Expression of the human salivary alpha-amylase gene in yeast and characterization of the secreted protein

Purification and high-resolution top-down mass spectrometric characterization of human salivary α-amylase

Human salivary α-amylase (HSAMY) is a major component of salivary secretions, possessing multiple important biological functions. Here we have established three methods to purify HSAMY in human saliva for comprehensive characterization of HSAMY by high-resolution top-down mass spectrometry (MS). Among the three purification methods, the affinity method based on the enzyme-substrate specific interaction between amylase and glycogen is preferred, providing the highest purity HSAMY with high reproducibility. Subsequently, we employed Fourier transform ion cyclotron resonance MS to analyze the purified HSAMY. The predominant form of α-amylase purified from saliva of various races and genders is nonglycosylated with the same molecular weight of 55,881.2, which is 1885.8 lower than the calculated value based on the DNA-predicted sequence. High-resolution MS revealed the truncation of the first 15 N-terminal amino acids (-1858.96) and the subsequent formation of pyroglutamic acid at the new N-terminus Gln (-17.03). More importantly, five disulfide bonds in HSAMY were identified (-10.08) and effectively localized by tandem MS in conjunction with complete and partial reduction by tris (2-carboxyethyl) phosphine. Overall, this study demonstrates that top-down MS combined with affinity purification and partial reduction is a powerful method for rapid purification and complete characterization of large proteins with complex and overlapping disulfide bond patterns.

Expression and Secretion of a Cellulomonas fimi Exoglucanase in Saccharomyces cerevisiae

We used the yeast MEL1 gene for secreted alpha-galactosidase to construct cartridges for the regulated expression of foreign proteins from Saccharomyces cerevisiae. The gene for a Cellulomonas fimi beta-1,4-exoglucanase was inserted into one cartridge to create a fusion of the alpha-galactosidase signal peptide to the exoglucanase. Yeast transformed with plasmids containing this construction produced active extracellular exoglucanase when grown under conditions appropriate to MEL1 promoter function. The cells also produced active intracellular enzyme. The secreted exoglucanase was N-glycosylated and was produced continuously during culture growth. It hydrolyzed xylan, carboxymethyl cellulose, 4-methylumbelliferyl-beta-d-cellobiose, and p-nitrophenyl-beta-d-cellobiose. A comparison of the recombinant S. cerevisiae enzyme with the native C. fimi enzyme showed the yeast version to have an identical K(m) and pH optimum but to be more thermostable.

Engineering of polyploid Saccharomyces cerevisiae for secretion of large amounts of fungal glucoamylase

We engineered Saccharomyces cerevisiae cells that produce large amounts of fungal glucoamylase (GAI) from Aspergillus awamori var. kawachi. To do this, we used the delta-sequence-mediated integration vector system and the heat-induced endomitotic diploidization method. delta-Sequence-mediated integration is known to occur mainly in a particular chromosome, and the copy number of the integration is variable. In order to construct transformants carrying the GAI gene on several chromosomes, haploid cells carrying the GAI gene on different chromosomes were crossed with each other. The cells were then allowed to form spores, which was followed by dissection. Haploid cells containing GAI genes on multiple chromosomes were obtained in this way. One such haploid cell contained the GAI gene on five chromosomes and exhibited the highest GAI activity (5.93 U/ml), which was about sixfold higher than the activity of a cell containing one gene on a single chromosome. Furthermore, we performed heat-induced endomitotic diploidization for haploid transformants to obtain polyploid mater cells carrying multiple GAI genes. The copy number of the GAI gene increased in proportion to the ploidy level, and larger amounts of GAI were secreted.

