Expression of L protein of vesicular stomatitis virus Indiana serotype from recombinant baculovirus in insect cells: requirement of a host factor(s) for its biological activity in vitro

Display of disparate transcription phenotype by the phosphorylation negative P protein mutants of vesicular stomatitis virus, Indiana serotype, expressed in E. coli and eucaryotic cells

The phosphoprotein (P) of vesicular stomatitis virus (VSV) is a subunit of the RNA polymerase (L) that transcribes the negative strand genome RNA into mRNAs both in vitro and in vivo. We have recently shown that the P protein of VSV, New Jersey serotype (PNJ), expressed in E. coli, is biologically inactive unless phosphorylated at specific serine residues by cellular casein kinase II (CKII). In the present work, we are studying the role of phosphorylation in the activation of the P protein of Indiana serotype (PIND), which is highly nonhomologous in amino acid sequence yet structurally similar to its New Jersey counterpart. Despite the fact that E. coli-expressed PIND required phosphorylation by CKII for activation, the phosphorylation negative P protein mutants generated by altering the phosphate acceptors S and T to alanine, surprisingly, showed transcription activity similar to wild-type in vitro. Alteration of S and T residues to phenylalanine, similarly, supported substantial transcription activity (approx. 60% of wild-type), whereas substitution with arginine residue abrogated transcription (approx. 5% of wild-type). In contrast, the same mutants, when expressed in eucaryotic cells, exhibited greatly reduced transcription activity in vitro. This disparate display of transcription phenotype by the PIND mutants expressed in bacteria and eucaryotic cells suggests that these mutants are unique in assuming different secondary structure or conformation when synthesized in two different cellular milieu. The findings that, unless phosphorylated by CKII, the bacterially expressed unphosphorylated (P0) form of PIND, as well as the phosphorylation negative mutants expressed in eucaryotic cells, demonstrates transcription negative phenotype indicate that, like PNJ, phosphorylation of PIND is essential for its activity.

HSP90 Chaperoning in Addition to Phosphoprotein Required for Folding but Not for Supporting Enzymatic Activities of Measles and Nipah Virus L Polymerases

Nonsegmented negative-stranded RNA viruses, or members of the order Mononegavirales, share a conserved gene order and the use of elaborate transcription and replication machinery made up of at least four molecular partners. These partners have coevolved with the acquisition of the permanent encapsidation of the entire genome by the nucleoprotein (N) and the use of this N-RNA complex as a template for the viral polymerase composed of the phosphoprotein (P) and the large enzymatic protein (L). Not only is P required for polymerase function, but it also stabilizes the L protein through an unknown underlying molecular mechanism. By using NVP-AUY922 and/or 17-dimethylaminoethylamino-17-demethoxygeldanamycin as specific inhibitors of cellular heat shock protein 90 (HSP90), we found that efficient chaperoning of L by HSP90 requires P in the measles, Nipah, and vesicular stomatitis viruses. While the production of P remains unchanged in the presence of HSP90 inhibitors, the production of soluble and functional L requires both P and HSP90 activity. Measles virus P can bind the N terminus of L in the absence of HSP90 activity. Both HSP90 and P are required for the folding of L, as evidenced by a luciferase reporter insert fused within measles virus L. HSP90 acts as a true chaperon; its activity is transient and dispensable for the activity of measles and Nipah virus polymerases of virion origin. That the cellular chaperoning of a viral polymerase into a soluble functional enzyme requires the assistance of another viral protein constitutes a new paradigm that seems to be conserved within the Mononegavirales order.

IMPORTANCE

Viruses are obligate intracellular parasites that require a cellular environment for their replication. Some viruses particularly depend on the cellular chaperoning apparatus. We report here that for measles virus, successful chaperoning of the viral L polymerase mediated by heat shock protein 90 (HSP90) requires the presence of the viral phosphoprotein (P). Indeed, while P protein binds to the N terminus of L independently of HSP90 activity, both HSP90 and P are required to produce stable, soluble, folded, and functional L proteins. Once formed, the mature P+L complex no longer requires HSP90 to exert its polymerase functions. Such a new paradigm for the maturation of a viral polymerase appears to be conserved in several members of the Mononegavirales order, including the Nipah and vesicular stomatitis viruses.

