Expression of a mammalian alpha 2,6-sialyltransferase gene in Pichia pastoris

Combining expression and process engineering for high-quality production of human sialyltransferase in Pichia pastoris

The human β-galactoside α2,6-sialyltransferase I, ST6Gal-I has drawn considerable interest for its use as biocatalyst for in-vitro glycoengineering of recombinantly produced therapeutic proteins. By attaching sialic acid onto the terminal galactoses of biantennary protein N-glycans, ST6Gal-I facilitates protein remodeling towards a humanized glycosylation and thus optimized efficacy in pharmacological use. Secreted expression of ST6Gal-I in Pichia pastoris is promising, but proteolysis restricts both the yield and the quality of the enzyme produced. Focusing on an N-terminally truncated (Δ108) variant of ST6Gal-I previously shown to represent a minimally sized, still active form of ST6Gal-I, we show here that protein expression engineering and optimization of bioreactor cultivation of P. pastoris KM71H (pPICZαB) synergized to enhance the maximum enzyme titer about 57-fold to 17units/L. N-Terminal fusion to the Flag-tag plus deletion of a potential proteolytic site (Lys(114)-Asn→Gln(114)-Asn) improved the intrinsic resistance of Δ108ST6Gal-I to degradation in P. pastoris culture. A mixed glycerol/methanol feeding protocol for P. pastoris growth and induction was key for enzyme production in high yield and quality. The sialyltransferase was recovered from the bioreactor culture in a yield of 70% using a single step of anion-exchange chromatography. Its specific activity was 0.05units/mg protein.

All-in-one assay for β-d-galactoside sialyltransferases: Quantification of productive turnover, error hydrolysis, and site selectivity

Sialyltransferases are important enzymes of glycobiology and the related biotechnologies. The development of sialyltransferases calls for access to quick, inexpensive, and robust analytical tools. We have established an assay for simultaneous characterization of sialyltransferase activity, error hydrolysis, and site selectivity. The described assay does not require expensive substrates, is very sensitive (limit of detection=0.3 μU), and is easy to perform. It is based on sialylation of nitrophenyl galactosides; the products thereof are separated and quantified by ion pair reversed phase high-performance liquid chromatography with ultraviolet detection.

High-quality production of human α-2,6-sialyltransferase in Pichia pastoris requires control over N-terminal truncations by host-inherent protease activities

BACKGROUND

α-2,6-sialyltransferase catalyzes the terminal step of complex N-glycan biosynthesis on human glycoproteins, attaching sialic acid to outermost galactosyl residues on otherwise fully assembled branched glycans. This "capping" of N-glycans is critical for therapeutic efficacy of pharmaceutical glycoproteins, making the degree of sialylation an important parameter of glycoprotein quality control. Expression of recombinant glycoproteins in mammalian cells usually delivers heterogeneous N-glycans, with a minor degree of sialylation. In-vitro chemo-enzymatic glycoengineering of the N-glycans provides an elegant solution to increase the degree of sialylation for analytical purposes but also possibly for modification of therapeutic proteins.

RESULTS

Human α-2,6-sialyltransferase (ST6Gal-I) was secretory expressed in P.pastoris KM71H. ST6Gal-I featuring complete deletion of both the N-terminal cytoplasmic tail and the transmembrane domain, and also partial truncation of the stem region up to residue 108 were expressed N-terminally fused to a His or FLAG-Tag. FLAG-tagged proteins proved much more resistant to proteolysis during production than the corresponding His-tagged proteins. Because volumetric transferase activity measured on small-molecule and native glycoprotein acceptor substrates did not correlate to ST6Gal-I in the supernatant, enzymes were purified and characterized in their action on non-sialylated protein-linked and released N-glycans, and the respective N-terminal sequences were determined by automated Edman degradation. Irrespective of deletion construct used (Δ27, Δ48, Δ62, Δ89), isolated proteins showed N-terminal processing to a highly similar degree, with prominent truncations at residue 108 - 114, whereby only Δ108ST6Gal-I retained activity. FLAG-tagged Δ108ST6Gal-I was therefore produced and obtained with a yield of 4.5 mg protein/L medium. The protein was isolated and shown by MS to be intact. Purified enzyme exhibited useful activity (0.18 U/mg) for sialylation of different substrates.

