Phosphoproteins of vesicular stomatitis virus: identity and interconversion of phosphorylated forms

Chandipura Virus: an emerging tropical pathogen

Chandipura Virus (CHPV), a member of Rhabdoviridae, is responsible for an explosive outbreak in rural areas of India. It affects mostly children and is characterized by influenza-like illness and neurologic dysfunctions. It is transmitted by vectors such as mosquitoes, ticks and sand flies. An effective real-time one step reverse-transcriptase PCR assay method is adopted for diagnosis of this virus. CHPV has a negative sense RNA genome encoding five different proteins (N, P, M, G, and L). P protein plays a vital role in the virus's life cycle, while M protein is lethal in nature. There is no specific treatment available to date, symptomatic treatment involves use of mannitol to reduce brain edema. A Vero cell based vaccine candidate against CHPV was evaluated efficiently as a preventive agent against it. Prevention is the best method to suppress CHPV infection. Containment of disease transmitting vectors, maintaining good nutrition, health, hygiene and awareness in rural areas will help in curbing the menace of CHPV. Thus, to control virus transmission some immense preventive measures need to be attempted until a good anti-CHPV agent is developed.

Functions of a protein kinase activity associated with purified capsids of the granulosis virus infecting Plodia interpunctella

Activation of a protein kinase associated with purified capsids of the granulosis virus of Plodia interpunctella resulted in release of the DNA from the nucleocapsid as determined by electron microscopy. Heat treatment of the virions (65 degrees for 10 min) inactivated the kinase and prevented this uncoating event. The basic viral core protein, VP12, is the predominant phosphate acceptor for the protein kinase and was the only DNA-binding protein present in nucleocapsids. VP12 binding to 32P-nick-translated granulosis virus DNA was determined by the hybridization of the nick-translated DNA to nucleocapsid proteins transferred electrophoretically to nitrocellolose after separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Profiles obtained when nick-translated DNA was added to sucrose gradients in the absence and presence of VP12 substantiated the DNA-binding capability of VP12. Comparison of the DNA-binding capability of phosphorylated and nonphosphorylated VP12 using sucrose gradient sedimentation provided evidence that phosphorylation of the basic protein reduced its capability to bind DNA. We propose the endogenous protein kinase activity of the granulosis virus may function in two ways: release of the DNA from the nucleocapsid (uncoating), and decondensation of the DNA due to phosphorylation of the basic core protein, VP12.

Reviewing Chandipura: a vesiculovirus in human epidemics

Chandipura virus, a member of the rhabdoviridae family and vesiculovirus genera, has recently emerged as human pathogen that is associated with a number of outbreaks in different parts of India. Although, the virus closely resembles with the prototype vesiculovirus, Vesicular Stomatitis Virus, it could be readily distinguished by its ability to infect humans. Studies on Chandipura virus while shed light into distinct stages of viral infection; it may also allow us to identify potential drug targets for antiviral therapy. In this review, we have summarized our current understanding of Chandipura virus life cycle at the molecular detail with particular interest in viral RNA metabolisms, namely transcription, replication and packaging of viral RNA into nucleocapsid structure. Contemporary research on otherwise extensively studied family member Vesicular Stomatitis Virus has also been addressed to present a more comprehensive picture of vesiculovirus life cycle. Finally, we reveal examples of protein economy in Chandipura virus life-cycle whereby each viral protein has evolved complexity to perform multiple tasks.

The host cell MAP kinase ERK-2 regulates viral assembly and release by phosphorylating the p6gag protein of HIV-1

