Characterization of some proteins associated with viruses in the potato Y group

The Potyviridae cylindrical inclusion helicase: a key multipartner and multifunctional protein

A unique feature shared by all plant viruses of the Potyviridae family is the induction of characteristic pinwheel-shaped inclusion bodies in the cytoplasm of infected cells. These cylindrical inclusions are composed of the viral-encoded cylindrical inclusion helicase (CI protein). Its helicase activity was characterized and its involvement in replication demonstrated through different reverse genetics approaches. In addition to replication, the CI protein is also involved in cell-to-cell and long-distance movements, possibly through interactions with the recently discovered viral P3N-PIPO protein. Studies over the past two decades demonstrate that the CI protein is present in several cellular compartments interacting with viral and plant protein partners likely involved in its various roles in different steps of viral infection. Furthermore, the CI protein acts as an avirulence factor in gene-for-gene interactions with dominant-resistance host genes and as a recessive-resistance overcoming factor. Although a significant amount of data concerning the potential functions and subcellular localization of this protein has been published, no synthetic review is available on this important multifunctional protein. In this review, we compile and integrate all information relevant to the current understanding of this viral protein structure and function and present a mode of action for CI, combining replication and movement.

Sequence determination of the capsid protein gene and flanking regions of tobacco etch virus: Evidence for synthesis and processing of a polyprotein in potyvirus genome expression

The nucleotide sequence of the 3'-terminal portion of the tobacco etch virus (TEV) genome was determined. The 2324-nucleotide sequence represented approximately one-fourth of the TEV genome and included the capsid protein gene and flanking regions. An open reading frame of 2135 nucleotides and an untranslated region of 189 nucleotides adjacent to a polyadenylate tract were identified. The sequence began within an open reading frame, indicating that the initiation codon was upstream of the available sequence data. The sequence of the 20 NH(2)-terminal amino acids of the TEV capsid protein was established chemically. An identical amino acid sequence, predicted from the nucleotide sequence, was located, commencing at amino acid - 263. These data indicated that maturation of the capsid protein required a post-translational cleavage of a larger protein precursor, with a probable cleavage site between the amino acids glutamine and glycine.

Expression of Potato virus Y cytoplasmic inclusion protein in tobacco results in disorganization of parenchyma cells, distortion of epidermal cells, and induces mitochondrial and chloroplast abnormalities, formation of membrane whorls and atypical lipid accumulation

Infection of plant cells by potyviruses induces the formation of cytoplasmic inclusions ranging in size from 200 to 1000 nm. To determine if the ability to form these ordered, insoluble structures is intrinsic to the potyviral cytoplasmic inclusion protein, we have expressed the cytoplasmic inclusion protein from Potato virus Y in tobacco under the control of the chrysanthemum ribulose-1,5-bisphosphate carboxylase small subunit promoter, a highly active, green tissue promoter. No cytoplasmic inclusions were observed in the leaves of transgenic tobacco using transmission electron microscopy, despite being able to clearly visualize these inclusions in Potato virus Y infected tobacco leaves under the same conditions. However, we did observe a wide range of tissue and sub-cellular abnormalities associated with the expression of the Potato virus Y cytoplasmic inclusion protein. These changes included the disruption of normal cell morphology and organization in leaves, mitochondrial and chloroplast internal reorganization, and the formation of atypical lipid accumulations. Despite these significant structural changes, however, transgenic tobacco plants were viable and the results are discussed in the context of potyviral cytoplasmic inclusion protein function.

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.

A comparison of pepper mottle virus with potato virus Y and evidence for their distinction

Pepper mottle virus (PepMOV) was identified as a distinct potyvirus infecting peppers in Arizona and Florida in the 1970's. The distinction of PepMoV from potato virus Y (PVY) has recently been challenged on the basis of sequence comparisons of the coat proteins and of the 3' nontranslated regions of the viral RNAs. We summarize the biological, cytological, serological, and in vitro translational studies which compare the apparent differences, and also similarities, between PepMoV and PVY. We conclude that although PepMoV may be more closely related to PVY than to other known potyvirus, PepMoV should be maintained as a separate virus on the basis of its distinctive characteristics.

