Cylindrical Inclusion Protein of Turnip Mosaic Virus Serves as a Docking Point for the Intercellular Movement of Viral Replication Vesicles.

Plant viruses move from the initially infected cell to adjacent cells through plasmodesmata (PDs). To do so, viruses encode dedicated protein(s) that facilitate this process. How viral proteins act together to support the intercellular movement of viruses is poorly defined. Here, by using an infection-free intercellular vesicle movement assay, we investigate the action of CI (cylindrical inclusion) and P3N-PIPO (amino-terminal half of P3 fused to Pretty Interesting Potyviridae open reading frame), the two PD-localized potyviral proteins encoded by Turnip mosaic virus (TuMV), in the intercellular movement of the viral replication vesicles. We provide evidence that CI and P3N-PIPO are sufficient to support the PD targeting and intercellular movement of TuMV replication vesicles induced by 6K2, a viral protein responsible for the generation of replication vesicles. 6K2 interacts with CI but not P3N-PIPO. When this interaction is impaired, the intercellular movement of TuMV replication vesicles is inhibited. Furthermore, in transmission electron microscopy, vesicular structures are observed in connection with the cylindrical inclusion bodies at structurally modified PDs in cells coexpressing 6K2, CI, and P3N-PIPO. CI is directed to PDs through its interaction with P3N-PIPO. We hypothesize that CI serves as a docking point for PD targeting and the intercellular movement of TuMV replication vesicles. This work contributes to a better understanding of the roles of different viral proteins in coordinating the intercellular movement of viral replication vesicles.