Secretion and processing of insulin precursors in yeast

Therapeutic insulins and their large-scale manufacture

Biotechnological innovations over the past 25 years have underpinned the rapid development of a thriving biopharmaceutical sector. Therapeutic insulin remains one of the most commonly used products of pharmaceutical biotechnology and insulin-based products command annual global sales in excess of $4.5 billion. Innovations in its method of production and in particular the advent of engineered insulin analogues provide a fascinating insight into how scientific and technological advances have impacted upon the pharmaceutical biotechnology sector as a whole. Current insulin-based diabetes research is increasingly focused not on the insulin molecule per se, but upon areas such as the development of non-parenteral insulin delivery systems, as well as organ-/cell-based and gene therapy-based approaches to controlling the disease.

Effects of gene dosage, promoters, and substrates on unfolded protein stress of recombinantPichia pastoris

The expression of heterologous proteins may exert severe stress on the host cells at different levels. Depending on the specific features of the product, different steps may be rate-limiting. For the secretion of recombinant proteins from yeast cells, folding and disulfide bond formation were identified as rate-limiting in several cases and the induction of the chaperone BiP (binding protein) is described. During the development of Pichia pastoris strains secreting human trypsinogen, a severe limitation of the amount of secreted product was identified. Strains using either the AOX1 or the GAP promoter were compared at different gene copy numbers. With the constitutive GAP promoter, no effect on the expression level was observed, whereas with the inducible AOX1 promoter an increase of the copy number above two resulted in a decrease of expression. To identify whether part of the product remained in the cells, lysates were fractionated and significant amounts of the product were identified in the insoluble fraction containing the endoplasmic reticulum, while the soluble cytosolic fraction contained product only in clones using the GAP promoter. An increase of BiP was observed upon induction of expression, indicating that the intracellular product fraction exerts an unfolded protein response in the host cells. A strain using the GAP promoter was grown both on glucose and methanol and trypsinogen was identified in the insoluble fractions of both cultures, but only in the soluble fraction of the glucose grown cultures, indicating that the amounts and distribution of intracellularly retained product depends on the culture conditions, especially the carbon source.

Assessing viability and cell-associated product of recombinant protein producing Pichia pastoris with flow cytometry

This paper describes the establishment of flow cytometric methods for recombinant Pichia pastoris strains, and their application to a lab scale fed batch fermentation. Using a strain which secretes human trypsinogen, the viability and the product which remained associated to the cell were measured with propidium iodide and immunofluorescent staining, respectively. Viability decreases significantly below 70% during the methanol fed batch phase, indicating a stress situation triggered by the fermentation conditions. Cell associated product is accumulated earlier after methanol induction than secreted product. These data demonstrate that flow cytometry is a powerful tool for the analysis and optimization of recombinant protein production processes, and they indicate the need to further improve a widely used fermentation protocol for P. pastoris.

Increased production of human proinsulin in the periplasmic space of Escherichia coli by fusion to DsbA

The production of human proinsulin in its disulfide-intact, native form in Escherichia coli requires disulfide bond formation and the periplasmic space is the favourable compartment for oxidative folding. However, the secretory expression of proinsulin is limited by its high susceptibility to proteolysis and by disulfide bond formation, which is rate-limiting for proinsulin folding. In this report we describe a method for the production of high amounts of soluble, native human proinsulin in E. coli. We fused proinsulin to the C-terminus of the periplasmic disulfide oxidoreductase DsbA via a trypsin cleavage site. As DsbA is the main catalyst of disulfide bond formation in E. coli, we expected increased yields of proinsulin by intra- or intermolecular catalysis of disulfide bond formation. In the context of the fusion protein, proinsulin was found to be stabilised, probably due to an increased solubility and faster disulfide bond formation. To increase the yield of DsbA-proinsulin in the periplasm, several parameters were optimised, including host strains and cultivation conditions, and in particular growth medium composition and supplement of low molecular weight additives. We obtained a further, about three-fold increase in the amount of native DsbA-proinsulin by addition of L-arginine or ethanol to the culture medium. The maximum yield of native human proinsulin obtained from the soluble periplasmic fraction after specific cleavage of the fusion protein with trypsin was 9.2 mg g(-1), corresponding to 1.8% of the total cell protein.

Intracellular retention of newly synthesized insulin in yeast is caused by endoproteolytic processing in the Golgi complex

An insulin-containing fusion protein (ICFP, encoding the yeast prepro-alpha factor leader peptide fused via a lysine-arginine cleavage site to a single chain insulin) has been expressed in Saccharomyces cerevisiae where it is inefficiently secreted. Single gene disruptions have been identified that cause enhanced immunoreactive insulin secretion (eis). Five out of six eis mutants prove to be vacuolar protein sorting (vps)8, vps35, vps13, vps4, and vps36, which affect Golgi<-->endosome trafficking. Indeed, in wild-type yeast insulin is ultimately delivered to the vacuole, whereas vps mutants secrete primarily unprocessed ICFP. Disruption of KEX2, which blocks intracellular processing to insulin, quantitatively reroutes ICFP to the cell surface, whereas loss of the Vps10p sorting receptor is without effect. Secretion of unprocessed ICFP is not based on a dominant secretion signal in the alpha-leader peptide. Although insulin sorting mediated by Kex2p is saturable, Kex2p functions not as a sorting receptor but as a protease: replacement of Kex2p by truncated secretory Kex2p (which travels from Golgi to cell surface) still causes endoproteolytic processing and intracellular insulin retention. Endoproteolysis promotes a change in insulin's biophysical properties. B5His residues normally participate in multimeric insulin packing; a point mutation at this position permits ICFP processing but causes the majority of processed insulin to be secreted. The data argue that multimeric assembly consequent to endoproteolytic maturation regulates insulin sorting in the secretory pathway.

