Spatio-temporal analysis of the RNAs, coat and movement (p7) proteins of Carnation mottle virus in Chenopodium quinoa plants

Time-course and in situ hybridization analyses were used to study the spatio-temporal distribution of Carnation mottle virus (CarMV) in Chenopodium quinoa plants. Genomic and subgenomic RNAs of plus polarity accumulated linearly with time, whereas the corresponding minus strands reached a peak during infection in inoculated leaves. Analyses of serial tissue sections showed that plus polarity strands were localized throughout the infection area, whereas minus strands were localized at the borders of the chlorotic lesions. The accumulation kinetics of the coat protein (CP) and the p7 movement protein (MP) as well as their subcellular localization were also studied. Unlike most MPs, CarMV p7 showed a non-transient expression and a mainly cytosolic location. However, as infection progressed the presence of p7 in the cell wall fraction increased significantly. These results are discussed on the basis of a recent model proposed for the mechanism of cell-to-cell movement operating in the genus Carmovirus.

The importance of alfalfa mosaic virus coat protein dimers in the initiation of replication

Deletion and substitution mutations affecting the oligomerization of alfalfa mosaic virus (AMV) coat protein (CP) were studied in protoplasts to determine their effect on genome activation, an early step in AMV replication. The CP mutants that formed dimers, CPDeltaC9 and CPC-A(R)F, were highly active in initiating replication with 63-84% of wild-type (wt) CP activity. However, all mutants that did not form dimers, CPDeltaC18, CPDeltaC19, CPC-WFP, and CPC-W, were much less active with 19-33% of wt CP activity. The accumulation and solubility of mutant CPs expressed from a virus-based vector in Nicotiana benthamiana were similar to that of wt CP. Analysis of CP-RNA interactions indicated that CP dimers and CP monomers interacted very differently with AMV RNA 3' ends. These results suggest that CP dimers are more efficient for replication than CP monomers because of differences in RNA binding rather than differences in expression and accumulation of the mutant CPs in infected cells.

Detection and subcellular localization of the turnip yellow mosaic virus 66K replication protein in infected cells

Turnip yellow mosaic virus (TYMV) encodes a 206-kDa (206K) polyprotein with domains of methyltransferase, proteinase, NTPase/helicase, and RNA-dependent RNA polymerase (RdRp). In vitro, the 206K protein has been shown to undergo proteolytic processing, giving rise to the synthesis of 140-kDa (140K) and 66-kDa (66K) proteins, the latter comprising the RdRp protein domain. Antibodies were raised against the 66K protein and were used to detect the corresponding viral protein in infected cells; both leaf tissues and protoplasts were examined. The antiserum specifically recognized a protein of approximately 66 kDa, indicating that the cleavage observed in vitro is also functional in vivo. The 66K protein accumulates transiently during protoplast infection and localizes to cellular membrane fractions. Indirect immunofluorescence assays and electron microscopy of immunogold-decorated ultrathin sections of infected leaf tissue using anti-66K-specific antibody revealed labeling of membrane vesicles located at the chloroplast envelope.

Serological detection of grapevine virus a using antiserum to a nonstructural protein, the putative movement protein

ABSTRACT Grapevine virus A (GVA) is implicated in the etiology of the rugose wood disease. The coat protein (CP) and the putative movement protein (MP) genes of GVA were cloned and expressed in Escherichia coli and used to produce antisera. Both the CP and the MP were detected with their corresponding antisera in GVA-infected Nicotiana benthamiana. The MP was first detected at an early stage of the infection, 6 to 12 h after inoculation, and the CP was detected 2 to 3 days after inoculation. The CP and MP were detected by immunoblot analysis in rugose wood-affected grapevines. The MP could be detected in GVA-infected grapevines that tested negative for CP, both with CP antiserum and with a commercially available enzyme-linked immunosorbent assay kit. The study shows that detection of the nonstructural MP may be an effective means for serological detection of GVA infection in grapevines.

