Identification of a Functional Plasmodesmal Localization Signal in a Plant Viral Cell-To-Cell-Movement Protein

Our fundamental knowledge of the protein-sorting pathways required for plant cell-to-cell trafficking and communication via the intercellular connections termed plasmodesmata has been severely limited by the paucity of plasmodesmal targeting sequences that have been identified to date. To address this limitation, we have identified the plasmodesmal localization signal (PLS) in the Tobacco mosaic virus (TMV) cell-to-cell-movement protein (MP), which has emerged as the paradigm for dissecting the molecular details of cell-to-cell transport through plasmodesmata. We report here the identification of a bona fide functional TMV MP PLS, which encompasses amino acid residues between positions 1 and 50, with residues Val-4 and Phe-14 potentially representing critical sites for PLS function that most likely affect protein conformation or protein interactions. We then demonstrated that this PLS is both necessary and sufficient for protein targeting to plasmodesmata. Importantly, as TMV MP traffics to plasmodesmata by a mechanism that is distinct from those of the three plant cell proteins in which PLSs have been reported, our findings provide important new insights to expand our understanding of protein-sorting pathways to plasmodesmata.

IMPORTANCE

The science of virology began with the discovery of Tobacco mosaic virus (TMV). Since then, TMV has served as an experimental and conceptual model for studies of viruses and dissection of virus-host interactions. Indeed, the TMV cell-to-cell-movement protein (MP) has emerged as the paradigm for dissecting the molecular details of cell-to-cell transport through the plant intercellular connections termed plasmodesmata. However, one of the most fundamental and key functional features of TMV MP, its putative plasmodesmal localization signal (PLS), has not been identified. Here, we fill this gap in our knowledge and identify the TMV MP PLS.

Localization by immunogold cytochemistry of the virus-coded 30K protein in plasmodesmata of leaves infected with tobacco mosaic virus

The 30K protein of tobacco mosaic virus (TMV) was localized to the plasmodesmata of infected tobacco leaves by immunogold cytochemistry. This protein has been reported to be in the nuclear fraction of TMV-infected protoplasts, but as it has been proposed to function in cell-to-cell transport of virus, probably via the plasmodesmata, intact tissue was investigated with particular attention directed to plasmodesmata and nuclei. Thin sections were made from leaves mechanically inoculated with TMV at different times. Affinity-purified antibodies against a synthetic peptide corresponding to the C-terminal sequence of the 30K protein were used in the incubations, and parallel sections were incubated with antibodies against TMV. The 30K protein label accumulated inside the plasmodesmata, with a maximum 24 hr after inoculation. No specific label was found in the nuclei or at any other site in the cells.

The informosome-like virus-specific ribonucleoprotein (vRNP) may be involved in the transport of tobacco mosaic virus infection

A new type of informosome-like virus-specific ribonucleoprotein (vRNP) differing from mature tobacco mosaic virus (TMV) particles in buoyant density and structure was found in TMV-infected cells (Yu. L. Dorokhov, N. M. Alexandrova, N. A. Miroshnichenko, and J. G. Atabekov, 1983, Virology 127, 237-252). Two groups of TMV ts mutants were used to discover whether there is a correlation between the vRNP formation and systemic spreading of virus infection (transport) over the infected plant. The first group of mutants (Ni118, flavum) contains a ts mutation in the coat protein gene but are capable of systemic spreading at nonpermissive temperature (tr transport); the second group of mutants (Ni2519, Ls1) cannot spread systemically at restrictive temperature (ts transport). It is shown that vRNP can be produced at restrictive temperature by tr-transport mutants but not by ts-transport mutants. The latter can produce vRNP only at a permissive temperature. The role of vRNP in long-distance transport of the virus infection is supported by two other observations: (a) upper leaves that were maintained at 5 degrees accumulate potentially infective material and material with the properties of vRNP but not virus particles and (b) plants that were simultaneously infected with Lsl and Ni118 at a non-permissive temperature exhibited long-distance transport and vRNP. These results also implicate coat protein in long-distance transport. It is suggested that vRNPs are novel types of virus-specific particles that are involved in both cell-to-cell and long-distance transport of TMV infections.

