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

Five questions on the cell-to-cell movement of Orthotospoviruses

Plant viruses employ Movement proteins (MP) for their cell to cell spread through plasmodesmata (PD). MP modifies the PD and increases its size exclusion limit (SEL). However, the mechanism by which MPs are targeted to the PD is still unresolved and there is a lack of consensus owing to limited studies on their biochemical and structural characters. The non structural protein m (NSm) functions as the MP in Orthotospoviruses. Tospoviral NSm associate with ER membrane. They also form tubules in protoplasts. Groundnut bud necrosis virus (GBNV), a tospovirus, infects several crop plants throughout India and is economically very important. GBNV NSm associates with the membrane strongly via the C-terminal coiled-coil domain, modifies the membrane and causes vesicle fusion in vitro and remodels the ER network into vesicles in vivo. These vesicles are in contrast to the tubules formed by other related tospovirus in cells lacking cell wall. In this review, five important questions on the cell-to-cell movement of tospoviruses have been addressed and based on the various reports, a plausible model on the cell-to-cell movement of Orthotospoviruses is presented.

Reticulons 3 and 6 interact with viral movement proteins

Plant reticulon (RTN) proteins are capable of constricting membranes and are vital for creating and maintaining tubules in the endoplasmic reticulum (ER), making them prime candidates for the formation of the desmotubule in plasmodesmata (PD). RTN3 and RTN6 have previously been detected in an Arabidopsis PD proteome and have been shown to be present in primary PD at cytokinesis. It has been suggested that RTN proteins form protein complexes with proteins in the PD plasma membrane and desmotubule to stabilize the desmotubule constriction and regulate PD aperture. Viral movement proteins (vMPs) enable the transport of viruses through PD and can be ER-integral membrane proteins or interact with the ER. Some vMPs can themselves constrict ER membranes or localize to RTN-containing tubules; RTN proteins and vMPs could be functionally linked or potentially interact. Here we show that different vMPs are capable of interacting with RTN3 and RTN6 in a membrane yeast two-hybrid assay, coimmunoprecipitation, and Förster resonance energy transfer measured by donor excited-state fluorescence lifetime imaging microscopy. Furthermore, coexpression of the vMP CMV-3a and RTN3 results in either the vMP or the RTN changing subcellular localization and reduces the ability of CMV-3a to open PD, further indicating interactions between the two proteins.

Plasmodesmata-Involved Battle Against Pathogens and Potential Strategies for Strengthening Hosts

Plasmodesmata (PD) are membrane-lined pores that connect adjacent cells to mediate symplastic communication in plants. These intercellular channels enable cell-to-cell trafficking of various molecules essential for plant development and stress responses, but they can also be utilized by pathogens to facilitate their infection of hosts. Some pathogens or their effectors are able to spread through the PD by modifying their permeability. Yet plants have developed various corresponding defense mechanisms, including the regulation of PD to impede the spread of invading pathogens. In this review, we aim to illuminate the various roles of PD in the interactions between pathogens and plants during the infection process. We summarize the pathogenic infections involving PD and how the PD could be modified by pathogens or hosts. Furthermore, we propose several hypothesized and promising strategies for enhancing the disease resistance of host plants by the appropriate modulation of callose deposition and plasmodesmal permeability based on current knowledge.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

A new virus, classifiable in the family Tombusviridae, found infecting Solanum tuberosum in the UK

A novel virus was discovered in a freeze-dried collection held at SASA, UK, originating from potato (Solanum tuberosum) cv. Nadine. The complete sequence of the viral RNA was determined to be 3674 nucleotides in length encoding five predicted proteins. Based on the deduced genome organization and phylogenetic analysis, this virus represents a putative new member of the genus Alphanecrovirus, family Tombusviridae, most closely related to isolates of Olive mild mosaic virus. The virus was easily transmitted to indicator plants with symptoms that were slower to develop and less severe than those of related viruses. To distinguish this virus, the clearest symptom differences occurred with Nicotiana debneyi, Chenopodium amaranticolor and Ch. quinoa. The virus was detected with antisera to the related viruses tobacco necrosis virus A and tobacco necrosis virus D. The close association to the tobacco necrosis viruses would suggest this virus is not a new introduction to potato but in the past has been misidentified as one of these viruses. The virus isolate has been named potato necrosis virus.