Expression, characterization, and biochemical properties of recombinant human salivary amylase

Human salivary amylase, a major component of human salivary secretions, possesses multiple functions in the oral cavity. It is the only enzyme in saliva capable of degrading oligosaccharides, which are used by the oral microflora for nutritional purposes. In order to understand its role in disease processes such as caries, we have undertaken the structure-function analyses of amylase. In this regard, the nonglycosylated human salivary amylase was expressed in a baculovirus expression system. The native and the recombinant amylases exhibit similar biochemical as well as biophysical properties. Unlike recombinant human pancreatic amylase, recombinant human salivary amylase is not glycosylated when expressed in a baculovirus system as determined from the crystal structure determination of the recombinant enzyme. Therefore, this system is suitable for further structure-function work without resorting to enzymatic removal of the carbohydrate chain. Details of the expression, purification, and biophysical properties will be presented.

Differential glycosylation produces heterogeneity in elevated esterases associated with insecticide resistance in the brown planthopper Nilaparvata lugens Stål

The major insecticide resistance mechanism in the brown planthopper Nilaparvata lugens involves overproduction of esterases. Esterases purified from a resistant strain appeared as a ladder of bands on isoelectric focussing (IEF) gels from pI 4.7 to 5.0. Two-dimensional electrophoresis showed that isozymes ranged in size from 66 to 68 kDa with those of lower pI being apparently smaller. All isozymes detected by two-dimensional electrophoresis were glycosylated. N-glycosidase A reduced the number of isozymes on IEF to two, with increased pI and an increased molecular weight of 69 kDa. No O-linked glycans were detected. Deglycosylation had no effect on esterase activity, hence glycosylation is not involved in active site conformation. As N-glycosidase F completely deglycosylated the esterases, none of the glycans has an alpha1,3-bound core fucose. Reactivity with the lectins GNA, MAA and DSA, combined with differential cleavage of N-linked glycans with endoglycosidases F1 and F2, indicated that terminally linked mannose is present in high mannose and/or hybrid type glycans and that terminally linked sialic acid and galactose-beta(1-4)-N-acetylglucosamine are present in biantennary complexes. Neuraminidase treatment had the same effect on pI of isozymes as complete deglycosylation. Therefore, the majority of the heterogeneity of elevated esterases on IEF is due to differential attachment of sialic acid to glycans of the two proteins.

Asparagine as a nitrogen source for improving the secretion of mouse alpha-amylase in Saccharomyces cerevisiae protease A-deficient strains

A modified chemically defined medium was achieved by using asparagine as a nitrogen source to increase the production of secreted mouse alpha-amylase in several Saccharomyces cerevisiae protease A-deficient (pep4) strains. The specific productivity (quantity) and the 53 kDa non-glycosylated active form (quality) of mouse salivary alpha-amylase in liquid medium containing asparagine was remarkably improved compared to media containing other nitrogen sources, including ammonium sulphate, glutamic acid, arginine, casamino acids, yeast extract and peptone. Similar improvement was also observed on starch solid agar regarding the clarity and size of the halo zone formed by alpha-amylase activity. Compared with ammonium sulphate, advantages of using asparagine as the nitrogen source in liquid or solid medium included increasing the cell mass of test strains, recovering the viability of protease-deficient strains to levels similar to the wild-type strain, and increasing the copy number of the mouse alpha-amylase expression vector in test strains. In turn, these advantages apparently contributed to the increase of secretion of mouse alpha-amylase in several test strains and especially in the protease A-deficient strains. In addition to demonstrating the use of modified chemically defined medium to improve the quality and quantity of secreted mouse alpha-amylase, this study also provides a new strategy to improve the secretion of heterologous proteins in protease A deficient strains.

The construction of a stable starch-fermenting yeast strain using genetic engineering and rare-mating

To develop a yeast strain that is able to produce ethanol directly from starch, alpha-amylase cDNA (originated from mouse salivary glands) was introduced into the hyploid Saccharomyces diastiticus cells secreting glucoamylase by using a linearized integrating vector. The integrating vector contains a LEU2 gene and the inside of the LEU2 gene was cut by KpnI to make the linearized vector. One of the transformants exhibited 100% mitotic stability after 100 generations of cell multiplication. To improve its ethanol-fermentability, the haploid transformant was rare-mated with a polyploid industrial strain having no amylase activity. The resulting hybrid RH51 produced 7.5 (w/v) ethanol directly from 20% (w/v) soluble starch and its mitotic stability was 100% at the end of fermentation.

