TRANSPORT OF PROTEINS AND NUCLEIC ACIDS THROUGH PLASMODESMATA

Quantification of Plasmodesmata Permeability in Arabidopsis Leaves by Tracing the Movement of GFP

Plasmodesmata (PD) play an important role in plant growth and development and defense. The permeability of PD is strictly regulated. Here, we describe an assay for measuring the permeability of PD in Arabidopsis thaliana leaves, which relies on tracing intercellular movement of green fluorescent protein (GFP) upon transient expression of the protein-encoding plasmid delivered by particle bombardment. The method allows to assess GFP movement at single-cell resolution.

Systemic signaling contributes to the unfolded protein response of the plant endoplasmic reticulum

The unfolded protein response (UPR) of the endoplasmic reticulum constitutes a conserved and essential cytoprotective pathway designed to survive biotic and abiotic stresses that alter the proteostasis of the endoplasmic reticulum. The UPR is typically considered cell-autonomous and it is yet unclear whether it can also act systemically through non-cell autonomous signaling. We have addressed this question using a genetic approach coupled with micro-grafting and a suite of molecular reporters in the model plant species Arabidopsis thaliana. We show that the UPR has a non-cell autonomous component, and we demonstrate that this is partially mediated by the intercellular movement of the UPR transcription factor bZIP60 facilitating systemic UPR signaling. Therefore, in multicellular eukaryotes such as plants, non-cell autonomous UPR signaling relies on the systemic movement of at least a UPR transcriptional modulator.

Transcriptional analysis of South African cassava mosaic virus-infected susceptible and tolerant landraces of cassava highlights differences in resistance, basal defense and cell wall associated genes during infection

BACKGROUND

Cassava mosaic disease is caused by several distinct geminivirus species, including South African cassava mosaic virus-[South Africa:99] (SACMV). To date, there is limited gene regulation information on viral stress responses in cassava, and global transcriptome profiling in SACMV-infected cassava represents an important step towards understanding natural host responses to plant geminiviruses.

RESULTS

A RNA-seq time course (12, 32 and 67 dpi) study, monitoring gene expression in SACMV-challenged susceptible (T200) and tolerant (TME3) cassava landraces, was performed using the Applied Biosystems (ABI) SOLiD next-generation sequencing platform. The multiplexed paired end sequencing run produced a total of 523 MB and 693 MB of paired-end reads for SACMV-infected susceptible and tolerant cDNA libraries, respectively. Of these, approximately 50.7% of the T200 reads and 55.06% of TME3 reads mapped to the cassava reference genome available in phytozome. Using a log2 fold cut-off (p<0.05), comparative analysis between the six normalized cDNA libraries showed that 4181 and 1008 transcripts in total were differentially expressed in T200 and TME3, respectively, across 12, 32 and 67 days post infection, compared to mock-inoculated. The number of responsive transcripts increased dramatically from 12 to 32 dpi in both cultivars, but in contrast, in T200 the levels did not change significantly at 67 dpi, while in TME3 they declined. GOslim functional groups illustrated that differentially expressed genes in T200 and TME3 were overrepresented in the cellular component category for stress-related genes, plasma membrane and nucleus. Alterations in the expression of other interesting genes such as transcription factors, resistance (R) genes, and histone/DNA methylation-associated genes, were observed. KEGG pathway analysis uncovered important altered metabolic pathways, including phenylpropanoid biosynthesis, sucrose and starch metabolism, and plant hormone signalling.

CONCLUSIONS

Molecular mechanisms for TME3 tolerance are proposed, and differences in patterns and levels of transcriptome profiling between T200 and TME3 with susceptible and tolerant phenotypes, respectively, support the hypothesis that viruses rearrange their molecular interactions in adapting to hosts with different genetic backgrounds.

Transgenic expression of pectin methylesterase inhibitors limits tobamovirus spread in tobacco and Arabidopsis

Plant infection by a virus is a complex process influenced by virus-encoded factors and host components which support replication and movement. Critical factors for a successful tobamovirus infection are the viral movement protein (MP) and the host pectin methylesterase (PME), an important plant counterpart that cooperates with MP to sustain viral spread. The activity of PME is modulated by endogenous protein inhibitors (pectin methylesterase inhibitors, PMEIs). PMEIs are targeted to the extracellular matrix and typically inhibit plant PMEs by forming a specific and stable stoichiometric 1:1 complex. PMEIs counteract the action of plant PMEs and therefore may affect plant susceptibility to virus. To test this hypothesis, we overexpressed genes encoding two well-characterized PMEIs in tobacco and Arabidopsis plants. Here, we report that, in tobacco plants constitutively expressing a PMEI from Actinidia chinensis (AcPMEI), systemic movement of Tobacco mosaic virus (TMV) is limited and viral symptoms are reduced. A delayed movement of Turnip vein clearing virus (TVCV) and a reduced susceptibility to the virus were also observed in Arabidopsis plants overexpressing AtPMEI-2. Our results provide evidence that PMEIs are able to limit tobamovirus movement and to reduce plant susceptibility to the virus.

The rubisco small subunit is involved in tobamovirus movement and Tm-2²-mediated extreme resistance

The multifunctional movement protein (MP) of Tomato mosaic tobamovirus (ToMV) is involved in viral cell-to-cell movement, symptom development, and resistance gene recognition. However, it remains to be elucidated how ToMV MP plays such diverse roles in plants. Here, we show that ToMV MP interacts with the Rubisco small subunit (RbCS) of Nicotiana benthamiana in vitro and in vivo. In susceptible N. benthamiana plants, silencing of NbRbCS enabled ToMV to induce necrosis in inoculated leaves, thus enhancing virus local infectivity. However, the development of systemic viral symptoms was delayed. In transgenic N. benthamiana plants harboring Tobacco mosaic virus resistance-2² (Tm-2²), which mediates extreme resistance to ToMV, silencing of NbRbCS compromised Tm-2²-dependent resistance. ToMV was able to establish efficient local infection but was not able to move systemically. These findings suggest that NbRbCS plays a vital role in tobamovirus movement and plant antiviral defenses.

Methyl esterification of pectin plays a role during plant-pathogen interactions and affects plant resistance to diseases

The cell wall is a complex structure mainly composed by a cellulose-hemicellulose network embedded in a cohesive pectin matrix. Pectin is synthesized in a highly methyl esterified form and is de-esterified in muro by pectin methyl esterases (PMEs). The degree and pattern of methyl esterification affect the cell wall structure and properties with consequences on both the physiological processes of the plants and their resistance to pathogens. PME activity displays a crucial role in the outcome of the plant-pathogen interactions by making pectin more susceptible to the action of the enzymes produced by the pathogens. This review focuses on the impact of pectin methyl esterification in plant-pathogen interactions and on the dynamic role of its alteration during pathogenesis.

