protein in P. pastoris.

Identifying carbohydrate-active enzymes of Cutaneotrichosporon oleaginosus using systems biology

Background

The oleaginous yeast Cutaneotrichosporon oleaginosus represents one of the most promising microbial platforms for resource-efficient and scalable lipid production, with the capacity to accept a wide range of carbohydrates encapsulated in complex biomass waste or lignocellulosic hydrolysates. Currently, data related to molecular aspects of the metabolic utilisation of oligomeric carbohydrates are sparse. In addition, comprehensive proteomic information for C. oleaginosus focusing on carbohydrate metabolism is not available.

Results

In this study, we conducted a systematic analysis of carbohydrate intake and utilisation by C. oleaginosus and investigated the influence of different di- and trisaccharide as carbon sources. Changes in the cellular growth and morphology could be observed, depending on the selected carbon source. The greatest changes in morphology were observed in media containing trehalose. A comprehensive proteomic analysis of secreted, cell wall-associated, and cytoplasmatic proteins was performed, which highlighted differences in the composition and quantity of secreted proteins, when grown on different disaccharides. Based on the proteomic data, we performed a relative quantitative analysis of the identified proteins (using glucose as the reference carbon source) and observed carbohydrate-specific protein distributions. When using cellobiose or lactose as the carbon source, we detected three- and five-fold higher diversity in terms of the respective hydrolases released. Furthermore, the analysis of the secreted enzymes enabled identification of the motif with the consensus sequence LALL[LA]L[LA][LA]AAAAAAA as a potential signal peptide.

Conclusions

Relative quantification of spectral intensities from crude proteomic datasets enabled the identification of new enzymes and provided new insights into protein secretion, as well as the molecular mechanisms of carbo-hydrolases involved in the cleavage of the selected carbon oligomers. These insights can help unlock new substrate sources for C. oleaginosus, such as low-cost by-products containing difficult to utilize carbohydrates. In addition, information regarding the carbo-hydrolytic potential of C. oleaginosus facilitates a more precise engineering approach when using targeted genetic approaches. This information could be used to find new and more cost-effective carbon sources for microbial lipid production by the oleaginous yeast C. oleaginosus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01692-2.

Fur4-mediated uracil-scavenging to screen for surface protein regulators

Cell surface membrane proteins perform diverse and critical functions and are spatially and temporally regulated by membrane trafficking pathways. Although perturbations in these pathways underlie many pathologies, our understanding of these pathways at a mechanistic level remains incomplete. Using yeast as a model, we have developed an assay that reports on the surface activity of the uracil permease Fur4 in uracil auxotroph strains grown in the presence of limited uracil. This assay was used to screen a library of haploid deletion strains and identified mutants with both diminished and enhanced comparative growth in restricted uracil media. Factors identified, including various multisubunit complexes, were enriched for membrane trafficking and transcriptional functions, in addition to various uncharacterized genes. Bioinformatic analysis of expression profiles from many strains lacking transcription factors required for efficient uracil-scavenging validated particular hits from the screen, in addition to implicating essential genes not tested in the screen. Finally, we performed a secondary mating factor secretion screen to functionally categorize factors implicated in uracil-scavenging.

Design of an improved universal signal peptide based on the α-factor mating secretion signal for enzyme production in yeast

Saccharomyces cerevisiae plays an important role in the heterologous expression of an array of proteins due to its easy manipulation, low requirements and ability for protein post-translational modifications. The implementation of the preproleader secretion signal of the α-factor mating pheromone from this yeast contributes to increase the production yields by targeting the foreign protein to the extracellular environment. The use of this signal peptide combined with enzyme-directed evolution allowed us to achieve the otherwise difficult functional expression of fungal laccases in S. cerevisiae, obtaining different evolved α-factor preproleader sequences that enhance laccase secretion. However, the design of a universal signal peptide to enhance the production of heterologous proteins in S. cerevisiae is a pending challenge. We describe here the optimisation of the α-factor preproleader to improve recombinant enzyme production in S. cerevisiae through two parallel engineering strategies: a bottom-up design over the native α-factor preproleader (αnat) and a top-down design over the fittest evolved signal peptide obtained in our lab (α9H2 leader). The goal was to analyse the effect of mutations accumulated in the signal sequence throughout iterations of directed evolution, or of other reported mutations, and their possible epistatic interactions. Both approaches agreed in the positive synergism of four mutations (Aα9D, Aα20T, Lα42S, Dα83E) contained in the final optimised leader (αOPT), which notably enhanced the secretion of several fungal oxidoreductases and hydrolases. Additionally, we suggest a guideline to further drive the heterologous production of a particular enzyme based on combinatorial saturation mutagenesis of positions 86th and 87th of the αOPT leader fused to the target protein.

The application of HPLC/MS analysis with a multi-enzyme digest strategy to characterize different interferon product variants produced from Pichia pastoris

Interferons are signaling proteins that belong to the large class of cytokines and human interferons which are classified based on the type of receptor interactions: type I, II and III. IFNα2b belongs to the type I interferon class with a major therapeutic application for the treatment of hepatitis B and C infections. A recombinant form of IFNα2b expressed in E. coli, known as IntronA, has been approved by US Food and Drug Administration (FDA). IFN γ, also known as type II interferon, plays a significant role in the inhibition of viral replication. Actimmune^® is a US Food and Drug Administration (FDA) approved version of IFN γ for the indication of reducing infections associated with chronic granulomatous disease and severe malignant osteopetrosis. In this study we have applied advanced analytical methods for the characterization of IFNα2b and IFN γ produced from Pichia pastoris. The multi-enzyme digestion approach has been developed to allow measurement of 100% sequence coverage and detailed analysis of post-translational variants and degradation products. In this manner, we identified the following variants in IFN α2b: N-terminal residual leader sequence, an amino acid substitution, oxidation of methionine residues and two sites of high mannose N-glycosylation. In the Pichia IFN γ produced material, our approach detected variants resulting from glycosylation, C-terminal proteolysis, oxidation of methionine residues and deamidation. In this manner, the analytical program was able to support rapid process development as well as identify product variants and degradation products in the resulting product.