The cellular protein TIP47 restricts Respirovirus multiplication leading to decreased virus particle production

The cellular tail-interacting 47-kDa protein (TIP47) acts positively on HIV-1 and vaccinia virus production. We show here that TIP47, in contrast, acts as a restriction factor for Sendai virus production. This conclusion is supported by the occurrence of increased or decreased virus production upon its suppression or overexpression, respectively. Pulse-chase metabolic labeling of viral proteins under conditions of TIP47 suppression reveals an increased rate of viral protein synthesis followed by increased incorporation of viral proteins into virus particles. TIP47 is here described for the first time as a viral restriction factor that acts by limiting viral protein synthesis.

Architecture and regulation of negative-strand viral enzymatic machinery

Negative-strand (NS) RNA viruses initiate infection with a unique polymerase complex that mediates both mRNA transcription and subsequent genomic RNA replication. For nearly all NS RNA viruses, distinct enzymatic domains catalyzing RNA polymerization and multiple steps of 5' mRNA cap formation are contained within a single large polymerase protein (L). While NS RNA viruses include a variety of emerging human and agricultural pathogens, the enzymatic machinery driving viral replication and gene expression remains poorly understood. Recent insights with Machupo virus and vesicular stomatitis virus have provided the first structural information of viral L proteins, and revealed how the various enzymatic domains are arranged into a conserved architecture shared by both segmented and nonsegmented NS RNA viruses. In vitro systems reconstituting RNA synthesis from purified components provide new tools to understand the viral replicative machinery, and demonstrate the arenavirus matrix protein regulates RNA synthesis by locking a polymerase-template complex. Inhibition of gene expression by the viral matrix protein is a distinctive feature also shared with influenza A virus and nonsegmented NS RNA viruses, possibly illuminating a conserved mechanism for coordination of viral transcription and polymerase packaging.

In vitro capping and transcription of rhabdoviruses

The RNA-dependent RNA polymerase L protein of vesicular stomatitis virus (VSV), a prototypic nonsegmented negative strand (NNS) RNA virus classified into the Rhabdoviridae family, has been used to investigate the fundamental molecular mechanisms of NNS RNA viral mRNA synthesis and processing. In vitro studies on mRNA cap formation with the VSV L protein eventually led to the discovery of the unconventional mRNA capping pathway catalyzed by the guanosine 5'-triphosphatase and RNA:GDP polyribonucleotidyltransferase (PRNTase) activities. The PRNTase activity is a novel enzymatic activity, which transfers 5'-monophosphorylated (p-) RNA from 5'-triphosphorylated (ppp-) RNA to GDP to form 5'-capped RNA (GpppRNA) in a viral mRNA-start sequence-dependent manner. This unconventional capping (pRNA transfer) reaction with PRNTase can be experimentally distinguished from the conventional capping (GMP transfer) reaction with eukaryotic GTP:RNA guanylyltransferase (GTase) on the basis of the following differences in their substrate specificity for the cap formation: PRNTase uses GDP and pppRNA, but not ppRNA, whereas GTase employs GTP, but not GDP, and ppRNA. The pRNA transfer reaction with PRNTase proceeds through a covalent enzyme-pRNA intermediate with a phosphoamide bond. Hence, to prove the PRNTase activity, it is necessary to demonstrate the following consecutive steps separately: (1) the enzyme forms a covalent enzyme-pRNA intermediate, and (2) the intermediate transfers pRNA to GDP. This article describes the methods for in vitro transcription and capping with the recombinant VSV L protein, which permit detailed characterization of its enzymatic reactions and mapping of active sites of its enzymatic domains. It is expected that these systems are adaptable to rhabdoviruses and, by extension, other NNS RNA viruses belonging to different families.