Tobacco mosaic virus replication in resistant and susceptible plants: in some resistant species virus is confined to a small number of initially infected cells

Only small amounts of tobacco mosaic virus (TMV) are recoverable from directly inoculated leaves of some plant species, a phenomenon investigated by P. C. Cheo (1970, Phytopathology 60, 41-46) and termed subliminal infection. To interpret this phenomenon in two varieties of cowpea (Vigna sinensis Emil.), primary leaves were inoculated on their lower surfaces with TMV (common strain), and at various times postinoculation, mesophyll protoplasts were isolated, incubated for 36 hr, and stained with a TMV-specific fluorescent-labeled antibody. It was determined that only 1 in 50,000 to 150,000 protoplasts contained TMV antigen; this number remained essentially unchanged for experimental periods of from immediately after inoculation to up to 11 days postinoculation (the longest period examined). Cytological staining of epidermis from another subliminally infected host, cotton, also revealed infection of only a few cells. These data suggest that leaves of subliminally infected plants support TMV replication in those cells which receive virus during mechanical inoculation, but that the infectious principle is unable to move from those original centers in these hosts. Control experiments with tobacco (Nicotiana tabacum L. cv. Turkish Samsun), in which virus spreads extensively in the inoculated leaves, suggest that a rapid cell-to-cell movement of the infectious entity begins after about 6 hr following inoculation. An unexpected observation was that some cowpea and tobacco mesophyll cells become infected immediately upon mechanical inoculation, suggesting that mesophyll cells can be primary sites of viral ingress into the leaf.

Second-site reversion of a dysfunctional mutation in a conserved region of the tobacco mosaic tobamovirus movement protein

The N-terminal two-thirds of tobamovirus movement proteins (MPs) contain two well conserved regions. Within region I (amino acids 56-96) is an area predicted by computer analysis to have loop secondary structure (amino acids 76-87). A single or two double amino acid mutations were introduced into the loop in region I of the TMV MP to destabilize the structure. The three mutant MPs were defective in movement function. The single amino acid mutation resulted in a Pro81-->Ser substitution. The mutant virus, TP81S, containing the Pro81-->Ser substitution, was propagated on a transgenic line of Nicotiana tabacum that expresses the sunn-hemp mosaic tobamovirus MP. Inoculation of virus progeny from the transgenic plants onto hypersensitive N. tabacum indicated the presence of infectious virus at a low frequency. Necrotic lesions were detected at 4 days postinoculation, 2 days later than those induced by wild-type TMV. Inoculation of virus extracted from necrotic lesions onto N. tabacum resulted in a delayed and attenuated systemic infection relative to that induced by TMV, indicating that a second-site mutation restored movement function rather than a reversion of the original mutation. Sequence analysis revealed that the revertant MP gene had two additional amino acid substitutions, a Thr104-->Ile and a Arg167-->Lys. Introduction of the amino acid substitutions individually or in combination into the MP of TP81S indicated that both substitutions were required for the revertant phenotype. The data indicate that structure within region I is important in maintaining an active conformation for functional MP, that changes outside region I can compensate for alterations within the region, and suggest that region I may interact with a distal portion of the protein.

Host-specificity restriction by bromovirus cell-to-cell movement protein occurs after initial cell-to-cell spread of infection in nonhost plants

The nonstructural 3a protein of the positive-strand RNA bromoviruses is required for infection spread in plants and is a crucial determinant of host specificity in systemic infection. To determine the paths of wild-type (wt) bromovirus infection spread, the step at which 3a mutants are arrested, and the nature of the host specificity associated with the 3a gene, we used in situ hybridization to examine infection spread by cowpea chlorotic mottle bromovirus (CCMV) and its derivatives at the level of individual cells in cowpea leaf epidermis. From 1 to 3 days post inoculation (dpi), wt CCMV spread from initially infected cells to adjacent cells, creating expanding infection foci whose radii grew by one additional epidermal cell diameter every 5 hr. By 3 to 4 dpi, vascular elements contacting such foci acted as conduits for further infection spread. By contrast, a 3a frameshift derivative multiplied in initially infected epidermal cells but failed to move into neighboring cells even by 4 dpi, showing that the 3a gene is essential for cell-to-cell spread. Most interestingly, a CCMV derivative with the 3a gene replaced by that of a bromovirus not adapted to cowpea, brome mosaic virus (BMV), initially spread from cell to cell in cowpea plants, but stopped spreading between 1 and 2 dpi, when most infection foci encompassed 40-80 epidermal cells. Thus, the host-specificity restriction imposed by BMV 3a protein did not result from an inability to direct the spread of infection out of initially infected cowpea cells, but from a much later block. The apparent absence of any preexisting anatomical boundary at the limit of infection spread and localized tissue changes at the infection foci suggested that induced host responses might have contributed to this block.