Characterizing the selectivity of ER α-glucosidase inhibitors

The endoplasmic reticulum (ER) contains both α-glucosidases and α-mannosidases which process the N-linked oligosaccharides of newly synthesized glycoproteins and thereby facilitate polypeptide folding and glycoprotein quality control. By acting as structural mimetics, iminosugars can selectively inhibit these ER localized α-glycosidases, preventing N-glycan trimming and providing a molecular basis for their therapeutic applications. In this study, we investigate the effects of a panel of nine iminosugars on the actions of ER luminal α-glucosidase I and α-glucosidase II. Using ER microsomes to recapitulate authentic protein N-glycosylation and oligosaccharide processing, we identify five iminosugars that selectively inhibit N-glycan trimming. Comparison of their inhibitory activities in ER microsomes against their effects on purified ER α-glucosidase II, suggests that 3,7a-diepi-alexine acts as a selective inhibitor of ER α-glucosidase I. The other active iminosugars all inhibit α-glucosidase II and, having identified 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) as the most effective of these compounds, we use in silico modeling to understand the molecular basis for this enhanced activity. Taken together, our work identifies the C-3 substituted pyrrolizidines casuarine and 3,7a-diepi-alexine as promising "second-generation" iminosugar inhibitors.

Challenging the workhorse: Comparative analysis of eukaryotic micro-organisms for expressing monoclonal antibodies

For commercial protein therapeutics, Chinese hamster ovary (CHO) cells have an established history of safety, proven capability to express a wide range of therapeutic proteins and high volumetric productivities. Expanding global markets for therapeutic proteins and increasing concerns for broadened access of these medicines has catalyzed consideration of alternative approaches to this platform. Reaching these objectives likely will require an order of magnitude increase in volumetric productivity and a corresponding reduction in the costs of manufacture. For CHO-based manufacturing, achieving this combination of targeted improvements presents challenges. Based on a holistic analysis, the choice of host cells was identified as the single most influential factor for both increasing productivity and decreasing costs. Here we evaluated eight wild-type eukaryotic micro-organisms with prior histories of recombinant protein expression. The evaluation focused on assessing the potential of each host, and their corresponding phyla, with respect to key attributes relevant for manufacturing, namely (a) growth rates in industry-relevant media, (b) adaptability to modern techniques for genome editing, and (c) initial characterization of product quality. These characterizations showed that multiple organisms may be suitable for production with appropriate engineering and development and highlighted that yeast in general present advantages for rapid genome engineering and development cycles.

Identification and characterization of N-glycosylation site on a Mucor circinelloides aspartic protease expressed in Pichia pastoris: effect on secretion, activity and thermo-stability

Methylotrophic yeasts have widely been used as model organisms for understanding cellular functions and biochemical activities in lower eukaryotes. The gene encoding an aspartic protease (MCAP) from Mucor circinelloides DSM 2183 was cloned and expressed into Pichia pastoris using both the native M. circinelloides signal peptide (mcSP) and α-factor secretion signal from Saccharomyces cerevisiae (α-MF). When expressed in P. pastoris using α-MF and mcSP, MCAP was secreted into the culture medium at a concentration 200 mg L-1 (410 MCU mL-1) and 110 mg L-1 (249 MCU mL-1), respectively. The SDS-PAGE analysis of each culture shows that the protein was secreted in the media in two forms with molecular weights of approximately 33 and 37 kDa. Upon digestion using endoglycosidase H (Endo H), only one band at 33 kDa was observed, indicating that the protein might be glycosylated. One putative N-glycosylation site was found and a site-directed mutagenesis at position Asn331-Gln of the sequence produce only one form of the protein of 33 kDa, similar to that obtained when digested with Endo H. The optimum temperature and pH activity of the expressed MCAP was found to be at 60 °C and 3.6, respectively.

The Long History of the Diverse Roles of Short ORFs: sPEPs in Fungi

Since the completion of the genome sequence of the model eukaryote Saccharomyces cerevisiae, there have been significant advancements in the field of genome annotation, in no small part due to the availability of datasets that make large-scale comparative analyses possible. As a result, since its completion there has been a significant change in annotated ORF size distribution in this first eukaryotic genome, especially in short ORFs (sORFs) predicted to encode polypeptides less than 150 amino acids in length. Due to their small size and the difficulties associated with their study, it is only relatively recently that these genomic features and the sORF-encoded peptides (sPEPs) they encode have become a focus of many researchers. Yet while this class of peptides may seem new and exciting, the study of this part of the proteome is nothing new in S. cerevisiae, a species where the biological importance of sPEPs has been elegantly illustrated over the past 30 years. Here the authors showcase a range of different sORFs found in S. cerevisiae and the diverse biological roles of their encoded sPEPs, and provide an insight into the sORFs found in other fungal species, particularly those pathogenic to humans.

Integrated Bottom-Up and Top-Down Liquid Chromatography-Mass Spectrometry for Characterization of Recombinant Human Growth Hormone Degradation Products

With the advent of biosimilars to the U.S. market, it is important to have better analytical tools to ensure product quality from batch to batch. In addition, the recent popularity of using a continuous process for production of biopharmaceuticals, the traditional bottom-up method, alone for product characterization and quality analysis is no longer sufficient. Bottom-up method requires large amounts of material for analysis and is labor-intensive and time-consuming. Additionally, in this analysis, digestion of the protein with enzymes such as trypsin could induce artifacts and modifications which would increase the complexity of the analysis. On the other hand, a top-down method requires a minimum amount of sample and allows for analysis of the intact protein mass and sequence generated from fragmentation within the instrument. However, fragmentation usually occurs at the N-terminal and C-terminal ends of the protein with less internal fragmentation. Herein, we combine the use of the complementary techniques, a top-down and bottom-up method, for the characterization of human growth hormone degradation products. Notably, our approach required small amounts of sample, which is a requirement due to the sample constraints of small scale manufacturing. Using this approach, we were able to characterize various protein variants, including post-translational modifications such as oxidation and deamidation, residual leader sequence, and proteolytic cleavage. Thus, we were able to highlight the complementarity of top-down and bottom-up approaches, which achieved the characterization of a wide range of product variants in samples of human growth hormone secreted from Pichia pastoris.