An Update on the Intracellular and Intercellular Trafficking of Carmoviruses

Despite harboring the smallest genomes among plant RNA viruses, carmoviruses have emerged as an ideal model system for studying essential steps of the viral cycle including intracellular and intercellular trafficking. Two small movement proteins, formerly known as double gene block proteins (DGBp1 and DGBp2), have been involved in the movement throughout the plant of some members of carmovirus genera. DGBp1 RNA-binding capability was indispensable for cell-to-cell movement indicating that viral genomes must interact with DGBp1 to be transported. Further investigation on Melon necrotic spot virus (MNSV) DGBp1 subcellular localization and dynamics also supported this idea as this protein showed an actin-dependent movement along microfilaments and accumulated at the cellular periphery. Regarding DGBp2, subcellular localization studies showed that MNSV and Pelargonium flower break virus DGBp2s were inserted into the endoplasmic reticulum (ER) membrane but only MNSV DGBp2 trafficked to plasmodesmata (PD) via the Golgi apparatus through a COPII-dependent pathway. DGBp2 function is still unknown but its localization at PD was a requisite for an efficient cell-to-cell movement. It is also known that MNSV infection can induce a dramatic reorganization of mitochondria resulting in anomalous organelles containing viral RNAs. These putative viral factories were frequently found associated with the ER near the PD leading to the possibility that MNSV movement and replication could be spatially linked. Here, we update the current knowledge of the plant endomembrane system involvement in carmovirus intra- and intercellular movement and the tentative model proposed for MNSV transport within plant cells.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

The Tobacco mosaic virus movement protein associates with but does not integrate into biological membranes

Plant positive-strand RNA viruses require association with plant cell endomembranes for viral translation and replication, as well as for intra- and intercellular movement of the viral progeny. The membrane association and RNA binding of the Tobacco mosaic virus (TMV) movement protein (MP) are vital for orchestrating the macromolecular network required for virus movement. A previously proposed topological model suggests that TMV MP is an integral membrane protein with two putative α-helical transmembrane (TM) segments. Here we tested this model using an experimental system that measured the efficiency with which natural polypeptide segments were inserted into the ER membrane under conditions approximating the in vivo situation, as well as in planta. Our results demonstrated that the two hydrophobic regions (HRs) of TMV MP do not span biological membranes. We further found that mutations to alter the hydrophobicity of the first HR modified membrane association and precluded virus movement. We propose a topological model in which the TMV MP HRs intimately associate with the cellular membranes, allowing maximum exposure of the hydrophilic domains of the MP to the cytoplasmic cellular components.

IMPORTANCE

To facilitate plant viral infection and spread, viruses encode one or more movement proteins (MPs) that interact with ER membranes. The present work investigated the membrane association of the 30K MP of Tobacco mosaic virus (TMV), and the results challenge the previous topological model, which predicted that the TMV MP behaves as an integral membrane protein. The current data provide greatly needed clarification of the topological model and provide substantial evidence that TMV MP is membrane associated only at the cytoplasmic face of the membrane and that neither of its domains is integrated into the membrane or translocated into the lumen. Understanding the topology of MPs in the ER is vital for understanding the role of the ER in plant virus transport and for predicting interactions with host factors that mediate resistance to plant viruses.

BB0172, a Borrelia burgdorferi outer membrane protein that binds integrin α3β1

Lyme disease is a multisystemic disorder caused by Borrelia burgdorferi infection. Upon infection, some B. burgdorferi genes are upregulated, including members of the microbial surface components recognizing adhesive matrix molecule (MSCRAMM) protein family, which facilitate B. burgdorferi adherence to extracellular matrix components of the host. Comparative genome analysis has revealed a new family of B. burgdorferi proteins containing the von Willebrand factor A (vWFA) domain. In the present study, we characterized the expression and membrane association of the vWFA domain-containing protein BB0172 by using in vitro transcription/translation systems in the presence of microsomal membranes and with detergent phase separation assays. Our results showed evidence of BB0172 localization in the outer membrane, the orientation of the vWFA domain to the extracellular environment, and its function as a metal ion-dependent integrin-binding protein. This is the first report of a borrelial adhesin with a metal ion-dependent adhesion site (MIDAS) motif that is similar to those observed in eukaryotic integrins and has a similar function.

Polar/Ionizable residues in transmembrane segments: effects on helix-helix packing

The vast majority of membrane proteins are anchored to biological membranes through hydrophobic α-helices. Sequence analysis of high-resolution membrane protein structures show that ionizable amino acid residues are present in transmembrane (TM) helices, often with a functional and/or structural role. Here, using as scaffold the hydrophobic TM domain of the model membrane protein glycophorin A (GpA), we address the consequences of replacing specific residues by ionizable amino acids on TM helix insertion and packing, both in detergent micelles and in biological membranes. Our findings demonstrate that ionizable residues are stably inserted in hydrophobic environments, and tolerated in the dimerization process when oriented toward the lipid face, emphasizing the complexity of protein-lipid interactions in biological membranes.

