Modifying the insect cell N-glycosylation pathway with immediate early baculovirus expression vectors

A New Bacmid for Customized Protein Glycosylation Pathway Engineering in the Baculovirus-Insect Cell System

One attractive feature of the baculovirus-insect cell system (BICS) is the baculoviral genome has a large capacity for genetic cargo. This enables construction of viral vectors designed to accept multigene insertions, which has facilitated efforts to produce recombinant multisubunit protein complexes. However, the large genetic capacity of baculovirus vectors has not yet been exploited for multistep pathway engineering. Therefore, we created PolyBac, which is a novel baculovirus shuttle vector, or bacmid, that can be used for this purpose. PolyBac was designed to accept multiple transgene insertions by three different mechanisms at three different sites within the baculovirus genome. After constructing and characterizing PolyBac, we used it to isolate nine derivatives encoding various combinations of up to eight different protein N-glycosylation pathway functions, or glycogenes. We then used these derivatives, which were designed to progressively extend the endogenous insect cell pathway, to assess PolyBac's utility for protein glycosylation pathway engineering. This assessment was enabled by engineering each derivative to produce a recombinant influenza hemagglutinin (rH5), which was used to probe the impact of each glycoengineered PolyBac derivative on the endogenous insect cell pathway. Genetic analyses of these derivatives confirmed PolyBac can accept large DNA insertions. Biochemical analyses of the rH5 products showed each had distinct N-glycosylation profiles. Finally, the major N-glycan on each rH5 product was the predicted end product of the engineered N-glycosylation pathways encoded by each PolyBac derivative. These results generally indicate that PolyBac has utility for multistep metabolic pathway engineering and directly demonstrate that this new bacmid can be used for customized protein glycosylation pathway engineering in the BICS.

Synthetic Glycobiology: Parts, Systems, and Applications

Protein glycosylation, the attachment of sugars to amino acid side chains, can endow proteins with a wide variety of properties of great interest to the engineering biology community. However, natural glycosylation systems are limited in the diversity of glycoproteins they can synthesize, the scale at which they can be harnessed for biotechnology, and the homogeneity of glycoprotein structures they can produce. Here we provide an overview of the emerging field of synthetic glycobiology, the application of synthetic biology tools and design principles to better understand and engineer glycosylation. Specifically, we focus on how the biosynthetic and analytical tools of synthetic biology have been used to redesign glycosylation systems to obtain defined glycosylation structures on proteins for diverse applications in medicine, materials, and diagnostics. We review the key biological parts available to synthetic biologists interested in engineering glycoproteins to solve compelling problems in glycoscience, describe recent efforts to construct synthetic glycoprotein synthesis systems, and outline exemplary applications as well as new opportunities in this emerging space.

The Coming Age of Insect Cells for Manufacturing and Development of Protein Therapeutics

Protein therapeutics is a rapidly growing segment of the pharmaceutical market. Currently, the majority of protein therapeutics are manufactured in mammalian cells for their ability to generate safe and efficacious human-like glycoproteins. The high cost of using mammalian cells for manufacturing has motivated a constant search for alternative host platforms. Insect cells have begun to emerge as a promising candidate, largely due to the development of the baculovirus expression vector system. While there are continuing efforts to improve insect-baculovirus expression for producing protein therapeutics, key limitations including cell lysis and the lack of homogeneous humanized glycosylation still remain. The field has started to see a movement toward virus-less gene expression approaches, notably the use of clustered regularly interspaced short palindromic repeats to address these shortcomings. This review highlights recent technological advances that are realizing the transformative potential of insect cells for the manufacturing and development of protein therapeutics.

Transient Expression of Foreign Genes in Insect Cells (sf9) for Protein Functional Assay

The transient gene expression system is one of the most important technologies for performing protein functional analysis in the baculovirus in vitro cell culture system. This system was developed to express foreign genes under the control of the baculoviral promoter in transient expression plasmids. Furthermore, this system can be applied to a functional assay of either the baculovirus itself or foreign proteins. The most widely and commercially available transient gene expression system is developed based on the immediate-early gene (IE) promoter of Orgyia pseudotsugata multicapsid nucleopolyhedrovirus (OpMNPV). However, a low expression level of foreign genes in insect cells was observed. Therefore, a transient gene expression system was constructed for improving protein expression. In this system, recombinant plasmids were constructed to contain the target sequence under the control of the Drosophila heat shock 70 (Dhsp70) promoter. This protocol presents the application of this heat shock-based pDHsp/V5-His (V5 epitope with 6 histidine)/Spodoptera frugiperda cell (sf9 cell) system; this system is available not only for gene expression but also for evaluating the anti-apoptotic activity of candidate proteins in insect cells. Furthermore, this system can be either transfected with one recombinant plasmid or co-transfected two potentially functionally antagonistic recombinant plasmids in insect cells. The protocol demonstrates the efficiency of this system and provides a practical case of this technique.

Alteration of a recombinant protein N-glycan structure in silkworms by partial suppression of N-acetylglucosaminidase gene expression

OBJECTIVE

To synthesize complex type N-glycans in silkworms, shRNAs against the fused lobe from Bombyx mori (BmFDL), which codes N-acetylglucosaminidase (GlcNAcase) in the Golgi, was expressed by recombinant B. mori nucleopolyhedrovirus (BmNPV) in silkworm larvae.

RESULTS

Expression was under the control of the actin promoter of B. mori or the U6-2 and i.e.-2 promoters from Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV). The reduction of specific GlcNAcase activity was observed in Bm5 cells and silkworm larvae using the U6-2 promoter. In silkworm larvae, the partial suppression of BmFDL gene expression was observed. When shRNA against BmFDL was expressed under the control of U6-2 promoter, the Man₃GlcNAc(Fuc)GlcNAc structure appeared in a main N-glycans of recombinant human IgG. These results suggested that the control of BmFDL expression by its shRNA in silkworms caused the modification of its N-glycan synthetic pathway, which may lead to the alteration of N-glycans in the expressed recombinant proteins.