Acid proteinase secreted by Candida tropicalis: functional analysis of preproregion cleavages in C. tropicalis and Saccharomyces cerevisiae

The 40 kDa secreted aspartyl proteinase (Sapt1) of Candida tropicalis is a pepsin-like enzyme encoded by the SAPT1 gene. According to the deduced amino acid sequence. Sapt1 has a putative preproregion of 60 amino acids preceding the mature enzyme. Maturation and processing of Sapt1 was analysed in C. tropicalis and Saccharomyces cerevisiae strains expressing wild-type or mutated forms of SAPT1. In S. cerevisiae, the glycosylated 46 kDa proenzyme was converted to the mature 40 kDa form of Sapt1 by KEX2-dependent proteolytic cleavage following the Lys59-Arg60 sequence. The replacement of Lys59-Arg60 by Lys59-Gly60 revealed that the precursor can be processed by an autocatalytic cleavage. This alternative processing pathway to produce mature Sapt1 is less efficient than the Kex2-mediated pathway. Finally, it was shown that in C. tropicalis and S. cerevisiae the removal of the proregion was a prerequisite for the secretion of Sapt1.

alpha-Amylase as a marker for evaluating the stability of recombinant yeast in long-term cultivation

We describe here a simple method for analyzing the stability of recombinant yeast. The mouse salivary alpha-amylase gene was used in this system as a marker since the stability of recombinant yeast can be detected easily by a halo zone-for-ming assay on starch-supplemented plates. We used this method to evaluate the stability of recombinant yeast harbouring a 2 mu directed episomal plasmid and of yeast harbouring the r-DNA directed integrative vector. Our results demonstrated clearly that the alpha-amylase gene was a convenient and reliable marker for evaluating the stability of recombinant yeasts in long-term cultivation.

Conversion of starch to ethanol in a recombinant Saccharomyces cerevisiae strain expressing rice alpha-amylase from a novel Pichia pastoris alcohol oxidase promoter

A recombinant Saccharomyces cerevisiae, expressing and secreting rice alpha-amylase, converts starch to ethanol. The rice alpha-amylase gene (OS103) was placed under the transcriptional control of the promoter from a newly described Pichia pastoris alcohol oxidase genomic clone. The nucleotide sequences of ZZA1 and other methanol-regulated promoters were analyzed. A highly conserved sequence (TTG-N3-GCTTCCAA-N5-TGGT) was found in the 5' flanking regions of alcohol oxidase, methanol oxidase, and dihydroxyacetone synthase genes in Pichia pastoris, Hansenula polymorpha, and Candida boidinii S2. The yeast strain containing the ZZA1-OS103 fusion secreted biologically active enzyme into the culture media while fermenting soluble starch.

Variation in gene copy number and polymorphism of the human salivary amylase isoenzyme system in Caucasians

The polymorphic patterns of human salivary amylase of a large number of individuals of Caucasian origin were determined by using isoelectric focusing and polyacrylamide gel electrophoresis. Nine different salivary amylase protein variants were found; three of them are recorded for the first time and their heredity is shown. Some of the variants are encoded by haplotypes expressing three allozymes. Most variants display low frequencies. Analysis of the relative intensities of variant-specific isozyme bands, combined with segregation analysis, show that extensive quantitative variation is present in the population. The numbers of salivary amylase genes in some families showing quantitative variation at the protein level have been estimated by the polymerase chain reaction. We present evidence that quantitative variations in amylase protein patterns do not always reflect variations in gene copy number but that other mechanisms are also involved.