Gene silencing in Arabidopsis spreads from the root to the shoot, through a gating barrier, by template-dependent, nonvascular, cell-to-cell movement

Upward long-distance mobile silencing has been shown to be phloem mediated in several different solanaceous species. We show that the Arabidopsis (Arabidopsis thaliana) seedling grafting system and a counterpart inducible system generate upwardly spreading long-distance silencing that travels not in the phloem but by template-dependent reiterated short-distance cell-to-cell spread through the cells of the central stele. Examining the movement of the silencing front revealed a largely unrecognized zone of tissue, below the apical meristem, that is resistant to the silencing signal and that may provide a gating or protective barrier against small RNA signals. Using a range of auxin and actin transport inhibitors revealed that, in this zone, alteration of vesicular transport together with cytoskeleton dynamics prevented or retarded the spread of the silencing signal. This suggests that small RNAs are transported from cell to cell via plasmodesmata rather than diffusing from their source in the phloem.

Cytoplasmic continuity revisited: closure of septa of the filamentous fungus Schizophyllum commune in response to environmental conditions

BACKGROUND

Mycelia of higher fungi consist of interconnected hyphae that are compartmentalized by septa. These septa contain large pores that allow streaming of cytoplasm and even organelles. The cytoplasm of such mycelia is therefore considered to be continuous.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we show by laser dissection that septa of Schizophyllum commune can be closed depending on the environmental conditions. The most apical septum of growing hyphae was open when this basidiomycete was grown in minimal medium with glucose as a carbon source. In contrast, the second and the third septum were closed in more than 50% and 90% of the cases, respectively. Interestingly, only 24 and 37% of these septa were closed when hyphae were growing in the absence of glucose. Whether a septum was open or closed also depended on physical conditions of the environment or the presence of toxic agents. The first septum closed when hyphae were exposed to high temperature, to hypertonic conditions, or to the antibiotic nourseothricin. In the case of high temperature, septa opened again when the mycelium was placed back to the normal growth temperature.

CONCLUSIONS/SIGNIFICANCE

Taken together, it is concluded that the septal pores of S. commune are dynamic structures that open or close depending on the environmental conditions. Our findings imply that the cytoplasm in the mycelium of a higher fungus is not continuous per se.

Identification of a novel tobacco DnaJ-like protein that interacts with the movement protein of tobacco mosaic virus

The movement protein (MP) of tobacco mosaic virus (TMV) mediates the transport of viral RNA from infected cells to neighboring uninfected cells via plasmodesmata by interacting with putative host factors. To find such host factors, we screened tobacco proteins using the yeast two-hybrid system. NtMPIP1, a novel subset of DnaJ-like proteins, was identified from a tobacco cDNA library, and its specific interaction with TMV MP was confirmed with an in vitro filter-binding assay. In a deletion analysis, using a series of truncated TMV MPs and NtMPIP1s, at least two regions of TMV MP, amino acid residues 65-86 and 120-185, conferred the ability to interact with the C-terminal domain of NtMPIP1, which is thought to be involved in substrate binding. Virus-induced gene silencing of NtMPIP1 significantly inhibited the spread of TMV. Therefore, it is reasonable to consider that endogenous NtMPIP1 is a host factor involved in virus cell-to-cell spread by interacting with TMV MP.

Nicotiana benthamiana protein, NbPCIP1, interacting with Potato virus X coat protein plays a role as susceptible factor for viral infection

The interactions of viral coat protein (CP) and host factors play an important role in viral replication and/or host defense mechanism. In this study, we constructed Nicotiana benthamiana cDNA library to find host factors interacting with Potato virus X (PVX) CP. Using yeast two-hybrid assay, we screened 3.3 x 10(6) independent yeast transformants from N. benthamiana cDNA library and identified six positive clones. One positive clone, named PVX CP-interacting protein 1 (NbPCIP1), is a plant-specific protein with homologue in N. tabacum (GenBank accession no. AB04049). We confirmed the PVX CP-NbPCIP1 interaction using yeast-two hybrid assay in yeast, protein-protein binding assay in vitro, and bimolecular fluorescent complementation assay in planta. Quantitative real-time RT-PCR analysis showed that the mRNA level of NbPCIP1 increased in PVX-infected N. benthamiana plants as compared to that of healthy plants. The green fluorescent protein (sGFP)-fused NbPCIP1 (NbPCIP1-sGFP) was localized in ER or ER-associated granular-like structure of cells. When we co-express NbPCIP1-sGFP and red fluorescent protein (RFP)-fused PVX CP (PVX CP-RFP), which were introduced by transiently expressing these proteins in N. benthamiana protoplasts and epidermal cells, however, we observed the co-localization of these proteins in the inclusion body-like complex in areas surrounding nucleus. Transient over-expression and transgene silencing of NbPCIP1 assay analysis indicated that NbPCIP1 plays a critical role in viral replication during PVX infection in host plant.

Pc4, a putative movement protein of Rice stripe virus, interacts with a type I DnaJ protein and a small Hsp of rice

Rice stripe virus (RSV) infects rice and causes great yield reduction in some Asian countries. In this study, rice cDNA library was screened by a Gal4-based yeast two-hybrid system using pc4, a putative movement protein of RSV, as the bait. A number of positive colonies were identified and sequence analysis revealed that they might correspond to ten independent proteins. Two of them were selected and further characterized. The two proteins were a J protein and a small Hsp, respectively. Interactions between Pc4 and the two proteins were confirmed using coimmunoprecipitation. Implications of the findings that pc4 interacted with two chaperone proteins were discussed.

Plasmodesmata transport of GFP and GFP fusions requires little energy and transitions during leaf expansion

Plasmodesmata (Pd) are symplastic channels between neighboring plant cells and are key in plant cell-cell signaling. Viruses of proteins, nucleic acids, and a wide range of signaling macromolecules move across Pd. Protein transport Pd is regulated by development and biotic signals. Recent investigations utilizing the Arrhenius equation or Coefficient of conductivity showed that fundamental energetic measurements used to describe transport of proteins across membrane pores or the nuclear pore can also apply to protein movement across Pd. As leaves continue to expand, Pd transport of proteins declines which may result from changes in cell volume, Pd density or Pd structure.