Wrapping membranes around plant virus infection

Positive strand RNA viruses cause membrane modifications which are microenvironments or larger intracellular compartments, also called 'viroplasms'. These compartments serve to concentrate virus and host factors needed to produce new genomes. Forming these replication sites often involves virus induced membrane synthesis, changes in fatty acid metabolism, and viral recruitment of cellular factors to subcellular domains. Interacting viral and host factors builds the physical scaffold for replication complexes. Such virus induced changes are a visible cytopathology that has been used by plant and mammalian virologists to describe virus disease. This article describes key examples of membrane modifications that are essential for plant virus replication and intercellular transport.

Intracellular transport of plant viruses: finding the door out of the cell

Plant viruses are a class of plant pathogens that specialize in movement from cell to cell. As part of their arsenal for infection of plants, every virus encodes a movement protein (MP), a protein dedicated to enlarging the pore size of plasmodesmata (PD) and actively transporting the viral nucleic acid into the adjacent cell. As our knowledge of intercellular transport has increased, it has become apparent that viruses must also use an active mechanism to target the virus from their site of replication within the cell to the PD. Just as viruses are too large to fit through an unmodified plasmodesma, they are also too large to be freely diffused through the cytoplasm of the cell. Evidence has accumulated now for the involvement of other categories of viral proteins in intracellular movement in addition to the MP, including viral proteins originally associated with replication or gene expression. In this review, we will discuss the strategies that viruses use for intracellular movement from the replication site to the PD, in particular focusing on the role of host membranes for intracellular transport and the coordinated interactions between virus proteins within cells that are necessary for successful virus spread.

Membrane integration of poliovirus 2B viroporin

Virus infections can result in a variety of cellular injuries, and these often involve the permeabilization of host membranes by viral proteins of the viroporin family. Prototypical viroporin 2B is responsible for the alterations in host cell membrane permeability that take place in enterovirus-infected cells. 2B protein can be localized at the endoplasmic reticulum (ER) and the Golgi complex, inducing membrane remodeling and the blockade of glycoprotein trafficking. These findings suggest that 2B has the potential to integrate into the ER membrane, but specific information regarding its biogenesis and mechanism of membrane insertion is lacking. Here, we report experimental results of in vitro translation-glycosylation compatible with the translocon-mediated insertion of the 2B product into the ER membrane as a double-spanning integral membrane protein with an N-/C-terminal cytoplasmic orientation. A similar topology was found when 2B was synthesized in cultured cells. In addition, the in vitro translation of several truncated versions of the 2B protein suggests that the two hydrophobic regions cooperate to insert into the ER-derived microsomal membranes.

A membrane-associated movement protein of Pelargonium flower break virus shows RNA-binding activity and contains a biologically relevant leucine zipper-like motif

Two small viral proteins (DGBp1 and DGBp2) have been proposed to act in a concerted manner to aid intra- and intercellular trafficking of carmoviruses though the distribution of functions and mode of action of each protein partner are not yet clear. Here we have confirmed the requirement of the DGBps of Pelargonium flower break virus (PFBV), p7 and p12, for pathogen movement. Studies focused on p12 have shown that it associates to cellular membranes, which is in accordance to its hydrophobic profile and to that reported for several homologs. However, peculiarities that distinguish p12 from other DGBps2 have been found. Firstly, it contains a leucine zipper-like motif which is essential for virus infectivity in plants. Secondly, it has an unusually long and basic N-terminal region that confers RNA binding activity. The results suggest that PFBV p12 may differ mechanistically from related proteins and possible roles of PFBV DGBps are discussed.

N-glycosylation efficiency is determined by the distance to the C-terminus and the amino acid preceding an Asn-Ser-Thr sequon

N-glycosylation is the most common and versatile protein modification. In eukaryotic cells, this modification is catalyzed cotranslationally by the enzyme oligosaccharyltransferase, which targets the β-amide of the asparagine in an Asn-Xaa-Ser/Thr consensus sequon (where Xaa is any amino acid but proline) in nascent proteins as they enter the endoplasmic reticulum. Because modification of the glycosylation acceptor site on membrane proteins occurs in a compartment-specific manner, the presence of glycosylation is used to indicate membrane protein topology. Moreover, glycosylation sites can be added to gain topological information. In this study, we explored the determinants of N-glycosylation with the in vitro transcription/translation of a truncated model protein in the presence of microsomes and surveyed 25,488 glycoproteins, of which 2,533 glycosylation sites had been experimentally validated. We found that glycosylation efficiency was dependent on both the distance to the C-terminus and the nature of the amino acid that preceded the consensus sequon. These findings establish a broadly applicable method for membrane protein tagging in topological studies.

