Recombinant Potato virus X Movement protein TGB2 (ORF3)

What is the structure of PVX TGBp2 protein and how does it relate to its function?

PVX TGBp2 is a membrane-associated protein of approximately 12 kDa encoded by the triple gene block (TGB), a genetic module consisting of three overlapping open reading frames (ORFs) . Structurally, TGBp2 contains multiple functional domains: an N-terminal domain (ND), a middle hydrophobic domain (MD), and a C-terminal hydrophilic domain (CD) . The protein's topology has been characterized with both the N-terminal and C-terminal domains positioned in the cytosol, while the middle domain resides in the endoplasmic reticulum (ER) lumen . This structure is critical for its dual role in viral replication and movement. TGBp2 forms distinctive "chain mail"-like aggregates around viral RNA-dependent RNA polymerase (RdRp) and double-stranded RNA (dsRNA) bodies, creating a specialized environment that potentially protects viral replication complexes while facilitating movement .

What are the key domains of PVX TGBp2 and their respective functions?

Research has identified three critical domains in TGBp2 with distinct functions:

• N-terminal domain (ND): Primarily involved in viral movement but not essential for viral replication. Deletion mutants lacking the N-terminal domain maintain replication efficiency similar to wild-type PVX but become defective in cell-to-cell movement .

• Middle domain (MD): Contains hydrophobic segments that mediate association with the ER membrane. Deletion of amino acids 52-60 within this domain affects the cellular localization of TGBp2 and significantly reduces both viral movement and replication efficiency . This domain is essential for proper protein folding and tertiary structure.

• C-terminal domain (CD): Critical for interaction with PVX RdRp. Unlike the ND, deletion of the CD severely attenuates both viral movement and replication . The CD may directly interact with the RepN domain of the viral RdRp, although it might also be involved in nonspecific RNA binding activity.

The combined functions of these domains enable TGBp2 to serve as a molecular adaptor between viral replication complexes and the cellular movement machinery .

How does TGBp2 localize within plant cells during infection?

TGBp2 exhibits dynamic subcellular localization during viral infection. When fused to green fluorescent protein (GFP) and expressed in plant cells, TGBp2 primarily localizes to vesicles and the endoplasmic reticulum (ER) network during early stages of infection . As infection progresses, the localization pattern changes, with fluorescence becoming increasingly cytosolic and nuclear in later stages .

Immunogold labeling and electron microscopy studies have confirmed that TGBp2 associates with ER-derived vesicles but not with the Golgi apparatus . Interestingly, TGBp2 also associates with actin filaments, suggesting cytoskeletal involvement in viral movement . During active viral replication, TGBp2 colocalizes with viral RdRp and dsRNA in specialized compartments known as X-bodies, where it forms characteristic "chain mail"-like structures that encase the viral replication machinery . This strategic positioning allows TGBp2 to connect viral replication sites with the cellular transport system.

How does TGBp2 interact with other viral proteins to facilitate movement?

TGBp2 functions as a crucial molecular adaptor within the PVX infection system through several key interactions:

• Interaction with viral RdRp: TGBp2 directly interacts with the C-terminal domain of PVX RdRp through its central and C-terminal hydrophilic domains . This interaction is essential for both proper localization of TGBp2 to viral replication sites and for enhancing viral replication activity.

• Recruitment of TGBp3: TGBp2 is responsible for recruiting TGBp3 to the viral replication/dsRNA bodies . Studies show that the localization of TGBp3 to the X-body is dependent on TGBp2, indicating a hierarchical relationship between these movement proteins .

• Coordination with TGBp1: While the direct interaction between TGBp2 and TGBp1 is less characterized in the provided search results, the TGB proteins collectively coordinate their functions for efficient viral movement. TGBp1 functions as an RNA helicase, suppressor of gene silencing, and plasmodesmata gating protein .

These interactions form an intricate network where TGBp2 bridges viral replication complexes with the cellular movement machinery, thereby connecting viral replication and intercellular movement processes .

What role does TGBp2 play in ER remodeling during viral infection?

TGBp2 has been shown to induce significant remodeling of the endoplasmic reticulum during PVX infection . Research using GFP-tagged TGBp2 has demonstrated that this protein specifically:

• Induces the formation of vesicles derived from the ER membrane .

• Changes the normal architecture of the ER network to form specialized compartments.

• Creates structures that associate with actin filaments but not with Golgi vesicles .

