Similarities in intracellular transport of plant viral movement proteins BMB2 and TGB3

Fine Structure of Plasmodesmata-Associated Membrane Bodies Formed by Viral Movement Protein

Cell-to-cell transport of plant viruses through plasmodesmata (PD) requires viral movement proteins (MPs) often associated with cell membranes. The genome of the Hibiscus green spot virus encodes two MPs, BMB1 and BMB2, which enable virus cell-to-cell transport. BMB2 is known to localize to PD-associated membrane bodies (PAMBs), which are derived from the endoplasmic reticulum (ER) structures, and to direct BMB1 to PAMBs. This paper reports the fine structure of PAMBs. Immunogold labeling confirms the previously observed localization of BMB1 and BMB2 to PAMBs. EM tomography data show that the ER-derived structures in PAMBs are mostly cisterns interconnected by numerous intermembrane contacts that likely stabilize PAMBs. These contacts predominantly involve the rims of the cisterns rather than their flat surfaces. Using FRET-FLIM (Förster resonance energy transfer between fluorophores detected by fluorescence-lifetime imaging microscopy) and chemical cross-linking, BMB2 is shown to self-interact and form high-molecular-weight complexes. As BMB2 has been shown to have an affinity for highly curved membranes at cisternal rims, the interaction of BMB2 molecules located at rims of adjacent cisterns is suggested to be involved in the formation of intermembrane contacts in PAMBs.

Kitaviruses: A Window to Atypical Plant Viruses Causing Nonsystemic Diseases

Kitaviridae is a family of plant-infecting viruses that have multiple positive-sense, single-stranded RNA genomic segments. Kitaviruses are assigned into the genera Cilevirus, Higrevirus, and Blunervirus, mainly on the basis of the diversity of their genomic organization. Cell-to-cell movement of most kitaviruses is provided by the 30K family of proteins or the binary movement block, considered an alternative movement module among plant viruses. Kitaviruses stand out for producing conspicuously unusual locally restricted infections and showing deficient or nonsystemic movement likely resulting from incompatible or suboptimal interactions with their hosts. Transmission of kitaviruses is mediated by mites of many species of the genus Brevipalpus and at least one species of eriophyids. Kitavirus genomes encode numerous orphan open reading frames but RNA-dependent RNA polymerase and the transmembrane helix-containing protein, generically called SP24, typify a close phylogenetic link with arthropod viruses. Kitaviruses infect a large range of host plants and cause diseases of economic concern in crops such as citrus, tomato, passion fruit, tea, and blueberry.

Interaction between Movement Proteins of Hibiscus green spot virus

Movement proteins (MPs) of plant viruses enable the translocation of viral genomes from infected to healthy cells through plasmodesmata (PD). The MPs functions involve the increase of the PD permeability and routing of viral genome both to the PD entrance and through the modified PD. Hibiscus green spot virus encodes two MPs, termed BMB1 and BMB2, which act in concert to accomplish virus cell-to-cell transport. BMB1, representing an NTPase/helicase domain-containing RNA-binding protein, localizes to the cytoplasm and the nucleoplasm. BMB2 is a small hydrophobic protein that interacts with the endoplasmic reticulum (ER) membranes and induces local constrictions of the ER tubules. In plant cells, BMB2 localizes to PD-associated membrane bodies (PAMBs) consisting of modified ER tubules and directs BMB1 to PAMBs. Here, we demonstrate that BMB1 and BMB2 interact in vitro and in vivo, and that their specific interaction is essential for BMB2-directed targeting of BMB1 to PAMBs. Using mutagenesis, we show that the interaction involves the C-terminal BMB1 region and the N-terminal region of BMB2.

Distinct Mechanisms of Endomembrane Reorganization Determine Dissimilar Transport Pathways in Plant RNA Viruses

Plant viruses exploit the endomembrane system of infected cells for their replication and cell-to-cell transport. The replication of viral RNA genomes occurs in the cytoplasm in association with reorganized endomembrane compartments induced by virus-encoded proteins and is coupled with the virus intercellular transport via plasmodesmata that connect neighboring cells in plant tissues. The transport of virus genomes to and through plasmodesmata requires virus-encoded movement proteins (MPs). Distantly related plant viruses encode different MP sets, or virus transport systems, which vary in the number of MPs and their properties, suggesting their functional differences. Here, we discuss two distinct virus transport pathways based on either the modification of the endoplasmic reticulum tubules or the formation of motile vesicles detached from the endoplasmic reticulum and targeted to endosomes. The viruses with the movement proteins encoded by the triple gene block exemplify the first, and the potyviral system is the example of the second type. These transport systems use unrelated mechanisms of endomembrane reorganization. We emphasize that the mode of virus interaction with cell endomembranes determines the mechanism of plant virus cell-to-cell transport.

