Transcriptional activities of different phosphorylated species of NS protein purified from vesicular stomatitis virions and cytoplasm of infected cells

p38MAPK- and GSK3-Mediated Phosphorylation of Snakehead Vesiculovirus Phosphoprotein at Threonine 160 Facilitates Viral Replication

Phosphoprotein (P), co-factor of the polymerase (large protein, L) of single-stranded negative-sense RNA viruses, is phosphorylated during viral infection and its phosphorylation has been reported to play important roles in viral replication. However, the function of P phosphorylation in viral replication is still far from clear. Snakehead vesiculovirus (SHVV) is a kind of fish rhabdovirus that has caused serious economic losses in snakehead fish culture in China without any effective preventive or therapeutical measures currently. In this study, 4D label-free phosphoproteomics sequencing of SHVV-infected cells identified five phosphorylated sites on SHVV P, among which threonine 160 (T160) was proved to be phosphorylated. Overexpression of wild-type P, but not P-T160A or P-T160E mutant, promoted SHVV replication, suggesting that the T160 phosphorylation on the P protein is critical for SHVV replication. Moreover, we found that T160A or T160E mutation on SHVV P had no effect on the interactions of P-nucleoprotein (N), P-P, or P-L. Further study revealed that p38 mitogen-activated protein kinase (p38MAPK) and glycogen synthase kinase 3 (GSK3) interacted with SHVV P and mediated the T160 phosphorylation. Besides, overexpression of p38MAPK or GSK3 facilitated, while knockdown or activity inhibition of p38MAPK or GSK3 suppressed, SHVV replication. Overall, p38MAPK- and GSK3-mediated phosphorylation of the P protein at T160 is required for SHVV replication, which provided targets for designing anti-SHVV drugs and developing live-attenuated SHVV vaccines. Our study helps understand the role of P phosphorylation in the replication of single-stranded negative-sense RNA viruses. IMPORTANCE Phosphorylation of viral proteins plays important roles in viral replication. Currently, the role of phosphorylation of phosphoprotein (P) in the replication of single-stranded negative-sense RNA viruses is far from clear. Identification of the phosphorylated sites on viral P protein and the related host kinases is helpful for developing live-attenuated vaccines and designing antiviral drugs. This study focused on identifying the phosphorylated sites on P protein of a fish rhabdovirus SHVV, determining the related host kinases, and revealing the effects of the phosphorylated sites and kinases on SHVV replication. We found that SHVV P was phosphorylated at T160, which was mediated by the kinases p38MAPK and GSK3 to promote SHVV replication. This study is the first time to study the role of P phosphorylation in fish rhabdovirus replication.

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.

Newly identified phosphorylation site in the vesicular stomatitis virus P protein is required for viral RNA synthesis

The vesicular stomatitis virus (VSV) RNA-dependent RNA polymerase consists of two viral proteins; the large (L) protein is the main catalytic subunit, and the phosphoprotein (P) is an essential cofactor for polymerase function. The P protein interacts with the L protein and the N-RNA template, thus connecting the polymerase to the template. P protein also binds to free N protein to maintain it in a soluble, encapsidation-competent form. Previously, five sites of phosphorylation were identified on the P protein and these sites were reported to be differentially important for mRNA synthesis or genomic replication. The previous studies were carried out by biochemical analysis of portions of the authentic viral P protein or by analysis of bacterium-expressed, exogenously phosphorylated P protein by mutagenesis. However, there has been no systematic biochemical search for phosphorylation sites on authentic, virus-expressed P protein. In this study, we analyzed the P protein isolated from VSV-infected cells for sites of phosphorylation by mass spectrometry. We report the identification of Tyr14 as a previously unidentified phosphorylation site of VSV P and show that it is essential for viral transcription and replication. However, our mass spectral analysis failed to observe the phosphorylation of previously reported C-terminal residues Ser226 and Ser227 and mutagenic analyses did not demonstrate a role for these sites in RNA synthesis.

Correlation between structure, protein composition, morphogenesis and cytopathology of Glossina pallidipes salivary gland hypertrophy virus

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is a dsDNA virus with rod-shaped, enveloped virions. Its 190 kb genome contains 160 putative protein-coding ORFs. Here, the structural components, protein composition and associated aspects of GpSGHV morphogenesis and cytopathology were investigated. Four morphologically distinct structures: the nucleocapsid, tegument, envelope and helical surface projections, were observed in purified GpSGHV virions by electron microscopy. Nucleocapsids were present in virogenic stroma within the nuclei of infected salivary gland cells, whereas enveloped virions were located in the cytoplasm. The cytoplasm of infected cells appeared disordered and the plasma membranes disintegrated. Treatment of virions with 1 % NP-40 efficiently partitioned the virions into envelope and nucleocapsid fractions. The fractions were separated by SDS-PAGE followed by in-gel trypsin digestion and analysis of the tryptic peptides by liquid chromatography coupled to electrospray and tandem mass spectrometry. Using the MaxQuant program with Andromeda as a database search engine, a total of 45 viral proteins were identified. Of these, ten and 15 were associated with the envelope and the nucleocapsid fractions, respectively, whilst 20 were detected in both fractions, most likely representing tegument proteins. In addition, 51 host-derived proteins were identified in the proteome of the virus particle, 13 of which were verified to be incorporated into the mature virion using a proteinase K protection assay. This study provides important information about GpSGHV biology and suggests options for the development of future anti-GpSGHV strategies by interfering with virus-host interactions.