CONCLUSIONS

Suppression of BmFDL gene expression by shRNA is not sufficient to synthesize complex N-glycans in silkworm larvae but can modify the N-glycan synthetic pathway.

In vitro amplification of scrapie and chronic wasting disease PrP(res) using baculovirus-expressed recombinant PrP as substrate

Protein misfolding cyclic amplification (PMCA) is an in vitro simulation of prion replication, which relies on the use of normal brain homogenate derived from host species as substrate for the specific amplification of abnormal prion protein, PrP(Sc). Studies showed that recombinant cellular PrP, PrP(C), expressed in Escherichia coli lacks N-glycosylation and an glycophosphatidyl inositol anchor (GPI) and therefore may not be the most suitable substrate in seeded PMCA reactions to recapitulate prion conversion in vitro. In this study, we expressed 2 PRNP genotypes of sheep, V136L141R154Q171 and A136F141R154Q171, and one genotype of white-tailed deer (Q95G96, X132,Y216) using the baculovirus expression system and evaluated their suitability as substrates in seeded-PMCA. It has been reported that host-encoded mammalian RNA molecules and divalent cations play a role in the pathogenesis of prion diseases, and RNA molecules have also been shown to improve the sensitivity of PMCA assays. Therefore, we also assessed the effect of co-factors, such as prion-specific mRNA molecules and a divalent cation, manganese, on protein conversion. Here, we report that baculovirus-expressed recombinant PrP(C) shows a glycoform and GPI-anchor profile similar to mammalian brain-derived PrP(C) and supports amplification of PrP(Sc) and PrP(CWD) derived from prion-affected animals in a single round of seeded PMCA in the absence of exogenous co-factors. Addition of species-specific in vitro transcribed PrP mRNA molecules stimulated the conversion efficiency resulting in increased PrP(Sc) or PrP(CWD) production. Addition of 2 to 20 μM of manganese chloride (MnCl2) to unseeded PMCA resulted in conversion of recombinant PrP(C) to protease-resistant PrP. Collectively, we demonstrate, for the first time, that baculovirus expressed sheep and deer PrP can serve as a substrate in protein misfolding cyclic amplification for sheep and deer prions in the absence of additional exogenous co-factors.

Analysis of the Bombyx mori nucleopolyhedrovirus ie-1 promoter in insect, mammalian, plant, and bacterial cells

The Bombyx mori nucleopolyhedrovirus (BmNPV) ie-1 promoter exhibits strong transcriptional activity and is used in transient foreign gene expression systems in insect cells. In a reporter assay experiment using the BmNPV ie-1 promoter, we found that it exhibited activity even in non-host mammalian BHK cells, plant BY-2 cells, and also bacterial Escherichia coli cells. An analysis using a deletion series of the BmNPV ie-1 promoter demonstrated that the core promoter region of this promoter was sufficient to display promoter activity in BHK cells, BY-2 cells, and E. coli cells, whereas upstream elements were required for higher activity in insect cells. Furthermore, we found that the BmNPV ie-1 promoter exhibited sufficient activity for a β-galactosidase assay in E. coli cells. The results obtained here suggest that the BmNPV ie-1 promoter has potential as a universal promoter for transient expression systems in insect, mammalian, plant, and bacterial cells.

9-O-Acetylation of sialic acids is catalysed by CASD1 via a covalent acetyl-enzyme intermediate

Sialic acids, terminal sugars of glycoproteins and glycolipids, play important roles in development, cellular recognition processes and host–pathogen interactions. A common modification of sialic acids is 9-O-acetylation, which has been implicated in sialoglycan recognition, ganglioside biology, and the survival and drug resistance of acute lymphoblastic leukaemia cells. Despite many functional implications, the molecular basis of 9-O-acetylation has remained elusive thus far. Following cellular approaches, including selective gene knockout by CRISPR/Cas genome editing, we here show that CASD1—a previously identified human candidate gene—is essential for sialic acid 9-O-acetylation. In vitro assays with the purified N-terminal luminal domain of CASD1 demonstrate transfer of acetyl groups from acetyl-coenzyme A to CMP-activated sialic acid and formation of a covalent acetyl-enzyme intermediate. Our study provides direct evidence that CASD1 is a sialate O-acetyltransferase and serves as key enzyme in the biosynthesis of 9-O-acetylated sialoglycans.

9-O-Acetylation is one of the most common modifications of sialic acids, implicated in sialoglycan recognition and ganglioside biology. Here, the authors show that the key enzyme for the biosynthesis of 9-O-acetylated sialoglycans is CASD1, which uses CMP-activated sialic acid as acceptor substrate.

An Overview and History of Glyco-Engineering in Insect Expression Systems

Insect systems, including the baculovirus-insect cell and Drosophila S2 cell systems are widely used as recombinant protein production platforms. Historically, however, no insect-based system has been able to produce glycoproteins with human-type glycans, which often influence the clinical efficacy of therapeutic glycoproteins and the overall structures and functions of other recombinant glycoprotein products. In addition, some insect cell systems produce N-glycans with immunogenic epitopes. Over the past 20 years, these problems have been addressed by efforts to glyco-engineer insect-based expression systems. These efforts have focused on introducing the capacity to produce complex-type, terminally sialylated N-glycans and eliminating the capacity to produce immunogenic N-glycans. Various glyco-engineering approaches have included genetically engineering insect cells, baculoviral vectors, and/or insects with heterologous genes encoding the enzymes required to produce various glycosyltransferases, sugars, nucleotide sugars, and nucleotide sugar transporters, as well as an enzyme that can deplete GDP-fucose. In this chapter, we present an overview and history of glyco-engineering in insect expression systems as a prelude to subsequent chapters, which will highlight various methods used for this purpose.

