A protein kinase binds the C-terminal domain of the readthrough protein of Turnip yellows virus and regulates virus accumulation

- The C-terminal domain of TuYV-RT is required for long-distance movement.
- CIPK7 from Arabidopsis interacts with RTCter in yeast and in plants.
- CIPK7 overexpression increases virus titer locally but not virus systemic movement.
- CIPK7 localizes to plasmodesmata.
- CIPK7 could be a defense protein regulating virus export.

Turnip yellows virus (TuYV), a phloem-limited virus, encodes a 74 kDa protein known as the readthrough protein (RT) involved in virus movement. We show here that a TuYV mutant deleted of the C-terminal part of the RT protein (TuYV-∆RTCter) was affected in long-distance trafficking in a host-specific manner. By using the C-terminal domain of the RT protein as a bait in a yeast two-hybrid screen of a phloem cDNA library from Arabidopsis thaliana we identified the calcineurin B-like protein-interacting protein kinase-7 (AtCIPK7). Transient expression of a GFP:CIPK7 fusion protein in virus-inoculated Nicotiana benthamiana leaves led to local increase of wild-type TuYV accumulation, but not that of TuYV-∆RTCter. Surprisingly, elevated virus titer in inoculated leaves did not result in higher TuYV accumulation in systemic leaves, which indicates that virus long-distance movement was not affected. Since GFP:CIPK7 was localized in or near plasmodesmata, CIPK7 could negatively regulate TuYV export from infected cells.