A protein that targets to curved sections of the ER enables a plant virus to spread between cells, Wu et al. show.
Animal viruses typically ride from cell to cell inside vesicles, but plant viruses spread via plasmodesmata, passageways that connect adjacent cells. Cortical ER tubules pass through the plasmodesmata, serving as highways for infection. Plant viruses often rely on so-called movement proteins to help them get around. In the group of viruses known as Potexviruses, the movement protein TGBp3 clusters with another movement protein, TGBp2, in the curved ER tubules near plasmodesmata. How the proteins home in on this location and whether their presence in the tubules is essential for infection wasn't clear.
Wu et al. tracked TGBp3 in yeast cells, which have a similar ER network as plants but lack plasmodesmata. They found that TGBp3 sports a sorting signal, a sequence of amino acids that directs the protein to the curved ER tubules. The sorting signal helps the virus spread: plants infected by viruses with mutations in the sequence stayed healthy or suffered only mild symptoms.
The researchers also found that the virus' third movement protein and its capsid, or protective protein coat, hook up with TGBp2. The virus' RNA might also connect to the TGBp3-TGBp2 complex. Curved ER tubules could be a staging point for the virus, where all of the viral components convene before moving on to the next cell. Researchers now need to determine how the virus triggers plasmodesmata to open up and let it through.