Possible members of the potyvirus group transmitted by mites or whiteflies share epitopes with aphid-transmitted definitive members of the group

Expression of a part of the Potato virus A non-structural protein P3 in Escherichia coli for the purpose of antibody preparation and P3 immunodetection in plant material

The N-terminal part of the Potato virus A (PVA) P3 protein was cloned into two E. coli fusion expression systems. An overexpression of the P3 fragment fused with thioredoxin was observed between 2 and 21 h after induction. The protein formed insoluble inclusions. Decreasing the cultivation temperature did not enhance its solubility. To obtain antigen for antibody preparation, inclusions were concentrated and purified by sucrose gradient centrifugation, and subjected to SDS-polyacrylamide gel electrophoresis. The band specific for the protein was excised from the gel and used for rabbit immunization. Obtained antibody tested positive with high specificity in immunoblots of expressed PVA P3 fused with either thioredoxin or GST. The antibody was also applied for the detection of P3 protein in plant material by immunoblot. Previous plant sap concentration was essential for most samples. Three concentration methods were tested: simple centrifugal size-exclusion filtration, the same preceded with high-speed centrifugation at 250,000 x g, and differential ammonium sulfate precipitation. The last approach was the most convenient. Plants tested included PVA P3-transgenic tobacco lines as well as PVA-infected wild-type tobacco. In all cases, mature P3 with a molecular mass of 40 kDa was detected.

Molecular evidence that the whitefly-transmitted sweetpotato mild mottle virus belongs to a distinct genus of the Potyviridae

Complementary DNA representing 2 108 nucleotides at the 3' end of the genomic RNA of the whitefly-transmitted sweetpotato mild mottle virus (SPMMV) was cloned after PCR. Sequence analysis revealed an open reading frame of 1 797 nucleotides which codes for a protein of 599 amino acids, followed by a 3' non-coding region of 311 nucleotides. Alignment of the deduced amino acid sequence with corresponding sequences of other members of the Potyviridae demonstrated that part of the presumptive RNA-dependent RNA polymerase and the coat protein coding regions of SPMMV are found at the 3' end of its genome, in that order. Alignment of the amino acid sequence of the core of SPMMV coat protein with those of selected members of the Potyviridae showed limited identity, thus demonstrating--with phylogenetic analysis--that SPMMV belongs to a distinct genus of the family Potyviridae.

Characterisation and epitope analysis of monoclonal antibodies to virions of clover yellow vein and Johnsongrass mosaic potyviruses

Mouse monoclonal antibodies (MAbs) against the Australian B strain of clover yellow vein (ClYVV-B) and the JG strain of Johnsongrass mosaic (JGMV) potyviruses were produced, characterised and the epitopes with which they reacted were deduced. Using intact particles of ClYVV a total of ten MAbs were obtained which reacted strongly with ClYVV-B in an enzyme-linked immunosorbent assay and Western blots. Four of these MAbs (1, 2, 4, and 13) were found to be ClYVV-specific, as they reacted with all five ClYVV strains from Australia and the U.S.A. but not with 11 strains of bean yellow mosaic (BYMV), pea mosaic (PMV), and white lupin mosaic (WLMV) viruses which, together with ClYVV, form the BYMV subgroup of potyvirses. These MAbs failed to react with eight other potyvirus species, including six which infect legumes like the viruses in the BYMV subgroup. The ClYVV MAb 10 was found to be BYMV subgroup-specific. It reacted strongly with 15 of the 16 strains of viruses in the subgroup and gave no reaction with eight other potyviruses. The other five ClYVV MAbs reacted with varying degrees of specificity with the BYMV subgroup viruses and also with other potyviruses. Eight of the ClYVV MAbs (1, 2, 4, 5, 13, 17, 21, and 22) reacted with the intact coat proteins only and not with the truncated (minus amino terminus) coat protein of ClYVV suggesting that the epitopes for these MAbs are located in the surface-exposed, amino-terminal region of the ClYVV coat protein. Comparison of published coat protein sequences of BYMV and ClYVV isolates indicated that the epitopes for the four ClYVV-specific MAbs may be in the amino-terminal region spanning amino acid residues 18 to 30, whereas those for the other four MAbs may be located in the first 17 amino-terminal amino acid residue region. The epitopes that reacted with BYMV subgroup-specific MAb 10 and MAb 30 which reacted with 20 of the 24 potyvirus isolates, are probably located in the core region of ClYVV coat protein as these MAbs reacted with the intact as well as truncated coat protein of ClYVV. Analysis, in Western blot immunoassay, of 17 MAbs raised against virions of JGMV revealed that only two MAbs (1-25 and 4-30) were JGMV-specific, whereas others displayed varying degrees of specificity to different potyviruses.(ABSTRACT TRUNCATED AT 400 WORDS)