Mutational analysis of PVX TGBp3 links subcellular accumulation and protein turnover

Potato virus X (PVX) TGBp3 is required for virus cell-to-cell transport, has an N-terminal transmembrane domain, and a C-terminal cytosolic domain. In the absence of virus infection TGBp3:GFP is seen in the cortical and perinuclear ER. In PVX infected cells the TGBp3:GFP fusion is also seen in the nucleoplasm indicating that events during PVX infection trigger entry into the nucleus. Mutational analysis failed to identify a nuclear targeting domain. Mutations inhibiting TGBp3 association with the ER and inhibiting virus movement did not block TGBp3:GFP in the nucleoplasm. A mutation disrupting the N-terminal transmembrane domain of TGBp3 caused the fusion to accumulate in the nucleus indicating that nuclear import is regulated by ER interactions. Tunicamycin, an ER-stress inducing chemical, caused lower levels of GFP and TGBp3:GFP to accumulate in virus infected protoplasts. MG115 and MG132 were used to demonstrate that wild-type and mutant TGBp3:GFP fusions were degraded by the 26S proteasome. These observations are consistent with an ER-associated protein degradation (ERAD) pathway suggesting that PVX TGBp3, similar to aberrant ER proteins, is translocated to the cytoplasm for degradation. Nuclear accumulation of mutant and wild-type TGBp3:GFP is independent of other PVX proteins and may be another feature of an ERAD pathway.

Role of plant virus movement proteins

Plant viruses spread from the initially infected cells to the rest of the plant in several distinct stages. First, the virus (in the form of virions or nucleic acid protein complexes) moves intracellularly from the sites of replication to plasmodesmata (PD, plant-specific intercellular membranous channels), the virus then transverses the PD to spread intercellularly (cell-to-cell movement). Long-distance movement of virus occurs through phloem sieve tubes. The processes of plant virus movement are controlled by specific viral movement proteins (MPs). No extensive sequence similarity has been found in MPs belonging to different plant virus taxonomic groups. Moreover, different MPs were shown to use different pathways and mechanisms for virus transport. Some viral transport systems require a single MP while others require additional virus-encoded proteins to transport viral genomes. In this review, we focus on the functions and properties of different classes of MPs encoded by RNA containing plant viruses.

Cellular response to unfolded proteins in the endoplasmic reticulum of plants

Secretory and transmembrane proteins are synthesized in the endoplasmic reticulum (ER) in eukaryotic cells. Nascent polypeptide chains, which are translated on the rough ER, are translocated to the ER lumen and folded into their native conformation. When protein folding is inhibited because of mutations or unbalanced ratios of subunits of hetero-oligomeric proteins, unfolded or misfolded proteins accumulate in the ER in an event called ER stress. As ER stress often disturbs normal cellular functions, signal-transduction pathways are activated in an attempt to maintain the homeostasis of the ER. These pathways are collectively referred to as the unfolded protein response (UPR). There have been great advances in our understanding of the molecular mechanisms underlying the UPR in yeast and mammals over the past two decades. In plants, a UPR analogous to those in yeast and mammals has been recognized and has recently attracted considerable attention. This review will summarize recent advances in the plant UPR and highlight the remaining questions that have yet to be addressed.

Integrative plant biology: role of phloem long-distance macromolecular trafficking

Recent studies have revealed the operation of a long-distance communication network operating within the vascular system of higher plants. The evolutionary development of this network reflects the need to communicate environmental inputs, sensed by mature organs, to meristematic regions of the plant. One consequence of such a long-distance signaling system is that newly forming organs can develop properties optimized for the environment into which they will emerge, mature, and function. The phloem translocation stream of the angiosperms contains, in addition to photosynthate and other small molecules, a variety of macromolecules, including mRNA, small RNA, and proteins. This review highlights recent progress in the characterization of phloem-mediated transport of macromolecules as components of an integrated long-distance signaling network. Attention is focused on the role played by these proteins and RNA species in coordination of developmental programs and the plant's response to both environmental cues and pathogen challenge. Finally, the importance of developing phloem transcriptome and proteomic databases is discussed within the context of advances in plant systems biology.

Cell-to-cell communication via plasmodesmata during Arabidopsis embryogenesis

In Arabidopsis embryogenesis, positional information establishes the overall body plan and lineage-dependent cell fate specifies local patterning. Position-dependent gene expression and responses to the plant hormone auxin are also crucial. Recently, another mechanism that delivers positional information has been uncovered. This pathway utilizes cell-to-cell communication via plasmodesmata. Plasmodesmata span the walls between neighboring plant cells. Groups of cells that allow intercellular transport of biotic and abiotic tracers form symplastic domains of shared communication. Initially, cells of the embryo form one symplast. As development proceeds, symplastic sub-domains that correspond to the major morphological regions of the plant (i.e. shoot apex, cotyledons, hypocotyl, and root) are formed. These sub-domains further resolve into tissue-specific domains of communication (such as protodermal and vascular regions). Cell-to-cell communication via plasmodesmata between embryonic and maternal tissues ceases as development proceeds.

Population structure within lineages of Wheat streak mosaic virus derived from a common founding event exhibits stochastic variation inconsistent with the deterministic quasi-species model

Structure of Wheat streak mosaic virus (WSMV) populations derived from a common founding event and subjected to serial passage at high multiplicity of infection (MOI) was evaluated. The founding population was generated by limiting dilution inoculation. Lineages of known pedigree were sampled at passage 9 (two populations) and at passage 15, with (three populations) or without mixing (four populations) of lineages at passage 10. Polymorphism within each population was assessed by sequencing 17-21 clones containing a 1371 nt region (WSMV-Sidney 81 nts 8001-9371) encompassing the entire coat protein cistron and flanking regions. Mutation frequency averaged approximately 5.0 x 10(-4)/nt across all populations and ranged from 2.4 to 11.6 x 10(-4)/nt within populations, but did not consistently increase or decrease with the number of passages removed from the founding population. Shared substitutions (19 nonsynonymous, 10 synonymous, and 3 noncoding) occurred at 32 sites among 44 haplotypes. Only four substitutions became fixed (frequency = 100%) within a population and nearly one third (10/32) never achieved a frequency of 10% or greater in any sampled population. Shared substitutions were randomly distributed with respect to genome position, with transitions outnumbering transversions 5.4:1 and a clear bias for A to G and U to C substitutions. Haplotype composition of each population was unique with complexity of each population varying unpredictably, in that the number and frequency of haplotypes within a lineage were not correlated with number of passages removed from the founding population or whether the population was derived from a single or mixed lineage. The simplest explanation is that plant virus lineages, even those propagated at high MOI, are subject to frequent, narrow genetic bottlenecks during systemic movement that result in low effective population size and stochastic changes in population structure upon serial passage.