Intracellular transport of viruses and their components: utilizing the cytoskeleton and membrane highways

Plant viruses are obligate organisms that require host components for movement within and between cells. A mechanistic understanding of virus movement will allow the identification of new methods to control virus systemic spread and serve as a model system for understanding host macromolecule intra- and intercellular transport. Recent studies have moved beyond the identification of virus proteins involved in virus movement and their effect on plasmodesmal size exclusion limits to the analysis of their interactions with host components to allow movement within and between cells. It is clear that individual virus proteins and replication complexes associate with and, in some cases, traffic along the host cytoskeleton and membranes. Here, we review these recent findings, highlighting the diverse associations observed between these components and their trafficking capacity. Plant viruses operate individually, sometimes within virus species, to utilize unique interactions between their proteins or complexes and individual host cytoskeletal or membrane elements over time or space for their movement. However, there is not sufficient information for any plant virus to create a complete model of its intracellular movement; thus, more research is needed to achieve that goal.

Near infrared spectroscopy and aquaphotomics: Novel approach for rapid in vivo diagnosis of virus infected soybean

Near infrared spectroscopy with aquaphotomics as a novel approach was assessed for the diagnosis of soybean plants (Glycine max) infected with soybean mosaic virus (SMV) at latent symptomless stage of the disease. Near infrared (NIR) leaf spectra (in the range of 730-1025nm) acquired from soybean plants with and without the inoculation of SMV were used. Leaf samples from all plants were assayed with enzyme-linked immunosorbent assay (ELISA) to confirm the infection. Previously reported NIR band for water at 970nm and two new bands at 910nm and 936nm in the water specific region of NIR were found to be markedly sensitive to the SMV infection 2weeks prior to the appearance of visual symptoms on infected leaves. The spectral calibration model soft independent modeling of class analogy (SIMCA), predicted the disease with 91.6% sensitivity and 95.8% specificity when the second order derivative of the individual plant averaged spectra were used. The study shows the potential of NIR spectroscopy with its novel approach to elucidate latent biochemical and biophysical information of an infection as it allowed successful discrimination of SMV infected plant from healthy at the early symptomless stage of the disease.

Membrane insertion and biogenesis of the Turnip crinkle virus p9 movement protein

Plant viral infection and spread depends on the successful introduction of a virus into a cell of a compatible host, followed by replication and cell-to-cell transport. The movement proteins (MPs) p8 and p9 of Turnip crinkle virus are required for cell-to-cell movement of the virus. We have examined the membrane association of p9 and found that it is an integral membrane protein with a defined topology in the endoplasmic reticulum (ER) membrane. Furthermore, we have used a site-specific photo-cross-linking strategy to study the membrane integration of the protein at the initial stages of its biosynthetic process. This process is cotranslational and proceeds through the signal recognition particle and the translocon complex.

Cellular remodeling during plant virus infection

This review focuses on the extensive membrane and organelle rearrangements that have been observed in plant cells infected with RNA viruses. The modifications generally involve the formation of spherules, vesicles, and/or multivesicular bodies associated with various organelles such as the endoplasmic reticulum and peroxisomes. These virus-induced organelles house the viral RNA replication complex and are known as virus factories or viroplasms. Membrane and organelle alterations are attributed to the action of one or two viral proteins, which additionally act as a scaffold for the assembly of a large complex of proteins of both viral and host origin and viral RNA. Some virus factories have been shown to align with and traffic along microfilaments. In addition to viral RNA replication, the factories may be involved in other processes such as viral RNA translation and cell-to-cell virus transport. Confining the process of RNA replication to a specific location may also prevent the activation of certain host defense functions.

A self-interacting carmovirus movement protein plays a role in binding of viral RNA during the cell-to-cell movement and shows an actin cytoskeleton dependent location in cell periphery

The p7A of Melon necrotic spot virus has been described to be a RNA-binding movement protein essential for cell-to-cell movement but its role in this process is still unknown. Here, we found that primary and secondary structure elements on p7A appear to form a composite RNA-binding site required for both RNA interaction and cell-to-cell movement in plants indicating a direct correlation between these activities. Furthermore, we found that fluorescent-tagged p7A was distributed in punctuate structures at the cell periphery but also in motile cytoplasmic inclusion bodies which were in close association with the actin MFs and most likely generated by self-interacting p7A molecules as shown by BiFC assays. Consistently, the p7A subcellular distribution was revealed to be sensitive to the actin inhibitor, latrunculin B. The involvement of the RNA-binding capabilities and the subcellular location of the p7A in the intracellular and intercellular virus movement is discussed.