The host cell MAP kinase ERK-2 incorporated within human immunodeficiency virus type 1 particles plays a critical role in virus infectivity by phosphorylating viral proteins. Recently, a fraction of the virus incorporated late (L) domain-containing p6(gag) protein, which has an essential function in the release of viral particles from the cell surface, was reported to be phosphorylated by an unknown virus-associated cellular protein kinase (Muller, B., Patschinsky, T., and Krausslich, H. G. (2002) J. Virol. 76, 1015-1024). The present study demonstrates the contribution of the MAP kinase ERK-2 in p6(gag) phosphorylation. According to mutational analysis, a single ERK-2-phosphorylated threonine residue, belonging to a highly conserved phosphorylation MAP kinase consensus site, was identified at position 23 within p6(gag). Substitution by an alanine of the Thr(23) phosphorylable residue within the pNL4.3 molecular clone was found to decrease viral release from various cell types. As observed from electron microscopy experiments, most virions produced from this molecular clone remained incompletely separated from the host cell membrane with an immature morphology and displayed a reduced infectivity in single round infection experiments. Analysis of protein processing by Western blotting experiments revealed an incomplete Pr55(gag) maturation and a reduction in the virion-associated reverse transcriptase proteins was observed that was not related to differences in intracellular viral protein expression. Altogether, these data suggest that phosphorylation of p6(gag) protein by virus-associated ERK-2 is involved in the budding stage of HIV-1 life cycle.

The late-domain-containing protein p6 is the predominant phosphoprotein of human immunodeficiency virus type 1 particles

The Gag-derived protein p6 of human immunodeficiency virus type 1 (HIV-1) plays a crucial role in the release of virions from the membranes of infected cells. It is presumed that p6 and functionally related proteins from other viruses act as adapters, recruiting cellular factors to the budding site. This interaction is mediated by so-called late domains within the viral proteins. Previous studies had suggested that virus release from the plasma membrane shares elements with the cellular endocytosis machinery. Since protein phosphorylation is known to be a regulatory mechanism in these processes, we have investigated the phosphorylation of HIV-1 structural proteins. Here we show that p6 is the major phosphoprotein of HIV-1 particles. After metabolic labeling of infected cells with [ortho-32P]phosphate, we found that phosphorylated p6 from infected cells and from virus particles consisted of several forms, suggesting differential phosphorylation at multiple sites. Apparently, phosphorylation occurred shortly before or after the release of p6 from Gag and involved only a minor fraction of the total virion-associated p6 molecules. Phosphoamino acid analysis indicated phosphorylation at Ser and Thr, as well as a trace of Tyr phosphorylation, supporting the conclusion that multiple phosphorylation events do occur. In vitro experiments using purified virus revealed that endogenous or exogenously added p6 was efficiently phosphorylated by virion-associated cellular kinase(s). Inhibition experiments suggested that a cyclin-dependent kinase or a related kinase, most likely ERK2, was involved in p6 phosphorylation by virion-associated enzymes.

The phosphoprotein of rabies virus is phosphorylated by a unique cellular protein kinase and specific isomers of protein kinase C

The phosphoprotein (P) gene of rabies virus (CVS strain) was cloned and expressed in bacteria. The purified protein was used as the substrate for phosphorylation by the protein kinase(s) present in cell extract prepared from rat brain. Two distinct types of protein kinases, staurosporin sensitive and heparin sensitive, were found to phosphorylate the P protein in vitro by the cell extract. Interestingly, the heparin-sensitive kinase was not the ubiquitous casein kinase II present in a variety of cell types. Further purification of the cell fractions revealed that the protein kinase C (PKC) isomers constitute the staurosporin-sensitive kinases alpha, beta, gamma, and zeta, with the PKCgamma isomer being the most effective in phosphorylating the P protein. A unique heparin-sensitive kinase was characterized as a 71-kDa protein with biochemical properties not demonstrated by any known protein kinases stored in the protein data bank. This protein kinase, designated RVPK (rabies virus protein kinase), phosphorylates P protein (36 kDa) and alters its mobility in gel to migrate at 40 kDa. In contrast, the PKC isoforms do not change the mobility of unphosphorylated P protein. RVPK appears to be packaged in the purified virions, to display biochemical characteristics similar to those of the cell-purified RVPK, and to similarly alter the mobility of endogenous P protein upon phosphorylation. By site-directed mutagenesis, the sites of phosphorylation of RVPK were mapped at S(63) and S(64), whereas PKC isomers phosphorylated at S(162), S(210), and S(271). Involvement of a unique protein kinase in phosphorylating rabies virus P protein indicates its important role in the structure and function of the protein and consequently in the life cycle of the virus.

