Engineering N-glycosylation pathways in the baculovirus-insect cell system

Can Virus-like Particles Be Used as Synergistic Agent in Pest Management?

Among novel strategies proposed in pest management, synergistic agents are used to improve insecticide efficacy through an elevation of intracellular calcium concentration that activates the calcium-dependent intracellular pathway. This leads to a changed target site conformation and to increased sensitivity to insecticides while reducing their concentrations. Because virus-like particles (VLPs) increase the intracellular calcium concentration, they can be used as a synergistic agent to synergize the effect of insecticides. VLPs are self-assembled viral protein complexes, and by contrast to entomopathogen viruses, they are devoid of genetic material, which makes them non-infectious and safer than viruses. Although VLPs are well-known to be used in human health, we propose in this study the development of a promising strategy based on the use of VLPs as synergistic agents in pest management. This will lead to increased insecticides efficacy while reducing their concentrations.

Cytoadherence Properties of Plasmodium knowlesi-Infected Erythrocytes

Plasmodium knowlesi is responsible for zoonotic malaria infections that are potentially fatal. While the severe pathology of falciparum malaria is associated with cytoadherence phenomena by Plasmodium falciparum-infected erythrocytes (IRBC), information regarding cytoadherence properties of P. knowlesi-IRBC remained scarce. Here, we characterized the cytoadherence properties of RBC infected with the laboratory-adapted P. knowlesi A1-H.1 strain. We found that late-stage IRBC formed rosettes in a human serum-dependent manner, and rosettes hampered IRBC phagocytosis. IRBC did not adhere much to unexposed (unstimulated) human endothelial cell lines derived from the brain (hCMEC/D3), lungs (HPMEC), and kidneys (HRGEC). However, after being "primed" with P. knowlesi culture supernatant, the IRBC-endothelial cytoadherence rate increased in HPMEC and HRGEC, but not in hCMEC/D3 cells. Both endothelial cytoadherence and rosetting phenomena were abrogated by treatment of P. knowlesi-IRBC with trypsin. We also found that different receptors were involved in IRBC cytoadherence to different types of endothelial cells. Although some of the host receptors were shared by both P. falciparum- and P. knowlesi-IRBC, the availability of glycoconjugates on the receptors might influence the capacity of P. knowlesi-IRBC to cytoadhere to these receptors.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

Glycans have been shown to play a key role in many biological processes, such as signal transduction, immunogenicity, and disease progression. Among the various glycosylation modifications found on cell surfaces and in biomolecules, sialylation is especially important, because sialic acids are typically found at the terminus of glycans and have unique negatively charged moieties associated with cellular and molecular interactions. Sialic acids are also crucial for glycosylated biopharmaceutics, where they promote stability and activity. In this regard, heterogenous sialylation may produce variability in efficacy and limit therapeutic applications. Homogenous sialylation may be achieved through cellular and molecular engineering, both of which have gained traction in recent years. In this paper, we describe the engineering of intracellular glycosylation pathways through targeted disruption and the introduction of carbohydrate active enzyme genes. The focus of this review is on sialic acid-related genes and efforts to achieve homogenous, humanlike sialylation in model hosts. We also discuss the molecular engineering of sialyltransferases and their application in chemoenzymatic sialylation and sialic acid visualization on cell surfaces. The integration of these complementary engineering strategies will be useful for glycoscience to explore the biological significance of sialic acids on cell surfaces as well as the future development of advanced biopharmaceuticals.

Secretion of Recombinant Human Annexin V in Fusion with the Super Folder GFP for Labelling Phosphatidylserine-Exposing Membranes

Annexin V (ANXV), mostly characterized by its ability to interact with biological membranes in a calcium-dependent manner. ANXV interacts mainly with phosphatidylserine (PS), for that fluorescent ANXV widely produced and used as a sensitive and specific probe to mark apoptotic cells or any PS-containing bilayers membranes. Many reports described the prokaryotic expression of recombinant human ANXV. To overcome some of E. coli expression limitations, we aimed in this work to investigate unconventional alternative expression system in mammalian cells for producing secreted human ANXV in fusion with the super folder green fluorescent protein (sfGFP). HEK239T cells were transfected using polyethylenimine (PEI) and pcDNA-sfGFP-ANXV plasmid. Forty-eight hours post transfection, direct fluorescence measurement, immunoblotting and ELISA confirmed the presence of secreted sfGFP-ANXV in cells supernatant. The yield of secreted 6 × His-tagged sfGFP-ANXV after affinity purification was estimated to be around 2 µg per 1 ml of cells supernatant. The secretion system was proper to produce a fully functional sfGFP-ANXV fusion protein in quantities enough to recognize and bind PS-containing surfaces or liposomes. Besides, biological assays such as flow cytometry and fluorescent microscopy confirmed the capacity of the secreted sfGFP-ANXV to detect PS exposure on apoptotic cells. Taken together, we present mammalian expression as a quick, affordable and endotoxin-free system to produce sfGFP-ANXV fusion protein. The secreted sfGFP-ANXV in eukaryotic system is a promising biotechnological tool, it opens up new horizons for additional applications in the detection of PS bearing surfaces and apoptosis in vitro and in vivo assays.

Expression of Cloned Genes Using the Baculovirus Expression System

The general strategy of the baculovirus expression system is to infect insect cells with a virus that expresses a foreign protein at a very late stage of infection. Almost all baculovirus expression systems use the procedures for insect cell transfection, baculovirus production, and protein expression given in the main portion of this protocol. This protocol also includes a method that uses molecular biology techniques to produce recombinant baculovirus DNA in E. coli before transfection of insect cells. It is important to quantify the viral titer to achieve optimal and reproducible expression of target proteins. Accordingly, the viral plaque assay is also described here.

