The role of the other TGBp1 activities (NTP binding, NTPase and RNA helicase) in cell-to-cell movement remains unclear. According to recent views, the cell-to-cell movement of viral genomes is an energy-dependent process (Carrington et al., 1996; Ghoshroy et al., 1997). There are at least two steps of MP-mediated translocation of nucleic acids where such ATP/NTP-dependent events may be involved: (i) intracellular transport of MP and virus-specific RNP to PD or to a region in the vicinity of PD and (ii) trafficking of proteins and RNP through PD involving both protein/RNA unfolding and microchannel dilation (Fig. 6) (Ghoshroy et al., 1997; Lazarowitz & Beachy, 1999; Lucas, 1999; Kragler et al., 1998; Tzfira et al., 2000; Haywood et al., 2002; Heinlein, 2002b; Roberts & Oparka, 2003).

No ability to modify PD and move cell to cell has been reported for individually expressed hordei-like TGBp1, suggesting that this protein is incapable of intracellular trafficking to PD by itself and depends on TGBp2 and TGBp3 for this function (discussed below). Conversely, potexviral TGBp1 is capable of interacting with PD and increasing PD SEL (Angell et al., 1996; Lough et al., 1998, 2000; Malcuit et al., 1999; Morozov et al., 1999; Yang et al., 2000). Mutations influencing the potexviral TGBp1 helicase motif I (disabling both helicase and NTPase activities of the protein) block the protein's ability to increase PD SEL, whereas mutations of motif VI (disabling the helicase but not NTPase activity) (Kalinina et al., 2002) affect neither the increase in TGBp1-induced SEL nor TGBp1 transport to the cell periphery (Lough et al., 1998; Morozov et al., 1999). Thus, ATP binding and/or hydrolysis rather than helicase activity per se are involved in PD dilation by potex-like TGBp1.

Potexviral TGBp1 is co-translocated with the CP and virus genomic RNA during virus movement through PD (Morozov et al., 1997; Lough et al., 1998, 2000, 2001). It is natural to propose that TGBp1 helicases couple RNA unwinding and translocation through PD microchannels. The discovery of a specialized RNA-translocating NTPase (P4 protein) that participates in RNA transfer and packaging into bacteriophage 6 virions (Juuti et al., 1998) further supports such a proposal.

TGBp1 of Potato virus X (PVX) has been reported to interact with one end of the virion and to induce energy-dependent conformational changes of virus particles in vitro (Atabekov et al., 2000). Soultanas & Wigley (2001) have suggested that energy generated by ATP hydrolysis can be used by helicases not only for separation of base-paired regions but also for displacement of other proteins from nucleic acids. Thus, by analogy with some SF-II cell and viral helicases ('RNPases') involved in disassembly and remodelling of RNP complexes (Tseng et al., 1998; Jankowsky et al., 2001; Schwer, 2001), TGBp1 could potentiate cell-to-cell transport of virions or movement-related RNPs through PD by disrupting both RNA–protein interactions and intramolecular RNA base-pairing. In this case, in addition to trafficking of viral genomes through PD, hordei-like TGBp1 may mediate a displacement of cell proteins prior to genomic RNA transport to PD. During viral genome replication, nascent RNA molecules are most likely packaged into RNPs by host cytosolic proteins which rapidly coat newly synthesized cellular and virus-specific mRNAs and facilitate their efficient translation and further turnover (Mitchell & Tollervey, 2001; Pilipenko et al., 2001; Fedoroff, 2002). Re-packaging of such RNPs by replacement of cell proteins with hordei-like TGBp1 may result in formation of movement-competent non-translatable RNPs (Karpova et al., 1999).

TGBp1 as a factor of whole plant infection

CPs are dispensable for systemic infection of hordeiviruses and pomoviruses, which are thus believed to enter the phloem and traffick along the sieve tubes as a non-virion RNP containing genomic RNA and TGBp1 (Brakke et al., 1988; Petty & Jackson, 1990; Donald et al., 1997; McGeachy & Barker, 2000; Lawrence & Jackson, 2001b). In hordeiviral TGBp1, two positively charged motifs responsible for RNA-binding activity of the N-terminal extension domain (Fig. 2) have been found to be dispensable for virus transport from cell to cell but, nevertheless, necessary for long-distance virus movement. Therefore, the RNA-binding activities of the helicase and extension domains of hordei-like TGBp1 could be specialized in either cell-to-cell or long-distance transport, respectively (Kalinina et al., 2001). However, the presence of the N-terminal extension in TGBp1 and its compatibility with the helicase domain are required to support both cell-to-cell and long-distance movement in hordeiviruses (Donald et al., 1995, 1997; Solovyev et al., 1999).

