Virus Memberane Proteins- Source of Their Movement inside Plant Cells

Virus MP provides the means for viruses to exploit inter and intracellular trafficking pathways: in the case of viroids, the movement determinants are provided by secondary RNA structures (Wang and Ding 2010) that presumably interact with host proteins. The “colors and flavors” of MP have been reviewed extensively (Lucas 2006; Melcher 2000; Talianskyet al. 2008). Briefly, they can be divided into a few broad classes based upon functional properties. i) MP that show RNA-binding properties and mediate the translocation of the viral ribonuclear protein complexes

(RNP) without obvious structural reorganization of PD (e.g.,TMV 30K). Many of these proteins, alone or as part of the infection, are themselves trafficked through PD and may accumulate within the channel. ii) MP that bring about a structural reorganization of the PD channel through the formation of tubules that provide the conduit for the movement of virus particles or RNP from cell to cell (e.g., Cowpea mosaic virus [CPMV] MP). iii) A loose group of subsidiary proteins defined genetically as MP but that have functions more aligned to intracellular trafficking of viral components to PD (e.g., Potexvirus TGBp2 and TGBp3); these may or may not show the ability to bind to viral nucleic acids. Despite the functional commonalities of MP in mediating virus spread through PD, there is increasing evidence that the subtleties of specific virus movement may differ, even between quite closely related viruses. Nevertheless, it is the case that viruses that move from cell to cell as RNP have MP that bind nonspecifically to single-stranded (ss)RNA, an activity not widely demonstrated for MP of viruses moving as particles.

Nevertheless, these generalizations are also complicated by sometimes oversimplistic definitions of MP function. Classically, the term has been used for proteins that are necessary for spreading infections but which have little impact on replication in single cells when mutated. However, the multifunctional nature of many viral proteins means that subsidiary movement functions may also be associated with other proteins. For example, the 126-kDa replicase protein from TMV was shown to have a role in movement (Hirashima and Watanabe 2001,2003), and the P3 protein from Cauliflower mosaic virus (CaMV) was shown to bridge the interaction between the viral MP (P1) and the coat protein (CP) (Kobayashi et al. 2002); P3also supports aphid transmission of the CaMV (Stavalone et al.2005). The multifunctional nature of the proteins also means that MP may show alternative functions; for example, the 25-kDa MP of Potato virus X (PVX) also shows silencing suppressor activity (Bayne et al. 2005).More systematically, MP can be divided into several large evolutionary groupings based upon functional properties and predicted secondary structures (Melcher 2000).

The 30Kgroup is a large family, a subset of which is represented by the proteins that form physical tubules, which contain and translocate virus particles through PD. These are distinct from other TMV 30K-like MP that form RNP and do not form tubules. An alternative functional classification of MP based on whether they transmit virus particles or RNP has also been proposed (Scholthof 2005).

30K family

Evidence to support the concept of the 30K family of MP has come from complementation experiments using transgenic plants expressing Alfalfa mosaic virus (AMV) RNAs 1 and 2.These plants complement the replication of incoming cognate RNA3, which expresses MP and CP, and GFP to allow monitoring of viral cell-to-cell movement. AMV MP has a C-terminal domain of 44 amino acids (A44) that interacts with cognate C-Por virus particles but is dispensable for tubule formation and AMV movement. Also, AMV CP has a C terminus dispensable for AMV cell-to-cell movement but necessary for virus encapsidation (Sanchez-Navarro and Bol 2001; Tenllado and Bol2000). AMV RNA3 movement still occurs when both MP and CP are deleted of their C-terminal domains, indicating that AMV RNA3 could also be translocated through tubules as aviral RNP complex (Sánchez-Navarro and Bol 2001; Sánchez-Navarro et al. 2006).

In the complementation system, replacement of AMV MP with TMV MP showed RNA3 movement whether or not the AMV A44 domain was retained. In this case, movement must again have occurred as a viral RNP complex (Sánchez-Navarro et al. 2006). When MP from Bromemosaic virus, Cucumber mosaic virus (CMV), CPMV, Grapevine fan leaf virus (GFLV), CaMV, or Prunus necrotic ringspot virus were used, movement was recorded but only upon addition of the A44 domain. Because this list includes viruses from within and outside (CMV) the 30K-tubule-forming group, it strengthens the broader concept of the 30K family. With the exception of CaMV MP, all these modified MP retained their capacity to promote RNA3 local movement whether or not AMV CP was competent for encapsidation (Sanchez-Navarroet al. 2006, 2010). This was taken to indicate that caulimoviral MP mediated only particle movement, whereas tubule-forming MP from RNA viruses promoted the transport of RNA3 as a viral RNA-CP complex. The unique limitation applying to CaMV may provide a necessary protection to the genomic DNA in the PD, an environment designed for the non–cell autonomous transport of RNA (Sanchez-Navarro et al. 2010).This conservation of function for tubule-forming 30K family MP is in accordance with analyses of their organization. Hence, although they show little direct sequence relatedness, common functional domains for CaMV, CPMV, AMV, GFLV and TSWV MP have been defined by mutagenesis (Belin et al. 1999) (Figure 8).

Generally, the core of the protein is required for tubule formation while the C terminus is dispensable for this function but necessary for the interaction with cognate CP (via P3 in the case of CaMV) (Carvalho et al.2003).Although the majority of the viruses represented in the 30Kgroup appear to have single MP, Geminivirus spp. separately encode functions that transfer the ssDNA genome from the nucleus to PD and for translocation through PD (Jeske 2009).Interestingly, in the assay described above, complementation of AMV movement with Geminivirus MP was not successful, perhaps questioning the inclusion of the latter within the 30Kgroup (Sanchez-Navarro et al. 2010).