Intercellular protein trafficking through plasmodesmata

Molecular cloning and partial characterization of a plant VAP33 homologue with a major sperm protein domain

In a search for proteins interacting with the resistance protein Cf9 from tomato, a new cDNA was cloned and characterized. Protein sequence database searches suggested that the 120 residue-N terminal domain of the encoded protein (named VAP27) is highly similar to the VAP33 protein family from animals, to uncharacterized plant proteins, and to a lower extent, to the major sperm protein (MSP) from nematodes. The second half of the protein is similar to VAMP and to the VAP33 N-terminus comprising a predicted coiled-coil region followed by a transmembrane segment. The sequence/structure comparison of VAP27 with the crystal structure of AsMSP1 from Ascaris suum, using molecular modeling with the threading method, suggested that the N-terminus of VAP27 does possess a MSP-like domain that might participate in the formation of a protein-protein network. The coiled-coil region of VAP27 was modeled based on the structure of the VAP- and VAMP-containing SNARE complex. The coiled-coil region might also be involved in protein-protein interactions similar to VAP-VAMP interactions.

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.

The '30K' superfamily of viral movement proteins

Relationships among the amino acid sequences of viral movement proteins related to the 30 kDa ('30K') movement protein of tobacco mosaic virus - the 30K superfamily - were explored. Sequences were grouped into 18 families. A comparison of secondary structure predictions for each family revealed a common predicted core structure flanked by variable N- and C-terminal domains. The core consisted of a series of beta-elements flanked by an alpha-helix on each end. Consensus sequences for each of the families were generated and aligned with one another. From this alignment an overall secondary structure prediction was generated and a consensus sequence that can recognize each family in database searches was obtained. The analysis led to criteria that were used to evaluate other virus-encoded proteins for possible membership of the 30K superfamily. A rhabdoviral and a tenuiviral protein were identified as 30K superfamily members, as were plant-encoded phloem proteins. Parsimony analysis grouped tubule-forming movement proteins separate from others. Establishment of the alignment of residues of diverse families facilitates comparison of mutagenesis experiments done on different movement proteins and should serve as a guide for further such experiments.

The 6K2 protein and the VPg of potato virus A are determinants of systemic infection in Nicandra physaloides

Infection with the isolate PVA-M of potato virus A (PVA; genus Potyvirus) is restricted to the inoculated leaves of Nicandra physaloides (Solanaceae), whereas the isolate PVA-B11 infects plants systemically by 10 days post inoculation. Resistance to systemic infection was shown to develop during plant growth. A recombinant virus (B11-M) in which a 1,208-nucleotide sequence of the full-length cDNA clone of PVA-B11 was replaced with the corresponding sequence from PVA-M displayed a phenotype similar to that of PVA-M. The replaced sequence contained four amino acid differences between the two isolates: one in the 6K2 protein and three in the viral genome-linked protein (VPg). Site-directed mutagenesis of the cDNA clones and inoculation of the mutants to N. physaloides indicated that the amino acid substitutions of Met5Val in the 6K2 protein or Leu185Ser in the VPg permitted vascular movement and systemic infection. However, resistance was only partially overcome by these changes, since systemic infection proceeded at a slower rate than with PVA-B11. The amino acid substitution Val116Met in the VPg alone was sufficient to overcome resistance and recover the phenotype of the isolate PVA-B11. These data indicated that both the 6K2 protein and the VPg were avirulence determinants of PVa-M in N. physaloides and suggested a possibly coordinated function of them in the vascular movement of PVA.

The first triple gene block protein of peanut clump virus localizes to the plasmodesmata during virus infection

The subcellular localization of the first triple gene block protein (TGBp1) of peanut clump pecluvirus (PCV) was studied by subcellular fractionation and immunogold cytochemistry using TGBp1-specific antibodies raised against a fusion protein expressed in and purified from bacteria. In the inoculated and apical leaves of virus-infected Nicotiana benthamiana, TGBp1 localized to the cell wall and P30 fractions. Electron microscopy of immunogold-decorated ultrathin sections of the infected leaf tissue revealed TGBp1-specific labeling of the plasmodesmata joining mesophyll cells. In longitudinal sections of the plasmodesmata, the TGBp1-specific labeling was most commonly associated with the plasmodesmal collar region. In transgenic N. benthamiana, which constitutively expressed TGBp1, no TGBp1-specific immunogold labeling of plasmodesmata was observed, but plasmodesmata were gold decorated when the transgenic plants were infected with a TGBp1-defective PCV mutant, indicating that factors induced by the virus infection target and/or anchor the transgene TGBp1 to the plasmodesmata.

Actin cytoskeleton in plants: from transport networks to signaling networks

The plant actin cytoskeleton is characterized by a high diversity in regard to gene families, isoforms, and degree of polymerization. In addition to the most abundant F-actin assemblies like filaments and their bundles, G-actin obviously assembles in the form of actin oligomers composed of a few actin molecules which can be extensively cross-linked into complex dynamic meshworks. The role of the actomyosin complex as a force generating system - based on principles operating as in muscle cells - is clearly established for long-range mass transport in large algal cells and specialized cell types of higher plants. Extended F-actin networks, mainly composed of F-actin bundles, are the structural basis for this cytoplasmic streaming of high velocities On the other hand, evidence is accumulating that delicate meshworks built of short F-actin oligomers are critical for events occurring at the plasma membrane, e.g., actin interventions into activities of ion channels and hormone carriers, signaling pathways based on phospholipids, and exo- and endocytotic processes. These unique F-actin arrays, constructed by polymerization-depolymerization processes propelled via synergistic actions of actin-binding proteins such as profilin and actin depolymerizing factor (ADF)/cofilin are supposed to be engaged in diverse aspects of plant morphogenesis. Finally, rapid rearrangements of F-actin meshworks interconnecting endocellular membranes turn out to be especially important for perception-signaling purposes of plant cells, e.g., in association with guard cell movements, mechano- and gravity-sensing, plant host-pathogen interactions, and wound-healing.

Plasmodesmata signaling: many roles, sophisticated statutes

Recent advances have increased our understanding of plasmodesmata function, their architecture as it relates to signaling capacity, the temporal and spatial regulation of their permeability, and their roles in systemic transport of macromolecules, non-cell autonomous development, and, potentially, plant defense.

Developmental regulation of intercellular protein trafficking through plasmodesmata in tobacco leaf epidermis

Plasmodesmata mediate direct cell-to-cell communication in plants. One of their significant features is that primary plasmodesmata formed at the time of cytokinesis often undergo structural modifications, by the de novo addition of cytoplasmic strands across cell walls, to become complex secondary plasmodesmata during plant development. Whether such modifications allow plasmodesmata to gain special transport functions has been an outstanding issue in plant biology. Here we present data showing that the cucumber mosaic virus 3a movement protein (MP):green fluorescent protein (GFP) fusion was not targeted to primary plasmodesmata in the epidermis of young or mature leaves in transgenic tobacco (Nicotiana tabacum) plants constitutively expressing the 3a:GFP fusion gene. Furthermore, the cucumber mosaic virus 3a MP:GFP fusion protein produced in planta by biolistic bombardment of the 3a:GFP fusion gene did not traffic between cells interconnected by primary plasmodesmata in the epidermis of a young leaf. In contrast, the 3a MP:GFP was targeted to complex secondary plasmodesmata and trafficked from cell to cell when a leaf reached a certain developmental stage. These data provide the first experimental evidence, to our knowledge, that primary and complex secondary plasmodesmata have different protein-trafficking functions and suggest that complex secondary plasmodesmata may be formed to traffic specific macromolecules that are important for certain stages of leaf development.