Synthesis of chicken major histocompatibility complex class II oligomers using a baculovirus expression system

Chicken major histocompatibility complex (MHC) B21 and B19 haplotypes are associated with resistance and susceptibility to Marek's disease (MD), respectively. T-cell-mediated immune response is crucial in coordinating protection against Marek's disease virus (MDV) infection, but it has been difficult to identify and characterize antigen-specific T-cells. MHC class II tetramers and oligomers have been widely used for characterization of antigen-specific T-cells in the context of infectious and autoimmune diseases. Thus, the objective of this study was to synthesize chicken MHC class II oligomers of B21 and B19 haplotypes for the future identification of antigen-specific T-cells. To achieve this objective, full-length coding sequences of chicken MHC class II B21 and B19 molecules were amplified and the molecules were expressed as fusion proteins, carrying Fos and Jun leucine zipper (LZ), histidine-tag and biotin ligase recognition site sequences, using a baculovirus expression system. Recombinant MHC-II were loaded with self-peptides, which stabilized the heterodimer in SDS-PAGE and allowed the detection of these molecules in Western blots with a conformation-specific anti-chicken MHC class II antibody. Biotinylated MHC molecules were conjugated to streptavidin to form oligomers, which were resolved under the transmission electron microscope through immuno-gold labelling, thus confirming success of oligomerization. In conclusion, chicken MHC class II oligomers may be used in the future to study the antigen-specific CD4+ T-cell compartment.     

Synthesis of viruslike particles by expression of the putative capsid protein of Leishmania RNA virus in a recombinant baculovirus expression system.

The putative capsid open reading frame (ORF2) of the Leishmania RNA virus LRV1-4 was expressed in a baculovirus expression system. The expressed protein was identified by Western immunoblot analysis with polyclonal antiserum raised to purified LRV1-4 virus. Electron microscopy and sedimentation analysis indicated that the expressed protein self-assembles into empty viruslike particles of similar size and shape to authentic virus particles, thus confirming that ORF2 encodes the viral capsid. The expressed particles are present exclusively in the cytoplasm of infected SF9 cells and are able to assemble in the absence of LRV1-4 RNA, viral polymerase, or any Leishmania host factors.     

Expression of recombinant myeloperoxidase using a baculovirus expression system.

Myeloperoxidase (MPO) is a glycosylated heme-containing enzyme present in the azurophilic granules of normal human polymorphonuclear neutrophils. This enzyme plays a major role in the microbicidal activity of the host defense system by catalyzing the formation of the potent oxidant, hypochlorous acid. Although the amino acid sequence of MPO has been deduced from the cDNA, the structural basis for the observed heterogeneity of this enzyme is not known. Furthermore, the nature of the prosthetic group and its mode of linkage to the apoprotein has not been determined. To address questions regarding the structural features of MPO, which arise during the complex posttranslational processing of this enzyme, we utilized a baculovirus system to express MPO in Sf9 insect cells. Two glycosylated, single-chain precursor species of MPO were observed: an 84 kDa species that was secreted and a 74 kDa species that was cell-associated. This is the first report of an expression system in which a cell-associated MPO precursor undergoes posttranslational proteolytic processing.     

Structural polymorphism of the major capsid protein of rotavirus

Rotaviruses are important human pathogens with a triple-layered icosahedral capsid. The major capsid protein VP6 is shown here to self-assemble into spherical or helical particles mainly depending upon pH. Assembly is inhibited either by low pH (<3.0) or by a high concentration (>100 mM) of divalent cations (Ca(2+) and Zn(2+)). The structures of two types of helical tubes were determined by electron cryomicroscopy and image analysis to a resolution of 2.0 and 2.5 nm. In both reconstructions, the molecular envelope of VP6 fits the atomic model determined by X-ray crystallography remarkably well. The 3-fold symmetry of the VP6 trimer, being incompatible with the helical symmetry, is broken at the level of the trimer contacts. One type of contact is maintained within all VP6 particles (tubes and virus), strongly suggesting that VP6 assemblies arise from different packings of a unique dimer of trimers. Our data show that the protonation state and thus the charge distribution are important switches governing the assembly of macromolecular assemblies.     

