Host-dependent phosphorylation and kinase activity associated with vesicular stomatitis virus

Comprehensive mutational analysis reveals p6Gag phosphorylation to be dispensable for HIV-1 morphogenesis and replication

The structural polyprotein Gag of human immunodeficiency virus type 1 (HIV-1) is necessary and sufficient for formation of virus-like particles. Its C-terminal p6 domain harbors short peptide motifs that facilitate virus release from the plasma membrane and mediate incorporation of the viral Vpr protein. p6 has been shown to be the major viral phosphoprotein in HIV-1-infected cells and virions, but the sites and functional relevance of p6 phosphorylation are not clear. Here, we identified phosphorylation of several serine and threonine residues in p6 in purified virus preparations using mass spectrometry. Mutation of individual candidate phosphoacceptor residues had no detectable effect on virus assembly, release, and infectivity, however, suggesting that phosphorylation of single residues may not be functionally relevant. Therefore, a comprehensive mutational analysis was conducted changing all potentially phosphorylatable amino acids in p6, except for a threonine that is part of an essential peptide motif. To avoid confounding changes in the overlapping pol reading frame, mutagenesis was performed in a provirus with genetically uncoupled gag and pol reading frames. An HIV-1 derivative carrying 12 amino acid changes in its p6 region, abolishing all but one potential phosphoacceptor site, showed no impairment of Gag assembly and virus release and displayed only very subtle deficiencies in viral infectivity in T-cell lines and primary lymphocytes. All mutations were stable over 2 weeks of culture in primary cells. Based on these findings, we conclude that phosphorylation of p6 is dispensable for HIV-1 assembly, release, and infectivity in tissue culture.

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.

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.

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.

Antiviral effect of cyclopentenone prostaglandins on vesicular stomatitis virus replication

Prostaglandins are potentially useful antiviral agents, however their mechanism of action is unclear. Recent evidence suggests that RNA transcription of vesicular stomatitis virus (VSV) is inhibited by prostaglandins (Bader and Ankel, J. Gen. Virol. 71, 2823-2832, 1990). Prostaglandins are known to have multiple effects on cells which may or may not be related to their antiviral action. We examined the effects of prostaglandins on cells and on VSV RNA polymerase in vitro to seek the mechanism of antiviral action. Actinomycin D inhibited cellular RNA synthesis but failed to block the antiviral activity of prostaglandins on VSV. Thus induction of host cell RNA transcription is not involved in the antiviral action. Neither modulation of the cellular glutathione level by prostaglandins nor formation of prostaglandin-glutathione conjugates was required for the antiviral action. The relative inhibition of VSV RNA polymerase in vitro by prostaglandins with different structures correlated to inhibition of VSV replication in infected cells. This result indicates that the same step in VSV replication is inhibited by prostaglandins both in the in vitro RNA polymerase assay and in the infected cell.

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.

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.

Intracellular distribution of input vesicular stomatitis virus proteins after uncoating

We have examined the fate of input viral proteins following the uncoating of vesicular stomatitis virus (VSV) by immunofluorescence microscopy, immunoelectron microscopy, and cell fractionation. VSV was adsorbed to BHK cells and allowed to become internalized in the presence of 100 mM NH4Cl; the NH4Cl was then removed to initiate synchronized uncoating. The three major structural proteins of VSV, the matrix protein (M), the nucleocapsid protein (N), and the glycoprotein (G), were each distributed uniquely after uncoating. Immunofluorescence microscopy showed that both G and N proteins retained a punctate distribution, whereas M protein was diffusely distributed throughout the cytoplasm, suggesting that it had become soluble. Immunoelectron microscopy showed that N protein was found in clusters (presumably in intact nucleocapsids) associated with the cell cytoskeleton and in unfused virions in endosomes and lysosomes. M protein was found diffusely distributed throughout the cytoplasm and also in endosomes and lysosomes. G protein was found only in association with endosomes and lysosomes after uncoating. Electrophoretic analysis of the high-speed cytosol fraction from infected cells showed that it contained chiefly M protein. The amount of M protein in the cytosol increased continuously during 90 min of uncoating, confirming its solubilization during uncoating. M protein was not covalently modified by phosphorylation upon uncoating, as evidenced by its mobility on nonequilibrium pH gradient gel electrophoresis. We suggest that those nucleocapsids associating with the cytoskeleton after uncoating may represent the sites of primary viral transcription.