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.

Host cell protein kinases in nonsegmented negative-strand virus (mononegavirales) infection

Phosphorylation of one or more viral proteins is probably an essential step in the life cycle of every member of the nonsegmented negative-strand RNA virus (mononegavirales [MNV]) group. Since no virally encoded protein kinases have been discovered in this group, phosphorylation is effected entirely by host cell kinases. The virally encoded P proteins of the MNV are the only ones consistently phosphorylated with a stoichiometry > or =1. The P protein of vesicular stomatitis virus (VSV), and perhaps also of respiratory syncytial virus, are the only ones for which a function of phosphorylation has been established. Phosphorylation by casein kinase 2 at one or more identified sites in the VSV P protein activates transcriptional activity by promoting formation of a homotrimer, which is then capable of binding the RNA polymerase and attaching it to the N protein-RNA template. A second phosphorylation of VSV P protein by a different kinase also occurs, dependent upon prior modification by casein kinase 2, but its function is not definitely known. Phosphorylation of the other MNV P proteins may serve a different purpose. No evidence has been obtained yet for any function for phosphorylation of any other MNV protein.

Optimal replication activity of vesicular stomatitis virus RNA polymerase requires phosphorylation of a residue(s) at carboxy-terminal domain II of its accessory subunit, phosphoprotein P

The phosphoprotein, P, of vesicular stomatitis virus (VSV) is a key subunit of the viral RNA-dependent RNA polymerase complex. The protein is phosphorylated at multiple sites in two different domains. We recently showed that specific serine and threonine residues within the amino-terminal acidic domain I of P protein must be phosphorylated for in vivo transcription activity, but not for replication activity, of the polymerase complex. To examine the role of phosphorylation of the carboxy-terminal domain II residues of the P protein in transcription and replication, we have used a panel of mutant P proteins in which the phosphate acceptor sites (Ser-226, Ser-227, and Ser-233) were altered to alanines either individually or in various combinations. Analyses of the mutant proteins for their ability to support replication of a VSV minigenomic RNA suggest that phosphorylation of either Ser-226 or Ser-227 is necessary for optimal replication activity of the protein. The mutant protein (P226/227) in which both of these residues were altered to alanines was only about 8% active in replication compared to the wild-type (wt) protein. Substitution of alanine for Ser-233 did not have any adverse effect on replication activity of the protein. In contrast, all the mutant proteins showed activities similar to that of the wt protein in transcription. These results indicate that phosphorylation of the carboxy-terminal domain II residues of P protein are required for optimal replication activity but not for transcription activity. Furthermore, substitution of glutamic acid residues for Ser-226 and Ser-227 resulted in a protein that was only 14% active in replication but almost fully active in transcription. Taken together, these results, along with our earlier studies, suggest that phosphorylation of residues at two different domains in the P protein regulates its activity in transcription and replication of the VSV genome.

Phosphorylation within the amino-terminal acidic domain I of the phosphoprotein of vesicular stomatitis virus is required for transcription but not for replication

Phosphorylation by casein kinase II at three specific residues (S-60, T-62, and S-64) within the acidic domain I of the P protein of Indiana serotype vesicular stomatitis virus has been shown to be critical for in vitro transcription activity of the viral RNA polymerase (P-L) complex. To examine the role of phosphorylation of P protein in transcription as well as replication in vivo, we used a panel of mutant P proteins in which the phosphate acceptor sites in domain I were substituted with alanines or other amino acids. Analyses of the alanine-substituted mutant P proteins for the ability to support defective interfering RNA replication in vivo suggest that phosphorylation of these residues does not play a significant role in the replicative function of the P protein since these mutant P proteins supported replication at levels > or = 70% of the wild-type P-protein level. However, the transcription function of most of the mutant proteins in vivo was severely impaired (2 to 10% of the wild-type P-protein level). The level of transcription supported by the mutant P protein (P(60/62/64)) in which all phosphate acceptor sites have been mutated to alanines was at best 2 to 3% of that of the wild-type P protein. Increasing the amount of P(60/62/64) expression in transfected cells did not rescue significant levels of transcription. Substitution with other amino acids at these sites had various effects on replication and transcription. While substitution with threonine residues (P(TTT)) had no apparent effect on transcription (113% of the wild-type level) or replication (81% of the wild-type level), substitution with phenylalanine (P(FFF)) rendered the protein much less active in transcription (< 5%). Substitution with arginine residues led to significantly reduced activity in replication (6%), whereas glutamic acid substituted P protein (P(EEE)) supported replication (42%) and transcription (86%) well. In addition, the mutant P proteins that were defective in replication (P(RRR)) or transcription (P(60/62/64)) did not behave as transdominant repressors of replication or transcription when coexpressed with wild-type P protein. From these results, we conclude that phosphorylation of domain I residues plays a major role in in vivo transcription activity of the P protein, whereas in vivo replicative function of the protein does not require phosphorylation. These findings support the contention that different phosphorylated states of the P protein regulate the transcriptase and replicase functions of the polymerase protein, L.