Synthesis of a human lysosomal enzyme, beta-hexosaminidase B, using the baculovirus expression system

Human lysosomal beta-hexosaminidase exists in two major forms: the A isoform is composed of both alpha and beta chains, while the B form is a homopolymer of beta chains. Deficiency of beta-hexosaminidase underlies the GM2 gangliosidoses. We have produced active beta-hexosaminidase B in cultured insect (Sf9) cells by isolation of a recombinant insect virus (baculovirus) containing the cDNA for the beta chain within the viral polyhedron gene and infection of Sf9 cells with this construct. That portion of the enzyme secreted into the medium, 50%, was purified with concanavalin A Sepharose and subsequent affinity chromatography to yield beta-hexosaminidase B that is 75% pure. The product has an N-terminal amino acid sequence, specific activity, and size (M(r) 62,000) similar to that of the enzyme present in cultured human fibroblasts. However, endo H sensitivity studies revealed that the oligosaccharide structures present on recombinant beta-hexosaminidase B differ from those found on the enzyme synthesized in the human system. In addition, these structures lack the mannose 6-phosphate recognition marker that targets degradative hydrolases to lysosomes. Despite these differences, recombinant beta-hexosaminidase B does serve as a specific substrate for the mannose phosphorylating enzyme, N-acetylglucosaminyl phosphotransferase. Furthermore, the oligosaccharide moieties phosphorylated in vitro match those phosphorylated in vivo, pointing to the conformational integrity of the recombinant enzyme. Generous amounts of easily obtained, easily purified, and properly folded beta-hexosaminidase B will facilitate physical structural analysis of the enzyme.     

Overexpression of plant histidinol dehydrogenase using a baculovirus expression vector system.

A cDNA encoding cabbage histidinol dehydrogenase, including the chloroplast transit peptide sequence, was overexpressed using a baculovirus expression vector system. The maximum level of the expression of histidinol dehydrogenase was reached 5 days after infection of the insect cells. Two forms of recombinant histidinol dehydrogenase with molecular masses of 53 and 52 kDa, respectively, were obtained by a one-step purification from the cell homogenate. Compared with the 52-kDa form, the 53-kDa form contained 10 additional amino acids at the N-terminus derived from the transit peptide. By incubating the cell homogenate for 2 h at 30 degrees C, the 53-kDa form could be completely converted to the 52-kDa form. This conversion was blocked by leupeptin. Eighty percent of the converted 52-kDa form had Cys at position 31 at the N-terminal amino acid and the rest had Met 33. Kinetic properties of the recombinant enzyme were virtually identical to those of histidinol dehydrogenase isolated from cabbage plants. The overexpression of recombinant cabbage histidinol dehydrogenase in insect cells, the proteolytic processing of the preprotein next to the N-terminus (compared to the mature cabbage enzyme), and its easy purification allow the preparation of large amounts of the active enzyme for structural and functional studies.     

Improvement of DNA vaccine immunogenicity by a dual antigen expression system

This study examined whether increased antigen expression resulted in enhanced antigen-specific immune responses in the context of DNA vaccines. To increase antigen expression, two copies of antigen expression cassettes were arranged in a plasmid pDX. BALB/c mice were intramuscularly immunized with various constructs that express influenza antigens and analysed for DNA-raised immunity. The plasmid pDX that expresses two copies of the antigen gene induced stronger antigen-specific immune responses than the plasmid pGA which expresses single antigen gene. To explore the in vivo transgene expression by pDX and pGA, luciferase activity was measured in the muscles transduced with luciferase expression plasmids. The pDX expressing two copies of luciferase induced the highest luciferase activity, which corresponded to the results from vaccination. We concluded that increasing the number of antigen expression cassettes in a vaccine construct improved antigen expression in the transduced tissue, which induced stronger DNA-raised immune responses.     

Polyomavirus major capsid protein VP1 is capable of packaging cellular DNA when expressed in the baculovirus system.

Using the p2Bac dual multiple cloning site transfer vector, the polyomavirus major capsid protein gene VP1 was cloned for expression in the baculovirus-insect cell expression system. The 5-day-infected cellular lysate from this recombinant preparation was purified by cesium chloride density gradient centrifugation. Capsid-like particles were observed in the resulting preparation. The purified particle preparation was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was shown to have accurately expressed the polyomavirus VP1 protein as cloned. It was found that the preparation revealed the presence of host histones in the stained gels, which is indicative of DNA packaging. To determine if cellular DNA was being packaged in the particles, Sf9 insect cells were prelabeled with [3H] thymidine. The label was removed, and the cells were subsequently infected with a recombinant Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) carrying the polyomavirus VP1 gene. Upon purification through three cesium chloride gradients and DNase I treatment, capsid-like particles, containing [3H]thymidine-labeled DNA, were isolated which were found to coincide with hemagglutination activity. Studies have indicated that the AcMNPV appears to have the ability to fragment Sf9 cellular DNA. When infected with the recombinant AcMNPV carrying the VP1 gene of polyomavirus, these host DNA fragments are being packaged by the VPI major capsid protein; further, these DNA fragments have been shown to be approximately 5 kb in size, which corresponds to the size of the native polyomavirus genome. These studies demonstrate that the recombinant polyomavirus VP1 protein has the ability to package DNA in the absence of the minor structural proteins VP2 and VP3 and independently of the polyomavirus T antigens.     

