Cell-free translation of tobacco vein mottling virus RNA

Aphids as transport devices for plant viruses

Plant viruses have evolved a wide array of strategies to ensure efficient transfer from one host to the next. Any organism feeding on infected plants and traveling between plants can potentially act as a virus transport device. Such organisms, designated vectors, are found among parasitic fungi, root nematodes and plant-feeding arthropods, particularly insects. Due to their extremely specialized feeding behavior - exploring and sampling all plant tissues, from the epidermis to the phloem and xylem - aphids are by far the most important vectors, transmitting nearly 30% of all plant virus species described to date. Several different interaction patterns have evolved between viruses and aphid vectors and, over the past century, a tremendous number of studies have provided details of the underlying mechanisms. This article presents an overview of the different types of virus-aphid relationships, state-of-the-art knowledge of the molecular processes underlying these interactions, and the remaining black boxes waiting to be opened in the near future.

The nucleotide sequence of the coding region of tobacco etch virus genomic RNA: evidence for the synthesis of a single polyprotein

The complete nucleotide sequence of the tobacco etch virus (TEV) RNA genome has been determined excepting only the nucleotide(s) present at the extreme 5' terminus. The assembled TEV genomic sequence is 9496 nucleotides in length followed by a polyadenylated tract ranging from 20 to 140 residues. A computer search of the sequence reveals the following. A 5' untranslated region, rich in adenosine and uridine, is present between nucleotides 1 and 144. A putative initiation codon, at nucleotides 145-147, marks the beginning of a large open-reading frame (ORF) which ends with an opal (UGA) termination codon at positions 9307-9309. A 186-nucleotide untranslated region is present between the termination codon of the ORF and the beginning of the 3' polyadenylated region. The predicted translation product of this ORF is a 3054 amino acid polyprotein with a mol wt of 345,943. A function for the large (54,000 Mr) nuclear inclusion protein is suggested by a comparison of the deduced amino acid sequence with a protein data bank. This protein displays biochemical similarities to other viral RNA-dependent, RNA polymerases.

Identification of potyviral amorphous inclusion protein as a nonstructural, virus-specific protein related to helper component

Antisera to amorphous inclusion (AI) proteins associated with infections by pepper mottle virus (PeMV) and the watermelon mosaic virus-1 strain of papaya ringspot virus (PRSV-W) were used to probe in vitro translation products of the viral RNAs. The major translation product of PeMV RNA in the rabbit reticulocyte lysate (RRL) system was a previously reported polypeptide of apparent molecular weight 78,000 (Mr 78K). It reacted with anti-AI serum, whereas the major translation product in the wheat germ (WG) system was a 30K polypeptide that did not react with the antiserum. These results, the Mr values, and analyses of peptides generated by partial digestion with proteinase indicate that the amino acid sequences of the 30K polypeptide and the (Mr) 51K AI protein are distinct subsets of the 78K polypeptide amino acid sequence. Similar results were obtained with PRSV-W except that the Mr values of the corresponding translation products are 110K (RRL) and 60K (WG). Thus the 5'-most region of the PeMV and PRSV-W RNAs (corresponding to 78K and 110K, respectively) appears to encode two proteins rather than one as previously supposed on the basis of RRL translation products. Reciprocal serological tests revealed that the tobacco vein mottling virus aphid transmission helper component protein was related to AI protein. There is direct evidence that the AI represent another potyviral-coded nonstructural protein and the first evidence that a biologically functional protein is related to a component of a potyviral inclusion.