Engineering β1,4-galactosyltransferase I to reduce secretion and enhance N-glycan elongation in insect cells

β1,4-galactosyltransferase I (B4GALT1) is a Golgi-resident enzyme that elongates glycoprotein glycans, but a subpopulation of this enzyme is secreted following proteolytic cleavage in its stem domain. We hypothesized that engineering B4GALT1 to block cleavage and secretion would enhance its retention and, therefore, its function. To test this hypothesis, we replaced the cytoplasmic/transmembrane/stem (CTS) domains of B4GALT1 with those from human α1,3-fucosyltransferase 7 (FUT7), which is not cleaved and secreted. Expression of FUT7-CTS-B4GALT1 in insect cells produced lower levels of secreted and higher levels of intracellular B4GALT1 activity than the native enzyme. We also noted that the B4GALT1 used in our study had a leucine at position 282, whereas all other animal B4GALT1 sequences have an aromatic amino acid at this position. Thus, we examined the combined impact of changing the CTS domains and the amino acid at position 282 on intracellular B4GALT1 activity levels and N-glycan processing in insect cells. The results demonstrated a correlation between the levels of intracellular B4GALT1 activity and terminally galactosylated N-glycans, N-glycan branching, the appearance of hybrid structures, and reduced core fucosylation. Thus, engineering B4GALT1 to reduce its cleavage and secretion is an approach that can be used to enhance N-glycan elongation in insect cells.

A new insect cell glycoengineering approach provides baculovirus-inducible glycogene expression and increases human-type glycosylation efficiency

Insect cells are often glycoengineered using DNA constructs encoding foreign glyocoenzymes under the transcriptional control of the baculovirus immediate early promoter, ie1. However, we recently found that the delayed early baculovirus promoter, 39K, provides inducible and higher levels of transgene expression than ie1 after baculovirus infection (Lin and Jarvis, 2013). Thus, the purpose of this study was to assess the utility of the 39K promoter for insect cell glycoengineering. We produced two polyclonal transgenic insect cell populations in parallel using DNA constructs encoding foreign glycoenzymes under either ie1 (Sfie1SWT) or 39K (Sf39KSWT) promoter control. The surface of Sfie1SWT cells was constitutively sialylated, whereas the Sf39KSWT cell surface was only strongly sialylated after baculovirus infection, indicating Sf39KSWT cells were inducibly-glycoengineered. All nine glycogene-related transcript levels were induced by baculovirus infection of Sf39KSWT cells and most reached higher levels in Sf39KSWT than in Sfie1SWT cells at early times after infection. Similarly, galactosyltransferase activity, sialyltransferase activity, and sialic acid levels were induced and reached higher levels in baculovirus-infected Sf39KSWT cells. Finally, two different recombinant glycoproteins produced by baculovirus-infected Sf39KSWT cells had lower proportions of paucimannose-type and higher proportions of sialylated, complex-type N-glycans than those produced by baculovirus-infected Sfie1SWT cells. Thus, the 39K promoter provides baculovirus-inducible expression of foreign glycogenes, higher glycoenzyme activity levels, and higher human-type N-glycan processing efficiencies than the ie1 promoter, indicating that this delayed early baculovirus promoter has great utility for insect cell glycoengineering.

A novel baculovirus vector for the production of nonfucosylated recombinant glycoproteins in insect cells

Glycosylation is an important attribute of baculovirus-insect cell expression systems, but some insect cell lines produce core α1,3-fucosylated N-glycans, which are highly immunogenic and render recombinant glycoproteins unsuitable for human use. To address this problem, we exploited a bacterial enzyme, guanosine-5'-diphospho (GDP)-4-dehydro-6-deoxy-d-mannose reductase (Rmd), which consumes the GDP-l-fucose precursor. We expected this enzyme to block glycoprotein fucosylation by blocking the production of GDP-l-fucose, the donor substrate required for this process. Initially, we engineered two different insect cell lines to constitutively express Rmd and isolated subclones with fucosylation-negative phenotypes. However, we found the fucosylation-negative phenotypes induced by Rmd expression were unstable, indicating that this host cell engineering approach is ineffective in insect systems. Thus, we constructed a baculovirus vector designed to express Rmd immediately after infection and facilitate the insertion of genes encoding any glycoprotein of interest for expression later after infection. We used this vector to produce a daughter encoding rituximab and found, in contrast to an Rmd-negative control, that insect cells infected with this virus produced a nonfucosylated form of this therapeutic antibody. These results indicate that our Rmd(+) baculoviral vector can be used to solve the immunogenic core α1,3-fucosylation problem associated with the baculovirus-insect cell system. In conjunction with existing glycoengineered insect cell lines, this vector extends the utility of the baculovirus-insect cell system to include therapeutic glycoprotein production. This new vector also extends the utility of the baculovirus-insect cell system to include the production of recombinant antibodies with enhanced effector functions, due to its ability to block core α1,6-fucosylation.

Comparative glycomics analysis of influenza Hemagglutinin (H5N1) produced in vaccine relevant cell platforms

Hemagglutinin (HA) is the major antigen in influenza vaccines, and glycosylation is known to influence its antigenicity. Embryonated hen eggs are traditionally used for influenza vaccine production, but vaccines produced in mammalian and insect cells were recently licensed. This raises the concern that vaccines produced with different cell systems might not be equivalent due to differences in their glycosylation patterns. Thus, we developed an analytical method to monitor vaccine glycosylation through a combination of nanoLC/MS(E) and quantitative MALDI-TOF MS permethylation profiling. We then used this method to examine glycosylation of HAs from two different influenza H5N1 strains produced in five different platforms, including hen eggs, three different insect cell lines (High Five, expresSF+ and glycoengineered expresSF+), and a human cell line (HEK293). Our results demonstrated that (1) sequon utilization is not necessarily equivalent in different cell types, (2) there are quantitative and qualitative differences in the overall N-glycosylation patterns and structures produced by different cell types, (3) ∼20% of the N-glycans on the HAs produced by High Five cells are core α1,3-fucosylated structures, which may be allergenic in humans, and (4) our method can be used to monitor differences in glycosylation during the cellular glycoengineering stages of vaccine development.