Overexpression and purification of human XPA using a baculovirus expression system

The xeroderma pigmentosum group A protein (XPA) is an essential component of the eukaryotic nucleotide excision repair (NER) process. Recombinant human XPA was expressed in baculovirus-infected insect cells as a [His](6)-tagged fusion protein. A two-column purification procedure resulted in greater than 90% purity for the recombinant protein with a final yield of 0.53 mg from 200 ml of infected cells. The recombinant protein migrated as a doublet of 44 and 42 kDa upon SDS-PAGE consistent with that observed for the native protein. XPA can interact with a number of proteins including replication protein A (RPA) which has been implicated in the initial recognition of damaged DNA. Using a modified ELISA, we demonstrate that the recombinant XPA fusion protein also forms a complex with RPA independent of DNA. The ability of XPA to bind damaged DNA was assessed in an electrophoretic mobility shift assay using globally cisplatin-damaged DNA. The results revealed a slight preference for DNA damaged with cisplatin consistent with its proposed role in the recognition of damaged DNA. The recombinant XPA fusion protein was able to complement cell-free extracts immunodepleted of XPA restoring NER-catalyzed incision of cisplatin-damaged DNA in an in vitro excision repair assay.     

Expression of a biologically active ovine trophoblastic interferon using a baculovirus expression system.

Ovine trophoblast protein (oTP) an embryonic interferon, which plays a key role in maternal recognition of pregnancy, has been expressed in insect cells using a baculovirus expression system. A cDNA coding for oTP was inserted downstream of the strong polyhedrin promoter. Cells infected with recombinant virus produced biologically active oTP and greater than 90% was secreted into the culture medium during infection. High amount of antiviral activity were produced (up to 5 x 10(5) IU per ml of culture medium). Recombinant oTP (roTP) was purified by immunoaffinity chromatography and found to be identical to authentic oTP with respect to molecular mass and N-terminal amino acid sequence.     

Expression and purification of functional G protein alpha subunits using a baculovirus expression system.

The alpha subunits of the heterotrimeric guanine nucleotide-binding proteins Gi1, Gi2, Gi3, G0, and Gs have been overexpressed in Sf9 cells using a baculovirus expression system. The Gi1 alpha, Gi2 alpha, Gi3 alpha, and G0 alpha have been purified to homogeneity from infected Spodoptera frugiperda (SF9) cells and characterized. Yields of up to 1.8 mg of purified recombinant G alpha have been obtained from 300-ml cultures of infected cells. The recombinant alpha subunits are myristoylated and are ADP-ribosylated by pertussis toxin only in the presence of beta gamma subunits. They bind guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) with low nM dissociation constants and stoichiometries of 0.8 mol/mol or greater. The rGi1 alpha, rGi2 alpha, and rGi3 alpha are capable of interacting with angiotensin II receptors based on their ability to restore high affinity angiotensin II binding in rat liver membranes shifted to a low affinity state with GTP gamma S.     

Purification and characterization of human Syk produced using a baculovirus expression system

The cytoplasmic tyrosine kinase p72syk (Syk) plays an essential role in signaling via a variety of immune and nonimmune cell receptors. Syk is activated in response to the engagement of the appropriate cell surface receptors and can phosphorylate downstream targets and recruit additional SH2-domain-containing proteins. In order to study the characteristics of Syk in vitro, we have overexpressed untagged, full-length human Syk in a recombinant baculovirus expression system. The enzyme was purified to 95% purity using a novel two-step affinity chromatography process using reactive yellow and phosphotyrosine columns. Yields of 3-10 mg purified Syk were obtained from 1 liter of infected insect cells. Western blotting, internal protein sequencing, and the specific tyrosine phosphorylation of a Syk peptide substrate indicated authenticity of the purified protein. The enzymatic properties of Syk were in good agreement with published data for the human enzyme, as the apparent K(m) of Syk for ATP was 10 microM and the peptide substrate was 3 microM. The recombinant protein also showed similar biochemical characteristics to the native protein isolated from B-cells such as autophosphorylation. Proteolytic cleavage of purified recombinant Syk was used to generate the kinase domain by micro-calpain. We therefore describe an efficient expression system and purification methodology to produce biologically active human Syk.     