In contrast to hordeiviruses, a functional CP is required for potexvirus long-distance movement (Santa Cruz et al., 1998). Potexviral TGBp1 is co-transported along the phloem sieve tube together with virions (or a non-virion CP-containing RNP). Because the sieve element contains no translational apparatus, this complex with TGBp1 must include all functional activities required for exiting from the sieve tube to the companion cells (Santa Cruz et al., 1998; Lough et al., 2001).

The ability of the virus to establish systemic infection in plants depends largely on the efficacy of plant defence response against infection versus the potential of the virus to escape or counter this defence (Carrington et al., 2001; Dangl & Jones, 2001; Vance & Vaucheret, 2001). One of the defence mechanisms in plants is gene silencing, which is mediated by sequence-specific degradation of viral RNAs in the cytoplasm (Baulcombe, 2002; Voinnet, 2001; Waterhouse et al., 2001). However, TGB-containing viruses, like many plant viruses, have evolved special mechanisms to suppress RNA silencing (Voinnet et al., 2000; Dunoyer et al., 2002a; Yelina et al., 2002). In particular, potexviral TGBp1 has been shown to suppress production or activity of the mobile silencing signal (Voinnet et al., 2000). Importantly, some of the sequences of TGBp1 involved in suppressing the silencing activity do not affect cell-to-cell movement (D. Baulcombe, The Sainsbury Laboratory, John Innes Centre, Norwich, UK, personal communication).

Another virus resistance mechanism is mediated by a large family of R gene-encoded proteins that recognize pathogen-encoded elicitors and trigger defence pathways, such as programmed cell death or hypersensitive response (Dangl & Jones, 2001; Holt et al., 2003). PVX TGBp1 has been shown recently to be such an elicitor recognized by the Nb gene-mediated resistance system in potatos. The TGBp1 region required for activation of the Nb response is located in the N terminus upstream of the helicase motif I (Malcuit et al., 1999).

Subcellular distribution of TGBp2 and TGBp3 and virus movement

In agreement with sequence analysis (Fig. 4) and in vitro studies predicting that TGBp2 and TGBp3 are integral membrane proteins (Morozov et al., 1987, 1990, 1991a), cell fractionation of plant tissues expressing these proteins demonstrates predominant association of both proteins with the P1 and P30 membranous fractions as well as with the cell wall (CW) fraction (Niesbach-Klösgen et al., 1990; Donald et al., 1993; Hefferon et al., 1997; Cowan et al., 2002; Gorshkova et al., 2003).

Further studies of subcellular localization of TGBp2 and TGBp3 employed their GFP fusions expressed in plant cells by a variety of techniques. Note that experiments on transient expression of MPs should be interpreted cautiously, since MPs expressed from vectors are likely produced in much larger quantities and in a non-regulated fashion compared to a virus infection. Also, methods of delivery of expression vectors such as high-pressure biolistic bombardment may perturb cell status (Crawford & Zambryski, 2001) and functional properties of proteins may be hindered by the fused fluorescent protein sequences (Thomas & Maule, 2000; Brandizzi et al., 2002a). Nevertheless, transient expression of GFP fusions is widely used to study MPs in live cells and the results of such studies do not usually contradict data obtained by other methods (Lazarowitz, 1999; Lazarowitz & Beachy, 1999; Brandizzi et al., 2002a; Heinlein, 2002a).

When transiently expressed in individual epidermal cells of Nicotiana benthamiana, GFP-tagged TGBp2s of Poa semilatent virus (PSLV) and Potato mop-top virus (PMTV) are localized to elements of the cell endomembrane system, mainly tubules of the cortical ER network (Solovyev et al., 2000; Cowan et al., 2002; Zamyatnin et al., 2003). In cells with higher levels of PSLV TGBp2 expression, a part of the protein is also associated with motile vesicles (Solovyev et al., 2000) that resemble plant Golgi stacks (Brandizzi et al., 2002b). The Golgi-like mobile vesicles have been found to contain most of the transiently expressed TGBp2 of PVX (our unpublished data). Subcellular localization of TGBp2 to the ER and Golgi is determined by the hydrophobic protein segments and, in particular, the length of the C-terminal hydrophobic segment (Solovyev et al., 2000; Zamyatnin et al., 2002; our unpublished data), which seems to act as a retrieval signal in Golgi-to-ER recycling by the host receptor Rer1 (Sato et al., 1999, 2001).