Recombinant human insulin. VIII. Isolation of fusion protein--S-sulfonate, biotechnological precursor of human insulin, from the biomass of transformed Escherichia coli cells

Various methods have been investigated for the isolation and purification of fusion proteins of precursors of human insulin in the form of S-sulfonates, from the biomass of transformed Escherichia coli cells. Fusion proteins were prepared with different sizes and structures of the leader peptide and the poly-His position (inserted for purification by metal chelate affinity chromatography). The fusion proteins contained an IgG-binding B domain of protein A from Staphylococcus aureus at the N-terminus and an Arg residue between the leader peptide of the molecule and the proinsulin sequence, for trypsin cleavage of the leader peptide. Six residues of Cys in proinsulin allow the chemical modification of the protein as a (Cys-S-SO(-)(3))(6) derivative (S-sulfonate), which increases its polyelectrolytic properties and improves the efficiency of its isolation. Various methods of oxidative sulfitolysis were compared with catalysis by sodium tetrathionate or cystine and Cu2+ or Ni2+ ions. An optimum scheme for the isolation and purification of S-sulfonated fusion proteins was developed by the combination of metal-chelating affinity and ion-exchange chromatography. Highly purified (95%) S-sulfonated fusion protein was recovered which was 85% of the fusion protein contained in the biomass of E. coli cells. Folding of fusion protein S-sulfonate occurred with high yield (up to 90-95%). We found that the fusion protein-S-sulfonate has proinsulin-like secondary structure. This structure causes highly efficient fusion protein folding.

Metabolic engineering of Saccharomyces cerevisiae

Comprehensive knowledge regarding Saccharomyces cerevisiae has accumulated over time, and today S. cerevisiae serves as a widley used biotechnological production organism as well as a eukaryotic model system. The high transformation efficiency, in addition to the availability of the complete yeast genome sequence, has facilitated genetic manipulation of this microorganism, and new approaches are constantly being taken to metabolicially engineer this organism in order to suit specific needs. In this paper, strategies and concepts for metabolic engineering are discussed and several examples based upon selected studies involving S. cerevisiae are reviewed. The many different studies of metabolic engineering using this organism illustrate all the categories of this multidisciplinary field: extension of substrate range, improvements of producitivity and yield, elimination of byproduct formation, improvement of process performance, improvements of cellular properties, and extension of product range including heterologous protein production.

The role of leaders in intracellular transport and secretion of the insulin precursor in the yeast Saccharomyces cerevisiae

Pulse-chase analysis of folded and misfolded insulin precursor (IP) expressed in Saccharomyces cerevisiae was performed to establish the requirements for intracellular transport and the influence of the secretory pathway quality control mechanisms on secretion. Metabolic labelling of the IP expressed in S. cerevisiae showed that the effect of a leader was to stabilise the IP in the endoplasmic reticulum (ER), and facilitate intracellular transport of the fusion protein and rapid secretion. The first metabolically labelled IP appeared in the culture supernatant within 2-4 min of chase, and most of the secreted IP appeared within the first 15 min of chase. After enzymatic removal of the leader in a late Golgi apparatus compartment, the IP followed one of two routes: (1) to the plasma membrane and hence to the culture supernatant, or (2) to a Golgi or post-Golgi compartment from which secretion was restricted. Combined secretion and intracellular retention of the IP reflected either saturation of a Golgi or post-Golgi compartment and secretion as a consequence of overexpression, or competition between secretion and intracellular retention. IP which was misfolded, either due to amino acid substitution or because disulphide bond formation had been prevented with dithiothreitol (DTT), was transported from the ER to the Golgi apparatus but then retained in a Golgi or post-Golgi compartment and not exported to the culture supernatant.

Use of the green fluorescent protein variant (YFP) to monitor MetArg human proinsulin production in Escherichia coli

Green fluorescent protein (GFP), a relatively new reporter gene, is making an impact on many aspects of science. The attributes of GFP could also be applied to the area of recombinant protein production. The work described here represents the first experiments using GFP as a tool to monitor recombinant protein production in real time in the fermentation process. We have constructed plasmids containing an operon fusion of the gene encoding MetArg-human proinsulin and reporter gene GFP (GFP, BFP, and YFP variants). The MetArg-proinsulin and GFP variant reporter protein were overexpressed in Escherichia coli BL21(DE3) after isopropyl beta-d-thiogalactoside induction. The MetArg-proinsulin to YFP protein ratio did not change in the cells during the bioprocess. Since there is a quantitative relationship between the level of MetArg-proinsulin concentration and YFP fluorescence, it is possible to measure only YFP fluorescence in order to monitor the production of MetArg-proinsulin during the bioprocess. The expression level of MetArg-proinsulin could reach 20-25%. Some 140 mg recombinant MetArg-human proinsulin could be obtained easily from 1 liter of fermentation medium. The MetArg-proinsulin could simply be changed into human insulin by trypsin and carboxypeptidase B treatment in later steps. These experiments provide possibilities for using the YFP reporter gene as a convenient tool to monitor protein expression in biotechnological processes. The proposed technique could reduce the time- and labor-intensive analysis of protein production and would improve the efficiency of process development.