Translation of papaya ringspot virus RNA in vitro: detection of a possible polyprotein that is processed for capsid protein, cylindrical-inclusion protein, and amorphous-inclusion protein

The genomic RNA of papaya ringspot virus (PRV), a member of the potyvirus group, was translated in a rabbit reticulocyte cell-free system as an approach to determining the translation strategy of the virus. The RNA directed synthesis of more than 20 distinct polypeptides ranging from apparent molecular weight of 26,000 (26K) to 220K. Antiserum to PRV capsid protein (CP) reacted with a subset of these polypeptides, including a 36K protein that comigrated with PRV CP during electrophoresis. Immunoprecipitation with antiserum to PRV cylindrical-inclusion protein (CIP) defined another set of polypeptides including 70K, 108K, 205K, and 220K proteins as major precipitates. The 70K protein comigrated with authentic CIP, and the 205K and 220K proteins were related to both CP and CIP. Immunoprecipitation with antiserum to PRV amorphous-inclusion protein (AIP) defined a unique set of polypeptides which contained a 112K protein as the major precipitate and 51K, 65K, and 86K proteins as minor precipitates. The 51K protein comigrated with authentic AIR A major product of 330K was observed when translation was done without the reducing agent, dithiothreitol. Immunological analyses and kinetic studies indicated that the 330K protein zone was related to the presumed CP, CIP, and AIP zones and 330K possibly is the common precursor for these viral proteins. The presence of a polyprotein of Mr corresponding to the entire coding capacity of the genomic RNA and its likely precursor relationship to the other polypeptides suggest that proteolytic processing is involved in the translation of PRV RNA.

Affinity purification of HC-Pro of potyviruses with Ni2+-NTA resin

The HC-Pro of zucchini yellow mosiac virus (ZYMV) was found to bind to Ni2+-NTA resin with or without His-tagging. The binding stringency was similar to that observed in proteins with a zinc finger motif like the HC-Pro. Using this characteristic we developed an efficient and rapid method (2-3 h) for purification of the HC-Pro of several potyviruses. A dominant protein of about 150 kDa was extracted and identified as the HC-Pro of ZYMV by means of immunoblotting. About 150 microg of HC-Pro were partially purified from the soluble fraction of 1 g of leaves. High titers of HC-Pro protein were obtained from plants infected with four potyviruses [ZYMV, watermelon mosaic virus II (WMVII), papaya ringspot virus (PRSV) and turnip mosaic virus (TuMV)]. The HC-Pros of potato virus Y (PVY) and tobacco vein mottling virus (TVMV) did not bind to the Ni2+-NTA resin. The ZYMV-HC-Pro purified by the Ni2+-NTA resin could bind in vitro to ZYMV virions blotted onto a membrane. All the HC-Pros which had been successfully purified by the Ni2+-NTA resin were bound in vitro to membrane-blotted ZYMV coat protein. However, only the HC-Pros of ZYMV and WMVII were able to mediate aphid transmission of purified ZYMV virions. The purification procedure described herein is efficient and convenient, and enables HC-Pro for a number of potyviruses to be obtained in larger amounts and at higher purity than possible by means of most existing methods, based on ultracentrifugation.

Characterization of a monoclonal antibody recognizing a DAG-containing epitope conserved in aphid transmissible potyviruses: evidence that the DAG motif is in a defined conformation

Two viral proteins, the helper component-protease and the coat protein, are required for the non-persistent aphid transmission of potyviruses. In the potyvirus coat protein, the tripeptide aspartate-alanine-glycine (DAG) has often been shown to be involved. A monoclonal antibody, raised against a synthetic decapeptide containing the DAG tripeptide, reacted with the peptide as well as with isolates of soybean mosaic, tobacco etch and tobacco vein mottling potyviruses. Experiments indicate that the monoclonal antibody recognizes a conformational rather than a sequential epitope. The data support the suggestion that the DAG region plays a structural role to determine a coat protein-helper component-protease conformation that influences aphid transmission.

Expression and assembly of the potato virus Y (PVY) coat protein (CP) in Escherichia coli cells

A clone harboring the full-length cDNA of potato virus Y in a lambda-DASH vector under the control of a T7 promoter was introduced into Escherichia coli carrying the T7-RNA-polymerase gene on a plasmid. The viral coat protein was expressed and the product was of the same size as the corresponding mature protein in infected plants. Immunoelectronmicroscopy of transfected cell extracts revealed virus-like particles, indicating that the proteins involved in its processing and the viral coat protein retained their native activity.

