Identification of a movement protein of rice yellow stunt rhabdovirus

Rice yellow stunt rhabdovirus (RYSV) encodes seven genes in its negative-sense RNA genome in the order 3'-N-P-3-M-G-6-L-5'. The existence of gene 3 in the RYSV genome and an analogous gene(s) of other plant rhabdoviruses positioned between the P and M genes constitutes a unique feature for plant rhabdoviruses that is distinct from animal-infecting rhabdoviruses in which the P and M genes are directly linked. However, little is known about the function of these extra plant rhabdovirus genes. Here we provide evidence showing that the protein product encoded by gene 3 of RYSV, P3, possesses several properties related to a viral cell-to-cell movement protein (MP). Analyses of the primary and secondary protein structures suggested that RYSV P3 is a member of the "30K" superfamily of viral MPs. Biolistic bombardment transcomplementation experiments demonstrated that RYSV P3 can support the intercellular movement of a movement-deficient potexvirus mutant in Nicotiana benthamiana leaves. In addition, Northwestern blot analysis indicated that the RYSV P3 protein can bind single-stranded RNA in vitro, a common feature of viral MPs. Finally, glutathione S- transferase pull-down assays revealed a specific interaction between the RYSV P3 protein and the N protein which is a main component of the ribonucleocapsid, a subviral structure believed to be involved in the intercellular movement of plant rhabdoviruses. Together, these data suggest that RYSV P3 is likely a MP of RYSV, thus representing the first example of characterized MPs for plant rhabdoviruses.