The Epichloë festucae antifungal protein has activity against the plant pathogen Sclerotinia homoeocarpa, the causal agent of dollar spot disease

Epichloë spp. are naturally occurring fungal endophytic symbionts of many cool-season grasses. Infection by the fungal endophytes often confers biotic and abiotic stress tolerance to their hosts. Endophyte-mediated disease resistance is well-established in the fine fescue grass Festuca rubra subsp. rubra (strong creeping red fescue) infected with E. festucae. Resistance to fungal pathogens is not an established effect of endophyte infection of other grass species, and may therefore be unique to the fine fescues. The underlying mechanism of the disease resistance is unknown. E. festucae produces a secreted antifungal protein that is highly expressed in the infected plant tissues and may therefore be involved in the disease resistance. Most Epichloë spp. do not have a gene for a similar antifungal protein. Here we report the characterization of the E. festucae antifungal protein, designated Efe-AfpA. The antifungal protein partially purified from the apoplastic proteins of endophyte-infected plant tissue and the recombinant protein expressed in the yeast Pichia pastoris was found to have activity against the important plant pathogen Sclerotinia homoeocarpa. Efe-AfpA may therefore be a component of the disease resistance seen in endophyte-infected strong creeping red fescue.

A high-throughput expression screening platform to optimize the production of antimicrobial peptides

Background

Antimicrobial peptides (AMPs) are promising candidates for the development of novel antibiotics, but it is difficult to produce sufficient quantities for preclinical and clinical studies due to their toxicity towards microbial expression hosts. To avoid laborious trial-and-error testing for the identification of suitable expression constructs, we have developed a small-scale expression screening platform based on a combinatorial plasmid library.

Results

The combinatorial library is based on the Golden Gate cloning system. In each reaction, six donor plasmids (each containing one component: a promoter, fusion partner 1, fusion partner 2, protease cleavage site, gene of interest, or transcriptional terminator) were combined with one acceptor plasmid to yield the final expression construct. As a proof of concept, screening was carried out in Escherichia coli and Pichia pastoris to study the expression of three different model AMPs with challenging characteristics, such as host toxicity or multiple disulfide bonds. The corresponding genes were successfully cloned in 27 E. coli and 18 P. pastoris expression plasmids, each in a one-step Golden Gate reaction. After transformation, small-scale expression screening in microtiter plates was followed by AMP quantification using a His₆ tag-specific ELISA. Depending on the plasmid features and the expression host, the protein yields differed by more than an order of magnitude. This allowed the identification of high producers suitable for larger-scale protein expression.

Conclusions

The optimization of recombinant protein production is best achieved from first principles by initially optimizing the genetic construct. The unrestricted combination of multiple plasmid features yields a comprehensive library of expression strains that can be screened for optimal productivity. The availability of such a platform could benefit all laboratories working in the field of recombinant protein expression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0637-5) contains supplementary material, which is available to authorized users.

Functionalized silk assembled from a recombinant spider silk fusion protein (Z-4RepCT) produced in the methylotrophic yeast Pichia pastoris

Functional biological materials are a growing research area with potential applicability in medicine and biotechnology. Using genetic engineering, the possibility to introduce additional functions into spider silk-based materials has been realized. Recently, a recombinant spider silk fusion protein, Z-4RepCT, was produced intracellularly in Escherichia coli and could after purification self-assemble into silk-like fibers with ability to bind antibodies via the IgG-binding Z domain. In this study, the use of the methylotrophic yeast Pichia pastoris for production of Z-4RepCT has been investigated. Temperature, pH and production time were influencing the amount of soluble Z-4RepCT retrieved from the extracellular fraction. Purification of secreted Z-4RepCT resulted in a mixture of full-length and degraded silk proteins that failed to self-assemble into fibers. A position in the C-terminal domain of 4RepCT was identified as being subjected to proteolytic cleavage by proteases in the Pichia culture supernatant. Moreover, the C-terminal domain was subjected to glycosylation during production in P. pastoris. These observed alterations of the CT domain are suggested to contribute to the failure in fiber assembly. As alternative approach, Z-4RepCT retrieved from the intracellular fraction, which was less degraded, was used and shown to retain ability to assemble into silk-like fibers after enzymatic deglycosylation.

High-temperature cultivation of recombinant Pichia pastoris increases endoplasmic reticulum stress and decreases production of human interleukin-10

Background

The yeast Pichia pastoris (P. pastoris) has become a popular ‘cell factory’ for producing heterologous proteins, but production widely varies among proteins. Cultivation temperature is frequently reported to significantly affect protein production; however, the underlying mechanisms of this effect remain unclear.

Results

A P. pastoris strain expressing recombinant human interleukin-10 (rhIL-10) under the control of the AOX1 promoter was used as the model in this study. This system shows high-yield rhIL-10 production with prolonged methanol-induction times when cultured at 20°C but low-yield rhIL-10 production and higher cell death rates when cultured at 30°C. Further investigation showed that G3-pro-rhIL10, an immature form of rhIL-10 that contains the glycosylation-modified signal peptide, remained in the ER for a prolonged period at 30°C. The retention resulted in higher ER stress levels that were accompanied by increased ROS production, Ca²⁺ leakage, ER-containing autophagosomes, shortened cortical ER length and compromised induction of the unfolded protein response (UPR). In contrast, G3-pro-rhIL10 was quickly processed and eliminated from the ER at 20°C, resulting in a lower level of ER stress and improved rhIL-10 production.