A modified form of the alfalfa mosaic virus movement protein induces stressed phenotypes in transgenic tobacco

A modified form of the movement protein (P3) of alfalfa mosaic virus, lacking amino acids 21 to 34, was transgenically expressed in Nicotiana tabacum cv. Xanthi (genotype nn) and cv. Xanthi nc (genotype NN). The modified protein (designated P3Δ[21–34]) was expressed more strongly than the full-length protein. The localization of P3Δ[21–34] was investigated by subcellular fractionation and immunocytochemistry. Immunolabelling was most frequent in vascular parenchymal cells, mainly in the cytoplasm (endoplasmic reticulum, Golgi, plasma membrane vesicles) but also in the cell wall. In contrast, full-length P3 accumulated almost exclusively in a cell-wall enriched fraction. Transgenically expressed P3Δ[21–34] increased the plasmodesmal gating capacity of epidermal cells, as did transgenically expressed P3. Thus, the plasmodesma-gating domain of P3 does not include amino acids 21 to 34. Plants expressing P3Δ[21–34] at a high level exhibited stressed phenotypes. Phenotype 1, only observed in 'Xanthi' NN lines, was characterized by stunting, small, thick, and hairy leaves, locally high starch accumulation, and occasional necrotic cells, mainly in the bundle sheath and vascular tissue. Phenotype 2, observed in both 'Xanthi' NN and nn lines, was characterized by short internodes, numerous small green leaves, sterile flowers, regular starch accumulation, and absence of necrotic cells. The stress-inducing activity of P3Δ[21–34] may be due to either its molecular conformation or its low efficiency of export toward the cell wall. Keywords: cell to cell movement, stress reaction, plasmodesmata, transgenic plant, ultrastructure, immunocytochemistry.

Accumulation kinetics of CMV RNA 3-encoded proteins and subcellular localization of the 3a protein in infected and transgenic tobacco plants

The complete nucleotide sequence of RNA 3 of a Spanish isolate of cucumber mosaic virus (CMV-24) has been determined. The encoded putative cell-to-cell movement protein (3a protein) and the coat protein are 279 and 218 amino acids long, respectively. The 3a protein was expressed in Escherichia coli using the vector pT7-7 and was used to raise an immunoserum. We have followed the time course of accumulation of the 3a protein, in parallel to that of the coat protein, and its subcellular localization as a function of time after CMV-24 infection on tobacco plants. The maximum accumulation level of the 3a protein was reached at early stages of infection, being detected in the cytosolic and the cell wall fractions. At later stages of infection, a decline in accumulation levels of the 3a protein was observed, and the protein was essentially associated with the cell wall fractions. These data were corroborated by immunocytochemistry performed in both infected and 3a-expressing transgenic tobacco plants.

Detection by immunogold labelling of P75 readthrough protein near an extremity of beet necrotic yellow vein virus particles

RNA 2 of beet necrotic yellow vein virus carries the cistron for the 21 kd coat protein at its 5'-extremity. During translation, the coat protein cistron termination codon is suppressed about 10% of the time so that translation continues into the adjacent open reading frame to produce a 75 kd species, known as P75, which contains the coat protein sequence at its N-terminus. Immunoblotting experiments with a P75-specific antiserum showed that P75 is present in only trace amounts in purified virus preparations. Electron microscopic visualization of immunogold-labelled virions in crude tissue extracts has provided evidence for an association between P75 and at least a fraction of the BNYVV particles, with P75 being predominantly located near one end of the rod-shaped virions. This finding is discussed in the context of the current model for the role of P75 in virus assembly and vector transmission.

In situ immunogold labeling analysis of the rice hoja blanca virus nucleoprotein and major noncapsid protein

Ribonucleoprotein particles (RNPs) of rice hoja blanca virus (RHBV) were purified and used for electron microscopic analysis and antibody production. Antibodies made to RNPs specifically decorated purified RNPs. The RNPs typically showed characteristic tenuivirus morphologies. They were approximately 8 nm in diameter, mostly circular in nature, and exhibited branching and a high degree of superhelicity. When the RNP antibodies were used for in situ immunogold labeling analysis of RHBV-infected tissues, no specific structures were identified, but gold particles were distributed throughout the cytosol of RHBV-infected but not healthy plants. However, amorphous semi-electron opaque inclusion bodies (ASO-IBs) were abundant in cells of RHBV-infected plants. While the ASO-IBs were not labeled with the anti-RNP antiserum, they were specifically labeled with antibodies to the RHBV major noncapsid protein (NCP) and with antibodies to the NCP of another tenuivirus, maize stripe virus.