Transiently expressed individual GFP–TGBp3 is found in membrane bodies of different sizes located at the cell periphery in close association with the CW (Solovyev et al., 2000; Cowan et al., 2002). TGBp3 expression has little effect on the basic structure of the ER in plant cells. However, TGBp3 synthesis results in the formation of new TGBp3-containing ER structures (peripheral bodies) connected with the cortical ER network (Zamyatnin et al., 2002; Gorshkova et al., 2003). The size of these bodies correlates with the amount of TGBp3 protein produced in a given cell (Zamyatnin et al., 2002). Thus, TGBp3 is able to induce formation (or proliferation) of a specific subdomain of the cortical ER.

A clue to the subcellular location of the TGBp3-containing bodies comes from fluorescent microscopy of leaves infected with a PMTV GFP–TGBp3-expressing virus vector (Cowan et al., 2002) and transgenic plants expressing PSLV GFP–TGBp3 (Gorshkova et al., 2003). When the protein is expressed in adjacent cells, the peripheral bodies formed in the two neighbouring cells are opposite to each other. The structural link that governs the formation of such twin bodies could be provided by PD (Cowan et al., 2002) and specific staining of PD-associated callose confirms the localization of TGBp3-containing bodies alongside of PD (Gorshkova et al., 2003).

Targeting of PSLV TGBp3 depends on a specific signal consisting of two parts, of which a central hydrophilic region conserved in all hordei-like TGBp3 (Fig. 4a) seems to be an oligomerization sequence (Cowan et al., 2002; Gorshkova et al., 2003; our unpublished data). Another part of the specific PD targeting signal of TGBp3 is located in the C-terminal transmembrane segment, which resembles a hydrophobic membrane-embedded segment that participates in forming a protein trafficking signal of mastrevirus MP (Kotlizky et al., 2000). Similarly, localization of PVX TGBp3 to peripheral bodies depends on the only protein transmembrane segment (our unpublished data). Mutations in the hordeivirus TGBp3 signal result in localization of the protein in a 'granular network' of tiny bodies visible as a reticulate pattern as if they are formed on the surface of cortical ER tubules (Solovyev et al., 2000; our unpublished data). Thus, it appears that mutations in either part of the bipartite signal permit protein segregation to ER-exit sites but cannot mediate further protein trafficking to the final destination at PD-associated compartments (Fig. 6).

Co-targeting of TGBp2 and TGBp3

In the presence of TGBp3, TGBp2 is re-targeted to peripheral bodies that resemble the structures observed in cells expressing TGBp3 (Solovyev et al., 2000). Perfect co-localization of co-expressed PSLV TGBp2 and TGBp3 in peripheral bodies has been demonstrated, confirming that the TGBp3 protein directs subcellular targeting of TGBp2 from the ER network to sites of TGBp3 location (Zamyatnin et al., 2002). PVX TGBp3 also targets PVX TGBp2 to peripheral bodies, showing that TGBp3-directed trafficking of TGBp2 occurs in both hordei- and potex-like TGBs (Solovyev et al., 2000).

Protein–protein interactions that result in the formation of TGBp2–TGBp3 complexes could be the mechanism by which the two proteins are co-targeted to peripheral bodies. However, co-expression of TGBp2 and TGBp3 mutants failed to identify regions potentially responsible for the interaction between TGBp2 and TGBp3 molecules (Solovyev et al., 2000). Further evidence for a sequence-independent co-targeting mechanism was obtained in experiments on co-expression of heterologous TGB proteins. Indeed, in spite of the absence of sequence similarity of TGBp3 proteins in hordei- and potex-like TGBs, PVX TGBp3 can target PSLV TGBp2 to peripheral bodies and PSLV TGBp3 can, likewise, target PVX TGBp2 (Solovyev et al., 2000). Moreover, PSLV TGBp3 can also target totally unrelated membrane-bound MPs, such as the C4 protein of Faba bean necrotic yellows virus (genus Nanovirus) and the 6K protein of Beet yellows virus (genus Closterovirus), to peripheral bodies (Zamyatnin et al., 2002). This suggests that a sequence-specific interaction of TGBp2 and TGBp3 molecules is unlikely to be involved in TGBp3-directed targeting of TGBp2 (Solovyev et al., 2000; Zamyatnin et al., 2002). Nevertheless, PMTV TGBp3 interacts physically with the homologous TGBp2 in a yeast two-hybrid system (Cowan et al., 2002). Presumably, this interaction may depend on the residue composition of hydrophobic segments, enabling side chain interaction between membrane-embedded helices of proteins (Scholze et al., 2002; Sjöberg & Garoff, 2003).

