Conversion in the Requirement of Coat Protein in Cell-to-Cell Movement Mediated by the Cucumber Mosaic Virus Movement Protein

Abstract

Plant viruses have movement protein (MP) gene(s) essential for cell-to-cell movement in hosts.Cucumber mosaic virus (CMV) requires its own coat protein (CP) in addition to the MP for intercellular movement. Our present results using variants of both CMV and a chimeric Brome mosaic virus with the CMV MP gene revealed that CMV MP truncated in its C-terminal 33 amino acids has the ability to mediate viral movement independently of CP. Coexpression of the intact and truncated CMV MPs extremely reduced movement of the chimeric viruses, suggesting that these heterogeneous CMV MPs function antagonistically. Sequential deletion analyses of the CMV MP revealed that the dispensability of CP occurred when the C-terminal deletion ranged between 31 and 36 amino acids and that shorter deletion impaired the ability of the MP to promote viral movement. This is the first report that a region of MP determines the requirement of CP in cell-to-cell movement of a plant virus.Plant viruses encode proteins that control their movement from cell to cell. These proteins are called movement proteins (MPs) and interact with the normal symplastic connections between plant cells, the plasmodesmata, by modifying the plasmodesmal structure and function. Consequently, the highly regulated passage of small molecules through plasmodesmata is altered to allow the passage of large nucleoprotein complexes containing the viral genome (8, 28).Some viruses, including Tobacco mosaic virus (TMV) and Red clover necrotic mosaic virus, do not require the viral coat protein (CP) for cell-to-cell movement. The MPs of these viruses have a nucleic acid binding activity (10, 37), and an MP-RNA nucleoprotein complex is thought to pass through the modified plasmodesmata to adjacent uninfected cells (8,26). Other viruses do not move from cell to cell in the absence of viral CP. Cauliflower mosaic virus and Cowpea mosaic virus are known to move as virus-like particles through tubules that pass through plasmodesmata into neighboring cells. These tubules are composed of MP (40, 49). Nepo-, Tospo-, and Fabaviruses are also thought to move similarly with the tubulemediated mechanism. There are viruses that are considered to move as a nucleoprotein complex different from virus particles despite their requirement of viral CP for cell-to-cell movement. Cucumber mosaic virus (CMV) is competent to induce tubules protruding from infected protoplasts like the viruses that move as virus-like particles (6). Nevertheless, no such tubules have been found in planta by electron microscopy (3), and mutants incapable of virion formation move successfully from cell to cell (22,44,46). CMV is the type member of the genus Cucumovirus and is one of the most common plant viruses of substantial agricultural significance. CMV infects more than 1,000 species of plants, shrubs, and trees and both monocots and dicots (41). The genomic RNAs of CMV are designated as RNAs 1, 2, and 3, by diminishing size (39). All the RNAs have a cap structure at the 5Ј terminus. The 3Ј portion of all the RNAs is ...

The C-terminal 33 amino acids of the cucumber mosaic virus 3a protein affect virus movement, RNA binding and inhibition of infection and translation

“…The above results confirmed and extended the observations of Nagano et al (2001), using a different strain of CMV in two other plant species. Moreover, infection in either N. benthamiana (Fig.…”

“…There was considerable variation in the number of local lesions obtained for the same concentration of inocula in different experiments and on different half leaves (Fig. 6A), but this is typical of CMV (Habili & Francki, 1974;Lakshman & Gonsalves, 1985;Nagano et al, 2001;Rao & Francki, 1981). Nevertheless, the data show that at high ratios of MP : RNA, the infectivity of both types of MP-RNA complex was inhibited.…”

“…We sought first to confirm the results of Nagano et al (2001), using Fny-CMV lacking the C-terminal 33 aa of the 3a MP (referred to here as 3aDC33), since the results concerning the movement of the 3a MP lacking the Cterminal 43 aa differed between Fny-CMV and Y-CMV Nagano et al, 2001). Specifically, the gene encoding the 3a MP of Fny-CMV was modified in a biologically active cDNA clone of CMV RNA 3 to express a truncated MP, in which the C-terminal 33 aa were absent.…”

“…If this were the case, then the CP might be required to alter the stability of MP-RNA complexes, while the C-terminal region of the 3a MP would be expected to have some effect on the binding of MP to RNA. No evidence could be obtained for a direct interaction between the CP and MP in vitro (Nagano et al, 2001; D. Szilassy, T. Canto & P. Palukaitis, unpublished data). Therefore, in this study we tested the hypothesis that deletion of the C-terminal 33 aa of the MP might alter its binding to viral RNA, and hence CP would not be required to effect the stability of the mutant 3a protein-RNA complexes.…”

“…It was speculated that the CP might have a role in altering the conformation of the 3a MP, facilitating its interaction with plasmodesmata (Ryabov et al, 1999). It is conceivable that the C-terminal region of the 3a MP is involved in altering the RNA-binding affinity, because this region was shown to be adjacent to the region involved directly in RNA binding (Vaquero et al, 1997) and was found to be dispensable for infection Nagano et al, 1997Nagano et al, , 2001. If this were the case, then the CP might be required to alter the stability of MP-RNA complexes, while the C-terminal region of the 3a MP would be expected to have some effect on the binding of MP to RNA.…”