Conclusions

High-temperature cultivation of an rhIL-10 expression strain leads to prolonged retention of immature G3-pro-rhIL10 in ER, causing higher ER stress levels and thus greater yeast cell death rates and lower production of rhIL-10.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0163-7) contains supplementary material, which is available to authorized users.

Protein expression in Pichia pastoris: recent achievements and perspectives for heterologous protein production

Pichia pastoris is an established protein expression host mainly applied for the production of biopharmaceuticals and industrial enzymes. This methylotrophic yeast is a distinguished production system for its growth to very high cell densities, for the available strong and tightly regulated promoters, and for the options to produce gram amounts of recombinant protein per litre of culture both intracellularly and in secretory fashion. However, not every protein of interest is produced in or secreted by P. pastoris to such high titres. Frequently, protein yields are clearly lower, particularly if complex proteins are expressed that are hetero-oligomers, membrane-attached or prone to proteolytic degradation. The last few years have been particularly fruitful because of numerous activities in improving the expression of such complex proteins with a focus on either protein engineering or on engineering the protein expression host P. pastoris. This review refers to established tools in protein expression in P. pastoris and highlights novel developments in the areas of expression vector design, host strain engineering and screening for high-level expression strains. Breakthroughs in membrane protein expression are discussed alongside numerous commercial applications of P. pastoris derived proteins.

Current methods for inducing pluripotency in somatic cells

The groundbreaking discovery of reprogramming fibroblasts towards pluripotency merely by introducing four transcription factors (OCT4, SOX2, KLF4 and c-MYC) by means of retroviral transduction has created a promising revolution in the field of regenerative medicine. These so-called induced pluripotent stem cells (iPSCs) can provide a cell source for disease-modelling, drug-screening platforms, and transplantation strategies to treat incurable degenerative diseases, while circumventing the ethical issues and immune rejections associated with the use of non-autologous embryonic stem cells. The risk of insertional mutagenesis, caused both by the viral and transgene nature of the technique has proven to be the major limitation for iPSCs to be used in a clinical setting. In view of this, a variety of alternative techniques have been developed to induce pluripotency in somatic cells. This review provides an overview on current reprogramming protocols, discusses their pros and cons and future challenges to provide safe and transgene-free iPSCs.

NAC functions as a modulator of SRP during the early steps of protein targeting to the endoplasmic reticulum

NAC acts as a modulator of SRP function. It can bind to signal sequences directly. SRP initially displaces NAC from RNCs; however, when the signal sequence emerges, trimeric NAC·RNC·SRP complexes form. Upon docking NAC·RNC·SRP complexes to the ER, NAC remains bound, allowing NAC to shield cytosolically exposed nascent chain domains.

Nascent polypeptide-associated complex (NAC) was initially found to bind to any segment of the nascent chain except signal sequences. In this way, NAC is believed to prevent mistargeting due to binding of signal recognition particle (SRP) to signalless ribosome nascent chain complexes (RNCs). Here we revisit the interplay between NAC and SRP. NAC does not affect SRP function with respect to signalless RNCs; however, NAC does affect SRP function with respect to RNCs targeted to the endoplasmic reticulum (ER). First, early recruitment of SRP to RNCs containing a signal sequence within the ribosomal tunnel is NAC dependent. Second, NAC is able to directly and tightly bind to nascent signal sequences. Third, SRP initially displaces NAC from RNCs; however, when the signal sequence emerges further, trimeric NAC·RNC·SRP complexes form. Fourth, upon docking to the ER membrane NAC remains bound to RNCs, allowing NAC to shield cytosolically exposed nascent chain domains not only before but also during cotranslational translocation. The combined data indicate a functional interplay between NAC and SRP on ER-targeted RNCs, which is based on the ability of the two complexes to bind simultaneously to distinct segments of a single nascent chain.

Fungal mating pheromones: choreographing the dating game

Pheromones are ubiquitous from bacteria to mammals - a testament to their importance in regulating inter-cellular communication. In fungal species, they play a critical role in choreographing interactions between mating partners during the program of sexual reproduction. Here, we describe how fungal pheromones are synthesized, their interactions with G protein-coupled receptors, and the signals propagated by this interaction, using Saccharomyces cerevisiae as a reference point. Divergence from this model system is compared amongst the ascomycetes and basidiomycetes, which reveals the wealth of information that has been gleaned from studying pheromone-driven processes across a wide spectrum of the fungal kingdom.

Erv26p-dependent export of alkaline phosphatase from the ER requires lumenal domain recognition

Active sorting at the endoplasmic reticulum (ER) drives efficient export of fully folded secretory proteins into coat protein complex II (COPII) vesicles, whereas ER-resident and misfolded proteins are retained and/or degraded. A number of secretory proteins depend upon polytopic cargo receptors for linkage to the COPII coat and ER export. However, the mechanism by which cargo receptors recognize transport-competent cargo is poorly understood. Here we examine the sorting determinants required for export of yeast alkaline phosphatase (ALP) by its cargo receptor Erv26p. Analyses of ALP chimeras and mutants indicated that Erv26p recognizes sorting information in the lumenal domain of ALP. This lumenal domain sorting signal must be positioned near the inner leaflet of the ER membrane for Erv26p-dependent export. Moreover, only assembled ALP dimers were efficiently recognized by Erv26p while an ALP mutant blocked in dimer assembly failed to exit the ER and was subjected to ER-associated degradation. These results further refine sorting information for ER export of ALP and show that recognition of folded cargo by export receptors contributes to strict ER quality control.