This ER remodeling appears to be a unique function of TGBp2, as comparative studies with TGBp3 showed that while TGBp3 also associates with the ER, it does not cause similar changes in endomembrane architecture . The vesicles induced by TGBp2 likely serve as specialized microenvironments for viral replication and/or as vehicles for transporting viral components to plasmodesmata. The ER reorganization process is proposed to be mechanistically linked to plasmodesmata gating, suggesting that structural changes in the ER network directly affect cell-to-cell communication channels .

How does TGBp2 contribute to viral replication enhancement?

TGBp2 enhances PVX replication through several mechanisms:

• Formation of protective compartments: TGBp2 forms "chain mail"-like structures around viral RdRp and dsRNA complexes, potentially creating a sheltered environment that protects viral replication machinery from host defense mechanisms .

• Direct interaction with viral replication machinery: The C-terminal domain of TGBp2 interacts with the RepN domain of PVX RdRp, which appears to stimulate viral replication activity . Knockout experiments have demonstrated that deletion of either the entire TGBp2 or just the domain involved in RdRp interaction significantly attenuates viral replication .

• Potential RNA binding activity: TGBp2 may enhance replication through nonspecific RNA binding activity, potentially stabilizing viral RNA or facilitating its incorporation into replication complexes .

Experimental evidence confirms the replication enhancement role of TGBp2 through multiple approaches: PVX clones lacking TGBp2 show greatly reduced replication compared to wild-type virus; mutations in the domains required for RdRp interaction diminish replication efficiency; and overexpression of TGBp2 noticeably enhances PVX replication in plant cells .

What are the most effective methods for visualizing TGBp2 subcellular localization?

Researchers have successfully employed several complementary techniques to visualize TGBp2 subcellular localization:

• Fluorescent protein fusions: The GFP gene fused to the TGBp2 gene (GFP:TGBp2) has been widely used to track protein localization in living cells . This approach allows for real-time monitoring of protein dynamics during infection. The fusion constructs can be expressed either from the viral genome (PVX-GFP:TGBp2) or from separate plasmids (pRTL2-GFP:TGBp2) .

• Immunogold labeling and electron microscopy: For higher resolution analysis of TGBp2-induced vesicles and subcellular structures, immunogold labeling followed by electron microscopy has proven valuable . This technique provides detailed ultrastructural information about the precise localization of TGBp2 in relation to cellular membranes.

• Confocal microscopy with double-labeling: Confocal microscopy combined with markers for cellular compartments (ER, Golgi, actin filaments) has enabled researchers to determine the relationship between TGBp2-containing structures and host cell components . This approach has revealed that TGBp2-induced vesicles associate with actin filaments but not with Golgi vesicles.

• In vivo dsRNA labeling system: A specialized system for labeling double-stranded RNA has been used in conjunction with fluorescently tagged TGBp2 to visualize the colocalization of TGBp2 with viral replication complexes .

These methods, particularly when used in combination, provide comprehensive insights into the dynamic localization and function of TGBp2 during viral infection.

What expression systems are most suitable for producing recombinant PVX TGBp2?

Based on the search results and research methodologies in the field, several expression systems have been successfully used for studying PVX TGBp2:

• Plant expression via viral vectors: Inserting TGBp2 (with or without fusion tags) into PVX infectious clones allows expression during authentic viral infection . This system provides the most physiologically relevant context for studying TGBp2 function but may be complicated by the effects of other viral proteins.

• Plant transient expression systems: Plasmids such as pRTL2 have been used to express GFP:TGBp2 fusion proteins in plant protoplasts and intact tissues . Agrobacterium-mediated transient expression (agroinfiltration) is particularly effective for expressing TGBp2 and its mutants in Nicotiana benthamiana leaves.

• Movement complementation assays: This experimental approach involves expressing TGBp2 in trans to complement movement-deficient viral vectors. This has been demonstrated not only for PVX but also for testing the function of movement proteins from other viruses like Hibiscus green spot virus .

When expressing TGBp2, researchers should consider the protein's membrane association properties and potential toxicity to host cells. Additionally, since protein stability appears to be affected by the cellular context (with greater instability observed in virus-infected cells compared to cells expressing TGBp2 alone) , experimental design should account for potential differences in protein accumulation and turnover rates.

What are the key experimental approaches for studying TGBp2-RdRp interactions?