Phosphorylated states of vesicular stomatitis virus P protein in vitro and in vivo

We have previously shown that the phosphoprotein (P) of vesicular stomatitis virus (VSV), New Jersey serotype (PNJ) is phosphorylated by casein kinase II, within the N-terminal domain I (P1 form), whereas the C-terminal domain II is phosphorylated by a protein kinase activity associated with the L protein (P2 form) (D. J. Chattopadhyay and A.K. Banerjee, Cell 49, 407, 1987; A.M. Takacs et al., J. Virol. 66, 5842, 1992). In the present studies, we have mapped the corresponding P1 and P2 phosphorylation sites in the P protein of the well-studied Indiana serotype (PIND) and compared that with the two previously designated NS1 and NS2 forms present in vivo. The PIND expressed in Escherichia coli in an unphosphorylated form (P0) was used as substrate for recombinant casein kinase II (CKII). By site-directed mutagenesis, the CKII-mediated phosphorylation sites in the P protein were mapped at S60, T62, and S64 within the acidic domain I in vitro. In contrast, using BHK cell extract as the source of CKII or expressing P protein in COS cells labeled with 32PI, the phosphorylation sites were mapped at S60 and S64 with no phosphorylation at T62 residue. We used a peptide mapping technique by which the phosphorylation sites within domain I and domain II were determined. Using this method we demonstrated that the P1 and P2 forms are similar, if not identical, to the previously designated NS1 and NS2 forms, respectively. The domain II phosphorylating kinase activity, associated with the L protein, is shown to be present also in the N-RNA complex, indicating that this activity is of cellular origin. By site-directed mutagenesis, we have shown that S226 and S227 are involved in phosphorylation within domain II. We also demonstrate that the P1 and P2 forms are interconvertible and arise by phosphorylation/dephosphorylation of the phosphate groups in domain II, confirming the precursor-product relationship between the two phosphorylated forms of P protein.

Constitutive phosphorylation of the vesicular stomatitis virus P protein modulates polymerase complex formation but is not essential for transcription or replication

As a subunit of both the P-L polymerase complex and the P-N assembly complex, the vesicular stomatitis virus (VSV) P protein plays a pivotal role in transcription and replication of the viral genome. Constitutive phosphorylation of this protein is currently thought to be essential for formation of the P-L complex. We recently identified the three relevant phosphate acceptor sites in the VSV Indiana serotype P protein (R. L. Jackson, D. Spadafora, and J. Perrault, Virology 214:189-197, 1995). We now report the effects of substituting Ala at these acceptor sites on transcription reconstitution in vitro and replication of defective interfering virus (DI) templates in vivo. The singly substituted S60A, T62A, and S64A mutants and the doubly substituted S60A/T62A and T62A/S64A mutants, all of which retain some constitutive phosphorylation, were nearly as active as the wild type in both assays. Surprisingly, the nonphosphorylated S60A/S64A protein was also active in transcription (> or = 28%)) and replication (> or = 50%) under optimal conditions. However, this mutant was much less active in in vitro transcription (< or = 5% of wild type) at low P concentrations (<27 nM). In addition, S60A/S64A required higher concentrations of L protein than did the wild type for optimal DI replication in vivo. DI replication efficiency and intracellular accumulation of L, P, and N proteins in the transfected system were very similar to those in VSV-infected cells. We conclude that P protein constitutive phosphorylation is not essential for VSV RNA synthesis per se but likely plays an important role in vivo in facilitating P multimerization and possibly P-L complex formation.

