Alpha-factor-leader-directed secretion of recombinant human-insulin-like growth factor I from Saccharomyces cerevisiae. Precursor formation and processing in the yeast secretory pathway

Directed evolution of a secretory leader for the improved expression of heterologous proteins and full-length antibodies in Saccharomyces cerevisiae

Because of its eukaryotic nature, simple fermentation requirements, and pliable genetics, there have been many attempts at improving recombinant protein production in Saccharomyces cerevisiae. These strategies typically involve altering the expression of a native protein thought to be involved in heterologous protein trafficking. Usually, these approaches yield three- to tenfold improvements over wild-type strains and are almost always specific to one type of protein. In this study, a library of mutant alpha mating factor 1 leader peptides (MFalpha1pp) is screened for the enhanced secretion of a single-chain antibody. One of the isolated mutants is shown to enhance the secretion of the scFv up to 16-fold over wild type. These leaders also confer a secretory improvement to two other scFvs as well as two additional, structurally unrelated proteins. Moreover, the improved leader sequences, combined with strain engineering, allow for a 180-fold improvement over previous reports in the secretion of full-length, functional, glycosylated human IgG(1). The production of full-length IgG(1) at milligram per liter titers in a simple, laboratory-scale system will significantly expedite drug discovery and reagent synthesis while reducing antibody cloning, production, and characterization costs.

Secretory protein trafficking: genetic and biochemical analysis

Acute insulin secretion from stimulated pancreatic beta-cells is derived from the intracellular pool of insulin secretory granules wherein insulin is packaged in a highly concentrated (and in some species, crystalline) state. Here we review experimental work, principally from our laboratory, on the question of biogenesis of mature secretory granules within the broader context of intracellular protein trafficking. Events occurring in the lumen of organelles at various stages of intracellular transport within the secretory pathway and events at the limiting membrane of newly forming secretory granules each contribute to formation of the insulin storage compartment comprising the readily releasable pool.

Overexpression of an anti-CD3 immunotoxin increases expression and secretion of molecular chaperone BiP/Kar2p by Pichia pastoris

We previously reported that the secretory capacity of Pichia pastoris is limited with respect to the secretion of a 96.5-kDa bivalent anti-CD3 immunotoxin; double-copy expression generated more translation products than single-copy expression but did not increase the secretion of the immunotoxin. In Saccharomyces cerevisiae heterologous protein secretion has been reported to increase the expression of molecular chaperones, most prominently BiP/Kar2p. We therefore investigated the relationships between immunotoxin secretion and Kar2p expression in P. pastoris. We found that expression of the immunotoxin in P. pastoris increased the expression of Kar2p to levels that surpassed the retrieval capacity of the cell, leading to secretion of Kar2p into the medium. The level of Kar2p secretion was correlated with the copy number of the immunotoxin gene. Intracellular Kar2p was found to bind exclusively to the unprocessed immunotoxin containing the prosequence of alpha-factor in the endoplasmic reticulum. These results show that Kar2p is intimately involved in immunotoxin secretion in P. pastoris. The limited capacity of P. pastoris to retain a sufficiently high level of intracellular Kar2p may be a factor restricting the production of the immunotoxin.

Behavior in the eukaryotic secretory pathway of insulin-containing fusion proteins and single-chain insulins bearing various B-chain mutations

In the secretory pathway, endoproteolytic cleavage of the insulin precursor protein promotes a change in the biophysical properties of the processed insulin product, and this may be relevant for its intracellular trafficking. We have now studied several independent point mutants contained within the insulin B-chain, S9D, H10D, V12E (called B9D, B10D, and B12E), as well as the double point mutant P28K,K29P (B28K,B29P), that have been reported to inhibit insulin oligomerization. In yeast cells, the unprocessed precursor of each of these mutants is secreted, whereas >90% of the endoproteolytically released single-chain insulin moiety is retained intracellularly; a large portion of the B9D, B10D, and B12E single-chain insulins exhibit abnormally slow mobility upon nonreducing SDS-PAGE, despite normal mobility upon reducing SDS-PAGE. Although no free thiols can be detected, each of these mutants exhibits increased disulfide accessibility to dithiothreitol. After dithiothreitol treatment, a portion of the molecules can reoxidize to a form more compact than the original single-chain insulin mutants formed in vivo (indicating initial disulfide mispairing). Disulfide mispairing of a fraction of B9D, B10D, and B12E mutants also occurs in the context of single-chain insulin and even in authentic proinsulin expressed within the secretory pathway of mammalian cells. We conclude that analyses of the intracellular trafficking of certain oligomerization-defective insulin mutants is complicated by the formation of disulfide isomers in the secretory pathway.