Several experimental approaches have proven effective for investigating the interactions between TGBp2 and viral RdRp:

• Co-localization studies: Fluorescently tagged TGBp2 and RdRp can be co-expressed to visualize their spatial relationship within cells. The in vivo dsRNA labeling system has been particularly valuable for showing that TGBp2 forms "chain mail"-like structures around RdRp/dsRNA bodies .

• Domain mapping through deletion analysis: Creating a series of TGBp2 deletion mutants has revealed which domains are essential for interaction with RdRp. Studies have shown that the central and C-terminal hydrophilic domains of TGBp2 are required for this interaction .

• Functional assays with infectious clones: PVX infectious clones harboring mutations in TGBp2 have demonstrated that disruption of the RdRp-interacting domains attenuates both viral replication and movement . Comparing replication levels between wild-type and mutant viruses provides quantitative assessment of the functional significance of these interactions.

• Overexpression studies: Experiments showing that overexpression of TGBp2 enhances PVX replication provide additional evidence for its role in replication enhancement .

• Protein-protein interaction assays: While not explicitly mentioned in the search results, standard techniques such as co-immunoprecipitation, yeast two-hybrid assays, or bimolecular fluorescence complementation would be appropriate for confirming and characterizing direct interactions between TGBp2 and RdRp.

These approaches collectively provide robust evidence for both the physical interaction between TGBp2 and RdRp and the functional significance of this interaction in the viral life cycle.

How can TGBp2 be exploited for developing virus-resistant plants?

TGBp2's critical role in both viral replication and movement makes it an attractive target for developing virus-resistant plants. Several strategies could be explored:

• Dominant negative mutants: Engineering plants to express modified versions of TGBp2 that can compete with viral TGBp2 but lack functional activity could interfere with viral infection . Mutations in the C-terminal domain that disrupt interaction with RdRp while maintaining protein stability would be particularly promising candidates.

• RNA silencing approaches: Designing RNA interference (RNAi) constructs targeting the TGBp2 gene could suppress viral infection. Since the search results indicate that knockout of TGBp2 severely attenuates viral replication and movement , this approach has considerable potential.

• Protein interaction disruption: Based on the finding that the interaction between TGBp2 and viral RdRp is essential for viral replication , developing peptides or small molecules that specifically disrupt this interaction could provide resistance without affecting plant cellular functions.

• Engineered R-gene recognition: Creating plant varieties where immune recognition has been engineered to specifically detect TGBp2 could trigger defense responses upon viral infection.

Research challenges include ensuring that resistance strategies are durable against viral evolution and confirming that introduced modifications don't adversely affect plant growth or yield.

What are the major unresolved questions regarding TGBp2 function?

Despite significant advances in understanding TGBp2 function, several important questions remain unresolved:

• Molecular mechanism of replication enhancement: While it's clear that TGBp2 enhances viral replication, the precise molecular mechanism remains unclear. Current hypotheses include providing a sheltered environment for replication or enhancing replication through nonspecific RNA binding, but the exact mechanism requires further investigation .

• Functional coordination with other TGB proteins: The hierarchical relationship between TGBp1, TGBp2, and TGBp3 in coordinating viral movement requires further clarification, particularly the temporal and spatial regulation of their activities during infection.

• Host factor interactions: The search results provide limited information about how TGBp2 interacts with host cellular factors. Identifying host proteins that interact with TGBp2 would provide insights into how this viral protein exploits cellular machinery.

• Mechanism of plasmodesmata gating: While TGBp2 is implicated in reorganization of the ER that is linked to plasmodesmata gating , the molecular details of how TGBp2-induced membrane modifications actually affect plasmodesmatal permeability remain to be elucidated.

• Evolutionary relationships with movement proteins from other viruses: The relationship between the TGB system and other viral movement systems like the binary movement block (BMB) or double gene block (DGB) systems requires further investigation to understand convergent or divergent evolutionary pathways .

Addressing these questions will require interdisciplinary approaches combining structural biology, advanced imaging, proteomics, and genetic analyses.

How do TGBp2 proteins from different potexviruses compare in structure and function?

A comprehensive comparative analysis would require sequence alignment, structural modeling, and functional testing of TGBp2 proteins from multiple potexviruses to identify conserved and variable regions that might correlate with host range, virulence, or other viral properties.

What are common difficulties in expressing functional recombinant TGBp2 and how can they be addressed?

