Expression and characterization of poliovirus proteins 3BVPg, 3Cpro, and 3Dpol in recombinant baculovirus-infected Spodoptera frugiperda cells

Expression of variable viruses as herpes simplex glycoprotein D and varicella zoster gE glycoprotein using a novel plasmid based expression system in insect cell

Several prokaryotic and eukaryotic expression systems have been used for in vitro production of viruses’ proteins. However eukaryotic expression system was always the first choice for production of proteins that undergo post-translational modification such as glycosylation. Recombinant baculoviruses have been widely used as safe vectors to express heterologous genes in the culture of insect cells, but the manipulation involved in creating, titrating, and amplifying viral stocks make it time consuming and laborious. Therefore, to facilitate rapid expression in insect cell, a plasmid based expression system was used to express herpes simplex type 1 glycoprotein D (HSV-1 gD) and varicella zoster glycoprotein E (VZV gE). Recombinant plasmids were generated, transfected into insect cells (SF9), and both glycoproteins were expressed 48 h post-infection. A protein with approximately molecular weight of 64-kDa and 98-kDa for HSV-1 gD and VZV gE respectively was expressed and confirmed by SDS. Proteins were detected in insect cells cytoplasm and outer membrane by immunofluorescence. The antigenicity and immunoreactivity of each protein were confirmed by immunoblot and ELISA. Results suggest that this system can be an alternative to the traditional baculovirus expression for small scale expression system in insect cells.

Factors affecting recombinant Western equine encephalitis virus glycoprotein production in the baculovirus system

In an effort to produce processed, soluble Western equine encephalitis virus (WEEV) glycoproteins for subunit therapeutic vaccine studies, we isolated twelve recombinant baculoviruses designed to express four different WEEV glycoprotein constructs under the transcriptional control of three temporally distinct baculovirus promoters. The WEEV glycoprotein constructs encoded full-length E1, the E1 ectodomain, an E26KE1 polyprotein precursor, and an artificial, secretable E2E1 chimera. The three different promoters induced gene expression during the immediate early (ie1), late (p6.9), and very late (polh) phases of baculovirus infection. Protein expression studies showed that the nature of the WEEV construct and the timing of expression both influenced the quantity and quality of recombinant glycoprotein produced. The full-length E1 product was insoluble, irrespective of the timing of expression. Each of the other three constructs yielded soluble products and, in these cases, the timing of expression was important, as higher protein processing efficiencies were generally obtained at earlier times of infection. However, immediate early expression did not yield detectable levels of every WEEV product, and expression during the late (p6.9) or very late (polh) phases of infection provided equal or higher amounts of processed, soluble product. Thus, while earlier foreign gene expression can provide higher recombinant glycoprotein processing efficiencies in the baculovirus system, in the case of the WEEV glycoproteins, earlier expression did not provide larger amounts of high quality, soluble recombinant glycoprotein product.

Nuclear entry of poliovirus protease-polymerase precursor 3CD: implications for host cell transcription shut-off

Host cell transcription mediated by all three RNA polymerases is rapidly inhibited after infection of mammalian cells with poliovirus (PV). Both genetic and biochemical studies have shown that the virus-encoded protease 3C cleaves the TATA-binding protein and other transcription factors at glutamine-glycine sites and is directly responsible for host cell transcription shut-off. PV replicates in the cytoplasm of infected cells. To shut-off host cell transcription, 3C or a precursor of 3C must enter the nucleus of infected cells. Although the 3C protease itself lacks a nuclear localization signal (NLS), amino acid sequence examination of 3D identified a potential single basic type NLS, KKKRD, spanning amino acids 125-129 within this polypeptide. Thus, a plausible scenario is that 3C enters the nucleus in the form of its precursor, 3CD, which then generates 3C by auto-proteolysis ultimately leading to cleavage of transcription factors in the nucleus. Using transient transfection of enhanced green fluorescent protein (EGFP) fusion polypeptides, we demonstrate here that both 3CD and 3D are capable of entering the nucleus in PV-infected cells. However, both polypeptides remain in the cytoplasm in uninfected HeLa cells. Mutagenesis of the NLS sequence in 3D prevents nuclear entry of 3D and 3CD in PV-infected cells. We also demonstrate that 3CD can be detected in the nuclear fraction from PV-infected HeLa cells as early as 2 h postinfection. Significant amount of 3CD is found associated with the nuclear fraction by 3-4 h of infection. Taken together, these results suggest that both the 3D NLS and PV infection are required for the entry of 3CD into the nucleus and that this may constitute a means by which viral protease 3C is delivered into the nucleus leading to host cell transcription shut-off.