MultiBac complexomics

Recombinant production of multiprotein complexes is an emerging focus in academic and pharmaceutical research and is expected to play a key role in addressing complex biological questions in health and disease. Here we describe MultiBac, a state-of-the-art eukaryotic expression technology utilizing an engineered baculovirus to infect insect cells. The robust and flexible concept of MultiBac allows for simultaneous expression of multiple proteins in a single cell, which can be used to produce protein complexes and to recapitulate metabolic pathways. The MultiBac system has been set up as an open-access platform technology at the European Molecular Biology Laboratory (EMBL) in Grenoble, France. The performance of this platform and its access modalities to the scientific community are detailed in this article. The MultiBac system has been instrumental for unlocking the function of a number of essential multiprotein complexes and recent examples are discussed. This article presents a novel concept for the customized production of glycosylated protein targets using SweetBac, a modified MultiBac vector system. Finally, this article outlines how MultiBac may further develop in the future to serve applications in both academic and industrial research and development.

Engineering the baculovirus genome to produce galactosylated antibodies in lepidopteran cells

Nowadays, recombinant proteins are used with great success for the treatment of a variety of medical conditions, such as cancer, autoimmune, and infectious diseases. Several expression systems have been developed to produce human proteins, but one of their most critical limitations is the addition of truncated or nonhuman glycans to the recombinant molecules. The presence of such glycans can be deleterious as they may alter the protein physicochemical properties (e.g., solubility, aggregation), its half-life, and its immunogenicity due to the unmasking of epitopes.The baculovirus expression system has long been used to produce recombinant proteins for research. Thanks to recent methodological advances, this cost-effective technology is now considered a very promising alternative for the production of recombinant therapeutics, especially vaccines. Studies on the lepidopteran cell metabolism have shown that these cells can perform most of the posttranslational modifications, including N- and O-glycosylation. However, these glycan structures are shorter compared to those present in mammalian proteins. Lepidopteran N-glycans are essentially of the oligomannose and paucimannose type with no complex glycan identified in both infected and uninfected cells. The presence of short N-glycan structures is explained by the low level of N-acetylglucosaminyltransferase I (GNT-I) activity and the absence of several other glycosyltransferases, such as GNT-II and β1,4-galactosyltransferase I (β1,4GalTI), and of sialyltransferases.In this chapter, we show that the glycosylation pathway of a lepidopteran cell line can be modified via infection with an engineered baculovirus to "humanize" the glycosylation pattern of a recombinant protein. This engineering has been performed by introducing in the baculovirus genome the cDNAs that encode three mammalian glycosyltransferases (GNT-I, GNT-II, and β1,4GalTI). The efficiency of this approach is illustrated with the construction of a recombinant virus that can produce a galactosylated antibody.

Innovative use of a bacterial enzyme involved in sialic acid degradation to initiate sialic acid biosynthesis in glycoengineered insect cells

The baculovirus/insect cell system is widely used for recombinant protein production, but it is suboptimal for recombinant glycoprotein production because it does not provide sialylation, which is an essential feature of many glycoprotein biologics. This problem has been addressed by metabolic engineering, which has extended endogenous insect cell N-glycosylation pathways and enabled glycoprotein sialylation by baculovirus/insect cell systems. However, further improvement is needed because even the most extensively engineered baculovirus/insect cell systems require media supplementation with N-acetylmannosamine, an expensive sialic acid precursor, for efficient recombinant glycoprotein sialylation. Our solution to this problem focused on E. coli N-acetylglucosamine-6-phosphate 2'-epimerase (GNPE), which normally functions in bacterial sialic acid degradation. Considering that insect cells have the product, but not the substrate for this enzyme, we hypothesized that GNPE might drive the reverse reaction in these cells, thereby initiating sialic acid biosynthesis in the absence of media supplementation. We tested this hypothesis by isolating transgenic insect cells expressing E. coli GNPE together with a suite of mammalian genes needed for N-glycoprotein sialylation. Various assays showed that these cells efficiently produced sialic acid, CMP-sialic acid, and sialylated recombinant N-glycoproteins even in growth media without N-acetylmannosamine. Thus, this study demonstrated that a eukaryotic recombinant protein production platform can be glycoengineered with a bacterial gene, that a bacterial enzyme which normally functions in sialic acid degradation can be used to initiate sialic acid biosynthesis, and that insect cells expressing this enzyme can produce sialylated N-glycoproteins without N-acetylmannosamine supplementation, which will reduce production costs in glycoengineered baculovirus/insect cell systems.

Genetic modification of baculovirus expression vectors

As a protein expression vector, the baculovirus demonstrates many advantages over other vectors. With the development of biotechnology, baculoviral vectors have been genetically modified to facilitate high level expression of heterologous proteins in both insect and mammalian cells. These modifications include utilization of different promoters and signal peptides, deletion or replacement of viral genes for increasing protein secretion, integration of polycistronic expression cassette for producing protein complexes, and baculovirus pseudotyping, promoter accommodation or surface display for enhancing mammalian cell targeting gene delivery. This review summarizes the development and the current state of art of the baculovirus expression system. Further development of baculovirus expression systems will make them even more feasible and accessible for advanced applications.