Disruption of the KEX1 gene in Pichia pastoris allows expression of full-length murine and human endostatin

Endostatin is a potent angiogenesis inhibitor. In order to isolate sufficient quantities of soluble protein for in vivo studies in mice, we expressed murine endostatin in Pichia pastoris. Analysis of the expressed protein by mass spectrometry indicated that the protein was truncated. N-terminal sequence analysis determined that the N-terminus was intact, suggesting that the C-terminal lysine was missing. In Saccharomyces cerevisiae, Kex1p can cleave lysine and arginine residues from the C-terminus of peptides and proteins. We hypothesized that the KEX1 homologue in P. pastoris is responsible for the loss of the C-terminal lysine of endostatin. To test this hypothesis, we cloned and disrupted the P. pastoris KEX1 gene. Although the overall amino acid identity between the P. pastoris and the S. cerevisae Kex1p is only 36%, the amino acid residues involved in the catalytic activity or close to the active residues are highly conserved. Disruption of the KEX1 reading frame allowed expression of murine and human endostatin with the C-terminal lysine. The KEX1 disruption strain may be a useful tool for the expression of other proteins with a C-terminal basic amino acid. Addition of a lysine to the C-terminus of recombinant proteins may protect the C-terminus from degradation by other carboxypeptidases.

Insulin analogs with improved pharmacokinetic profiles

The aim of insulin replacement therapy is to normalize blood glucose in order to reduce the complications of diabetes. The pharmacokinetics of the traditional insulin preparations, however, do not match the profiles of physiological insulin secretion. The introduction of the rDNA technology 20 years ago opened new ways to create insulin analogs with altered properties. Fast-acting analogs are based on the idea that an insulin with less tendency to self-association than human insulin would be more readily absorbed into the systemic circulation. Protracted-acting analogs have been created to mimic the slow, steady rate of insulin secretion in the fasting state. The present paper provides a historical review of the efforts to change the physicochemical and pharmacological properties of insulin in order to improve insulin therapy. The available clinical studies of the new insulins are surveyed and show, together with modeling results, that new strategies for optimal basal-bolus treatment are required for utilization of the new fast-acting analogs.

Prepro-leaders lacking N-linked glycosylation for secretory expression in the yeast Saccharomyces cerevisiae

Synthetic prepro-leaders lacking consensus N-linked glycosylation sites confers secretion competence of correctly folded insulin precursor expressed in the yeast species Saccharomyces cerevisiae with a yield comparable to, or better than the alpha-factor prepro-leader. In contrast, the S. cerevisiae alpha-factor prepro-leader's three N-linked oligosaccharide chains are necessary for the ability to facilitate secretion of the insulin precursor from S. cerevisiae (T. Kjeldsen et al., Biotechnol. Appl. Biochem. 27, 109-115, 1998). Synthetic prepro-leader lacking both N-glycosylation and the dibasic Kex2 endoprotease processing site also efficiently facilitated secretion of a pro-leader/insulin precursor fusion protein in which the insulin precursor was correctly folded. The unprocessed pro-leader/insulin-precursor fusion protein was purified from culture medium and matured in vitro to desB30 insulin by Achromobacter lyticus lysyl-specific protease providing an alternative yeast expression system not dependent on the Kex2 endoprotease. The synthetic prepro-leader lacking N-linked glycosylation provides the opportunity for secretory expression in yeast utilizing either in vivo Kex2 endoprotease maturation of the fusion protein during secretion or in vitro maturation of the purified fusion protein with a suitable enzyme.

Leptin does not fully account for the satiety activity of adipose tissue-conditioned medium

To determine whether leptin alone accounts for the satiety activity secreted by native adipose tissue, we prepared culture media conditioned by microdissected adipose tissue from overfed Long-Evans rats, fa/fa rats, or db/db mice (media A, B, and C, respectively). Medium A significantly suppressed food intake following intracerebroventricular delivery to Long-Evans rats (2-h chow intake = 68 +/- 5% of baseline, P < 0.001). Media B and C significantly suppressed food intake following intraperitoneal delivery to ob/ob mice (24-h chow intake = 56 +/- 7% of baseline for medium B, P = 0. 001; 4-day chow intake = 78 +/- 3% of baseline for medium C, P = 0. 004). Using a leptin receptor-based bioassay, we determined that the leptin concentration of medium C was 392 +/- 18 ng/ml. This concentration was 20-fold lower than the concentration of recombinant murine leptin required to produce a similar degree of feeding suppression following 5 days of administration to ob/ob mice. Neither medium conditioned by adipose tissue from ob/ob mice nor medium conditioned by adipose tissue from fa/fa rats and subsequently immunodepleted of leptin had significant satiety activity. We conclude that leptin is necessary but not sufficient to account for the satiety activity of native adipose tissue, perhaps due to the production by adipocytes of a cofactor that augments the ability of leptin to suppress feeding.