Ribose 2'-O methylation of the vesicular stomatitis virus mRNA cap precedes and facilitates subsequent guanine-N-7 methylation by the large polymerase protein

During conventional mRNA cap formation, two separate methyltransferases sequentially modify the cap structure, first at the guanine-N-7 (G-N-7) position and subsequently at the ribose 2'-O position. For vesicular stomatitis virus (VSV), a prototype of the nonsegmented negative-strand RNA viruses, the two methylase activities share a binding site for the methyl donor S-adenosyl-l-methionine and are inhibited by individual amino acid substitutions within the C-terminal domain of the large (L) polymerase protein. This led to the suggestion that a single methylase domain functions for both 2'-O and G-N-7 methylations. Here we report a trans-methylation assay that recapitulates both ribose 2'-O and G-N-7 modifications by using purified recombinant L and in vitro-synthesized RNA. Using this assay, we demonstrate that VSV L typically modifies the 2'-O position of the cap prior to the G-N-7 position and that G-N-7 methylation is diminished by pre-2'-O methylation of the substrate RNA. Amino acid substitutions in the C terminus of L that prevent all cap methylation in recombinant VSV (rVSV) partially retain the ability to G-N-7 methylate a pre-2'-O-methylated RNA, therefore uncoupling the effect of substitutions in the C terminus of the L protein on the two methylations. In addition, we show that the 2'-O and G-N-7 methylase activities act specifically on RNA substrates that contain the conserved elements of a VSV mRNA start at the 5' terminus. This study provides new mechanistic insights into the mRNA cap methylase activities of VSV L, demonstrates that 2'-O methylation precedes and facilitates subsequent G-N-7 methylation, and reveals an RNA sequence and length requirement for the two methylase activities. We propose a model of regulation of the activity of the C terminus of L protein in 2'-O and G-N-7 methylation of the cap structure.

Opposing effects of inhibiting cap addition and cap methylation on polyadenylation during vesicular stomatitis virus mRNA synthesis

The multifunctional large (L) polymerase protein of vesicular stomatitis virus (VSV) contains enzymatic activities essential for RNA synthesis, including mRNA cap addition and polyadenylation. We previously mapped amino acid residues G1154, T1157, H1227, and R1228, present within conserved region V (CRV) of L, as essential for mRNA cap addition. Here we show that alanine substitutions to these residues also affect 3'-end formation. Specifically, the cap-defective polymerases produced truncated transcripts that contained A-rich sequences at their 3' termini and predominantly terminated within the first 500 nucleotides (nt) of the N gene. To examine how the cap-defective polymerases respond to an authentic VSV termination and reinitiation signal present at each gene junction, we reconstituted RNA synthesis using templates that contained genes inserted (I) at the leader-N gene junction. The I genes ranged in size from 382 to 1,098 nt and were typically transcribed into full-length uncapped transcripts. In addition to lacking a cap structure, the full-length I transcripts synthesized by the cap-defective polymerases lacked an authentic polyadenylate tail and instead contained 0 to 24 A residues. Moreover, the cap-defective polymerases were also unable to copy efficiently the downstream gene. Thus, single amino acid substitutions in CRV of L protein that inhibit cap addition also inhibit polyadenylation and sequential transcription of the genome. In contrast, an amino acid substitution, K1651A, in CRVI of L protein that completely inhibits cap methylation results in the hyperpolyadenylation of mRNA. This work reveals that inhibiting cap addition and cap methylation have opposing effects on polyadenylation during VSV mRNA synthesis and provides evidence in support of a link between correct 5' cap formation and 3' polyadenylation.