Note that TGBp3 apparently does not traffick any integral membrane protein, since GFP derivatives statically retained in ER membranes by synthetic hydrophobic anchors are not targeted by TGBp3 (Zamyatnin et al., 2002). Thus, it appears that some functional feature(s), rather then a specific sequence, is responsible for efficient trafficking of membrane proteins by TGBp3. Such features could include specific localization and dynamics of the membrane proteins in the cell endomembrane system, including their ability to cycle between the ER and the Golgi (our unpublished data).

As noted above, an intermediate step of translocation of TGBp3 to PD involves its segregation in hypothetical 'TGBp3 islands' in ER membranes (ER-exit sites) (Fig. 6). These protein islands, which also include TGBp2, can be translocated using the targeting signal of TGBp3 to a specific receptor near PD in specific membrane containers (vesicles or tubules) (Stephens & Pepperkok, 2001; Nebenführ, 2002) delivered to the neck region of PD and fused there to cortical ER tubules (Fig. 6) (Solovyev et al., 2000; Cowan et al., 2002; Zamyatnin et al., 2002; Gorshkova et al., 2003).

Targeting of TGBp1 by TGBp2/TGBp3

Unlike potexviral TGBp1, which is capable of moving intracellularly to a peripheral layer of cytoplasm and PD (Lough et al., 1998; Malcuit et al., 1999; Morozov et al., 1999; Yang et al., 2000), hordei-like TGBp1 expressed individually is not targeted to specific sites at the cell periphery. However, when TGBp1 is expressed in the presence of other virus products, it localizes to the punctate structures at the CW (Erhardt et al., 1999b, 2000; Lawrence & Jackson, 2001a). At higher magnification, these structures are visible as pairs of disconnected bodies on opposite sides of the CW, closely resembling the structures formed by GFP–TGBp3 in close vicinity to PD (see above). Accordingly, GFP–TGBp1 punctate bodies co-localized with callose (Erhardt et al., 2000), confirming the immuno-gold detection of TGBp1 in PD of infected leaves (Erhardt et al., 1999b). Experiments with chimeric virus genomes suggested TGBp2 and TGBp3 as the most probable components responsible for this localization. Indeed, a combination of TGBp1 with homologous TGBp2/TGBp3 was required for TGBp1 function, particularly trafficking to PD (Lauber et al., 1998; Lough et al., 1998, 2000; Erhardt et al., 1999a, 2000; Solovyev et al., 1999; Lawrence & Jackson, 2001a; Zamyatnin et al., 2003). Hence, the role of TGBp2/TGBp3 may be primarily a matter of intracellular delivery of TGBp1-formed transport-competent RNPs to PD (Fig. 6).

There are indications that TGBp1 is actively, rather than passively, transported by TGBp2/TGBp3 to PD and that this process requires enzymatic activities of the protein. Mutations in the conserved sequence motifs in the NTPase/helicase domains of hordei-like TGBp1 not only blocked cell-to-cell movement of the virus but also abolished protein targeting to PD in the presence of TGBp2 and TGBp3 (Erhardt et al., 2000; Lawrence & Jackson, 2001a; Zamyatnin et al., 2003).

TGBp2-induced increase in PD permeability and other putative movement-related activities of TGBp2/p3 proteins

Some of the point and insertion mutants of TGBp2 are dominant–negative, i.e. they inhibit cell-to-cell movement and diminish virus accumulation (Beck et al., 1994; Seppanen et al., 1997; Lauber et al., 2001). The recently discovered ability of potex- and hordei-like TGBp2 to facilitate movement of GFP between adjacent epidermal cells (Tamai & Meshi, 2001; our unpublished data) suggests that co-expression of a non-functional TGBp2 mutant during infection can interfere not only with viral RNP trafficking to PD but also with some additional movement-related function(s).