Secretory expression of human albumin domains in Saccharomyces cerevisiae and their binding of myristic acid and an acylated insulin analogue

Albumin is organized in three homologous domains formed by double loops stabilized by disulfide bonds. Utilizing a secretory expression system based on a synthetic secretory prepro-leader, the three human serum albumin domains were expressed in the yeast Saccharomyces cerevisiae. Human serum albumin domains I and III were efficiently expressed and secreted, indicating that these domains can form independent structural units capable of folding into stable tertiary structures. In contrast, albumin domain II was not secreted and disappeared early in the secretory pathway. Human serum albumin has the ability to bind a large number of small molecule ligands, including fatty acids, presumably due to its structure and structural flexibility. Purified albumin domain III bound myristic acid, whereas purified albumin domain I did not bind myristic acid. A new soluble long-acting insulin an alogue acylated with myristic acid (Markussen J., et al., Diabetologia 39, 281-288, 1996) bound to domain III and bound markedly more weakly to domain I.

Human insulin production from a novel mini-proinsulin which has high receptor-binding activity

To increase the folding efficiency of the insulin precursor and the production yield of insulin, we have designed a mini-proinsulin (M2PI) having the central C-peptide region replaced with a sequence forming a reverse turn. The mini-proinsulin was fused at the N-terminus to a 21-residue fusion partner containing a His10 tag for affinity purification. The gene for the fusion protein was inserted downstream of the T7 promoter of the expression plasmid pET-3a, and the fusion proteins were produced as inclusion bodies in the Escherichia coli cytoplasm at levels up to 25% of the total cell protein. The protein was sulphonated, cleaved by CNBr and the M2PI mini-proinsulin was purified using ion-exchange chromatography. The refolding yield of M2PI was 20-40% better than that of proinsulin studied at the same molar concentrations, indicating that the short turn-forming sequence is more effective in the refolding process than the much longer C-peptide. Native human insulin was successfully generated by subsequent enzymic conversion of mini-proinsulin. The mini-proinsulin exhibited high receptor-binding activity, about 50% as potent as insulin, suggesting that this single-chained mini-proinsulin may provide a foundation in understanding the receptor-bound structure of insulin as well as the role of C-peptide in the folding and activity of proinsulin.

Failure of high-dose insulin treatment to increase beta-cell insulin content in diabetic non obese diabetic (NOD) mice

High-dose insulin treatment in the first period after clinical onset of insulin-dependent diabetes mellitus (IDDM) has been found to reduce diabetic manifestations in humans. The aim of the present study was to examine whether high-dose insulin treatment of newly diagnosed diabetic non obese diabetic (NOD) mice would increase beta-cell insulin content after termination of treatment in this experimental IDDM animal model. Newly diagnosed diabetic female NOD mice were randomized into three groups composed of a low-dose insulin treated group (n = 10) injected subcutaneously with 15 IU/kg per day of NPH for 14 days followed by 5 days without insulin, a high-dose insulin treated group (n = 8) injected subcutaneously with 150 IU/kg per day of Actrapid for 14 days followed by 5 days without insulin and an untreated group sacrificed 3 days after diagnosis (n = 11). A reference group of age matched non-diabetic untreated female NOD mice (n = 11) was included in the study and sacrificed at the same time as the untreated diabetic mice. No significant difference in the amount of insulin extracted from the total pancreas was found by comparison of the three diabetic groups, consisting of the newly diagnosed untreated mice, the low-dose insulin treated mice and the high-dose insulin treated mice, respectively. The level was about 100-fold less than in the non-diabetic group. Blood glucose values in the two treated diabetic groups were at a high level (median > 18 mM) throughout the study. We conclude that no increase in beta-cell insulin content could be demonstrated in newly diagnosed diabetic NOD mice after early high-dose insulin treatment, at least not in the presence of high blood glucose values.

Alanine scanning mutagenesis of insulin

Alanine scanning mutagenesis has been used to identify specific side chains of insulin which strongly influence binding to the insulin receptor. A total of 21 new insulin analog constructs were made, and in addition 7 high pressure liquid chromatography-purified analogs were tested, covering alanine substitutions in positions B1, B2, B3, B4, B8, B9, B10, B11, B12, B13, B16, B17, B18, B20, B21, B22, B26, A4, A8, A9, A12, A13, A14, A15, A16, A17, A19, and A21. Binding data on the analogs revealed that the alanine mutations that were most disruptive for binding were at positions TyrA19, GlyB8, LeuB11, and GluB13, resulting in decreases in affinity of 1,000-, 33-, 14-, and 8-fold, respectively, relative to wild-type insulin. In contrast, alanine substitutions at positions GlyB20, ArgB22, and SerA9 resulted in an increase in affinity for the insulin receptor. The most striking finding is that B20Ala insulin retains high affinity binding to the receptor. GlyB20 is conserved in insulins from different species, and in the structure of the B-chain it appears to be essential for the shift from the alpha-helix B8-B19 to the beta-turn B20-B22. Thus, replacing GlyB20 with alanine most likely modifies the structure of the B-chain in this region, but this structural change appears to enhance binding to the insulin receptor.

