Movement of potexviruses requires species-specific interactions among the cognate triple gene block proteins, as revealed by a trans-complementation assay based on the bamboo mosaic virus satellite RNA-mediated expression system

Abstract

The intra-and intercellular transport of potexviruses require interactions among viral RNA, coat protein and elements of the triple gene block proteins (TGBps). In this study, the requirement of bamboo mosaic virus (BaMV) TGBps for movement functions and the compatibilities with those of two potexviruses, Potato virus X (PVX) and Foxtail mosaic virus (FoMV), were examined using a satellite RNA-mediated trans-complementation assay system. Single or multiple TGBps of BaMV, PVX and FoMV were expressed from BaMV satellite RNA (satBaMV RNA) vectors to complement the functions of green fluorescent protein-tagged, movement-defective BaMV with mutation(s) in the matching gene(s). It was found that individual BaMV TGBps expressed from the satellite vector could function normally in trans, whereas bi-gene BaMV TGBp constructs in which the expression of TGBp3 might be impaired and individual TGBp genes from PVX or FoMV could not complement the movement functions of the defective helper viruses. Furthermore, alterations of the ratio among TGBps by ectopic expression of individual components of TGBps from satBaMV RNA vectors did not affect the cell-to-cell movement capabilities of wild-type BaMV significantly. The results indicate that species-specific interactions among movement proteins are obligatory for the cell-to-cell movement of BaMV and possibly other potexviruses. INTRODUCTIONSuccessful establishment of systemic infection by plant viruses requires that the viruses spread from the initially infected cells to adjacent cells and subsequently into and out of the vascular systems. Virus-encoded proteins are essential for cell-to-cell movement of most plant viruses through plasmodesmata. Based on the requirement of coat proteins (CPs), formation of virions and the involvement of tubule structures, at least three different models for cell-to-cell movement of plant viruses have been proposed (for recent reviews, see Morozov & Solovyev, 2003;Lucas & Lee, 2004;Verchot-Lubicz, 2005).For viruses that do not require virion formation for cellular movement, many have been found to contain three overlapping open reading frames (ORFs) termed the triple gene block (TGB), which encode three movement proteins (MPs) designated TGBp1-3 (Mushegian & Koonin, 1993;Morozov & Solovyev, 2003). According to primary structure comparisons and involvement of the CP in cell-to-cell movement, the TGB-containing viruses have been classified further into hordei-like and potex-like classes (Solovyev et al., 1996;Wong et al., 1998;Morozov & Solovyev, 2003).For hordei-like viruses, TGBp1 contains a stretch of positively charged amino acids in the N-terminal portion and the CP is not required for cell-to-cell movement (Petty & Jackson, 1990;Schmitt et al., 1992;Sit & Abouhaidar, 1993;Solovyev et al., 1996;Herzog et al., 1998;Wong et al., 1998;Erhardt et al., 1999;McGeachy & Barker, 2000). Cell-tocell movement of hordei-like viruses requires highly specific interactions among the cognate TGBps. It has been shown that the accumulation level of TGBp1 of Beet...

Satellite RNAs and Satellite Viruses of Plants

“…Being a messenger-type satRNA, satBaMV RNA encodes a P20 gene, which, unlike all other large messenger-type satRNAs, was shown to be nonessential for the replication of satRNA. Therefore, P20 was replaced with a chloramphenicol acetyltransferase (CAT) gene, which resulted in high expression of the CAT protein in infected Chenopodium quinoa and demonstrated that satBaMV RNA could be useful as a satellite-based expression vector.As vectors for functional studies:satBaMV RNA has been used for the expression of individual TGBps in a complementary experiment for study of cell-to-cell movement of BaMV [123]. To dissect the functions of individual TGBps, single or multiple TGBps of BaMV, PVX and Foxtail mosaic virus were expressed by use of satBaMV RNA-based vectors to complement the functions of green fluorescent protein-tagged, movement-defective BaMV with mutation(s) in the matching gene(s).…”

Understanding the intracellular trafficking and intercellular transport of potexviruses in their host plants

“…The mutations of the chloroplast-targeting signal in AltMV TGB3 protein resulted in very limited cell-to-cell movement, suggesting that this signal is required for the systemic movement of AltMV (Lim et al, 2010a). Together, these data indicate that the mechanisms for viral movement may differ among potexviruses (Lin et al, 2006). …”

Function and Structural Organization of the Replication Protein of Bamboo mosaic virus

“…As many positive strand RNA viruses, BaMV must encode its own enzymes for replication/transcription and 5′ cap formation because it replicates only in the cytoplasm. ORF2, 3 and 4 are overlapped, often referred to as the triple gene block (TGB), and their translated proteins, TGBp1, TGBp2, and TGBp3, respectively, are indispensable for BaMV movement in plants (Lin et al, 2004, 2006). In-depth discussions about the functions of each of the TGB proteins of PVX in the intracellular trafficking and intercellular transport can be referred in a couple of recent reviews (Verchot-Lubicz et al, 2010; Park et al, 2014).…”