Peculiar properties of DsbA in its export across the Escherichia coli cytoplasmic membrane

Export of DsbA, a protein disulfide bond-introducing enzyme, across the Escherichia coli cytoplasmic membrane was studied with special reference to the effects of various mutations affecting translocation factors. It was noted that both the internalized precursor retaining the signal peptide and the periplasmic mature product fold rapidly into a protease-resistant structure and they exhibited anomalies in sodium dodecyl sulfate-polyacrylamide gel electrophoresis in that the former migrated faster than the latter. The precursor, once accumulated, was not exported posttranslationally. DsbA export depended on the SecY translocon, the SecA ATPase, and Ffh (signal recognition particle), but not on SecB. SecY mutations, such as secY39 and secY205, that severely impair translocation of a number of secretory substrates by interfering with SecA actions only insignificantly impaired the DsbA export. In contrast, secY125, affecting a periplasmic domain and impairing a late step of translocation, exerted strong export inhibition of both classes of proteins. These results suggest that DsbA uses not only the signal recognition particle targeting pathway but also a special route of translocation through the translocon, which is hence suggested to actively discriminate pre-proteins.

Intermolecular contact between globular N-terminal fold and C-terminal domain of ApoA-I stabilizes its lipid-bound conformation: studies employing chemical cross-linking and mass spectrometry

The structure of apoA-I on discoidal high density lipoprotein (HDL) was studied using a combination of chemical cross-linking and mass spectrometry. Recombinant HDL particles containing 145 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and two molecules of apoA-I with a 96-A diameter were treated with the lysine-specific cross-linker, dithiobis(succinimidylpropionate) at varying molar ratios from 2:1 to 200:1. At low molar ratios of dithiobis(succinimidylpropionate) to apoA-I, two products were obtained corresponding to approximately 53 and approximately 80 kDa. At high molar ratios, these two products merged, yielding a product of approximately 59 kDa, close to the theoretical molecular mass of dimeric apoA-I. To identify the intermolecular cross-links giving rise to the two different sized products, bands were excised from the gel, digested with trypsin, and then analyzed by liquid chromatography-electrospray-tandem mass spectrometry. In addition, tandem mass spectrometry of unique cross-links found in the 53- and 80-kDa products suggested that a distinct conformation exists for lipid-bound apoA-I on 96-A recombinant HDL, emphasizing the inherent flexibility and malleability of the N termini and its interaction with its C-terminal domain.

Expression of an anti-CD33 single-chain antibody by Pichia pastoris

CD33 is a cell surface glycoprotein expressed on cells of myelomonocytic lineage, leukaemic cells, but not haematopoietic stem cells. By virtue of its expression pattern, CD33 has become a popular target for new immunotherapeutic approaches to treat acute myeloid leukaemia. The methylotrophic yeast Pichia pastoris strain KM71H was used to produce an anti-CD33 single chain variable fragment (scFv), with the intention of conjugation to a radioisotope, for therapeutic use. To direct secreted expression of the anti-CD33-scFv the alpha-mating factor secretory signal sequence (alpha-MF) was used, with constructs containing a complete (CS) and incomplete (INCS) cleavage site to accommodate the potential outcomes of dibasic endopeptidase, Kex2, and dipeptidyl amino peptidase, Ste13, processing. The anti-CD33-scFv was expressed in BMMY cultures using both constructs, with a final yield of 48 mg/l (CS) and 11 mg/l (INCS). N-terminal sequencing showed that the CS-scFv had not been cleaved by Ste13, leaving amino acids EAEA at the N-terminus. The INCS-scFv construct produced a mixture of 50% authentic scFv and 50% with 11 amino acids from the alpha-MF remaining at the N-terminus. Despite the aberrations in alpha-MF processing, the anti-CD33-scFv's produced from both constructs were found to be functional. Flow cytometry and Biacore analysis demonstrated binding to target antigen CD33 on the surface of human leukaemic cell line HL-60, and to recombinant soluble CD33 respectively.

Functional characterization of an alpha-factor-like Sordaria macrospora peptide pheromone and analysis of its interaction with its cognate receptor in Saccharomyces cerevisiae

The homothallic filamentous ascomycete Sordaria macrospora possesses genes which are thought to encode two pheromone precursors and two seven-transmembrane pheromone receptors. The pheromone precursor genes are termed ppg1 and ppg2. The putative products derived from the gene sequence show structural similarity to the alpha-factor precursors and a-factor precursors of the yeast Saccharomyces cerevisiae. Likewise, sequence similarity has been found between the putative products of the pheromone receptor genes pre2 and pre1 and the S. cerevisiae Ste2p alpha-factor receptor and Ste3p a-factor receptor, respectively. To investigate whether the alpha-factor-like pheromone-receptor pair of S. macrospora is functional, a heterologous yeast assay was used. Our results show that the S. macrospora alpha-factor-like pheromone precursor PPG1 is processed into an active pheromone by yeast MATalpha cells. The S. macrospora PRE2 protein was demonstrated to be a peptide pheromone receptor. In yeast MATa cells lacking the endogenous Ste2p receptor, the S. macrospora PRE2 receptor facilitated all aspects of the pheromone response. Using a synthetic peptide, we can now predict the sequence of one active form of the S. macrospora peptide pheromone. We proved that S. macrospora wild-type strains secrete an active pheromone into the culture medium and that disruption of the ppg1 gene in S. macrospora prevents pheromone production. However, loss of the ppg1 gene does not affect vegetative growth or fertility. Finally, we established the yeast assay as an easy and useful system for analyzing pheromone production in developmental mutants of S. macrospora.

Supplying copper to the cuproenzyme peptidylglycine alpha-amidating monooxygenase

We explored the role of known copper transporters and chaperones in delivering copper to peptidylglycine-alpha-hydroxylating monooxygenase (PHM), a copper-dependent enzyme that functions in the secretory pathway lumen. We examined the roles of yeast Ccc2, a P-type ATPase related to human ATP7A (Menkes disease protein) and ATP7B (Wilson disease protein), as well as yeast Atx1, a cytosolic copper chaperone. We expressed soluble PHMcc (catalytic core) in yeast using the yeast pre-pro-alpha-mating factor leader region to target the enzyme to the secretory pathway. Although the yeast genome encodes no PHM-like enzyme, PHMcc expressed in yeast is at least as active as PHMcc produced by mammalian cells. PHMcc partially co-migrated with a Golgi marker during subcellular fractionation and partially co-localized with Ccc2 based on immunofluorescence. To determine whether production of active PHM was dependent on copper trafficking pathways involving the CCC2 or ATX1 genes, we expressed PHMcc in wild-type, ccc2, and atx1 mutant yeast. Although ccc2 and atx1 mutant yeast produce normal levels of PHMcc protein, it lacks catalytic activity. Addition of exogenous copper yields fully active PHMcc. Similarly, production of active PHM in mouse fibroblasts is impaired in the presence of a mutant ATP7A gene. Although delivery of copper to lumenal cuproproteins like PAM involves ATP7A, lumenal chaperones may not be required.