Electrophoretic characterization of posttranslational modifications of human parotid salivary alpha-amylase

Human salivary alpha-amylase displays multiple bands upon native polyacrylamide gel electrophoresis. In fresh saliva, due to posttranslational modifications, a pattern of 5-6 isozymes is observed. The isozymes are designated 1-6, in the order of increasing anodal mobility. As a result of the development of a rapid and sensitive electrophoresis system, with markedly higher resolution than previously reported, we concluded that a previously proposed model (Karn et al., Biochem. Genet. 1973, 10, 341-350) is inadequate to explain the origin of the various bands. We propose an alternative model that fits in with our new and previously made observations. According to this model, band 2 is the primary gene product and band 1 is its glycosylated counterpart--with only one neutral oligosaccharide present on each molecule. Band 3 originates from band 1 by the transialidase-catalyzed incorporation of sialic acid into the biantennary chain. Bands 4 and 6 originate from bands 2 and 4, respectively, by deamidation; band 5 is the deamidation product of amylase with an acidic oligosaccharide (band 3). Only a minor part of band 3 consists of the deamidation product of band 1. Peptide Asn-Gly-Ser (residues 427-429) is the most probable candidate for glycosylation; literature data suggests that deamidation occurs in the stretch Glu-Asn-Gly-Lys-Asp (residues 364-368) and Asn-Gly-Asn-Cys (residues 474-477). Both glycosylation and deamidation might play a role in the clearance of amylase from the systemic circulation. The electrophoresis system described is a powerful tool to determine amylase isozyme distributions in health and disease, especially for the screening of alterations seen in ectopically produced amylase.

Amylase synthesis as a simple model system for translation and hybrid arrest in Xenopus oocytes

A human alpha-amylase-encoding cDNA has been cloned in a transcription vector. When messenger RNA (mRNA) made in vitro from this construct was injected into Xenopus oocytes, amylase (AMY) activity was detected both in oocyte homogenates and in the incubation medium, indicating that the oocyte machinery correctly translated and processed the protein. Because AMY activity is easy to detect with a blue-starch assay, this expression system was used to determine the parameters of antisense oligodeoxyribonucleotide (oligo) inhibition of translation in the oocytes. Unique oligos complementary to the AMY mRNA sequence were effective in arresting translation, at approximately stoichiometric levels. Mixed oligos also inhibited translation, at levels that suggest that some mismatches may be tolerated in the formation of DNA-RNA hybrids. The AMY system provides a convenient probe of oocyte protein synthesis and processing machinery and can serve as a control substrate in investigations of other mRNAs.

Expression and secretion of rice alpha-amylase by Saccharomyces cerevisiae

We report the high level expression and secretion of rice alpha-amylase isozyme by Saccharomyces cerevisiae. Transcription of this gene was under control of the yeast enolase promoter. The synthesized protein had an approximate molecular size of 45 kDa and a pI of approx 4.7 to 5.0. The rice alpha-amylase signal peptide was recognized and efficiently processed by yeast and the active, glycosylated enzyme was secreted into the culture media. This enzyme was purified to homogeneity by affinity chromatography and its enzymatic properties were characterized. The Km and Vmax were found to be similar to those of alpha-amylases from other organisms. The high level of secretion observed in these studies may be due to the unique features of the rice signal peptide and/or to the glycosylation of the recombinant enzyme.

Identification of a novel alpha-amylase by expression of a newly cloned human amy3 cDNA in yeast

A novel amylase gene (amy3) that differs in nucleotide sequence from salivary amylase gene (amy1) and pancreatic amylase gene (amy2) has been described [Tomita et al., Gene 76 (1989) 11-18], but whether this gene can ever code for an active enzyme has not been shown. We prepared cDNA of this gene from an mRNA obtained from lung carcinoid tissue, and expressed it in Saccharomyces cerevisiae under the control of an acid phosphatase promoter. The product was secreted into culture media, and showed enzymatic activity, demonstrating that this novel alpha-amylase gene (amy3) can code for a functional isozyme. We purified this enzyme, and compared its biological properties with those of salivary and pancreatic human amylases similarly expressed in yeast. We observed that the novel amylase isozyme is more heat-sensitive than others, and that its substrate specificity is different from the other two isozymes.