Baculovirus-Derived Vectors for Immunization and Therapeutic Applications

Baculoviruses are arthropod-specific, enveloped viruses with circular, supercoiled double-stranded deoxyribonucleic acid genomes. While many viruses are studied to seek solutions for their adverse impact on human, veterinary, and plant health, the study of baculoviruses was stimulated initially by their potential utility to control insect pests. Later, the utility of baculovirus as gene expression vectors was evidenced leading to numerous applications. Several strategies are employed to obtain recombinant viruses that express large quantities of heterologous proteins. A major step forward was the development of bacmid technology (the construction of bacterial artificial chromosomes containing the genome of the baculovirus) which allows the manipulation of the baculovirus genome in bacteria. With this technology, foreign genes can be introduced into the bacmid by homologous and site-directed recombination or by transposition. Baculoviruses have been used to explore fundamental questions in molecular biology such as the nature of programmed cell-death. Moreover, the ability of baculoviruses to transduce mammalian cells led to the consideration of their use as gene-therapy and vaccine vectors. Strategies for genetic engineering of baculoviruses have been developed to meet the requirements of new application areas. Display of foreign proteins on the surface of virions or in nucleocapsid structures, the assembly of expressed proteins to form virus-like particles or protein complexes have been explored and validated as vaccines. The aim of this chapter is to update the areas of application of the baculoviruses in protein expression, alternative vaccine designs and gene therapy of infectious diseases and genetic disorders. Finally, we review the baculovirus-derived products on the market and in the pipeline for biomedical and veterinary use.

Evaluation of Diagnostic Performance of Three Indirect Enzyme-Linked Immunosorbent Assays for the Detection of IgG Antibodies to Ebola Virus in Human Sera

Filovirus serological diagnosis and epidemiological investigations are hampered due to the unavailability of validated immunoassays. Diagnostic performance of three indirect enzyme-linked immunosorbent assays (I-ELISA) was evaluated for the detection of IgG antibody to Ebola virus (EBOV) in human sera. One I-ELISA was based on a whole EBOV antigen (WAg) and two utilized recombinant nucleocapsid (NP) and glycoproteins (GP), respectively. Validation data sets derived from individual sera collected in South Africa (SA), representing an EBOV non-endemic country, and from sera collected during an Ebola disease (EBOD) outbreak in Sierra Leone (SL), were categorized according to the compounded results of the three I-ELISAs and real time reverse-transcription polymerase chain reaction (RT-PCR). At the cut-off values selected at 95% accuracy level by the two-graph receiver operating characteristic analysis, specificity in the SA EBOV negative serum panel (n = 273) ranged from 98.17% (GP ELISA) to 99.27% (WAg ELISA). Diagnostic specificity in the SL EBOV negative panel (n = 676) was 100% by the three ELISAs. The diagnostic sensitivity in 423 RT-PCR confirmed EBOD patients was dependent on the time when the serum was collected after onset of disease. It significantly increased 2 weeks post-onset, reaching 100% sensitivity by WAg and NP and 98.1% by GP I-ELISA.

A Bacterial Expression Platform for Production of Therapeutic Proteins Containing Human-like O-Linked Glycans

We have developed an Escherichia coli strain for the in vivo production of O-glycosylated proteins. This was achieved using a dual plasmid approach: one encoding a therapeutic protein target, and a second encoding the enzymatic machinery required for O-glycosylation. The latter plasmid encodes human polypeptide N-acetylgalactosaminyl transferase as well as a β1,3-galactosyl transferase and UDP-Glc(NAc)-4-epimerase, both from Campylobacter jejuni, and a disulfide bond isomerase of bacterial or human origin. The effectiveness of this two-plasmid synthetic operon system has been tested on three proteins with therapeutic potential: the native and an engineered version of the naturally O-glycosylated human interferon α-2b, as well as human growth hormone with one engineered site of glycosylation. Having established proof of principle for the addition of the core-1 glycan onto proteins, we are now developing this system as a platform for producing and modifying human protein therapeutics with more complex O-glycan structures in E. coli.

Characterization of native-like HIV-1 gp140 glycoprotein expressed in insect cells

The trimeric HIV-1 envelope glycoprotein (Env) is critical for vaccine development aimed at achieving broadly-neutralizing antibody responses. The use of various recombinant expression systems and construct designs are associated with the resultant nature of produced proteins, especially in terms of glycosylation, antigenicity, and immunogenicity of the glycoprotein. Here, we explored an otherwise baculovirus cassette than classical one designed to express HIV-1 Env protein, including SOSIP mutation and Foldon moiety involvement. This improved design increased the ratio of the Env trimer fraction from ∼40% to ∼60% with respect to that of prototypical design, as indicated by high-performance size-exclusion chromatography and sedimentation velocity analysis. In addition, the design prolonged cell viability and enhanced the final yield (approximately 13-15 mg/L) after affinity purification. gp140 produced from insect cells mimicked the native-like trimer and mainly adopted glycosylation pattern of oligomannose glycans. The native-like Env proteins conferred cross-clade neutralizing antibody production in BALB/c mice. In summary, the expression of Env in insect cells by optimizing the baculovirus vector provides an alternative strategy for HIV-1 immunogen production and may benefit future Env-based HIV vaccine design.