SweetBac: a new approach for the production of mammalianised glycoproteins in insect cells

Recombinant production of therapeutically active proteins has become a central focus of contemporary life science research. These proteins are often produced in mammalian cells, in order to obtain products with post-translational modifications similar to their natural counterparts. However, in cases where a fast and flexible system for recombinant production of proteins is needed, the use of mammalian cells is limited. The baculoviral insect cell system has proven to be a powerful alternative for the expression of a wide range of recombinant proteins in short time frames. The major drawback of baculoviral systems lies in the inability to perform mammalian-like glycosylation required for the production of therapeutic glycoproteins. In this study we integrated sequences encoding Caenorhabditis elegans N-acetylglucosaminyltransferase II and bovine β1,4-galactosyltransferase I into the backbone of a baculovirus genome. The thereby generated SweetBac virus was subsequently used for the production of the human HIV anti-gp41 antibody 3D6 by integrating heavy and light chain open reading frames into the SweetBac genome. The parallel expression of target genes and glycosyltransferases reduced the yield of secreted antibody. However, the overall expression rate, especially in the recently established Tnao38 cell line, was comparable to that of transient expression in mammalian cells. In order to evaluate the ability of SweetBac to generate mammalian-like N-glycan structures on 3D6 antibody, we performed SDS-PAGE and tested for the presence of terminal galactose using Riccinus communis agglutinin I. The mammalianised variants of 3D6 showed highly specific binding to the lectin, indicating proper functionality. To confirm these results, PNGase A released N-glycans were analyzed by MALDI-TOF-MS and shown to contain structures with mainly one or two terminal galactose residues. Since the presence of specific N-glycans has an impact on antibodies ability to exert different effector functions, we tested the binding to human Fc gamma receptor I present on U937 cells.

MultiBac turns sweet

The baculovirus/insect cell system has proven to be a powerful tool for the expression of eukaryotic proteins. Therapeutics, especially in the field of vaccinology, are often composed of several different protein subunits. Conventional baculoviral expression schemes largely lack efficient strategies for simultaneous multi-gene expression. The MultiBac technology which is based on an engineered genome of Autographa californica nuclear polyhedrosis virus in combination with specially designed transfer vectors is an elegant way for flexible generation of multi-subunit proteins in insect cells. Yet, the glycosylation pattern of insect cell-derived products is not favorable for many applications. Therefore, a modified version of MultiBac, SweetBac, was generated allowing for a flexible glycosylation of target proteins in insect cells. Beyond the SweetBac technology MultiBac can further be designed for bridging the gap between cell engineering and transient modulation of host genes for improved and product tailored expression of recombinant proteins.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

Expression of the hIGF-I gene driven by the Fhx/P25 promoter in the silk glands of germline silkworm and transformed BmN cells

The expression of the human insulin-like growth factor (hIGF-I) gene driven by the Fhx/P25 promoter in the silk glands of transgenic silkworms (Bombyx mori) and in transformed silkworm cells, was achieved using BmN cells transfected with a piggyBac vector, pigA3GFP-Fhx/P25-hIGF-ie-neo containing a neomycin-resistance gene (neo), a green fluorescent protein gene (gfp), an hIGF-I gene, and a helper plasmid containing the piggyBac transposase sequence under the control of the B. mori actin 3 (A3) promoter. We selected stably transformed BmN cells expressing hIGF-I using the antibiotic G418. The expression level of hIGF-I was about 450 pg in 3 × 10(6) cells, determined by ELISA. The piggyBac vector was transferred into the silkworm eggs using sperm-mediated gene transfer. The expression level of hIGF-I per gram fresh posterior silk glands of G4 transgenic silkworms was approx. 150 ng.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Egg-independent vaccine strategies for highly pathogenic H5N1 influenza viruses

The emergence of a highly pathogenic H5N1 influenza virus in Hong Kong in 1997 and the subsequent appearance of other H5N1 strains and their spread to several countries in southeast Asia, Africa, the Middle East and Europe has evoked fear of a global influenza pandemic. Vaccines offer the best hope to combat the threat of an influenza pandemic. However, the global demand for a pandemic vaccine cannot be fulfilled by the current egg-based vaccine manufacturing strategies, thus creating a need to explore alternative technologies for vaccine production and delivery. Several egg-independent vaccine approaches such as cell culture-derived whole virus or subvirion vaccines, recombinant protein-based vaccines, virus-like particle (VLP) vaccines, DNA vaccines and viral vector-based vaccines are currently being investigated and appear promising both in preclinical and clinical studies. The present review will highlight the various egg-independent alternative vaccine approaches for pandemic influenza.

Mammalian membrane protein expression in baculovirus-infected insect cells

A system of expression of mammalian membrane proteins in baculovirus-infected insect cells is described, allowing analytical or preparative production in the milligram range of such type of proteins. This is illustrated by the setup of the expression system of the human breast cancer resistance protein (BCRP), which is a homodimeric multidrug ABC (adenosine triphosphate-binding cassette) transporter. The system used is Bac to Bac, which allows generation of a viral genome that includes the protein of interest by using a shuttle plasmid and a bacterial host carrying the bacmid. In the present case, the use of a molecular chaperon, ninaA, is illustrated, co-expressed with BCRP by using a plasmid allowing the expression of two proteins, pFastBac Dual. The method is detailed to allow the expression of such proteins and membrane protein in general.

Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context.

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

Initial size and dynamics of viral fusion pores are a function of the fusion protein mediating membrane fusion

BACKGROUND INFORMATION

Protein-mediated merger of biological membranes, membrane fusion, is an important process. To investigate the role of fusogenic proteins in the initial size and dynamics of the fusion pore (a narrow aqueous pathway, which widens to finalize membrane fusion), two different fusion proteins expressed in the same cell line were investigated: the major glycoprotein of baculovirus Autographa californica (GP64) and the HA (haemagglutinin) of influenza X31.

RESULTS

The host Sf9 cells expressing these viral proteins, irrespective of protein species, fused to human RBCs (red blood cells) upon acidification of the medium. A high-time-resolution electrophysiological study of fusion pore conductance revealed fundamental differences in (i) the initial pore conductance; pores created by HA were smaller than those created by GP64; (ii) the ability of pores to flicker; only HA-mediated pores flickered; and (iii) the time required for pore formation; HA-mediated pores took much longer to form after acidification.