Inhibition of vesicular stomatitis virus RNA synthesis by protein hyperphosphorylation

Vesicular stomatitis virus (VSV) RNA synthesis requires the template nucleocapsid, the polymerase (L) protein, and the cofactor phosphorylated (P/NS) protein. To determine whether the degree of phosphorylation regulated VSV RNA synthesis, infected Chinese hamster ovary cells were treated with okadaic acid (OKA), a serine/threonine phosphatase inhibitor. OKA reduced viral penetration and uncoating but had little or no effect on primary transcription or viral protein synthesis. However, approximately 80% of total viral RNA synthesis was inhibited when 2 microM or more OKA was added to infected cells after viral uncoating had taken place. Analysis of proteins and RNA species in infected cells labeled with 32P showed that OKA led to hyperphosphorylation of two viral phosphoproteins, the P/NS protein and matrix protein (M), resulting in inhibition of full-length RNA synthesis and subsequent secondary transcription. Pulse-chase experiments demonstrated that the hyperphosphorylated P/NS species was converted rapidly from the less phosphorylated form. Hyperphosphorylated P/NS as well as the less phosphorylated form, but not M, were found to be associated with nucleocapsids isolated from cytoplasmic extracts. These results suggest that phosphorylation played an important role in the regulation between viral transcription and viral RNA replication as well as the turning off of RNA replication. Thus, phosphatase inhibitors promise to be a valuable tool for dissecting the regulatory mechanisms involving phosphorylated viral proteins.

Sites of in vivo phosphorylation of vesicular stomatitis virus matrix protein

We mapped the in vivo phosphorylation sites for the matrix (M) protein of the Orsay and San Juan strains of vesicular stomatitis virus, Indiana serotype, using limited proteolysis and phosphoamino acid analysis. M protein was solubilized from 32P-labeled virions by using detergent and high-salt conditions, then treated with either trypsin or Staphylococcus aureus V8 protease, and analyzed by polyacrylamide gel electrophoresis and autoradiography to determine which fragments contained phosphate residues. The M protein fragment extending from amino acid 20 to the carboxy terminus contained approximately 70% of the control 32P label, while the fragment extending from amino acid 35 to the carboxy terminus had only trace amounts of label. These data indicate that the major phosphorylation site was between amino acids 20 and 34 in the Orsay strain M protein. Phosphoamino acid analysis of M protein by thin-layer electrophoresis showed the presence of phosphothreonine and phosphoserine and that phosphothreonine continued to be released after prolonged vapor-phase acid hydrolysis. These data identify Thr-31 as the primary in vivo phosphate acceptor for M protein of the Orsay strain of vesicular stomatitis virus. The San Juan strain M protein has serine at position 32, which may also be an important phosphate acceptor. In addition, phosphorylation at Ser-2, -3, or -17 occurs to a greater extent in the San Juan strain M protein than in the Orsay strain M protein. The subcellular distribution of phosphorylated M protein was investigated to determine a probable intracellular site(s) of phosphorylation. Phosphorylated M protein was associated primarily with cellular membranes, suggesting phosphorylation by a membrane-associated kinase. Virion M protein was phosphorylated to a greater extent than membrane-bound M protein, indicating that M protein phosphorylation occurs at a late stage in virus assembly. Phosphorylation of wild-type and temperature-sensitive mutant M protein was studied in vivo at the nonpermissive temperature. The data show that phosphorylated M protein was detected only in wild-type virus-infected cells and virions, suggesting that association with nucleocapsids may be required for M protein phosphorylation or that misfolding of mutant M protein at the nonpermissive temperature prevents phosphorylation.

Stepwise phosphorylation of vesicular stomatitis virus P protein by virion-associated kinases and uncoupling of second step from in vitro transcription

Transcription-competent cores of vesicular stomatitis virus (VSV) contain two tightly bound protein kinase activities capable of phosphorylating the viral P protein (Beckes and Perrault, Virology 184, 383-386, 1991). We examined here the specificity of these kinases for the P protein substrate and their activity during the in vitro transcription process. Conditions favoring the VSVK1 kinase activity resulted in phosphorylation of the P1 species predominantly whereas conditions favoring VSVK2, or transcription conditions, led to an increase in the proportion of the faster migrating P2 and P3 species. A minimum of 2 mol phosphate/mol P protein was incorporated in 1 hr under optimal transcription conditions. Pulse-chase experiments revealed that the VSVK2 activity converted phosphorylated P1 to P2/P3 species. Most or all of the sites modified by VSVK1 (serines only) mapped to the 78 amino acid-long N-terminal fragment of the P protein; additional serine acceptor sites of undetermined location were also phosphorylated under VSVK2 conditions. Pretreatment of virion cores with 5'-p-fluorosulfonylbenzoyl adenosine had little or no effect on P1 phosphorylation but inhibited P1 to P2/P3 conversion nearly completely, with no effect on subsequent transcription. Likewise, the addition of cell extracts had relatively little effect on P1 phosphorylation but strongly inhibited the appearance of P2/P3, without affecting concurrent transcription. We conclude that phosphorylation of the P protein during transcription in vitro is a two-step process carried out by two distinct kinase activities, but only the first step may be essential for viral mRNA synthesis.