N-Glycosylation of Seasonal Influenza Vaccine Hemagglutinins: Implication for Potency Testing and Immune Processing

Prior to each annual flu season, health authorities recommend three or four virus strains for inclusion in the annual influenza vaccine: a type A:H1N1 virus, a type A:H3N2 virus, and one or two type B viruses. Antigenic differences between strains are found in the glycosylation patterns of the major influenza virus antigen, hemagglutinin (HA). Here we examine the glycosylation patterns of seven reference antigens containing HA used in influenza vaccine potency testing. These reagents are supplied by the Center for Biologics Evaluation and Research (CBER) or the National Institute for Biological Standards and Control (NIBSC) for use in vaccine testing. Those produced in hen egg, Madin-Darby canine kidney (MDCK), and insect (Sf9) expression systems were examined. They are closely related or identical to antigens used in commercial vaccines. The reference antigens studied were used in the 2014-2015 influenza season and included A/California/07/2009 H1N1, A/Texas/50/2012 H3N2, and B/Massachusetts/02/2012. Released glycan and HA-specific glycopeptide glycosylation patterns were examined. We also examined the sensitivity of the single radial immunodiffusion (SRID) potency test to differences in HA antigen glycosylation. Based on deglycosylation studies applied using standard assay procedures, the SRID assay was not sensitive to any HA antigen glycosylation status from any cell system. Mapping of glycosites with their occupying glycan to functional regions, including antigenic sites, lectin interaction regions, and fusion domains, was performed and has implications for immune processing, immune responses, and antigenic shielding. Differences in glycosylation patterns, as dictated by the cell system used for expression, may impact these functions.IMPORTANCE In the present study, the glycosylation patterns of the 2014-2015 influenza vaccine season standard antigens A/California/07/2009 H1N1, A/Texas/50/2012 H3N2, and B/Massachusetts/02/2012 were revealed, and the sensitivity of the single radial immunodiffusion (SRID) potency test to glycosylation was tested. Differences in hemagglutinin glycosylation site composition and heterogeneity seen in antigens produced in different cell substrates suggest differences in processing and downstream immune responses. The SRID potency test used for vaccine release is not sensitive to differences in glycosylation under standard use conditions. This work reveals important differences in vaccine antigens and may point out areas where improvements may be made concerning vaccine antigen preparation, immune processing, and testing.

Enhanced Production of Recombinant Protein by Fusion Expression with Ssp DnaB Mini-Intein in the Baculovirus Expression System

The baculovirus expression system (BES) is considered to be a very powerful tool for the expression of numerous difficult to express vertebrate proteins. Ssp DnaB mini-intein is a useful fusion partner for the production of recombinant proteins because it can be self-cleaved by controlling the pH and temperature, without additional treatment. To evaluate the utility of Ssp DnaB mini-intein in the BES, recombinant viruses were generated to express the enhanced green fluorescent protein, the VP2 protein of porcine parvovirus, and the E2 protein of classical swine fever virus fused to a mini-intein. As expected, intracellular self-cleavage of the mini-intein occurred during virus infection, but the cleavage initiation time varied depending on the target protein. Significantly enhanced protein production was observed for all of the target proteins that were fused to the mini-intein. This increase was enough to overcome the decrease in the fusion protein due to intracellular self-cleavage. The mini-intein in all of the recombinant fusion proteins was successfully cleaved by controlling the pH and temperature. These results suggest that the Ssp DnaB mini-intein is a useful fusion partner in the BES for easy purification and enhanced production of target proteins.

A fusion-protein approach enabling mammalian cell production of tumor targeting protein domains for therapeutic development

A single chain Fv fragment (scFv) is a fusion of the variable regions of heavy (VH ) and light (VL ) chains of immunoglobulins. They are important elements of chimeric antigen receptors for cancer therapy. We sought to produce a panel of 16 extracellular protein domains of tumor markers for use in scFv yeast library screenings. A series of vectors comprising various combinations of expression elements was made, but expression was unpredictable and more than half of the protein domains could not be produced using any of the constructs. Here we describe a novel fusion expression system based on mouse TEM7 (tumor endothelial marker 7), which could facilitate protein expression. With this approach we could produce all but one of the tumor marker domains that could not otherwise be expressed. In addition, we demonstrated that the tumor associated antigen hFZD10 produced as a fusion protein with mTEM7 could be used to enrich scFv antibodies from a yeast display library. Collectively our study demonstrates the potential of specific fusion proteins based on mTEM7 in enabling mammalian cell production of tumor targeting protein domains for therapeutic development.

Recombinant expression and purification of functional vascular endothelial growth factor-121 in the baculovirus expression system

OBJECTIVE

To express human vascular endothelial growth factor121 (VEGF121) in insect cells.

METHODS

A gene construct containing VEGF was cloned in the pFastBac-HTA vector, followed by transformation in DH10BAC. The recombinant bacmid was then extracted, and transfected into Sf9 insect cells. The transfected cells were harvested, and then VEGF expression was confirmed by western blotting using specific antibodies. The tube formation assay was used for functional assessment of VEGF.

RESULTS

Our results showed that VEGF could be successfully expressed in the baculovirus system. Purified VEGF was able to stimulate in vitro tube formation of human endothelial cells.

CONCLUSIONS

Results from this study demonstrated that the recombinantly-produced VEGF can be considered as a promising candidate for therapeutic purposes.

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.