Different effect of proteasome inhibition on vesicular stomatitis virus and poliovirus replication

Proteasome activity is an important part of viral replication. In this study, we examined the effect of proteasome inhibitors on the replication of vesicular stomatitis virus (VSV) and poliovirus. We found that the proteasome inhibitors significantly suppressed VSV protein synthesis, virus accumulation, and protected infected cells from toxic effect of VSV replication. In contrast, poliovirus replication was delayed, but not diminished in the presence of the proteasome inhibitors MG132 and Bortezomib. We also found that inhibition of proteasomes stimulated stress-related processes, such as accumulation of chaperone hsp70, phosphorylation of eIF2α, and overall inhibition of translation. VSV replication was sensitive to this stress with significant decline in replication process. Poliovirus growth was less sensitive with only delay in replication. Inhibition of proteasome activity suppressed cellular and VSV protein synthesis, but did not reduce poliovirus protein synthesis. Protein kinase GCN2 supported the ability of proteasome inhibitors to attenuate general translation and to suppress VSV replication. We propose that different mechanisms of translational initiation by VSV and poliovirus determine their sensitivity to stress induced by the inhibition of proteasomes. To our knowledge, this is the first study that connects the effect of stress induced by proteasome inhibition with the efficiency of viral infection.

Production of recombinant orange-spotted grouper (Epinephelus coioides) luteinizing hormone in insect cells by the baculovirus expression system and its biological effect

The cDNA sequence encoding orange-spotted grouper lhb (LHbeta) and cga (GTHalpha) subunits were cocloned into baculovirus transfer vectors and expressed in insect Sf9 cells. The results showed that two bands of 15.6 kDa and 11.4 kDa could be detected by SDS-PAGE and a band of 30 kDa could be detected by native PAGE. The recombinant grouper Lh (rgLh) could stimulate the secretion of testosterone (T) and estradiol-17beta (E2) from the gonad in a static incubation system in a time-dependent, but not a dose-dependent, manner. Using in vivo bioassay, the mRNA levels of two aromatases (cyp19a1a [P450aromA] and cyp19a1b [P450aromB]), gnrh (GnRH), lhb, and cga in the pituitary, gonad, and hypothalamus were determined in different groups of orange-spotted groupers treated respectively with rgLh, human chorionic gonadotropin (hCG), and a culture medium of insect cells transformed with an expression vector without lhb and cga subunits. The mRNA levels of cyp19a1a and cyp19a1b rose dramatically after injecting rgLh intraperitoneally, which was consistent with the secretion of sex steroid hormones. Interestingly, the mRNA levels of gnrh dropped in the pituitary, hypothalamus, and gonad, and the mRNA levels of lhb and cga in the pituitary of the experimental group expressed at a higher level than that of the hCG group. These results are in accord with the long positive feedback loop of Lh on gonad sex steroid hormones and the short negative feedback loop of Lh on gnrh mRNA levels. These results indicate that the rgLh is successfully expressed by the baculovirus-insect expression system and that the rgLh has biological activity.

Leader RNA of Rinderpest virus binds specifically with cellular La protein: a possible role in virus replication

Rinderpest virus (RPV) is an important member of the Morbillivirus genus in the family Paramyxoviridae and employs a similar strategy for transcription and replication of its genome as that of other negative sense RNA viruses. Cellular proteins have earlier been shown to stimulate viral RNA synthesis by isolated nucleocapsids from purified virus or from virus-infected cells. In the present work, we show that plus sense leader RNA of RPV, transcribed from 3' end of genomic RNA, specifically interacts with cellular La protein employing gel mobility shift assay as well as UV cross-linking of leader RNA with La protein. The leader RNA synthesized in virus-infected cells was shown to interact with La protein by immunoprecipitation of leader RNA bound to La protein and detecting the leader RNA in the immunoprecipitate by Northern hybridization with labeled antisense leader RNA. Employing a minireplicon system, we demonstrate that transiently expressed La protein enhances the replication/transcription of the RPV minigenome in cells. Sub-cellular immunolocalization shows that La protein is redistributed from nucleus to the cytoplasm upon infection. Our results strongly suggest that La protein may be involved in regulation of Rinderpest virus replication.

