The prepro-peptide of Mucor rennin directs the secretion of human growth hormone by Saccharomyces cerevisiae

Saccharomyces cerevisiae in directed evolution: An efficient tool to improve enzymes

Over the past 20 years, directed evolution has been seen to be the most reliable approach to protein engineering. Emulating the natural selection algorithm, ad hoc enzymes with novel features can be tailor-made for practical purposes through iterative rounds of random mutagenesis, DNA recombination and screening. Of the heterologous hosts used in laboratory evolution experiments, the budding yeast Saccharomyces cerevisiae has become the best choice to express eukaryotic proteins with improved properties. S. cerevisiae not only allows mutant enzymes to be secreted but also, it permits a wide range of genetic manipulations to be employed, ranging from in vivo cloning to the creation of greater molecular diversity, thanks to its efficient DNA recombination apparatus. Here, we summarize some successful examples of the use of the S. cerevisiae machinery to accelerate artificial evolution, complementing the traditional in vitro methods to generate tailor-made enzymes.

A molecular approach to optimize hIFN α2b expression and secretion in Yarrowia lipolytica

In this work, we investigated the effect of codon bias and consensus sequence (CACA) at the translation initiation site on the expression level of heterologous proteins in Yarrowia lipolytica; human interferon alpha 2b (hIFN-α2b) was studied as an example. A codon optimized hIFN-α2b gene was synthesized according to the frequency of codon usage in Y. lipolytica. Both wild-type (IFN-wt) and optimized hIFN-α2b (IFN-op) genes were expressed under the control of a strong inducible promoter acyl-co-enzyme A oxidase (POX2). Protein secretion was directed by the targeting sequence of the extracellular lipase (LIP2): pre-proLIP2. Codon optimization increased protein production by 11-fold, whereas the insertion of CACA sequence upstream of the initiation codon of IFN-op construct resulted in 16.5-fold increase of the expression level; this indicates that translational efficiency plays an important part in the increase of hIFN-α2b production level. The replacement of the pre-proLIP2 signal secretion with the LIP2 pre-region sequence followed by the X-Ala/X-Pro stretch but without the pro-region also increased the secretion of the target protein by twofold, suggesting therefore that the LIP2 pro-region is not necessary for extracellular secretion of small heterologous proteins in Yarrowia lipolytica.

Construction of a protease-deficient strain set for the fission yeast Schizosaccharomyces pombe, useful for effective production of protease-sensitive heterologous proteins

One of the major problems hindering effective production and purification of heterologous proteins from the fission yeast Schizosaccharomyces pombe is proteolytic degradation of the recombinant gene products by host-specific proteases. As an initial solution to this problem, we constructed a protease-deficient disruptant set by respective disruption of 52 Sz. pombe protease genes. Functional screening of the resultant set was performed by observing secretory production of a proteolytically sensitive model protein, human growth hormone (hGH). The results indicated that some of the resultant disruptants were effective in reducing hGH degradation, as observed during the hGH expression procedure and mainly as a result of unknown serine- and/or cysteine-type proteases in the culture medium. These findings also demonstrated that construction of a protease-deficient strain set is not only useful for practical application in protein production, but also for functional screening, specification and modification of proteases in Sz. pombe, where further investigations of proteolytic processes and improvement through multiple gene manipulations are required.

