Cell-to-Cell Movement of Potexviruses: Evidence for a Ribonucleoprotein Complex Involving the Coat Protein and First Triple Gene Block Protein

Abstract

The triple gene block proteins (TGBp1-3) and coat protein (CP) of potexviruses are required for cell-to-cell movement. Separate models have been proposed for intercellular movement of two of these viruses, transport of intact virions, or a ribonucleoprotein complex (RNP) comprising genomic RNA, TGBp1, and the CP. At issue therefore, is the form(s) in which RNA transport occurs and the roles of TGBp1-3 and the CP in movement. Evidence is presented that, based on microprojectile bombardment studies, TGBp1 and the CP, but not TGBp2 or TGBp3, are co-translocated between cells with viral RNA. In addition, cell-to-cell movement and encapsidation functions of the CP were shown to be separable, and the rate-limiting factor of potexvirus movement was shown not to be virion accumulation, but rather, the presence of TGBp1-3 and the CP in the infected cell. These findings are consistent with a common mode of transport for potexviruses, involving a non-virion RNP, and show that TGBp1 is the movement protein, whereas TGBp2 and TGBp3 are either involved in intracellular transport or interact with the cellular machinery/docking sites at the plasmodesmata.

Mutation of a chloroplast-targeting signal in Alternanthera mosaic virus TGB3 impairs cell-to-cell movement and eliminates long-distance virus movement

“…TGB2-and CP-defective viruses (AltMVDTGB2-eGFP, AltMVDCP-eGFP) infected only single epidermal cells without further movement, and diffusion of free GFP from infected cells was not detected (Fig. 4c, f), similar to studies with WClMV and PVX (Beck et al, 1991;Lough et al, 2000). Surprisingly, limited epidermal cell-to-cell movement of AltMVDTGB3-eGFP and AltMV.TGB3(VL17/18AR)-eGFP was detected, with infection foci expanding to 700 mm or less at 7 days p.i.…”

“…Deletion or mutation of TGB3 restricts replication of PVX (Lough et al, 2000), WClMV (Lough et al, 1998) and BaMV (Lin et al, 2006) to the inoculated cells, whereas systemic TGB3 complementation in transgenic plants enabled cell-to-cell and systemic movement (Lough et al, 1998). In the case of AltMVDTGB3-eGFP, infection spread beyond initially infected cells, but was restricted to the epidermis.…”

“…Several reports have addressed the contributions of TGB proteins to movement for WClMV (Lough et al, 1998), PVX (Ju et al, 2005;Krishnamurthy et al, 2003;Lough et al, 2000;Solovyev et al, 2000;Tamai & Meshi, 2001) and BaMV (Lin et al, 2006), and describe trans complementation of potexviral genomes with defects within the TGB through delivery of functional TGB constructs by using particle bombardment (Morozov et al, 1997), transgenic plants (Lough et al, 1998) or satellite RNA (Lin et al, 2006). Despite this body of work, 'a full understanding of the specific requirement of TGB3 and the functional interchangeability of TGB3 among different potexviruses for intercellular movement remains elusive' (Lin et al, 2006).…”

Arg-16 and Arg-21 in the N-terminal region of the triple-gene-block protein 1 of Bamboo mosaic virus are essential for virus movement

“…It is highly possible that the loss of the ATPase activity of TGBp1 leads to inactivation of its helicase activity, which is essential for unwinding the duplex region of the viral RNA (Kalinina et al, 2002). In addition, loss of RNAbinding activity of a TGBp1 mutant would probably negatively affect the ability of the mutant TGBp1 to form a streamlined nucleoprotein complex with the viral RNA suitable for movement (Lough et al, 1998(Lough et al, , 2000. Thus, the movement process of the virus, at least in the initial stage of transport, is probably hampered by the loss of each of these two activities.…”

“…Among the three TGB proteins, TGBp1 appears to exist in soluble and cytoplasmic inclusion forms. The soluble form of TGBp1 binds cooperatively to ssRNA, possesses ATPase (Rouleau et al, 1994;Wung et al, 1999) and RNA helicase activities (Kalinina et al, 2002), increases the SEL of plasmodesmata (Angell et al, 1996;Lough et al, 1998) and forms a ribonucleoprotein complex (Lough et al, 1998(Lough et al, , 2000. TGBp1 is also a gene silencing suppressor, which prevents spread of the gene silencing signal in Nicotiana benthamiana (Voinnet et al, 2000).…”