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.

Improved secretion of native human insulin-like growth factor 1 from gas1 mutant Saccharomyces cerevisiae cells

We studied the secretion of recombinant human insulin-like growth factor 1 (rhIGF-1) from transformed yeast cells. The hIGF-1 gene was fused to the mating factor alpha prepro- leader sequence under the control of the constitutive ACT1 promoter. We found that the inactivation of the GAS1 gene in the host strain led to a supersecretory phenotype yielding a considerable increase, from 8 to 55 mg/liter, in rhIGF-1 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.

Enhancement of protein secretion in Saccharomyces cerevisiae by overproduction of Sso protein, a late-acting component of the secretory machinery

Increased production of secreted proteins in Saccharomyces cerevisiae was achieved by overexpressing the yeast syntaxins. Sso1 or Sso2 protein, the t-SNAREs functioning at the targeting/fusion of the Golgi-derived secretory vesicles to the plasma membrane. Up to four- or six-fold yields of a heterologous secreted protein, Bacillus alpha-amylase, or an endogenous secreted protein, invertase, were obtained respectively when expressing either one of the SSO genes, SSO1 or SSO2, from the ADH1 promoter on a multicopy plasmid. Direct correlation between the Sso protein level and the amount of secreted alpha-amylase was demonstrated by modulating the expression level of the SSO2 gene. Quantitation of the alpha-amylase activity in the culture medium, periplasmic space and cytoplasm suggests that secretion into the periplasmic space is the primary stage at which the SSO genes exert the secretion-enhancing function. Pulse-chase data also support enhanced secretion efficiently obtained by SSO overexpression. Our data suggest that the Sso proteins may be rate-limiting components of the protein secretion machinery at the exocytosis step in yeast.

Expression and secretion of antifreeze peptides in the yeast Saccharomyces cerevisiae

The antifreeze peptide AFP6 from the polar fish Pseudopleuronectus americanus has been expressed in and secreted by the yeast Saccharomyces cerevisiae as a biologically active molecule. The gene for the 37 amino acid long peptide has been chemically synthesized using yeast preferred codons. Subsequently, the gene has been cloned into an episomal expression vector as well as in a multicopy integration vector, which is mitotically more stable. The expression is under the control of the inducible GAL7 promoter. The enzyme alpha-galactosidase has been investigated as a carrier protein to facilitate expression and secretion of AFP. In order to reach increased expression levels, tandem repeats of the AFP gene (up to eight copies) have been cloned. In most cases the genes are efficiently expressed and the products secreted. The expression level amounts to approximately 100 mg/l in the culture medium. In a number of genetic constructs the genes are directly linked and expressed as AFP multimers. In other constructs linker regions have been inserted between the AFP gene copies, that allow the peptide to be processed by specific proteinases, either from the endogenous yeast proteolytic system or from a non-yeast source. The latter requires a separate processing step after yeast cultivation to obtain mature AFP. In all these cases proteolytic processing is incomplete, generating a heterogeneous mixture of mature AFP, carrier and chimeric protein, and/or a mixture of AFP-oligomers. The antifreeze activity has been demonstrated for such mixtures as well as for AFP multimers.

A C-terminal domain, which prevents secretion of the neuroendocrine protein 7B2 in Saccharomyces cerevisiae, inhibits Kex2 yet is processed by the Yap3 protease

Recent reports reveal that the C-terminal half of the neuroendocrine polypeptide 7B2 selectively inhibits and binds PC2, a mammalian prohormone converting enzyme that is homologous to the yeast pro-alpha-factor processing protease Kex2. During attempted secretion of the 185 amino-acid human 7B2 in Saccharomyces cerevisiae, we observe that the protein is mostly retained inside the cell. However a mutant polypeptide (7B2 delta 1), where the C-terminal 48 amino acids of 7B2 are deleted, is efficiently secreted. Two shorter C-terminal truncations either permit poor secretion or no secretion at all. Surprisingly, full-length 7B2 but not 7B2 delta 1 abolishes the catalytic activity of Kex2, indicating that C-terminal residues of 7B2 might also be important for inhibition of the yeast protease. When the KEX2 gene is disrupted, yeast cells unexpectedly secrete a 7B2 variant similar in size to 7B2 delta 1, suggesting involvement of the alternate yeast prohormone convertase Yap3 in processing. Secretion is enhanced by overexpression of Yap3 and by the presence of a Lys-Arg residue at the processing site of precursor 7B2. These results purport that, in neuroendocrine cells too, secretion of 7B2 could be mediated by a homologue of Yap3.