Increased expression of the N protein of respiratory syncytial virus stimulates minigenome replication but does not alter the balance between the synthesis of mRNA and antigenome

A popular model for RNA synthesis by nonsegmented negative-strand RNA viruses is that transcription and RNA replication are executed by the same polymerase complex and that there is a dynamic balance between the two processes that is mediated by the nucleocapsid N protein. According to this model, transcription occurs until sufficient soluble N protein accumulates to initiate encapsidation of the nascent RNA product, which somehow switches the polymerase into a readthrough replicative mode. This model was examined for respiratory syncytial virus (RSV) using a reconstituted transcription and RNA replication system that involves a minireplicon and viral proteins that are expressed intracellularly from transfected plasmids. Preliminary experiments showed that reconstituted RNA replication was highly productive, such that on average each molecule of plasmid-supplied minigenome that became encapsidated was amplified 10- to 50-fold. N protein was increased on its own or in concert with the phosphoprotein P and in the presence or absence of the M2 ORF1 transcription elongation factor. The maximum level of N and P protein expression achieved from plasmids equalled or exceeded that obtained in RSV-infected cells. Increased levels of N protein stimulated RNA replication. This is consistent with the idea that RNA replication is dependent on the availability of N protein for encapsidation, which is one postulate of the model. The M2 ORF1 protein had no detectable effect on RNA replication under the various conditions of expression of N and P, which confirmed and extended previous results. However, there was no evidence of a significant switch in positive-sense RNA synthesis from transcription (synthesis of mRNAs) to RNA replication (synthesis of antigenome). The synthesis of positive-sense antigenome and mRNA appeared to occur at a fixed ratio, with mRNA being by far the more abundant product.

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.

Ribosomal frameshifting during translation of measles virus P protein mRNA is capable of directing synthesis of a unique protein

Members of the Paramyxoviridae family utilize a variety of different strategies to increase coding capacity within their P cistrons. Translation initiation at alternative 5'-proximal AUG codons is used by measles virus (MV) to express the virus-specific P and C proteins from overlapping reading frames on their mRNAs. Additional species of mRNAs are transcribed from the MV P cistron by the insertion of extra nontemplated G residues at a specific site within the P transcript. Addition of only a single nontemplated G residue results in the expression of the V protein, which contains a unique carboxyl terminus. We have used an Escherichia coli system to express MV P cistron-related mRNAs and proteins. We have found that ribosomal frameshifting on the MV P protein mRNA is capable of generating a previously unrecognized P cistron-encoded protein that we have designated R. Some ribosomes which have initiated translation of the P protein mRNA use the sequence TCC CCG AG (24 nucleotides upstream of the V protein stop codon) to slip into the -1 reading frame, thus translating the sequence as TC CCC GAG. The resulting R protein terminates five codons downstream of the frameshift site at the V protein stop codon. We have gone on to use a chloramphenicol acetyltransferase reporter system to demonstrate that this MV-specific sequence is capable of directing frameshifting during in vivo translation in eukaryotic cells. Analysis of immunoprecipitated proteins from MV-infected cells by two-dimensional gel electrophoresis allowed detection of a protein species consistent with R protein in MV-infected cells. Quantitation of this protein species allowed a rough estimation of frameshift frequency of approximately 1.8%. Significant stimulation of ribosomal frameshift frequency at this locus of the MV P mRNA was mediated by a downstream stimulator element which, although not yet fully defined, appeared to be neither a conventional stem-loop nor an RNA pseudoknot structure.

Role of cellular casein kinase II in the function of the phosphoprotein (P) subunit of RNA polymerase of vesicular stomatitis virus

The phosphorylation of the P protein of vesicular stomatitis virus by cellular casein kinase II (CKII) is essential for its activity in viral transcription. Recent in vitro studies have demonstrated that CKII converts the inactive unphosphorylated form of P (P0) to an active phosphorylated form P1, after phosphorylation at two serine residues, Ser-59 and Ser-61. To gain insight into the role of CKII-mediated phosphorylation in the structure and function of the P protein, we have carried out circular dichroism (CD) and biochemical analyses of both P0 and P1. The results of CD analyses reveal that phosphorylation of P0 to P1 significantly increases the predicted alpha-helical structure of the P1 protein from 27 to 48%. The phosphorylation defective double serine mutant (P59/61), which is transcriptionally inactive, possesses a secondary structure similar to that of P0. P1, at a protein concentration of 50 micrograms/ml, elutes from a gel filtration column apparently as a dimer, whereas both P0 and the double serine mutant elute as a monomer at the same concentration. Interestingly, unlike wild-type P1 protein, the P mutants in which either Ser-59 or Ser-61 is altered to alanine required a high concentration of CKII for optimal phosphorylation. We demonstrate here that phosphorylation of either Ser-59 or Ser-61 is necessary and sufficient to transactivate L polymerase although alteration of one serine residue significantly decreases its affinity for CKII. We have also shown that P1 binds to the N-RNA template more efficiently than P0 and the formation of P1 is a prerequisite for the subsequent phosphorylation by L protein-associated kinase. In addition, mutant P59/61 acts as a transdominant negative mutant when used in a transcription reconstitution assay in the presence of wild-type P protein.