Identification of an interactor of cadmium ion-induced glycine-rich protein involved in regulation of callose levels in plant vasculature

Cadmium-induced glycine-rich protein (cdiGRP) is a cell wall-associated factor that increases callose levels in plant vasculature. To better understand the cdiGRP/callose regulation system, we identified a tobacco protein, GrIP (cdiGRP-interacting protein, GrIP), that associates with cdiGRP and localizes at the plant cell wall. Constitutive overexpression of GrIP enhanced the accumulation of the cdiGRP protein and callose in vasculature-associated cells with or without treatment with cadmium ions. That GrIP gene expression was not affected by cadmium ions indicated that GrIP does not directly modulate the callose levels induced by the treatment. Instead, GrIP most likely functions by further elevating the accumulated amount of cdiGRP, the expression of which is up-regulated by the cadmium ions. Interestingly, the levels of cdiGRP mRNA were not affected by constitutive expression of GrIP, demonstrating that the enhancement in cdiGRP protein accumulation by GrIP overexpression occurs posttranslationally. Collectively, these observations suggest that GrIP interacts with cdiGRP and increases its level of accumulation; in turn, the elevated amounts of cdiGRP induce callose deposits in the plant cell walls. Therefore, GrIP and cdiGRP represent sequentially acting factors in a biochemical pathway that regulates callose accumulation in the plant vasculature.

Efficient bacterial expression of recombinant potato mop-top virus non-structural triple gene block protein 1 modified by progressive deletion of its N-terminus

To obtain strong bacterial expression of proteins that seem to be hard to express in bacteria or are highly toxic for bacteria, it is possible to create a palette of similar constructs, differing only by several nucleotides, gradually deleted from the full-length clone by exonuclease III. When a construct is equipped with the 6xHis tag, a simple colony-blot procedure can be performed and a colony giving strong and efficient expression can easily be selected for high range protein expression. We utilized this procedure to produce one of potato mop-top virus (PMTV) movement proteins, namely triple gene block protein 1 (TGBp1) which was very hard to express in bacteria in its original length. The TGBp1 gene was digested with exonuclease III and nuclease S1 from its 5' terminus, leaving 6xHis tag intact. The clone that showed the strongest signal with anti-His antibodies in colony-blot procedure was found to have 44 amino acids (of total 463) deleted. The SDS-PAGE and Western blot of high range bacterial culture lysate confirmed the efficient expression of this deleted 6xHis tagged TGBp1 fragment.

Direct role of a viroid RNA motif in mediating directional RNA trafficking across a specific cellular boundary

The plasmodesmata and phloem form a symplasmic network that mediates direct cell-cell communication and transport throughout a plant. Selected endogenous RNAs, viral RNAs, and viroids traffic between specific cells or organs via this network. Whether an RNA itself has structural motifs to potentiate trafficking is not well understood. We have used mutational analysis to identify a motif that the noncoding Potato spindle tuber viroid RNA evolved to potentiate its efficient trafficking from the bundle sheath into mesophyll that is vital to establishing systemic infection in tobacco (Nicotiana tabacum). Surprisingly, this motif is not necessary for trafficking in the reverse direction (i.e., from the mesophyll to bundle sheath). It is not required for trafficking between other cell types either. We also found that the requirement for this motif to mediate bundle sheath-to-mesophyll trafficking is dependent on leaf developmental stages. Our results provide genetic evidence that (1) RNA structural motifs can play a direct role in mediating trafficking across a cellular boundary in a defined direction, (2) the bundle sheath-mesophyll boundary serves as a novel regulatory point for RNA trafficking between the phloem and nonvascular tissues, and (3) the symplasmic network remodels its capacity to traffic RNAs during plant development. These findings may help further studies to elucidate the interactions between RNA motifs and cellular factors that potentiate directional trafficking across specific cellular boundaries.

The movement protein of cowpea mosaic virus binds GTP and single-stranded nucleic acid in vitro

The movement protein (MP) of Cowpea mosaic virus forms tubules in plasmodesmata to enable the transport of mature virions. Here it is shown that the MP is capable of specifically binding riboguanosine triphosphate and that mutational analysis suggests that GTP binding plays a role in the targeted transport of the MP. Furthermore, the MP is capable of binding both single-stranded RNA and single-stranded DNA in a non-sequence-specific manner, and the GTP- and RNA-binding sites do not overlap.

Triple gene block: modular design of a multifunctional machine for plant virus movement

Many plant virus genera encode a 'triple gene block' (TGB), a specialized evolutionarily conserved gene module involved in the cell-to-cell and long-distance movement of viruses. The TGB-based transport system exploits the co-ordinated action of three polypeptides to deliver viral genomes to plasmodesmata and to accomplish virus entry into neighbouring cells. Although data obtained on both the TGB and well-studied single protein transport systems clearly demonstrate that plant viruses employ host cell pathways for intra- and intercellular trafficking of genomic nucleic acids and proteins, there is no integral picture of the details of molecular events during TGB-mediated virus movement. Undoubtedly, understanding the molecular basis of the concerted action of TGB-encoded proteins in transporting viral genomes from cell to cell should provide new insights into the general principles of movement protein function. This review describes the structure, phylogeny and expression of TGB proteins, their roles in virus cell-to-cell movement and potential influence on host antiviral defences.

The catalytic subunit of protein kinase CK2 phosphorylates in vitro the movement protein of Tomato mosaic virus

The movement protein (MP) of Tomato mosaic virus (ToMV) was reported previously by us to be phosphorylated in vitro by a cellular protein kinase(s) that exhibited several characteristics of casein kinase 2 (CK2). To characterize further this CK2-like cellular kinase, we have cloned cDNAs encoding the CK2 catalytic subunit from tobacco and compared the properties of the recombinant protein with those of the CK2-like cellular kinase. The recombinant CK2 catalytic subunit formed a complex with ToMV MP and phosphorylated it, similar to the CK2-like cellular kinase. Phosphoamino acid analyses of various mutant MPs altered near the C terminus revealed that the recombinant CK2 catalytic subunit phosphorylated serine-261, while the CK2-like cellular kinase phosphorylated both serine-261 and threonine-256. Both kinases were suggested to phosphorylate an additional serine residue(s) in regions other than the C-terminal peptide. The results are consistent with our previous prediction of involvement of CK2 in phosphorylation of ToMV MP.