Nucleic acid-binding properties of the alfalfa mosaic virus movement protein produced in yeast

The movement protein of alfalfa mosaic virus (P3) was purified from yeasts transformed with an expression vector containing the P3 gene. Its nucleic acid-binding properties were tested by electrophoretic retardation, nitrocellulose retention, and RNA-protein cross-linking. The recombinant protein had a higher affinity for single-stranded RNA and DNA than for double-stranded nucleic acids. Each nucleic acid molecule bound several protein molecules without sequence specificity. The binding was 80% inhibited by 0.2 M NaCl. These properties are qualitatively similar, but not strictly identical, to those of two other viral movement proteins, the 30-kDa protein of tobacco mosaic virus and the gene I product of cauliflower mosaic virus.

Nucleic acid binding properties of the 92-kDa replicase subunit of alfalfa mosaic virus produced in yeast

The 92-kDa non-structural protein of alfalfa mosaic virus (one of the replicase subunits) was synthesized by Saccharomyces cerevisiae transformed with a recombinant expression vector. The yeast-expressed protein had the immunological and size characteristics of the naturally made viral protein. It was partially purified and its nucleic acid binding properties were tested by gel-retardation electrophoresis and nitrocellulose adsorption. The protein interacted with single-stranded RNA, double-stranded RNA and double-stranded DNA in a salt-dependent manner, with a slight preference for RNA. These properties may be related to its putative function as a core RNA polymerase.

Analysis of the protein composition of alfalfa mosaic virus RNA-dependent RNA polymerase

RNA-dependent RNA polymerase (RdRp) was solubilized and purified from cellular membranes isolated from alfalfa mosaic virus (AIMV)-infected tobacco by employing a procedure recently described for brome mosaic virus RdRp [R. Quadt and E.M.J. Jaspars, 1990, Virology 178, 189-194]. The purified AIMV RdRp is completely dependent on added template RNAs and exhibits a high degree of template specificity. Analysis of the protein composition of AIMV RdRp showed that AIMV-encoded proteins P1 and P2 and the coat protein (CP) are present in the active enzyme complex. Minus-strand synthesis by the AIMV RdRp is inhibited by AIMV CP. Native double-stranded AIMV RNAs are utilized as template for viral RNA synthesis by AIMV RdRp indicating that a helicase activity is present in the purified AIMV RdRp preparation.

The pattern of accumulation of cauliflower mosaic virus-specific products in infected turnips

The concentrations of cauliflower mosaic virus (CaMV) DNA and protein products in the developing leaves of a host, turnip, have been measured and the results have been correlated with symptom production. Virus-specific products were limited to the symptomatic leaves. CaMV DNA was detected in the youngest foliar tissues showing full systemic symptoms and continued to accumulate as the leaf expanded, indicating that virus multiplication was not restricted to meristematic tissues of the host plant and that virus concentration was not a primary determinant for symptom production. Using specific antisera for Western blot analysis, the distribution of CaMV-specific proteins (P1-P6) in a range of subcellular fractions of infected tissue was determined. The protein products (P2-P6) of genes II-VI were all detected in fractions enriched for virus inclusion bodies, although P5 was present only at low levels. A high-speed pellet fraction enriched for virus replication complexes revealed P5 in higher concentrations, and also contained P4 and small amounts of P6 in proportions which indicated that replication complexes had been released from inclusion bodies. In the different leaves of the host, P2, 3, 4, 5, and 6 all increased in concentration in parallel with viral DNA, although there appeared to be a bias toward protein rather than DNA synthesis in the very young leaves. P1 showed a different pattern of accumulation; it was most concentrated in the very young and the oldest infected tissues, and showed a different spectrum of products between leaves. The experiments described provide a more complete picture of the relationship between CaMV multiplication and expression, and leaf development, and an increased understanding of how the disease syndrome is established.