Movement protein of tobacco mosaic virus modifies plasmodesmatal size exclusion limit

The function of the 30-kilodalton movement protein (MP) of tobacco mosaic virus is to facilitate cell-to-cell movement of viral progeny in an infected plant. A novel method for delivering non-plasmalemma-permeable fluorescent probes to the cytosol of spongy mesophyll cells of tobacco leaves was used to study plasmodesmatal size exclusion limits in transgenic plants that express the MP gene. Movement of fluorescein isothiocyanate-labeled dextran (F-dextran) with an average molecular mass of 9400 daltons and an approximate Stokes radius of 2.4 nanometers was detected between cells of the transgenic plants, whereas the size exclusion limit for the control plants was 700 to 800 daltons. No evidence of F-dextran metabolism in the leaves of the transgenic plants was found. Thus, the tobacco mosaic virus movement protein has a direct effect on a plasmodesmatal function.

The 30-Kilodalton Gene Product of Tobacco Mosaic Virus Potentiates Virus Movement

The proposed role of the 30-kilodalton(kD) protein of tobacco mosaic virus is to facilitate cell-to-cell spread of the virus-during infection. To directly define the function of the protein, a chimeric gene containing a cloned complementary DNA of the 30-kD protein gene was introduced into tobacco cells via a Ti plasmid-mediated transformation system of Agrobacterium tumefaciens. Transgenic plants regenerated from transformed tobacco cells expressed the 30-kD protein messenger RNA and accumulated 30-kD protein. Seedlings expressing the 30-kD protein gene complemented the Lsl mutant of TMV, a mutant that is temperature-sensitive in cell-to-cell movement. In addition, enhanced movement of the Lsl virus at the permissive temperature was detected in seedlings that express the 30-kD protein gene. These results conclusively demonstrate that the 30-kD protein of tobacco mosaic virus potentiates the movement of the virus from cell to cell.

Subcellular localization of the 30K protein in TMV-inoculated tobacco protoplasts

We investigated the intracellular localization of the 30K protein in TMV-inoculated tobacco protoplasts by means of pulse-labeling and pulse-chase experiments with [35S]methionine. Protoplasts were lysed with a nonionic detergent and the extracts were centrifuged to yield soluble and crude nuclear fractions. Most of the 30K protein was found in the crude nuclear fraction. The nuclear fraction was further purified by centrifugation in a step-wise Percoll gradient. Nuclei and the 30K protein were found in the same fractions. The results of pulse-chase experiments indicated that the 30K protein is synthesized in the soluble fraction and then translocated to the crude nuclear fraction. The 30K protein of Ls1, a temperature-sensitive (ts) mutant affecting the cell-to-cell viral transport function, was also found in the nuclei, even at a nonpermissive temperature. These results suggested that the 30K protein has to be localized in the nuclei to function, and that impaired translocation of the 30K protein to the nuclei is not responsible for the ts lesion of mutant Ls1.

Homologous proteins encoded by yeast mitochondrial introns and by a group of RNA viruses from plants

Hensgens et al. (1983a) have demonstrated the existence of distant homology (averaging 19.6%) between the central sections of seven proteins encoded by introns (and one product of an apparently independent gene) in yeast mitochondrial DNA. The homologous regions are typically segments of about 115 amino acids within open reading frames of about 10(3) bases. Genetic studies indicate that at least two of these proteins are required for the splicing of mitochondrial transcripts. This paper reports that two distantly related proteins of Mr 30,000 that are encoded by different strains of tobacco mosaic virus both contain central sections whose amino acid sequences are 15% to 23% identical in a single alignment to those of one group of four intron-encoded proteins, and possess certain groups of conserved residues also characteristic of the mitochondrial proteins. Genetic studies implicate these proteins in the spreading of viral lesions. While this level of identity cannot establish conclusively that the proteins are related, it suggests the possibility of a functional and/or evolutionary connection that would, if borne out, have important implications.

Single amino acid substitution in 30K protein of TMV defective in virus transport function

Involvement of the tobacco mosaic virus (TMV) coded 30K protein in a virus transport function within the infected plant has been suggested. Previously a temperature sensitive mutant, TMV Ls 1, that is defective in cell-to-cell movement at a restrictive temperature, was reported. To demonstrate a relationship between the 30K protein and the transport function, the nucleotide sequences of the 30K and coat protein cistrons of the mutant, TMV Ls 1, and the wild type, TMV L (tomato strain) were compared. A single base substitution which causes replacement of a proline codon in the L strain by a serine codon was found in the Ls 1 mutant. Results support the notion that the 30K protein is responsible for the virus transport function.