The molecular nature of the TGBp2-directed increase in PD permeability is enigmatic. However, a relationship between this phenomenon and modifications of the tissue stress-response system can be proposed. In particle bombardment studies, the ability of GFP, which is a 27 kDa protein, to spread from an initially transfected epidermal cell of source N. benthamiana leaves to neighbouring cells depends on experimental conditions. When the leaves of intact plants are bombarded, GFP spreads over multiple cell boundaries to give a focus of more than 30 fluorescent cells. However, GFP is confined mostly to single cells after bombardment of detached leaves in a vacuum chamber (Oparka et al., 1999; Crawford & Zambryski, 2000, 2001; Itaya et al., 2000; Krishnamurthy et al., 2002). Under the latter conditions, TGBp2 potentiates the spread of GFP to adjacent epidermal cells (Tamai & Meshi, 2001). Various stress factors, including leaf detachment, are known to reduce PD SEL due to rapid callose deposition (Sivaguru et al., 2000; Crawford & Zambryski, 2001; Radford & White, 2001; Roberts & Oparka, 2003). Hence, it is possible that TGBp2 expression is not involved directly in increasing PD permeability but rather significantly decreases callose deposition in the CW and can thus inhibit or reverse the stress-induced decrease in PD SEL (Fig. 6). In line with this hypothesis, potexvirus TGBp2 has been shown recently to interact with TIP, a host protein regulator of -1,3-glucanase, which is a key enzyme of callose turnover (Fridborg et al., 2003). Thus, keeping the PD neck region open by callose degradation (or prevention of callose accumulation) is a possible function of TGBp2 at the early stage of infection (Fridborg et al., 2003). In this context, it is interesting that co-expression of TGBp3 and TGBp2 completely blocks the TGBp2-induced 'increase' of PD SEL (our unpublished data). Probably, trapping of TGBp2 in the peripheral membrane bodies formed by TGBp3 in the vicinity of PD (see above) can either prevent interaction between TGBp2 and TIP or directly block intracellular trafficking of TIP.

Apart from trafficking of TGBp1 and genomic RNA and PD SEL control, small TGB proteins could be involved directly in regulating a hypersensitive response (Bleykasten-Grosshans et al., 1997; Solovyev et al., 1999; Lauber et al., 2001; Kobayashi et al., 2001). We believe that this activity of TGBp2/p3 could also be related to the regulation of callose turnover in view of the fact that (i) limitation of PVX spread in a hypersensitive response is accompanied by heavy callose deposits in the vicinity of PD (Allison & Shalla, 1974) and (ii) callose deposition may regulate virus movement in hypersensitive hosts by affecting the PD SEL (Iglesias & Meins, 2000; Bucher et al., 2001; Crawford & Zambryski, 2001; Radford & White, 2001).

There is increasing evidence that viruses exploit endogenous intra- and intercellular trafficking pathways for spread of proteins and nucleic acids within plants. A growing number of plant cell proteins ['non-cell-autonomously acting plant proteins' (NCAPs)] has been demonstrated to have properties of plant virus MPs, such as the ability to increase PD SEL and traffic between cells. Moreover, some of these proteins are able to transport RNA through PD (Lucas, 1999; Crawford & Zambryski, 2000; Tzfira et al., 2000; Blackman & Overall, 2001; Lucas et al., 2001; Haywood et al., 2002; Heinlein, 2002b; Lee et al., 2003; Roberts & Oparka, 2003).

Importantly, TMV MP, similar to NCAP CmPP16, is not capable of PD modification and trafficking between cells in the absence of an assisting cell protein, NCAPP1 (Lee et al., 2003). The dependence of TMV MP on NCAPP1 emphasizes the idea that viral MPs act in concert with a number of as yet undiscovered cell proteins required to accomplish intra- and intercellular steps in cell-to-cell movement. Thus, analysis of dissimilar virus transport systems that comprise several MPs may suggest possible roles of viral proteins that mimic components of host intracellular trafficking machinery.