Plant virus cell-to-cell movement is not dependent on the transmembrane disposition of its movement protein

The cell-to-cell transport of plant viruses depends on one or more virus-encoded movement proteins (MPs). Some MPs are integral membrane proteins that interact with the membrane of the endoplasmic reticulum, but a detailed understanding of the interaction between MPs and biological membranes has been lacking. The cell-to-cell movement of the Prunus necrotic ringspot virus (PNRSV) is facilitated by a single MP of the 30K superfamily. Here, using a myriad of biochemical and biophysical approaches, we show that the PNRSV MP contains only one hydrophobic region (HR) that interacts with the membrane interface, as opposed to being a transmembrane protein. We also show that a proline residue located in the middle of the HR constrains the structural conformation of this region at the membrane interface, and its replacement precludes virus movement.

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.

Intracellular targeting of a hordeiviral membrane-spanning movement protein: sequence requirements and involvement of an unconventional mechanism

The membrane-spanning protein TGBp3 is one of the three movement proteins (MPs) of Poa semilatent virus. TGBp3 is thought to direct other viral MPs and genomic RNA to peripheral bodies located in close proximity to plasmodesmata. We used the ectopic expression of green fluorescent protein-fused TGBp3 in epidermal cells of Nicotiana benthamiana leaves to study the TGBp3 intracellular trafficking pathway. Treatment with inhibitors was used to reveal that the targeting of TGBp3 to plasmodesmata does not require a functional cytoskeleton or secretory system. In addition, the suppression of endoplasmic reticulum-derived vesicle formation by a dominant negative mutant of small GTPase Sar1 had no detectable effect on TGBp3 trafficking to peripheral bodies. Collectively, these results suggested the involvement of an unconventional pathway in the intracellular transport of TGBp3. The determinants of targeting to plasmodesmata were localized to the C-terminal region of TGBp3, including the conserved hydrophilic and terminal membrane-spanning domains.

Membrane insertion and topology of the p7B movement protein of Melon Necrotic Spot Virus (MNSV)

Cell-to-cell movement of the Melon Necrotic Spot Virus (MNSV) is controlled by two small proteins working in trans, an RNA-binding protein (p7A) and an integral membrane protein (p7B) separated by an amber stop codon. p7B contains a single hydrophobic region. Membrane integration of this region was observed when inserted into model proteins in the presence of microsomal membranes. Furthermore, we explored the topology and targeting mechanisms of full-length p7B. Here we present evidence that p7B integrates in vitro into the ER membrane cotranslationally and with an Nt-cytoplasmic/Ct-luminal orientation. The observed topology was monitored in vivo by fusing GFP to the Ct of p7B, enabling the overexpression in Escherichia coli cultures. Finally, the topology of a putative p14 movement protein was established by replacing the amber stop codon located between p7A and p7B.

Sec61α and TRAM are Sequentially Adjacent to a Nascent Viral Membrane Protein during its ER Integration

Co-translational integration of a nascent viral membrane protein into the endoplasmic reticulum membrane takes place via the translocon. We have been studying the early stages of the integration of a double-spanning plant viral movement protein to gain insights into how viral membrane proteins are transferred from the hydrophilic interior of the translocon into the hydrophobic environment of the bilayer, where the transmembrane (TM) segments of the viral proteins can diffuse freely. Photocrosslinking experiments reveal that this integration involves the sequential passage of the TM segments past Sec61alpha and translocating chain-associating membrane protein (TRAM). Each TM segment is first adjacent to Sec61alpha and subsequently is adjacent to TRAM. TRAM crosslinking extends for a long period during nascent chain biogenesis. In addition, the replacement of the first viral TM segment with a non-viral TM sequence still yields nascent chain photo-adducts with TRAM. TRAM therefore appears to be involved in viral membrane protein integration, and nascent chain recognition by TRAM does not appear to rely solely on the TM domains.