Phospholipid scramblase 1 potentiates the antiviral activity of interferon

Phospholipid scramblase 1 (PLSCR1) is an interferon (IFN)- and growth factor-inducible, calcium-binding protein that either inserts into the plasma membrane or binds DNA in the nucleus depending on its state of palmyitoylation. In certain hematopoietic cells, PLSCR1 is required for normal maturation and terminal differentiation from progenitor cells as regulated by select growth factors, where it promotes recruitment and activation of Src kinases. PLSCR1 is a substrate of Src (and Abl) kinases, and transcription of the PLSCR1 gene is regulated by the same growth factor receptor pathways in which PLSCR1 potentiates afferent signaling. The marked transcriptional upregulation of PLSCR1 by IFNs led us to explore whether PLSCR1 plays an analogous role in cellular responses to IFN, with specific focus on antiviral activities. Accordingly, human cells in which PLSCR1 expression was decreased with short interfering RNA were rendered relatively insensitive to the antiviral activity of IFNs, resulting in higher titers of vesicular stomatitis virus (VSV) and encephalomyocarditis virus. Similarly, VSV replicated to higher titers in mouse PLSCR1(-/-) embryonic fibroblasts than in identical cells transduced to express PLSCR1. PLSCR1 inhibited accumulation of primary VSV transcripts, similar to the effects of IFN against VSV. The antiviral effect of PLSCR1 correlated with increased expression of a subset of IFN-stimulated genes (ISGs), including ISG15, ISG54, p56, and guanylate binding proteins. Our results suggest that PLSCR1, which is itself an ISG-encoded protein, provides a mechanism for amplifying and enhancing the IFN response through increased expression of a select subset of potent antiviral genes.

Two RNA polymerase complexes from vesicular stomatitis virus-infected cells that carry out transcription and replication of genome RNA

By immunoaffinity column chromatography, we have purified two RNA polymerase complexes, the transcriptase and replicase, from vesicular stomatitis virus-infected baby hamster kidney cells. The transcriptase is a multiprotein complex, containing the virus-encoded RNA polymerase L and P proteins, and two cellular proteins, translation elongation factor-1alpha and heat-shock protein 60. In addition, the complex contains a submolar amount of cellular mRNA cap guanylyltransferase. The replicase, on the other hand, is a complex containing the viral proteins, L, P, and the nucleocapsid (N), but lacking elongation factor-1alpha, heat-shock protein 60, and guanylyltransferase. The transcriptase complex synthesizes capped mRNAs and initiates transcription at the first gene (N) start site, whereas the replicase complex initiates RNA synthesis at the precise 3' end of the genome RNA and synthesizes encapsidated replication products in the presence of the N-P complex. We propose that two RNA polymerase complexes that differ in their content of virally and host-encoded proteins are separately responsible for transcription and replication of vesicular stomatitis virus genome RNA.

Identification of a novel tripartite complex involved in replication of vesicular stomatitis virus genome RNA

Our laboratory's recent observations that transcriptionally inactive phosphoprotein (P) mutants can efficiently function in replicating vesicular stomatitis virus (VSV) defective interfering particle in a three-plasmid-based (L, P, and N) reverse genetics system in vivo (A. K. Pattnaik, L. Hwang, T. Li, N. Englund, M. Mathur, T. Das, and A. K. Banerjee, J. Virol. 71:8167-8175, 1997) led us to propose that a tripartite complex consisting of L-(N-P) protein may represent the putative replicase for synthesis of the full-length genome RNA. In this communication we demonstrate that such a complex is indeed detectable in VSV-infected BHK cells. Furthermore, coexpression of L, N, and P proteins in Sf21 insect cells by recombinant baculovirus containing the respective genes also resulted in the formation of a tripartite complex, as shown by immunoprecipitation with specific antibodies. A basic amino acid mutant of P protein, P260A, previously shown to be inactive in transcription but active in replication (T. Das, A. K. Pattnaik, A. M. Takacs, T. Li, L. N. Hwang, and A. K. Banerjee, Virology 238:103-114, 1997) was also capable of forming the mutant [L-(N-Pmut)] complex in both insect cells and BHK cells. Sf21 extract containing either the wild-type P protein or the mutant P protein along with the L and N proteins was capable of synthesizing 42S genome-sense RNA in an in vitro replication reconstitution reaction. Addition of N-Pmut or wild-type N-P complex further stimulated the synthesis of the genome-length RNA. These results indicate that the transcriptase and replicase complexes of VSV are possibly two distinct entities involved in carrying out capped mRNAs and uncapped genome and antigenome RNAs, respectively.