Protein interactions entered into by the measles virus P, V, and C proteins

Measles virus (MV) expresses at least 3 proteins from the phosphoprotein (P) cistron. Alternative translation initiation directs synthesis of the C protein from the +1 reading frame, while so-called RNA editing generates a second population of mRNAs which express the V protein from the -1 reading frame which lies within and overlaps the larger P reading frame. While the P protein has been demonstrated to be an essential cofactor for the L protein in the formation of active transcriptase complexes, the functions of the V and C proteins remain unknown. In order to investigate these functions, we have expressed the MV P, V and C proteins as GST fusions in E. coli for affinity purification and use in an in vitro binding assay with other viral and cellular proteins. The P protein was found to interact with L, NP, and with itself. These interactions were mapped to the carboxy-terminal half of the protein which is absent in the V protein. In contrast, both the V and C proteins failed to interact with any other viral proteins, but were each found to interact specifically with one or more cellular proteins. Appropriate aspects of these results were confirmed in vivo using the yeast two-hybrid system. These observations suggest that the V and C proteins may be involved in modulation of the host cellular environment within MV-infected cells. Such activity would be distinct from their previously proposed role in the possible down-regulation of virus-specific RNA transcription and replication.

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.

Viral protein kinases and protein phosphatases

Certain large DNA viruses (e.g. herpesviruses and poxviruses) encode proteins related to cellular protein-serine/threonine kinases, and Hepatitis B virus and vesicular stomatitis virus may encode structurally different protein kinases. Other viruses activate cellular protein kinases, e.g. interferon-induced eukaryotic initiation factor-2 kinase, growth factor-induced kinases and protein kinases that regulate mitosis. Protein phosphatases are encoded by vaccinia virus and bacteriophage lambda and must also play a role in viral infection--as do cellular protein phosphatases. The functions of many of these viral enzymes remain to be determined, but they represent possible new targets for anti-viral therapy.

Viral liposomes released from insect cells infected with recombinant baculovirus expressing the matrix protein of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) has been found to promote assembly and budding of virions as well as down-regulating of VSV transcription. Large quantities of M protein can be produced in insect cells infected with recombinant baculovirus expressing the VSV M gene under control of the polyhedrin promoter. Analysis by pulse-chase experiments and density gradient centrifugation revealed that the [35S]methionine-labeled M protein synthesized in insect cells is released into the extracellular medium in association with lipid vesicles (liposomes). Electron microscopy and immunogold labeling showed that M protein expressed in insect cells induced the formation on plasma membrane of vesicles containing M protein, which are released from the cell surface in the form of liposomes. The baculovirus vector itself or recombinants expressing VSV glycoprotein (G) or nucleocapsid (N) protein did not produce the formation of vesicles in infected cells. The baculovirus-expressed M protein retains biological activity as demonstrated by its capacity to inhibit transcription when reconstituted with VSV nucleocapsids in vitro. These data suggest that M protein has the capacity to associate with the plasma membrane of infected cells and, in so doing, causes evagination of the membrane to form a vesicle which is released from the cell. This observation leads to the postulate, which requires further proof, that the VSV M protein can induce the formation and budding of liposomes from the cell membrane surface.

Phosphorylation of specific serine residues within the acidic domain of the phosphoprotein of vesicular stomatitis virus regulates transcription in vitro

The phosphorylated state of the vesicular stomatitis virus phosphoprotein (P), an essential component of the virion-associated RNA polymerase complex, has been shown to be important for the transcriptional activity of the complex. Recent studies indicate that phosphorylation within the acidic domain of the P protein by cellular casein kinase II is necessary for its activity. In an attempt to identify the exact location of the cell kinase-mediated phosphorylation, we altered specific serine and threonine residues within the acidic domain of the New Jersey serotype of P protein by site-directed mutagenesis. The altered P proteins were then tested to determine what effect these mutations had on the phosphorylated state of the protein in vivo as well as its transcriptional activity in vitro. We report that serine residues 59 and 61 within the acidic domain of the P protein must be phosphorylated for it to be functionally active in a reconstituted transcription assay. These results demonstrate the importance of site-specific phosphorylation in the transcriptional activity of a negative-strand RNA viral phosphoprotein and the crucial role played by a cell protein kinase in this process.

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.

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.