Heterologous expression of human cholecystokinin in Saccharomyces cerevisiae. Evidence for a lysine-specific endopeptidase in the yeast secretory pathway

Precursors of the human regulatory peptide cholecystokinin (CCK) have been expressed in Saccharomyces cerevisiae, and the post-translational processing of secreted CCK-related products analyzed. Recombinant plasmids expressing native human prepro-CCK and a hybrid molecule encompassing the prepro leader of the yeast alpha-mating pheromone fused to pro-CCK were examined. The latter construct resulted in considerably higher levels of pro-CCK secretion and was therefore analyzed in more detail. Two of the protein modifications essential for CCK bioactivity, C-terminal alpha-amidation and tyrosyl sulfation, were not detected in S. cerevisiae. Proteolytic cleavage of pro-CCK occurred C-terminally of three basic sites; (i) Arg105-Arg106 which, upon exposure to carboxypeptidase activity, leads to the production of glycine-extended CCK; (ii) Arg95 to produce CCK-8 related processing intermediates; and (iii) Lys81 resulting in CCK-22 related products. To elucidate which protease(s) are involved in these endoproteolytic cleavage events, pro-CCK was expressed in yeast mutants lacking various combinations of the Mkc7, Yap3, and Kex2 proteases. Only in S. cerevisiae strains deficient in Kex2 function was any of the above mentioned pro-CCK cleavages abolished, namely processing at the Arg105-Arg106 and Arg95 sites. This suggests that mammalian Kex2-like serine proteases may process pro-CCK at single arginine residues. Our data suggests that an as yet uncharacterized endopeptidase(s) in the S. cerevisiae secretory pathway is responsible for the lysine-specific cleavage of pro-CCK.

Synthetic leaders with potential BiP binding mediate high-yield secretion of correctly folded insulin precursors from Saccharomyces cerevisiae

Secretion leaders are essential for expression of many heterologous proteins including insulin in yeast. The function of secretion leaders and their interaction with the secretory pathway is not clear. To determine what constitutes functional pre-pro-leader sequences in Saccharomyces cerevisiae, synthetic leader sequences for secretion of the insulin precursor were developed by a combination of rational design and stepwise systematic optimization. The synthetic leaders efficiently facilitate secretion of the insulin precursor from S. cerevisiae when compared with the alpha-factor leader, leading to a high yield of correctly folded insulin precursor in the culture supernatant. The synthetic leaders feature two potential N-linked glycosylation sites which are efficiently glycosylated during secretion. Pulse-chase analysis indicates that the synthetic leaders/insulin precursor fusion protein have a prolonged residence in the endoplasmic reticulum compared to the alpha-factor leader/insulin precursor fusion protein. The longer transition time in the endoplasmic reticulum mediated by the synthetic leaders might provide additional time for correct folding of the insulin precursor and account for the increased fermentation yield.

Characterization of the heterologous invertase produced by Schizosaccharomyces pombe from the SUC2 gene of Saccharomyces cerevisiae

In order to gain information on the ability of Schizosaccharomyces pombe to process heterologous glycoproteins, the heterologous invertase, obtained from the expression in Schiz. pombe of the SUC2 gene of Saccharomyces cerevisiae, was characterized. In Schiz. pombe the heterologous invertase is secreted into the cell wall and seems to be firmly bound to this structure. After the isolation of the heterologous invertase the study of its enzymatic characteristics revealed that it is more similar to the Sacch. cerevisiae external invertase than to the Schiz. pombe invertase. However, it is glycosylated like the Schiz. pombe invertase since it reacts with the lectin from Bandeiraea simplicifolia seeds conjugated to fluorescein isothiocyanate, which indicates the presence of terminal galactose residues in the enzyme. Moreover, the presence of galactose in the heterologous invertase has been confirmed after analysis of the sugars present in its carbohydrate moiety by gas liquid chromatography.

Host cell properties and external pH affect proinsulin production by Saccharomyces yeast

The expression of a hybrid gene encoding an alpha-factor prepro leader peptide-miniproinsulin (MPI) fusion [MPI is the same as the LysArg human insulin precursor described by Thim et al. (1986)] was tested in a series of isogenic yeast strains to investigate the influence of some genetic and physiological factors on heterologous production in yeast. We found that: (i) an MF alpha 1 gene disruption in haploid cells, as well as MF alpha 1 gene product expression in diploid cells, do not affect the MPI secretion level; (ii) under conditions of exogenous leucine availability, MPI production is hindered by leucine auxotrophy (a leu2 mutation); (iii) rho- mutations increase the per-cell MPI yield approximately three-fold; (iv) the MPI yield is apparently dependent on the pH of the culture medium: the higher the external pH, the larger the per-cell MPI yield.

Endoproteolytic processing of recombinant proalbumin variants by the yeast Kex2 protease

The yeast Kex2 protease is regarded as the prototype of the eukaryotic family of subtilisin-like serine proteases involved in processing after dibasic amino acid sequences. Here we investigate the specificity of Kex2 using recombinant human proalbumin variants. Proalbumins with the processing site sequences Arg-Arg and Lys-Arg were cleaved after the dibasic sequence at approximately the same rate by Kex2 in vitro, and yeast expressing either of these sequences secreted mature albumin into the culture medium. As expected, the Arg-Gly-Val-Phe-His-Arg-albumin (proalbumin Lille) was not a substrate for Kex2 and neither was the Arg-Gly-Arg-Phe-His-Arg-albumin. In contrast to the mammalian endoproteases furin and the hepatic proalbumin convertase, the Kex2 protease was adversely affected by a P4 arginine. There was an 85% decrease in the cleavage of Arg-Gly-Arg-Phe-Arg-Arg-albumin compared with normal; also chicken proalbumin with an Arg-Phe-Ala-Arg processing site sequence was not a substrate for Kex2. A P1' arginine had a marked negative effect on processing and N-terminal sequence analysis confirmed that cleavage was occurring at the P1-P1' bond. The sequence context surrounding the classical dibasic site is critical in determining susceptibility to cleavage by the Kex2 protease.