Minimizing fucosylation in insect cell-derived glycoproteins reduces binding to IgE antibodies from the sera of patients with allergy

The baculovirus/insect cell system has proven to be a very powerful tool for the expression of several therapeutics. Nevertheless, these products sometimes suffer from reduced biological activity and unwanted side effects. Several studies have demonstrated that glycosylation can greatly influence the structure, function, half-life, antigenicity and immunogenicity of various glycoproteins. Yet, the glycosylation pattern of insect cell-derived products is not favorable for many applications. Especially, the presence of core α1,3-linked fucose bears the risk of causing immediate hypersensitivity reactions in patients with allergy. In this study, we evaluated the impact of fucose residues on the allergenic potential of an insect cell-expressed vaccine candidate. In order to block the GDP-L-fucose de novo synthesis pathway, we integrated the Pseudomonas aeruginosa GDP-6-deoxy-D-lyxo-4-hexulose reductase (RMD) gene into a baculovirus backbone. This virus was then used for the expression of soluble influenza A virus hemagglutinin (HA). Expression studies showed that the co-expression of RMD did not influence the overall level of recombinant protein secretion. We confirmed the result of our strategy by analyzing PNGase A-released N-glycans using MALDI-TOF-MS. In order to evaluate the biological impact of defucosylation of influenza HA we tested the binding activity of IgE derived from the sera of patients with allergy to the purified antigen. The non-fucosylated HA showed a 10-fold decrease in IgE binding levels as compared to wildtype variants.

Large scale expression and purification of mouse melanopsin-L in the baculovirus expression system

Melanopsin is the mammalian photopigment that primarily mediates non-visual photoregulated physiology. So far, this photopigment is poorly characterized with respect to structure and function. Here, we report large-scale production and purification of the intact long isoform of mouse melanopsin (melanopsin-L) using the baculovirus/insect cell expression system. Exploiting the baculoviral GP67 signal peptide, we obtained expression levels that varied between 10-30pmol/10(6)cells, equivalent to 2-5mg/L. This could be further enhanced using DMSO as a chemical chaperone. LC-MS analysis confirmed that full-length melanopsin-L was expressed and demonstrated that the majority of the expressed protein was N-glycosylated at Asn(30) and Asn(34). Other posttranslational modifications were not yet detected. Purification was achieved exploiting a C-terminal deca-histag, realizing a purification factor of several hundred-fold. The final recovery of purified melanopsin-L averaged 2.5% of the starting material. This was mainly due to low extraction yields, probably since most of the protein was present as the apoprotein. The spectral data we obtained agree with an absorbance maximum in the 460-500nm wavelength region and a significant red-shift upon illumination. This is the first report on expression and purification of full length melanopsin-L at a scale that can easily be further amplified.

Different immunity elicited by recombinant H5N1 hemagglutinin proteins containing pauci-mannose, high-mannose, or complex type N-glycans

Highly pathogenic avian influenza H5N1 viruses can result in poultry and occasionally in human mortality. A safe and effective H5N1 vaccine is urgently needed to reduce the pandemic potential. Hemagglutinin (HA), a major envelope protein accounting for approximately 80% of spikes in influenza virus, is often used as a major antigen for subunit vaccine development. In this study, we conducted a systematic study of the immune response against influenza virus infection following immunization with recombinant HA proteins expressed in insect (Sf9) cells, insect cells that contain exogenous genes for elaborating N-linked glycans (Mimic) and mammalian cells (CHO). While the antibody titers are higher with the insect cell derived HA proteins, the neutralization and HA inhibition titers are much higher with the mammalian cell produced HA proteins. Recombinant HA proteins containing tri- or tetra-antennary complex, terminally sialylated and asialyated-galactose type N-glycans induced better protective immunity in mice to lethal challenge. The results are highly relevant to issues that should be considered in the production of fragment vaccines.

Mucin-type proteins produced in the Trichoplusia ni and Spodoptera frugiperda insect cell lines carry novel O-glycans with phosphocholine and sulfate substitutions

The O-glycans of a recombinant mucin-type protein expressed in insect cell lines derived from Trichoplusia ni (Hi-5) and Spodoptera frugiperda (Sf9) were characterized. The P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG2b) fusion protein carrying 106 potential O-glycosylation sites and 6 potential N-glycosylation sites was expressed and purified from the Hi-5 and Sf9 cell culture medium using affinity chromatography and gel filtration. Liquid chromatography mass spectrometry (LC-MS) of O-glycans released from PSGL-1/mIgG2b revealed a large repertoire of structurally diverse glycans, which is in contrast to previous reports of only simple glycans. O-Glycans containing hexuronic acid (HexA, here glucuronic acid and galacturonic acid) were found to be prevalent. Also sulfate (Hi-5 and Sf9) and phosphocholine (PC; Sf9) O-glycan substitutions were detected. Western blotting confirmed the presence of O-linked PC on PSGL-1/mIG2b produced in Sf9 cells. To our knowledge, this is the first structural characterization of PC-substituted O-glycans in any species. The MS analyses revealed that Sf9 oligosaccharides consisted of short oligosaccharides (<6 residues) low in hexose (Hex) and with terminating N-acetylhexosamine (HexNAc) units, whereas Hi-5 produced a family of large O-glycans with (HexNAc-HexA-Hex) repeats and sulfate substitution on terminal residues. In both cell lines, the core N-acetylgalactosamine was preferentially non-branched, but small amounts of O-glycan cores with single fucose or hexose branches were found.