Antiviral effects of xanthate D609 on the human respiratory syncytial virus growth cycle

The antiviral compound tricyclo-decan-9-yl-xanthogenate (D609) inhibits respiratory syncytial (RS) virus growth in human epithelial (Hep 2) cells. D609 treatment resulted in a decrease in the accumulation of viral proteins, in the phosphorylation of the viral phosphoprotein, and in the amount of extracellular antigens and infectious particles. The relative accumulation of viral proteins was also unbalanced, however no differences were found in the amount of viral RNA with plus or minus polarity. In addition nucleocapsids formation was not inhibited. These observations suggested that this antiviral compound affects the relative proportion of viral proteins and the phosphorylation of P protein. Both features appear to be important in RS virus morphogenesis.

Gene expression of nonsegmented negative strand RNA viruses

Nonsegmented negative strand RNA viruses comprise major human and animal pathogens in nature. This class of viruses is ubiquitous and infects vertebrates, invertebrates, and plants. Our laboratory has been working on the gene expression of two prototype nonsegmented negative strand RNA viruses, vesicular stomatitis virus (a rhabdovirus) and human parainfluenza virus 3 (a paramyxovirus). An RNA-dependent RNA polymerase (L and P protein) is packaged within the virion which faithfully copies the genome RNA in vitro and in vivo; this enzyme complex, in association with the nucleocapsid protein (N), is also involved in the replication process. In this review, we have presented up-to-date information of the structure and function of the RNA polymerases of these two viruses, the mechanisms of transcription and replication, and the role of host proteins in the life-cycle of the viruses. These detailed studies have led us to a better understanding of the roles of viral and cellular proteins in the viral gene expression.

Phosphorylation of NS protein by vesicular stomatitis virus nucleocapsids: lack of effect during RNA synthesis and separation of kinase from L protein

The relationship between NS protein phosphorylation and RNA polymerase activities was determined in nucleocapsids purified from vesicular stomatitis virus grown in BHK cells. Phosphate incorporation into endogenous NS protein under transcription conditions reached a maximum value of 0.06 mol/mol of NS within 20 to 30 min, while RNA synthesis remained linear for 90 min. Phosphate incorporation into NS increased further upon addition of kinase-free NS protein but not upon addition of nucleocapsid kinase (prepared as described below), indicating that cessation of NS phosphorylation under transcribing conditions was due to substrate exhaustion. When NS was phosphorylated with 32P, less than 8% of the radiolabel was lost during subsequent transcription, indicating that this phosphate did not turn over. Treatment of nucleocapsids with 5'-p-fluorosulfonylbenzoyl adenosine resulted in greater than 90% inhibition of NS phosphorylation but had no effect on RNA polymerase activity. Fast protein liquid (Superose-6) chromatography of a nucleocapsid (L + NS) fraction resulted in complete separation of the viral (L + NS) protein from NS-phosphorylating activity. The addition of this kinase-free (L + NS) fraction to a kinase-deficient N-RNA fraction reconstituted an active RNA polymerase containing less than 20% of the original NS-phosphorylating activity. These results demonstrate that NS-phosphorylating activity is unnecessary during vesicular stomatitis virus RNA synthesis and indicate that all of the protein kinase(s) present in purified nucleocapsids is probably of cellular rather than viral origin.

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.

Altered ATP utilization by the poIR mutants of vesicular stomatitis virus maps to the N-RNA template

In a continuation of previous efforts to study the modified ATP requirements for RNA synthesis by poIR mutants of vesicular stomatitis virus (VSV), we have used a novel reconstitution assay to show that it is the template moiety of the mutants, not the polymerase proteins, which governs both the increased utilization of the ATP analog, beta, gamma-imido ATP (AMP-PMP), and the loss of a positive cooperativity-like response to varying ATP concentrations. Assays utilized uv-irradiated virus as a source of polymerase proteins and purified N-RNA as templates. Homologous and heterologous transcriptase reactions were carried out with wild-type (wt) virus and each of the two independently isolated poIR mutants. We show that in the presence of wt N-RNA template, substitution of AMP-PNP for ATP resulted in only approximately 5% of control RNA synthesis regardless of which source of polymerase was used. Furthermore, all reactions containing wt N-RNA template responded to varying ATP concentrations with a concave, upward-shaped Lineweaver-Burke plot generally indicative of positive cooperativity effects. In contrast, all reactions which utilized N-RNA templates from the poIR mutants showed an increased utilization of AMP-PNP (greater than 20%) and a more characteristic Michaelis-Menten response to changing ATP concentrations. These findings strongly support the notion that the template-associated nucleocapsid protein modulates the utilization of an ATP site which is directly or indirectly involved in VSV RNA synthesis.