Constriction of endoplasmic reticulum tubules by the viral movement protein BMB2 is associated with local BMB2 anchorage at constriction sites

Plant virus-encoded movement proteins (MPs) interact with endoplasmic reticulum (ER) membranes, the cytoskeleton, and plasmodesmata (PD) to mediate intracellular delivery of the virus genome to PD and its further transport through PD from infected to healthy cells. The Hibiscus green spot virus MP termed BMB2 has been shown to induce constrictions of ER tubules and to occur at highly curved membranes, thus showing properties similar to those of reticulons, a class of cellular proteins inducing membrane curvature and shaping the ER tubules. Consistent with this BMB2 function, mRFP-BMB2 localizes to discrete, constricted regions scattered along the ER tubules. Here, using BMB2-mRFP fusion protein as a BMB2 derivative with partially disabled functionality, we demonstrate that the focal localization of BMB2 to discrete sites along the ER tubules is insufficient to induce local tubule constrictions at these sites, suggesting that the formation of ER tubule constrictions represents a specific BMB2 function and is not simply a mechanistic consequence of its localization to the ER. The presented data suggest that the formation of ER-residing BMB2-containing distinct small aggregates, or protein platforms, can be uncoupled from BMB2-induced ER tubule constrictions, whereas the anchoring of platforms at particular ER sites appears to be linked to the constriction of ER tubules at these sites.

Reticulon-like properties of a plant virus-encoded movement protein

Plant viruses encode movement proteins (MPs) that ensure the transport of viral genomes through plasmodesmata (PD) and use cell endomembranes, mostly the endoplasmic reticulum (ER), for delivery of viral genomes to PD and formation of PD-anchored virus replication compartments. Here, we demonstrate that the Hibiscus green spot virus BMB2 MP, an integral ER protein, induces constrictions of ER tubules, decreases the mobility of ER luminal content, and exhibits an affinity to highly curved membranes. These properties are similar to those described for reticulons, cellular proteins that induce membrane curvature to shape the ER tubules. Similar to reticulons, BMB2 adopts a W-like topology within the ER membrane. BMB2 targets PD and increases their size exclusion limit, and these BMB2 activities correlate with the ability to induce constrictions of ER tubules. We propose that the induction of ER constrictions contributes to the BMB2-dependent increase in PD permeability and formation of the PD-associated replication compartments, therefore facilitating the virus intercellular spread. Furthermore, we show that the ER tubule constrictions also occur in cells expressing TGB2, one of the three MPs of Potato virus X (PVX), and in PVX-infected cells, suggesting that reticulon-like MPs are employed by diverse RNA viruses.

Intercellular movement of plant RNA viruses: Targeting replication complexes to the plasmodesma for both accuracy and efficiency

Replication and movement are two critical steps in plant virus infection. Recent advances in the understanding of the architecture and subcellular localization of virus-induced inclusions and the interactions between viral replication complex (VRC) and movement proteins (MPs) allow for the dissection of the intrinsic relationship between replication and movement, which has revealed that recruitment of VRCs to the plasmodesma (PD) via direct or indirect MP-VRC interactions is a common strategy used for cell-to-cell movement by most plant RNA viruses. In this review, we summarize the recent advances in the understanding of virus-induced inclusions and their roles in virus replication and cell-to-cell movement, analyze the advantages of such coreplicational movement from a viral point of view and discuss the possible mechanical force by which MPs drive the movement of virions or viral RNAs through the PD. Finally, we highlight the missing pieces of the puzzle of viral movement that are especially worth investigating in the near future.

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

Non-replicative Integral Membrane Proteins Encoded by Plant Alpha-Like Viruses: Emergence of Diverse Orphan ORFs and Movement Protein Genes

Fast accumulation of sequencing data on plant virus genomes and plant transcriptomes demands periodic re-evaluation of current views on the genome evolution of viruses. Here, we substantiate and further detail our previously mostly speculative model on the origin and evolution of triple gene block (TGB) encoding plant virus movement proteins TGB1, TGB2, and TGB3. Recent experimental data on functional competence of transport gene modules consisting of two proteins related to TGB1 and TGB2, as well as sequence analysis data on similarity of TGB2 and TGB3 encoded by a viral genome and virus-like RNAs identified in a plant transcriptomes, suggest that TGB evolution involved events of gene duplication and gene transfer between viruses. In addition, our analysis identified that plant RNA-seq data assembled into RNA virus-like contigs encode a significant variety of hydrophobic proteins. Functions of these orphan proteins are still obscure; however, some of them are obviously related to hydrophobic virion proteins of recently sequenced invertebrate (mostly insect) viruses, therefore supporting the current view on a common origin for many groups of plant and insect RNA-containing viruses. Moreover, these findings may suggest that the function of at least some orphan hydrophobic proteins is to provide plant viruses with the ability to infect insect hosts. In general, our observations emphasize that comparison of RNA virus sequences in a large variety of land plants and algae isolated geographically and ecologically may lead to experimental confirmation of previously purely speculative schemes of evolution of single genes, gene modules, and whole genomes.