CONCLUSION

HA and GP64 have divergent electrophysiological phenotypes even when they fuse identical membranes, and fusion proteins play a crucial role in determining initial fusion pore characteristics. The structure of the initial fusion pore detected by electrical conductance measurements is sensitive to the nature of the fusion protein.

Gating of the shaker potassium channel is modulated differentially by N-glycosylation and sialic acids

N-linked glycans, including sialic acids, are integral components of ion channel complexes. To determine if N-linked sugars can modulate a rapidly inactivating K+ channel, the glycosylated Drosophila melanogaster Shaker K+ channel (ShB) and the N-glycosylation-deficient mutant (ShNQ), were studied under conditions of full and reduced sialylation. Through an apparent electrostatic mechanism, full sialylation induced uniform and significant hyperpolarizing shifts in all measured voltage-dependent ShB gating parameters compared to those measured under conditions of reduced sialylation. Steady-state gating of ShNQ was unaffected by changes in sialylation and was nearly identical to that observed for ShB under conditions of reduced sialylation, indicating that N-linked sialic acids were wholly responsible for the observed effects of sialic acid on ShB gating. Interestingly, the rates of transition among channel states and the voltage-independent rates of activation and inactivation were significantly slower for ShNQ compared to ShB. Both effects were independent of sialylation, indicating that N-linked sugars other than sialic acids alter ShB gating kinetics but have little to no effect on the steady-state distribution of channels among states. The effect of sialic acids on channel gating, particularly inactivation gating, and the impact of other N-linked sugars on channel gating kinetics are unique to the ShB isoform. Thus, ShB gating is modulated by two complementary but distinct sugar-dependent mechanisms, (1) an N-linked sialic acid-dependent surface charge effect and (2) a sialic acid-independent effect that is consistent with N-linked sugars affecting the stability of ShB among its functional states.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

Expression of recombinant cyclooxygenase 1 in Drosophila melanogaster S2 cells transformed with human beta1,4-galactosyltransferase and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase

We examined the expression of human cyclooxygenase-1 (COX-1) in Drososphila melanogaster S2 (S2) cells transformed with cDNAs encoding beta1,4-galactosyltransferase (GalT) and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase (ST). Southern blot analysis indicated that multiple copies of the glycosyltransferases genes were integrated into the S2 cell genome. A lectin blot analysis also indicated that recombinant COX-1 from S2COX-1/GalT-ST cells contained the glycan residues of beta1,4-linked galactose and alpha2,6-linked sialic acid. The specific peroxidase activity of recombinant sialylated COX-1 from S2COX-1/GalT-ST cells was 41,250 U mg(-1), indicating an increase of approximately 22% compared with a non-sialylated control (33,850 U mg(-1)) from S2COX-1 cells.

Heterologous expression of wildtype and mutant myocilin in High Five insect cells shows comparable effects to cultivated trabecular meshwork cells

Myocilin (MYOC, TIGR) variations are associated with juvenile and primary open angle glaucoma (POAG). To investigate consequences of MYOC wildtype overexpression and selected mutations, we established a heterologous insect cell system (High Five). Wildtype, Pro370Leu, Gln368X and Lys423Glu were cloned into a modified pIB/V5-His (pEXIV) vector with and without downstream GFP in frame fusion. Mutations were introduced by in vitro mutagenesis. Heterologous expression was shown and analysed by RT-PCR, Western blotting, immunocytochemistry and fluorescence microscopy. Extended cultivation (>14 days) resulted in accumulation of MYOC protein for all variants in growing dilated cisterns of the rough endoplasmic reticulum. Finally cell death for overexpressed wildtype and mutants occurs. A direct attachment of ribosomes to these growing vesicles preceding the cell death was observed by electron microscopy. Our observations indicate that this system is suitable to trace the intracellular effects of MYOC mutants.

The insect metalloproteinase inhibitor gene of the lepidopteran Galleria mellonella encodes two distinct inhibitors

The insect metalloproteinase inhibitor (IMPI) from the greater wax moth, Galleria mellonella, represents the first and to date only specific inhibitor of microbial metalloproteinases reported from animals. Here, we report on the characterization including carbohydrate analysis of two recombinant constructs encoded by impi cDNA either upstream or downstream of the furin cleavage site identified. rIMPI-1, corresponding to native IMPI purified from hemolymph, is encoded by the N-terminal part of the impi sequence, whereas rIMPI-2 is encoded by its C-terminal part. rIMPI-1 is glycosylated at N48 with GlcNAc2Man3, showing fucosylation to different extents. Similarly, rIMPI-2 is glycosylated at N149 with GlcNAc2Man3, but is fully fucosylated. rIMPI-1 represents a promising template for the design of second-generation antibiotics owing to its specific activity against thermolysin-like metalloproteinases produced by human pathogenic bacteria such as Vibrio vulnificus. In contrast, rIMPI-2 does not inhibit bacterial metalloproteinases, but is moderately active against recombinant human matrix metalloproteinases (MMPs). Both microbial metalloproteinases and MMPs induce expression of the impi gene when injected into G. mellonella larvae. These findings provide evidence that the impi gene encodes two distinct inhibitors, one inhibiting microbial metalloproteinases and contributing to innate immunity, the other putatively mediating regulation of endogenous MMPs during metamorphosis.

Recombinant IgA Antibodies

The production of monoclonal antibodies and the development of recombinant antibody technology have made antibodies one of the largest classes of drugs in development for prophylactic, therapeutic and diagnostic purposes. Currently, all of the Food and Drug Administration (FDA)- approved antibodies are immunoglobulin Gs (IgGs). However, more than 95%of the infections are initiated at the mucosal surfaces, where IgA is the primary immune effector antibody.