Roles of cytosolic Hsp70 and Hsp40 molecular chaperones in post-translational translocation of presecretory proteins into the endoplasmic reticulum

Hsp70 molecular chaperones and their co-chaperones work together in various cellular compartments to guide the folding of proteins and to aid the translocation of proteins across membranes. Hsp70s stimulate protein folding by binding exposed hydrophobic sequences thereby preventing irreversible aggregation. Hsp40s stimulate the ATPase activity of Hsp70s and target unfolded proteins to Hsp70s. Genetic and biochemical evidence supports a role for cytosolic Hsp70s and Hsp40s in the post-translational translocation of precursor proteins into endoplasmic reticulum and mitochondria. To gain mechanistic insight, we measured the effects of Saccharomyces cerevisiae Ssa1p (Hsp70) and Ydj1p (Hsp40) on the translocation of histidine-tagged prepro-alpha-factor (ppalphaF6H) into microsomes. Radiolabeled ppalphaF6H was affinity purified from wheat germ translation reactions (or Escherichia coli) to remove endogenous chaperones. We demonstrated that either Ssa1p or Ydj1p stimulates post-translational translocation by preventing ppalphaF6H aggregation. The binding and/or hydrolysis of ATP by Ssa1p were required to maintain the translocation competence of ppalphaF6H. To clarify the contributions of membrane-bound and cytosolic Ydj1p, we compared the efficiency of chaperone-dependent translocation into wild-type and Ydj1p-deficient microsomes. Neither soluble nor membrane-bound Ydj1p was essential for post-translational protein translocation. The ability of Ssa1p, Ydj1p, or both chaperones to restore the translocation competence of aggregated ppalphaF6H was negligible.

Quantitative assessment of enzyme specificity in vivo: P2 recognition by Kex2 protease defined in a genetic system

The specificity of the yeast proprotein-processing Kex2 protease was examined in vivo by using a sensitive, quantitative assay. A truncated prepro-alpha-factor gene encoding an alpha-factor precursor with a single alpha-factor repeat was constructed with restriction sites for cassette mutagenesis flanking the single Kex2 cleavage site (-SLDKR downward arrowEAEA-). All of the 19 substitutions for the Lys (P2) residue in the cleavage site were made. The wild-type and mutant precursors were expressed in a yeast strain lacking the chromosomal genes encoding Kex2 and prepro-alpha-factor. Cleavage of the 20 sites by Kex2, expressed at the wild-type level, was assessed by using a quantitative-mating assay with an effective range greater than six orders of magnitude. All substitutions for Lys at P2 decreased mating, from 2-fold for Arg to >10(6)-fold for Trp. Eviction of the Kex2-encoding plasmid indicated that cleavage of mutant sites by other cellular proteases was not a complicating factor. Mating efficiencies of strains expressing the mutant precursors correlated well with the specificity (kcat/KM) of purified Kex2 for comparable model peptide substrates, validating the in vivo approach as a quantitative method. The results support the conclusion that KM, which is heavily influenced by the nature of the P2 residue, is a major determinant of cleavage efficiency in vivo. P2 preference followed the rank order: Lys > Arg > Thr > Pro > Glu > Ile > Ser > Ala > Asn > Val > Cys > AsP > Gln > Gly > His > Met > Leu > Tyr > Phe > Trp.

A cell-free translation-translocation system reconstituted with subcellular fractions from the wall-less variant fz;sg;os-1V of Neurospora crassa

A translation-translocation system reconstituted with subcellular fractions from the wall-less variant fz;sg;os-1V of Neurospora crassa reproduces in vitro translocation and processing of a secretory protein. The translation extract was isolated from the wall-less variant by gently lysing cells by a freeze-thaw procedure. This method yielded more extract then the method developed previously (R. Addison, J. Biol. Chem. 262: 17031, 1987) as well as reducing microsomal contamination. The microsomal fraction was isolated from lysed cells using a series of discontinuous sucrose gradients. The resultant microsomes were less inhibitory to translation of various transcripts and consisted of a more homogenous mixture of vesicles then microsomes prepared previously. Polyclonal antibodies directed against a polypeptide of approximately 75 kDa from the microsomes were used in indirect-immunofluorescence microscopy. The resultant fluorescent pattern shows a network of tubulo-reticular structures in a juxtanuclear region, which is the pattern expected of the rough endoplasmic reticulum.

Binding of secretory precursor polypeptides to a translocon subcomplex is regulated by BiP

The translocation of a secretory precursor protein across the ER membrane comprises three phases: docking of the precursor at the membrane, insertion into the translocation pore, and exit from the pore into the ER lumen. We demonstrate that Sec62p, Sec71p and Sec72p form a translocon subcomplex that engages secretory precursors at the membrane site of the ER translocation machinery. Binding of a precursor to the subcomplex depends on the presence of an intact signal sequence and occurs only in the absence of ATP. In the presence of ATP, the precursor is released from the subcomplex in a reaction mediated by the lumenal hsp70, BiP. This release reaction, which is specific to BiP and requires interaction between BiP and the DnaJ homolog Sec63p, defines a role for BiP and Sec63p early in the ER translocation process.