Effect of a pmr 1 disruption and different signal sequences on the intracellular processing and secretion of Cyamopsis tetragonoloba alpha-galactosidase by Saccharomyces cerevisiae

We fused the yeast-derived sequences encoding the invertase, acid phosphatase and alpha-factor pre- and prepro-signal peptides (SP) to the Cyamopsis tetragonoloba (guar plant) alpha-galactosidase(alpha Gal)-encoding gene and expressed these gene fusions in yeast. Whereas the amount of fusion protein produced by each of the constructs did not vary significantly, the secretion efficiency of the fusion protein that carried the SP of the prepro-alpha-factor (MF alpha 1) was consistently found to be about 10% higher than that of the other fusions (99% vs. 90%). Furthermore, when the secretion of alpha Gal was directed by the invertase (SUC2) SP, the intracellular enzyme localized to the endoplasmic reticulum (ER), whereas use of the MF alpha 1 SP caused the intracellular enzyme to be outer-chain-glycosylated and processed by the KEX2 endoproteinase, implying that it had passed the ER. These results suggest that the pro-peptide of MF alpha 1 stimulates the efflux of the heterologous protein from the ER. Null mutants of PMR1 (encoding a Ca(2+)-dependent ATPase) are known to give higher secretion efficiencies for a number of different heterologous proteins. Therefore, we also studied the secretion of alpha Gal in a pmr 1 disruption mutant. Structural analysis of the enzyme secreted by the mutant cells showed that it was completely processed by KEX2 and outer-chain-glycosylated, although the length of the outer-chain carbohydrate moiety was reduced when compared with the enzyme secreted by wild-type cells. These results contradict the hypothesis advanced by Rudolph et al. [Cell 58 (1989) 133-145] that disruption of PMR1 causes the secretory pathway to bypass the Golgi apparatus.

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.

The pro-region of the yeast prepro-alpha-factor is essential for membrane translocation of human insulin-like growth factor 1 in vivo

Four yeast secretion signals, the 19-amino-acid invertase signal sequence, the 17-amino-acid acid-phosphatase signal sequence, and the pre-sequence and prepro-sequence of prepro-alpha-factor have been used to look for the secretion of recombinant human insulin-like growth factor 1 (IGF1) from Saccharomyces cerevisiae. Only the prepro-sequence, often referred to as the alpha-factor leader and consisting of an N-terminal 19-amino-acid pre-sequence or signal sequence attached to a 66-amino-acid pro-region, permits secretion of IGF1. The signal sequences alone do not allow the translocation of IGF1 into the endoplasmic reticulum. This is evident from the fact that IGF1-like molecules, to which the signal sequences are still attached, accumulate intracellularly in the cytosol. Fusion of the pro-region of the alpha-factor leader to the C-terminus of the acid-phosphatase and invertase signal sequences allows IGF1 to be secreted once again. These results reveal the essential role of the pro-region of the alpha-factor leader in the secretion of IGF1 and indicate that it may have a function in guiding a nascent IGF1 polypeptide to a state in which translocation can occur.

A modified Kex2 enzyme retained in the endoplasmic reticulum prevents disulfide-linked dimerisation of recombinant human insulin-like growth factor-1 secreted from yeast

The majority of the recombinant human insulin-like growth factor-1 (IGF1) molecules, secreted from yeast using the prepro sequence of the prepro-alpha-factor, are not active monomers but inactive, disulfide-linked dimers. The prepro sequence of the prepro-alpha-factor, usually referred to as the alpha-factor leader (alpha FL), consists of a pre or signal sequence and a proregion. After signal sequence removal during translocation into the endoplasmic reticulum (ER) the proregion is still attached to IGF1 when it folds to acquire a tertiary structure. Mature IGF1 is released only in a late Golgi compartment by the membrane-bound endoprotease Kex2p. We find that co-expression of a novel ER-retained Kex2p variant, soluble Kex2pHDEL, can prevent intermolecular disulfide bond formation between two IGF1 molecules, implying that the presence of the proregion during the folding of IGF1 in the ER could be a reason for disulfide-linked dimerisation. This result indicates that the proregion of the alpha FL may have a role in the folding of some heterologous proteins in yeast, and that the ER-retained Kex2p mutant could be used as a convenient tool to study the cellular function of the proregions present naturally in various eucaryotic precursor proteins.

