The product of ORF III in cauliflower mosaic virus interacts with the viral coat protein through its C-terminal proline rich domain

Cauliflower mosaic virus: Virus-host interactions and its uses in biotechnology and medicine

Cauliflower mosaic virus (CaMV) was the first discovered plant virus with genomic DNA that uses reverse transcriptase for replication. The CaMV 35S promoter is a constitutive promoter and thus, an attractive driver of gene expression in plant biotechnology. It is used in most transgenic crops to activate foreign genes which have been artificially inserted into the host plant. In the last century, producing food for the world's population while preserving the environment and human health is the main topic of agriculture. The damage caused by viral diseases has a significant negative economic impact on agriculture, and disease control is based on two strategies: immunization and prevention to contain virus spread, so correct identification of plant viruses is important for disease management. Here, we discuss CaMV from different aspects: taxonomy, structure and genome, host plants and symptoms, transmission and pathogenicity, prevention, control and application in biotechnology as well as in medicine. Also, we calculated the CAI index for three ORFs IV, V, and VI of the CaMV virus in host plants, the results of which can be used in the discussion of gene transfer or antibody production to identify the CaMV.

Complete genome sequence of pueraria virus A, a new member of the genus Caulimovirus

The complete genome sequence of a new caulimovirus in Pueraria montana was determined using high-throughput sequencing. The 7,572 nucleotide genome of pueraria virus A (PVA) contains genes that encode a movement protein, an aphid transmission factor, a virion-associated protein, a coat protein, a protease + reverse transcriptase + ribonuclease H, and a transactivator/viroplasmin protein, as well as two intergenic regions, which are all common features of members of the genus Caulimovirus. A sequence alignment revealed that the complete genome of PVA shares 66.82% nucleotide sequence identity with strawberry vein banding virus (GenBank accession no. KX249738.1). The results of phylogenetic analysis and the observation that the nucleotide sequence of the polymerase coding region differed by more than 20% indicated that PVA is a member of a new species the genus Caulimovirus, family Caulimoviridae.

Efficient immunodiagnosis of Citrus yellow mosaic virus using polyclonal antibodies with an expressed recombinant virion-associated protein

Citrus yellow mosaic virus (CYMV) is a member of genus Badnavirus of the family Caulimoviridae. It is the causal agent of citrus yellow mosaic disease in citrus and causes reduction in yield. As the virus is vegetative propagated by grafting, development of high-throughput diagnosis methods based on serological techniques is a prerequisite for production of healthy virus-free planting material. The current study describes the development of polyclonal antibodies raised in rabbits against purified recombinant virion-associated protein (rVAP) encoded by ORF-II of CYMV. The specificity of developed antiserum was evaluated in immunosorbent electron microscopy (ISEM), antigen-coated plate-enzyme linked immunosorbent assay (ACP-ELISA) and immunocapture PCR (IC-PCR). The antiserum specifically reacted up to a dilution of 1:2000 in ACP-ELISA for detection of CYMV-infected plants. The antiserum was validated by screening CYMV-infected plants maintained in the glass house through ACP-ELISA. To the best for our knowledge, this is the first report on production of polyclonal antiserum using recombinant virion-associated protein as fusion protein, which could be used for screening CYMV-infected plants by ELISA and IC-PCR. These immunodiagnostic methods can be effectively employed in routine indexing of citrus and in quarantine process.

A Sequence-Independent Strategy for Amplification and Characterisation of Episomal Badnavirus Sequences Reveals Three Previously Uncharacterised Yam Badnaviruses

Yam (Dioscorea spp.) plants are potentially hosts to a diverse range of badnavirus species (genus Badnavirus, family Caulimoviridae), but their detection is complicated by the existence of integrated badnavirus sequences in some yam genomes. To date, only two badnavirus genomes have been characterised, namely, Dioscorea bacilliform AL virus (DBALV) and Dioscorea bacilliform SN virus (DBSNV). A further 10 tentative species in yam have been described based on their partial reverse transcriptase (RT)-ribonuclease H (RNaseH) sequences, generically referred to here as Dioscorea bacilliform viruses (DBVs). Further characterisation of DBV species is necessary to determine which represent episomal viruses and which are only present as integrated badnavirus sequences in some yam genomes. In this study, a sequence-independent multiply-primed rolling circle amplification (RCA) method was evaluated for selective amplification of episomal DBV genomes. This resulted in the identification and characterisation of nine complete genomic sequences (7.4-7.7 kbp) of existing and previously undescribed DBV phylogenetic groups from Dioscorea alata and Dioscorea rotundata accessions. These new yam badnavirus genomes expand our understanding of the diversity and genomic organisation of DBVs, and assist the development of improved diagnostic tools. Our findings also suggest that mixed badnavirus infections occur relatively often in West African yam germplasm.