Double labeling of KNOTTED1 mRNA and protein reveals multiple potential sites of protein trafficking in the shoot apex

Recent reports indicate that several plant mRNAs and proteins are able to traffic intercellularly through plasmodesmata. Localization studies can reveal differences between mRNA and protein localization that would be indicative of such a process. However, subtle differences could be missed when comparing localization in adjacent sections, especially in developmental studies where adjacent sections through immature apical regions may be one or more cells removed from each other. Therefore, we have developed a novel method for double localization of KNOTTED1 mRNA and protein in sections through the maize (Zea mays) shoot apex. The advantage of double labeling is revealed in our demonstration of novel potential sites of cell-to-cell trafficking of KNOTTED1 protein in the shoot apical region. The technique should be applicable to any gene products where the appropriate probes are available and will, therefore, help to determine the extent of protein and/or mRNA trafficking in plants.

A novel plant homeodomain protein interacts in a functionally relevant manner with a virus movement protein

Tomato bushy stunt virus and its cell-to-cell movement protein (MP; P22) provide valuable tools to study trafficking of macromolecules through plants. This study shows that wild-type P22 and selected movement-defective P22 amino acid substitution mutants were equivalent for biochemical features commonly associated with MPs (i.e. RNA binding, phosphorylation, and membrane partitioning). This generated the hypothesis that their movement defect was caused by improper interaction between the P22 mutants and one or more host factors. To test this, P22 was used as bait in a yeast (Saccharomyces cerevisiae) two-hybrid screen with a tobacco (Nicotiana tabacum) cDNA library, which identified a new plant homeodomain leucine-zipper protein that reproducibly interacted with P22 but not with various control proteins. These results were confirmed with an independent in vitro binding test. An mRNA for the host protein was detected in plants, and its accumulation was enhanced upon Tomato bushy stunt virus infection of two plant species. The significance of this interaction was further demonstrated by the failure of the homeodomain protein to interact efficiently with two of the well-defined movement-deficient P22 mutants in yeast and in vitro. This is the first report, to our knowledge, that a new plant homeodomain leucine-zipper protein interacts specifically and in a functionally relevant manner with a plant virus MP.

Insertion and topology of a plant viral movement protein in the endoplasmic reticulum membrane

Virus-encoded movement proteins (MPs) mediate cell-to-cell spread of viral RNA through plant membranous intercellular connections, the plasmodesmata. The molecular pathway by which MPs interact with viral genomes and target plasmodesmata channels is largely unknown. The 9-kDa MP from carnation mottle carmovirus (CarMV) contains two potential transmembrane domains. To explore the possibility that this protein is in fact an intrinsic membrane protein, we have investigated its insertion into the endoplasmic reticulum membrane. By using in vitro translation in the presence of dog pancreas microsomes, we demonstrate that CarMV p9 inserts into the endoplasmic reticulum without the aid of any additional viral or plant host components. We further show that the membrane topology of CarMV p9 is N(cyt)-C(cyt) (N and C termini of the protein facing the cytoplasm) by in vitro translation of a series of truncated and full-length constructs with engineered glycosylation sites. Based on these results, we propose a topological model in which CarMV p9 is anchored in the membrane with its N- and C-terminal tail segments interacting with its soluble, RNA-bound partner CarMV p7, to accomplish the viral cell-to-cell movement function.

Intercellular trafficking of a KNOTTED1 green fluorescent protein fusion in the leaf and shoot meristem of Arabidopsis

Dominant mutations in the maize homeobox gene knotted1 (kn1) act nonautonomously during maize leaf development, indicating that Kn1 is involved in the generation or transmission of a developmental signal that passes from the inner layers of the leaf to epidermal cells. We previously found that this nonautonomous activity is correlated with the presence of KN1 protein in leaf epidermal cells, where KN1 mRNA could not be detected. Furthermore, KN1 protein expressed in Escherichia coli and labeled with a fluorescent dye can traffic between leaf mesophyll cells in microinjection assays. Here we show that green fluorescent protein (GFP)-tagged KN1 is able to traffic between epidermal cells of Arabidopsis and onion. When expressed in vivo, the GFP approximately KN1 fusion trafficked from internal tissues of the leaf to the epidermis, providing the first direct evidence, to our knowledge, that KN1 can traffic across different tissue layers in the leaf. Control GFP fusions did not show this intercellular trafficking ability. GFP approximately KN1 also trafficked in the shoot apical meristem, suggesting that cell-to-cell trafficking of KN1 may be involved in its normal function in meristem initiation and maintenance.

Athila4 of Arabidopsis and Calypso of soybean define a lineage of endogenous plant retroviruses

The Athila retroelements of Arabidopsis thaliana encode a putative envelope gene, suggesting that they are infectious retroviruses. Because most insertions are highly degenerate, we undertook a comprehensive analysis of the A. thaliana genome sequence to discern their conserved features. One family (Athila4) was identified whose members are largely intact and share >94% nucleotide identity. As a basis for comparison, related elements (the Calypso elements) were characterized from soybean. Consensus Calypso and Athila4 elements are 12-14 kb in length and have long terminal repeats of 1.3-1.8 kb. Gag and Pol are encoded on a single open reading frame (ORF) of 1801 (Calypso) and 1911 (Athila4) amino acids. Following the Gag-Pol ORF are noncoding regions of ~0.7 and 2 kb, which, respectively, flank the env-like gene. The env-like ORF begins with a putative splice acceptor site and encodes a protein with a predicted central transmembrane domain, similar to retroviral env genes. RNA of Athila elements was detected in an A. thaliana strain with decreased DNA methylation (ddm1). Additionally, a PCR survey identified related reverse transcriptases in diverse angiosperm genomes. Their ubiquitous nature and the potential for horizontal transfer by infection implicates these endogenous retroviruses as important vehicles for plant genome evolution.