Kinetic, quantitative, and functional analysis of multiple forms of the vesicular stomatitis virus nucleocapsid protein in infected cells

Multiple forms of the vesicular stomatitis virus nucleocapsid protein N have been detected in infected cells. One form is complexed with the viral NS protein in a 1:1 molar ratio, and the other forms are distinguished by their more rapid sedimentation rates on glycerol gradients. I performed a series of experiments designed to analyze the relationships between these forms of the N protein. Pulse-chase experiments demonstrate that the N protein is made first as the form which binds to the NS protein, forming a 1-to-1 molar complex, and that with increasing times of chase it is either assembled into nucleocapsids or converted to the two higher sedimenting forms. Using a newly developed quantitative immunoblotting procedure, I have quantitated the three differentially sedimenting species of the N protein and have shown that at later times postinfection (6 to 7 h), the faster-sedimenting forms of the N protein account for as much as 50% of the soluble N protein in the cell. The activity of these forms has been assessed, with only the 1-to-1 molar N-NS complex demonstrating the ability to support the replication and encapsidation of viral genomic RNA. A model for the conversion of the N protein from the active N-NS complex into the other forms of the protein is presented, and the possible function of the N-protein self-complexes is discussed.

Mapping of monoclonal antibodies to the Sendai virus P protein and the location of its phosphates

The epitopes of a panel of five monoclonal antibodies to the Sendai virus P protein were mapped by generating modified forms of the P mRNA via SP6 expression followed by in vitro translation. The epitopes were found to be clustered in the C-terminal region of the protein. Two epitopes were within the last 30 residues, two were within the next 65, and one was between residues 308 and 451 of this 568-residue-long protein. By a combination of partial proteolysis and Western immunoblotting with one of these antibodies, the sites at which phosphates are added in vitro by the virion-associated kinase were mapped to the second quarter of the molecule from the N terminus.

Deletion mapping analyses indicate that epitopes for monoclonal antibodies to the NS phosphoprotein of VSV are linear and clustered

Thirteen distinct monoclonal antibodies to the 30-kDa NS phosphoprotein of vesicular stomatitis virus were isolated and assayed by Western blot analysis and immune precipitation reactions. Epitopes recognized by the antibodies were mapped by immune precipitation of NS deletion proteins synthesized in vitro from cloned NS gene constructs. None of the epitopes recognized by the 13 antibodies could be mapped to the phosphorylated amino-terminal half of the NS protein. Twelve of the antibodies recognized epitopes located within the carboxy-terminal 142 amino acids of the protein. The great majority of epitopes appeared to consist of a linear array of amino acids.

The vesicular stomatitis virus L protein possesses the mRNA methyltransferase activities

We have previously shown that the vesicular stomatitis virus (VSV) host range mutant, hr 1, is completely defective for the mRNA methyltransferase activities, but can synthesize full-length, unmethylated mRNAs in vitro [S. M. Horikami and S. A. Moyer (1982). Proc. Natl. Acad. Sci. USA 79, 7694-7698] and in vivo [S. M. Horikami, F. De Ferra, and S. A. Moyer (1984). Virology 138, 1-15]. Here we have used the hr 1 mutant to identify the viral protein which possesses the methyltransferase activities. The wild-type VSV L and NS proteins, subunits of the viral RNA polymerase, were separately purified and added to high salt dissociated mutant hr 1 nucleocapsids for in vitro transcription reactions. The results show that the purified wild-type L protein, but not the NS protein, restores methylation and thus possesses the viral mRNA methyltransferase activities.

Viral proteins required for the in vitro replication of vesicular stomatitis virus defective interfering particle genome RNA

The viral proteins required for VSV RNA replication have been partially purified. With the use of monoclonal antibodies specific for the VSV N protein we have identified a putative N/NS complex present in the soluble protein fraction of infected cells. The complex is stable upon partial purification, contains the N and NS proteins in a 1:1 molar ratio, and has an elongated shape based on its hydrodynamic properties. Depletion of the N/NS complex from the infected cell soluble protein fraction results in the loss of the ability of this fraction to support RNA replication suggesting that the complex is required for this reaction. The ability to support viral genome RNA replication indeed cochromatographs with the N/NS protein complex through several steps of purification. Only the N protein of the N/NS complex appears to be bound to RNA during encapsidation with the release of NS protein.

Viral RNA polymerases

Recent progress in molecular biological techniques revealed that genomes of animal viruses are complex in structure, for example, with respect to the chemical nature (DNA or RNA), strandedness (double or single), genetic sense (positive or negative), circularity (circle or linear), and so on. In agreement with this complexity in the genome structure, the modes of transcription and replication are various among virus families. The purpose of this article is to review and bring up to date the literature on viral RNA polymerases involved in transcription of animal DNA viruses and in both transcription and replication of RNA viruses. This review shows that the viral RNA polymerases are complex in both structure and function, being composed of multiple subunits and carrying multiple functions. The functions exposed seem to be controlled through structural interconversion.