H5N1 influenza virus-like particles produced by transient expression in mammalian cells induce humoral and cellular immune responses in mice

Vaccination is an effective way to protect from influenza virus infection. Among the new candidates of influenza vaccines, influenza virus-like particles (VLPs) seem to be promising. Here, we generated 2 types of H5N1 influenza VLPs by co-expressing influenza virus Env (envelope protein) and murine leukemia virus (MLV) Gag–Pol. VLPs generated by co-transfection of pHCMV-wtH5 or pHCMV-mtH5 with pSV-Mo-MLVgagpol and pHCMV-N1 were named as wtH5N1 VLPs or mtH5N1 VLPs. The plasmid of pHCMV-wtH5 encoded the wild-type hemagglutinin (HA) (wtH5) from A/swine/Anhui/ca/2004 (H5N1) with a multibasic cleavage site, while pHCMV-mtH5 encoded the modified mutant-type (mtH5) with a monobasic cleavage site. Influenza virus HA VLPs were characterized and equal amounts of them were used to immunize mice subcutaneously, intraperitoneally, or intramuscularly. The levels of HA-specific IgG1, IFN-γ, and neutralization antibodies were significantly induced in mice immunized with wtH5N1 VLPs or mtH5N1 VLPs via all 3 routes, while HA-specific IgG2a was barely detectable. IL-4 secretion was detected in mice subcutaneously immunized with wtH5N1 VLPs or mtH5N1 VLPs, or intramuscularly immunized with mtH5N1 VLPs. Our results indicated that both H5N1 influenza VLPs could induce specific humoral and cellular immune responses in immunized mice. In conclusion, our study provides helpful information for designing new candidate vaccines against H5N1 influenza viruses.

Glycosylation of Twisted Gastrulation is Required for BMP Binding and Activity during Craniofacial Development

Twisted gastrulation (TWSG1) is a conserved, secreted glycoprotein that modulates signaling of bone morphogenetic proteins (BMPs) in the extracellular space. Deletion of exon 4 of mouse Twsg1 (mTwsg1) is associated with significant craniofacial defects. However, little is understood about the biochemical properties of the corresponding region of the protein. We have uncovered a significant role for exon 4 sequences as encoding the only two glycosylation sites of the mTWSG1 protein. Deletion of the entire exon 4 or mutation of both glycosylation sites within exon 4 abolishes glycosylation of mTWSG1. Importantly, we find that constructs with mutated glycosylation sites have significantly reduced BMP binding activity. We further show that glycosylation and activity of TWSG1 recombinant proteins vary markedly by cellular source. Non-glycosylated mTWSG1 made in E. coli has both reduced affinity for BMPs, as shown by surface plasmon resonance analysis, and reduced BMP inhibitory activity in a mandibular explant culture system compared to glycosylated proteins made in insect cells or murine myeloma cells. This study highlights an essential role for glycosylation in Twisted gastrulation action.

Semi-synthesis of glycoproteins from E. coli through native chemical ligation

Sufficient quantities of homogeneous samples of post-translationally modified proteins are often not readily available from biological sources to facilitate structure-function investigations. Native chemical ligation (NCL) is a convenient method for the production of biologically active proteins from smaller fragments. Such an approach allows protein modifications to be introduced in a controlled fashion into smaller peptide fragments which are amenable to total chemical synthesis. These fragments of defined sequence and structure can be elaborated to full-length proteins through NCL reactions with suitable components derived from bacterial origin. This report describes methods for the bacterial production of components for NCL and their use in typical reactions.

Enzyme-linked immunosorbent assay for detection of filovirus species-specific antibodies

Several enzyme-linked immunosorbent assays (ELISAs) for the detection of filovirus-specific antibodies have been developed. However, diagnostic methods to distinguish antibodies specific to the respective species of filoviruses, which provide the basis for serological classification, are not readily available. We established an ELISA using His-tagged secreted forms of the transmembrane glycoproteins (GPs) of five different Ebola virus (EBOV) species and one Marburg virus (MARV) strain as antigens for the detection of filovirus species-specific antibodies. The GP-based ELISA was evaluated by testing antisera collected from mice immunized with virus-like particles as well as from humans and nonhuman primates infected with EBOV or MARV. In our ELISA, little cross-reactivity of IgG antibodies was observed in most of the mouse antisera. Although sera and plasma from some patients and monkeys showed notable cross-reactivity with the GPs from multiple filovirus species, the highest reactions of IgG were uniformly detected against the GP antigen homologous to the virus species that infected individuals. We further confirmed that MARV-specific IgM antibodies were specifically detected in specimens collected from patients during the acute phase of infection. These results demonstrate the usefulness of our ELISA for diagnostics as well as ecological and serosurvey studies.

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.

Dissecting cross-reactivity in hymenoptera venom allergy by circumvention of alpha-1,3-core fucosylation

Hymenoptera venom allergy is known to cause life-threatening and sometimes fatal IgE-mediated anaphylactic reactions in allergic individuals. About 30-50% of patients with insect venom allergy have IgE antibodies that react with both honeybee and yellow jacket venom. Apart from true double sensitisation, IgE against cross-reactive carbohydrate determinants (CCD) are the most frequent cause of multiple reactivities severely hampering the diagnosis and design of therapeutic strategies by clinically irrelevant test results. In this study we addressed allergenic cross-reactivity using a recombinant approach by employing cell lines with variant capacities of alpha-1,3-core fucosylation. The venom hyaluronidases, supposed major allergens implicated in cross-reactivity phenomena, from honeybee (Api m 2) and yellow jacket (Ves v 2a and its putative isoform Ves v 2b) as well as the human alpha-2HS-glycoprotein as control, were produced in different insect cell lines. In stark contrast to production in Trichoplusia ni (HighFive) cells, alpha-1,3-core fucosylation was absent or immunologically negligible after production in Spodoptera frugiperda (Sf9) cells. Consistently, co-expression of honeybee alpha-1,3-fucosyltransferase in Sf9 cells resulted in the reconstitution of CCD reactivity. Re-evaluation of differentially fucosylated hyaluronidases by screening of individual venom-sensitised sera emphasised the allergenic relevance of Api m 2 beyond its carbohydrate epitopes. In contrast, the vespid hyaluronidases, for which a predominance of Ves v 2b could be shown, exhibited pronounced and primary carbohydrate reactivity rendering their relevance in the context of allergy questionable. These findings show that the use of recombinant molecules devoid of CCDs represents a novel strategy with major implications for diagnostic and therapeutic approaches.