Functional analysis of proteins involved in movement of the monopartite begomovirus, Tomato yellow leaf curl virus

The functional properties of proteins [capsid protein (CP), V1, and C4] potentially involved with movement of the monopartite begomovirus, Tomato yellow leaf curl virus (TYLCV), were investigated using microinjection of Escherichia coli expressed proteins and transient expression of GFP fusion proteins. The TYLCV CP localized to the nucleus and nucleolus and acted as a nuclear shuttle, facilitating import and export of DNA. Thus, the CP serves as the functional homolog of the bipartite begomovirus BV1. The TYLCV V1 localized around the nucleus and at the cell periphery and colocalized with the endoplasmic reticulum, whereas C4 was localized to the cell periphery. Together, these patterns of localization were similar to that of the bipartite begomovirus BC1, known to mediate cell-to-cell movement. However, in contrast to BC1, V1 and C4, alone or in combination, had a limited capacity to move and mediate macromolecular trafficking through mesophyll or epidermal plasmodesmata. Immunolocalization and in situ PCR experiments, conducted with tomato plants at three stages of development, established that TYLCV infection was limited to phloem cells of shoot apical, leaf, stem, and floral tissues. Thus, the V1 and/or C4 may be analogs of the bipartite begomovirus BC1 that have evolved to mediate TYLCV movement within phloem tissue.

Rapid screening for dominant negative mutations in the beet necrotic yellow vein virus triple gene block proteins P13 and P15 using a viral replicon

Point mutations were introduced into the genes encoding the triple gene bock movement proteins P13 and P15 of beet necrotic yellow vein virus (BNYVV). Mutations which disabled viral cell-to-cell movement in Chenopodium quinoa were then tested for their ability to act as dominant negative inhibiters of movement of wild-type BNYVV when expressed from a co-inoculated BNYVV RNA 3-based replicon. For P13, three types of mutation inhibited the movement function: non-synomynous mutations in the N- and C-terminal hydrophobic domains, a mutation at the boundary between the N-terminal hydrophobic domain and the central hydrophilic domain (mutant P13-A12), and mutations in the conserved sequence motif in the central hydrophilic domain. However, only the 'boundary' mutant P13-A12 strongly inhibited movement of wild-type virus when expressed from the co-inoculated replicon. Similar experiments with P15 detected four movement-defective mutants which strongly inhibited cell-to-cell movement of wild-type BNYVV when the mutants were expressed from a co-inoculated replicon. Beta vulgaris transformed with two of these P15 mutants were highly resistant to fungus-mediated infection with BNYVV.

Visualization by atomic force microscopy of tobacco mosaic virus movement protein-RNA complexes formed in vitro

The structure of complexes formed in vitro by tobacco mosaic virus (TMV)-coded movement protein (MP) with TMV RNA and short (890 nt) synthetic RNA transcripts was visualized by atomic force microscopy on a mica surface. MP molecules were found to be distributed along the chain of RNA and the structure of MP-RNA complexes depended on the molar MP:RNA ratios at which the complexes were formed. A rise in the molar MP:TMV RNA ratio from 20:1 to 60-100:1 resulted in an increase in the density of the MP packaging on TMV RNA and structural conversion of complexes from RNase-sensitive 'beads-on-a-string' into a 'thick string' form that was partly resistant to RNAse. The 'thick string'-type RNase-resistant complexes were also produced by short synthetic RNA transcripts at different MP:RNA ratios. The 'thick string' complexes are suggested to represent clusters of MP molecules cooperatively bound to discrete regions of TMV RNA and separated by protein-free RNA segments.

Nucleic acid transport in plant-microbe interactions: the molecules that walk through the walls

Many microbes "genetically invade" plants by introducing DNA or RNA molecules into the host cells. For example, plant viruses transport their genomes between host cells, whereas Agrobacterium spp. transfer T-DNA to the cell nucleus and integrate it into the plant DNA. During these events, the transported nucleic acids must negotiate several barriers, such as plant cell walls, plasma membranes, and nuclear envelopes. This review describes the microbial and host proteins that participate in cell-to-cell transport and nuclear import of nucleic acids during infection by plant viruses and Agrobacterium spp. Possible molecular mechanisms by which these transport processes occur are discussed.

Primary and secondary plasmodesmata: structure, origin, and functioning

In the multicellular organisms of higher plants, plasmodesmata provide pathways for intimate symplasmic communication between neighboring cells. The arguments summarized in the present review demonstrate that plasmodesmata are diverse and highly dynamic structures. Differences in the plasmodesmal origin and modifications of the plasmodesmal structure and functioning at the various cell interfaces are the basic means which give rise to a complicated and flexibile symplasmic network. This complex communication system is discussed to serve a significant role in the coordinated development and in the concerted physiological functioning of the cells within the plant tissues, organs, and organisms.

Mutational analysis of Turnip crinkle virus movement protein p8

Summary Turnip crinkle virus encodes two proteins, p8 and p9, that are both required for cell-to-cell movement. The p8 movement protein has been demonstrated to bind RNA in a cooperative manner, although, similar to many other plant virus movement proteins, it contains no canonical RNA binding domain(s). However, three positively charged regions of p8 may potentially form ionic interactions with the RNA backbone. To identify functional regions of p8, a series of alanine and deletion scanning mutations were produced. The effects of these mutations were analysed using both in vitro RNA binding assays and in vivo infections of susceptible (Di-3) and resistant (Di-17) Arabidopsis thaliana plants. Several mutants that have reduced RNA binding ability were also demonstrated to be movement deficient and replication competent. Based on these results, there appear to be two regions, located between amino acids 18 and 31, and 50 and 72, that are required for RNA binding. Furthermore, additional regions (amino acids 12-15, and 34-37) appear to play a role in vivo unrelated to in vitro RNA binding activity.

The multifunctional capsid proteins of plant RNA viruses

This article summarizes studies of viral coat (capsid) proteins (CPs) of RNA plant viruses. In addition, we discuss and seek to interpret the knowledge accumulated to data. CPs are named for their primary function; to encapsidate viral genomic nucleic acids. However, encapsidation is only one feature of an extremely diverse array of structural, functional, and ecological roles played during viral infection and spread. Herein, we consider the evolution of viral CPs and their multitude of interactions with factors encoded by the virus, host plant, or viral vector (biological transmission agent) that influence the infection and epidemiological facets of plant disease. In addition, applications of today's understanding of CPs in the protection of crops from viral infection and use in the manufacture of valuable compounds are considered.