Expression of functional recombinant TGBp2 presents several challenges due to its membrane-associated nature and potential cytotoxicity. Based on the research methodologies described in the search results, researchers should consider these issues and solutions:

• Membrane association challenges: TGBp2's hydrophobic domains can cause protein aggregation or misfolding when expressed in conventional systems. Solutions include:

  • Using specialized vectors designed for membrane protein expression

  • Incorporating detergents or membrane-mimicking environments during purification

  • Expressing fusion proteins with solubility-enhancing tags that can be later removed

• Protein instability: The search results indicate that TGBp2 exhibits enhanced instability in virus-infected cells compared to when expressed alone . Researchers might:

  • Include proteasome inhibitors when studying the protein in infected cells

  • Monitor protein turnover rates using cycloheximide chase experiments

  • Consider pulse-chase labeling to track protein stability over time

• Maintaining proper folding: The tertiary structure of TGBp2 is essential for its function, as demonstrated by studies showing that mutations affecting protein morphology impair function . Approaches to address this include:

  • Careful design of fusion proteins to minimize structural disruption

  • Expression at lower temperatures to promote proper folding

  • Use of molecular chaperones as co-expression partners

• Visualization challenges: When using fluorescent protein fusions, researchers should be aware that the large size of GFP or similar tags might affect TGBp2 localization or function. Controls using smaller epitope tags or comparing N-terminal versus C-terminal fusions can help validate observations.

How can researchers effectively study the dynamics of TGBp2 trafficking in live cells?

Studying the dynamic trafficking of TGBp2 in live cells requires specialized approaches that balance temporal resolution with minimal disruption of protein function:

• Photoactivatable and photoconvertible fluorescent proteins: These tools enable pulse-chase experiments in living cells, allowing researchers to track specific populations of TGBp2 as they traffic through the cell. This approach would be valuable for determining the kinetics of TGBp2 movement from the ER to plasmodesmata.

• Fluorescence recovery after photobleaching (FRAP): This technique can assess the mobility of TGBp2 within cellular compartments by bleaching fluorescence in a defined region and measuring the rate of fluorescence recovery. The search results suggest that TGBp2 forms aggregates , and FRAP could determine whether these structures are static or dynamic.

• Time-lapse imaging: The search results describe changes in TGBp2 localization during progression of viral infection, with increasing cytosolic and nuclear fluorescence in late stages . Time-lapse imaging of cells expressing fluorescently tagged TGBp2 during the course of infection would provide valuable insights into these dynamics.

• Correlative light and electron microscopy (CLEM): This approach combines the temporal resolution of fluorescence microscopy with the ultrastructural detail of electron microscopy, which would be particularly valuable for characterizing the TGBp2-induced vesicles and their relationship to viral replication sites.

• Multi-color imaging with compartment markers: Co-expressing TGBp2 with markers for different cellular compartments (ER, Golgi, actin, etc.) enables tracking of its relationship with dynamic cellular structures over time. The search results already indicate that double-labeling has revealed TGBp2's association with actin filaments .

These approaches can provide comprehensive information about TGBp2 trafficking dynamics that static imaging methods cannot capture.

How does the TGB movement system compare with other viral movement strategies?

The triple gene block (TGB) system represents one of several distinct viral movement strategies that have evolved in plant viruses. Based on the search results, we can compare the TGB system with other movement strategies:

• TGB versus BMB (Binary Movement Block): The Binary Movement Block is a specialized transport module found in viruses like Hibiscus green spot virus (HGSV), consisting of two movement proteins (BMB1 and BMB2) . Similar to the TGB system, the BMB system involves membrane-associated proteins, but with only two components instead of three. BMB2, like TGBp2 and TGBp3, contains hydrophobic segments but shows only marginal similarity to TGB proteins .

• TGB versus DGB (Double Gene Block): The search results mention DGB as another alternative to tubule-based transport systems , but specific details on its structure and function are not provided in the available information.

• TGB versus Single Protein Systems: Some plant viruses utilize a single movement protein for cell-to-cell trafficking. The search results note that the single gene-coded transport system likely evolved independently from the TGB, DGB, and BMB systems .

• Common Evolutionary Pattern: Despite their differences, these movement systems share a common pattern in their trafficking mechanism: initial delivery to membrane compartments adjacent to plasmodesmata, subsequent entry into the plasmodesmata cavity, and finally, transport to adjacent cells . This suggests convergent evolution toward similar functional solutions despite different protein components.

The diversity of movement strategies reflects the evolutionary pressure on plant viruses to overcome the cell wall barrier while adapting to different host environments. The TGB system, with its specialized division of labor among three proteins, represents one successful evolutionary solution to this challenge.