Direct detection of recombinant gene expression by two genetically engineered yeasts in soil on the transcriptional and translational levels

The expression of a recombinant gene by yeasts seeded into soil samples was directly measured by analyzing transcripts and gene product occurrences in soil extracts. Two yeast species, Saccharomyces cerevisiae WHL292 and Hansenula polymorpha LR9-Apr4, both engineered by a synthetic gene sequence encoding the mammalian peptide aprotinin, produced and secreted this peptide in batch cultures at concentrations of 90 and 64 ng ml-1, respectively. In S. cerevisiae, the aprotinin gene was located on plasmid p707 and expressed constitutively. H. polymorpha carried the gene chromosomally integrated, and its expression was inducible by methanol. To detect aprotinin transcripts, cells were directly lysed in the soil samples and the crude lysates were hybridized to oligo(dT)-coated magnetized polystyrene beads (Dynabeads). After separation and purification in a magnetic field, aprotinin mRNA was detected by reverse transcriptase PCR with aprotinin gene-specific primers. Transcripts from 10 cells g of soil-1 were sufficient for detection. When 10(7) cells of S. cerevisiae were inoculated into soil, aprotinin mRNA was detectable during the first 4 days. Addition of methanol and a combined nutrient solution was necessary to induce aprotinin gene expression of H. polymorpha in soil. Aprotinin could be detected directly in soil extracts by an indirect enzyme-linked immunosorbent assay with monoclonal aprotinin-specific antibodies. The detection threshold was 45 pg g of soil-1. In presterilized soil inoculated with S. cerevisiae (10(6) CFU g-1), aprotinin accumulated during the first 10 days to 12 ng g of soil-1 and then remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Fungal lipopeptide mating pheromones: a model system for the study of protein prenylation

In a variety of fungal species, mating between haploid cells is initiated by the action of peptide pheromones. The identification and characterization of several fungal pheromones has revealed that they have common structural features classifying them as lipopeptides. In the course of biosynthesis, these pheromones undergo a series of posttranslational processing events prior to export. One common modification is the attachment of an isoprenoid group to the C terminus of the pheromone precursor. Genetic and biochemical investigations of this biosynthetic pathway have led to the elucidation of genes and enzymes which are responsible for isoprenylation of other polypeptides including the nuclear lamins, several vesicular transport proteins, and the oncogene product Ras. The alpha-factor of Saccharomyces cerevisiae serves as a model for studying the biosynthesis, export, and bioactivity of lipopeptide pheromones. In addition to being isoprenylated with a farnesyl group, the alpha-factor is secreted by a novel peptide export pathway utilizing a yeast homolog of the mammalian multidrug resistance P-glycoprotein. The identification of putative lipopeptide-encoding loci within other fungi, including the human immunodeficiency virus-associated opportunistic pathogen Cryptococcus neoformans and the plant pathogen Ustilago maydis, has stimulated much interest in understanding possible roles for pheromones in fungal proliferation and pathogenicity. Knowledge of variations within the processing, export, and receptor-mediated signal transduction pathways associated with different fungal lipopeptide pheromones will continue to provide insights into similar mechanisms which exist in higher eukaryotes.

Interference of humic acids and DNA extracted directly from soil in detection and transformation of recombinant DNA from bacteria and a yeast

A two-step protocol for the extraction and purification of total DNA from soil samples was developed. Crude DNA extracts (100 microliters from 5 g of soil) were contaminated with humic acids at concentrations of 0.7 to 3.3 micrograms/microliters, depending on the type of soil extracted. The coextracted humic acid fraction of a clay silt was similar to a commercially available standard humic acid mixture, as determined by electrophoretic mobility in agarose gels, UV fluorescence, and inhibition assays with DNA-transforming enzymes. Restriction endonucleases were inhibited at humic acid concentrations of 0.5 to 17.2 micrograms/ml for the commercial product and 0.8 to 51.7 micrograms/ml for the coextracted humic acids. DNase I was less susceptible (MIC of standard humic acids, 912 micrograms/ml), and RNase could not be inhibited at all (MIC, > 7.6 mg/ml). High inhibitory susceptibilities for humic acids were observed with Taq polymerase. For three Taq polymerases from different commercial sources, MICs were 0.08 to 0.64 micrograms of the standard humic acids per ml and 0.24 to 0.48 micrograms of the coextracted humic acids per ml. The addition of T4 gene 32 protein increased the MIC for one Taq polymerase to 5.12 micrograms/ml. Humic acids decreased nonradioactive detection in DNA-DNA slot blot hybridizations at amounts of 0.1 micrograms and inhibited transformation of competent Escherichia coli HB101 with a broad-host-range plasmid, pUN1, at concentrations of 100 micrograms/ml. Purification of crude DNA with ion-exchange chromatography resulted in removal of 97% of the initially coextracted humic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Biologically active human and mouse nerve growth factors secreted by the yeast Saccharomyces cerevisiae

Nerve growth factor (NGF) is a trophic agent that is essential for the development and survival of sympathetic and sensory nerves. A chemically-synthesized DNA fragment encoding human NGF (hNGF) and a cDNA encoding mouse NGF (mNGF) were engineered for expression in the yeast, Saccharomyces cerevisiae. Expression and secretion of hNGF and mNGF was attempted under the direction of the yeast PGK promoter and with various leader sequences. Among the leader sequences tested, that of the yeast alpha-factor successfully directed secretion of both hNGF and mNGF that were correctly processed. The content of the recombinant NGF (reNGF) in the culture supernatant was estimated to be 1 microgram/ml. The yeast-produced reNGF was able to bind to NGF receptors in rat pheochromocytoma (PC12) cells as efficiently as the standard mNGF, and partially purified reNGF could induce neurite outgrowth of PC12 cells. Thus, we have demonstrated that biologically active human and mouse reNGF can be produced in yeast cells.