Cellular targets of functional and dysfunctional mutants of tobacco mosaic virus movement protein fused to green fluorescent protein

Intercellular transport of tobacco mosaic virus (TMV) RNA involves the accumulation of virus-encoded movement protein (MP) in plasmodesmata (Pd), in endoplasmic reticulum (ER)-derived inclusion bodies, and on microtubules. The functional significance of these interactions in viral RNA (vRNA) movement was tested in planta and in protoplasts with TMV derivatives expressing N- and C-terminal deletion mutants of MP fused to the green fluorescent protein. Deletion of 55 amino acids from the C terminus of MP did not interfere with the vRNA transport function of MP:GFP but abolished its accumulation in inclusion bodies, indicating that accumulation of MP at these ER-derived sites is not a requirement for function in vRNA intercellular movement. Deletion of 66 amino acids from the C terminus of MP inactivated the protein, and viral infection occurred only upon complementation in plants transgenic for MP. The functional deficiency of the mutant protein correlated with its inability to associate with microtubules and, independently, with its absence from Pd at the leading edge of infection. Inactivation of MP by N-terminal deletions was correlated with the inability of the protein to target Pd throughout the infection site, whereas its associations with microtubules and inclusion bodies were unaffected. The observations support a role of MP-interacting microtubules in TMV RNA movement and indicate that MP targets microtubules and Pd by independent mechanisms. Moreover, accumulation of MP in Pd late in infection is insufficient to support viral movement, confirming that intercellular transport of vRNA relies on the presence of MP in Pd at the leading edge of infection.

Bromovirus movement protein conditions for the host specificity of virus movement through the vascular system and affects pathogenicity in cowpea

Previously, we reported that CCMV(B3a), a hybrid of bromovirus Cowpea chlorotic mottle virus (CCMV) with the 3a cell-to-cell movement protein (MP) gene replaced by that of cowpea-nonadapted bromovirus Brome mosaic virus (BMV), can form small infection foci in inoculated cowpea leaves, but that expansion of the foci stops between 1 and 2 days postinoculation. To determine whether the lack of systemic movement of CCMV(B3a) is due to restriction of local spread at specific leaf tissue interfaces, we conducted more detailed analyses of infection in inoculated leaves. Tissue-printing and leaf press-blotting analyses revealed that CCMV(B3a) was confined to the inoculated cowpea leaves and exhibited constrained movement into leaf veins. Immunocytochemical analyses to examine the infected cell types in inoculated leaves indicated that CCMV(B3a) was able to reach the bundle sheath cells through the mesophyll cells and successfully infected the phloem cells of 50% of the examined veins. Thus, these data demonstrate that the lack of long-distance movement of CCMV(B3a) is not due to an inability to reach the vasculature, but results from failure of the virus to move through the vascular system of cowpea plants. Further, a previously identified 3a coding change (A776C), which is required for CCMV(B3a) systemic infection of cowpea plants, suppressed formation of reddish spots, mediated faster spread of infection, and enabled the virus to move into the veins of inoculated cowpea leaves. From these data, and the fact that CCMV(B3a) directs systemic infection in Nicotiana benthamiana, a permissive systemic host for both BMV and CCMV, we conclude that the bromovirus 3a MP engages in multiple activities that contribute substantially to host-specific long-distance movement through the phloem.

Opening up the communication channels: recent insights into plasmodesmal function

The past year has seen significant advances in our understanding of the function and regulation of plasmodesmata. Notably, we have learned that plasmodesmata undergo dynamic changes during development and may participate in long-range communication through the transmission of RNA signals. Biochemical studies have enriched our understanding of a putative plasmodesmal receptor and of plant factors involved in viral cell-to-cell movement.

Regulation of plasmodesmal transport by phosphorylation of tobacco mosaic virus cell-to-cell movement protein

Cell-to-cell spread of tobacco mosaic virus (TMV) through plant intercellular connections, the plasmodesmata, is mediated by a specialized viral movement protein (MP). In vivo studies using transgenic tobacco plants showed that MP is phosphorylated at its C-terminus at amino acid residues Ser258, Thr261 and Ser265. When MP phosphorylation was mimicked by negatively charged amino acid substitutions, MP lost its ability to gate plasmodesmata. This effect on MP-plasmodesmata interactions was specific because other activities of MP, such as RNA binding and interaction with pectin methylesterases, were not affected. Furthermore, TMV encoding the MP mutant mimicking phosphorylation was unable to spread from cell to cell in inoculated tobacco plants. The regulatory effect of MP phosphorylation on plasmodesmal permeability was host dependent, occurring in tobacco but not in a more promiscuous Nicotiana benthamiana host. Thus, phosphorylation may represent a regulatory mechanism for controlling the TMV MP-plasmodesmata interactions in a host-dependent fashion.

Cell-to-cell movement of potexviruses: evidence for a ribonucleoprotein complex involving the coat protein and first triple gene block protein

The triple gene block proteins (TGBp1-3) and coat protein (CP) of potexviruses are required for cell-to-cell movement. Separate models have been proposed for intercellular movement of two of these viruses, transport of intact virions, or a ribonucleoprotein complex (RNP) comprising genomic RNA, TGBp1, and the CP. At issue therefore, is the form(s) in which RNA transport occurs and the roles of TGBp1-3 and the CP in movement. Evidence is presented that, based on microprojectile bombardment studies, TGBp1 and the CP, but not TGBp2 or TGBp3, are co-translocated between cells with viral RNA. In addition, cell-to-cell movement and encapsidation functions of the CP were shown to be separable, and the rate-limiting factor of potexvirus movement was shown not to be virion accumulation, but rather, the presence of TGBp1-3 and the CP in the infected cell. These findings are consistent with a common mode of transport for potexviruses, involving a non-virion RNP, and show that TGBp1 is the movement protein, whereas TGBp2 and TGBp3 are either involved in intracellular transport or interact with the cellular machinery/docking sites at the plasmodesmata.

Nuclear localization of turnip crinkle virus movement protein p8

Turnip crinkle virus (TCV) is a single-stranded positive-sense RNA virus of the Carmovirus genus. Two of its five open reading frames (ORFs), encoding proteins of 8 and 9 kDa, are required for cell-to-cell movement of the virus. These movement proteins (MPs) were fused to green fluorescent protein (GFP) to determine their cellular localization. In protoplasts, p9-GFP, like GFP itself, is found throughout the cytoplasm, as well as in cell nuclei. In contrast, p8-GFP was confined to the cell nucleus. Similar localization patterns were observed when specific small peptide epitopes were fused to p8 and p9 proteins instead of GFP. The cytoplasmic localization of p9-GFP and nuclear localization of p8-GFP were also detected in leaves after particle bombardment of DNA encoding these fusion proteins or after overexpression of p8-GFP in transgenic Arabidopsis seedlings. The expression of the GFP fusion proteins by recombinant TCV viruses in infected protoplasts or on inoculated Arabidopsis leaves produced similar patterns. Unlike TMV-MP and other MPs studied to date, no obvious punctuate expression in the cell wall or association with the cytoskeleton was detected. The sequence analysis of p8 revealed two unique nuclear localization signals (NLSs), which were not conserved within p8 homologues of other viruses in the genus Carmovirus. Mutation in either of these NLSs did not disrupt the nuclear localization of p8-GFP. However, when both NLSs were mutated, p8-GFP expression was no longer restricted to cell nuclei. The NLSs are not required for cell-to-cell movement; TCV recombinant viruses mutated in one or both NLSs could still facilitate cell-to-cell movement of the virus. The nuclear localization of p8 suggests a novel function for this protein in the cell nucleus.