A single-chain insulin-like growth factor I/insulin hybrid binds with high affinity to the insulin receptor

1. To investigate the structure/function relationship of the interaction between ligand and receptor in the insulin-like growth factor I (IGF-I) and insulin receptor systems we have prepared and characterized a single-chain insulin/IGF-I hybrid. The single-chain hybrid consists of the insulin molecule combined with the C domain of IGF-I. The single-chain hybrid was found to bind with high affinity to both truncated soluble insulin receptors and membrane-bound holoreceptors. The affinity for interacting with the soluble truncated insulin receptors was 55-94% relative to insulin, and affinity for membrane-bound insulin receptors was 113% of that of insulin. Furthermore we found that the affinity of the single-chain hybrid molecule for IGF-I receptors was 19-28% relative to IGF-I. 2. The affinity of the single-chain hybrid for chimeric insulin/IGF-I receptors exceeded that of either natural ligand. This indicates that coordinately changing domains of the receptors and the ligands can induce higher affinity of ligand for receptor, supporting the idea that these receptors have a common ligand-binding site [Kjeldsen, Andersen, Wiberg, Rasmussen, Schäffer, Balschmidt, Møller and Møller (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 4404-4408]. 3. In contrast with what was generally assumed about the ligand structure required for binding to the insulin receptor we demonstrate the first single-chain insulin analogue that can bind with high affinity to the insulin receptor.

Production of active, insect-specific scorpion neurotoxin in yeast

A cDNA encoding the Androctonus australis Hector insect toxin 1 (AaH IT1) was expressed in yeast leading to secretion of fully biologically active protein. Three different multicopy plasmids were constructed using PCR. Expression was directed by the strong PGK1 promoter of the yeast vector pMA 91. Plasmid pMA 91-AaH IT1 encodes AaH IT1 and its own signal peptide. In the two other constructions, the cDNA encoding the mature part of AaH IT1 is fused to the prepro-signal sequence of the yeast alpha-mating-factor precursor; the pBAL 7-alpha-KREAEA-AaH IT1 includes the cDNA sequence encoding the KR(EAEA) processing sequence of the alpha-mating factor, and pBAL 7-alpha-KR-AaH IT1 encodes the KR fused directly to the AaH IT1 gene. The yeast alpha-mating-factor signal peptide launched the pro-alpha-mating-factor-AaH IT1 fusion protein into the secretory pathway. The fusion proteins are expected to be cleaved in the Golgi by the KEX2 endopeptidase and the STE13 dipeptidyl aminopeptidase, leading to release of mature AaH IT1. Pulse/chase labelling of transformed yeast protoplasts, followed by SDS/PAGE analysis of proteins immunoprecipitated from either the lysate or the extracellular fluid, showed that AaH IT1 was produced. The highest concentration of recombinant AaH IT1 in the culture medium, as determined using a 125I-AaH IT1 specific radioimmunoassay, was 4 micrograms/l (0.5 nM). The recombinant toxin was fully biologically active against cockroaches as assessed by injection and comparison to native AaH IT1. Moreover, it competed with radiolabelled native toxin for its receptor on the voltage-sensitive Na+ channel with a dissociation constant of 0.5 nM.

A note on the primary structure and expression of an Erwinia carotovora polygalacturonase-encoding gene (peh1) in Escherichia coli and Saccharomyces cerevisiae

A 1209-base pair (bp) DNA fragment containing the endopolygalacturonase-encoding gene (peh1) from Erwinia carotovora subsp. carotovora was amplified by the polymerase chain reaction (PCR) technique and expressed in Escherichia coli. The nucleotide sequence of the PCR product was determined and found to be highly homologous to the primary structures of other polygalacturonase-encoding genes. The peh1 DNA fragment encoding the mature polygalacturonase was inserted between two different yeast expression-secretion cassettes and a yeast gene terminator, generating recombinant yeast-integrating shuttle plasmids pAMS10 and pAMS11. These YIp5-derived plasmids were transformed and stably integrated into the genome of a laboratory strain of Saccharomyces cerevisiae. Transcription initiation signals present in these expression-secretion cassettes were derived from the yeast alcohol dehydrogenase (ADC1P) or mating pheromone alpha-factor (MF alpha 1P) gene promoters. The transcription termination signals were derived from the yeast tryptophan synthase gene terminator (TRP5T). Secretion of polygalacturonase was directed by the signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S). Northern blot analysis revealed the presence of peh1 mRNA in the yeast transformants and a polypectate agarose test was used to monitor polygalacturonase production.