In vivo and in vitro models of demyelinating disease: activation of the adenylate cyclase system influences JHM virus expression in explanted rat oligodendrocytes

The specificity of JHM virus (JHMV) tropism for rat oligodendrocytes, as one of the primary host cells in the central nervous system, is maintained after explanation (S. Beushausen and S. Dales, Virology 141:89-101, 1985). The temporal correlation between onset of resistance to JHMV infection in vivo, completion of myelination, and maturation of the central nervous system can be simulated in vitro by inducers of oligodendrocyte differentiation (Beushausen and Dales, Virology, 1985). Stimulation of differentiation through the elevation of intracellular cyclic AMP (cAMP) levels suggests a possible connection between activation of the adenylate cyclase system and coronavirus expression. Chromatographic analysis of cAMP-dependent protein kinase activity in cytosol extracts prepared from astrocytes or oligodendrocytes revealed that both glial cell types were deficient in protein kinase I, indicating that expression of coronavirus in differentiated cells was not contingent upon the presence of protein kinase I. However, treatment with N6,2'-O-dibutyryladenosine-3',5'-cyclic monophosphate (dbcAMP) resulted in a severalfold enhancement of the free regulatory subunit (RI) in oligodendrocytes but not in astrocytes. The RII subunit in both neural cell types was relatively unaffected. Rapid increase in RI due to dbcAMP treatment was correlated with inhibition of JHMV expression. Other differentiation inducers, including 8-Br cAMP and forskolin which, by contrast, caused a decrease in detectable RI, also blocked JHMV expression. This apparent anomaly can be attributed to an increased turnover of RI due to destabilization of the molecule which occurs upon site-specific binding of the cyclic nucleotides. On the basis of these observations, we conclude that the state of oligodendrocyte differentiation manifested with the modulation of RI regulates JHMV expression. The differentiation process did not affect either virus adsorption or sequestration but appeared to inhibit the expression of viral RNA and proteins, implying that replication was inhibited at some step between penetration and initiation of genomic functions, perhaps at the stage of uncoating. We therefore examined the possibility that protein kinases and phosphatases, which influence cellular regulation during cAMP-induced differentiation, may be responsible for the phenomenon of coronavirus suppression in oligodendrocytes. Evidence was obtained indicating that normal processing of the phosphorylated nucleocapsid protein is inhibited in differentiated oligodendrocytes, consistent with the notion that JHMV replication might be arrested during uncoating.

Herpes simplex virus 1 protein kinase is encoded by open reading frame US3 which is not essential for virus growth in cell culture

Earlier reports have described a novel protein kinase in cells infected with herpes simplex or pseudorabies viruses. These novel enzymes were characterized by their acceptance of protamine as a substrate and by their differential chromatographic behavior in anion-exchange chromatography. We report that this activity was not present in extracts of uninfected cells or of cells infected with a mutant constructed so as to contain a deletion in the US3 open reading frame mapping in the small component of herpes simplex virus 1 DNA. The activity was present in extracts of cells infected with wild-type virus and with a recombinant in which the US3 open reading frame had been rescued. Our results are consistent with the observation reported earlier that the coding sequences predict an amino acid motif common to protein kinases and lead to the conclusion that the US3 open reading frame encodes a virus-specific protein kinase that is not required for virus growth in cells in culture.

In vitro replication of mouse hepatitis virus strain A59

An in vitro replication system for mouse hepatitis virus (MHV) strain A59 was developed using lysolecithin to produce cell extracts. In extracts of MHV-infected cells, radiolabeled UMP was incorporated at a linear rate for up to 1 h into RNA, which hybridized to MHV-specific cDNA probes and migrated in denaturing formaldehyde-agarose gels to the same position as MHV genomic RNA. The incorporation of [32P]UMP into genome-sized RNA in vitro correlated with the observed increase of [3H]uridine incorporation in MHV-infected cells labeled in vivo. Incorporation of [32P]UMP into genome-sized RNA was inhibited when extracts were incubated with puromycin. The addition to the assay of antiserum to the MHV-A59 nucleocapsid protein N inhibited synthesis of genome-sized RNA by 90% compared with the addition of preimmune serum. In contrast, antiserum to the E1 or E2 glycoproteins did not significantly inhibit RNA replication. In vitro-synthesized RNA banded in cesium chloride gradients as a ribonucleoprotein complex with the characteristic density of MHV nucleocapsids isolated from virions. These experiments suggest that ongoing protein synthesis is necessary for replication of MHV genomic RNA and indicate that the N protein plays an important role in MHV replication.

Determination of the orientation of an integral membrane protein and sites of glycosylation by oligonucleotide-directed mutagenesis: influenza B virus NB glycoprotein lacks a cleavable signal sequence and has an extracellular NH2-terminal region

The membrane orientation of the NB protein of influenza B virus, a small (Mr, approximately 18,000) glycoprotein with a single internal hydrophobic domain, was investigated by biochemical and genetic means. Cell fractionation and protein solubility studies indicate NB is an integral membrane protein, and NB has been shown to be a dimer under nonreducing conditions. Treatment of infected-cell surfaces with proteinase K and endoglycosidase F and immunoprecipitation with a site-specific antibody suggests that the 18-amino-acid NH2-terminal region of NB is exposed at the cell surface. Oligonucleotide-directed mutagenesis to eliminate each of the four potential sites of N-linked glycosylation and expression of the mutant NB proteins in eucaryotic cells suggest that the two sites adjacent to the NH2 terminus are glycosylated. This provides further evidence that NB, which lacks a cleavable NH2-terminal signal sequence, has an exposed NH2 terminus at the cell surface.