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.

Deciphering Dorin M glycosylation by mass spectrometry

The soft tick, Ornithodoros moubata, is a vector of several bacterial and viral pathogens including Borrelia duttoni, a causative agent of relapsing fever and African swine fever virus. Previously, a sialic acid-specific lectin Dorin M was isolated from its hemolymph. Here, we report on the complete characterization of the primary sequence of Dorin M. Using liquid chromatography coupled to mass spectrometry, we identified three different glycopeptides in the tryptic digest of Dorin M. The peptide, as well as the glycan part of all glycopeptides, were further fully sequenced by means of tandem mass spectrometry (MS2) and multiple-stage mass spectrometry (MS3). Two classical N-glycosylation sites were modified by high-mannose-type glycans containing up to nine mannose residues. The third site bore a glycan with four to five mannose residues and a deoxyhexose (fucose) attached to the proximal N-acetylglycosamine. The microheterogeneity at each site was estimated based on chromatographic behavior of different glycoforms. The fourth, a non-classical N-glycosylation site (Asn-Asn-Cys), was not glycosylated, probably due to the involvement of the cysteine residue in a disulfide bridge.

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 and functional characterization of classical swine fever virus E(rns) protein

E(rns) is an envelope glycoprotein of classical swine fever virus (CSFV) with unusual RNase activity. Recently, E(rns) was found to have a new function of counteracting the beta-interferon (IFN-beta) induction pathway. In this study, wildtype ErnsSM and two mutated E(rns) proteins ErnsH297k and ErnsH346k were expressed in insect cells and purified for RNase activity and function analysis. RNase activity assay in vitro demonstrated that only wildtype E(rns) protein had RNase activity. However, both wildtype ErnsSM and the two mutated E(rns)ErnsH297k and ErnsH346k as exogenous proteins had a block effect on Newcastle disease virus (NDV)-mediated IFN-beta promoter induction.

Viral envelope protein gp64 transgenic mouse facilitates the generation of monoclonal antibodies against exogenous membrane proteins displayed on baculovirus

We have been investigating the functional display of multipass membrane protein such as transporter or G-protein coupled receptor on the budded baculovirus (BV). We tested the use of a viral envelope protein gp64 transgenic mouse for the direct immunization of these membrane proteins displayed on BVs. The gp64 transgenic mice showed only a weak response to virus compared to wild type BALB/c mice. Immunizing gp64 transgenic mice with the BV expressing peptide transporter PepT1, we obtained 47 monoclonal antibodies (mAbs). These mAbs were specific to the PepT1 on the pancreatic cancer cells AsPC-1 by fluorocytometric analysis and exhibited antibody-dependent cellular cytotoxicity or complement-dependent cytotoxicity to AsPC-1. We also generated 7 mAbs by immunizing gp64 transgenic mice on a CCR2-deficient background with the BV expressing chemokine receptor CCR2 together with partially purified CCR2. These mAbs possessed specific binding to CCR2 in CHO cells on fluorocytometric analysis, and exhibited neutralizing activities for ligand-dependent inhibition of cyclic AMP production. This method provides a powerful tool for the generation of therapeutic/diagnostic mAbs against membrane proteins.

Post-translational modifications in the context of therapeutic proteins

The majority of protein-based biopharmaceuticals approved or in clinical trials bear some form of post-translational modification (PTM), which can profoundly affect protein properties relevant to their therapeutic application. Whereas glycosylation represents the most common modification, additional PTMs, including carboxylation, hydroxylation, sulfation and amidation, are characteristic of some products. The relationship between structure and function is understood for many PTMs but remains incomplete for others, particularly in the case of complex PTMs, such as glycosylation. A better understanding of such structural-functional relationships will facilitate the development of second-generation products displaying a PTM profile engineered to optimize therapeutic usefulness.

New tools for G-protein coupled receptor (GPCR) drug discovery: combination of baculoviral expression system and solid state NMR

Biotechnology using molecular biology, biochemistry, biophysics, and computational approaches provides an alternative approach for classical pharmacological screening to look at ligand-receptor interactions and receptor specificity, which should support the design of selective drugs based on detailed structural principles. This review addresses specific approaches to study function, structure and relevance of a major pharmaceutical target, namely the G-Protein Coupled Receptors (GPCRs). The main aim of this review has been to exploit and combine GPCR over-expression in a baculoviral expression system with solid-state MAS NMR (ssNMR) approaches for the elucidation of electronic structures of the coordinating ligands/drugs and their modes of interactions with the GPCRs. This review summarizes the approaches, possible future experiments and developments using the above combination of tools for GPCR drug discovery.