Sequential phosphorylation of the phosphoprotein of vesicular stomatitis virus by cellular and viral protein kinases is essential for transcription activation

The phosphoprotein (P) and the large protein (L) constitute the RNA-dependent RNA polymerase of vesicular stomatitis virus (VSV). We show that phosphate-free P protein expressed in bacteria is transcriptionally inactive when reconstituted with L protein and viral N-RNA template free of cellular protein kinase. Phosphorylation of P protein by a cellular kinase(s) was essential for transcription as well as for further phosphorylation by an L-associated kinase, the two kinases acting in a sequential (cascade) manner. Phosphate groups introduced by cell kinase were stable, whereas those due to L kinase underwent a turnover which was coupled to ongoing transcription. We present a model for the phosphorylation pathway of P protein and propose that continued phosphorylation and dephosphorylation of P protein may represent a transcriptional regulatory (on-off) switch of nonsegmented negative-strand RNA viruses.

Phosphorylation of vaccinia virus core proteins during transcription in vitro

The phosphorylation of vaccinia virus core proteins has been studied in vitro during viral transcription. The incorporation of [gamma-32P]ATP into protein is linear for the first 2 min of the reaction, whereas incorporation of [3H]UTP into RNA lags for 1 to 2 min before linear synthesis. At least 12 different proteins are phosphorylated on autoradiograms of acrylamide gels, and the majority of label is associated with low-molecular-weight proteins. If the transcription reaction is reduced by dropping the pH to 7 from its optimal of 8.5, two proteins (70 and 80 kDa) are no longer phosphorylated. RNA isolated from the pH 7 transcription reaction hybridized primarily to the vaccinia virus HindIII DNA fragments D to F, whereas the transcripts synthesized at pH 8.5 hybridized to almost all of the HindIII-digested vaccinia virus DNA fragments. The differences between the pH 7.0 and 8.5 transcription reactions in phosphorylation and transcription could be eliminated by preincubating the viral cores with 2 mM ATP. In sum, the results suggest that the phosphorylation of the 70- and 80-kDa peptides may contribute to the regulation of early transcription.

Cloning and expression of the vesicular stomatitis virus phosphoprotein gene in Escherichia coli: analysis of phosphorylation status versus transcriptional activity

The phosphoprotein (P, previously known as NS) genes of vesicular stomatitis virus serotypes New Jersey and Indiana have been cloned in the Escherichia coli expression vector pET-3a. Transcription of P genes in these clones initiated from a phage T7 RNA polymerase promoter, whereas translation was driven by the Shine-Dalgarno sequence and the initiator AUG codon of the T7 gene 10 message. The clones were introduced into an appropriate E. coli strain in which T7 RNA polymerase was expressed under the control of the lac promoter. Under optimal conditions of induction with isopropylthiogalactopyranoside, P protein made in these bacterial strains constituted 5 to 20% of total cellular protein. P protein expressed in bacteria was unphosphorylated and transcriptionally active in an in vitro reconstitution assay with viral L protein and an N-RNA template. However, the P protein was phosphorylated in vitro by the kinase activities associated with L and the N-RNA template.