Co-expression of an Erwinia chrysanthemi pectate lyase-encoding gene (pelE) and an E. carotovora polygalacturonase-encoding gene (peh1) in Saccharomyces cerevisiae

A pectate lyase (PL)-encoding gene (pelE) from Erwinia chrysanthemi and a polygalacturonase (PG)-encoding gene (peh1) from E. carotovora were each inserted between a novel yeast expression-secretion cassette and a yeast gene terminator, and cloned separately into a yeast-centromeric shuttle vector (YCp50), generating recombinant plasmids pAMS12 and pAMS13. Transcription initiation signals present in the expression-secretion cassette were derived from the yeast alcohol dehydrogenase gene promoter (ADC1P), whereas the transcription termination signals were derived from the yeast tryptophan synthase gene terminator (TRP5T). Secretion of PL and PG was directed by the signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1s). A pectinase cassette comprising ADC1P-MF alpha 1s-pelE-TRP5T and ADC1P-MF alpha 1s-peh1-TRP5T was subcloned into YCp50, generating plasmid pAMS14. Subsequently, the dominant selectable Geneticin G418-resistance (GtR) marker, APH1, inserted between the yeast uridine diphosphoglucose 4-epimerase gene promoter (GAL10P) and yeast orotidine-5'-phosphate carboxylase gene terminator (URA3T), was cloned into pAMS14, resulting in plasmid pAMS15. Plasmids pAMS12, pAMS13 and pAMS14 were transformed into a laboratory strain of Saccharomyces cerevisiae, whereas pAMS15 was stably introduced into two commercial wine yeast strains. DNA-DNA and DNA-RNA hybridization analyses revealed the presence of these plasmids, and the pelE and peh1 transcripts in the yeast transformants, respectively. A polypectate agarose assay indicated the extracellular production of biologically active PL and PG by the S. cerevisiae transformants and confirmed that co-expression of the pelE and peh1 genes synergistically enhanced pectate degradation.

Expression and secretion of Bacillus amyloliquefaciens alpha-amylase by using the yeast pheromone alpha-factor promoter and leader sequence in Saccharomyces cerevisiae

Replacement of the regulatory and secretory signals of the alpha-amylase gene (AMY) from Bacillus amylolique-faciens with the complete yeast pheromone alpha-factor prepro region (MF alpha 1p) resulted in increased levels of extracellular alpha-amylase production in Saccharomyces cerevisiae. However, the removal of the (Glu-Ala)2 peptide from the MF alpha 1 spacer region (Lys-Arg-Glu-Ala-Glu-Ala) yielded decreased levels of extracellular alpha-amylase.

Purification and characterization of the trefoil peptide human spasmolytic polypeptide (hSP) produced in yeast

Recombinant human spasmolytic polypeptide (r-hSP) has been produced in relatively large amounts in Saccharomyces cerevisiae. The two intronless trefoil domains of the hSP-DNA were cloned separately by PCR from human genomic DNA, and the remaining parts of the gene synthesized. Recombinant plasmids were constructed to encode a fusion protein consisting of a hybrid leader sequence and the hSP sequence. The leader sequence serves to direct the fusion protein into the secretory pathway of the cell and to expose it to the Kex 2 processing enzyme system. The secreted r-hSP was found in a glycosylated and an non-glycosylated form. The two forms of r-hSP were purified from the yeast fermentation broth by a combination of ion-exchange chromatography and preparative HPLC. The overall yield from 8 litres of fermentation broth was 160 mg r-hSP and 219 mg glycosylated r-hSP corresponding to 50% and 34%, respectively. The structure of the r-hSP and the glycosylated r-hSP was determined by amino acid analysis and carbohydrate composition analysis as well as by peptide mapping, amino acid sequencing and mass spectrometric analysis.

Processing of proopiomelanocortin by insulin secretory granule proinsulin processing endopeptidases

A lysed preparation of isolated insulin secretory granules efficiently cleaved murine proopiomelanocortin (mPOMC) at physiologically important Lys-Arg processing sites. This processing was mostly attributed to an activity that co-eluted with the proinsulin processing type-II endopeptidase from anion exchange chromatography (Lys-Arg-directed; Davidson, H. W., Rhodes, C. J., and Hutton, J. C. (1988) Nature 333, 93-96). The principal peptide hormone products generated by the insulin secretory granule lysate were identified by specific radioimmunoassay and NH2-terminal microsequencing analysis of high performance liquid chromatography-separated products as alpha-melanocyte-stimulating hormone, corticotropin-like intermediate, gamma-lipotropin, beta-endorphin-(1-31), 18-kDa NH2-terminal fragment and, to a lesser extent, adrenocorticotrophin and beta-lipotropin. This processing had an acidic pH optimum (pH 5-5.5) and was Ca(2+)-dependent (K0.5 activation = 5-80 microM). With increasing Ca2+ concentrations there was an increase in the extent to which mPOMC was processed. The in vitro processing of mPOMC by the insulin secretory granule endopeptidase activity reported here is in excellent agreement with the in vivo processing of this prohormone by a combination of PC2 and PC3, candidates of prohormone endpeptidase, in gene transfer studies with cells that express the regulated secretory pathway (Thomas, L., Leduc, R., Thorne, B. A., Smeekens, S. S., Steiner, D. F., and Thomas, G. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 5297-5301).