TGBp1s share some features with NCAPs involved in plant development. First, ectopic expression of potexviral TGBp1 appears to cause defects in the cell-to-cell communications that control lateral organ development (Foster et al., 2002). Second, TGBp1 competes directly for intercellular trafficking pathways with NCAP Knotted-1 (Lough et al., 2000). Third, the ability of NCAP CmPP16 to increase PD SEL and traffic through PD depends on an intracellular trafficking step directed by a membrane protein, NCAPP1, which localizes to cortical ER compartments in the vicinity of PD (Lee et al., 2003), a pathway that parallels TGBp1 transport to PD-associated ER structures directed by TGBp2/TGBp3 (Fig. 6) (Zamyatnin et al., 2002, 2003; Gorshkova et al., 2003).

Carmo- and necroviruses (family Tombusviridae) have a transport system of two small MPs, an RNA-binding protein and a membrane protein (Fig. 7) (Hacker et al., 1992; Marcos et al., 1999; Vilar et al., 2002). Therefore, in members of the family Tombusviridae, all energy-utilizing steps of movement likely depend on host proteins, a situation that is in contrast to TGB and other multicomponent transport systems. Particularly, in members of the family Potyviridae, cell-to-cell movement requires the CI protein (Fig. 7), which is an SF-II helicase able to interact with PD and form conical deposits guiding potyviral filamentous virions to and through PD (Rodriguez-Cerezo et al., 1997; Carrington et al., 1998; Roberts et al., 1998). However, it is not known whether the functions of the helicases in potexviruses and potyviruses (SF-I helicase TGBp1 and SF-II helicase CI) are similar. Viruses of the family Closteroviridae have no movement-related helicase but encode another ATPase, Hsp70h (Fig. 7), which is related to a large group of cell chaperones (Hsp70s) involved in energy-coupled processes of protein folding, degradation and transport (Agranovsky et al., 1991, 1997; Ellis & Hartl, 1999; Pilon & Schekman, 1999). Hsp70h is required for cell-to-cell and long-distance movement as well as for infectivity of virus particles and assembly of movement-competent virions (Agranovsky et al., 1998; Medina et al., 1999; Peremyslov et al., 1999; Napuli et al., 2000; Alzhanova et al., 2001; Prokhnevsky et al., 2002; Dolja, 2003). Similarly to TGBp1 and TGBp2/TGBp3 proteins (which mimic functions of NCAPs and NCAPP1, respectively), closteroviral Hsp70h may be the virus counterpart of a host component of a cell-to-cell movement machine.

Fig. 7. Comparison of multicomponent cell-to-cell transport systems encoded by plant viruses. Genes are shown as boxes with names of encoded proteins. Genes of proteins involved in cell-to-cell movement are shown in colour. PRO, proteinase domain; PVX, Potato virus X (X05198); CarMV, Carnation mottle virus (X02986); PVY, Potato virus Y (M95491); BYV, Beet yellows virus (X73476). Potyviral proteins implicated in virus spread in addition to CI protein (see text) are the genome-linked proteins (VPg), HCpro and CP (Callaway et al., 2001; Rajamaki & Valkonen, 1999; Revers et al., 1999; Rojas et al., 1997; Saenz et al., 2002). BYV proteins involved in cell-to-cell movement in addition to Hsp70h (see text) include CP and its distant homologue (dCP), the minor CP 64K, the papain-like leader proteinase responsible for processing replicative protein precursors and the 6K small hydrophobic protein resembling potex-like TGBp3 (Alzhanova et al., 2000; Peng et al., 2001, 2002, 2003; Napuli et al., 2003; Dolja, 2003).

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Recently, Aoki et al. (2002) identified a new subfamily of cell Hsp70 proteins that exhibit properties of NCAPs, including the ability to interact with PD. Another type of cell chaperone has been shown to interact with the MP of Tomato spotted wilt virus (von Bargen et al., 2001). Additionally, translocation of NCAP Knotted-1 through PD requires partial protein unfolding (Kragler et al., 1998; Roberts & Oparka, 2003), suggesting the role of cell chaperones at this step of host- and virus-specific cell-to-cell movement (Fig. 6). In general, it can be speculated that plant viruses have acquired and adopted distinct components of the cell trafficking machinery. As a result, these adaptive evolutionarily events have allowed viruses to recruit the existing host pathways of intra- and intercellular transport.

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We are grateful to Dr D. Baulcombe for providing unpublished data and to Dr N. S. Vassetzky for critical reading of the manuscript. Work of the authors was supported in part by Volkswagen-Stiftung, Federal Republic of Germany.

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