Coat protein phylogeny and systematics of potyviruses

The feasibility of applying molecular phylogenetic methods of analysis to aligned coat-protein sequences and other molecular data derived from coat proteins or genomic sequences of members of the proposed taxonomic family of Potyviridae, is discussed. We show that comparative sequence analysis of whole coat-protein sequences may be used reliably to differentiate between sequences of closely related strains, and to show groupings of more distantly related viruses; that coat proteins of putative Potyviridae cluster according to the proposed generic divisions, and, even if some are only very distantly related, the members of the family form a cluster distinct from coat proteins of other filamentous and rod-shaped viruses. Taxonomic revisions based on perceived evolutionary relationships, and the lack of feasibility of erecting higher taxa for these viruses, are discussed.

Clustering Potyviridae species on the basis of four major traits

Cluster analysis was used to examine taxonomic relationships among 31 potyviruses, using four categorical variables; genome segmentation, vector, inclusion bodies produced and host range. Analysis showed that regardless of weight given to genome segmentation, the fungus-transmitted viruses clustered in one group and the rest of the viruses in another at 60% level of similarity. It has been concluded that the creation of one family to include both the bymoviruses and potyviruses seems to be a reasonable compromise at the present time.

Potyviruses, chaos or order?

At first potyviruses were easily distinguished by biological and serological properties because only a few were known and information on their host ranges was limited. The first evidence of serological cross reaction between two of these viruses was reported in 1951 and was further corroborated for three obviously distinct members of the group in 1960. In 1968 attention was drawn to the fact that some legume and non-legume potyviruses have much wider host ranges than previously known and that within the potyvirus group there is as much biological variation within viruses and overlap between viruses as there is in serology. The concept of continuity within the group was soon supported by others and became known as the "continuum hypothesis." Results with highly sensitive serological methods using polyclonal antisera were conflicting, and nucleic acid hybridization techniques did not unambiguously discriminate between potyviruses. Recent results, obtained with antibodies directed toward epitopes located in the N-termini of the coat proteins of potyviruses, suggest that there are ways to more definitely group strains of one potyvirus and distinguish them from other potyviruses. However, there are exceptions to this rule, as in the case of bean yellow mosaic virus and clover yellow vein virus which are clearly distinct in host range, inclusion bodies, and migration velocity of coat protein, but which still react with antibodies to the N-terminal epitopes of one virus. So the question remains of whether coat-protein properties, especially the serological reactivity of N-termini, which do not alter overall virus integrity when lost, sufficiently represent the genome of a pathogenic virus entity as a single criterion for classification.

Serology of potyviruses: current problems and some solutions

The serological relationships among members of the family Potyviridae are extremely complex and inconsistent. Variable cross-reactivity of polyclonal antisera, unexpected paired relationships between distinct viruses, and lack of cross-reactions between some strains are the major problems associated with the serology of potyviruses. Recent biochemical and immunochemical investigations of coat proteins have established the molecular basis for potyvirus serology and provided explanations for most of the problems with serology of potyviruses. Information from these studies has also formed the basis for the development of several novel approaches to the accurate detection and identification of potyviruses. However, even these novel approaches are not without drawbacks and some of them cannot be applied easily in plant virus laboratories, since they require prior sequence information and facilities for peptide synthesis. These findings suggest that serology is an imperfect criterion for the identification and classification of potyviruses.