Mutations in the central domain of potato virus X TGBp2 eliminate granular vesicles and virus cell-to-cell trafficking

Most RNA viruses remodel the endomembrane network to promote virus replication, maturation, or egress. Rearrangement of cellular membranes is a crucial component of viral pathogenesis. The PVX TGBp2 protein induces vesicles of the granular type to bud from the endoplasmic reticulum network. Green fluorescent protein (GFP) was fused to the PVX TGBp2 coding sequence and inserted into the viral genome and into pRTL2 plasmids to study protein subcellular targeting in the presence and absence of virus infection. Mutations were introduced into the central domain of TGBp2, which contains a stretch of conserved amino acids. Deletion of a 10-amino-acid segment (m2 mutation) overlapping the segment of conserved residues eliminated the granular vesicle and inhibited virus movement. GFP-TGBp2m2 proteins accumulated in enlarged vesicles. Substitution of individual conserved residues in the same region similarly inhibited virus movement and caused the mutant GFP-TGBp2 fusion proteins to accumulate in enlarged vesicles. These results identify a novel element in the PVX TGBp2 protein which determines vesicle morphology. In addition, the data indicate that vesicles of the granular type induced by TGBp2 are necessary for PVX plasmodesmata transport.

RNA-binding properties and membrane insertion of Melon necrotic spot virus (MNSV) double gene block movement proteins

Advances in structural and biochemical properties of carmovirus movement proteins (MPs) have only been obtained in p7 and p9 from Carnation mottle virus (CarMV). Alignment of carmovirus MPs revealed a low conservation of amino acid identity but interestingly, similarity was elevated in regions associated with the functional secondary structure elements reported for CarMV which were conserved in all studied proteins. Nevertheless, some differential features in relation with CarMV MPs were identified in those from Melon necrotic virus (MNSV) (p7A and p7B). p7A was a soluble non-sequence specific RNA-binding protein, but unlike CarMV p7, its central region alone could not account for the RNA-binding properties of the entire protein. In fact, a 22-amino acid synthetic peptide whose sequence corresponds to this central region rendered an apparent dissociation constant (K(d)) significantly higher than that of the corresponding entire protein (9 mM vs. 0.83-25.7 microM). This p7A-derived peptide could be induced to fold into an alpha-helical structure as demonstrated for other carmovirus p7-like proteins. Additionally, in vitro fractionation of p7B transcription/translation mixtures in the presence of ER-derived microsomal membranes strongly suggested that p7B is an integral membrane protein. Both characteristics of these two small MPs forming the double gene block (DGB) of MNSV are discussed in the context of the intra- and intercellular movement of carmovirus.

Functional analysis of the five melon necrotic spot virus genome-encoded proteins

Function of the melon necrotic spot virus (MNSV) genome-encoded proteins (p29, p89, p7A, p7B and p42) has been studied. Protein-expression mutants of an infectious, full-length cDNA clone of a Spanish MNSV-Al isolate and a recombinant green fluorescent protein (GFP)-expressing virus were used in infection bioassays on melon plants. Results revealed that p29 and p89 are both essential for virus replication, whereas small proteins p7A and p7B are sufficient to support viral movement between adjacent cells operating in trans. It is also demonstrated that, in addition to its structural role as coat protein, p42 is an important factor controlling symptoms and is required for systemic transport. Moreover, both p42 and p7B, among all of the MNSV-encoded proteins, were able to delay RNA silencing in transient-expression assays on GFP-transgenic Nicotiana benthamiana plants. Finally, the presence of p42 also produced an enhancing effect on local spread similar to that of potyviral helper component proteinase (HC-Pro), probably due to its RNA silencing-suppression ability.

Domains of tobacco mosaic virus movement protein essential for its membrane association

A series of deletion mutants of tobacco mosaic virus movement protein (TMV-MP) was used to identify domains of the protein necessary for membrane association. A membrane fraction was isolated from tobacco BY-2 protoplasts infected with wild-type and mutant TMV that produce MP carrying a 3 aa deletion. Deletions that affected membrane association were clustered around the two major hydrophobic regions of MP that are predicted to be transmembrane. Deletions in other hydrophobic regions also reduced membrane association. In addition, a non-functional mutant of MP, in which one of the known phosphorylation sites was eliminated, was not associated with cellular membranes, while a functional second site revertant restored membrane association. This indicates that MP function requires interaction with membrane; however, membrane association was not sufficient for function. Results are consistent with the hypothesis that TMV-MP is an integral or tightly associated membrane protein that includes two hydrophobic transmembrane domains.