Determination of the orientation of an integral membrane protein and sites of glycosylation by oligonucleotide-directed mutagenesis: influenza B virus NB glycoprotein lacks a cleavable signal sequence and has an extracellular NH2-terminal region

The membrane orientation of the NB protein of influenza B virus, a small (Mr, approximately 18,000) glycoprotein with a single internal hydrophobic domain, was investigated by biochemical and genetic means. Cell fractionation and protein solubility studies indicate NB is an integral membrane protein, and NB has been shown to be a dimer under nonreducing conditions. Treatment of infected-cell surfaces with proteinase K and endoglycosidase F and immunoprecipitation with a site-specific antibody suggests that the 18-amino-acid NH2-terminal region of NB is exposed at the cell surface. Oligonucleotide-directed mutagenesis to eliminate each of the four potential sites of N-linked glycosylation and expression of the mutant NB proteins in eucaryotic cells suggest that the two sites adjacent to the NH2 terminus are glycosylated. This provides further evidence that NB, which lacks a cleavable NH2-terminal signal sequence, has an exposed NH2 terminus at the cell surface.

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.

Protein kinase in nondiabetogenic coxsackievirus B4

Alkali-dissociated, purified preparations of prototype coxsackievirus B4 release a protein kinase that catalyzes the incorporation of gamma-phosphate from 32P-labeled ATP into three virus capsid proteins (VP1, VP3, VP4), several additional proteins of the particle, and exogenous acceptor proteins. Using protamine sulfate as an acceptor protein, we detected nearly 20-fold more enzyme activity in membrane-bound virions (MBV) than in virions of the virus. The activity in the MBV is cyclic nucleotide-independent, divalent cation-dependent, and has a pH optimum of 8.0. Phosphoserine is labeled with 32P. The enzyme activity sediments at about 5S and is separated into at least two peaks of heterogeneous proteins by ion-exchange chromatography. The patterns of phosphorylation by these enzyme peaks are somewhat similar. Coxsackievirus-associated protein kinase appears to be located internally in the virus and may be host-cell-coded. The enzyme appears to be lacking in a variant of the virus that produced diabetes in mice.

Vesicular stomatitis virus NS proteins: structural similarity without extensive sequence homology

The complete nucleotide sequence of the NS mRNA of vesicular stomatitis virus (New Jersey serotype) was established from two cDNA clones spanning the entire coding region of the mRNA. The gene is 856 nucleotides long and can code for a polypeptide of 274 amino acids. Comparison with the nucleotide sequence of the NS gene of the Indiana serotype revealed only 41% sequence homology. The deduced amino acid sequences of the NS proteins were only 32% homologous, with no identical stretches of more than five amino acids. However, at the C-terminal domain there was a conserved region of 21 amino acids with greater than 90% homology. Surprisingly, relative hydropathicity plots also demonstrated the presence of a large number of hydrophilic amino acids sequestered similarly over the N-terminal half of the protein. In addition, the total number of serine and threonine residues, presumptive phosphorylation sites, was similar and included seven serine and three threonine residues located at identical positions. It appears that during divergent evolution of these two vesicular stomatitis virus serotypes from a common ancestor, considerable mutation occurred in the main body of the gene but the overall structure of the protein was retained. The function of the NS protein in relation to the evolution of the two viruses is discussed.

Phosphorylation sites on phosphoprotein NS of vesicular stomatitis virus

The phosphoprotein NS of vesicular stomatitis virus which accumulates within the infected cell cytoplasm is phosphorylated at multiple serine and threonine residues (G. M. Clinton and A. S. Huang, Virology 108:510-514, 1981; Hsu et al., J. Virol. 43:104-112, 1982). Using incomplete chemical cleavage at tryptophan residues, we mapped the major phosphorylation sites to the amino-terminal half of the protein. Analysis of phosphate-labeled tryptic peptides suggests that essentially all of the label is within the large trypsin-resistant fragment predicted from the sequence of Gallione et al. (J. Virol. 39:52-529, 1981). A similar result has been obtained for NS protein isolated from the virus particle by C.-H. Hsu and D. W. Kingsbury (J. Biol. Chem., in press). Analysis of phosphodipeptides utilizing the procedures of C. E. Jones and M. O. J. Olson (Int. J. Pept. Protein Res. 16:135-142, 1980) enabled us to detect as many as six distinct phosphate-containing dipeptides. From these studies, together with the known sequence data, we conclude that the major phosphate residues on cytoplasmic NS protein are located in the amino third of the NS molecule and most probably between residues 35 and 106, inclusive. The studies also provide formal chemical proof that NS protein has a structure consistent with a monomer of the sequence of Gallione et al. as modified by J. K. Rose (personal communication). The low electrophoretic mobility of this protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis is not therefore due to dimerization.