The VPg of tobacco etch virus RNA is the 49-kDa proteinase or the N-terminal 24-kDa part of the proteinase

Preparations of tobacco etch virus (TEV) RNA which were purified by sucrose gradient centrifugation, digested with RNase, and analyzed by SDS-polyacrylamide gel electrophoresis contained proteins of 49, 32, and 24 kDa. The 49- and 24-kDa proteins reacted with polyclonal antiserum to the TEV 49-kDa proteinase while the 32-kDa protein reacted with anti-TEV serum. Further purification of the RNA by centrifugation through CsCl removed the coat protein (32 kDa), but not the 49- and 24-kDa proteins. The 49- and 24-kDa proteins did not migrate into a polyacrylamdie gel when the RNA was not digested with RNase. These results indicate that the VPg of TEV is either the 49-kDa proteinase or the 24 kDa that represents the amino-terminal half thereof.

Production of antiserum to a non-structural potyviral protein and its use to detect narcissus yellow stripe and other potyviruses

A protein, of apparent molecular weight 72,000, was purified from experimentally infected narcissus plants with yellow stripe symptoms utilising SDS-polyacrylamide gel electrophoresis. This protein was excised from the gels and used to prepare antiserum, which reacted specifically with cytoplasmic cylindrical inclusions in ultra-thin sections of virus-infected cells and, in immunoblots, with the 72 kDa protein in preparations containing cytoplasmic inclusions. The antiserum reacted in ELISA with leaf extracts from yellow stripe diseased plants of four narcissus cultivars but not with extracts from comparable symptomless plants. In tests with extracts of plants infected with seven definitive potyviruses, reactions were obtained with bean yellow mosaic and iris mild mosaic viruses. Virus-specific reactions in dot-blot ELISA were dependent on the presence of Tween 20 in the extraction buffer. In contrast, an antiserum to the putative cytoplasmic inclusion protein of alstroemeria mosaic virus reacted only with SDS-treated leaf extracts of infected plants. In limited tests, the method of purifying cytoplasmic inclusion protein was successfully applied to four definitive potyviruses, suggesting that it may be generally applicable to potyviruses and of use for preparing antisera when purification of virus particles is difficult.

An improved purification procedure for preparing potyviruses and cytoplasmic inclusions from the same tissue

Twelve different potyviruses and cytoplasmic inclusion proteins were purified from a range of plant species utilizing a single purification protocol. Highly purified preparations have been obtained with yields that reflect the relative concentrations in the starting material; virus yields of up to 15 mg per 100 g of tissue were obtained. In some cases aggregation resulted in losses of significant amounts of virus to the inclusion fraction; this varied among preparations of the same virus. Preparations obtained from cesium gradients were typically unaggregated and essentially free of host materials. Purified virus was suitable for the production of antisera with high specific titers and low titers against healthy plant antigens. Both purified virus and RNA prepared from the virus retained infectivity. Purified RNA was free of detectable host plant nucleic acids, as complementary DNA preparations synthesized using virion RNA as template were highly virus-specific.

Pinwheel inclusions in morphogenesis: a possible alternative to induction by viruses

Pinwheel inclusions (PWs) were found in cells of callus tissue derived from explants of secondary phloem parenchyma of carrot (Daucus carota) storage root and grown on a basal medium containing zeatin and indoleacetic acid or coconut milk, naphthalene acetic acid, or combinations of these. Preliminary attempts to demonstrate the presence of viruses in the callus tissue failed. The possibility that the tissles were infected by a low titer or unstable conventional virus or by a defective mutant has not been ruled out. However, two lines of evidence suggest that the PWs in these tissues may be a result of culture conditions and not of virus infection. First, no PWs or other cytoplasmic inclusions were found in cells of otherwise similar tissue cultured on basal medium alone, and multifibrillar bundles (MFBs) but not PWs were found when the tissues were cultured on a medium that stimulates differentiation and morphogenesis. Second, culture stimulated to differentiate and containing MFBs only were returned to the supplemented basal medium and subsequently found to contain both PWs and MFBs.