Subcellular localization and membrane topology of the melon ethylene receptor CmERS1

Ethylene receptors are multispanning membrane proteins that negatively regulate ethylene responses via the formation of a signaling complex with downstream elements. To better understand their biochemical functions, we investigated the membrane topology and subcellular localization of CmERS1, a melon (Cucumis melo) ethylene receptor that has three putative transmembrane domains at the N terminus. Analyses using membrane fractionation and green fluorescent protein imaging approaches indicate that CmERS1 is predominantly associated with the endoplasmic reticulum (ER) membrane. Detergent treatments of melon microsomes showed that the receptor protein is integrally bound to the ER membrane. A protease protection assay and N-glycosylation analysis were used to determine membrane topology. The results indicate that CmERS1 spans the membrane three times, with its N terminus facing the luminal space and the large C-terminal portion lying on the cytosolic side of the ER membrane. This orientation provides a platform for interaction with the cytosolic signaling elements. The three N-terminal transmembrane segments were found to function as topogenic sequences to determine the final topology. High conservation of these topogenic sequences in all ethylene receptor homologs identified thus far suggests that these proteins may share the same membrane topology.

Transient structural ordering of the RNA-binding domain of carnation mottle virus p7 movement protein modulates nucleic acid binding

Plant viral movement proteins bind to RNA and participate in the intra- and intercellular movement of the RNAs from plant viruses. However, the role and magnitude of the conformational changes associated with the formation of RNA-protein complexes are not yet defined. Here we describe studies on the relevance of a preexisting nascent alpha-helix at the C terminus of the RNA-binding domain of p7, a movement protein from carnation mottle virus, to RNA binding. Synthetic peptide analogues and single amino acid mutation at the RNA-binding domain of recombinant p7 protein were used to correlate the transient structural order in aqueous solution with RNA-binding potential.

Double-spanning plant viral movement protein integration into the endoplasmic reticulum membrane is signal recognition particle-dependent, translocon-mediated, and concerted

The current model for cell-to-cell movement of plant viruses holds that transport requires virus-encoded movement proteins that intimately associate with endoplasmic reticulum membranes. We have examined the early stages of the integration into endoplasmic reticulum membranes of a double-spanning viral movement protein using photocross-linking. We have discovered that this process is cotranslational and proceeds in a signal recognition particle-dependent manner. In addition, nascent chain photocross-linking to Sec61alpha and translocating chain-associated membrane protein reveal that viral membrane protein insertion takes place via the translocon, as with most eukaryotic membrane proteins, but that the two transmembrane segments of the viral protein leave the translocon and enter the lipid bilayer together.

Cell wall localization of Red clover necrotic mosaic virus movement protein is required for cell-to-cell movement

The Red clover necrotic mosaic virus movement protein (MP) is essential for cell-to-cell movement. Eight previously characterized alanine-scanning mutants of the MP were fused to the green fluorescent protein (GFP) and expressed from viral infectious transcripts. Inoculated plants were assayed for movement and intracellular accumulation of MP by confocal laser-scanning microscopy. A strict correlation was observed between the targeting to the cell wall (presumably the plasmodesmata) and cell-to-cell movement. Complementation of dysfunctional MP mutants with either wild-type MP or other null mutants in some cases rescued intracellular targeting and movement. The data suggest the presence of distinct domains in the MP for virus movement (near residues 27-31), complementarity (near residues 122 and 128), and intracellular localization (near residue 161). These data support a model of MP interacting cooperatively with itself to bind viral RNA, localize to and modify plasmodesmata and effect virus movement.

Membrane-insertion fragments of Bcl-xL, Bax, and Bid

Apoptosis regulators of the Bcl-2 family associate with intracellular membranes from mitochondria and the endoplasmic reticulum, where they perform their function. The activity of these proteins is related to the release of apoptogenic factors, sequestered in the mitochondria, to the cytoplasm, probably through the formation of ion and/or protein transport channels. Most of these proteins contain a C-terminal putative transmembrane (TM) fragment and a pair of hydrophobic alpha helices (alpha5-alpha6) similar to the membrane insertion fragments of the ion-channel domain of diphtheria toxin and colicins. Here, we report on the membrane-insertion properties of different segments from antiapoptotic Bcl-x(L) and proapoptotic Bax and Bid, that correspond to defined alpha helices in the structure of their soluble forms. According to prediction methods, there are only two putative TM fragments in Bcl-x(L) and Bax (the C-terminal alpha helix and alpha-helix 5) and one in activated tBid (alpha-helix 6). The rest of their sequence, including the second helix of the pore-forming domain, displays only weak hydrophobic peaks, which are below the prediction threshold. Subsequent analysis by glycosylation mapping of single alpha-helix segments in a model chimeric system confirms the above predictions and allows finding an extra TM fragment made of helix alpha1 of Bax. Surprisingly, the amphipathic helices alpha6 of Bcl-x(L) and Bax and alpha7 of Bid do insert in membranes only as part of the alpha5-alpha6 (Bcl-x(L) and Bax) or alpha6-alpha7 (Bid) hairpins but not when assayed individually. This behavior suggests a synergistic insertion and folding of the two helices of the hairpin that could be due to charge complementarity and additional stability provided by turn-inducing residues present at the interhelical region. Although these data come from chimeric systems, they show direct potentiality for acquiring a membrane inserted state. Thus, the above fragments should be considered for the definition of plausible models of the active, membrane-bound species of Bcl-2 proteins.