Characterisation of the influenza virus associated protein kinase and its resemblance to casein kinase II

The protein kinase activity associated with purified influenza virus has been examined. By use of a radiolabelled photoreactive ATP analogue (3'-O-(4-benzoyl) benzoyl adenosine triphosphate) a 47 kD polypeptide has been identified that binds ATP. A comparison of the sensitivity of the kinase activity and the 47 kDa polypeptide labelling to inhibitors indicate that the 47 kDa polypeptide is likely to represent the major protein kinase activity of the virus. The virus associated protein kinase phosphorylates the synthetic peptide RREEETEEE, a peptide substrate for casein kinase II. Antiserum directed against casein kinase II revealed a positive signal in immunoblots of purified virus. We propose that the major protein kinase activity associated with purified virus preparations is host cell casein kinase II.

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 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.

Viral products in cells infected with vesicular stomatitis virus and superinfected with Rous sarcoma virus

In cells infected with Vesicular Stomatitis virus (VSV) ts 1026 and superinfected with Rous Sarcoma virus (RSV) synthesis of vsrc mRNA and RSV env mRNA decreases. In these cells post-translational processing of RSV precursor proteins is impaired and small amounts of VSV antigens are detected.

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.

Cancer-associated lactate dehydrogenase is a tyrosylphosphorylated form of human LDH-A, skeletal muscle isoenzyme

Cancer-associated lactate dehydrogenase is a tyrosylphosphorylated form of human skeletal muscle isoenzyme, since the partial amino acid sequences of human liver LDH-K/A protein were found to be identical with the known primary structure of human LDH-A isoenzyme and the LDH-A isoenzymes from human placenta and bovine muscle were shown to be tyrosylphosphorylated. This tyrosylphosphorylated LDH-K/A protein was also found to be complexed with 21 kD, 30 kD, and 56 kD proteins.

Differential effect of ATP concentration on synthesis of vesicular stomatitis virus leader RNAs and mRNAs

Cleavage of the beta-gamma bond of ATP is required for wild-type (wt) vesicular stomatitis virus transcription in vitro. Recent findings have established that a domain-specific phosphorylation of the virus NS protein is necessary for activity. We report here that RNA synthesis catalyzed by purified standard wt virions responded cooperatively to various ATP concentrations, with half-maximal activity at approximately 500 microM. In contrast, mutant polR1 standard virions and wt defective interfering particles both showed conventional Michaelis-Menten kinetic profiles with Km values of approximately 143 and approximately 133 microM, respectively. The former synthesize readthrough products of the leader-N gene junction in addition to plus-strand leader RNA and mRNAs, whereas the latter synthesize only minus-strand leader RNA. The cooperative response of wt virus products, however, was specific to mRNAs; the small fraction of the total products corresponding to plus-strand leader approximated Michaelis-Menten behavior. Since the unique phenotype of the polR mutants correlates with the synthesis of replicationlike products in vitro, the affected ATP-requiring function most likely regulates both transcription and replication. We suggest that this mutated function involves phosphorylation of viral proteins.

Protein kinase activity associated with immunoprecipitates of the vesicular stomatitis virus phosphoprotein NS

Monospecific antisera prepared to denatured N and NS proteins of the Indiana serotype of VSV were used to investigate the protein associations in extracts of virus-infected cells. Complexes precipitated from the cytoplasm of infected cells with either anti-N or anti-NS serum both contained N and NS proteins but could be differentiated by the absence of any trace of M protein in the complexes precipitated with anti-NS serum. Immunoprecipitation with anti-NS serum of NS protein from the soluble cytoplasmic protein pool always resulted in coprecipitation of N protein suggesting a possible functional association of these proteins in the soluble fraction. The association of protein kinase activity with complexes containing NS protein was demonstrated by the phosphorylation of NS serine residues when immunoprecipitates containing NS protein were incubated with [32P]ATP in vitro. In protein aggregates precipitated with antibody after high salt dissociation of viral proteins it was also possible to demonstrate the presence of a c-src-like protein kinase activity as previously shown by G.M. Clinton, N.G. Guerina, H. Guo, and H.S. Huang (J. Biol. Chem. 257, 3313-3319 (1982) ).