Vesicular stomatitis virus and its defective interfering particles exhibit in vitro transcriptional and replicative competition for purified L-NS polymerase molecules

We have quantitated replication of the RNA genomes of vesicular stomatitis virus (VSV) and its defective interfering (DI) particles in a BHK21 cell-free system into which nucleocapsids were introduced in varying amounts and ratios, with or without addition of purified virus polymerase components. The quantitative transcriptional and replicative competition observed in vitro between virus and DI genomes resembled DI particle interference observed in vivo in infected cells. The effects of an added polymerase protein (L-NS) complex from purified virions showed that this competition varies with polymerase availability. When DI nucleocapsids were added in small amounts, addition of L-NS polymerase protein complex stimulated a linear increase in viral mRNA transcription until the viral templates' transcription capacity became saturated; then there was a reproducible sudden switch toward RNA replication (mainly of DI genomes). Purified L or NS proteins added separately produced different effects than the L-NS complex. These findings support earlier evidence for replicative competition as the mechanism of DI particle interference with standard virus, and suggest that the major competition is for limiting amounts of L-NS molecules involved in transcription and replication, and in facilitation of encapsidation.

Phosphorylation of vesicular stomatitis virus M protein: evidence for a second virion-associated protein serine kinase activity

The vesicular stomatitis virus (VSV) NS and M proteins are not only phosphorylated in vivo but are also further modified by the virion-associated protein kinase(s) concomitantly with the in vitro transcription process. Although NS phosphorylation is necessary for this transcription, no function has yet been ascribed for M protein phosphorylation. We show here that all phosphates added to M protein in vitro mapped to the trypsin-sensitive N-terminal basic domain (residues 1-43). The major site(s) (approximately 93%) corresponded to one or more of three serine residues within the first 17 amino acids. Nearly 1 mol phosphate/mol protein was added in vitro under optimal conditions suggesting that only one of these three candidate serine residues corresponds to the major site. This same M protein domain is thought to play an important role in virus RNA synthesis by inhibiting transcription. We show here that in vitro phosphorylation did not appear to affect this function. Two critical serine residues in the VSV NS protein were previously reported to be phosphorylated during in vitro transcription (D. Chattopadhyay and A. K. Banerjee, 1987, Cell 49, 407-414). The sequence flanking these NS serines is very acidic while that of all three candidate phosphoserines in the M protein is very basic. We therefore predict that at least two distinct serine-specific kinase activities are packaged in virions, one specific for M and one specific for NS.

Continuing coevolution of virus and defective interfering particles and of viral genome sequences during undiluted passages: virus mutants exhibiting nearly complete resistance to formerly dominant defective interfering particles

We quantitatively analyzed the interference interactions between defective interfering (DI) particles and mutants of cloned vesicular stomatitis virus passaged undiluted hundreds of times in BHK-21 cells. DI particles which predominated at different times in these serial passages always interfered most strongly (and very efficiently) with virus isolated a number of passages before the isolation of the DI particles. Virus isolated at the same passage level as the predominant DI particles usually exhibited severalfold resistance to these DI particles. Virus mutants (Sdi- mutants) isolated during subsequent passages always showed increasing resistance to these DI particles, followed by decreasing resistance as new DI particles arose to predominate and exert their own selective pressures on the virus mutant population. It appears that such coevolution of virus and DI particle populations proceeds indefinitely through multiple cycles of selection of virus mutants resistant to a certain DI particle (or DI particle class), followed by mutants resistant to a newly predominant DI particle, etc. At the peak of resistance, virus mutants were isolated which were essentially completely resistant to a particular DI particle; i.e., they were several hundred thousand-fold resistant, and they formed plaques of normal size and numbers in the presence of extremely high multiplicities of the DI particle. However, they were sensitive to interference by other DI particles. Recurring population interactions of this kind can promote rapid virus evolution. Complete sequencing of the N (nucleocapsid) and NS (polymerase associated) genes of numerous Sdi- mutants collected at passage intervals showed very few changes in the NS protein, but the N gene gradually accumulated a series of stable nucleotide and amino acid substitutions, some of which correlated with extensive changes in the Sdi- phenotype. Likewise, the 5' termini (and their complementary plus-strand 3' termini) continued to accumulate extensive base substitutions which were strikingly confined to the first 47 nucleotides. We also observed addition and deletion mutations in noncoding regions of the viral genome at a level suggesting that they probably occur at a high frequency throughout the genome, but usually with lethal or debilitating consequences when they occur in coding regions.