Production and secretion of porcine urokinase in Saccharomyces cerevisiae: characterization of the secreted gene product

The properties of porcine urokinase plasminogen activator (u-PA), produced and secreted by Saccharomyces cerevisiae, were studied to evaluate processing of the enzyme by yeast. Porcine u-PA cDNA was positioned behind the triosephosphate isomerase promoter and the yeast alpha-mating factor secretion signal sequences in a yeast expression vector, pZV125. Greater than 99% of the secreted PA activity was found to be single chain (pro-urokinase). The secreted gene product could be converted to two-chain (tc) with plasmin and then purified to homogeneity on benzamidine sepharose. Plasmin cleavage resulted in the formation of high Mr (HMW) and low Mr moieties representing HMW tc and free catalytic domain, respectively, as detected by N-terminal amino acid sequence analysis. Approximately 60-70% of the secreted activity was found to be associated with hyperglycosylated fractions from G-75 sizing columns. Approximately 30% of the total activity was secreted into the culture medium, where levels of activity approached 200 I.U./ml.

The conformation of mature human alpha-amylase conditions its secretion from yeast

The yeast Saccharomyces cerevisiae expresses the cloned cDNA (Amy) encoding human salivary alpha-amylase (Amy) under control of the yeast PHO5 promoter, and secretes the active enzyme into the culture medium. Two approaches were utilized to define the moiety of Amy, which is required for proper secretion and glycosylation. In one approach, chimeras were constructed with a variety of secretion signal sequences (yeast mating factor precursor sequence, yeast acid phosphatase signal sequence and human gastrin signal sequence) fused to the secretion signal-deleted Amy cDNA. The other approach involved analysis of a set of deletion series and a set of point mutations in the Amy-encoding region. The results showed that heterologous signal sequences were sufficient for proper secretion in yeast, irrespective of the insertion of some extra amino acids. In most cases, enzymes with deletions and Cys-465 substitution were not secreted, even though they had complete secretion signal sequences. Instead, they accumulated in the cell in a glycosylated form. Thus, proper secretion seems to require an appropriate conformation in the polypeptide moiety to be secreted.

A full length cDNA clone for the alkaline protease from Aspergillus oryzae: structural analysis and expression in Saccharomyces cerevisiae

We have cloned and determined the nucleotide sequence of a cDNA fragment for the entire coding region of the alkaline protease (Alp) from a filamentous ascomycete Aspergillus oryzae. According to the deduced amino acid sequence, Alp has a putative prepro region of 121 amino acids preceding the mature region, which consists of 282 amino acids. A consensus sequence of a signal peptide consisting of 21 amino acids is found at the N-terminus of the prepro region. The primary structure of the mature region shares extensive homology (29%-44%) with those of subtilisin families, and the three residues (Asp 32, His 64 and Ser 221 in subtilisin BPN') composing the active site are preserved. The entire cDNA, coding for prepro Alp, when introduced into the yeast Saccharomyces cerevisiae, directed the secretion of enzymatically active Alp into the culture medium, with its N-terminus and specific activity identical to native Aspergillus Alp.

Synthesis and secretion of bacterial alpha-amylase by the yeast Saccharomyces cerevisiae

Alpha-amylase from Bacillus amyloliquefaciens, synthesized in yeast Saccharomyces cerevisiae without substitution of the signal sequence, is efficiently secreted from yeast cells: 60-70% of the overall amount of the enzyme is found in the culture fluid. In contrast to many yeast secretory proteins, which accumulate in the periplasmic space and in the cell wall, intracellular alpha-amylase is localized mainly in the cytoplasm. Obviously, transfer across the cell wall is not a rate-limiting step in alpha-amylase export from the cell. The glycosylated forms of proteins are predominantly found both inside the cell and in the culture medium.