Immunodetection of the plum pox virus helper component in infected plants and expression of its gene in transgenic plants

Tobacco plants (Nicotiana tabacum cv. Xanthi) have been transformed via Agrobacterium tumefaciens vectors, with cDNAs corresponding to the plum pox virus (PPV) cistron 2 encoding helper component (HC-Pro) and with the first two and half cistrons of the PPV genome. Presence of the HC-Pro in PPV-infected plants and transgenic plants transformed with the gene coding for this protein was investigated using specific polyclonal antibodies produced against the PPV HC-Pro. The results suggest that two proteases are involved in the processing of the PPV N-terminal polyprotein to yield a protein of 48 k (HC-Pro). HC-Pro autolytically cleaves at its carboxyl-terminus and a proteolytic activity, probably associated with the protein (P1) encoded by the cistron 1, is required for the cleavage in planta between the proteins derived from cistrons 1 and 2.

Construction of a chimeric viral gene expressing plum pox virus coat protein

The capsid-encoding gene of plum pox virus (PPV) was fused with the leader sequence of the coat protein mRNA (cp) of tobacco mosaic virus by a novel mutagenesis technique which involves reverse transcription of minus-strand RNA [synthesized by in vitro transcription of a double-stranded (ds) cDNA clone], using an ad hoc synthetic oligodeoxynucleotide as primer. The resulting cDNA was rendered ds and cloned into the plasmid, pBluescribe M13+. Transcription of this chimeric construction produced RNA molecules of 1250 nucleotides in length, which were used as messengers in the in vitro protein-synthesizing systems. The major product of this transcript consists of a 36-kDa polypeptide and was identified as the PPV coat protein (CP) by molecular weight estimation and by immunoprecipitation with a polyclonal antiserum to PPV. Transfer of this cDNA via Agrobacterium tumefaciens into plants was successfully performed. Transgenic Nicotiana plants producing the PPV CP were subsequently obtained.

Serological relationships involving potyviral nonstructural proteins

This report represents a compilation of many of the publications on antigenic properties of potyviral-specified nonstructural proteins. Polyclonal antisera have been prepared for use in characterization of six nonstructural proteins. These include antisera to the cylindrical inclusion proteins of at least 28 potyviruses, to small nuclear inclusion protein (protease) of four potyviruses, to large nuclear inclusion protein (putative replicase) of three viruses, helper component-protease or amorphous inclusion protein of at least four viruses, to the P1 protein (located at the N-terminus of the polyprotein) of one virus, and to the P3 protein (located between helper component protease and cylindrical inclusion protein) of one virus. Monoclonal antibodies also have been prepared to several of these nonstructural proteins. The evidence thus far indicates that cylindrical inclusions of different potyviruses have both conserved and unique epitopes. Nuclear inclusion proteins and amorphous inclusion proteins also may have conserved and unique epitopes. Antigenic relationships of potyviral nonstructural proteins have potential for the identification and classification of potyviruses.

The general properties of potyviruses

The criteria used during the past three decades for including viruses in the potyvirus group are briefly discussed and evaluated. The biological and physico-chemical properties of the viruses transmitted by aphids, mites, whiteflies, or the fungus Polymyxa graminis are reviewed, and the taxonomic value of their molecular properties in regrouping the viruses into four groups or genera within the family Potyviridae is discussed.

