Effects of movement protein mutations on the formation of tubules in plant protoplasts expressing a fusion between the green fluorescent protein and Cauliflower mosaic virus movement protein

The ORF3-encoded proteins of vitiviruses GVA and GVB induce tubule-like and punctate structures during virus infection and localize to the plasmodesmata

The genomic RNA of vitiviruses contains 5 open reading frames (ORF). ORF3 encodes a protein to which the function of a movement protein (MP) was assigned, based on sequence homology with other viral proteins. The aim of the research described in this paper was to gain further insight in distribution profile of the ORF3 product encoded by the vitiviruses Grapevine virus A (GVA) and Grapevine virus B (GVB). Expression of the GVA MP-GFP fusion protein via the virus genome in Nicotiana benthamiana leaves resulted in the formation of irregular spots and fibrous network structures on the outermost periphery of epidermal cells. Expression of GVA MP-GFP and GVB MP-GFP was involved in the formation of the tubule-like and punctate structures on the periphery of N. benthamiana and Vitis vinifera protoplasts. Co-expression of the GVA MP-GFP and GVA MP-RFP in protoplasts resulted in co-localization of these proteins into the same punctate structures, indicating that the MP is not accumulated randomly onto the cell surface, but targeted to particular sites at the cell periphery, where punctate and tubule-like structures are likely formed. With the use of cytoskeleton and secretory pathway inhibitors, we showed that the cytoskeletal elements are not likely to be involved in targeting of the MP-GFP to the punctate cellular structures. In addition to MP, a functional coat protein was found to be essential for virus spread within inoculated leaves.

Identification of domains of the Tomato spotted wilt virus NSm protein involved in tubule formation, movement and symptomatology

Deletion and alanine-substitution mutants of the Tomato spotted wilt virus NSm protein were generated to identify domains involved in tubule formation, movement and symptomatology using a heterologous Tobacco mosaic virus expression system. Two regions of NSm, G(19)-S(159) and G(209)-V(283), were required for both tubule formation in protoplasts and cell-to-cell movement in plants, indicating a correlation between these activities. Three amino acid groups, D(154), EYKK(205-208) and EEEEE(284-288) were linked with long-distance movement in Nicotiana benthamiana. EEEEE(284-288) was essential for NSm-mediated long-distance movement, whereas D(154) was essential for tubule formation and cell-to-cell movement; indicating separate genetic controls for cell-to-cell and long-distance movement. The region I(57)-N(100) was identified as the determinant of foliar necrosis in Nicotiana benthamiana, and mutagenesis of HH(93-94) greatly reduced necrosis. These findings are likely applicable to other tospovirus species, especially those within the 'New World' group as NSm sequences are highly conserved.

Cauliflower mosaic virus gene VI product N-terminus contains regions involved in resistance-breakage, self-association and interactions with movement protein

Cauliflower mosaic virus (CaMV) gene VI encodes a multifunctional protein (P6) involved in the translation of viral RNA, the formation of inclusion bodies, and the determination of host range. Arabidopsis thaliana ecotype Tsu-0 prevents the systemic spread of most CaMV isolates, including CM1841. However, CaMV isolate W260 overcomes this resistance. In this paper, the N-terminal 110 amino acids of P6 (termed D1) were identified as the resistance-breaking region. D1 also bound full-length P6. Furthermore, binding of W260 D1 to P6 induced higher beta-galactosidase activity and better leucine-independent growth in the yeast two-hybrid system than its CM1841 counterpart. Thus, W260 may evade Tsu-0 resistance by mediating P6 self-association in a manner different from that of CM1841. Because Tsu-0 resistance prevents virus movement, interaction of P6 with P1 (CaMV movement protein) was investigated. Both yeast two-hybrid analyses and maltose-binding protein pull-down experiments show that P6 interacts with P1. Although neither half of P1 interacts with P6, the N-terminus of P6 binds P1. Interestingly, D1 by itself does not interact with P1, indicating that different portions of the P6 N-terminus are involved in different activities. The P1-P6 interactions suggest a role for P6 in virus transport, possibly by regulating P1 tubule formation or the assembly of movement complexes.

Plant virus transport: motions of functional equivalence

Plant virus cell-to-cell movement and subsequent systemic transport are governed by a series of mechanisms involving various virus and plant factors. Specialized virus encoded movement proteins (MPs) control the cell-to-cell transport of viral nucleoprotein complexes through plasmodesmata. MPs of different viruses have diverse properties and each interacts with specific host factors that also have a range of functions. Most viruses are then transported via the phloem as either nucleoprotein complexes or virions, with contributions from host and virus proteins. Some virus proteins contribute to the establishment and maintenance of systemic infection by inhibiting RNA silencing-mediated degradation of viral RNA. In spite of all the different movement strategies and the viral and host components, there are possible functional commonalities in virus-host interactions that govern viral spread through plants.

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.

Identification of distinct steps during tubule formation by the movement protein of Cowpea mosaic virus

The movement protein (MP) of Cowpea mosaic virus (CPMV) forms tubules through plasmodesmata in infected plants thus enabling virus particles to move from cell to cell. Localization studies of mutant MPs fused to GFP in protoplasts and plants identified several functional domains within the MP that are involved in distinct steps during tubule formation. Coinoculation experiments and the observation that one of the C-terminal deletion mutants accumulated uniformly in the plasma membrane suggest that dimeric or multimeric MP is first targeted to the plasma membrane. At the plasma membrane the MP quickly accumulates in peripheral punctuate spots, from which tubule formation is initiated. One of the mutant MPs formed tubules containing virus particles on protoplasts, but could not support cell-to-cell movement in plants. The observations that this mutant MP accumulated to a higher level in the cell than wt MP and did not accumulate in the cell wall opposite infected cells suggest that breakdown or disassembly of tubules in neighbouring, uninfected cells is required for cell-to-cell movement.

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.