Identification of a domain within the phosphoprotein of vesicular stomatitis virus that is essential for transcription in vitro

A full-length cDNA copy of the phosphoprotein (NS) mRNA of vesicular stomatitis virus (New Jersey serotype) was inserted into pGEM4 vector downstream of the promoter for bacteriophage SP6 RNA polymerase. Transcription of the cDNA in vitro resulted in the synthesis of NS mRNA, which was subsequently translated into NS protein in a cell-free rabbit reticulocyte system. The biological activity of the expressed NS protein was demonstrated by in vitro synthesis of mRNA by transcription-reconstitution with purified viral L protein and N-RNA template. Deletion mapping of the NS gene defined a specific domain between amino acid residues 213 and 247, which was essential for in vitro transcription. Removal of the COOH-terminal 21 amino acids, on the other hand, did not have a significant effect on transcription. This domain appears to be involved in efficient binding of NS protein to the N protein-RNA template.

Phosphoprotein NS of vesicular stomatitis virus: phosphorylated states and transcriptional activities of intracellular and virion forms

The phosphorylation and transcriptional competence of the free cytoplasmic form and the virion form of NS protein of vesicular stomatitis virus (VSV-Indiana/Mudd-Summers) were compared. NS protein is known to exist in two distinct phosphorylated states, NS1 and NS2, that are resolvable by gel electrophoresis. In vitro phosphorylation of virion NS protein by the viral L protein-associated protein kinase resulted in the phosphorylation of both NS1 and NS2. However, in the presence of the N-RNA complex, the NS2 form was preferentially phosphorylated. A cellular protein kinase activity, found in cytoplasmic extracts from VSV-infected or uninfected cells, preferentially phosphorylated NS1, which did not undergo dephosphorylation by cellular phosphatase and also did not convert to NS2. In contrast, the virion or cellular NS2 which had been phosphorylated in vivo or in vitro could be rapidly dephosphorylated by a cellular phosphatase. Cytoplasmic NS protein was found to be fully capable of binding to the virion N-RNA template, and in conjunction with L protein, it participated in synthesis of the leader RNA and five mRNA species of VSV. Moreover, under these conditions, neither cellular phosphatase nor cellular ribonuclease was able to bind to reconstituted nucleocapsids. Binding of cytoplasmic NS to the virion N-RNA template in the presence of L protein resulted in a large and preferential enhancement of NS2 phosphorylation. A protein kinase activity, which phosphorylated NS protein in vitro, was found to be associated with the N-RNA template. This activity appeared to be very tightly bound to N-RNA and exhibited absolute specificity for NS protein of the homologous serotype.

Role of the nucleocapsid protein in regulating vesicular stomatitis virus RNA synthesis

We describe experiments with two monoclonal antibodies to the vesicular stomatitis virus (VSV) nucleocapsid protein N with strikingly different characteristics. Antibody 1 binds to nucleocapsids and probably the pool of free (unbound) N protein; it inhibits transcription in vitro, and when microinjected into cells, protects the cells against VSV. Antibody 2 binds poorly to nucleocapsids, does not inhibit transcription, but when microinjected into cells, binds selectively to the free N and delays the appearance of progeny virus. We have confirmed these results by analyzing the effect of these antibodies on in vitro genomic RNA synthesis. The results of both the in vivo and in vitro experiments show that the replication of the VSV genome is controlled by the availability of the nucleocapsid protein, even when the polymerase has access to the host factors and multiple phosphorylated forms of the NS protein thought to be involved in genomic RNA synthesis.