Biochemical characterization of rhinovirus RNA-dependent RNA polymerase

Human rhinoviruses (HRV) represent the single most important causative agent of the common cold. The HRV genome encodes an RNA-dependent RNA polymerase (RdRp) designated 3D polymerase that is required for replication of the HRV RNA genome. We have expressed and purified recombinant HRV-16 3D polymerase to near homogeneity from Escherichia coli transformed with an expression plasmid containing the full-length 460 amino acid HRV-16 3D sequence with a methionine at the N-terminus and a glycine-serine linker followed by a 6-histidine affinity tag at the C-terminus. The purified recombinant protein has rifampicin-resistant activity in a poly(A)-dependent poly(U) polymerase assay while corresponding fractions similarly purified from E. coli transformed with an expression plasmid without the HRV-16 3D sequence showed no activity. The optimal conditions for temperature, pH, divalent cations Mg(2+) and Mn(2+), and KCl were determined. The recombinant protein has RNA polymerase activity on homopolymeric templates poly(A) and poly(C) and heteropolymeric RNA templates primed with either RNA or DNA oligonucleotide primers or self-primed by a copy-back mechanism. A unique, secondary structureless heteropolymeric RNA template that is an efficient substrate was developed to facilitate kinetic characterizations of the enzyme. In the presence of Mg(2+), the enzyme displayed strong base and sugar specificity. However, when Mg(2+) was replaced by Mn(2+) specificity for ribonucleotides was lost, utilization of deoxynucleotides became possible and primer-independent activity was observed on the poly(C) template. Zn(2+) was found to inhibit HRV-16 3D polymerase with an IC(50) as low as 0.6 microM by a mechanism distinct from the magnesium ion stimulation. The activity of this 6His-tagged HRV-16 3D polymerase was compared with that of a recombinant HRV-16 3D polymerase expressed without the 6His-tag and was found to be identical. The availability of recombinant rhinovirus RdRp in a purified form will facilitate the structure-function analysis of this enzyme as well as the identification of specific inhibitors to the rhinovirus 3D polymerase that have therapeutic value in the treatment of the common cold.

Role of the 3'-untranslated regions of alfalfa mosaic virus RNAs in the formation of a transiently expressed replicase in plants and in the assembly of virions

Alfalfa mosaic virus (AMV) RNAs 1 and 2 encode the replicase proteins P1 and P2, respectively, whereas RNA 3 encodes the movement protein and the coat protein (CP). When RNAs 1 and 2 were transiently expressed from a T-DNA vector (R12 construct) by agroinfiltration of Nicotiana benthamiana, the infiltrated leaves accumulated minus-strand RNAs 1 and 2 and relatively small amounts of plus-strand RNAs. In addition, RNA-dependent RNA polymerase (RdRp) activity could be detected in extracts of the infiltrated leaves. After transient expression of RNAs 1 and 2 with the 3'-untranslated regions (UTRs) of both RNAs deleted (R1Delta/2Delta construct), no replication of RNAs 1 and 2 was observed, while the infiltrated leaves supported replication of RNA 3 after inoculation of the leaves with RNA 3 or expression of RNA 3 from a T-DNA vector (R3 construct). No RdRp activity could be isolated from leaves infiltrated with the R1Delta/2Delta construct, although P1 and P2 sedimented in a region of a glycerol gradient where active RdRp was found in plants infiltrated with R12. RdRp activity could be isolated from leaves infiltrated with constructs R1Delta/2 (3'-UTR of RNA 1 deleted), R1/2Delta (3'-UTR of RNA 2 deleted), or R1Delta/2Delta plus R3. This demonstrates that the 3'-UTR of AMV RNAs is required for the formation of a complex with in vitro enzyme activity. RNAs 1 and 2 with the 3'-UTRs deleted were encapsidated into virions by CP expressed from RNA 3. This shows that the high-affinity binding site for CP at the 3'-termini of AMV RNAs is not required for assembly of virus particles.