A model for intracellular movement of Cauliflower mosaic virus: the concept of the mobile virion factory

The genomes of many plant viruses have a coding capacity limited to <10 proteins, yet it is becoming increasingly clear that individual plant virus proteins may interact with several targets in the host for establishment of infection. As new functions are uncovered for individual viral proteins, virologists have realized that the apparent simplicity of the virus genome is an illusion that belies the true impact that plant viruses have on host physiology. In this review, we discuss our evolving understanding of the function of the P6 protein of Cauliflower mosaic virus (CaMV), a process that was initiated nearly 35 years ago when the CaMV P6 protein was first described as the 'major inclusion body protein' (IB) present in infected plants. P6 is now referred to in most articles as the transactivator (TAV)/viroplasmin protein, because the first viral function to be characterized for the Caulimovirus P6 protein beyond its role as an inclusion body protein (the viroplasmin) was its role in translational transactivation (the TAV function). This review will discuss the currently accepted functions for P6 and then present the evidence for an entirely new function for P6 in intracellular movement.

Plant pararetroviruses: interactions of cauliflower mosaic virus with plants and insects

Virion associated protein (VAP) binds to the icosahedral capsid of cauliflower mosaic virus (CaMV) - a plant pararetrovirus. The interactive coiled-coil domains of this protein can interact with the coiled-coils of either the movement protein or the aphid transmission factor, thereby mediating both cell-to-cell movement and aphid transmission. The host counters CaMV infection with two lines of defense: innate immunity and silencing. The viral protein 'transactivator/viroplasmin' (TAV) is recognized as an effector and either initiates the innate immunity reaction in a non-permissive host or interferes with it in a permissive host. As a silencing suppressor, TAV interferes with dicing of dsRNAs.

Cauliflower mosaic virus major inclusion body protein interacts with the aphid transmission factor, the virion-associated protein, and gene VII product

The Cauliflower mosaic virus (CaMV) gene VI product (P6) is a multifunctional protein essential for viral infection. In order to perform its various tasks, P6 interacts with both viral and host factors, as well as forming electron-dense cytoplasmic inclusion bodies. Here we investigate the interactions of P6 with three CaMV proteins: P2 (aphid transmission factor), P3 (virion-associated protein), and P7 (protein of unknown function). Based on yeast two-hybrid and maltose-binding protein pull-down experiments, P6 interacted with all three of these CaMV proteins. P2 helps to stabilize P6 inclusion bodies. Although the P2s from two CaMV isolates (W260 and CM1841) differ in the ability to stabilize inclusion bodies, both interacted similarly with P6. This suggests that inclusion body stability may not be dependent on the efficiency of P2-P6 interaction. However, neither P2 nor P3 interacted with P7 in yeast two-hybrid assays.

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.

Structural insights into the molecular mechanisms of cauliflower mosaic virus transmission by its insect vector

Cauliflower mosaic virus (CaMV) is transmitted from plant to plant through a seemingly simple interaction with insect vectors. This process involves an aphid receptor and two viral proteins, P2 and P3. P2 binds to both the aphid receptor and P3, itself tightly associated with the virus particle, with the ensemble forming a transmissible viral complex. Here, we describe the conformations of both unliganded CaMV P3 protein and its virion-associated form. X-ray crystallography revealed that the N-terminal domain of unliganded P3 is a tetrameric parallel coiled coil with a unique organization showing two successive four-stranded subdomains with opposite supercoiling handedness stabilized by a ring of interchain disulfide bridges. A structural model of virus-liganded P3 proteins, folding as an antiparallel coiled-coil network coating the virus surface, was derived from molecular modeling. Our results highlight the structural and biological versatility of this coiled-coil structure and provide new insights into the molecular mechanisms involved in CaMV acquisition and transmission by the insect vector.

A coiled-coil interaction mediates cauliflower mosaic virus cell-to-cell movement

The function of the virion-associated protein (VAP) of cauliflower mosaic virus (CaMV) has long been only poorly understood. VAP is associated with the virion but is dispensable for virus morphogenesis and replication. It mediates virus transmission by aphids through simultaneous interaction with both the aphid transmission factor and the virion. However, although insect transmission is not fundamental to CaMV survival, VAP is indispensable for spreading the virus infection within the host plant. We used a GST pull-down technique to demonstrate that VAP interacts with the viral movement protein through coiled-coil domains and surface plasmon resonance to measure the interaction kinetics. We mapped the movement protein coiled-coil to the C terminus of the protein and proved that it self-assembles as a trimer. Immunogold labeling/electron microscopy revealed that the VAP and viral movement protein colocalize on CaMV particles within plasmodesmata. These results highlight the multifunctional potential of the VAP protein conferred by its efficient coiled-coil interaction system and show a plant virus possessing a surface-exposed protein (VAP) mediating viral entry into host cells.