Respiratory syncytial virus mRNA coding assignments

The polypeptide coding assignments for six of the respiratory syncytial virus-specific mRNAs were determined by translation of the individual mRNAs in vitro. The coding assignments of the RNAs are as follows. RNA band 1 is complex and can be separated into at least two components on the basis of electrophoretic mobility (molecular weights [MWs] approximately equal to 0.21 X 10(6) and 0.31 X 10(6), respectively) that code for three polypeptides of 9.5, 11, and 14 kilodaltons (K). RNA 2 (MW, 0.39 X 10(6)) codes for a 34K polypeptide; RNA 3 (MW, 0.40 X 10(6)) codes for a 26K polypeptide; RNA 4 (MW, 0.47 X 10(6)) codes for a 42K polypeptide; and RNA 5 (MW, 0.74 X 10(6)) codes for a 59K polypeptide. By limited-digest peptide mapping, the 34, 26, and 42K polypeptides synthesized in vitro appeared to be unique. Additionally, peptide mapping showed that the 34, 26, and 42K polypeptides synthesized in vitro were indistinguishable from their counterparts synthesized in infected cells. Thus, the 34, 26, and 42K polypeptides coded for by mRNAs 2, 3, and 4, respectively, were identified as the respiratory syncytial virus phosphoprotein (34K), matrix protein (26K), and nucleocapsid protein (42K), respectively. RNA 5 was shown to code for a 59K polypeptide. The 59K polypeptide synthesized in vitro did not comigrate with any polypeptide specific to infected cells, suggesting that it is a candidate for co- or post-translational modification.

Site-specific phosphorylation regulates the transcriptive activity of vesicular stomatitis virus NS protein

In vitro transcription by vesicular stomatitis virus nucleocapsids is inhibited by enzymatic dephosphorylation of the NS protein. We provide evidence that specific, partial dephosphorylation of NS molecules is the only detectable change in nucleocapsids treated with bacterial alkaline phosphatase under conditions that prevent the action of adventitious protease. Dephosphorylation appeared to affect only the rate of transcription; there were no changes in sedimentation rates of transcripts. To identify the sites of phosphorylation required for NS activity in transcription, we examined phosphopeptides produced by chymotrypsin digestion of the two electrophoretic classes of NS molecules found in virions and infected cells. The electrophoretically slower class, NS1, abundant in the intracellular soluble pool, has a lower activity in transcription; it contained six chymotryptic phosphopeptides. Five of these peptides contained both phosphoserine and phosphothreonine, indicating that this peptide cluster represents at least 11 separate sites of phosphorylation. In the electrophoretically faster nucleocapsid-associated NS2 class of molecules, which support a higher rate of transcription, another group of eight phosphopeptides was superimposed on this pattern. Two of these peptides contained both phosphoserine and phosphothreonine, so this cluster of peptides represents at least 10 additional phosphorylation sites. These sites were especially sensitive to dephosphorylation by bacterial alkaline phosphatase. One or more of them appears to be responsible for the higher transcription rates medicated by NS2 molecules.

NS phosphoprotein of vesicular stomatitis virus: subspecies separated by electrophoresis and isoelectric focusing

The NS protein of vesicular stomatitis virus is the only phosphorylated nucleocapsid protein. The amount of NS phosphorylation appears to regulate the activity of the protein in the transcription of the virus genome. Several methods have been used to separate NS subspecies containing different amounts of phosphate, but the relationships among the subspecies separated by different workers have been unclear. We report that the isoelectric points of NS molecules were abnormally acidic in some commercial ampholytes, but favorable ampholytes resolved multiple phosphorylated NS subspecies with isoelectric points ranging from pH 6.8 to 7.2. The most highly phosphorylated NS molecules had more acidic isoelectric points, and they exhibited greater electrophoretic mobilities in two previously employed electrophoretic systems.

Identification of a protein kinase activity in purified foot- and-mouth disease virus

Purified preparations of foot-and-mouth disease virus types A, O, and C contain a protein kinase activity which can transfer the gamma phosphate of [32P]ATP to virion structural proteins VP2 and VP3 and exogenous acceptor proteins. Utilizing protamine sulfate as an acceptor, the kinase activity can be demonstrated in disrupted virus but not in intact virus. The enzyme is heat labile with optimal activity at pH 7 or greater. Serine residues of protamine sulfate were identified as the amino acid phosphorylated by the protein kinase. Treatment of purified virus with trypsin, which cleaves VP3, did not affect the protein kinase activity. The results indicate that the protein kinase activity found in FMDV is present in an internally located protein of viral or host origin.