Unique capping activity of the recombinant RNA polymerase (L) of vesicular stomatitis virus: association of cellular capping enzyme with the L protein

Vesicular stomatitis virus (VSV), a prototype of non-segmented negative strand RNA viruses, packages an RNA-dependent RNA polymerase (L) which, together with an associated phosphoprotein (P), transcribes the genome RNA, in vitro and in vivo, into mRNAs that are capped at the 5'-ends. However, unlike cellular guanlylyltransferase (GT), the RNA polymerase incorporates GDP in the capped structure, as Gp(alpha)p(beta)-p(alpha)A. In an effort to characterize the capping activity of the RNA polymerase, we have purified recombinant L (rL) protein expressed in insect cells. The rL, like the virion L polymerase, also caps transcribed mRNAs with identical unique cap structure. Interestingly, the purified rL is found to be tightly bound to the GT of the insect cell during all stages of purification. VSV grown in baby hamster kidney cells also packages cellular GT of the murine cell, suggesting that VSV L protein or its associated proteins may have a strong affinity for the cellular GT. The GT bound to rL, however, formed E-GMP complex, whereas no such complex was detected with the rL protein. It appears that the L protein may contain the putative active site for the unique capping reaction or the tightly bound cellular GT may by some unknown mechanism participate in the unique capping reaction.

Novel binding of GTP to the phosphoprotein (P) of vesicular stomatitis virus

The phosphoprotein (P) of vesicular stomatitis virus (VSV) is a subunit of the RNA polymerase (L) that transcribes the negative strand genome RNA into mRNAs both in vitro and in vivo. We have previously shown that the P protein of VSV, expressed in E. coli, is biologically inactive unless phosphorylated at specific serine residues by cellular casein kinase II (CKII). In the present study we present evidence that the P protein, in addition to being phosphorylated, binds covalently to GTP only when it is phosphorylated. Competition experiments show that ATP, ADP, GTP, and GDP can compete for the binding site(s) of GTP but not AMP, GMP, CTP, or UTP. Interestingly, once GTP is bound to P protein it cannot be displaced by unlabeled GTP. The GTP binding site has been mapped within the domain where the phosphorylation of P protein by CKII occurs. Finally, we show that phosphorylation negative P mutants P3A (P60A, P62A, P64A), P3E (P60E, P62E, P64E), and P3R (P60R, P62R, P64R) failed to bind to GTP, indicating that phosphorylation of P is indeed essential for binding to GTP. Although the precise role of binding of GTP to P is unclear, it appears that phosphorylation of P may initiate a structural change within the P protein allowing GTP to bind, thus manifesting biological function to the transcription factor.

High-level expression of biologically active human prolactin from recombinant baculovirus in insect cells

We examined the feasibility of high-level production of recombinant human prolactin, a multifunctional protein hormone, in insect cells using a baculovirus expression system. The human prolactin cDNA with and without the secretory signal sequence was cloned into pFastBac1 baculovirus vector under the control of polyhedrin promoter. Prolactin was produced upon infection of either Sf9 or High-Five cells with the recombinant baculovirus containing the human prolactin cDNA. The production of recombinant prolactin varied from 20 to 40 mg/L of monolayer culture, depending on the cell types. The prolactin polypeptide with its own secretory signal was secreted into the medium. N-terminal amino acid sequence analysis of the recombinant polypeptide purified from the culture medium indicated that the protein was processed similar to human pituitary prolactin. Carbohydrate analysis of the purified protein indicated that a fraction of the recombinant prolactin made in insect cells appeared to be glycosylated. Also, both secreted and nonsecreted forms of the recombinant prolactin in insect cells were biologically equivalent to the native human prolactin (pituitary derived) in the Nb2 lymphoma cell proliferation assay.