In vitro cleavage at or near the N-terminus of the helper component protein in the tobacco vein mottling virus polyprotein

Translation of tobacco vein mottling virus (TVMV) RNA in a wheat germ system resulted in two products that were not observed in a rabbit reticulocyte system. One of these was the N-terminal protein, based on its being the most abundant product and its migration on SDS-PAGE at about 34 kDa. The second product was similar or identical to helper component (HC) isolated from TVMV-infected plants, based on co-migration with HC on SDS-PAGE and immunoprecipitation with anti-HC antibodies. The N-terminus of this product was determined by radiochemical Edman degradation to be Ser-257 of the polyprotein. This assignment was supported by peptide mapping with a tryptophan-specific reagent. A similar cleavage was observed when tobacco etch virus was translated in a wheat germ system. Comparison with homologous regions in five other potyviruses indicated conservation of amino acid residues on both sides of the proposed cleavage site. Conversion of Phe-256 to Met, Pro, Arg, His, or Trp by site-directed mutagenesis of a TVMV RNA transcription template inhibited cleavage in the wheat germ system. These results suggest that in vitro cleavage occurs between Phe-256 and Ser-257 and that this cleavage is the same as the in vivo cleavage which liberates the N-terminus of HC.

Expression of a potyvirus non-structural protein in transgenic tobacco

A cDNA fragment encoding the cytoplasmic inclusion protein of tobacco vein mottling virus was inserted into the plant expression cassette of a Ti plasmid-based binary vector. The vector was transferred to Agrobacterium tumifaciens, and following a modified leaf disc procedure, transformed tobacco plants were obtained. Analysis of poly(A)+ RNA from transgenic plants revealed a novel RNA of approximately 2100 nucleotides possessing tobacco vein mottling virus sequences. Also, immunoprecipitation of protein extracts of [35S]methionine-labeled transformed callus using anti-cytoplasmic inclusion protein antiserum revealed a polypeptide of approximately 70 kDa. This size is consistent with that predicted from the inserted tobacco vein mottling virus coding sequences. Together these data demonstrate the expression of the cytoplasmic inclusion protein in the absence of viral infections.

In vitro analysis of tobacco vein mottling virus NIa cistron: evidence for a virus-encoded protease

Potential protease functions associated with the NIa nuclear inclusion protein of tobacco vein mottling virus (TVMV) were investigated. In the absence of treatments, in vitro translation of viral RNA produced various polypeptides representing each of the proposed TVMV cistrons--28K-HC-42K-CI-5.5K-NIa-NIb-CP. When viral RNA was first hybridized to DNA probes complementary to the NIa cistron, and then treated with RNase H prior to translation, a 48-kDa processing product, immunologically identified as the NIa protein, was abolished. In its place was observed a series of larger polypeptides, immunologically identified as fusion products of the cylindrical inclusion (CI) and NIa cistrons. The use of probes which permitted translation through as few as 15 nucleotide residues beyond the sequences encoding the proposed carboxyl terminus of NIa resulted in normal processing. None of the DNA probes affected an apparent cleavage between the helper component (HC) and 42K proteins. Cloned cDNA regions representing the NIa cistron and flanking sequences were inserted in transcription vectors. Translation of the in vitro transcript resulted in synthesis, not of a large fusion polyprotein, but, of a mature-sized NIa polypeptide. In vitro transcripts, lacking the 3'-most sequences that were expected to encode the protease active site of the NIa protein, were translated. These generated an apparent fusion polypeptide that reacted with antisera to both CI and NIa. The results indicate that the NIa gene product functions as a protease responsible for some but not all of the cleavage events which lead to the production of the mature forms of TVMV proteins.

Mapping of the tobacco vein mottling virus VPg cistron

The location of the cistron encoding the genome-linked protein (VPg) in the potyvirus tobacco vein mottling virus (TVMV) was investigated. Precipitation of 125I-labeled VPg with anti-tobacco etch virus 49K nuclear inclusion protein antiserum (which reacts with the NIa nuclear inclusion protein of TVMV) indicated that the TVMV VPg is immunologically related to NIa. Lysyl residues were found to be present at positions 2, 11, and 16 of the amino-terminal region of the VPg. A search of the TVMV polyprotein sequence for this distribution of lysyl residues revealed a unique location beginning at amino acid residue 1801, the proposed amino-terminus of the NIa protein.