Stably Transformed Insect Cell Lines: Tools for Expression of Secreted and Membrane‐anchored Proteins and High‐throughput Screening Platforms for Drug and Insecticide Discovery

Insect cell-based expression systems are prominent amongst current expression platforms for their ability to express virtually all types of heterologous recombinant proteins. Stably transformed insect cell lines represent an attractive alternative to the baculovirus expression system, particularly for the production of secreted and membrane-anchored proteins. For this reason, transformed insect cell systems are receiving increased attention from the research community and the biotechnology industry. In this article, we review recent developments in the field of insect cell-based expression from two main perspectives, the production of secreted and membrane-anchored proteins and the establishment of novel methodological tools for the identification of bioactive compounds that can be used as research reagents and leads for new pharmaceuticals and insecticides.

Demonstration by heterologous expression that the Leishmania SCA1 gene encodes an arabinopyranosyltransferase

In part of the life cycle within their sand fly vector, Leishmania major parasites first attach to the fly's midgut through their main surface adhesin lipophosphoglycan (LPG) and later resynthesize a structurally distinct LPG that results in detachment and eventual transmission. One of these structural modifications requires the addition of alpha1,2-D-arabinopyranose caps to beta1,3-galactose side chains in the phosphoglycan repeat unit domain of LPG. We had previously identified two side chain arabinose genes (SCA1/2) that were involved in the alpha1,2-D-Arap capping. SCA1/2 exhibit canonical glycosyltransferase motifs, and overexpression of either gene leads to elevated microsomal alpha1,2-D-ArapT activity, resulting in arabinopyranosylation of beta1,3-Gal side chains in LPG (hereafter called side chain D-arabinopyranosyltransferase [sc-D-ArapT]). Heterologous expression in a null arabinose background was used to determine whether the SCA1 gene encodes the actual sc-D-ArapT. SCA1 expression constructs introduced into both mammalian COS-7 cells and the baculovirus-sf9 cell system exhibited considerable expression of the protein. However, functional sc-D-ArapT activity was observed only in the latter. In in vitro assays incubated with guanidine 59-diphosphate (GDP)-D-[3H]Arap as the sugar donor and utilizing exogenous LPG as an acceptor, significant sc-D-ArapT activity was observed when microsomes from the baculovirus-sf9 cells were incubated in presence of the LPG acceptor. No activity was observed in the absence of LPG. These results demonstrate that SCA1 encodes a sc-D-ArapT and provide the first example of heterologous expression of a D-ArapT gene.

Baculovirus as versatile vectors for protein expression in insect and mammalian cells

Today, many thousands of recombinant proteins, ranging from cytosolic enzymes to membrane-bound proteins, have been successfully produced in baculovirus-infected insect cells. Yet, in addition to its value in producing recombinant proteins in insect cells and larvae, this viral vector system continues to evolve in new and unexpected ways. This is exemplified by the development of engineered insect cell lines to mimic mammalian cell glycosylation of expressed proteins, baculovirus display strategies and the application of the virus as a mammalian-cell gene delivery vector. Novel vector design and cell engineering approaches will serve to further enhance the value of baculovirus technology.

The N-linked oligosaccharides of aminopeptidase N from Manduca sexta: site localization and identification of novel N-glycan structures

Mass spectrometric studies on the N-linked glycans of aminopeptidase 1 from Manduca sexta have revealed unusual structures not previously observed on any insect glycoprotein. Structure elucidation of these oligosaccharides was carried out by high-energy collision-induced dissociation (CID) using a matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI-TOF/TOF) tandem mass spectrometer. These key experiments revealed that three out of the four N-linked glycosylation sites in this protein (Asn295, Asn623 and Asn752) are occupied with highly fucosylated N-glycans that possess unusual difucosylated cores. Cross-ring fragment ions and 'internal' fragment ions observed in the CID spectra, showed that these fucoses are found at the 3-position of proximal GlcNAc and at the 3-position of distal GlcNAc in the chitobiose unit. The latter substitution has only been previously observed in nematodes. In addition, these core structures can be decorated with novel fucosylated antennae composed of Fucalpha(1-3)GlcNAc. Key fragment ions revealed that these antennae are predominantly found on the upper 6-arm of the core mannose. The paucimannosidic N-glycan (Man(3)GlcNAc(2)), commonly found on other insect glycoproteins, is the predominant oligosaccharide found at the remaining N-glycosylation site (Asn609).

A glutathione-S-transferase-FSHalpha subunit hybrid associates with FSHbeta, retains biological activity, and facilitates purification

The glycoprotein hormones are heterodimeric proteins that share a common alpha subunit and have unique beta subunits that confer receptor selectivity. One member of this family, follicle-stimulating hormone (FSH), is secreted by the pituitary and is involved in the control of male and female reproduction. Herein, we describe the construction of baculoviruses for glutathione-S-transferase (GST) fusions of the human FSH (hFSH) subunits and their expression in insect cells, either alone or with the complementary non-fused FSH subunits (FSHalpha or FSHbeta). Only the GST-BV-hFSHalpha monomer and the GST-BV-hFSHalpha/BV-hFSHbeta (GST-BV-hFSH) heterodimer were efficiently secreted into the culture supernatant. The hybrid molecule, GST-BV-hFSH, was affinity purified in one step, and demonstrated activity in receptor-radioligand binding assays and in a cAMP accumulation assay. The use of GST-BV-hFSHalpha provides a novel and efficient method for purifying and studying members of the glycoprotein hormone family derived from the culture supernatant or subcellular fractions of the cell.

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.