CONCLUSIONS

Functional expression of human ST6Gal-I as secretory protein in P.pastoris necessitates that N-terminal truncations promoted by host-inherent proteases be tightly controlled. N-terminal FLAG-Tag contributes extra stability to the N-terminal region as compared to N-terminal His-Tag. Proteolytic degradation proceeds up to residues 108 - 114 and of the resulting short-form variants, only Δ108ST6Gal-I seems to be active. FLAG-Δ108ST6Gal-I transfers sialic acids to monoclonal antibody substrate with sufficient yields, and because it is stably produced in P.pastoris, it is identified here as an interesting glycoengineering catalyst.

Analysis of the Role of Oligosaccharides in the Apoptotic Activity of Glycodelin A

Glycodelin A, also known as placental protein-14, is a multifunctional glycosylated protein secreted by the uterine endometrium during the early phases of pregnancy. It is a known suppressor of T cell proliferation, inducer of T cell apoptosis, and inhibitor of sperm zona binding. Unlike in contraceptive activity, where the glycans on the molecule have been shown to play a crucial role, mutagenesis of the asparagines at sites of N-linked glycosylation (Asn(28) and Asn(63)) to glutamine shows that the apoptogenic activity of glycodelin A is executed by the protein backbone. Glycosylation at Asn(28) appears to play a role in the extracellular secretion of the molecule, as mutation of Asn(28) resulted in a significant decrease in the amount of secreted protein, and loss of both glycosylation sites reduced the secretion drastically. Our results also suggest that the loss of glycosylation does not affect the dimerization status of the molecule.

Glycodelin A, not glycodelin S, is apoptotically active. Relevance of sialic acid modification

Glycodelin, previously known as PP14 (placental protein-14), is a kernel lipocalin secreted by the glandular epithelium of the endometrium upon progesterone stimulation and by the seminal vesicles. The isoform of the protein present in female reproductive tissue, glycodelin A (GdA), and the male counterpart, glycodelin S (GdS), have identical amino acid sequences, but strikingly different N-linked glycans. It is well documented in literature that GdA is an immunosuppressive protein, and we have shown that this activity is due to its ability to induce apoptosis in activated T cells. The precise role of GdS in seminal plasma is not known. In this study, we report that GdS is not apoptotically active. We observe that the apoptotic activity requires the presence of sialic acid residues on the complex glycans, as in the case of GdA; however, complex glycans of GdS are non-sialylated. We have expressed the wild-type protein in Pichia pastoris, which does not add sialic acid to the secreted proteins, and confirmed our observations that the protein is apoptotically inactive in the non-sialylated form. Our results indicate that differential glycosylation modulates the function of the different glycodelin isoforms.

Nuclear magnetic resonance-based dissection of a glycosyltransferase specificity for the mucin MUC1 tandem repeat

The human glycoprotein MUC1 mucin plays a critical role in cancer progression. Breast, ovarian, and colon cancer cells often display unique cell-surface antigens corresponding to aberrantly glycosylated forms of the MUC1 tandem repeat. In this report, (15)N- and (13)C-labeled forms of a recombinant MUC1 construct containing five tandem repeats were used as substrates to define the order and kinetics of addition of N-acetylgalactosamine (GalNAc) moieties by a recombinant active form of the human enzyme UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase I (ppGalNAc-T1; residues 40-559). Heteronuclear NMR experiments were performed to assign resonances associated with the two serines (Ser5 and Ser15) and three threonines (Thr6, Thr14, and Thr19) present in the 20-residue long MUC1 repeat. The kinetics and order of addition of GalNAc moieties (Tn antigen) on the MUC1 construct by human ppGalNAc-T1 were subsequently dissected by NMR spectroscopy. Threonine 14 was shown to be rapidly glycosylated by ppGalNAc-T1 with an initial rate of 25 microM/min, followed by Thr6 (8.6 microM/min). The enzyme also modified Ser5 at a slower rate (1.7 microM/min), an event that started only after the glycosylation of Thr14 and Thr6 side chains was mostly completed. Ser15 and Thr19 remained unglycosylated by ppGalNAc-T1. Corresponding O-glycosylation sites within all five tandem repeats were simultaneously modified by ppGalNAc-T1, suggesting that each repeat behaves as an independent substrate unit. This study demonstrated that the hydroxyl oxygens of Thr14 and to a lesser extent Thr 6 represent the two dominant substrates modified by ppGalNAc-T1 within the context of a complex MUC1 peptide substrate. More importantly, the availability of defined isotopically labelled MUC1 glycopeptide substrates and the relative simplicity of their NMR spectra will facilitate the analysis of other transferases within the O-glycosylation pathways and the rational design of tumor-associated MUC1 antigens.