RNA polymerase of vesicular stomatitis virus specifically associates with translation elongation factor-1 alphabetagamma for its activity

An RNA-dependent RNA polymerase is packaged within the virions of purified vesicular stomatitis virus, a nonsegmented negative-strand RNA virus, which carries out transcription of the genome RNA into mRNAs both in vitro and in vivo. The RNA polymerase is composed of two virally encoded polypeptides: a large protein L (240 kDa) and a phosphoprotein P (29 kDa). Recently, we obtained biologically active L protein from insect cells following infection by a recombinant baculovirus expressing L gene. During purification of the L protein from Sf21 cells, we obtained in addition to an active L fraction an inactive fraction that required uninfected insect cell extract to restore its activity. The cellular factors have now been purified, characterized, and shown to be beta and gamma subunits of the protein synthesis elongation factor EF-1. We also demonstrate that the alpha subunit of EF-1 remains tightly bound to the L protein in the inactive fraction and betagamma subunits associate with the L(alpha) complex. Further purification of L(alpha) from the inactive fraction revealed that the complex is partially active and is significantly stimulated by the addition of betagamma subunits purified from Sf21 cells. A putative inhibitor(s) appears to co-elute in the inactive fraction that blocked the L(alpha) activity. The purified virions also package all three subunits of EF-1. These findings have a striking similarity with Qbeta RNA phage, which also associates with the bacterial homologue of EF-1 for its replicase function, implicating a possible evolutionary relationship between these host proteins and the RNA-dependent RNA polymerase of RNA viruses.

Basic amino acid residues at the carboxy-terminal eleven amino acid region of the phosphoprotein (P) are required for transcription but not for replication of vesicular stomatitis virus genome RNA

The phosphoprotein (P) of vesicular stomatitis virus (VSV) serotypes New Jersey [P(NJ)] and Indiana [P(I)] contains a highly conserved carboxy-terminal domain which is required for binding to the cognate N-RNA template as well as to form a soluble complex with the nucleocapsid protein N in vivo. We have shown that the deletion of 11 amino acids from the C terminal end of the P(I) protein abolishes both the template binding and the complex forming activity with the N protein. Within this region, there are conserved basic amino acid residues (R260 and K262) that are potential candidates for such interactions. We have generated mutant P proteins by substitution of these basic amino acid residues with alanine and studied their role in both transcription and replication. We have found that the R260A mutant failed to bind to the N-RNA template, whereas the K262A mutant bound efficiently as the wild-type protein. The R260A mutant, as expected, was unable to support mRNA synthesis in vitro in a transcription reconstitution reaction as well as transcription in vivo of a minigenome using a reverse genetic approach. However, the K262A mutant supported low level of transcription (12%) both in vitro and in vivo, suggesting that direct template binding of P protein through the C-terminal domain is necessary but not sufficient for optimal transcription. Using a two-hybrid system we have also shown that both R260A and K262A mutants interact inefficiently with the L protein, suggesting further that the two point mutants display differential phenotype with respect to binding to the template. In addition, both R260A and K262A mutants were shown to interact efficiently with the N protein in vivo, indicating that these mutants form N-P complexes which are presumably required for replication. This contention is further supported by the demonstration that these mutants support efficient replication of a DI RNA in vivo. Since the transcription defective P mutants can support efficient replication, we propose that the transcriptase and the replicase are composed of two distinct complexes containing (L-P2-3) and L-(N-P), respectively.

Involvement of host factors in transcription from baculovirus very late promoters -- a review

The baculovirus expression vector system has emerged as the system of choice for the expression of a number of heterologous genes of both prokaryotic and eukaryotic origin. This system utilizes the baculovirus very late, hyperactive polyhedrin and p10 promoters to drive the transcription of foreign genes. Regulation of transcription from these promoters is presently not well understood even though a number of viral gene products that may be important for transcription have been identified. Fresh insight into host-virus interactions during baculovirus pathogenesis is now offered by the identification of insect host factors that interact with transcriptionally essential motifs of these promoters as well as cis-acting enhancer-like elements upstream from the promoter.