Evidence for phosphorylation and ubiquitinylation of the turnip yellow mosaic virus RNA-dependent RNA polymerase domain expressed in a baculovirus-insect cell system

All RNA viruses known to date encode an RNA-dependent RNA polymerase (RdRp) that is required for replication of the viral genome. We have expressed and purified the turnip yellow mosaic virus (TYMV) RdRp in insect cells using a recombinant baculovirus, either in its native form, or fused to an hexa-histidine tag. Phosphorylation of the protein was demonstrated by labelling experiments in vivo, as well as phosphatase treatment of the purified protein in vitro. Phospho amino acid analysis and immunoblotting experiments identified serine and threonine residues as being the subject of phosphorylation. Peptide mass mapping using MS analysis of a protein digest revealed that phosphorylation sites are localized within a putative PEST sequence [a sequence rich in proline (P), glutamic acid (E), serine (S) and threonine (T) residues] in the N-terminal region of the protein. Using monoclonal antibodies specific for ubiquitin conjugates, we were able to demonstrate that the TYMV RdRp is conjugated to ubiquitin molecules when expressed in insect cells. These observations suggest that the TYMV RdRp may be processed selectively by the ubiquitin/proteasome degradation system upon phosphorylation of the PEST sequence.

Production of "authentic" poliovirus RNA-dependent RNA polymerase (3D(pol)) by ubiquitin-protease-mediated cleavage in Escherichia coli

The first amino acid of "authentic" poliovirus RNA-dependent RNA polymerase, 3D(pol), is a glycine. As a result, production of 3D(pol) in Escherichia coli requires addition of an initiation codon; thus, a formylmethionine is added to the amino terminus. The formylmethionine should be removed by the combined action of a cellular deformylase and methionine aminopeptidase. However, high-level expression of 3D(pol) in E. coli yields enzyme with a heterogeneous amino terminus. To preclude this problem, we developed a new expression system for 3D(pol). This system exploits the observation that proteins fused to the carboxyl terminus of ubiquitin can be processed in E. coli to produce proteins with any amino acid as the first residue when expressed in the presence of a ubiquitin-specific, carboxy-terminal protease. By using this system, authentic 3D(pol) can be obtained in yields of 30-60 mg per liter of culture. While addition of a single glycine, alanine, serine, or valine to the amino terminus of 3D(pol) produced derivatives with a specific activity reduced by at least 25-fold relative to wild-type enzyme, addition of a methionine to the amino terminus resulted in some processing to yield enzyme with a glycine amino terminus. Addition of a hexahistidine tag to the carboxyl terminus of 3D(pol) had no deleterious effect on the activity of the enzyme. The utility of this expression system for production of other viral polymerases and accessory proteins is discussed.

Cellular factors in the transcription and replication of viral RNA genomes: a parallel to DNA-dependent RNA transcription

Viral RNA replication and transcription involves not only viral RNA-dependent RNA polymerases, but also cellular proteins, the majority of which are subverted from the RNA-processing or translation machineries of host cells. These factors interact with viral RNA or polymerases to form transcription or replication ribonucleoprotein complexes and may provide template specificity for RNA-dependent RNA synthesis, suggesting a close parallel to the mechanism of DNA-dependent RNA synthesis. The types of cellular proteins involved and their modes of action are reviewed.

Comparison of the replication of positive-stranded RNA viruses of plants and animals

It is clear from the experimental data that there are some similarities in RNA replication for all eukaryotic positive-stranded RNA viruses—that is, the mechanism of polymerization of the nucleotides is probably similar for all. It is noteworthy that all mechanisms appear to utilize host membranes as a site of replication. Membranes appear to function not only as a way of compartmentalizing virus RNA replication but also appear to have a central role in the organization and functioning of the replication complex, and further studies in this area are needed. Within virus supergroups, similarities are evident between animal and plant viruses—for example, in the nature and arrangements of replication genes and in sequence similarities of functional domains. However, it is also clear that there has been considerable divergence, even within supergroups. For example, the animal alpha-viruses have evolved to encode proteinases which play a central controlling function in the replication cycle, whereas this is not common in the plant alpha-like viruses and even when it occurs, as in the tymoviruses, the strategies that have evolved appear to be significantly different. Some of the divergence could be host-dependent and the increasing interest in the role of host proteins in replication should be fruitful in revealing how different systems have evolved. Finally, there are virus supergroups that appear to have no close relatives between animals and plants, such as the animal coronavirus-like supergroup and the plant carmo-like supergroup.

Expression and subcellular localization of poliovirus VPg-precursor protein 3AB in eukaryotic cells: evidence for glycosylation in vitro

The poliovirus-encoded, membrane-associated VPg-precursor polypeptide 3AB has been implicated in the initiation of viral RNA synthesis. We have expressed 3AB and 3A polypeptides in eukaryotic cells and examined their localization using indirect immunofluorescence and a direct in vitro membrane-binding assay. Results presented here demonstrate that both 3AB and 3A are capable of localizing in the endoplasmic reticulum and the Golgi apparatus in transfected HeLa cells in the absence of any other poliovirus protein. We have also shown that the carboxy-terminal 18 amino acids of 3A that constitute an amphipathic domain are important in membrane binding of 3A and 3AB. Additionally, we demonstrate that a significant fraction of both 3A and 3AB can be glycosylated in a membrane-dependent fashion during in vitro translation in reticulocyte lysate. We demonstrate that 6-diazo-5-oxo-L-norleucine, an inhibitor of glycoprotein synthesis, significantly inhibits poliovirus RNA synthesis in vivo. The implications of glycosylation of 3AB (and 3A) in viral replication are discussed.