Mutants of human insulin-like growth factor II (IGF II). Expression and characterization of truncated IGF II and of two naturally occurring variants

Insulin-like growth factor II (IGF II) and four structural analogs, constructed by site-directed mutagenesis, were expressed as protein A fusion proteins in Escherichia coli BL21pLysS cells, cleaved with cyanogen bromide and purified by affinity chromatography and HPLC. Two mutants (Ser29 substituted by Arg-Leu-Pro-Gly, and Ser33 substituted by Cys-Gly-Asp) represent two naturally occurring variants of IGF II. The other two mutants, (7-67)IGF II and (9-67)IGF II, are truncated at the amino-terminus in analogy to the naturally occurring des(1-3)IGF I ('truncated IGF I'). These mutants were tested for their binding affinities to type-1 and type-2 IGF receptors, to IGF binding protein-3 (IGFBP-3) and for their stimulation of thymidine incorporation into DNA. The affinities of the Ser29 and Ser33 mutants to the type-1 IGF receptor were 85% and 39%, respectively, compared to wild-type IGF II, those of (7-67)IGF II and (9-67)IGF II 96% and 15%, respectively. The potencies of the Ser33 and the (9-67) mutant to stimulate thymidine incorporation into DNA correlated closely with the affinities to the type-1 IGF receptor, whereas the bioavailability of the Ser29 mutant was lower and that of the (7-67) mutant higher than the type-1 receptor binding, possibly due to interferences with endogenously secreted IGFBPs. The affinities of the Ser29 and Ser33 mutants to the type-2 IGF receptor were 110% and 71%, respectively, those of the two truncated mutants 25% and 23%, respectively. The affinity of the Ser29 mutant to IGFBP-3 was increased to 171%, whereas those of the Ser33 mutant and the two truncated mutants were reduced (34%, 10% and 19%, respectively).

In-vitro processing of yeast alpha-factor leader fusion proteins using a soluble yscF (Kex2) variant

The Saccharomyces cerevisiae KEX2 gene encodes the membrane-bound endoprotease yscF, which is responsible for the site-specific endoproteolytic cleavages at pairs of basic amino acid residues in the alpha-factor precursor. In order to obtain soluble yscF activity, a mutant KEX2 gene lacking 600 bp coding for the C-terminal 200 amino acids was constructed. Expression of the truncated KEX2 gene in yeast led to the secretion of an active soluble yscF protein (yscFs). The soluble yscF protein is able to efficiently cleave heterologous protein precursors in-vitro, as demonstrated for alpha-factor leader-hIGF1 and alpha-factor leader-hirudin fusion proteins.

A novel Kex2 enzyme can process the proregion of the yeast alpha-factor leader in the endoplasmic reticulum instead of in the Golgi

The prepro sequence of the yeast prepro-alpha-factor, usually referred to as the alpha-factor leader, has often been used for the efficient secretion of heterologous proteins from the yeast Saccharomyces cerevisiae. The alpha-factor leader consists of a 19-amino acid N-terminal pre or signal sequence followed by a 66-amino acid proregion. After removal of the signal sequence during membrane translocation, the proregion is cleaved from the precursor protein by the Kex2 endoprotease only in a late Golgi compartment. Here we report that a modified Kex2 enzyme, containing at the C-terminus the HDEL tetrapeptide, cleaves the proregion from the alpha-factor leader--human insulin like growth factor-1 fusion protein in the endoplasmic reticulum. The processing of pro-proteins earlier in the secretion pathway could be helpful in defining the cellular function of the proregions present naturally in various eucaryotic precursor proteins.

A Lys27-to-Glu27 mutation in the human insulin-like growth factor-1 prevents disulfide linked dimerization and allows secretion of BiP when expressed in yeast

Recombinant human insulin-like growth factor-1 (IGF1) secreted from yeast contains only 10-15% of the active monomer. A majority of the IGF1-like molecules are disulfide bonded dimers. These dimers are not formed in an IGF1 mutant where Lys27 has been replaced by glutamic acid. However, increased levels of secreted BiP (the yeast KAR2 gene product) are seen in cells expressing the mutant. These results imply that by preventing ionic interactions between two IGF1 molecules, intermolecular disulfide bonds do not form in yeast, and that in the mutant there is a structural change which induces BiP, allowing its secretion.

Gene expression in yeast: protein secretion

Maximizing efficiency for the secretion of proteins from yeast requires an understanding of the rate limiting stages in secretion that can result from high levels of gene expression. Recent progress in this area has produced a number of improvements in yeast expression systems for protein secretion.