Protein N-glycosylation in the baculovirus-insect cell expression system and engineering of insect cells to produce "mammalianized" recombinant glycoproteins

Baculovirus expression vectors are frequently used to express glycoproteins, a subclass of proteins that includes many products with therapeutic value. The insect cells that serve as hosts for baculovirus vector infection are capable of transferring oligosaccharide side chains (glycans) to the same sites in recombinant proteins as those that are used for native protein N-glycosylation in mammalian cells. However, while mammalian cells produce compositionally more complex N-glycans containing terminal sialic acids, insect cells mostly produce simpler N-glycans with terminal mannose residues. This structural difference between insect and mammalian N-glycans compromises the in vivo bioactivity of glycoproteins and can potentially induce allergenic reactions in humans. These features obviously compromise the biomedical value of recombinant glycoproteins produced in the baculovirus expression vector system. Thus, much effort has been expended to characterize the potential and limits of N-glycosylation in insect cell systems. Discoveries from this research have led to the engineering of insect N-glycosylation pathways for assembly of mammalian-style glycans on baculovirus-expressed glycoproteins. This chapter summarizes our knowledge of insect N-glycosylation pathways and describes efforts to engineer baculovirus vectors and insect cell lines to overcome the limits of insect cell glycosylation. In addition, we consider other possible strategies for improving glycosylation in insect cells.

Production and N-glycan analysis of secreted human erythropoietin glycoprotein in stably transfected Drosophila S2 cells

Schneider 2 (S2) cells from Drosophila melanogaster have been used as a plasmid-based, non-lytic expression system for foreign proteins. Here, a plasmid encoding the human erythropoietin (hEPO) gene fused with a hexahistidine (His(6)) tag under the control of the Drosophila metallothionein (MT) promoter was stably transfected into Drosophila S2 cells. After copper sulfate induction, transfected S2 cells were found to secrete hEPO with a maximum expression level of 18 mg/L and a secretion efficiency near 98%. The secreted hEPO from Drosophila S2 had an apparent molecular weight of about 23-27 kDa which was significantly lower than a recombinant hEPO expressed in Chinese hamster ovary (CHO) cells (about 36 kDa). N-glycosidase F digestion almost completely eliminated the difference and resulted in the same molecular weight ( approximately 20 kDa) of de-N-glycosylated hEPO proteins. These data suggest that recombinant hEPO from S2 cells was modified with smaller N-glycans. Subsequently, the major N-glycans were identified following glycoamidase A digestion, labeling with 2-aminopyridine (PA), and two-dimensional high-performance liquid chromatography (HPLC) analysis in concert with exoglycosidase digestion. This analysis of N-glycans revealed that hEPO was modified to include paucimannosidic glycans containing two or three mannose residues with or without core fucose. A similar glycosylation pattern was observed on a recombinant human transferrin expressed in S2 cells. These results provide a detailed analysis of multiple N-glycan structures produced in a Drosophila cell line that will be useful in the subsequent application of these cells for the generation of heterologous glycoproteins.

Expression and characterization of recombinant beta-secretase from Trichoplusia ni BTI Tn5B1-4 cells transformed with cDNAs encoding human beta1,4-galactosyltransferase and Gal beta1,4-GlcNAc alpha 2,6-sialytransferase

Beta-Secretase (betaSEC) was expressed in Trichoplusia ni BTI Tn5B1-4 (Tn5B1-4) cells transformed with cDNAs encoding beta1,4-galactosyltransferase (GalT) and Gal beta1,4-GlcNAc alpha 2,6-sialyltransferase (ST). The apparent molecular weight of recombinant beta-secretase was increased from 57 to 59 k Da. A lectin blot analysis indicated that recombinant beta-secretase from Tn5B1-4 betaSEC/GalT-ST cells (Tn5B1-4 cells co-transformed with cDNAs encoding beta-secretase, glycosyltransferases, GalT, and ST) contained the glycan residues of beta1,4-linked galactose and alpha2,6-linked sialic acid. Two-dimensional electrophoresis revealed that recombinant beta-secretase from Tn5B1-4 beta SEC/GalT-ST cells had a lower isoelectric point than beta-secretase from control Tn5B1-4 betaSEC cells (Tn5B1-4 cells transformed only with beta-secretase cDNA). The enzyme activity of recombinant beta-secretase from Tn5B1-4 betaSEC/GalT-ST cells was enhanced up to 77% compared to control Tn5B1-4 betaSEC cells. The concentrations at half-maximum inhibition (IC(50)) values estimated from inhibition analyses using purified beta-secretases from Tn5B1-4/betaSEC and Tn5B1-4/betaSEC/GalT-ST cells were 32 and 290 nM, respectively.

Catalytic Residues and Substrate Specificity of Recombinant Human Tripeptidyl Peptidase I (CLN2)

Tripeptidyl peptidase I (TTP-I), also known as CLN2, a member of the family of serine-carboxyl proteinases (S53), plays a crucial role in lysosomal protein degradation and a deficiency in this enzyme leads to fatal neurodegenerative disease. Recombinant human TPP-I and its mutants were analyzed in order to clarify the biochemical role of TPP-I and its mechanism of activity. Ser280, Glu77, and Asp81 were identified as the catalytic residues based on mutational analyses, inhibition studies, and sequence similarities with other family members. TPP-I hydrolyzed most effectively the peptide Ala-Arg-Phe*Nph-Arg-Leu (*, cleavage site) (k(cat)/K(m) = 2.94 microM(-1).s(-1)). The k(cat)/K(m) value for this substrate was 40 times higher than that for Ala-Ala-Phe-MCA. Coupled with other data, these results strongly suggest that the substrate-binding cleft of TPP-I is composed of only six subsites (S(3)-S(3)'). TPP-I prefers bulky and hydrophobic amino acid residues at the P(1) position and Ala, Arg, or Asp at the P(2) position. Hydrophilic interactions at the S(2) subsite are necessary for TPP-I, and this feature is unique among serine-carboxyl proteinases. TPP-I might have evolved from an ancestral gene in order to cleave, in cooperation with cathepsins, useless proteins in the lysosomal compartment.