Coupled translation and replication of poliovirus RNA in vitro: synthesis of functional 3D polymerase and infectious virus

Poliovirus RNA polymerase and infectious virus particles were synthesized by translation of virion RNA in vitro in HeLa S10 extracts. The in vitro translation reactions were optimized for the synthesis of the viral proteins found in infected cells and in particular the synthesis of the viral polymerase 3Dpol. There was a linear increase in the amount of labeled protein synthesized during the first 6 h of the reaction. The appearance of 3Dpol in the translation products was delayed because of the additional time required for the proteolytic processing of precursor proteins. 3Dpol was first observed at 1 h in polyacrylamide gels, with significant amounts being detected at 6 h and later. Initial attempts to assay for polymerase activity directly in the translation reaction were not successful. Polymerase activity, however, was easily detected by adding a small amount (3 microliters) of translation products to a standard polymerase assay containing poliovirion RNA. Full-length minus-strand RNA was synthesized in the presence of an oligo(U) primer. In the absence of oligo(U), product RNA about twice the size of virion RNA was synthesized in these reactions. RNA stability studies and plaque assays indicated that a significant fraction of the input virion RNA in the translation reactions was very stable and remained intact for 20 h or more. Plaque assays indicated that infectious virus was synthesized in the in vitro translation reactions. Under optimal conditions, the titer of infectious virus produced in the in vitro translation reactions was greater than 100,000 PFU/ml. Virus was first detected at 6 h and increased to maximum levels by 12 h. Overall, the kinetics of poliovirus replication (protein synthesis, polymerase activity, and virus production) observed in the HeLa S10-initiation factor in vitro translation reactions were similar to those observed in infected cells.

Cloning and inducible synthesis of poliovirus nonstructural proteins

The poliovirus nonstructural protein-encoding genes have been cloned and expressed in Escherichia coli using the inducible system described by Studier and Moffat [J. Mol. Biol. 189 (1986) 113-130] and Studier [J. Mol. Biol. 219 (1991) 37-44]. The two genes encoding the poliovirus proteases, 2Apro and 3Cpro, were cloned together with their flanking regions in order to test the ability of the polyprotein precursors synthesized to cause proteolytic cleavage and generate mature forms. Both proteases were synthesized and showed activity upon induction in this system. Previously, it had not been possible to produce the three poliovirus nonstructural proteins, 2B, 2C and 3A, and some of their precursors, 2C3AB, 2C3A and 3AB, at high levels in E. coli cells. We report the cloning of their genes using PCR techniques and their efficient expression from pET vectors upon induction with IPTG (isopropyl-beta-D-thiogalactopyranoside). Moreover, some of these proteins, e.g., 3AB, 3A and 2B, are quite toxic for E. coli cells and lysed them upon production. Our results demonstrate the usefulness of this inducible system using the pET vectors to express these toxic poliovirus proteins.

Temperature-sensitive polioviruses containing mutations in RNA polymerase

Site-directed mutagenesis was performed to change the wild-type residue (asparagine) to aspartate, histidine, or tyrosine at amino acid 424 of the poliovirus RNA polymerase, 3Dpol. The mutations were introduced into plasmids containing full-length viral cDNA and plasmids which direct the expression of 3Dpol in Escherichia coli. Mutant viruses, recovered after transfection of HeLa cells with RNA transcripts of the full-length clones, produced small plaques at 32 degrees. In addition, the plaquing efficiency was decreased for all three mutants at 37 degrees, compared to 32 degrees. The polyprotein processing of all mutant viruses was normal at the temperatures tested, suggesting that the mutant plaque phenotypes were not due to incorrect processing of viral proteins. Analyses of viral RNA synthesis in infected cells and of the polymerase activities of mutant enzymes produced in E. coli suggested the following: (1) The his424 mutant enzyme appeared to be defective in the initiation of plus-strand RNA synthesis in HeLa cells. (2) The asp424 mutant enzyme appeared unable to assume proper conformation for active polymerase function when synthesized at 37 degrees. (3) The tyr424 mutant enzyme was totally inactive when synthesized in E. coli at 37 degrees.