A mutant Kex2 enzyme with a C-terminal HDEL sequence releases correctly folded human insulin-like growth factor-1 from a precursor accumulated in the yeast endoplasmic reticulum

Mutations in the pro region of the yeast DNA hybrid of prepro-alpha-factor and human insulin-like growth factor-1 (IGF-1) cause the accumulation, in the yeast Saccharomyces cerevisiae, of an unglycosylated precursor protein where the pre sequence is missing. The prepro sequence of the prepro-alpha-factor consists of a pre or signal sequence and a proregion which possesses three sites for N-glycosylation. Isolation of a precursor, where the pro region is still linked to IGF-1 through a pair of dibasic amino acid residues, implies that the polypeptide may have translocated into the endoplasmic reticulum (ER) but has not been processed by the Golgi membrane-bound Kex2 endoprotease. However, the lack of any N-glycosylation in the translocated polypeptide is surprising. The mutated pro region, can be processed, in vitro, by treatment with a soluble form of the Kex2 enzyme. It is also possible to release the pro region, in vivo, by coexpressing a mutant Kex2 protease which is partially retained in the ER with the help of the C-terminal tetrapeptide sequence, HDEL. The mature IGF-1, which is secreted from the intracellular pool of precursor proteins, is predominantly an active, monomeric molecule, corroborating observations that early removal of the pro region before folding in the ER helps to prevent aberrant intermolecular disulfide-bond formation in IGF-1. These results have revealed the utility of the ER-retained Kex2 enzyme as a novel in vivo biochemical tool.

Heterologous protein production in yeast

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

Efficient synthesis of secreted murine interleukin-2 by Saccharomyces cerevisiae: influence of 3'-untranslated regions and codon usage

Several expression vectors were compared which directed the synthesis of secreted murine interleukin-2 (mIL2) in the culture medium of Saccharomyces cerevisiae. We used the prepro-sequence of the alpha 1 mating-factor precursor as a secretion signal in S. cerevisiae in combination with different promoters. The yield of mature mIL2 was significantly improved by deleting the major part of the 3'-untranslated region (UTR). In Northern-blotting experiments we showed that a destabilizing sequence present in the 3' UTR might be responsible for rapid degradation of the mIL2 mRNA. The highest expression (about 10 micrograms/ml) was obtained under control of the GAL1 promoter in an S. cerevisiae strain where the regulatory GAL4 gene was overexpressed. No difference in expression level was observed in a construct wherein twelve consecutive codons were replaced by optimal codons for S. cerevisiae.

High-level expression of foreign genes in Hansenula polymorpha

The methylotrophic yeast Hansenula polymorpha belongs to a limited number of non-Saccharomyces yeast species used as hosts for heterologous gene expression. It has successfully been applied for the production of hormones, antigens and enzymes. The system excells by mitotically stable recombinant strains, high productivity and faithful processing of the produced polypeptides. The favourable characteristics of this microorganism for protein production at an industrial scale are described in the following article focusing on some recent representative examples.

Protein transport and compartmentation in yeast

Many newly synthesized proteins must be translocated across one or more membranes to reach their destination in the individual organelles or membrane systems. Translocation, mostly requiring an energy source, a signal on the protein itself, loose conformation of the protein and the presence of cytosolic and/or membrane receptor-like proteins, is often accompanied by covalent modifications of transported proteins. In this review I discuss these aspects of protein transport via the classical secretory pathway and/or special translocation mechanisms in the unicellular eukaryotic organism Saccharomyces cerevisiae.

Nucleotide sequence of the exo-1,3-beta-glucanase-encoding gene, EXG1, of the yeast Saccharomyces cerevisiae

The nucleotide (nt) sequence of the Saccharomyces cerevisiae gene (EXG1) encoding extracellular exo-1,3-beta-glucanases (EXG) I and II was determined. An open reading frame of 1344 bp codes for a 448-amino acid (aa) polypeptide, with a calculated Mr of 51,307, which contains two potential N-glycosylation sites. The EXG1 DNA hybridizes to a 1.7-kb transcript whose 5' end maps to a position 98 bp upstream from the site of initiation of protein synthesis. Comparison of the N-terminal aa sequence deduced from the nt sequence with that of the purified EXGII revealed the existence of an extra 40-aa peptide in the precursor protein containing a Lys-Arg peptidase-processing site at the junction with the mature, extracellular form. The N-terminal region of the putative precursor is a very hydrophobic segment with structural features resembling those of signal peptides of secreted proteins. The Mr of the mature EXG polypeptide deduced from the nt sequence is 46,385. The 5'- and 3'-flanking regions of the EXG1 gene have structural features in common with other yeast genes.

Glycosylation and structure of the yeast MF alpha 1 alpha-factor precursor is important for efficient transport through the secretory pathway

The MF alpha 1 gene encodes a precursor, prepro-alpha-factor, that undergoes several proteolytic processing steps within the classical secretory pathway to produce the mature peptide pheromone, alpha-factor. To investigate the role of structural features of the MF alpha 1 precursor in alpha-factor production, we analyzed the effect of mf alpha 1 mutations that alter precursor structure in a number of ways. These mutations resulted in decreased alpha-factor secretion and intracellular accumulation of pro-alpha-factor. With the exception of the mutant lacking all three N glycosylation sites, the pro-alpha-factor forms that accumulated were core glycosylated but had not yet undergone the addition of outer chain carbohydrate. The delay, therefore, occurred at a step prior to the first proteolytic processing step involved in maturation of the precursor and was probably due to inefficient endoplasmic reticulum-to-Golgi transport. Elimination of all three N-glycosylation sites caused a delay in disappearance of intracellular precursor, and alpha-factor secretion was also slowed. These data indicate that N glycosylation is important but not essential for transport of the precursor through the secretory pathway. The decreased alpha-factor secretion and increased precursor accumulation seen with many different structural changes of pro-alpha-factor indicate that the secretory pathway is extremely sensitive to changes in precursor structure. This sensitivity could cause inefficient secretion of heterologous proteins and hybrids between MF alpha 1 and heterologous proteins in yeast cells.