A novel type of human alpha-amylase produced in lung carcinoid tumor

A novel type of alpha-amylase was detected in a lung carcinoid tissue after surveying the cDNA library constructed from this tumor mRNA. Nucleotide sequence analysis showed that the amylase expressed in this carcinoid tumor has 13 and 6 amino acid substitutions when compared with salivary amylase (Amy1) and pancreatic amylase (Amy2), respectively. The nucleotide sequence homologies of cDNAs between this carcinoid amylase and amy1, amy2 are 97.5% and 98.2%, respectively. The nucleotide sequence comparison strongly suggests that this new amylase is the product of the amy3 gene that has been detected in human genome [Emi et al., Gene 62 (1988) 229-235]

Secretion of the proenzyme and active bovine pancreatic phospholipase A2 enzyme by Saccharomyces cerevisiae: design and use of a synthetic gene

We have developed a phospholipase A2(PLA2)-producing system using Saccharomyces cerevisiae. A 456-bp synthetic DNA fragment was constructed encoding bovine pancreatic phospholipase A2 (proPLA2; zymogen) along with the signal sequence of dog pancreatic PLA2. Yeast-preferred codons were chosen and unique restriction enzyme sites were incorporated. 22 oligodeoxynucleotides that varied in size from 33 to 48 nucleotides were chemically synthesized and assembled into the DNA fragment, which was then placed under the control of the yeast acid phosphatase repressible promoter. The resulting plasmid, transformed into S. cerevisiae, directed the synthesis of about 2.8 micrograms/ml of PLA2, most of which was secreted into the culture fluid. The secreted PLA2 comprised 18 to 26% of active enzyme, the remainder being proenzyme. Both had the expected N-terminal amino acid sequences, indicating that the yeast accurately released the signal peptide and the activation peptide (N-terminal heptapeptide of proPLA2). The specific activity of PLA2 thus produced is the same as that of the authentic bovine enzyme.

Human lysozyme: sequencing of a cDNA, and expression and secretion by Saccharomyces cerevisiae

A cDNA encoding human lysozyme was isolated from a human placenta cDNA library. The cDNA was 1.5 kb in size and coded for a signal peptide consisting of 18 amino acids and mature lysozyme. The amino acid sequence of the mature lysozyme, deduced from the nucleotide sequence, was identical with the published sequence. In the 3'-noncoding region of the cDNA, an Alu sequence was found in the reverse orientation. In a protein coding region, the human lysozyme cDNA shows 60.1% and 51.3% similarity with chicken lysozyme and human alpha-lactalbumin cDNAs, respectively. When the cDNA was expressed in Saccharomyces cerevisiae, an active and correctly processed human lysozyme was secreted efficiently into the culture medium.

Expression of human pancreatic secretory trypsin inhibitor in Saccharomyces cerevisiae

Human pancreatic secretory trypsin inhibitor (PSTI) cDNA was expressed in Saccharomyces cerevisiae using the yeast acid phosphatase PHO5 promoter. The product encoded by the PSTI-coding cDNA was correctly processed in yeast cells, and the PSTI molecules were efficiently secreted into the medium. The amino acid composition and the N-terminal amino acid sequence of the secreted PSTI molecules were identical to those of the authentic PSTI polypeptides from human pancreas, and the product exhibited trypsin-inhibitory activity.

Efficient secretion of Bacillus amyloliquefaciens alpha-amylase by [corrected] its own signal peptide from Saccharomyces cerevisiae host cells [corrected]

The expression and secretion of Bacillus amyloliquefaciens alpha-amylase was studied in yeast Saccharomyces cerevisiae. The Bacillus promoter was removed by BAL 31 digestion and three forms of the alpha-amylase gene were constructed: the Bacillus signal sequence was either complete (YEp alpha a1), partial (YEp alpha a2) or missing (YEp alpha a3). Secretion of alpha-amylase into the culture medium was obtained with the complete signal sequence only. The secreted alpha-amylase was glycosylated and its signal peptide was apparently processed. The glycosylated alpha-amylase remained active. The enzyme produced by the other constructions was not glycosylated and thus probably remained in the cytoplasm.