Enhanced activity of recombinant beta-secretase from Drosophila melanogaster S2 cells transformed with cDNAs encoding human beta1,4-galactosyltransferase and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase

beta-Secretase (betaSEC) was expressed in Drososphila melanogaster Schneider 2 (S2) cells transformed with cDNAs encoding beta1,4-galactosyltransferase (GalT) and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase (ST). The apparent molecular weight of recombinant beta-secretase was increased from 56kDa to 61kDa. A lectin blot analysis indicated that recombinant beta-secretase from S2betaSEC/GalT-ST cells (S2 cells co-transformed with cDNAs encoding beta-secretase, glycosyltransferases, GalT, and ST) contained the glycan residues of beta1,4-linked galactose and alpha2,6-linked sialic acid. Two dimensional electrophoresis revealed that recombinant beta-secretase from S2betaSEC/GalT-ST cells had a lower isoelectric point compared to beta-secretase from control S2betaSEC cells (S2 cells transformed only with beta-secretase cDNA). Recombinant beta-secretase from transformed S2 cells was also present as heterogeneous forms. The enzyme activity of recombinant beta-secretase from S2betaSEC/GalT-ST cells was enhanced up to 260% compared to control S2betaSEC cells. We have shown that an exogeneous human glycosyltransferases cDNA can be introduced into S2 cells to extend the N-glycan processing capabilities of the insect cell line, and that the extended glycosylation improves the activity of recombinant beta-secretase.

Comparing N-glycan processing in mammalian cell lines to native and engineered lepidopteran insect cell lines

In the past decades, a large number of studies in mammalian cells have revealed that processing of glycoproteins is compartmentalized into several subcellular organelles that process N-glycans to generate complex-type oligosaccharides with terminal N -acetlyneuraminic acid. Recent studies also suggested that processing of N-glycans in insect cells appear to follow a similar initial pathway but diverge at subsequent processing steps. N-glycans from insect cell lines are not usually processed to terminally sialylated complex-type structures but are instead modified to paucimannosidic or oligomannose structures. These differences in processing between insect cells and mammalian cells are due to insufficient expression of multiple processing enzymes including glycosyltransferases responsible for generating complex-type structures and metabolic enzymes involved in generating appropriate sugar nucleotides. Recent genomics studies suggest that insects themselves may include many of these complex transferases and metabolic enzymes at certain developmental stages but expression is lost or limited in most lines derived for cell culture. In addition, insect cells include an N -acetylglucosaminidase that removes a terminal N -acetylglucosamine from the N-glycan. The innermost N -acetylglucosamine residue attached to asparagine residue is also modified with alpha(1,3)-linked fucose, a potential allergenic epitope, in some insect cells. In spite of these limitations in N-glycosylation, insect cells have been widely used to express various recombinant proteins with the baculovirus expression vector system, taking advantage of their safety, ease of use, and high productivity. Recently, genetic engineering techniques have been applied successfully to insect cells in order to enable them to produce glycoproteins which include complex-type N-glycans. Modifications to insect N-glycan processing include the expression of missing glycosyltransferases and inclusion of the metabolic enzymes responsible for generating the essential donor sugar nucleotide, CMP- N -acetylneuraminic acid, required for sialylation. Inhibition of N -acetylglucosaminidase has also been applied to alter N-glycan processing in insect cells. This review summarizes current knowledge on N-glycan processing in lepidopteran insect cell lines, and recent progress in glycoengineering lepidopteran insect cells to produce glycoproteins containing complex N-glycans.

Molecular cloning and functional characterization of a Lepidopteran insect beta4-N-acetylgalactosaminyltransferase with broad substrate specificity, a functional role in glycoprotein biosynthesis, and a potential functional role in glycolipid biosynthesis

A degenerate PCR approach was used to isolate a lepidopteran insect cDNA encoding a beta4-galactosyl-transferase family member. The isolation and initial identification of this cDNA was based on bioinformatics, but its identification as a beta4-galactosyltransferase family member was experimentally confirmed. The newly identified beta4-galactosyltransferase family member had unusually broad donor and acceptor substrate specificities in vitro, as transferred galactose, N-acetylglucosamine, and N-acetylgalactosamine to carbohydrate, glycoprotein, and glycolipid acceptors. However, the enzyme preferentially utilized N-acetylgalactosamine as the donor for all three acceptors, and its derived amino acid sequence was closely related to a known N-acetylgalactosaminyltransferase. These data suggested that the newly isolated cDNA encodes a beta4-N-acetylgalactosaminyltransferase that functions in insect cell glycoprotein biosynthesis, glycolipid biosynthesis, or both. The remainder of this study focused on the role of this enzyme in N-glycoprotein biosynthesis. The results showed that the purified enzyme transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to a synthetic N-glycan in vitro. The structure of the reaction product was confirmed by chromatographic, mass spectroscopic, and nuclear magnetic resonance analyses. Co-expression of the new cDNA product in insect cells with an N-glycoprotein reporter showed that it transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to this N-glycoprotein in vivo. Confocal microscopy showed that a GFP-tagged version of the enzyme was localized in the insect cell Golgi apparatus. In summary, this study demonstrated that lepidopteran insect cells encode and express a beta4-N-acetylgalactosaminyltransferase that functions in N-glycoprotein biosynthesis and perhaps in glycolipid biosynthesis, as well. The isolation and characterization of this gene and its product contribute to our basic understanding of insect protein N-glycosylation pathways and to the growing body of evidence that insects can produce glycoproteins with complex N-glycans.