A mutant of human insulin-like growth factor II (IGF II) with the processing sites of proinsulin. Expression and binding studies of processed IGF II

A mutant of human insulin-like growth factor II (IGF II) was constructed by site-directed mutagenesis: the nucleotides coding for Ser33 and Ser39 were changed to yield Arg and Lys, respectively, thus creating two pairs of basic residues, Arg-Arg and Lys-Arg, as flanking sequences of the remaining C domain. [Arg33, Lys39]IGF II was expressed in NIH-3T3 cells as a processed two-chain peptide with a deletion of amino acid residues 37-40 and crosslinked by three disulfide bonds. This des(37-40)[Arg33]IGF II showed 3.6-fold and 7.4-fold reduced affinities to the type 1 and type 2 IGF receptor overexpressing cells, respectively, whereas the thymidine incorporation potency was the same as that of wild-type IGF II. We speculate that the discrepancy between the reduced binding to the type 1 IGF receptor and the full thymidine incorporation potency is due to the 6.1-fold reduced affinity of the expressed mutant to the co-expressed IGF binding protein 3 (IGFBP-3). The results suggest that des(37-40)[Arg33]IGF II assumes a conformation very similar to IGF II, and that the entire length of the C domain is not essential for biological activity.

Proinsulin processing by the subtilisin-related proprotein convertases furin, PC2, and PC3

Experiments using recombinant vaccinia viruses expressing rat proinsulin I coinfected into COS-7 cells with recombinant vaccinia virus expressing human furin, human PC2, mouse PC3 (subtilisin-related proprotein convertases 1-3, respectively), or yeast Kex2 indicate that in this system both Kex2 and furin produce mature insulin, whereas PC2 selectively cleaves proinsulin at the C-peptide-A-chain junction. This is a property consistent with its probable identity with the rat insulinoma granule type II proinsulin processing activity as described by Davidson et al. [Davidson, H. W., Rhodes, C. J. & Hutton, J. C. (1988) Nature (London) 333, 93-96]. PC3 generates mature insulin but cleaves preferentially at the proinsulin B-chain-C-peptide junction. This pattern of cleavage by PC3 is similar, but not identical, to that of the highly B-chain-C-peptide junction-selective type I activity as described by Davidson et al., perhaps due to the presence of a P4 arginine residue near the C-peptide-A-chain junction unique to the rat proinsulins. These results along with data presented on the expression of both PC2 and PC3 in islet beta cells strongly support the conclusion that these proteases are involved in the conversion of proinsulin to insulin in vivo.

Sequence requirements for processing of proinsulin in transfected mouse pituitary AtT20 cells

To investigate the sequence requirements for proteolytic processing of prohormones at pairs of basic amino acids, normal and mutant proinsulins were expressed in the mouse pituitary corticotrophic cell line AtT20. The extent of processing was determined by h.p.l.c. analysis of insulin-like immunoreactivity secreted into the media of transfected cells. In this model system, normal proinsulin was efficiently processed to insulin. The mutant des-38-62-proinsulin, in which all but six amino acids of the C-peptide were deleted, was also processed to insulin but less efficiently than the wild-type. The mutant Lys64-Arg65 to Thr64-Arg65 was partially processed to insulin, while the mutant Arg31-Arg32 to Arg31-Gly32 was not processed at either site. These results indicate: (i) that a six-amino-acid spacer between the two pairs of basic amino acids in proinsulin is sufficient to permit processing at both sites; (ii) that the endoproteinase responsible for cleavage at the Lys64-Arg65 site will also recognize Thr64-Arg65; (iii) that the endoproteinase responsible for cleavage at the Arg31-Arg32 site will not recognize Arg31-Gly32; and (iv) that the change Arg31-Arg32 to Arg31-Gly32 affects processing at the Lys64-Arg65 site.

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.

A member of the eukaryotic subtilisin family (PC3) has the enzymic properties of the type 1 proinsulin-converting endopeptidase

PC3, a mammalian homologue of the yeast subtilisin-like proteinase Kex2, was expressed in Xenopus oocytes and its activity was characterized. PC3 cleaved human proinsulin at one of the two dibasic sites (KTRR32 but not LQKR65). The specificity, inhibitor profile, pH optimum (5.5) and Ca(2+)-dependence (K0.5 = 2.5-3 mM) paralleled those of the insulin-granule type 1 endopeptidase activity, suggesting a role for PC3 in the conversion of prohormones.

Large-scale purification and characterization of recombinant tick anticoagulant peptide

Recombinant tick anticoagulant peptide (r-TAP), a potent and specific inhibitor of blood coagulation factor Xa, was purified to greater than 99% homogeneity at the multi-gram scale. Genetically engineered yeast secreted 200-250 mg/l of the heterologous protein into the medium. Cells were separated from broth by diafiltration and purification was done by two chromatographic steps, both conducive to operation on a large scale. Analysis of the purified protein by several methods indicated that it was greater than 99% homogeneous and no incompletely processed or truncated proteins were detected. Physico-chemical characterization data of r-TAP show that it exists as a monomer in solution and no evidence of post-translational modification was observed. The purified protein was fully active in inhibiting human coagulation factor Xa.

Characterization of growth hormone releasing factor analog expression in Saccharomyces cerevisiae

An analog of growth hormone releasing factor (GRF), [Leu27]GRF(1-40)-OH, has been expressed and secreted in Saccharomyces cerevisiae under the control of the alpha-factor gene promoter and prepro sequence. A single pair of consecutive basic residues served as a processing site between the alpha-factor sequences and the GRF sequences. [Leu27]GRF(1-40)-OH from fermentor broth containing 20-30 mg/L of immunoreactive peptides was shown to be correctly processed and to possess biological activity as measured in vitro and in vivo. Additional peptides purified from broth appear to result from proteolytic degradation of the original translation product. Analysis of the amino acid compositions and sequences of these peptides suggests that processing enzymes may be responsible for some of the degradation.