Potyviral 6K2 protein long-distance movement and symptom-induction functions are independent and host-specific

Deletion of various portions, or insertion of six histidine residues (6xHis) into various positions of the membrane-bound 6K2 protein (53 amino acids) of Potato virus A (PVA, genus Potyvirus), inhibited systemic infection in Nicotiana tabacum and N. benthamiana plants. However, a spontaneous mutation (Gly2Cys) that occurred in 6K2 adjacent to the 6xHis insert placed between Ser1 and Gly2 enabled systemic infection in a single N. benthamiana plant. No symptoms were observed, but virus titers were similar to the symptom-inducing wild-type (wt) PVA. N. tabacum plants were not systemically infected, albeit virus propagation was observed in inoculated protoplasts. The 6xHis/Gly2Cys mutant was reconstructed in vitro and serially propagated by mechanical inoculation in N. benthamiana. Following the third passage, a novel viral mutant appeared, lacking the last four His residues of the insert, as well as the Gly2 and Thr3 of 6K2. It infected N. tabacum plants systemically, and in the systemically infected N. benthamiana leaves, vein chlorosis and mild yellowing symptoms were observed, typical of wt PVA infection. The mutant virus accumulated to titers similar to wt PVA in both hosts. These results show that the PVA 6K2 protein affects viral long-distance movement and symptom induction independently and in a host-specific manner.

Movement protein of a closterovirus is a type III integral transmembrane protein localized to the endoplasmic reticulum

Cell-to-cell movement of beet yellows closterovirus requires four structural proteins and a 6-kDa protein (p6) that is a conventional, nonstructural movement protein. Here we demonstrate that either virus infection or p6 overexpression results in association of p6 with the rough endoplasmic reticulum. The p6 protein possesses a single-span, transmembrane, N-terminal domain and a hydrophilic, C-terminal domain that is localized on the cytoplasmic face of the endoplasmic reticulum. In the infected cells, p6 forms a disulfide bridge via a cysteine residue located near the protein's N terminus. Mutagenic analyses indicated that each of the p6 domains, as well as protein dimerization, is essential for p6 function in virus movement.

Influence of Proline Residues in Transmembrane Helix Packing

Integral membrane proteins often contain proline residues in their alpha-helical transmembrane (TM) fragments, which may strongly influence their folding and association. Pro-scanning mutagenesis of the helical domain of glycophorin A (GpA) showed that replacement of the residues located at the center abrogates helix packing while substitution of the residues forming the ending helical turns allows dimer formation. Synthetic TM peptides revealed that a point mutation of one of the residues of the dimerization motif (L75P) located at the N-terminal helical turn of the GpA TM fragment, adopts a secondary structure and oligomeric state similar to the wild-type sequence in detergents. In addition, both glycosylation mapping in biological membranes and molecular dynamics showed that the presence of a proline residue at the lipid/water interface has as an effect the extension of the helical end. Thus, helix packing can be an important factor that determines appearance of proline in TM helices. Membrane proteins might accumulate proline residues at the two ends of their TM segments in order to modulate the exposition of key amino acid residues at the interface for molecular recognition events while allowing stable association and native folding.

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.

Spatio-temporal analysis of the RNAs, coat and movement (p7) proteins of Carnation mottle virus in Chenopodium quinoa plants

Time-course and in situ hybridization analyses were used to study the spatio-temporal distribution of Carnation mottle virus (CarMV) in Chenopodium quinoa plants. Genomic and subgenomic RNAs of plus polarity accumulated linearly with time, whereas the corresponding minus strands reached a peak during infection in inoculated leaves. Analyses of serial tissue sections showed that plus polarity strands were localized throughout the infection area, whereas minus strands were localized at the borders of the chlorotic lesions. The accumulation kinetics of the coat protein (CP) and the p7 movement protein (MP) as well as their subcellular localization were also studied. Unlike most MPs, CarMV p7 showed a non-transient expression and a mainly cytosolic location. However, as infection progressed the presence of p7 in the cell wall fraction increased significantly. These results are discussed on the basis of a recent model proposed for the mechanism of cell-to-cell movement operating in the genus Carmovirus.