The immunogenicity of VP7, a rotavirus antigen resident in the endoplasmic reticulum, is enhanced by cell surface expression.

The glycoprotein VP7, the major serotype antigen of rotaviruses, is localized to the endoplasmic reticulum (ER) of the cell, where it is retained as a membrane-associated protein before assembly into mature virus particles. Wild-type VP7 expressed by a recombinant vaccinia virus was also located internally and was poorly antigenic. Using recombinant techniques, a correctly processed, secreted form of VP7 (S.C. Stirzaker and G.W. Both, Cell 56:741-747, 1989) was modified by addition to its C terminus of the membrane anchor and cytoplasmic domains from the influenza virus hemagglutinin. The hybrid protein was directed to the surface of cells, where it was anchored in the plasma membrane. When expressed in mice and rabbits by a recombinant vaccinia virus, the surface-anchored antigen stimulated a level of rotavirus-specific antibodies that was greater than 100-fold above the level induced by wild-type VP7. T-cell responses to the novel antigen were also elevated in comparison with the wild-type, intracellular protein. Cell surface anchoring may provide a strategy to increase the immunogenicity of intracellular antigens from other parasites and viruses.     

Expression of Yb1 glutathione S-transferase using a baculovirus expression system

A full-length cDNA clone was isolated for rat liver Yb1 glutathione S-transferase (EC 2.5.1.18). The coding sequence of Yb1 cDNA was inserted into a baculovirus vector for infection of Spodoptera frugiperda (SF9) cells. The enzymatically active recombinant Yb1 glutathione S-transferase protein has a native molecular weight of 42,000 daltons (by molecular sieve chromatography), a subunit molecular weight of 26,500 daltons (by SDS-polyacrylamide gel electrophoresis), a pI of 8.4 and an extinction coefficient E1%280 of 5.6 +/- 0.4.     

Synthesis, purification, and characterization of the cytoplasmic domain of the human insulin receptor using a baculovirus expression system

The cytoplasmic domain of the beta subunit of the human insulin receptor has been overexpressed in insect cells using the baculovirus expression system. A recombinant baculovirus (BIR-2) was constructed by inserting the human insulin proreceptor cDNA fragment that encodes the cytoplasmic domain of the receptor into the genome of Autographa californica nuclear polyhedrosis virus adjacent to the strong polyhedrin promoter. Synthesis of the protein (baculovirus insulin receptor kinase (BIRK), Mr 48,000) in BIR-2-infected Spodoptera frugiperda (Sf9) cells was detected 24 h after infection and maximal accumulation (2-3% of the cytosolic protein) was achieved 48-72 h post-infection. The expressed protein is active as a soluble protein tyrosine kinase, both in Sf9 cells and in vitro. Rapid purification to near homogeneity was accomplished by sequential chromatography on Fast-Q-Sepharose and phenyl-Superose with an overall yield of 35% and a specific activity with histone as substrate of 20 nmol/min/mg protein. Autophosphorylation activated the intrinsic kinase activity of BIRK and decreased its mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using a combination of tryptic digestion and immunoprecipitation with specific antipeptide antisera, it was ascertained that 30-40% of the 32P incorporated into BIRK by autophosphorylation is in the carboxyl-terminal domain (that includes tyrosyl residues 1316 and 1322 of the human proreceptor). Of the remaining radioactivity, 75% is in the amino-terminal domain (that includes tyrosyl residues 953, 960, 972, 999, and 1075) and 25% is in the conserved autophosphorylation domain (including tyrosyl residues 1146, 1150, and 1151). Limited digestion of BIRK with trypsin yielded a fragment, Mr 38,000, that lacks the carboxyl-terminal domain. This fragment exhibits protein tyrosine kinase activity that is stimulated by autophosphorylation. The properties of the soluble, monomeric BIRK are similar to those of the intact, activated, oligomeric insulin receptor kinase with respect to specificity, immunoreactivity, divalent cation requirements, and specific activity. These observations coupled with the ease of producing 0.4 mg of purified enzyme from 100 ml of suspension culture suggest that BIRK will be useful for biochemical and biophysical analysis of the insulin receptor protein tyrosine kinase.