Structure of the Mature P3-virus Particle Complex of Cauliflower Mosaic Virus Revealed by Cryo-electron Microscopy

The cauliflower mosaic virus (CaMV) has an icosahedral capsid composed of the viral protein P4. The viral product P3 is a multifunctional protein closely associated with the virus particle within host cells. The best-characterized function of P3 is its implication in CaMV plant-to-plant transmission by aphid vectors, involving a P3-virion complex. In this transmission process, the viral protein P2 attaches to virion-bound P3, and creates a molecular bridge between the virus and a putative receptor in the aphid's stylets. Recently, the virion-bound P3 has been suggested to participate in cell-to-cell or long-distance movement of CaMV within the host plant. Thus, as new data accumulate, the importance of the P3-virion complex during the virus life-cycle is becoming more and more evident. To provide a first insight into the knowledge of the transmission process of the virus, we determined the 3D structures of native and P3-decorated virions by cryo-electron microscopy and computer image processing. By difference mapping and biochemical analysis, we show that P3 forms a network around the capsomers and we propose a structural model for the binding of P3 to CaMV capsid in which its C terminus is anchored deeply in the inner shell of the virion, while the N-terminal extremity is facing out of the CaMV capsid, forming dimers by coiled-coil interactions. Our results combined with existing data reinforce the hypothesis that this coiled-coil N-terminal region of P3 could coordinate several functions during the virus life-cycle, such as cell-to-cell movement and aphid-transmission.

Characterization of Cestrum yellow leaf curling virus: a new member of the family Caulimoviridae

Cestrum yellow leaf curling virus (CmYLCV) has been characterized as the aetiological agent of the Cestrum parqui mosaic disease. The virus genome was cloned and the clone was proven to be infectious to C. parqui. The presence of typical viroplasms in virus-infected plant tissue and the information obtained from the complete genomic sequence confirmed CmYLCV as a member of the Caulimoviridae family. All characteristic domains conserved in plant pararetroviruses were found in CmYLCV. Its genome is 8253 bp long and contains seven open reading frames (ORFs). Phylogenetic analysis of the relationships with other members of the Caulimoviridae revealed that CmYLCV is closely related to the Soybean chlorotic mottle virus (SbCMV)-like genus and particularly to SbCMV. However, in contrast to the other members of this genus, the primer-binding site is located in the intercistronic region following ORF Ib rather than within this ORF, and an ORF corresponding to ORF VII is missing.

Cauliflower mosaic virus: still in the news

SUMMARY Taxonomic relationship: Cauliflower mosaic virus (CaMV) is the type member of the Caulimovirus genus in the Caulimoviridae family, which comprises five other genera. CaMV replicates its DNA genome by reverse transcription of a pregenomic RNA and thus belongs to the pararetrovirus supergroup, which includes the Hepadnaviridae family infecting vertebrates. Physical properties: Virions are non-enveloped isometric particles, 53 nm in diameter (Fig. 1). They are constituted by 420 capsid protein subunits organized following T= 7 icosahedral symmetry (Cheng, R.H., Olson, N.H. and Baker, T.S. (1992) Cauliflower mosaic virus: a 420 subunit (T= 7), multilayer structure. Virology, 16, 655-668). The genome consists of a double-stranded circular DNA of approximately 8000 bp that is embedded in the inner surface of the capsid. Viral proteins: The CaMV genome encodes six proteins, a cell-to-cell movement protein (P1), two aphid transmission factors (P2 and P3), the precursor of the capsid proteins (P4), a polyprotein precursor of proteinase, reverse transcriptase and ribonuclease H (P5) and an inclusion body protein/translation transactivator (P6). Hosts: The host range of CaMV is limited to plants of the Cruciferae family, i.e. Brassicae species and Arabidopsis thaliana, but some viral strains can also infect solanaceous plants. In nature, CaMV is transmitted by aphids in a non-circulative manner.

The cauliflower mosaic virus virion-associated protein is dispensable for viral replication in single cells

Cauliflower mosaic virus (CaMV) open reading frame III (ORF III) codes for a virion-associated protein (Vap), which is one of two viral proteins essential for aphid transmission. However, unlike the aphid transmission factor encoded by CaMV ORF II, Vap is also essential for systemic infection, suggesting that it is a multifunctional protein. To elucidate the additional function or functions of Vap, we tested the replication of noninfectious ORF III-defective mutants in transfected turnip protoplasts. PCR and Western blot analyses revealed that CaMV replication had occurred with an efficiency similar to that of wild-type virus and without leading to reversions. Electron microscopic examination revealed that an ORF III frameshift mutant formed normally structured virions. These results demonstrate that Vap is dispensable for replication in single cells and is not essential for virion morphogenesis. Analysis of inoculated turnip leaves showed that the ORF III frameshift mutant does not cause any detectable local infection. These results are strongly indicative of a role for Vap in virus movement.

Tetramerization is a conserved feature of the virion-associated protein in plant pararetroviruses

All plant pararetroviruses belong to the Caulimoviridae family. This family contains six genera of viruses with different biological, serological, and molecular characteristics. Although some important mechanisms of viral replication and host infection are understood, much remains to be discovered about the many functions of the viral proteins. The focus of this study, the virion-associated protein (VAP), is conserved among all members of the group and contains a coiled-coil structure that has been shown to assemble as a tetramer in the case of cauliflower mosaic virus. We have used the yeast two-hybrid system to characterize self-association of the VAPs of four distinct plant pararetroviruses, each belonging to a different genus of Caulimoviridae. Chemical cross-linking confirmed that VAPs assemble into tetramers. Tetramerization is thus a common property of these proteins in plant pararetroviruses. The possible implications of this conserved feature for VAP function are discussed.