What evolutionary insights can be gained from studying TGBp2 across different virus species?

Comparative analysis of TGBp2 across different virus species can provide valuable evolutionary insights, although the search results offer limited direct comparative data:

Comprehensive evolutionary studies of TGBp2 would provide insights not only into viral adaptation but also into fundamental mechanisms of membrane protein evolution and host-pathogen co-evolution.

What new technological advances are improving our understanding of TGBp2 function?

Recent technological advances have significantly enhanced our ability to study TGBp2 function, though not all are explicitly mentioned in the search results:

• In vivo dsRNA labeling system: This technique has enabled researchers to visualize the colocalization of TGBp2 with viral replication complexes containing dsRNA and RdRp, revealing the "chain mail"-like structures formed by TGBp2 around these complexes . This approach provides unprecedented insights into the spatial organization of viral replication sites.

• Advanced imaging techniques: While not explicitly described in the search results, super-resolution microscopy techniques like STORM, PALM, or STED would enable visualization of TGBp2-containing structures below the diffraction limit, providing more detailed information about their organization and relationship to cellular structures.

• Cryo-electron microscopy and tomography: These techniques could provide structural information about TGBp2 in its native membrane environment and reveal the three-dimensional organization of TGBp2-induced vesicles and their relationship to viral replication complexes.

• Proteomics approaches: Techniques like proximity labeling (BioID, APEX) could identify host proteins that interact with TGBp2 in different cellular compartments, providing insights into how this viral protein exploits cellular machinery.

• CRISPR-Cas9 genome editing: This technology enables precise manipulation of host factors potentially involved in TGBp2 function, allowing researchers to test the functional significance of specific host-virus interactions.

These technological advances, especially when used in combination, promise to reveal new aspects of TGBp2 function and its role in viral replication and movement.

What are the most promising directions for developing antiviral strategies targeting TGBp2?

Based on our understanding of TGBp2 function, several promising antiviral strategies could be developed:

• Small molecule inhibitors of TGBp2-RdRp interaction: Since the interaction between TGBp2 and viral RdRp is essential for both viral replication and movement , small molecules that specifically disrupt this interaction could effectively inhibit viral infection.

• Peptide-based inhibitors: Synthetic peptides derived from the interaction interfaces between TGBp2 and its viral or host partners could competitively inhibit these interactions. The search results identify the C-terminal domain of TGBp2 as critical for interaction with viral RdRp , making this domain a prime target for peptide design.

• RNA aptamers: RNA aptamers that specifically bind to TGBp2 could interfere with its function in viral replication or movement. Since TGBp2 may have RNA binding activity , aptamers could compete with viral RNA for binding.

• Host factor modulation: If essential host factors that interact with TGBp2 are identified, strategies to modulate these factors without adversely affecting plant health could provide resistance against a broad spectrum of viruses that use similar movement mechanisms.

• Structure-based drug design: While the three-dimensional structure of TGBp2 is not described in the search results, determining this structure would enable rational design of inhibitors that target specific functional domains of the protein.

The dual role of TGBp2 in viral replication and movement makes it a particularly attractive antiviral target, as strategies targeting this protein could simultaneously inhibit two essential viral processes, potentially reducing the likelihood of resistance development.

How can knowledge of TGBp2 function contribute to crop protection strategies?

Understanding TGBp2 function can inform several crop protection strategies against potexviruses and possibly other TGB-containing plant viruses:

• Engineered resistance: Knowledge of TGBp2's critical domains can guide the development of transgenic plants expressing dominant negative mutants or antibodies that specifically interfere with TGBp2 function . Plants expressing TGBp2 mutants that lack the ability to interact with viral RdRp but can still incorporate into viral movement complexes might effectively inhibit viral spread.

• RNA silencing-based approaches: Targeting conserved regions of the TGBp2 gene with RNA interference (RNAi) constructs could provide resistance against multiple related viruses. The search results confirm that TGBp2 is essential for robust viral replication and movement , suggesting that silencing this gene would effectively suppress viral infection.

• Novel screening methods: Understanding the molecular mechanisms of TGBp2 function enables the development of high-throughput screening systems to identify chemical compounds that disrupt its activity. These screens could target TGBp2-RdRp interactions or TGBp2's membrane-modifying activities.

• Diagnostic tools: Knowledge of TGBp2 sequence conservation across related viruses can inform the design of broad-spectrum diagnostic tools for early detection of viral infections in crops.

