Membrane rafts and virus movement in plant cells.

Lipid rafts are liquid-ordered subdomains within cell membranes that are hypothesized to be highly dynamic assemblies that play important roles in signal transduction and membrane trafficking in the plasma membrane of both animal and plant cells (reviewed in Bhat and Panstruga, 2005; Brown, 2006). However, solid evidence of their importance and functions remains sketchy (Shaw, 2006). Studies in plants have identified a number of proteins enriched in detergent-insoluble membranes (believed to represent the raft component of the membrane), but such raft domains and localization of proteins to them have not been demonstrated directly. In this issue, Raffaele et al. (pages nnn) show that a group of proteins specific to vascular plants, called Remorins (REMs), have biochemical properties of membrane raft proteins and localize to discrete patches in the plasma membrane and plasmodesmata in tobacco and tomato. The authors also show that REM interferes with cell-to-cell movement of Potato virus X (PVX) in tomato plants. REMs are small, hydrophilic plasma membrane proteins originally identified in potato and shown to be phosphorylated in response to oligogalacturonide signals that stimulate a defense response to certain pathogens (Jacinto et al., 1993; Reymond et al., 1996). They have since been identified in other plant species. Raffaele et al. show that detergentinsoluble membranes purified from tobacco and tomato are highly enriched in REM and that the protein is anchored to the cytosolic side of the plasma membrane in purified vesicles. The authors used electron microscopy with immunogold labeling and a rigorous pattern-identifying statistical analysis to show that the REM label is clustered into microdomains ;70 nm in diameter in the plasma membrane (see figure). A series of immunolabeling and colocalization experiments using a plasmodesmatal marker showed that REM accumulates along the length of the plasmodesmatal channel. The authors constructed transgenic tomato plants with altered levels of REM by expression of sense or antisense REM constructs and obtained a range of individual lines that accumulated between 0.5 and 3.3 times the wild type amount of REM. They found that, after infection with PVX, there was a significant increase in viral accumulation in plants that underaccumulated REM, whereas plants overexpressing REM showed reduced viral accumulation in both inoculated and distal leaves. These results, together with localization data, suggest that REM function inhibits the movement of PVX through the plasmodesmata. Further experiments showed that REM binds directly to the virus movement protein TGBp1 from PVX and that it impairs cell-to-cell movement of the virus rather than viral replication inside cells. The work of Raffaele et al. adds weight to the hypothesis that membrane rafts play an important role in macromolecular trafficking. The authors speculate that REM binding to TGBp1 within membrane rafts could effectively titrate out TGBp1 activity and prevent it from carrying out its role in virus movement. Future studies will seek to clarify the effect of REM phosphorylation on TGBp1 binding and putative membrane raft function and whether REM has a specific role in relation to plant defense or a more generalized role in plasmodesmatal trafficking.