Thermophilic fungi: their physiology and enzymes

Thermophilic fungi are a small assemblage in mycota that have a minimum temperature of growth at or above 20 degrees C and a maximum temperature of growth extending up to 60 to 62 degrees C. As the only representatives of eukaryotic organisms that can grow at temperatures above 45 degrees C, the thermophilic fungi are valuable experimental systems for investigations of mechanisms that allow growth at moderately high temperature yet limit their growth beyond 60 to 62 degrees C. Although widespread in terrestrial habitats, they have remained underexplored compared to thermophilic species of eubacteria and archaea. However, thermophilic fungi are potential sources of enzymes with scientific and commercial interests. This review, for the first time, compiles information on the physiology and enzymes of thermophilic fungi. Thermophilic fungi can be grown in minimal media with metabolic rates and growth yields comparable to those of mesophilic fungi. Studies of their growth kinetics, respiration, mixed-substrate utilization, nutrient uptake, and protein breakdown rate have provided some basic information not only on thermophilic fungi but also on filamentous fungi in general. Some species have the ability to grow at ambient temperatures if cultures are initiated with germinated spores or mycelial inoculum or if a nutritionally rich medium is used. Thermophilic fungi have a powerful ability to degrade polysaccharide constituents of biomass. The properties of their enzymes show differences not only among species but also among strains of the same species. Their extracellular enzymes display temperature optima for activity that are close to or above the optimum temperature for the growth of organism and, in general, are more heat stable than those of the mesophilic fungi. Some extracellular enzymes from thermophilic fungi are being produced commercially, and a few others have commercial prospects. Genes of thermophilic fungi encoding lipase, protease, xylanase, and cellulase have been cloned and overexpressed in heterologous fungi, and pure crystalline proteins have been obtained for elucidation of the mechanisms of their intrinsic thermostability and catalysis. By contrast, the thermal stability of the few intracellular enzymes that have been purified is comparable to or, in some cases, lower than that of enzymes from the mesophilic fungi. Although rigorous data are lacking, it appears that eukaryotic thermophily involves several mechanisms of stabilization of enzymes or optimization of their activity, with different mechanisms operating for different enzymes.

Secretion by Saccharomyces cerevisiae of rat apolipoprotein E as a fusion to Mucor rennin

As the first step for production of rat apolipoprotein E (rApoE) in Saccharomyces cerevisiae, the rApoE cDNA was cloned and its nucleotide sequence was determined. When the intact rApoE gene including the presequence-encoding region was expressed under the control of the yeast GAL7 promoter, no protein immunoreactive with anti-rApoE antibody was detected either in the culture medium or inside the cells. For the purpose of the extracellular production of rApoE, three fusion genes were constructed in which the mature rApoE-encoding sequence was connected after the pre, prepro, and whole regions of the gene encoding a fungal aspartic proteinase, Mucor pusillus rennin (MPP), since MPP is efficiently secreted from recombinant S. cerevisiae containing the MPP gene. When these three fusion genes were expressed under the control of the GAL7 promoter, only one, encoding the mature rApoE connected to the whole MPP sequence, directed efficient secretion of the fused protein. The maximum yield of the fused protein secreted into the medium reached 11.8 mg/l and the calculated rApoE part was 5.3 mg in the fused protein. The excreted fusion protein was glycosylated at the original two sites in the MPP part. The fused protein was gradually degraded in the medium probably by proteases of the host cell, because no such degradation occurred in a yeast pep4mutant strain.

Optimization of the expression system using galactose-inducible promoter for the production of anticoagulant hirudin in Saccharomyces cerevisiae

We have tried to optimize the galactose-inducible gene expression system for the overproduction of the potent thrombin-specific inhibitor, hirudin, in a genetically engineered yeast, Saccharomyces cerevisiae. The expression and secretion of hirudin were directed by the galactose-inducible promoter, GAL10, and the mating factor alpha pre-pro leader sequence. The initial hirudin expression level in shake-flask culture was 2.3 mg 1-1. Modification of the expression vector and optimization of culture conditions, including the induction conditions, improved the level of hirudin gene expression and secretion into the culture supernatant more than 20-fold (50 mg 1-1) in a 4-1 scale batch cultivation. The expression and secretion level of hirudin seemed to be partially dependent on cell growth when galactose was used as a carbon source. Overexpression of the transcriptional activator, GAL4, appeared to have only negative effects on the expression of the hirudin gene and lacZ directed by the GAL10 promoter in the strain used in this study, unlike the previously reported examples. The complex medium containing yeast extract used for the increase of the cell mass and hirudin level did not show any detrimental effect on plasmid stability and did not complicate the downstream purification of hirudin from the culture supernatant. Moreover, the complex medium could greatly improve the hirudin productivity and reduce the degradation of hirudin produced in the culture supernatants.