Peptide antagonists of the plasmodesmal macromolecular trafficking pathway

In plants, cell-to-cell transport of endogenous and viral proteins and ribonucleoprotein complexes (RNPCs) occurs via plasmodesmata. Specificity of this transport pathway appears to involve interaction between such proteins/RNPCs and plasmodesmal chaperones/receptors. Here, KN1 and the cucumber mosaic virus movement protein (CMV-MP) were used, in a modified phage-display screening system, to identify peptides capable of interacting with proteins present in a plasmodesmal-enriched cell wall fraction. Binding/competition assays and microinjection experiments revealed that these phage-displayed peptides and homologous synthetic oligopeptides function as ligand-specific antagonists of macromolecular trafficking through plasmodesmata. A KN1 peptide antagonist had the capacity to interact with a motif involved in the dilation of plasmodesmal microchannels. Although KN1 could still achieve limited movement through plasmodesmata when this SEL motif was blocked, KN1-mediated transport of KN1-sense RNA was fully inhibited. These findings provide direct support for the hypothesis that KN1 requires, minimally, two physically separated signal motifs involved in the dilation of, and protein translocation through, plasmodesmal microchannels, and provide direct proof that plasmodesmal dilation is a prerequisite for the cell-to-cell transport of an RNPC.

Host suppressors in Arabidopsis thaliana of mutations in the movement protein gene of Cauliflower mosaic virus

A novel genetic screen was used to identify host factors in Arabidopsis thaliana that suppress mutations in the Cauliflower mosaic virus (CaMV) movement protein gene (gene I). A series of small mutations was made in gene I and the mutations were tested for their suitability in a suppressor screen. The first round of screening yielded only revertants or second-site mutations in gene I. A derivative of one of the second-site mutant viruses (N7) that was delayed in symptom production was used in a second round of screening for suppressor plants that accelerated symptom production. Two candidate suppressor plants were found that accelerated by 1 to 4 days the first appearance of symptoms caused by the mutant viruses. One of the suppressors (5-2), called asc1 (acceleration of symptoms by CaMV N7), was mapped to chromosome 1. Two additional loci that differentially affect N7 virus susceptibility in the parental Columbia and Ler ecotypes were mapped to chromosomes 3 and 4 by quantitative trait locus (QTL) analysis.

Complementation of the movement-deficient mutations in potato virus X: potyvirus coat protein mediates cell-to-cell trafficking of C-terminal truncation but not deletion mutant of potexvirus coat protein

The cell-to-cell movement of the GUS-tagged potato virus X (PVX) coat protein (CP) movement-deficient mutant was restored by potyviral CPs of potato virus A (PVA) and potato virus Y (PVY) in Nicotiana benthamiana leaves in transient cobombardment experiments. Viral cell-to-cell movement of PVX CP mutant was complemented in Nicotiana tabacum cv. SR1 transgenic plants expressing PVY CP: PVX RNA and polymerase were detected in the PVX CP mutant-inoculated leaves of transgenic plants. These findings demonstrated the ability of the PVX CP-deficient mutant to move from cell to cell but not long distances in the transgenic plants and suggest that CPs of potex- and potyviruses display complementary activities in the movement process. Potyviral CP alone is not able to carry out these activities, since the mutated PVX CP is indispensable for restored movement. No trans-encapsidation between potyviral CP and PVX RNA was observed. Therefore, potyviral CP facilitates the PVX CP mutant movement by the mechanism that cannot be explained by coat protein substitution. Our data also suggest that CP functioning in cell-to-cell movement is not restricted to a simple passive role in forming virions.

A genetic system for detection of protein nuclear import and export

We have developed a simple genetic assay to detect active nuclear localization (NLS) and export signals (NES) on the basis of their function within yeast cells. The bacterial LexA protein was modified (mLexA) to abolish its intrinsic NLS and fused to the activation domain of the yeast Gal4p (Gal4AD) with or without the SV40 large T-antigen NLS. In the import assay, if a tested protein fused to mLexA-Gal4AD contains a functional NLS, it will enter the cell nucleus and activate the reporter gene expression. In the export assay, if a tested protein fused to mLexA-SV40 NLS-Gal4AD contains a functional NES, it will exit into the cytoplasm, decreasing the reporter gene expression. We tested this system with known NLS and NES and then used it to demonstrate a NES activity of the capsid protein of a plant geminivirus. This approach may help to identify, analyze, and select for proteins containing functional NLS and NES.

Subcellular sorting of small membrane-associated triple gene block proteins: TGBp3-assisted targeting of TGBp2

We studied subcellular distribution of green fluorescent protein (GFP)-tagged movement proteins encoded by the second and the third genes of poa semilatent hordeivirus (PSLV) triple gene block (TGB), 15K TGBp2 and 18K TGBp3. GFP-15K transiently expressed in Nicotiana benthamiana leaf epidermal cells was associated with the endomembrane system elements. GFP-18K appeared in the membrane bodies at cell periphery. Mutation analysis demonstrated that subcellular targeting of GFP-15K depended on the protein transmembrane segment(s), whereas the TGBp3 central hydrophilic region was responsible for targeting of GFP-18K. Coexpression of GFP-15K with the intact 18K protein induced drastic changes in the TGBp2 localization: GFP-15K appeared in the cell peripheral bodies similar to those in the cells expressing GFP-18K alone. Coexpression experiments with mutant forms of both proteins argue against involvement of direct interaction between small TGB proteins in the TGBp3-assisted targeting of TGBp2 to the cell peripheral compartments. This conclusion was further confirmed by similar effects on the PSLV 15K TGBp2 localization induced by TGBp3 proteins of PSLV and potato virus X, which have no detectable sequence similarity to each other.