• Natural resistance identification: Understanding how TGBp2 interacts with host factors could guide efforts to identify natural resistance genes in crop germplasm collections. Plants with variations in these host factors might exhibit reduced compatibility with viral TGBp2, providing a basis for breeding programs.

These strategies could contribute to developing durable resistance against economically important potexviruses in various crop species, reducing yield losses and decreasing reliance on chemical control measures.

What experimental systems best model TGBp2 function in crop plants?

Several experimental systems can effectively model TGBp2 function in crop plants, each with particular advantages:

• Nicotiana benthamiana as a model host: This plant species has been extensively used in studies of PVX TGBp2 , offering advantages of ease of transformation, amenability to virus-induced gene silencing, and compatibility with transient expression systems. While not a crop itself, N. benthamiana provides a well-characterized system for initial mechanistic studies.

• Crop-specific cell cultures and protoplasts: Cell cultures derived from relevant crop species can be used to study TGBp2 function in a more crop-relevant cellular context. The search results describe the use of protoplasts for studying TGBp2 localization and function , and this approach could be adapted to crop-specific protoplasts.

• Virus movement complementation assays: The search results describe the use of movement complementation assays to verify the function of viral movement proteins . This approach can be adapted to crop-specific viruses and hosts to study how TGBp2 functions in economically important plant-virus interactions.

• Heterologous expression in model crop systems: Expression of fluorescently tagged TGBp2 in model crop systems (e.g., rice, tomato, wheat) using transient expression or stable transformation can reveal crop-specific aspects of TGBp2 function and localization.

• Cross-protection systems: Utilizing attenuated virus strains with modified TGBp2 proteins could provide insights into how alterations in TGBp2 function affect virus-crop interactions under field conditions.

The choice of experimental system should be guided by the specific research question and the relevance to particular crop-virus combinations. Ideally, initial mechanistic studies in model systems would be validated in crop-specific contexts to ensure relevance to agricultural applications.

How does TGBp2 manipulate host membrane systems during infection?

TGBp2 induces significant remodeling of host membrane systems during viral infection, particularly the endoplasmic reticulum. Based on the search results, these manipulations include:

• ER-derived vesicle formation: TGBp2 induces the formation of vesicles derived from the ER . Immunogold labeling and electron microscopy have confirmed that these vesicles are ER-derived and not associated with the Golgi apparatus .

• Association with actin cytoskeleton: TGBp2-induced vesicles associate with actin filaments, suggesting that TGBp2 recruits cytoskeletal elements to facilitate trafficking of viral components . Double-labeling studies using confocal microscopy have visualized this association with actin, but not with Golgi vesicles .

• Plasmodesmata targeting: TGBp2 is implicated in a model where reorganization of the ER is linked to plasmodesmata gating . This suggests that TGBp2-induced membrane modifications directly affect the structure or function of these intercellular channels.

• Formation of viral replication compartments: TGBp2 forms "chain mail"-like structures around viral RdRp and dsRNA, potentially creating specialized membrane-associated compartments for viral replication . These structures may provide a protected environment that shields viral replication from host defense mechanisms.

These membrane manipulations serve dual purposes: creating an optimal environment for viral replication and facilitating the movement of viral components to and through plasmodesmata. The ability of TGBp2 to induce these significant membrane alterations despite its small size (approximately 12 kDa) highlights the remarkable efficiency of viral proteins in reprogramming host cellular architecture.

What host factors interact with TGBp2 during viral infection?

• ER membrane proteins: Given TGBp2's association with the ER and its ability to induce ER-derived vesicles , it likely interacts with proteins involved in ER membrane shaping and vesicle formation. Candidates might include reticulons, SNARE proteins, or components of the COPII vesicle formation machinery.

• Cytoskeletal components: The association of TGBp2-induced vesicles with actin filaments suggests interactions with actin itself or with actin-binding proteins that mediate attachment of membranes to the cytoskeleton.

• Plasmodesmata-associated proteins: Since TGBp2 is involved in viral movement and ER reorganization linked to plasmodesmata gating , it likely interacts with host proteins located at or near plasmodesmata, such as plasmodesmata-located proteins (PDLPs) or callose synthases/glucanases that regulate plasmodesmatal permeability.

• Protein degradation machinery: The observation that TGBp2 shows enhanced instability in virus-infected protoplasts suggests interaction with components of the cellular protein degradation machinery, possibly including ubiquitin ligases or proteasome components.