Recombinant replication-restricted VSV as an expression vector for murine cytokines

Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus that rapidly and efficiently shuts down the production of host cell-encoded proteins and utilizes the cell's protein production machinery to express high levels of virally encoded proteins. In an effort to take advantage of this characteristic of VSV, we have employed a reverse genetics system to create recombinant forms of VSV encoding a variety of murine cytokines. Previous studies have revealed that cells infected with recombinant VSV that lack expression of the surface glycoprotein (G protein), designated deltaG-VSV, more efficiently express and secrete recombinant proteins than do recombinant "wild-type" VSV. Therefore, murine cytokine-expressing recombinants were produced as deltaG viruses. Propagation of these deltaG viruses in cells that transiently express G protein in vitro results in G-complemented virions that can infect cells, shut down host protein synthesis, and express at high levels each virally encoded protein (including the designated cytokine). We assessed the ability of each deltaG-VSV construct to express recombinant cytokine by infecting BHK cells and then monitoring/measuring the production of the desired cytokine. When possible, the bioactivity of the cytokine products was also measured. The results presented here reveal that large quantities of bioactive cytokines can be produced rapidly and inexpensively using deltaG-VSV as a protein expression system.

Targeting macrophages with baculovirus-produced lysosomal enzymes: implications for enzyme replacement therapy of the glycoprotein storage disorder galactosialidosis

Lysosomal storage diseases (LSDs) are monogenic disorders of metabolism caused by deficiency of hydrolytic enzymes. In several LSDs, cells of the reticuloendothelial (RE) system are the primary targets of the disease. Exogenous administration of recombinant enzymes overproduced in mammalian cells has proved effective for treating the systemic phenotype in nonneuropathic patients with LSDs. However, for the treatment of diseases with primary involvement of the RE system, the production of the therapeutic enzyme in insect cells could be an alternative and advantageous method because glycoproteins expressed in insect cells carry carbohydrates of the pauci-mannose or core-type. These recombinant enzymes are in principle already poised to be internalized by cells that express mannose receptors, including macrophages. Here, we demonstrate that three baculovirus-expressed enzymes, protective protein/cathepsin A (PPCA), neuraminidase (Neu1), and beta-glucosidase, were readily taken up and restored lysosomal function in enzyme-deficient mouse macrophages. The capacity of recombinant PPCA and Neu1 to clear the lysosomal storage in target cells was assessed in PPCA-/- mice, a model of galactosialidosis. Intravenously injected PPCA-/- mice efficiently internalized the corrective enzymes in resident macrophages of many organs. In addition, treated mice showed overall clearance of lysosomal storage in the most affected systemic organs, kidney, liver, and spleen. Our results suggest that ERT with baculovirus-expressed enzymes might be an effective treatment for nonneuropathic patients with galactosialidosis and possibly for others with LSDs that primarily involve the RE system.

A functional assay for G-protein-coupled receptors using stably transformed insect tissue culture cell lines

Insect cells are an underexplored resource for functional G-protein-coupled receptor (GPCR) assays, despite a strong record in biochemical (binding) assays. Here we describe the use of vectors capable of creating stably transformed insect cell lines to generate a cell-based functional GPCR assay. This assay employs the luminescent photoprotein aequorin and the promiscuous G-protein subunit Galpha16 and is broadly applicable to human GPCRs. We demonstrate that the assay can quantitate ligand concentration-activity relationships for seven different human GPCRs, can differentiate between partial and full agonists, and can determine rank order potencies for both agonists and antagonists that match those seen with other assay systems. Human Galpha16 improves signal strength but is not required for activity with some receptors. The coexpression of human and bovine betagamma subunits and/or phospholipase Cbeta makes no difference to agonist efficacy or potency. Two different receptors expressed in the same cell line respond to their specific agonists, and two different cell lines (Sf9 and High 5) are able to functionally detect the same expressed GPCR. Sf9 cells have the capability to produce fully functional human receptors, allied to a low background of endogenous receptors, and so are a valuable system for investigating orphan GPCRs and receptor dimerization.

Developing baculovirus-insect cell expression systems for humanized recombinant glycoprotein production

The baculovirus-insect cell expression system is widely used to produce recombinant glycoproteins for many different biomedical applications. However, due to the fundamental nature of insect glycoprotein processing pathways, this system is typically unable to produce recombinant mammalian glycoproteins with authentic oligosaccharide side chains. This minireview summarizes our current understanding of insect protein glycosylation pathways and our recent efforts to address this problem. These efforts have yielded new insect cell lines and baculoviral vectors that can produce recombinant glycoproteins with humanized oligosaccharide side chains.

Improvement of glycosylation in insect cells with mammalian glycosyltransferases

The N-glycans of recombinant glycoproteins expressed in insect cells mainly contain high mannose or tri-mannose structures, which are truncated forms of the sialylated N-glycans found in mammalian cells. Because asialylated glycoproteins have a shorter half-life in blood circulation, we investigated if sialylated therapeutic glycoprotein can be produced from insect cells by enhancing the N-glycosylation machinery of the cells. We co-expressed in two insect cell lines, Sf9 and Ea4, the human alpha1-antitrypsin (halpha1AT) protein with a series of key glycosyltransferases, including GlcNAc transferase II (GnT2), beta1,4-galactosyltransferase (beta14GT), and alpha2,6-sialyltransferase (alpha26ST) by a single recombinant baculovirus. We demonstrated that the enhancement of N-glycosylation is cell type-dependent and is more efficient in Ea4 than Sf9 cells. Glycan analysis indicated that sialylated halpha1AT proteins were produced in Ea4 insect cells expressing the above-mentioned exogenous glycosyltransferases. Therefore, our expression strategy may simplify the production of humanized therapeutic glycoproteins by improving the N-glycosylation pathway in specific insect cells, with an ensemble of exogenous glycosyltransferases in a single recombinant baculovirus.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.