Potyviruses, monoclonal antibodies, and antigenic sites

Virus-specific and cross-reactive monoclonal antibodies have been produced to at least 19 different aphid-transmitted potyviruses. This report summarizes the development of these monoclonal antibodies as well as presents information on the delineation of the virus-specific and group-common epitopes defined by these monoclonal antibodies. Virus-specific and group-common antigenic determinants were mapped by a variety of techniques, including analysis of antigen: antibody reactivity patterns, determination of N-terminal vs. trypsin-resistant core peptide-specificity, immunoanalysis of overlapping synthetic peptides, and immunoanalysis of bacterially expressed coat-protein gene products. Of those monoclonal antibodies that have been examined, monoclonal antibody-defined virus-specific epitopes are virion surface-located within the 30+ amino acid amino terminus, whereas the group-common epitopes are found in the trypsin-resistant core protein not usually located on the virion surface, as has been shown previously with certain polyclonal antibodies. New information is presented on the analysis of bean yellow mosaic virus amino terminal epitopes as well as on the identification of amino terminal antigenic determinants shared between strains of bean yellow mosaic virus and pepper mottle virus. A recommendation on the evaluation and use of a panel of potyvirus broad-spectrum reacting monoclonals as reference monoclonal antibodies for the detection and classification of aphid-transmitted potyviruses is also presented.

Sequence data as the major criterion for potyvirus classification

Recent knowledge of the structure of the potyvirus particle and its components appears to have resolved what was thought to be an intractable problem of plant virology. This review describes how coat-protein and gene sequence data can be used to provide an hierarchical classification of potyviruses. This classification puts the aphid and non-aphid-transmitted potyviruses into a single family, divides this family into four genera that correspond to the four modes of vector transmission, discriminates distinct potyvirus species from strains, and provides a basis for the formation of subgroups composed of closely related species within a genus.

Viruses of the Potyviridae with non-aphid vectors

The large majority of members of the family Potyviridae are aphid-transmitted. However, 17 viruses whose vectors are unknown have been classified as members of the genus Potyvirus. Loss of aphid transmissibility has been observed in some strains of several potyviruses. There are currently 11 members of the Potyviridae whose vectors are not aphids. These viruses with non-aphid vectors exhibit most of the characteristics of the family. Viruses of the Potyviridae induce cytoplasmic cylindrical inclusions in their hosts whether their vectors are aphids, non-aphids, or are unknown. The virus genome produces the inclusion protein and thus the viruses have related inclusion body gene sequences. Non-aphid-transmitted viruses of the Potyviridae also are serologically related to aphid-transmitted potyviruses.

Potyviridae: genus Rymovirus

The genus Rymovirus of the family Potyviridae is comprised of seven rod-shaped viruses with the shared characteristic of being transmitted by mites. Aside from this distinguishing feature, rymoviruses are similar to aphid-transmitted potyviruses in that they share a similar particle morphology, some similar antigenic determinants, similar physico-chemical properties, the ability to induce the formation of cytoplasmic cylindrical inclusions, and the ability to infect only graminaceous hosts. In vitro translation studies with wheat streak mosaic virus (WSMV) suggest that this rymovirus uses a potyviral proteolytic processing strategy to express the 3' terminal capsid protein. At the molecular level, limited nucleotide sequence data for WSMV show similarities with aphid-transmitted potyviruses in the potyviral capsid protein, large nuclear inclusion and cylindrical inclusion regions. Thus, given the similarities between the rymoviruses and the potyviruses, it is appropriate to include this genus within the family Potyviridae.

Identification and classification of potyviruses on the basis of coat protein sequence data and serology. Brief review

The identification and classification of potyviruses has been in a very unsatisfactory state due to the large size of the group, the apparent vast variation among the members and the lack satisfactory taxonomic parameters that will distinguish distinct viruses from strains. In the past, use of classical methods, such as host range and symptomatology, cross-protection, morphology of cytoplasmic inclusions and conventional serology, revealed a "continuum" implying that the "species" and "strain" concepts cannot be applied to potyviruses. In contrast nucleic acid and amino acid sequence data of coat proteins has clearly demonstrated that potyviruses can be divided into distinct members and strains. This sequence data in combination with information of the structure of the potyvirus particle has been used to develop simple techniques such as HPLC peptide profiling, serology (using polyclonal antibody probes obtained by cross-adsorption with core protein from trypsin treated particles) and cDNA hybridization. These findings, along with immunochemical analyses of overlapping synthetic peptides have established the molecular basis for potyvirus serology; explained many of the problems associated with the application of conventional serology; and provided a sound basis for the identification and classification of potyviruses. As a result, the virus/strain status of some potyviruses has been redefined, requiring a change in the potyvirus nomenclature. These new developments necessitate a re-evaluation of the earlier literature on symptomatology, cross-protection, cytoplasmic inclusion body morphology and serology.