Mutation of a fungal aspartic proteinase, Mucor pusillus rennin, to decrease thermostability for use as a milk coagulant

Mutagenesis of a fungus Mucor pusillus, a producer of an aspartic proteinase named Mucor pusillus rennin (MPR), was performed to obtain the mutated enzymes with decreased thermostability, which is desirable for practical use of the enzyme as a milk coagulant for cheese manufacturing. A fungal mutant strain which produced the mutant enzyme with distinctly reduced thermostability was isolated. Two different mutant alleles of the mpr gene, one with a single amino acid exchange of Ala101 for Thr and the other of Gly186 for Asp, were cloned out of this mutant strain. The mutated mpr genes were expressed in Saccharomyces cerevisiae under the control of the yeast GAL7 promoter to produce the active enzymes in extracellular medium. Both of the mutations, especially Gly186Asp, were confirmed to cause a marked decrease in thermostability of the enzyme. All mutants possessing exchanges of Gly186 for various amino acids by site-directed mutagenesis showed a decrease in thermostability, indicating involvement of this residue to maintain a conformation of the enzyme. A double mutant having the both exchanges, Ala101Thr and Gly186Asp, in a single molecule showed the lowest thermostability without decrease in the enzymatic activity as well as the relative ratio of clotting to proteolytic activity.

Yeast extracellular proteases

Many species of yeast secrete significant amounts of protease(s). In this article, results of numerous surveys of yeast extracellular protease production have been compiled and inconsistencies in the data and limitations of the methodology have been examined. Regulation, purification, characterization, and processing of yeast extracellular proteases are reviewed. Results obtained from the sequences of cloned genes, especially the Saccharomyces cerevisiae Bar protease, the Candida albicans acid protease, and the Yarrowia lipolytica alkaline protease, have been emphasized. Biotechnological applications and the medical relevance of yeast extracellular proteases are covered. Yeast extracellular proteases have potential in beer and wine stabilization, and they probably contribute to pathogenicity of Candida spp. Yeast extracellular protease genes also provide secretion and processing signals for yeast expression systems designed for secretion of heterologous proteins. Coverage of the secretion of foreign proteases such as prochymosin, urokinase, and tissue plasminogen activator by yeast in included.

The secretion leader of Mucor pusillus rennin which possesses an artificial Lys-Arg sequence directs the secretion of mature human growth hormone by Saccharomyces cerevisiae

The prepro-peptide of fungal aspartic proteinase, Mucor pusillus rennin, is useful as a secretion leader for efficient secretion of human growth hormone (HGH) from Saccharomyces cerevisiae. For secretion by yeast cells of HGH with the same NH2 terminus as native HGH, an artificial Lys-Arg linker, which is one of the potential KEX2 recognition sequences, was introduced at the junction between the M. pusillus rennin secretion leader and mature HGH. The HGH directed by this construction was the same size as native HGH, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and amino acid sequencing of its NH2 terminus revealed that the secretion leader peptide was removed correctly at the COOH-terminal side of the Lys-Arg linker. On the other hand, when the same plasmid was expressed in a kex2 mutant strain, unprocessed HGH of a higher molecular weight was secreted, indicating that no proteolytic cleavage at the Lys-Arg site occurred. These results clearly showed that the leader peptide with the Lys-Arg linker was recognized and specifically cleaved by the yeast KEX2 protease. The mature HGH purified from yeast culture medium was indistinguishable from native HGH in biological activity, determined by the adipocyte conversion assay, and in secondary structure, determined by circular dichroism spectroscopy.

Isolation and characterization of human pro-urokinase and its mutants accumulated within the yeast secretory pathway

Human pro-urokinase (pro-UK) and two pro-UK deletion mutants, one lacking the epidermal growth factor(EGF)-like domain, and the other lacking both the EGF-like domain and the kringle domain, were produced in Saccharomyces cerevisiae. This was done using the yeast GAL7 promoter and the prepeptide sequence of a fungal aspartic proteinase, Mucor pusillus rennin (MPR). Although biologically active and heavily glycosylated pro-UKs were secreted into the culture medium, the amounts were extremely small. On the other hand, large amounts of pro-UKs of a single-chain form were accumulated inside the cells, exceeding 3-4% of total cellular proteins. The intracellular pro-UKs were N-glycosylated, probably with a single core carbohydrate unit, and amino acid sequencing of their N termini revealed that the secretion signal of MPR was correctly processed. Biologically active pro-UKs were recovered in high yields by means of solubilization with 4.5 M guanidine.HCl and subsequent dialysis for refolding. The refolded yeast pro-UK was indistinguishable from human kidney-derived pro-UK in terms of specific enzymatic activity and its secondary structure, as determined by circular dichroism spectroscopy.